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 What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 1 / their 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 that generate the functionality, but rather the physics at the interfaces, where different materials meet. Modern electronics for example is not possible without semiconductor heterostructures. In contrast to semiconductors, oxide materials have strongly correlated electrons and exhibit in bulk a different intrinsic functionalities, like (anti)feroomagnetism, (anti)ferroelectricity, piezoeelectricity, multiferroicity and superconductivity. Oxide heterostructures can be even more interesting than solely exploiting their bulk properties. Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 2 / their 54
Outline What are perovskites? Polar catastrophe 2DEG at the LaAlO 3 /SrTiO 3 interface LAO/STO interface: Conducting properties LAO/STO interface: magnetic properties Superconductivity at the LAO/STO interface Coexistence of magnetic ordering and superconductivity at the interface Spin dynamics at the interface Conclusions Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 3 / their 54
Perovskites The general formula: ABO 3, where A is large cation and B a small cation. ABO 3 compounds can condense in a large diversity of crystal structures like Cubic, Orthorhombic, Tetragonal, Rhombohedral, and Monoclinic. Additionally, structural phase transitions are often observed as a function of external parameters like Temperature or Pressure. Cubic perovskites are made of alternating AO and BO 2 planes. Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 4 / their 54
Perovskites SrTiO 3 (STO) Standard substrate for many oxide materials due to the similarities in structure and lattice constant. Cubic at Room temperature (space group Pm 3m) with lattice constant 3.905 Å At about 105 K neighboring TiO 6 octahedra are slightly rorated in opposite directions and the system transforms to tetragonal structure (space group I 4 /mcm) at lower temerature. STO has a relatively large, indirect band gap of 3.25 ev this implies it is a band insulator. STO can be made metallic by creating the oxygen vacancies after heating the sample to more than 800 C in low oxygen pressures 10 6 mbar. Metallic STO is also formed upon substituting small amounts of La for Sr or Nb for Ti, or other similar substitutions. Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 5 / their 54
Perovskites LaAlO 3 (LAO) At high temperature it is cubic (space group Pm 3m) and at temperatures less than 813 K it is in rhombohedra phase (space group R 3c) LAO is also a band insulator with large optical band around 5 ev The reasonably small lattice mismatch of 3% to STO and similarity of the thermal expansion coefficients allow the epitaxial growth of LAO films on STO. LAO is a polar material as LaO layer has +1 charge and AlO 2 layer has -1 charge Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 6 / their 54
Perovskites LAO: Polar Alternating layers have opposite charge Electric field is calculated using Gauss law. The electric potential increases with the distance. Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 7 / their 54
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 Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 8 / their 54
LAO STO heterointerface Polar catastrophe A process in electrons transfer from the LAO valence bands to the STO conduction bands, mitigating the divergence of the build-up electric potential as n LAO increases. J. Mannhart et. al., MRS BULLETIN, 33, 2008 Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, Superconductivity July 24, 2014 and 9 / their 54
LAO STO heterointerface Techniques to grow LAO on STO: Pulsed Laser Deposition From Wikipedia Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 10 and/ their 54
LAO STO heterointerface Below critical thickness Above critical thickness 2DEG is characterized by a quantum well where electrons coming from some donor states are trapped. Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 11 and/ their 54
LAO STO heterointerface Electron reconstruction Cation intermixing Oxygen vacancies Where do electrons come from? One of the still debated issue regarding LAO/STO interfaces is just the nature of donor states. Oxygen vacancies can actually dominate the transport properties of samples that are fabricated at too low oxygen partial pressures. In that case we can even get bulk conductance. Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 12 and/ their 54
Conducting properties at the interface A high-mobility electron gas at the LAO/STO heterointerface A. Ohtomo et. al., NATURE 427 (2004) Growth and schematic models of the two possible interfaces between LAO and STO in the (001) orientation Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 13 and/ their 54
Conducting properties at the interface Transport properties of the (LaO) + /(TiO 2 ) 0 interface for different oxygen partial pressures during growth Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 14 and/ their 54
Conducting properties at the interface The dependence of -1/R H e at 2K on the po 2 during growth Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 15 and/ their 54
Conducting properties at the interface Field tuned 2DEG Thiel et. al., Science 313, 1942 (2006) Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 16 and/ their 54
Conducting properties at the interface Influence of LAO thickness on electronic properties of LAO/STO interface Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 17 and/ their 54
Conducting properties at the interface Transport properties of 2DEG as a function of applied gate field Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 18 and/ their 54
Conducting properties at the interface Supression of the critical thickness threshold for the conductivity at the LAO/STO interface E. Lesne et. al., nat. comm., 2014. Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 19 and/ their 54
Conducting properties at the interface Hall effect measurements Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 20 and/ their 54
Conducting properties at the interface X-Ray absorbtion spectroscopy at Ti L edge Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 21 and/ their 54
Conducting properties at the interface Layer-resolved DOS Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 22 and/ their 54
Conducting properties at the interface Two-dimensional quantum oscillations of the conductance at LAO/STO interfaces A. D. Caviglia et. al., PRL 105, 236802 (2010) Shubnikov-de Haas oscillations of the LAO/STO interface Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 23 and/ their 54
Conducting properties at the interface Angular dependence of the quantum oscillations Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 24 and/ their 54
Conducting properties at the interface Temperature dependence of the Shubnikov-de Haas oscillations Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 25 and/ their 54
Conducting properties at the interface Field effect modulation of the Shubnikov-de Haas oscillations Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 26 and/ their 54
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 ions. Theoretically, the magnetism has been postulated in STO side, involving the occupation of the low-energy Ti-d xy -like sub-bands caused by the interfacial splitting of orbital degenaracies, or interfacial disorder, or interfacial oxygen vacancies. Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 27 and/ their 54
Magnetism at the interface Electronic phase separation at LAO/STO interface Ariando et. al., Nat. Comm. 2, 188 (2011) Electrical properties Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 28 and/ their 54
Magnetism at the interface Electronic phase separation at LAO/STO interface Magnetic properties Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 29 and/ their 54
Magnetism at the interface Electronic phase separation at LAO/STO interface Magnetoresitance Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 30 and/ their 54
Magnetism at the interface From first-principles perspectives: Magnetism of LAO/STO superlattices Karolina Janicka et. al., J. Aplied. Phys. 103, 07B508 (2008) DOS for majority and minority spin electrons on the Ti atom at the interface for (LAO) 3 /(STO) n superlattices Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 31 and/ their 54
Magnetism at the interface Table : Magnetic moments on the interface Ti atom (m Ti ), the total moment of the unit cell (m total ) n m Ti (µ B ) m total (µ B ) 3 0.21 0.53 5 0.07 0.22 7 0.023 0.10 9 0.016 0.07 Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 32 and/ their 54
Magnetism at the interface Magnetic effects at the interface between non-magnetic oxides A. Brinkman et. al., nat. mat. 6, 2007. Sheet resistance of n-type LAO/STO interfaces Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 33 and/ their 54
Magnetism at the interface Large negative magnetoresistance Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 34 and/ their 54
Magnetism at the interface Magnetic hysterisis Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 35 and/ their 54
Superconductivity at the LAO/STO interface Superconductivity at the LAO/STO interface S Gariglio et. al., J. Phys. Condens. Matter 21 (2009). Structural characterization of LAO thin films Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 36 and/ their 54
Superconductivity at the LAO/STO interface Transport properties in the normal state for an LAO/STO structure Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 37 and/ their 54
Superconductivity at the LAO/STO interface Transport properties at low temperatures Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 38 and/ their 54
Coexistence of magnetic ordering and superconductivity at the interface Direct imaging of the coexistence of ferromagnetism and superconductivity at the LAO/STO interface A. Bert Julie et. al., nat. phys., 7, 2011. Comparison of SQUID images on LAO/STO and δ-doped STO samples Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 39 and/ their 54
Coexistence of magnetic ordering and superconductivity at the interface Dipole distribution Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 40 and/ their 54
Coexistence of magnetic ordering and superconductivity at the interface Paramagnetic signal on patterned LAO/STO sample Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 41 and/ their 54
Coexistence of magnetic ordering and superconductivity at the interface Coexistence of magnetic order and two-dimensional superconductivity at LAO/STO interfaces Lu Li et. al., nat. phys., 7, 2011. Torque magnetometry of LAO/STO oxide interface Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 42 and/ their 54
Coexistence of magnetic ordering and superconductivity at the interface Coexistence of superconductivity and magnetic ordering in a 5 u.c. LA/STO interface sample Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 43 and/ their 54
Coexistence of magnetic ordering and superconductivity at the interface Magnetic ordering persisting to elevated temperature Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 44 and/ their 54
Coexistence of magnetic ordering and superconductivity at the interface Angular dependence of the interface torque indicating an in-plane saturation magnetic moment Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 45 and/ their 54
Origin of spin-rashba coupling at the interface and its dynamics Inversion symmetry breaking at the interface gives rise to the Spin-Rashba coupling Ferromagnetic exchange, spin-orbit coupling and spiral magnetism at the LAO/STO interface Sumilan et. al., nat. phys., 9, 2013. Electronic orbitals and charge localization Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 46 and/ their 54
Origin of spin-rashba coupling at the interface and its dynamics Spiral ground state and its evolution with field Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 47 and/ their 54
Origin of spin-rashba coupling at the interface and its dynamics Torque magnetometry Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 48 and/ their 54
Origin of spin-rashba coupling at the interface and its dynamics Quasiparticle dynamics and spin-orbit textures of the STO 2DEG P.D.C King et. al., nat. comm., 2014. Orbital ordering of a 2DEG in STO Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 49 and/ their 54
Origin of spin-rashba coupling at the interface and its dynamics Orbitally enhanced spin splitting Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 50 and/ their 54
Origin of spin-rashba coupling at the interface and its dynamics Quasiparticle dynamics of the subband states Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 51 and/ their 54
Acknowledgements HRK Sumilan Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 52 and/ their 54
Thank you Pramod Verma (Indian Institute of Science, Bangalore What so special 560012) about LaAlO3/SrTiO3 interface? Magnetism, July Superconductivity 24, 2014 53 and/ their 54