What so special about LaAlO3/SrTiO3 interface? Magnetism...

What so special about LaAlO3/SrTiO3 interface?Magnetism, Superconductivity and their coexistence at

the interface

Pramod Verma

Indian Institute of Science, Bangalore 560012

July 24, 2014

Pramod Verma (Indian Institute of Science, Bangalore 560012)What so special about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity and their coexistence at the interfaceJuly 24, 2014 1 / 54

”Often, it may be said that the interface is the device”

H. Kroemer (Nobel Laureate)

In many devices it is not the bulk properties of a material thatgenerate the functionality, but rather the physics at the interfaces,where different materials meet. Modern electronics for example is notpossible without semiconductor heterostructures.

In contrast to semiconductors, oxide materials have stronglycorrelated electrons and exhibit in bulk a different intrinsicfunctionalities, like (anti)feroomagnetism, (anti)ferroelectricity,piezoeelectricity, multiferroicity and superconductivity.

Oxide heterostructures can be even more interesting than solelyexploiting their bulk properties.

Outline

What are perovskites?

Polar catastrophe

2DEG at the LaAlO3/SrTiO3 interface

LAO/STO interface: Conducting properties

LAO/STO interface: magnetic properties

Superconductivity at the LAO/STO interface

Coexistence of magnetic ordering and superconductivity at theinterface

Spin dynamics at the interface

Conclusions

Perovskites

The general formula: ABO3, where A is large cation and B a smallcation.

ABO3 compounds can condense in a large diversity of crystalstructures like Cubic, Orthorhombic, Tetragonal, Rhombohedral, andMonoclinic.

Additionally, structural phase transitions are often observed as afunction of external parameters like Temperature or Pressure.

Cubic perovskites are made of alternating AO and BO2 planes.

Perovskites

SrTiO3 (STO)

Standard substrate for many oxide materials due to the similarities instructure and lattice constant.

Cubic at Room temperature (space group Pm3m) with latticeconstant 3.905 A

At about 105 K neighboring TiO6 octahedra are slightly rorated inopposite directions and the system transforms to tetragonal structure(space group I4/mcm) at lower temerature.

STO has a relatively large, indirect band gap of 3.25 eV this implies itis a band insulator.

STO can be made metallic by creating the oxygen vacancies afterheating the sample to more than 800 ◦C in low oxygen pressures≤ 10−6 mbar. Metallic STO is also formed upon substituting smallamounts of La for Sr or Nb for Ti, or other similar substitutions.

Perovskites

LaAlO3 (LAO)

At high temperature it is cubic (space group Pm3m) and attemperatures less than 813 K it is in rhombohedra phase (spacegroup R3c)

LAO is also a band insulator with large optical band around 5 eV

The reasonably small lattice mismatch of 3% to STO and similarity ofthe thermal expansion coefficients allow the epitaxial growth of LAOfilms on STO.

LAO is a polar material as LaO layer has +1 charge and AlO2 layerhas -1 charge

Perovskites

LAO: Polar

Alternating layers have opposite charge

Electric field is calculated using Gauss’ law. The electric potentialincreases with the distance.

Perovskites

LAO as capacitor

Macroscopic sample: Charges on top and bottom surface provdes’capacitor’, cancels average internal field.

Why LAO/STO interface has cariiers: the polar discontinuity

LAO STO heterointerface

Polar catastrophe

A process in electrons transfer from the LAO valence bands to theSTO conduction bands, mitigating the divergence of the build-upelectric potential as nLAO increases.

J. Mannhart et. al., MRS BULLETIN, 33, 2008

Techniques to grow LAO on STO: Pulsed Laser Deposition

From Wikipedia

Below critical thickness

Above critical thickness

2DEG is characterized by a quantum well where electrons coming fromsome donor states are trapped.

Where do electrons come from?

Electron reconstruction

Cation intermixing

Oxygen vacancies

One of the still debated issue regarding LAO/STO interfaces is just thenature of donor states.

Oxygen vacancies can actually dominate the transport properties ofsamples that are fabricated at too low oxygen partial pressures. In that

case we can even get bulk conductance.

Conducting properties at the interface

A high-mobility electron gas at the LAO/STO heterointerfaceA. Ohtomo et. al., NATURE 427 (2004)Growth and schematic models of the two possible interfaces between LAOand STO in the (001) orientation

Transport properties of the (LaO)+/(TiO2)0 interface for different oxygenpartial pressures during growth

The dependence of -1/RHe at 2K on the po2 during growth

Field tuned 2DEGThiel et. al., Science 313, 1942 (2006)

Influence of LAO thickness on electronic properties of LAO/STO interface

Transport properties of 2DEG as a function of applied gate field

Supression of the critical thickness threshold for the conductivity at theLAO/STO interfaceE. Lesne et. al., nat. comm., 2014.

Hall effect measurements

X-Ray absorbtion spectroscopy at Ti L edge

Layer-resolved DOS

Two-dimensional quantum oscillations of the conductance at LAO/STOinterfacesA. D. Caviglia et. al., PRL 105, 236802 (2010)Shubnikov-de Haas oscillations of the LAO/STO interface

Angular dependence of the quantum oscillations

Temperature dependence of the Shubnikov-de Haas oscillations

Field effect modulation of the Shubnikov-de Haas oscillations

Magnetism at the interface

Where does magnetism come from?

The origin of interface magnetism is also unclear and under debate.

Experimentally it is found that the magnetism is associated with Ti+3

Theoretically, the magnetism has been postulated in STO side,involving the occupation of the low-energy Ti-dxy -like sub-bandscaused by the interfacial splitting of orbital degenaracies, or interfacialdisorder, or interfacial oxygen vacancies.

Electronic phase separation at LAO/STO interfaceAriando et. al., Nat. Comm. 2, 188 (2011)Electrical properties

Electronic phase separation at LAO/STO interfaceMagnetic properties

Electronic phase separation at LAO/STO interfaceMagnetoresitance

From first-principles perspectives: Magnetism of LAO/STO superlatticesKarolina Janicka et. al., J. Aplied. Phys. 103, 07B508 (2008)

DOS for majority and minority spin electrons on the Ti atom at theinterface for (LAO)3/(STO)n superlattices

Table : Magnetic moments on the interface Ti atom (mTi ), the total moment ofthe unit cell (mtotal)

n mTi (µB) mtotal (µB)

3 0.21 0.535 0.07 0.227 0.023 0.109 0.016 0.07

Magnetic effects at the interface between non-magnetic oxidesA. Brinkman et. al., nat. mat. 6, 2007.Sheet resistance of n-type LAO/STO interfaces

Large negative magnetoresistance

Magnetic hysterisis

Superconductivity at the LAO/STO interfaceS Gariglio et. al., J. Phys. Condens. Matter 21 (2009).Structural characterization of LAO thin films

Transport properties in the normal state for an LAO/STO structure

Transport properties at low temperatures

Direct imaging of the coexistence of ferromagnetism and superconductivityat the LAO/STO interfaceA. Bert Julie et. al., nat. phys., 7, 2011.Comparison of SQUID images on LAO/STO and δ-doped STO samples

Dipole distribution

Paramagnetic signal on patterned LAO/STO sample

Coexistence of magnetic order and two-dimensional superconductivity atLAO/STO interfacesLu Li et. al., nat. phys., 7, 2011.Torque magnetometry of LAO/STO oxide interface

Coexistence of superconductivity and magnetic ordering in a 5 u.c.LA/STO interface sample

Magnetic ordering persisting to elevated temperature

Angular dependence of the interface torque indicating an in-planesaturation magnetic moment

Origin of spin-Rashba coupling at the interface and its dynamics

Inversion symmetry breaking at the interface gives rise to the Spin-RashbacouplingFerromagnetic exchange, spin-orbit coupling and spiral magnetism at theLAO/STO interfaceSumilan et. al., nat. phys., 9, 2013.Electronic orbitals and charge localization

Spiral ground state and its evolution with field

Torque magnetometry

Quasiparticle dynamics and spin-orbit textures of the STO 2DEGP.D.C King et. al., nat. comm., 2014.Orbital ordering of a 2DEG in STO

Orbitally enhanced spin splitting

Quasiparticle dynamics of the subband states

Acknowledgements

HRKSumilan

Thank you

What so special about LaAlO3/SrTiO3 interface? Magnetism...

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