BALTIMORE,MD August16,2010

Plenary talksLeon Balents (Santa Barbara) Steven Bramwell (London) Zenji Hiroi (Tokyo) T. Senthil (Boston) Yoshinori Tokura (Tokyo)

Invited talksTaka-hisa Arima (Tohoku) Claudio Castelnovo (Oxford) John Chalker (Oxford) Radu Coldea (Oxford) Bruce Gaulin (Hamilton) George Jackeli (Stuttgart) Reizo Kato (RIKEN) Hikaru Kawamura (Osaka) Yong Baek Kim (Toronto) Tsuyoshi Kimura (Osaka) Bella Lake (Berlin) Philippe Mendels (Paris) Frederic Mila (Lausanne) Jonathan Morris (Berlin) Satoru Nakatsuji (Tokyo) Shivaji Sondhi (Princeton) Yasu Takano (Gainesville) Mike Zhitomirsky (Grenoble)

Program Co-chairsRoderich Moessner (Dresden) Hidenori Takagi (Tokyo)

Organizing CommitteeCollin Broholm (Baltimore) Jason Gardner (Gaithersburg) Seunghun Lee (Charlotteville) Oleg Tchernyshyov (Baltimore)

Tutorials (August 1)

Gabriel Aeppli (London) Michel Gingras (Waterloo) Subir Sachdev (Boston) Masashi Takigawa (Tokyo)

AdministratorSharon P. Karsk (Baltimore)

Conference ObjectivesThe International Conference on Highly Frustrated Magnetism HFM 2010 is the fifth conference in a series of meetings that began with the conference June 2000 in Waterloo, Canada, succeeded by the second conference held in Grenoble, France in June 2003. The third conference was held in Osaka, Japan in August 2006 and the fourth, HFM 2008 in Braunschweig, Germany. We are expecting to welcome approximately 200 participants, working in all areas of highly frustrated magnetism. The program comprises plenary and invited lectures, contributed talks and poster presentations. The topics of the HFM2010 conference will include: Quantum frustrated magnetism and spin liquids. Magnetic order in geometrically frustrated magnets. Itinerant frustrated systems and novel superconductivity. Spin glasses and random magnets. Coupling between lattice, orbital and charge degrees of freedom. Exotic phenomena induced by macroscopic degeneracy. Artificial and molecular frustrated magnets.

The HFM2010 Conference site is the Bloomberg Center for Physics and Astronomy at The Johns Hopkins University in Baltimore, Maryland. The Conference opens with a tutorial on Sunday 8/1 from 9-5:30 pm by leaders of the field for graduate students, post docs and newcomers to the field. There will be a Welcome reception and Registration on Sunday evening, 5:30-7:00 p.m. in the Bloomberg Center. The scientific sessions will start the morning of August 2nd, and end after a closing session on Friday, August 6, 2010 at 3 pm. There will be no proceedings but with the speakers permission talks will be recorded and posted on the conference web site.

Organizing CommitteeCollin Broholm (USA, Johns Hopkins University), Chair. Jason Gardner (USA, NIST) Seunghun Lee (USA, University of Virginia) Oleg Tchernyshyov (USA, Johns Hopkins University)

International Advisory BoardLeon Balents (USA) Stephen Bramwell (UK) Bruce Gaulin (Canada) Hikaru Kawamura (Japan) Reinhard Kremer (Germany) Claire Lhuillier (France) Roderich Moessner (Germany) Bruce Normand (China) Arthur Ramirez (USA) Hidenori Takagi (Japan) Hirokazu Tsunetsugu (Japan)

International Program CommitteeRoderich Moessner (Germany), Co-Chair Hidenori Takagi (Japan), Co-Chair Leon Balents (USA) John Chalker (UK) Radu Coldea (UK) John Cumings (USA) Michel Gingras (Canada) Zenji Hiroi (Japan) Peter Holdsworth (France) Tsuyoshi Kimura (Japan) Philippe Mendels (France) Maxim Mostovoy (Netherlands) Yukitoshi Motome (Japan) Satoru Nakatsuji (Japan) Subir Sachdev (USA) B. Sriram Shastry (USA) Oleg Starykh (USA) Andrey Zheludev (Switzerland)

Financial Support

The National Science Foundation (NSF) is an independent federal agency created by Congress in 1950 to promote the progress of science; to advance the national health, prosperity, and welfare; to secure the national defense. With an annual budget of about $6.9 billion (FY 2010), [NSF] is the funding source for approximately 20 percent of all federally supported basic research conducted by America's colleges and universities. In many fields such as mathematics, computer science and the social sciences, NSF is the major source of federal backing.

The NCNR is part of the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland. Its activities focus on providing neutron measurement capabilities to the U.S. research community. It is a national center for research using thermal and cold neutrons, offering its instrumentation for use by all qualified applicants. Many of its instruments rely on intense beams of cold neutrons emanating from an advanced liquid hydrogen moderator.

The Neutron Sciences Directorate at Oak Ridge National Laboratory (ORNL) operates two of the worlds most advanced neutron scatter research facilities: the Spallation Neutron Source (SNS) and the High Flux Isotope Reactor (HFIR). Funding is provided by the U.S. Department of Energy Office of Basic Energy Sciences.

The Johns Hopkins University opened in 1876, with the inauguration of its first president, Daniel Coit Gilman.The mission laid out by Gilman remains the university's mission today"Knowledge for the world." What Gilman created was a research university, dedicated to advancing both students' knowledge and the state of human knowledge through research and scholarship.The realization of Gilman's philosophy at Johns Hopkins, and at other institutions that later attracted Johns Hopkins-trained scholars, revolutionized higher education in America, leading to the research university system as it exists today. After more than 130 years, Johns Hopkins remains a world leader in both teaching and research.in the arts and music, the humanities, the social and natural sciences, engineering, international studies, education, business and the health professions.

The Institute for Complex Adaptive Matter (ICAM-I2CAM) is a distributed experimentbased multi-institutional partnership whose purpose is to identify major new research themes in complex adaptive matterthe search for an understanding of emergent behavior in hard, soft, and living matter.*

*To be confirmed

Instructions for the PresentationsOral presentations (Plenary, Invited and Contributed Talks)The scheduled presentation time is: Plenary Lectures (50 min.) Invited Talks (30 min.) Contributed Talks (20 min.) Common PC projection devices will be available for all oral presentations. Speakers are asked to bring their talk on a USB device and load it onto the presentation computer during breaks. A PC and a MAC with the latest version of PowerPoint, Acrobat & Keynote will be available.

Poster PresentationsThe Poster Sessions will be on Tuesday, August 3rd and Thursday, August 5th. Posters are to be mounted on the walls and poster boards provided, on the 2nd and 4th floor Rotunda areas of the Bloomberg Center. The space available for each poster is 122 cm wide by 152 cm tall. Please prepare your posters so they can easily be read from a few meters distance. The boards will be labeled with poster ID numbers. Posters will be ordered according to the numbers assigned by the organizers as given in the Program and Abstract Book.

BANQUETThe HFM2010 Excursion and Conference Banquet will be held on Wednesday, August 4, 2010 beginning at 15:20. The Banquet will be preceded by a walking tour of Ft. McHenry and tours of Piers 3 and 4 of the Baltimore National Aquarium (BNA). Dinner will be served at approx. 20:00 at the BNA. Bus transportation will be provided between the JHU Homewood Campus and downtown Baltimore destinations. The Banquet fee is included in the Conference Registration fee.

BALTIMORE, MD August 1-6 2010

15:25 16:00-17:15 17:15-17:30 17 15 17 30 18:00-19:00 19:00-20:00 20:00-21:30 22:00

Depart JHU for Baltimore Inner Harbor Ft. McHenry Board buses for trip B db f t ip to Baltimore National Aquarium Self guided tour of Pier 3 Reception, Pier 4 Lobby & Atrium Banquet Dinner, Pier 4 Atrium Depart for JHU

*HFM2010 Conference ID (name tags) will be required to board buses, for participation in all activities and to attend the Banquet.

General Contact Information for Participants HFM2010 Local Organizing Committee: Dr. Collin Broholm (c) Dr. Jason Gardner Dr. Oleg Tchernyshyov (c) Staff: Sharon Karsk (c) Woodland Pomeroy (c) Physics & Astronomy Dept. Chair: Dr. Daniel H. Reich Physics & Astronomy Admin. Norma Berry Security/JHU (Emergencies) Charles Commons Inn at the Colonnade Radisson at Cross Keys Taxi/Limo service:u e ( o po S oe Washington International[BWI], Dulles Int'l([IDA], Ronald Reagan national [DCA] to BWI or DCA: SupperShuttle City Transportation(limo service) Yellow Taxi Diamond Cab Columbia Taxi Service Amtrak (443) 779-5592 (301) 975-8391 (410) 215-2585 (410) 591-3948 (443) 803-3659 (410) 516-7346 (410) 516-7815 (410) 516-7777 (410) 516-0160 (410) 235-5400 (866) 757-0610 (800)776-0323 3301 N. Charles St. 4 West University Parkway 5100 Falls Road JHU N.I.S.T JHU

For Local Bus, Metro Subway, Light Rail, MARC Train and Commuter Bus: Union Memorial Hospital

(800) 258-3826 (443) 520-0338 (410) 685-1212 (410) 947-3333 (443) 355-4607 (800) 872-7245 or TDD/TTY (800) 5236590 Monday-Friday from 6:00 (410) 539-5000 am - 7:00 pm. (410) 554-2000 201 East University Parkway

Azafran

HFM2010 Short ProgramTime 9:00 9:45 Aug.1, Sunday Sachdev I Unified theory of spin liquids I Gingras I Spin ice I 9:50 Time 8:50 9:00 Aug. 2, Monday Opening P-1 Anisotropic triangular AFM Balents (I) I-1 Ising chains 10:30 Coffee Break 10:20 Coldea (I) I-2 Spont. Hall effect 11:00 Takigawa I Applications of NMR to QM I 10:50 11:20 Nakatsuji Coffee Break (C) I-1 Criticality and frustration 11:40 11:45 Aeppli I Magnetic correlations in nonclassical magnets I 12:00 Lee (C) I-2 Metal-insulator trans. Becca (C) I-3 Spin-charge-orbital 12:30 14:00 Lunch provided Sachdev II Unified theory of spin liquids II 12:20 14:00 Motome Lunch provided (I) II-1 Spin dynamics 14:30 14:45 Gringas II Spin ice II Chalker (I) II-2 Planar pyrochlores a a py oc o es 15:00 Gaulin (C) II-1 2D spins in Yb2Ti2O7 15:20 15:30 16:00 Coffee Break Takigawa II Applications of NMR to QM II Thompson (C) II-2 Coupled spins & phonons 15:40 16:00 Ruff Coffee Break (I) III-1 Triangular AFM 16:30 16:45 Aeppli II Magnetic correlations in nonclassical magnets II 17:00 Zhitomirsky (I) III-2 Kagome Mendels (C) III-1 Spinons on kagome Hao 17:30 -18:30 Welcome Reception and Registration 17:20-17:40 (C) III-2 RE magnetism on kagome Zorko Ft. McHenry & Baltimore National Aquarium POSTER I Excursion & Banquet Dinner POSTER II Aug. 3, Tuesday P-2 Skymions and multiferroics Tokura (C) Invited IV-1 Dielectric measurements Kimura (C) IV-1 Multiferroic Ni3V2O8 Cabrera Coffee break (C) IV-2 Chirality in langasite Loire (C) IV-3 Quasi-1D Azurite Honecker (C) IV-4 FREE Lunch provided (I) V-1 Spin liquid in 2D Kawamura (C) V-1 Slow spins in NiGa2S4 p Nambu (C) V-2 Organic spin liquid Itou (C) V-3 Classical kagome Taillefumer 15:40-17:50 15:20 to 22:00 Aug. 4, Wednesday P-3 New magnets Hiroi (I) VI-1 Molecular Mott insulator Kato (I) VI-2 Quantum dimers Lake Coffee break (C) VI-1 Spin texture Moessner (C) VI-2 High fields, volborthite Takigawa (C) VI-3 Spin liquid excitations Yamashita Lunch provided (I) VII-1 X rays studies Arima (I) VII-1 Sp o b t entanglement Spin-orbit e ta g e e t Jackeli (C) VII-2 RXS in Ir oxides Fujiyama Aug. 5, Thursday P-4 Spin ice Bramwell (I) VIII-1 Spin ice, monopoles Castelnovo (I) VIII-2 Dirac strings in spin ice Morris Coffee break (C) VIII-1 Quantum spin ice Shannon (C) VIII-2 QCP in itinerant ice Udagawa (C) VIII-3 Topology Holdsworth Lunch provided (I) IX-1 Plateaus in dimerized magnets Mila (I) IX-2 Romantic t eo y o a t c theory Sondhi (C) IX-1 Non-abelian height models Henley (C) IX-2 Nematic valence-bond order Paramekanti 15:40-17:50 Aug. 6, Friday P-5 Quantum spin liquids Senthil (I) X-1 Topological insulator Y.B.Kim (I) X-2 Triang. antiferromagnet Takano Coffee break (C) X-1 Orbitals and trimerization Katsufuji (C) X-2 Artificial spin ice Daunheimer (C) X-3 Artificial spin ice Li Lunch provided Summary (experiment) Broholm Summary (theory) Tchernyshyov Award Ceremony & Closing Remarks Broholm

Program of Oral PresentationsSunday, August 1, 2010Tutorial lectures will begin at 9:00 and continue until 18:30 with breaks for coffee and lunch. 9:00-9:45 9:45-10:30 Unified theory of spin liquids I Spin ice: A fascinating playground for the experimental and theoretical study of frustrated magnetism I S. Sachdev M. Gingras

Coffee break 11:00-11:45 11:45-12:30 Lunch 14:00-14:45 14:45-15:30 Unified theory of spin liquids II Spin ice: A fascinating playground for the experimental and theoretical study of frustrated magnetism II S. Sachdev M. Gingras Applications of NMR to quantum magnetism I Magnetic correlations in non-classical magnets I M. Takigawa G. Aeppli

Coffee break 16:00-16:45 16:45-17:30 17:30-19:00 Applications of NMR to quantum magnetism II Magnetic correlations in non-classical magnets II Welcome Reception & Registration M. Takigawa G. Aeppli

Monday, August 2, 20108:50-9:00 9:00-9:50 (p) Opening Quantum phases and excitations of spatially anisotropic triangular antoferromagnets. C. Broholm L. Balents

9:50-10:20 (i)

Quantum criticality in weakly-coupled Ising chains in transverse field. Frustrated magnetism and spontaneous Hall effect in Pr2Ir2O7.

R. Coldea

10:20-10:50 (i)

S. Nakatsuji

Coffee break 11:20-11:40 (c) Interplay of quantum criticality and geometric frustration in columbite. Metal-insulator transition and Fermi surface evolution in frustrated triangular-based lattices. Spin-charge-orbital coupled phenomena in Mo pyrochlore oxides: Monte Carlo study of the pyrochlore double-exchange model. S.B. Lee

11:40-12:00 (c)

F. Becca

12:00-12:20 (c)

Y. Motome

Lunch 14:00-14:30 (i) Spin dynamics in pyrochlore Heisenberg antiferromagnets. Phase transitions in planar pyrochlores. Anisotropic exchange and emergence of quasi-two-dimensional spin-spin correlations in the Yb2Ti2O7 pyrochlore. Magnetoelastics of a spin liquid: X-ray diffraction studies of Tb2Ti2O7 in pulsed magnetic fields. J. Chalker

14:30-15:00 (i) 15:00-15:20 (c)

B. Gaulin J. Thompson

15:20-15:40 (c)

J. Ruff

Coffee break 16:00-16:30 (i) Triangular antiferromagnet : magnon decays and the fate of discrete symmetries. Herbertsmithite: A quantum Heisenberg "spin liquid" on the kagome network. Spinons in the S=1/2 Heisenberg antiferromagnet on kagome. Rare-earth based magnetism on kagome lattice. M. Zhitomirsky

16:30-17:00 (i)

P. Mendels

17:00-17:20 (c)

Z. Hao

17:20-17:40 (c)

A. Zorko

Tuesday, August 3, 20109:00-9:50 (p) Multiferroics and Skyrmion crystals as derived from helimagnets. Dielectric measurement of frustrated spin systems. Complex magnetic structures and electric field studies of kagome-staircase compound Ni3V2O8. Y. Tokura

9:50-10:20 (i)

T. Kimura

10:20-10:50 (c)

I. Cabrera

Coffee break 11:20-11:40 (c) 11:40-12:00 (c) Chirality and frustration in a Fe langasite. Azurite: a highly frustrated quasi-1D antiferromagnet. M. Loire A. Honecker

Lunch 14:00-14:30 (i) Spin liquid and novel order in twodimensional frustrated Heisenberg antiferromagnets. Slow spin dynamics in 2D magnetism of NiGa2S4. Spin liquid state and its instability in the organic triangular 1/2-spin system EtMe3Sb[Pd(dmit)2]2. Spin dynamics of classical kagome antiferromagnets. H. Kawamura

14:30-15:00 (c)

Y. Nambu

15:00-15:20 (c)

T. Itou

15:20-15:40 (c)

M. Taillefumer

Poster Session I

Wednesday, August 4, 20109:00-9:50 (p) 9:50-10:20 (i) Highly frustrated magnets of current interest. A molecular Mott system with a quasi-triangular lattice. Z. Hiroi Reizo Kato

10:20-10:50 (i)

Magnetic excitations of the quantum dimer system Sr3Cr2O8.

B. Lake

Coffee break 11:20-11:40 (c) 11:40-12:00 (c) 12:00-12:20 (c)

Fractional spin textures from dilution. High-field phases in volborthite. Bipartite spin excitations in a two-dimensional quantum spin liquid state.

R. Moessner M. Takigawa M. Yamashita

Lunch 14:00-14:30 (i) Synchrotron X rays as a useful probe for frustrated magnets. Spin-orbit entanglement in Mott insulators: new route to unusual phases. Spin correlations of Sr2IrO4 and Eu2Ir2O7 observed by resonant magnetic x-ray diffuse scattering. T.H. Arima

14:30-15:00 (i)

G. Jackeli

15:00-15:20 (c)

S. Fujiyama

Excursion and Banquet

Thursday, August 5, 20109:00-9:50 (p) 9:50-10:20 (i) Spin ice. Magnetic monopole physics in spin ice materials. Dirac strings and magnetic monopoles in spin ice Dy2Ti2O7. S. Bramwell C. Castelnovo

10:20-10:50 (i)

J. Morris

Coffee break 11:20-11:40 (c) Numerical evidence for U(1) liquid phases in three dimensions: Does quantum ice have monopoles, too? Quantum criticality in itinerant ice-rule systems. N. Shannon

11:40-12:00 (c)

M. Udagawa

12:00-12:20 (c) Lunch 14:00-14:30 (i)

Topological sector fluctuations in Ho2Ti2O7.

P. Holdsworth

Magnetization plateaus in dimer-based frustrated magnets close to a quantum phase transition. TBA Classical topological order in non-abelian height models. Nematic valence-bond order and magnetic supersolidity in highly frustrated magnets.

F. Mila

14:30-15:00 (i) 15:00-15:20 (c)

S. Sondhi C. Henley

15:20-15:40 (c)

A. Paramekanti

Poster Session II

Friday, August 6, 20109:00-9:50 (p) 9:50-10:20 (i) Theory of quantum spin liquids. Spin liquid and topological insulator in frustrated magnets. High-field phases of the spin-1/2 triangular antiferromagnet Cs2CuBr4. T. Senthil Y. B. Kim

10:20-10:50 (i)

Y. Takano

Coffee break 11:20-11:40 (c) Trimerization with orbital ordering in various vanadates. Lorentz force microscopy of artificial spin ice. Geometry tuning and clusters in artificial frustrated magnets. T. Katsufuji

11:40-12:00 (c) 12:00-12:20 (c)

S. Daunheimer J. Li

Lunch 14:00-14:20 14:20-14:40 14:40-15:00 Conference Summary (experiment) Conference Summary (theory) Award Ceremony and Closing Remarks. C. Broholm O. Tchernyshyov C. Broholm

Oral Presentations: S d O lP t ti Sunday

Abstract ID T104

Unied theory of spin liquidsSubir Sachdev Department of Physics, Harvard University, Cambridge, MA, USA I will give a general introduction to the theory of quantum spin liquids. A large number of proposals have been made, most relying on a description in terms of neutral fermions or bosons. I will show how all of these can be unied in a common formalism, which exposes the relationship between the various distinct phases. Majorana fermions play a central role in this unifying approach.

Abstract ID T019

Spin Ice: a Fascinating Playground for the Experimental and Theoretical Study of Frustrated MagnetismMichel Gingrasa,b a Dept. of Physics and Astronomy, U. of Waterloo, Waterloo, Ontario N2L 3G1, Canada b Canadian Institute for Advanced Research, Toronto, Ontario M5G 1Z8, Canada Geometric frustration usually arises in systems that comprise magnetic moments (spins) which reside on the sites of a lattice made up of elementary triangular or tetrahedral units and which interact via antiferromagnetic nearest-neighbor exchange. Albeit much less common, geometric frustration can also arise in systems with strong non-collinear single-ion easyaxis (Ising-like) anisotropy and ferromagnetically coupled spins. This is what happens in some pyrochlore oxide materials where Ising-like magnetic rare earth moments (Ho3+ , Dy3+ ) sit on a lattice of corner-shared tetrahedra and are coupled via eectively ferromagnetic (dipolar) interactions [1]. These systems possess a macroscopic number of quasi-degenerate classical ground states and display an extensive low-temperature entropy closely related to the extensive proton disorder entropy in common water ice. For this reason, these magnetic systems are called spin ice [1-5]. In this talk, I shall review the essential ingredients of spin ice phenomenology in the A2 B2 O7 magnetic pyrochlore oxides [6] and discuss some of the most recent theoretical and experimental developments in the study of these systems. [1] M. J. Harris et al., Phys. Rev. Lett. 79, 2554(1997). [2] A. P. Ramirez et al., Nature 399, 333 (1999). [3] S. T. Bramwell and M. J. P. Gingras, Science 294, 1495(2001). [4] S. T. Bramwell, M. J. P. Gingras and P. C. W.Holdsworth, in Frustrated Spin Systems, ed. H. T. Diep, (World Scientic Publishing, Singapore, 2004); p.367. [5] M. J. P. Gingras, Spin Ice, to appear as a chapter in Highly Frustrated Magnetism, Eds. C. Lacroix, P. Mendels, F. Mila, (Springer, in press); preprint at http://arXiv.org/abs/0903.2772 (2009). [6] J. S. Gardner, M. J. P. Gingras and J. E. Greedan, Rev. Mod. Phys. 82, 53 (2010).

Abstract ID T041

Application of NMR to quantum magnetismMasashi Takigawa Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan In this tutorial lecture, I will illustrate how the nuclear magnetic resonance (NMR) technique can be used to identify various symmetry broken phases and characterize the magnetic dynamics in correlated electron systems with particular emphasis on highly frustrated materials. I will start with the basic description of the hyperne interaction between electrons and nuclei. I will then demonstrate how one can get information on the static microscopic structure of the electronic degrees of freedom from the observation of hyperne elds at the nuclei, using the knowledge of local symmetry properties of the nuclear sites. One advantage of the NMR technique is the local nature of the probe. When there are more than two types of order in a given phase, for example, one can determine whether they coexist uniformly or occur in spatially distinct regions. It is also very powerful for impurity problems, since NMR provides information on the spatial distribution of spin or charge density near impurities of defects. Another advantage is that one can study dynamics by NMR. I will show several examples to illustrate how the magnetic dynamics can be understood from the measurements of longitudinal and transverse nuclear relaxation rates.

Abstract ID W062 Magnetic correlations in non-classical magnetsG. Aeppli Department of Physics and Astronomy, University College London and London Centre for Nanotechnology, 17-19 Gordon Street London WC1H 0AH, London , UK

We describe how magnetic susceptibility as well as neutron and X-ray techniques can measure directly correlation functions in non-classical magnets. Particular emphasis will be given to nontraditional order parameters, including those associated with both frustrated classical magnets and quantum spin fluids, and non-linear response functions.

Oral Presentations: M d O lP t ti Monday

Abstract ID T026

Quantum phases and excitations of spatially anisotropic triangular antiferromagnetsLeon Balents,a Hosho Katsura,a and Oleg Starykh,b Kavli Institute for Theoretical Physics, University of California, Santa Barbara, CA-93106-9530 b Department of Physics and Astronomy, University of Utah, Salt Lake City, UT 84112a

Fig. 1. One of many possible ground states of the spatially anisotropic triangular lattice antiferromagnet in an applied eld.

The S = 1/2 quantum antiferromagnet on the triangular lattice is one of the archetypal models of frustrated magnetism. It exhibits a rich phase diagram as a function of temperature, magnetic eld, and spatial anisotropy, many aspects of which have been and are being studied experimentally. I will review theory and experiments on such systems, and then zero in on issues of particular relevance to recent experiments on Cs2 CuCl4 [1,2,3] and Cs2 CuBr4 [4,5], which are materials in this class. I plan to specically report on a recent theoretical study of the problem based on a quasi-onedimensional approach, which is in excellent agreement with experiment. The analysis reveals some surprising physics. First, we nd that, when the magnetic eld is oriented within the triangular layer, spins are actually most strongly correlated within planes perpendicular to the triangular layers. Second, the phase diagram in such orientations is exquisitely sensitive to tiny interactions neglected before this study. These interactions, which we describe in detail, induce entirely new phases, and a novel commensurate-incommensurate transition, the signatures of which are identied in NMR experiments. We discuss the dierences between the behavior of Cs2 CuCl4 and an ideal two-dimensional triangular model, and in particular the occurrence of magnetization plateaux in the latter as observed in Cs2 CuBr4 . We acknowledge useful discussions with J. Alicea, A. Chubukov, R. Coldea, V. Mitrovic, M. Takigawa, and Y. Takano. This research was supported in part by NSF grants PHY0551164, DMR-0804564 (LB), DM-0808842 (OS), the Packard Foundation (LB) and the JSPS Postdoctoral Fellowship for Research Abroad (HK). [1] [2] [3] [4] [5] R. Coldea et al, Phys. Rev. Lett. 86, 1335 (2001). Y. Tokiwa et al, Phys. Rev. B 73, 134414 (2006). M. Yoshida et al., Poster presentation, HFM2008 (2008); and private communication. T. Ono et al., J. Phys.: Cond. Matt. 16,S773 (2004). N. A. Fortune et al., Phys. Rev. Lett. 102, 257201 (2009).

Abstract ID W048 Quantum criticality in weakly-coupled Ising chains in transverse fieldR. Coldea,a D.A. Tennant,b E.M. Wheeler,a E. Wawrzynska,c D. Prabhakaran,a M. Telling,d K. Habicht,b P. Smeibidl,b and K. Kieferb a Clarendon Laboratory, Oxford University Physics Department, Parks Road, Oxford OX1 3PU,UK b Helmholtz-Zentrum Berlin fr Materialien und Energie, Lise Meitner Campus, Glienicker Str. 100, D-14109 Berlin, Germany c H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, UK d ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, OX11 0QX, UK

a)

b) Co2+ O2-

Fig. 1. a) Phase diagram of the Ising chain in transverse field showing a continuous quantum phase transition between the low-field, magnetically-ordered phase and the high-field paramagnet. The magnetic fields required to access this transition are experimentally attainable in CoNb2O6 where Co2+ Ising spins form ferromagnetic zig-zag chains (b). The one-dimensional chain of Ising spins in transverse magnetic field is one of the theoretically most studied paradigms for a continuous zero-temperature phase transition [see Fig 1a)] and is one of the cornerstones of the general theory of quantum criticality [1]. We have recently realized this system experimentally by applying strong magnetic fields to the quasi-one-dimensional, low-exchange Ising ferromagnet CoNb2O6 [2]. Using high-resolution single crystal neutron scattering we have probed how the spin dynamics evolves with the applied field and have observed a dramatic change in the character of spin excitations at the quantum critical point, from pairs of domain-wall (kink) quasiparticles in the magnetically ordered phase to sharp spin-flip quasiparticles in the paramagnetic phase. We have also observed how the small and frustrated couplings between the chains significantly enrich the physics, and in particular stabilize a complex structure of two-kink bound states due to mean-field confinement effects. The data is found to be in good agreement with analytic theories of confinement in zero field [3] and also in the scaling limit near the quantum critical point [4]. [1] S. Sachdev, in Quantum Phase Transitions (Cambridge, 1999). [2] R. Coldea et al., Science 327, 177 (2010). [3] B. M. McCoy and T. T. Wu, Phys. Rev. D 18, 1259 (1978). [4] A.B. Zamolodchikov, Int. J. Mod. Phys. A4, 4235 (1989).

Abstract ID W051 Frustrated Magnetism and Spontaneous Hall Effect in Pr2Ir2O7Satoru Nakatsuji,a Yo Machida, a Yasuo Ohtaa, and Jun Ishikawaa a Institute for Solid State Physics, University of Tokyo, Kashiwa 277-8581, Japan An electric current flowing through a conductor in a magnetic field produces a transverse voltage drop known as the Hall effect. In the absence of the field, this effect also appears in ferromagnets in a plane normal to its spontaneous magnetization vector owing to the spin-orbit coupling. Generally, it may also detect a nontrivial order parameter breaking the time-reversal symmetry on a macroscopic scale, for example, scalar spin chirality. In this talk, we present our recent results in the study of the frustrated magnetism and Hall transport of the metallic magnet Pr2Ir2O7 [1,2,3]. Strikingly, a spontaneous Hall effect is observed in the absence of both an external magnetic field and conventional magnetic long-range order [3]. This strongly suggests the existence of a spin-liquid phase breaking the time-reversal symmetry. Both our measurements suggest that spin-ice correlations in the liquid phase lead to a non-coplanar spin texture forming a uniform but hidden order parameter: the spin chirality. This is the work performed in collaboration with S. Onoda, T. Sakakibara, T. Tayama, Y. Uwatoko, D. E. MacLaughlin, C. Broholm, Y. Nambu, C. Stock.

[1] S. Nakatsuji, Y. Machida, Y. Maeno, T. Tayama, T. Sakakibara, J. van Duijn, L. Balicas, J. N. Millican, R. T. Macaluso, and Julia Y. Chan, Phys. Rev. Lett. 96, 087204 (2006). [2] Y. Machida, S. Nakatsuji, Y. Maeno, T. Tayama, T. Sakakibara, S. Onoda, Phys. Rev. Lett. 98, 057203 (2007). [3] Yo Machida, Satoru Nakatsuji, Shigeki Onoda, Takashi Tayama, Toshiro Sakakibara, Nature, 463, 210 (2010).

Abstract ID T002

Interplay of Quantum Criticality and Geometric Frustration in ColumbiteSungBin Lee,a Ribhu K. Kaul,b and Leon Balents,c a Department of Physics, University of California, Santa Barbara, CA-93106-9530 b Microsoft Station Q, University of California, Santa Barbara, CA-93106-6105 c Kavli Institute for Theoretical Physics, Univeristy of California, Santa Barbara, CA-93106-9530 Two of the most exciting themes in condensed matter physics are the destruction of magnetic order at absolute zero temperature, i.e quantum criticality [1], and quantum uctuations in geometrically frustrated magnets. CoNb2 O6 , one of the columbite family, forms weakly coupled Ising chains that sit on a triangular lattice in the plane perpendicular to the chain direction as shown in Fig.1. This material exhibits both quantum criticality (by applying a transverse eld to Ising chains) and geometric frustration (Ising chains form a triangular lattice). With the chain mean eld approximation, we apply the exact solutions of one dimensional Ising eld theory in a magnetic eld, studied by Zamolodchikov [2]. When there is an isosceles distortion, the T = 0 phase diagram is very rich with ve dierent states: ferrimagnetic, Nel, anti-ferromagnetic, paramagnetic and incommensurate phases. We also discuss the e excitation spectrums based on these phases and a nite temperature phase transitions, which are expected to be measured by experiment.

Fig. 1. Schematic structure of Co2+ ions in CoNb2 O6 . Ferromagnetic coupling along c direction (J0 < 0) and anti-ferromagnetic coupling (J1 , J2 > 0, J2 /J1 1) on a-b plane.

We thank Radu Coldea for stimulating discussions and acknowledge support from the Packard Foundation and National Science Foundation through grants DMR-0804564 and PHYS0551164. [1] Subir Sachdev, Quantum Phase Transition (1999). [2] A. B. Zamolodchikov, Int. J. Mod. Phys. A 4, 4235 (1989).

Abstract ID T092

Metal-insulator transition and Fermi surface evolution in frustrated triangular-based latticesFederico Becca,a Luca Tocchio,b and Claudius Grosb a CNR-IOM-Democritos National Simulation Centre and International School for Advanced Studies (SISSA), Trieste, Italy b Institute for Theoretical Physics, Frankfurt University, Frankfurt a.M., Germany The triangular lattice is the simplest two-dimensional network where magnetic frustration takes place. In this talk we present two important aspects of correlated itinerant systems in triangular-based lattices. The rst one deals with the possibility to stabilize a pure Mott phase when the on-site Coulomb interaction is suciently strong. In the second part, instead, we will show the interesting evolution of the Fermi surface across the Mott-Hubbard transition. The two-dimensional Hubbard model on the anisotropic triangular lattice, with two dierent hopping amplitudes t and t , is relevant to describe the low-energy physics of -(ET)2 X, a family of organics salts (whose building block is the so-called BEDT-TTF or ET molecule and X is a monovalent anion).[1] Recent ab-initio calculations, based upon local-density approximation (LDA) and generalized gradient approximation (GGA), made it possible to have an accurate estimate of the microscopic parameters that dene the low-energy Hubbard model.[2,3] The ground-state properties of this model are studied by using Monte Carlo techniques, on the basis of a recent denition of backow correlations for strongly-correlated lattice systems.[4] The results show that there is no magnetic order for reasonably large values of the electron-electron interaction U and frustrating ratio t /t = 0.85, suitable to describe the non-magnetic compound with X=Cu2 (CN)3 .[5] On the contrary, Neel magnetic order takes place for weaker frustrations, namely t /t = 0.4 0.6, suitable for materials with X=Cu2 (SCN)2 , Cu[N(CN)2 ]Cl or Cu[N(CN)2 ]Br. By using a similar variational approach, we analyze the evolution of the renormalized excitation spectra in the one-dimensional tt Hubbard model (that can be viewed as a two-leg triangular strip).[6] We nd that the Fermi surface renormalizes to perfect nesting right at the Mott-Hubbard transition in the insulating state, with a rst-order reorganization when crossing into the conducting state. We also report preliminary results for two-dimensional lattices and argue that this is a genuine aspect of the Fermi surface in frustrated systems.[7] [1] K. Kanoda, J. Phys. Soc. Jpn. 75, 051007 (2006). [2] K. Nakamura, Y. Yoshimoto, T. Kosugi, R. Arita, and M. Imada, J. Phys. Soc. Jpn. 78, 083710 (2009). [3] H. C. Kandpal, I. Opahle, Y.-Z. Zhang, H. O. Jeschke, and R. Valenti, Phys. Rev. Lett. 103, 067004 (2009). [4] L.F. Tocchio, F. Becca, A. Parola, and S. Sorella, Phys. Rev. B 78, 041101(R) (2008). [5] L.F. Tocchio, A. Parola, C. Gros, and F. Becca, Phys. Rev. B 80, 064419 (2009). [6] L.F. Tocchio, F. Becca, and C. Gros, Phys. Rev. B 81, 205109 (2010). [7] L.F. Tocchio, F. Becca, and C. Gros, in preparation.

Abstract ID T018

Spin-charge-orbital coupled phenomena in Mo pyrochlore oxides: Monte Carlo study of the pyrochlore double-exchange modelYukitoshi Motomea and Nobuo Furukawab,c a Department of Applied Physics, University of Tokyo, Tokyo, Japan b Department of Physics and mathematics, Aoyama Gauin University, Kanagawa, Japan c Multiferroics Project, ERATO, Japan Science and Technology Agency (JST) Mo pyrochlore oxides R2 Mo2 O7 is known to show an interesting competition between ferromagnetic metal and spin-glass insulator by changing the rear-earth element R [1,2]. Recent experiments by applying external pressure revealed some new aspects on the phase competition. In particular, it was shown that the ferromagnetic metal turns into a peculiar diusive metal under pressure, with showing an intervening spin-glass metallic state [3]. This is surprising because in many transition metal compounds the chemical pressure by substitution and the external pressure often lead to qualitatively similar behaviors. We theoretically investigate the origin of these complex behaviors by studying the doubleexchange model on the frustrated pyrochlore lattice by Monte Carlo simulation [4,5]. We nd that the experimental results under pressure are well explained by the keen competition under strong frustration between the double-exchange ferromagnetic interaction induced by the kinetic motion of electrons and the super-exchange antiferromagnetic interaction between localized spins: Our model exhibits an incoherent metallic state with extremely suppressed spin correlation and an electronic phase separation between two metallic states which potentially corresponds to the spin-glassy metallic behavior. We also extend the model to incorporate the orbital degeneracy under the trigonal distortion as well as the Coulomb repulsion between itinerant electrons in order to explain the metal-insulator transition by the R-site substitution. Our study indicates that the wide variety of phase transitions in Mo pyrochlore oxides is comprehensively understood from the extended double-exchange model with varying the super-exchange antiferromagnetic interaction and the Coulomb repulsion. We acknowledge fruitful discussions with Y. Tokura, S. Iguchi, K. Penc, and A. Georges. This work was supported by Grants-in-Aid for Scientic research (Nos. 17071003, 17740244, 19052008, and 21340090), by Global COE Program the Physical Sciences Frontier, and by the Next Generation Super Computing Project, Nanoscience Program, MEXT, Japan. [1] T. Katsufuji, H. Y. Hwang, and S-W. Cheong, Phys. Rev. Lett. 84, 1998 (2000). [2] I. V. Solovyev, Phys. Rev. B 67, 174406 (2003). [3] S. Iguchi, N. Hanasaki, M. Kinuhara, N. Takeshita, C. Terakura, Y. Taguchi, H. Takagi, and Y. Tokura, Phys. Rev. Lett. 102, 136407 (2009). [4] Y. Motome and N. Furukawa, Phys. Rev. Lett. 104, 106407 (2010). [5] Y. Motome and N. Furukawa, J. Phys.: Conf. Ser. 200, 012131 (2010).

Abstract ID T007

Spin Dynamics in Pyrochlore Heisenberg Antiferromagnetsa

John Chalkera and Peter Conlona Theoretical Physics, Oxford University, 1, Keble Road, Oxford OX1 3NP, United Kingdom

We study the low temperature dynamics of the classical Heisenberg antiferromagnet with nearest neighbour interactions on the pyrochlore lattice. We present extensive results for the wavevector and frequency dependence of the dynamical structure factor, obtained from simulations of the precessional dynamics. We also construct a solvable stochastic model for dynamics with conserved magnetisation, which accurately reproduces most features of the precessional results. Spin correlations over most of the Brillouin zone relax at a rate independent of wavevector and proportional to temperature. Magnetisation uctuations are small at low temperature, but make the dominant contributions to correlations near the Brillouin zone centre. Their dynamics is diusive, with a temperature-independent diusion constant. [1] P. Conlon and J. T. Chalker, Phys. Rev. Lett. 102, 237206 (2009).

Abstract ID T102

Phase transitions in planar pyrochloresBruce D. Gaulin Brockhouse Institute for Materials Research, McMaster University, Hamilton, ON, L8S 4M1, Canada Rare earth titanate pyrochlore magnets with the chemical composition A2 B2 O7 have been a playground for the physics of geometrical frustration. Many magnetic trivalent rare earth ions can reside on the A site of this structure, and such compounds give rise to physical realizations of magnetic moments decorating a network of corner-sharing tetrahedra. Such networks can be combined with antiferromagnetism and ferromagnetism along with dierent spin anisotropies to produce materials with varied and exotic, sometimes disordered, ground states. I will discuss new neutron scattering work on two magnetic pyrochlores Er2 Ti2 O7 and Yb2 Ti2 O7 , which can be thought of in terms of XY moments decorating a network of corner sharing tetrahedra. The ferromagnet, Yb2 Ti2 O7 displays an unexpected disordered ground state which can be brought to order in an applied magnetic eld [1]. The antiferromagnet, Er2 Ti2 O7 , displays a long range ordered state at low temperatures which can be driven into a disordered state by application of a magnetic eld. In collaboration with K. A. Ross, J. P. C. Ru, J. P. Clancy, C. P. Adams, J. S. Gardner, H. A. Dabkowska, M. Ramazanoglu, A. Bourque, M. A. White, Y. Qiu, and J. R. D. Copley. This work was supported by NSERC of Canada. [1] K. A. Ross, J. P. C. Ru, C. P. Adams, J. S. Gardner, H. A. Dabkowska, Y. Qiu, J. R. D. Copley, and B. D. Gaulin, Phys. Rev. Lett. 103, 227202 (2009). [2] J. P. C. Ru, J. P. Clancy, A. Bourque, M. A. White, M. Ramazanoglu, J. S. Gardner, Y. Qiu, J. R. D. Copley, M. B. Johnson, H. A. Dabkowska, and B. D. Gaulin, Phys. Rev. Lett. 101, 147205 (2008).

Abstract ID T006

Anisotropic Exchange and Emergence of Quasi-Two-Dimensional Spin-Spin Correlations in the Yb2 Ti2 O7 PyrochloreJ.D. Thompsona , P.A. McClartya , H.M. Rnnowb , L.-P. Regnaultc , A. Sorged , and M.J.P. Gingrasa,e a Dept. of Physics and Astronomy, U. of Waterloo, Waterloo, Ontario N2L 3G1, Canada b Laboratory for Quantum Magnetism, EPFL Station 3, PH D2 455, CH-1015 Lausanne, Switzerland c CEA-Grenoble, INAC-SPSMS-MDN, 38054 Grenoble, cedex 9, France d Network Dynamics Group, MPIDS, Bunsenstr. 10, 37073 Gttingen, Germany o e Canadian Institute for Advanced Research, Toronto, Ontario M5G 1Z8, Canada Yb2 Ti2 O7 is a rare earth pyrochlore material with many interesting low temperature properties. Heat capacity measurements on powder samples indicate a rst order phase transition at Tc 240mK [1]. Mssbauer absorption spectroscopy and muon spin resonance measureo ments on a single crystal sample [2] nd a sudden decrease in the uctuation rate of the Yb3+ moments at Tc . Yet, powder and single crystal neutron diraction below Tc show no evidence of long range order [2,3] The inability of all measurements to date to determine the nature of the low temperature phase is reminiscent of the hidden order in URu2 Si2 [4]. Measurements of the diuse neutron scattering above Tc have yielded interesting results including 111 rods of scattering intensity. It has been suggested that such rods are indicative of 2D correlations in kagome planes perpendicular to the 111 directions, and that these correlations may indicate a lattice distortion [3]. With the aim to ultimately shed light on the physics at play in this material, we consider the paramagnetic neutron scattering to probe the type of magnetic interactions present in the system. We consider all symmetry allowed nearest-neighbour exchange interactions, Jex , in addition to long-range dipole-dipole interactions. By varying the relative strengths of each of the Jex interactions, a set of exchange couplings is found that reproduce experimental diuse neutron scattering measurements quite well. The resulting couplings are highly anisotropic in nature, consisting mainly of Ising and isotropic exchange terms. By Fourier transforming the q space correlations generated by our model Hamiltonian, we conrm that the diuse neutron scattering results from planar correlations, yet without the existence of lattice distortions. This is a rare example of dimensional reduction of the spin correlations in a highly frustrated spin system [5,6] involving no structural distortion. [1] [2] [3] [4] [5] [6] H. W. J. Blte et al., Physica (Amsterdam) 43, 549 (1969). o J. A. Hodges et al., Phys. Rev. Lett. 88, 077204 (2002). K. A. Ross et al., Phys. Rev. Lett. 103, 227202 (2009). S. Elgazzar et al., Nature Materials 8, 337 (2009). A.S. Wills et al. Chem. Mater. 11, 1936 (1999). C. Stock et al., Phys. Rev. Lett. 103, 077202 (2009).

Abstract ID T056

Magnetoelastics of a spin liquid: X-ray diraction studies of Tb2 Ti2 O7 in pulsed magnetic eldsJ.P.C. Ru,a,b Z. Islam,a J.P. Clancy,b K.A. Ross,b H. Nojiri,c Y.H. Matsuda,d H.A. Dabkowska,b and B.D. Gaulin.b a Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, USA b Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, L8S 4M1, Canada c Institute for Materials Research, Tohoku University, Katahira, Sendai 980-8577, Japan d Institute for Solid State Physics, University of Tokyo, Chiba 277-8581, Japan

Fig. 1. Time resolved shift in lattice constant during a magnetic eld pulse in Tb2 Ti2 O7 .

We report high resolution single crystal x-ray diraction measurements of the frustrated pyrochlore magnet Tb2 Ti2 O7 , collected using a novel low temperature pulsed magnet system. This unique instrument allows for thorough characterization of structural degrees of freedom to temperatures as low as 4.4 K, and in applied magnetic elds as large as 30 Tesla. We show that Tb2 Ti2 O7 manifests a host of intriguing structural eects under the application of magnetic elds, including strongly anisotropic giant magnetostriction, a restoration of perfect pyrochlore symmetry in low magnetic elds, and ultimately a structural phase transition in high magnetic elds. It is suggested that the magnetoelastic coupling thus revealed plays a signicant role in the spin liquid physics of Tb2 Ti2 O7 at low temperatures. This research was supported by D.O.E. and NSERC.

Abstract ID T081

Triangular antiferromagnet: magnon decays and the fate of discrete symmetriesMike Zhitomirsky Service de Physique Statistique, Magntisme et Supraconductivit, CEA-Grenoble, France e e The Heisenberg antiferromagnet on a triangular lattice is the paradigm of magnetic frustration. Being the source of various fruitful concepts such as the RVB spin liquid and magnetic chirality this spin model continues to supply us with new interesting results and ideas. We will discuss two recent theoretical developments in our understanding of the triangular antiferromagnet. First, excitations in the ordered 120 spin-structure of the quantum triangular antiferromagnet appear to have a nite life-time even at T = 0. This is related to spontaneous two-magnon decays, which are intrinsic to all noncollinear antiferromagnets [1,2]. The distinct feature of the 2D quantum model is a logarithmic enhancement of the magnon decay rate along certain contours in the momentum space. We illustrate the above eects by presenting the spin-wave results for the dynamics of the spin-1/2 triangular antiferromagnet. Second, we consider nite-temperature transitions in the easy-axis triangular antiferromagnet with the single-ion anisotropy. Interplay of anisotropy and frustration leads to three successive Berezinskii-Kosterlitz-Thouless transitions [3]. The discrete Z6 symmetry breaks down in two steps with the intermediate phase exhibiting algebraic longitudinal spin correlations. We present the classical Monte Carlo results for the phase diagram of the anisotropic triangular antiferromagnet in applied magnetic eld. [1] A. L. Chernyshev and M. E. Zhitomirksy, Phys. Rev. Lett. 97, 207202 (2006). [2] A. L. Chernyshev and M. E. Zhitomirksy, Phys. Rev. Lett. 79, 144416 (2009). [3] P.-E. Melchy and M.E. Zhitomirsky, Phys. Rev. B 80, 064411 (2009).

Abstract ID T103

Herbertsmithite: A quantum Heisenberg spin liquid on the kagome network.Philippe Mendels,a F. Bert,a , A. Olariu,a , A. Zorko,1,b , E. Kermarrec,a and M. Chung,a a Lab. de Physique des Solides, Univ Paris-Sud, UMR CNRS 8502, 91405 Orsay, France b Joef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia z The discovery of herbertsmithite ZnCu3 (OH)6 Cl2 has been recently described as the end to the drought of spin liquids [1]. It has triggered an intense activity on new kagome materials and related theories for the ground state of the kagome Heisenberg antiferromagnet which has eluded any denitive conclusion for the last twenty years. This is indeed the rst experimental example of a kagome lattice where no order has been found at any temperature well below J through all experimental techniques, including SR [2]. This illustrates the power of the association of an enhancement of quantum uctuations for S = 1/2 spins with the frustration of antiferromagnetic interactions on the loosely connected kagome lattice to stabilize novel ground states of magnetic matter. Various experimental results on Herbertsmithite will be reviewed and discussed, including kagome susceptibility as determined by 17 O NMR lineshift [3], the impact of inter-site Cu/Zn mixing [3,4] on the physics of defects and Dzyaloshinskii-Moriya anisotropy, as evidenced through high-eld ESR [5]. Recent developments on new families and debates which have opened in the comparison with models (e.g. quantum criticality) as well as pending issues on the theoretical and experimental side will be reviewed [6]. This research was supported in part by the EC MC Grant MEIFCT- 2006-041243, the ANR Project OxyFonda NT05-4-41913, the ARRS Project No. Z1-9530 and the ESF network Highly Frustrated Magnetism. [1] P. A. Lee, Science 321, 1306 (2008). [2] P. Mendels et al., Phys. Rev. Lett. 98, 077204 (2007). [3] A. Olariu et al., Phys. Rev. Lett. 100, 087202 (2008). [4] F. Bert et al., Phys. Rev. B 76, 132411 (2007). [5] A. Zorko et al., Phys. Rev. Lett. 101, 026405 (2008). [6] P. Mendels and F.Bert, in Special Topics Section on Novel States of Matter Induced by Frustration, J. Phys. Soc. Jpn 79, 011001 (2010).

Abstract ID T100

Spinons in the S = 1/2 Heisenberg antiferromagnet on kagomeZhihao Hao and Oleg Tchernyshyov Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD, USA1/2 background +1 antikink 1 kink +2 defect triangle 1/2 E flux

Fig. 1. Left: a defect triangle (red) and spinons of the kink (green) and antikink (blue) avors on kagome. Right: a compact U(1) gauge theory on the honeycomb lattice.

The S = 1/2 Heisenberg antiferromagnet on kagome [1] has emerged as a favorite place to search for exotic quantum physics [2,3]. The ground state of this model is a valence-bond crystal: it preserves the spin-rotation symmetry but breaks the symmetry of the lattice [4,5]; spin excitations have a small but nite energy gap [6,7]. Building on the previous work of Elser and co-workers [1,8] we oer a simple picture of the low-energy physics in this model [9]. Triangles with a singlet bond represent a vacuum of the theory. Defect triangles lacking a bond act as sources of quantum uctuations. Spin excitations are spinons, S = 1/2 fermions closely resembling kinks and antikinks of the chain [10]. A defect triangle is a S = 0 bound state of two antikinks. Their binding energy, 0.06J, determines the spin gap. Elsers arrow representation [8] provides a mapping of this physics onto a compact U(1) gauge theory with half-integer values of electric ux (Fig. 1). Research supported by the U.S Department of Energy, Oce of Basic Energy Sciences, Division of Materials Sciences and Engineering under the Grant No. DE-FG02-08ER46544. [1] V. Elser, Phys. Rev. Lett. 62, 2405 (1989). [2] P. A. Lee, Science 321, 1306 (2008). [3] L. Balents, Nature 464, 199 (2010). [4] J. B. Marston and C. Zeng, J. Appl. Phys. 69, 5962 (1991). [5] P. Nikolic and T. Senthil, Phys. Rev. B 68, 214415 (2003). [6] H. C. Jiang, Z. Y. Weng, and D. N. Sheng, Phys. Rev. Lett. 101, 117203 (2008). [7] R. R. P. Singh and D. A. Huse, Phys. Rev. B 77, 144415 (2008). [8] V. Elser and C. Zeng, Phys. Rev. B 48, 13647 (1993). [9] Z. Hao and O. Tchernyshyov, Phys. Rev. Lett. 103, 187203 (2009). [10] T. Nakamura and K. Kubo, Phys. Rev. B 53, 6393 (1996).

Abstract ID T048

Rare-earth based magnetism on the kagom lattice eAndrej Zorko,a Fabrice Bert,b and Philippe Mendelsb a Joef Stefan Institute, Ljubljana, Slovenia z b Laboratoire de Physique des Solides, Universit Paris-Sud 11, UMR CNRS 8502, e 91405 Orsay, France The Ising kagom lattice is established in theory as the archetypal model for geometrical e frustration, because of exponentially large degeneracy of the ground state and spin-spin correlation that remain short-ranged down to T = 0. Theoretical predictions trace back to the early 50s when the seminal paper was published by I. Syzi [1]. Beyond the Ising o model, recent theoretical studies have revealed that transverse quantum dynamics favor an unconventional semiclassical spin liquid [2]. However, due to the scarcity of suitable systems no confrontation between theory and experiments was possible until very recently. The recently discovered family of rare-earth (RE) based Langasites RE3 Ga5 SiO14 [3] has opened a new eld of experimental research on the kagom lattice, where magnetism is due to e rare earths rather than transition metals. Quite generally, rare earths tend to exhibit highly anisotropic magnetism because of strong crystal-eld eects. Due to large magnetocrystalline anisotropy present in the Langasite family [3], competition between single-ion anisotropy and magnetic frustration is expected. Additionally, various compositions with the same crystal structure could be synthesized. We have performed a comprehensive local-probe experimental investigation (SR, NMR and NQR), oering a unique insight to local-scale magnetism of three members of the Langasite family that lack long-range magnetic ordering down to 20 mK. For Kramers Nd3+ ions (J = 9/2) a uctuating ground state was observed [4]. Spin dynamics are strongly sensitive to magnetic eld. We unveil the origin of these unusual ndings by comparing this case to another Kramers case that of Sm3+ ions (J = 5/2). For non-Kramers Pr3+ ions (J = 4), on the other hand, hyperne enhanced quasistatic magnetism is withnessed at low temperatures [5]. [1] [2] [3] [4] [5] I. Syzi, Prog. Theor. Phys. 6, 306 (1951). o A. Sen et al., Phys. Rev. Lett. 102, 227001 (2009). P. Bordet et al., J. Phys.: Condens. Matter 18, 5147 (2006). A. Zorko et al., Phys. Rev. Lett. 100, 147201 (2008). A. Zorko et al., Phys. Rev. Lett. 104, 057202 (2010).

Oral Presentations: T d O lP t ti Tuesday

Abstract ID W043 Multiferroics and Skyrmion crystals as derived from helimangetsYoshinori Tokuraa,b,c Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan b ERATO Multiferroics Project, JST, Tokyo 113-8656, Japan c Cross-Correlated Materials Research Group (CMRG) and Correlated Electron Research Group (CERG), ASI, RIKEN, Wako 351-0198, Japana

The frustration in spin exchange interactions as well as the Dzyaloshinskii-Moriya(DM) interaction on noncentrosymmetric lattice can often produce the helical spin structure. Among them, the transverse-spiral (cycloidal) structure in an insulating magnet can almost always produce the ferroelectricity whose polarization direction depends on the vector spin chirality. Even for the case of the longitudinal-spiral or proper screw structure, the coupling of such a spin texture to the underlying chemical lattice can produce the ferroelectric polarization via the spiorbit interaction; this can be widely observed in actual triangular-lattice magnets like delafossites CuMO2 and layered halides MX2 (M=transition metal element, X=halogen). The magnetically flexible spin structure immediately leads to the magnetic control of the ferroelectric polarization vector, both in magnitude and direction. In some cases, external electric field can well modulate magnetic states via the coupling to the spontaneous polarization. Another prototype of helimagnets is derived from the DM interaction on the noncentrosymmetric crystal; prototypical examples are the B20 type (FeSi type) transition-metal silicide and germanide families. Recently, the Skrymion lattice was confirmed to form in a narrow temperature(T) -magnetic field(B) region of the helimagnetic phase. By contrast, thin films of B20 type MSi (M=Mn or Fe1-xCox ) or MGe (M=Mn, Fe) , whose thickness is smaller than the helical spin modulation period (=10-100nm), ubiquitously form the two-dimensional (2D) Skyrmion crystal with magnetic fields (B) applied normal to the film plane over a wide T-B region down to the lowest temperature. The implication of such a 2D Skyrmion crystal in the magneto-transport properties is discussed, such as the spin-chirality- induced anomalous Hall effect

Abstract ID W009 Dielectric measurement of frustrated spin systems: A way to study frustrated magnetismT. Kimuraa, K. Kimuraa,*, M. Sodaa,c, T. Otania, Y. Yamaguchia, H. Nakamuraa, Y. Wakabayashia, H. Yamaguchib, S. Kimurab, M. Hagiwarab, M. Matsuurac,, and K. Hirotac a Division of Materials Physics, Graduate School of Eng. Sci., Osaka University, Japan b Center for Quantum Sci. and Tech. under Extreme Conditions, Osaka University, Japan c Department of Earth and Space Science, Graduate School of Science, Osaka University, Japan

Geometrically frustrated spin systems have attracted considerable attention because of their exotic properties such as spin liquid, spin ice, and noncollinear magnetic ordering. Spin chirality, i.e. handedness of an ordered magnetic state, often plays an important role in physical properties of frustrated magnets. However, it is not straightforward to detect and control the chirality. Thus, establishing a technique to study the spin chirality in frustrated magnets is greatly desirable. The polarized neutron diffraction technique has been developed as a powerful tool to detect the chirality. However, it has not yet been fully substantiated owing to limitation of experimental facilities and requirement of a large single crystal. In this presentation, we discuss another approach to study frustrated magnetism (e.g., detection and control of spin chirality), that is, dielectric measurement. Here, we mostly focus on one of typical frustrated spin systems, a triangular lattice antiferromagnet (TLA), CuCrO2 with the delafossite structure. In CuCrO2, magnetic properties are dominated by Cr3+ ions (3d3, S = 3/2) forming triangular lattice planes, and are well represented by a S = 3/2 Heisenberg TLA. An early neutron powder diffraction study has indicated that CuCrO2 undergoes magnetic ordering into the out-of-plane 120 structure characterized by the commensurate propagation vector (1/3,1/3,0) below TN24K [1]. Recently, magnetically-induced ferroelectricity has been reported for a polycrystalline sample below TN [2]. More lately, we have successfully grown single crystals of CuCrO2 [3]. We report magnetic, magnetostrictive, and magnetoelectric properties of the single crystal samples [4-8], and discuss detection and control of spin chirality, in (multiferroic) frustrated magnets. This work was supported in part by Grants-in-Aid for Scientific Research (20674005, 20001004, 17072005, 20340089, 19052002, and 19052001) and Global COE Program (G10) from MEXT, Japan. Present address: Institute for Solid State Physics, University of Tokyo, Kashiwa, Japan Present address: Institute for Materials Research, Tohoku University, Sendai, Japan [1] H. Kadowaki et al., J. Phys.: Condens. Matter 2, 4485 (1990). [2] S. Seki et al., Phys. Rev. Lett. 101, 067204 (2008). [3] K. Kimura et al., Phys. Rev. B 78, 140401 (2008). [4] K. Kimura et al., Phys. Rev. Lett. 103, 107201 (2009). [5] K. Kimura et al., J. Phys. Soc. Jpn. 78, 113710 (2009). [6] M. Soda et al., J. Phys. Soc. Jpn. 78, 124703 (2009). [7] H. Yamaguchi et al., Phys. Rev. B 81, 033104 (2010). [8] M. Soda et al., Phys. Rev. B 81, 100406(R) (2010).

*

Abstract ID T090

Complex magnetic structures and electric eld studies of Kagom-staircase compound Ni3 V2 O8 eIvelisse Cabrera,a,b Oksana Zaharko,c Michel Kenzelmann,d and Collin Broholma,b a Department of Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland USA b NIST Center for Neutron Research, Gaithersburg, Maryland USA c Laboratory for Neutron Scattering, ETH Zurich & Paul Scherrer Institut, Villigen, Switzerland d Laboratory for Developments and Methods, Paul Scherrer Institut, Villigen, Switzerland In Kagom-related magnets, interactions that are otherwise considered weak usually become e important, and are responsible for the emergence of novel ordered states. A special case is the Kagom-staircase compound Ni3 V2 O8 (NVO), where competing nearest and nexte nearest neighbor interacions, anisotropy, and Dzyaloshinskii-Moriya interactions yield long range magnetic order as the temperature is lowered below 10 K [1]. The system undergoes four magnetic phase transitions, including a transition to a multiferroic phase with concurrent magnetic and ferroelectric order. A comprehensive qualitative and quantitative analysis indicates that magnetic and ferroelectric domains are strongly coupled in this phase, collectively behaving as single multiferroic domains [2]. By complementing polarized neutron scattering with electric polarization measurements, we demonstrate direct control of multiferroic domains in NVO using an external electric eld. We present evidence for an unusual memory eect, where a previously electrically-polarized sample remembers its polarization direction upon exiting and re-entering the multiferroic phase in the absence of a eld. Finally, we discuss results on the magnetic structure renement of NVOs ordered phases using diraction and spherical neutron polarimetry that reveal new details on the magnetic structure. [1] M. Kenzelmann et al., Phys. Rev. B 74, 014429 (2006). [2] I. Cabrera et al., Phys. Rev. Lett. 103, 087201 (2009).

Abstract ID T101

Chirality and frustration in an Fe langasiteMickael Loire,a Karol Marty,a,b Virginie Simonet,c Rak Ballou,a Eric Ressouche,a Pierre Bordet,a and Pascal Lejaya a Institut Nel, CNRS & Universit Joseph Fourier, BP166, 38042 Grenoble, France e e b Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, USA c Institut Nanosciences et Cryogenie, SPSMS/MDN, CEA-Grenoble, 38054 Grenoble, France

Fig. 1. Magnetic structure of Ba3 NbFe3 Si2 O14 .

Ba3 NbFe3 Si2 O14 belongs to the family of non-centrosymmetric langasite materials providing with interesting geometrically frustrated networks of spins [1]. In this layered compound, a triangular array of magnetic Fe3+ triangles are found in the (a,b) planes. Neutron studies have shown that it orders magnetically below TN =27.5 K, the magnetic moments forming a 120 spins structure within each triangle as imposed by the geometrical frustration, and showing a helical modulation along the perpendicular direction with a period of 7 lattice parameters (see gure) [2]. This magnetic structure is remarkable: it was proposed to give rise to multiferroism [3, 4]; it is single-domain, with a single helicity and a single triangular chirality. Both are related to each other and to the structural chirality which is expressed by a twist of the exchange paths between spins in adjacent layers [2]. Recent Inelastic neutron scattering experiments revealed novel features in the magnon excitations emerging from this ground state, answerable in part to the lack of inversion symmetry and in part to the underlying magnetic chirality. This work was nancially supported by the ANR 06-BLAN-01871. [1] [2] [3] [4] P. Bordet et al., J. Phys.: Condens. Matter 18, 5147 (2006). K. Marty et al., Phys. Rev. Lett. 101, 247201 (2008). K. Marty et al., Phys. Rev. B 81, 054416 (2010). H. D. Zhou et al., Chem. Mater. 21, 156 (2009).

Abstract ID T036

Azurite: a highly frustrated quasi-1D antiferromagnetAndreas Honecker,a Robert Peters,a Thomas Pruschke,a Roser Valent b Harald Jeschke,b , b b c c Ingo Opahle, Hem Kandpal, Tanusri Saha-Dasgupta, Hena Das, Johannes Richter,d Helge Rosner,e Oleg Janson,e Shijie Hu,f Xiaoqun Wang,f Bernd Wolf,g and Michael Langg a Institut fr Theoretische Physik, Georg-August-Universitt Gttingen, Germany u a o b Institut fr Theoretische Physik, Goethe-Universitt Frankfurt am Main, Germany u a c Bose National Centre for Basic Sciences, Kolkata, India d Institut fr Theoretische Physik, Otto-von-Guericke Universitt Magdeburg, Germany u a e Max-Planck-Institut fr Chemische Physik fester Stoe, Dresden, Germany u f Department of Physics, Renmin University of China, Beijing, China g Physikalisches Institut, Goethe-Universitt Frankfurt am Main, Germany a

Fig. 1. Crystal structure of azurite: spin-1/2 copper ions (orange), oxygen (blue), and carbon (gray). Bold lines indicate the exchange paths of the generalized diamond chain.

The natural mineral azurite has been used as a blue pigment since ancient times. More recently, a plateau at 1/3 of the saturation magnetization has been discovered [1] in the high-eld magnetization curve of azurite Cu3 (CO3 )2 (OH)2 . By ab-initio computations, we have identied eight relevant exchange constants. We argue that, to a rst approximation, the interchain exchange can be neglected for the low-energy properties and derive an eective generalized spin-1/2 diamond chain model (see bold lines in Fig. 1). Using numerical results, we then demonstrate that a consistent description can be obtained for various physical properties of azurite: (i) the low-temperature magnetization curve [1], (ii) inelastic neutron scattering on the 1/3 magnetization plateau [2], (iii) nuclear magnetic resonance measurements on the 1/3 magnetization plateau [3], and (iv) the magnetic susceptibility [1] as well as the specic heat [1,2]. Our results not only resolve previous controversies on the modeling of azurite but place it in a highly frustrated parameter regime, thus earmarking it as an interesting system for further investigation. This research was supported in part by the DFG via a Heisenberg fellowship for A.H., SFB602, and SFB/TR 49. [1] H. Kikuchi et al., Phys. Rev. Lett. 94, 227201 (2005). [2] K. C. Rule et al., Phys. Rev. Lett. 100, 117202 (2008). [3] F. Aimo et al.. Phys. Rev. Lett. 102, 127205 (2009).

Abstract ID W015 Spin liquid and novel order in two-dimensional frustrated Heisenberg antiferromagnetsa

Hikaru Kawamura, Graduate School of Science, Osaka University,Toyonaka 560-0043 Japan

Ordering of geometrically frustrated magnets has attracted much recent interest, since it often leads to intriguing phenomena like a spin-liquid behavior and an unconventional magnetic ordering. In this talk, I wish to take up two such examples. One is an exotic vortex order and a spin-gel state realized in the antiferromagnetic (AF) Heisenberg model on the two-dimensional (2D) triangular lattice [1], and the other is a ring-liquid or a pancake-liquid state realized in the AF Heisenberg model on the 2D honeycomb lattice with the next-nearest-neighbor interaction [2]. Interest in the ordering of triangular-lattice AFs has been enhanced by recent experiments on various triangular-lattice materials including, S=3/2 NaCrO2, S=1 NiGa2S4 and S=1/2 organic compounds, -(BEDT-TTF)2Cu2(CN)3 and EtMe3Sb[Pd(dmit)2]2. While most of these compounds exhibit a spin-liquid-like behavior at low temperatures without the conventional magnetic long-range order, all of them turn out to exhibit a weak but clear transition-like anomaly at a finite temperature. Motivated by these recent experiments, we study here the ordering of the classical Heisenberg AF on the triangular lattice by means of mean-field calculation, scaling argument, Monte Carlo and spindynamics simulations, with special attention to its vortex degree of freedom. The model exhibits a thermodynamic transition driven by the Z2-vortex binding-unbinding [3], at which various thermodynamic quantities exhibit an essential singularity. The low-temperature state is a spin-gel state with a long but finite spin correlation length where the ergodicity is broken topologically. Signature of a free Z2-vortex can be captured as a characteristic central peak of the dynamical spin structure factor [1]. In contrast to the triangular counterpart, the 2D honeycomb lattice is a bipartite lattice and is unfrustrated if the interaction is limited only to the nearest neighbors. Very recent experiment by Azuma et al on S=3/2 honeycomb-lattice Heisenberg AF Bi3Mn4O12(NO3), however, has revealed an intriguing spin-liquid-like behavior at low temperatures. Frustration effect induced by the AF nextnearest-neighbor interaction is likely to be the cause of such exotic behaviors. Motivated by the recent experiment, we study here the ordering of the classical Heisenberg AF on the honeycomb lattice with the AF next-nearest-neighbor interaction by means of a low-temperature expansion and a Monte Carlo simulation. Order from disorder mechanism is operative leading to the symmetrybroken state and the associated phase transition, which, however, are suppressed down to very low temperatures near the AF phase boundary. There, exotic spin-liquid states like the ring liquid and pancake-liquid states are stabilized down to very low temperatures, accompanied by the fieldinduced antiferromagnetism. Implications to recent experiments on the honeycomb Heisenberg AF are discussed.

[1] H. Kawamura, A. Yamamoto and T. Okubo, J. Phys. Soc. Jpn. 79 (2010) 023701; T. Okubo and H. Kawamura, unpublished. [2] S. Okumura, H. Kawamura, T. Okubo and Y. Motome, unpublished. [3] H. Kawamura and S. Miyashita, J. Phys. Soc. Jpn. 53 (1984) 4138.

Abstract ID T082

Slow spin dynamics in 2D magnetism of NiGa2 S4Yusuke Nambu,a,b,c Jiajia Wen,a,b Satoru Nakatsuji,c and Collin Broholma,b a Institute for Quantum Matter and Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA b NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA c Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan!"!'$ !"!'#

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Fig. 1. Temperature dependence of in-phase component of of the ac susceptibility. Enlarged low-T region is also plotted in the inset. Vertical dashed line indicates T = 8.5 K.

Spin dynamics and excitations of 2D triangular antiferromagnet NiGa2 S4 [1] were examined through ac and nonlinear susceptibilities and neutron experiments. This compound does not form a conventional antiferromagnetic order at least down to the lowest measured temperature. Instead, NMR/NQR [2] and SR [3] experiments clarify critical slowing down of spin dynamics toward T = 8.5 K that has a highly extended region down to another characteristic temperature T0 3 K. Spins can be dominantly uctuating with speed comparable to the resonance frequency, 106 sec, in this temperature regime (T0 < T < T ). Low frequency ac susceptibility observes frequency dependence at around T0 rather than T (Fig. 1), indicating that the majority of spins is faster than kHz above T0 and then becomes quasi-static below T0 . From nonlinear susceptibilities, it cannot be found an evidence for complete freezing at T as characterized by negative divergence of 3 in alloy spin glasses. Below T0 , neutron measurements reveal a linear dispersion, suggesting a spin-wave-like excitation in 2D despite the absence of a long-range order. We will also present recent results of inelastic neutron scattering experiment on the newly developed Multi Axis Crystal Spectrometer (MACS) at NIST Center for Neutron Research. [1] S. Nakatsuji, Y. Nambu, and S. Onoda, J. Phys. Soc. Jpn. 79, 011003 (2010). [2] H. Takeya et al., Phys. Rev. B 77, 054429 (2008). [3] D.E. MacLaughlin et al., Phys. Rev. B 78, 220403(R) (2008).

Abstract ID T066

Spin liquid state and its instability in the organic triangular 1/2-spin system EtMe3 Sb[Pd(dmit)2 ]2T. Itou,a T. Kubota,a K. Yamashita,a M. Nishiyama,a A. Oyamada,a S. Maegawa,a K. Kubo,b,c H. M. Yamamoto,b,d and R. Kato,b a Graduated School of Human and Environmental Studies, Kyoto University, Japan b Condensed Molecular Materials Laboratory, RIKEN, Japan c Department of Chemistry, Graduated School of Science, Tohoku University, Japan d PRESTO, Japan Science and Technology Agency, Japan Quantum liquidsknown to be realized in 3 He, 4 He, and electrons in metalsgenerally exhibit instabilities unforeseen under classical Newtonian dynamics such as the superuid / superconducting transition. Recently, a new quantum liquid has been discovered in organic frustrated antiferromagnetic spin-1/2 systems, called the quantum spin liquid. The fundamental question is whether instabilities other than classical ordering occur in such a quantum spin liquid, as well as in the typical well-known quantum liquids. Indeed, theorists have proposed several possible instabilities, such as spinon pairing and chiral ordering. In the most studied organic spin-liquid material -(BEDT-TTF)2 Cu2 (CN)3 , however, experimental reports on its low-temperature nature are controversial and this question has remained an open issue. We report the discovery of spin-liquid instability found in a further organic spin-liquid material, EtMe3 Sb[Pd(dmit)2 ]2 , which is a 2D triangular-lattice spin system with antiferromagnetic interactions J = 220-250 K. It has previously been found that this spin system maintains a gapless spin-liquid state down to at least 1.37 K [1]. We performed 13 C-NMR measurements at ultra-low temperatures and found an obvious kink at around 1.0 1 K in the temperature dependence of T1 . This strongly suggests that a continuous phase transition occurs at this temperature. Since continuous transitions always involve essential changes of states, that is, symmetry breaking and/or topological ordering, our result indicates that the gapless spin liquid changes to an essentially dierent spin state with symmetry breaking and/or topological ordering. The low-temperature state clearly diers from the classical magnetic-ordered phase, because the 13 C-NMR spectra do not broaden at all. 1 The steep decrease in T1 in the low-temperature phase suggests the appearance of the spin gap. We point out that the decrease does not follow an exponential law but a power law of temperature at suciently low temperatures, which implies that this gap may be nodal one. We will discuss possible scenarios to explain the instability at around 1.0 K and the nature of the low-temperature phase, which is possibly a new quantum state of matter. This work was supported by Grant-In-Aid for Scientic Research from MEXT, Japan (numbers 16GS0219, 18740199, 19052005, 20110003, and 21740255). [1] T. Itou et al., Phys. Rev. B 77, 104413 (2008).

Abstract ID T086

Spin dynamics of classical kagome antiferromagnetsMathieu Taillefumier,a Julien Robert,b and Benjamin Canalsc Department of Physics, UIO, Postboks 1048 - Blindern 0316 Oslo (Norway) b Laboratoire Lon Brillouin - CEA Saclay, bt. 563, F-91191 Gif-sur-Yvette cedex, France. e a c Institut Nel - 25 avenue des Martyrs, bt. K, BP 166, 38042 Grenoble cedex 9, France. e aa

By combining monte carlo and spin dynamics simulations, we investigate the possibility of coherent excitations in classical kagome antiferromagnets through the calculation of the dynamical structure factors S(Q, ) (resp. the powder averaged S(|Q|, )). In the dense, non disordered case, evidences for spin wave like excitations are given: we show that the Heisenberg kagome antiferromagnet has two distincts low temperature dynamical regimes, whose temperature ranges are given by the entropically driven onset of spin plane coplanarity at T /J 5 103 . We furthermore show that the low temperature manifold is very fragile and that the spin dynamics strongly depends on the ratio of disorder, whatever its type (non-magnetic impurities or bond distortions). Antisymmetric interactions are nally taken into account as another source of perturbation. [1] J. Robert, B. Canals, V. Simonet and R. Ballou, Phys. Rev. Lett. 101, 117207 (2008). [2] M. Taillefumier, J. Robert, and B. Canals (unpublished).

Oral Presentations: Wednesday O lP t ti Wd d

Abstract ID W019 Highly Frustrated Magnets of Current InterestZenji Hiroi Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan

I would like to give a brief review on various highly-frustrated antiferromagnets that recently have attracted many researchers in the field of quantum magnetism from the materials point of view. They are realized in the pyrochlore, triangular and kagome lattices, or even in the diamond and honeycomb lattices in the presence of next-nearest-neighbor interactions. Particularly, emphasis is on the spin-1/2 kagome antiferromagnet (KAF). There has been a controversial issue on the ground state of the spin-1/2 KAF. Theoretically predicted is a tiny gap of the order of a few percent of a nearest-neighbor antiferromagnetic interaction J in the magnetic excitation spectrum [1]. In contrast, experimentalists have always observed gapless features in various kagome compounds including herbertsmithite ZnCu3(OH)6Cl2 [2], volborthite Cu3V2O7(OH)22H2O [3] and vesignieite BaCu3V2O8(OH)2 [4]. In actual materials, however, some unfavorable effects such as a structural distortion, chemical disorder, interplane or higher-order intra-plane interactions are more or less inevitable and may disrupt or mask the intrinsic ground state of the spin-1/2 KAF. Thus, one has to investigate their properties carefully and systematically. Herbertsmithite is a structurally perfect but compositionally imperfect kagome compound, suffering from an exchange between Cu and Zn ions. Vesignieite presents an almost isotropic kagome lattice, but the sample quality seems to be poor at present probably due to certain chemical disorder. In contrast, volborthite presents a structurally distorted kagome lattice with much less effect of randomness. Our group has studied volborthite using high-quality polycrystalline samples and found spin-liquid like behavior with no gap but with a peculiar transition at 1 K, where NMR 1/T1T exhibits a sharp peak [5] and specific heat exhibits not a peak but a clear kink. On the other hand, three magnetization steps are observed at magnetic fields of 4.3, 25 and 46 T, which are followed by a magnetization plateau above 60 T. It seems that interesting physics is involved there, although many mysteries to be clarified have been left for volborthite. I acknowledge useful discussions with Y. Okamoto, M. Yoshida and M. Takigawa. This research was supported in part by the Grant-in-Aid for Scientific Research on Priority Areas "Novel States of Matter Induced by Frustration" (19052003). [1] C. Waldtmann et al., Eur. Phys. J. B 2 (1998) 501. [2] M. P. Shores et al., J. Am. Chem. Soc. 127 (2005) 13462. [3] H. Yoshida et al., J. Phys. Soc. Jpn. 78, 043704 (2009) [4] Y. Okamoto et al., J. Phys. Soc. Jpn. 78, 033701 (2009). [5] M. Yoshida et al., Phys. Rev. Lett. 103 (2009) 077207.

Abstract ID W023 A molecular Mott system with a quasi-triangular latticeReizo Kato Condensed Molecular Materials Laboratory, RIKEN, Hirosawa, Wako-shi, Saitama, 351-0198, Japan

Fig. 1. Crystal structure of -type Pd(dmit)2 salt, quasi-triangular lattice, and temperature dependence of magnetic susceptibility for Me4Sb, EtMe3Sb, and Et2Me2Sb salts. A series of -type anion radical salts of Pd(dmit)2 (dmit=1,3-dithiole-2-thione-4,5-dithiolate; Fig. 1) with monovalent cations EtxMe4-xZ+ (Et = C2H5-, Me = CH3-, Z= P, As, Sb; x = 02) belong to a 2D Mott system with a quasi-triangular lattice [1]. At ambient pressure, they are dimer Mott insulators where one spin (S=1/2) is localized on each dimer unit [Pd(dmit)2]2-. Choice of the cation provides fine tuning of the interdimer interactions and leads to various magnetic states: an antiferromagnetic ordered state (Me4Sb), a charge ordered state (Et2Me2Sb), and a quantum spin liquid state (EtMe3Sb) as shown in Fig. 1 [2]. We can observe the effects of frustration in various situations, including the spin dynamics in a thermal transition and a pressure-induced Mott transition toward a metallic (and frequently superconducting) state. This work has been done in collaboration with M. Tamura, A. Tajima, A. Nakao, T. Fukunaga, K. Kubo, H. M. Yamamoto, S. Yamashita (RIKEN), T. Yamamoto, Y. Nakazawa (Osaka Univ.), T. Itou, M. Yamashita, and Y. Matsuda (Kyoto Univ.). This work was supported by Grant-in-Aid for Scientific Research on Innovative Areas (No. 20110003) from the Ministry of Education, Culture, Sports, Science and Technology, Japan [1] R. Kato, Chem. Rev. 104, 5319 (2004). [2] M. Tamura and R. Kato, Sci. Technol. Adv. Mater., 10, 024304 (2009).

Abstract ID W030 Magnetic excitations of the quantum dimer system Sr3Cr2O8Diana L. Quintero-Castro,a Bella Lake,a,b Elisa M. Wheeler,a A.T.M. Nazmul Islam,a Tatiana Guidi,c Kirrily C. Rule,a Zunbeltz Izaola, a Margarita Russina, a Klaus Kiefer, a Yurii Skourski d a Helmholtz Zentrum Berlin fr Materialien und Energie, D-14109 Berlin, Germany b Institut fr Festkrperphysik, Technische Universitt Berlin, D-10623 Berlin, Germany c ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K. d Hochfeld Magnetlab, Forschungszentrum Dresden-Rossendorf EV, D-01314 Dresden, Germany

Sr3Cr2O8 is an interesting and rich system for a number of reasons. It is a dimerised quantum magnet with gapped excitations and displays Bose-Einstein condensation in an applied magnetic field. The interdimer interactions are frustrated at high temperatures and the frustration is lifted by orbital ordering. Furthermore it displays substantial magnonphonon coupling at low temperatures. Sr3Cr2O8 consists of triangular bilayers of Chromium ions in the a-b plane that are stacked along the c direction in a ABCABC sequence. The magnetic ions are paired into dimers by the dominant antiferromagnetic bilayer coupling and the system can therefore be viewed as a frustrated triangular network of dimers. The Chromium ions are in the unusual 5+ valence state resulting in one electron in the 3d shell and a spin value of while the tetrahedral crystal field ensures that this electron occupies the doubly degenerate eg orbitals. At 285K Sr3Cr2O8 undergoes a cooperative Jahn-Teller distortion that lifts the orbital degeneracy so that only the 3z2-r2 orbital is occupied. The low temperature structure is characterized by monoclinic crystal symmetry and antiferroorbital ordering [1]. The transition also lifts the frustration giving rise to spatially anisotropic exchange paths and results in three crystal twins. Furthermore there is evidence from Raman and ESR measurements of strong magnon-phonon coupling around ~110K [2,3]. We have grown single crystals of Sr3Cr2O8 and have performed DC susceptibility measurements, high field magnetisation and powder and single crystal inelastic neutron scattering. The data reveals a singlet ground state and gapped triplet excitations consisting of three modes arising from the three twins. The magnetic exchange interactions were extracted using the first moment sum rule and a random phase approximation. At base temperature the intradimer coupling is found to be J0=5.6 meV and the ratio of interdimer to intradimer exchange constants is J/J0=0.64. Our findings reveal the interplay of spin, lattice and orbital degrees of freedom in Sr3Cr2O8 and our results will be compared with the related compounds Ba3Cr2O8 (where the Jahn-Teller distortion is dynamical) and Ba3Mn2O8 (where the Jahn-Teller distortion is absent).

[1] L.C. Chapon, C. Stock, P.G. Radaelli, and C. Martin, cond-mat arXiv:0807.0877v2 (2008). [2] P. Lemmens, (private communication). [3] J. Deisenhofer (private communication).

Abstract ID T062

Fractional Spin Textures from DilutionArnab Sen,a Kedar Damle,b and Roderich Moessnerc a Boston University b Tata Institute of Fundamental Research, Mumbai, India c Max-Planck-Institut fr Physik komplexer Systeme, Dresden, Germany u

orphan spin y x Kagome layer 3 5 4 6 Apical spin 1 2 0 Kagome layer Cr(3+) S=3/2 moment Isolated dimer layer

Fig. 1. The SCGO lattice

We discuss how a classical spin fractionalises by dressing itself with an extended texture which screens exactly half the spins moment in the limit of zero temperature. This screening provided by the surrounding spin liquid is not entirely unlike the screening of an impurity in the Kondo eect. In detail, we consider non-magnetic substitutions in the quasi two-dimensional kagometriangle sandwich (Fig.1), motivated by experiments on SCGO [1]. Correlated defects, where impurities substitute for all but one spin on a defective triangle or tetrahedron, are particularly interesting in this context. The resulting orphan spin [2], on the simplex acts like a paramagnetic spin as T 0 and induces a spin texture around it [1,3]. We provide a detailed analysis of the properties of this texture as a function of eld and temperature. We also numerically analyse the issue of interactions between these extended textures, which allows us to address the NMR experiments by Limot et al. [1].

[1] L. Limot et. al, Phys. Rev. B 65, 14447 (2002). [2] P. Schier and R. Daruka, Phys. Rev. B 56, 13712 (1997); R. Moessner and A. J. Berlinsky, Phys. Rev. Lett. 83, 3293 (1999) [3] C. L. Henley, Can. J. Phys. 79, 1307 (2001)

Abstract ID T079

High-eld phases in VolborthiteMasashi Takigawa,a Makoto Yoshida,a Mladen Horvati,b Steen Krmer,b Sutirtha c a Mukhopadhyay,b Claude Berthier,b Yoshihiko Okamoto,a and Zenji Hiroia a Insitute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan Laboratoire National des Champ Magntique Intenses, CNRS, BP 166 - 38042 Grenoble, e France

b

Fig. 1. Magnetic eld - temperature phase diagram obtained from V-NMR measurements in Volborthite

Volborthite Cu3 V2 O7 (OH)2 2H2 O is a quasi 2D compound, in which Cu2+ ions with spin-1/2 make a spacially anisotorpic kagom lattice formed by isosceles triangles. Consequently, it e has two Cu sites and two kinds of exchange interactions. H. Yoshida et al. found a sequence of magnetization steps at 4, 25, and 46 tesla in a high quality powder sample [1]. M. Yoshida et al. performed V-NMR experiments, which revealed a magnetic transition at 1 K for the elds below 4 T accompanied by a sharp peak of nuclear relaxation rate and appearance of internal magnetic eld due to some kind of magnetic order [2]. A sudden change in the NMR spectral shape and relaxation behavior was found at 4 T, where the rst magnetization step occurs. Recently we have extended NMR measurements to higher elds up to 31 T. The obtained H-T phase diagram is shown in Fig. 1. All the phase boundaries are marked by a sudden line broadening or a change of spectral shape. There are three distinct magnetic phases at low temperatures, consistent with the magnetization results. The phase I is characterized by anomalous Lorentzian line shape and highly enhanced spin-echo decay rate. Phase II exhibits two kinds of V sites with distinct relaxation behavior, indicating a heterogeneous magnetic structure. The eld region between phase II and III is likely to correspond to the broadened transition due to anisotorpy of g-facor in the powder smaple. [1] H. Yoshida et al., J. Phys. Soc. Jpn. 78, 043704 (2009). [2] M. Yoshida et al., Phys. Rev. Lett. 103, 077207 (2009).

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Abstract ID W007 Bipartite Spin Excitations in a Two-dimensional Quantum Spin-liquid State Minoru Yamashita,a Norihito Nakata,a Yoshinori Senshu,a Masaki Nagata,a Hiroshi M. Yamamoto,b,c Reizo Kato,b Takasada Shibauchi,a Yuji Matsudaaa

Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan b RIKEN, Wako-shi, Saitama 351-0198, Japan c JST-PRESTO, Wako-shi, Saitama 351-0198, Japan

Quantum spins under geometrical frustration are expected to host novel quantum spin liquid states where strong fluctuation destroys magnetic long-range order even at the zero temperature. By now, some promising candidates have been reported to have no long-range order down to the lowest temperature. The detail nature of the QSLs, however, has remained elusive. One promising method to unveil the ground state is to measure the thermal conductivity because it is very sensitive to delocalized low-lying quasiparticles and can be applied to one single organic crystal in which it is very difficult to adopt neutron scattering measurements. We report thermal conductivity xx measurements of a Pd(dmit)2-compound down to 100 mK. This organic insulator with nearly identical 2D triangular lattice of S = 1/2 has been reported to possess a spin liquid state down to ~ J/12,000 by NMR. We found a clear enhancement of thermal conductivity of the spin liquid material from a non-magnetic cousin compound, showing a substantial contribution of spin thermal transport. Most remarkably, a sizable xx/T is clearly resolved in the zero-temperature limit, in contrast to the absence of the linear term found in -(BEDTTTF)2Cu2(CN)3 [1]. This finite residual indicates gapless spin excitations, in analogy to excitations near the Fermi surface in normal metals. Meanwhile, its magnetic-field dependence suggests a concomitant appearance of spin-gap at low temperatures. These findings expose a highly unusual dichotomy which characterizes the low-energy physics of this quantum spin liquid. This work was supported by KAKENHI (20224008, 22740224) from JSPS. [1] M. Yamashita, et al., Nature Physics 5, 44-47 (2009).

Abstract ID T023

Synchrotron X-ray as a Useful Probe for Frustrated MagnetsT. Arima,a,b H. Ohsumi,b S. Takeshita,b H. Sagayama,a D. Okuyama,c S. Fujiyama,c B. J. Kim,d and Hidenori Takagic,d a Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan b RIKEN SPring-8 Center, Sayo 679-5148, Japan c Advanced Science Institute, RIKEN, Wako 351-0198, Japan d Universi