Laboratory for

Quantum Magnetism

TP lab presentation 2009

Henrik M. Ronnow (EPFL since Jan. 2007)

How many body physics?

One of the simplest problems:

H = J Si Sj

1 spin: trivial

2 spins: singlet state |↑↓ - |↓↑

4 spins: back-of-the-envelope calc.

16 spins: 10 seconds on computer

40 spins: World record ! (A. Läuchli, EPFL)

1023 spins: Antiferromagnet (Louis Neel 1932)

Fluctuating singlets (PW Anderson 1973,1987)

1023 – some electrons: High-Tc superconductivity

– THE enigma of modern solid state physics

CuO S= 1/22

A small contribution

There are fluctuating singlets in the

ground state !

Quantum Magnetism

Theoretical models Novel materials

theNeutron Bulk methods scattering big magnets,

low temperature,

high pressure

of physics

Physics of Interacting Systems

• A challenge on all length scales Classical n-body problem (from 3 to

galaxies)

Neural networks

Spin-models

= QUANTUM EFFECT ?

Maybe the “Big Bang” was powered by

“Vacuum Quantum Fluctuations” ?

(Hawkins et al.)

Novel electronic materials

• Strongly correlated electrons

• Often magnetism plays a (leading?) role - e.g.:High-Tc superconductors Colossal magnetoresistance

La2-xBaxCuO4 La2-2xSr1+2xMn2O7

Doped spin ½ antiferromagnets Intrinsic spin valves

Building models

• SpinsLength: |S|=1/2

Quantum / classical

Dimension: Ising, XY, Heisenberg

• Architecture

Dimension

Connectivity

• InteractionsCu2+ O 2px Cu 3dx2-y2

H = J Si Sj

Anti-/Ferromagnetic

• Extentions

Randomness

Charge, orbit, lattice...

HsatCuGeO3

(Hpip)2CuBr4

(d6-5CAP)2CuCl42DHAF

CuGeO3

Magnetic measurementsM

ag

ne

tization

S

usceptibili

ty

NM

R, μ

SR

etc

. S

pecific

heat

Neutron Scattering

intensity cross-section correlation function wave-function overlap

2

f

2

0)(),(f

fE

dEd qSfSdEd

dI Q

Experiment Theory

fi kkQ

m

k

m

k

22

2

f

22

i

2

(Crystal) momentum transfer

Energy transfer

Mais les Neutrons, ils sont où ?

All ways lead to Rome…

Reactor or spallation sources:

6-10 in Europe

~2008 next-generationin US & Japan

European Spallation

Source (ESS) ?

Last decade:x10 in fluxx10 in detection

Can study samples and phenomena not previously possible

ILL, Grenoble

EPFL

SINQ, PSI

Bern

Start: Villigen

Via: Lausanne

Ziel: Grenoble

400.2 km 3:04 h

Quantum Magnetism - Quo vadis ?

• Entanglement & quantum information theory

– New notation or new resource ?

• Quantum phase transitions:

– Different quantum phases, universal behaviour etc.

• Controlled quantum magnets:

– “Pump” dynamically to obtain and control “new semiconductor”

• Bulk Restricted geometries

– Finite size quantization devices ?

Driver of new theories and pictorial explanations

Correlated Electron Technologies ?

Quantum Phase Transitions

Classical phase

transition –

thermal fluctuations

Correlation

length ξT

Power-law scaling

Universality classes

Quantum phase transition

– quantum fluctuations (T 0)

Coherence length ξc

Universal scaling ?

QPT?

LiHoF4

Transverse field

Ising model

The world‟s

simplest QPT ?

(Sachdev ‟99)

Quantum Phase Transition in a Spin Bath !

Expected soft-mode transition at Hc

Incomplete softening

QC-scalingis fragile

Hyperfine coupling to nuclear-spin „bath‟

Minimum

gap at

finite T

„closest‟

to QCP

Controlled Quantum Magnets

• Conventional parameters: Temperature, Field, Pressure

• Dynamical “pumping”:

– Laser orbitals (exchange, valence…)

– Light phonons

– Radio-frequency / microwaves nuclear & electronic spins

Tuned systems, non-equilibrium physics, time-dependence

NMR saturated

nuclear spins

in LiHoF4

Recover

“world‟s simplest

quantum critical point”

LiY0.95Ho0.05F4

spin glass

-hole burning

Clusters

Image with SANS

Correlated electron technologies

Metals Insulators

Discover, understand and control “new semiconductors”

Semiconductors

Bardeen‟s transistor

Quantum Magnetism

Theoretical models Novel materials

theNeutron Bulk methods scattering big magnets,

low temperature,

high pressure

of physics

The laboratory

• Activities:

~ 40% neutron scattering (at international facilities)

~ 60% in-house activities

– Sample synthesis and study of new materials

– Sub-kelvin measurements (susceptibility etc.)

– High-pressure cells (quantum phase transitions)

People !

Henrik M Ronnow (that‟s me)

Caroline Pletscher, secretary

Mark de Vries, visitor, Frustrated quantum magnets

Ivica Zivkovic, Pdoc, Ruthanates, non-linear susceptibility

Julio Larrea, Pdoc, high-pressure measurements

Mohamed Zayed, PhD, SrCu2(BO3)2, high pressure neutron

Goran Nilsen, PhD, Chemistry, new system synthesis

Conradin Kraemer, PhD (PSI) LiReF4, quantum phase transitions

Neda Nikseresht, PhD LiReF4, quantum phase transitions

Martin Mourigal, PhD (ILL), low-D quantum magnets, neutron scatt.

Arash Omrani, PhD, nano-transport devices of novel electronic materials

Julian Piatek, TP4, Masters, PhD, low-T susceptibility, Li(Ho/Er)F4

Bastien DallaPiazza TP4, Masters, PhD, inhomogenous meanfield theory

Laurent Cevey Staggiere TP4, Master novel superconductors

The laboratories:

• Halle Bernard Vittoz

– 9 tesla cryomagnet

– Dilution fridge

– Dip-stick, 3He

Susceptometry,

specific heat,

high-pressure

Future:

– New magnetometer

– 18 tesla system

– 400μW fridge

The laboratories:

• The abyss (hosting the SQUID magnetometer)

– SQUID magnetometer

– Synthesis lab.

Copper

acetate

Cu(C5D5NO)6(11BF4)2

Synthesis

Crystal growth

SamplesMeasurements

TP-projects• General philosophy:

– Foreseeable outcome in one semester

– Related to real research (linked to ongoing projects)

– Can be extended to Dimploma/Master‟s project

– Defined together with student

• Past projects and present suggestions:

1. Synthesis of spin-dimer systems (Farley)

2. Adiabatic cooling for magnetometer (dalla Piazza)

3. Low-T susceptometer (Piatek)

4. High-pressure susceptometry of SrCu2(BO3)2 (Cevey)

5. Simulation of novel neutron spectrometer

6. New iron-pnictide superconductors

7. Quantum criticality under pressure in CeCoGe3-xSix

8. Magnetometer design for Swiss company (non-disclosure restriction)

9. Nano-devices of correlated electron materials (collaboration with STI)

10. Quantitative crystal growth

Example: High-pressure susceptometry

• Current TPIV

project:

Laurent Cevey

Quantum phase transition

at 20-25kbar !

• design and build coils for

high-pressure susceptometer

• Measure SrCu2(BO3)2

• Compare to neutron and ESR

New cell:

30kbar

Square lattice antiferromagnet

Quantitative crystal growth

• Single crystals are prerequisite to most projects

• Crystal growth often a chemist‟s secret fingerspitzgefühl

• Apply physical approach: measure and control

– Growth by evaporating solvent from saturated solution

– Controlled temperature gradient – crystal grows on cold finger

– Optical monitoring – transmission decrease towards saturation

Parallel processing simulation

• Last year Bastien dalla Piazza wrote inhomogenous meanfield simulation of quantum magnets

• To speed up, we want to use modern graphics card for parallelized simulation

• Need soemone with good computing skills

Simulation of Novel Neutron Spectrometer

Continuous Angle Multiple Energy Analysis (CAMEA)

Hybrid for mapping excitation spectectra:

• 60º continuous angle coverage x15(over conventional TAS)

• 5 successive analysers x 4.5

• Better resolution x 3

• Estimated improvement x 200 !

• Prove improvement

• Develop actual design

Sample analysers

detectors

TP-projects

• General philosophy:

– Foreseeable outcome in one semester

– Related to real research (linked to ongoing projects)

– Can be extended to Dimploma/Master‟s project

– Defined together with student

• Any questions?

• If interested, schedule a discussion

– henrik.ronnow@epfl.ch