Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session F12: Focus Session: Complex Oxide Interfaces - Polar interfaces II |
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Sponsoring Units: DMP Chair: Jeremy Levy, University of Pittsburgh Room: 314 |
Tuesday, March 19, 2013 8:00AM - 8:12AM |
F12.00001: Mott p-n junctions in layered materials Maxime Charlebois, Syed Hassan, Rajesh Karan, David Senechal, A.-M.S. Tremblay Correlated electron heterostructure became a possible alternative when thin film deposition techniques achieved structures with a sharp interface transition [1]. We study here the electronic reconstruction of doped Mott insulator p-n junctions based on a Cluster Dynamical Mean Field Theory (CDMFT) calculation of the Hubbard model in the limit where electrostatic energy dominates over the kinetic energy associated with transport across layers. The grand potential of individual layers is first computed within CDMFT and then the electrostatic potential energy is taken into account in the Hartree approximation. The charge reconstruction in an ensemble of stacked planes of different nature can lead to a distribution of electron charge [2], density of states, and optical properties that are unique to doped-Mott insulators.\\[4pt] [1] J. Mannhart, D. G. Schlom, Science 327, 1607 (2010)\\[0pt] [2] T. Oka, N. Nagaosa, PRL 95, 266403 (2005) [Preview Abstract] |
Tuesday, March 19, 2013 8:12AM - 8:24AM |
F12.00002: Strong electron correlation enhancement to capacitance via frustrated phase separation James Freericks, Simon Hale Recent experiments on strongly correlated capacitors from Mannhart's group have shown that the capacitance can be enhanced by about 50\% over the geometric capacitance when the metallic leads are gated to be nearly depleted of electrons. More recently, direct measurements of the electron compressibility in those leads show that they become phase separated in this regime. It has long been known that proximity to phase separation, or equivalently negative electron compressibility, should lead to an enhancement of capacitance. In this work, we show that this phenomenon is quite general. By employing a microscopic model of a strongly correlated capacitor composed of multilayers of electronic leads and a Mott insulating dielectric, we show that by tuning the barrier to lie in the regime where it is phase separated in the bulk, it exhibits a type of frustrated phase separation in the multilayer, which gives rise to an enhancement in the capacitance with capacitance curves versus gate voltage resembling quite close to those of experiment. In the calculations, the enhancement effect is lower (on the order of 10\%), and the mechanism is different, because here the phase separation is in the dielectric instead of the metallic plates. Nevertheless, this behavior seems to be ubiquitous. [Preview Abstract] |
Tuesday, March 19, 2013 8:24AM - 8:36AM |
F12.00003: Theory of optical response and ellipsometry spectra of LaAlO$_3$/SrTiO$_3$ heterostructures Se Young Park, Andrew Millis We present a theory of the optical and ellipsometric properties of the electron gas at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface. The reflectivity and ellipsometry angles are obtained by calculating the random phase approximation (RPA) dielectric constant including the optical phonon of STO and the charge response of the electron gas. We find a dip in the ellipsometry angle at the plasma edge of STO phonon that is related with in-plane Drude response and a peak in high energy from the plasmon excitation of yz and xz electrons and show how these may be related to subband occupancy and scattering rates. Comparison of the theory to published data indicates that about 80\% of electrons in xy band are inert to optical transition, possibly explaining the discrepancy in charge density between transport measurements and polar catastrophe scenario. [Preview Abstract] |
Tuesday, March 19, 2013 8:36AM - 8:48AM |
F12.00004: Optical conductivity of SrTiO$_3$ based interfaces Ming Xie, Guru Khalsa, Allan MacDonald Since the discovery of a high mobility two-dimensional electron gas at the interface of LaAlO$_{3}$/SrTiO$_{3}$, there has been a large scientific effort to understand the properties of perovskite interfaces. Naturally, this effort has focused on magneto-transport and photoemission studies. Here we use the Kubo formalism to study the optical conductivity of SrTiO$_3$ based interfaces and discuss its implications on the underlying physical properties of these systems. In particular, the response to light polarized in- and out-of-plane will be contrasted. [Preview Abstract] |
Tuesday, March 19, 2013 8:48AM - 9:24AM |
F12.00005: Oxygen vacancies and magnetism at titanate interfaces Invited Speaker: Natalia Pavlenko Breaking the translation or inversion symmetry at surfaces and interfaces may lead to the formation of new charge, spin and orbital electronic states which are different than the bulk states. The emergence of these states is particularly relevant for oxides where the balance of competing interactions and the resulting stable electronic phase crucially depend on the local oxidation state near the interface. A prominent example is the interface of LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO), which exhibits a two-dimensional electron liquid state in the structures with LaAlO$_{3}$-layers more than 4uc thick, and undergoes a transition into a superconducting state below 0.2 K. Depending on growth conditions LAO/STO has also been found to display pronounced magnetotransport effects indicating the existence of local moments. Recently, even a coexistence of ferromagnetism and superconductivity has been reported, possibly due to an electronic phase separation within the interface. We analyze the magnetic state at the LAO/STO interface within density functional theory and provide evidence that it is caused by the spin polarization of Ti 3d interface electrons. The magnetic state depends strongly on the oxidation state of the interfaces. We show that oxygen vacancies at titanate interfaces induce a complex multiorbital reconstruction which involves a lowering of the local symmetry and an inversion of t$_{2g}$ and e$_{g}$ orbitals resulting in the occupation of the e$_{g}$ orbitals of Ti atoms neighboring the O vacancy. In contrast to stoichiometric nonmagnetic interfaces of LaAlO$_{3}$ and SrTiO$_{3}$, the vacancy-induced orbital reconstruction at LAO/STO interfaces generates a two-dimensional interface magnetic state not observed in bulk SrTiO$_{3}$. We demonstrate that oxygen vacancies in the TiO$_{2}$ interface layer enhance the tendency for ferromagetism considerably. This allows for the notion that areas with increased density of oxygen vacancies produce ferromagnetic puddles and account for the previous observation of a superparamagnetic behavior in the superconducting state. Using generalized gradient approximation (LSDA) with intra-atomic Coulomb repulsion (GGA$+$U), we find that this magnetic state is common for titanate surfaces and interfaces. [Preview Abstract] |
Tuesday, March 19, 2013 9:24AM - 9:36AM |
F12.00006: Local Moment Formation and Magnetism at LaAlO$_3$/SrTiO$_3$ Interfaces Onur Erten, Sumilan Banerjee, Mohit Randeria One of the most exciting observations at oxide interfaces relate to the observation of magnetism at the LaAlO$_3$/SrTiO$_3$ (LAO/STO) interface, since neither material is magnetic in the bulk even with doping. Experiments [1,2] give incontrovertible evidence for local moments at the LAO/STO interface, consistent with an areal density close to 0.5 per interfacial Ti atom. The particular splitting of the $t_{2g}$ orbitals at the interface, leads to a quarter-filled $d_{xy}$ band on the top band. Using a slave-rotor approach for the on-site Coulomb interaction U and Hartree-Fock for nearest neighbor V, we show that local moments form in a checkerboard charge-ordered insulating (COI) state, even for a very modest values of $U$. Phonons further stabilize the COI state as the breathing mode couples cooperatively to the charge order. To understand the magnetic interactions between moments, we examine both the small superexchange and the dominant kinetic exchange mediated by conduction electrons. We show that this leads to a ferromagnetic double exchange model with some very interesting twists arising from the Rashba SOC of the conduction electron due to broken inversion at the interface. Ref: [1] L. Li et al., Nature Phys. 7 762. [2] J. A. Bert et al., Nature Phys. 7 76. [Preview Abstract] |
Tuesday, March 19, 2013 9:36AM - 9:48AM |
F12.00007: Theory for magnetic exchange and anisotropy at the LaAlO$_3$/SrTiO$_3$ interface Sumilan Banerjee, Onur Erten, Mohit Randeria The LaAlO$_3$/SrTiO$_3$ interface exhibits unusual magnetic properties with a large density of local moments seen by both torque and scanning SQUID experiments. We develop a model where local moments are formed on a 2D checkerboard lattice due to correlation-driven charge ordered insulator. We focus on the double exchange interaction of these moments via conduction electrons with a large Rashba spin-orbit coupling (SOC) due to broken inversion at the interface. We derive an effective Hamiltonian for the local moments that has an unusual double square-root ferromagnetic exchange, previously seen in a different context [1]. Two new features arise from SOC, direction-dependent anisotropic exchanges and Dzyaloshinskii-Moriya type terms, which can be tuned by gating. We show that SOC accounts for the the unusually large easy-plane magnetic anisotropy seen in experiment. We will explore the phase diagram, as a function of the strength of Rashba SOC, to see what unusual magnetic states might be stabilized. We will comment on the possibility of reconciling the apparently different conclusions reached by torque and scanning SQUID measurements regarding the magnetic ordering and ordered moment.\\[4pt] [1] O.~Erten et al., Phys. Rev. Lett. 107, 257201 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 9:48AM - 10:00AM |
F12.00008: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 10:00AM - 10:12AM |
F12.00009: Intrinsic spin Hall effect at oxide interfaces: a simple model Lorien Hayden, Roberto Raimondi, Michael Flatte', Giovanni Vignale An asymmetric triangular potential well provides one of the simplest model for the confinement of mobile electrons at the interface between two insulating oxides, such as LaAlO$_3$ and SrTiO$_3$ (LAO/STO). In this paper we study the intrinsic spin Hall effect due to Rashba coupling in an asymmetric triangular potential well. Besides splitting each subband into two branches of opposite chirality, the spin-orbit interaction causes the wave function in the direction perpendicular to the plane of the quantum well (i.e., the growth direction) to depend on the in plane wave vector $kv$. In contrast to the extreme asymmetric case, i.e., the wedge-shaped quantum well, for which the intrinsic spin Hall effect is known to vanish due to vertex corrections, we find that the asymmetric well supports a non-vanishing intrinsic spin Hall conductivity, proportional to the square of the spin-orbit coupling constant. Its origin lies in the non-vanishing matrix elements of the spin current between subbands corresponding to different states of quantized motion perpendicular to the plane of the well. Vertex corrections are carefully considered, both for the intra-band and the inter-band contributions to the spin Hall conductivity. [Preview Abstract] |
Tuesday, March 19, 2013 10:12AM - 10:24AM |
F12.00010: Theory of spin-orbit effects in the t$_{2g}$ band of pseudo-cubic perovskites Guru Khalsa, Byounghak Lee, Allan MacDonald Epitaxial interfaces of perovskite systems have recently been the focus of an enormous amount of research due to their novel properties and potential for integration with silicon based technologies. Although the role of spin-orbit effects has been discussed in the literature, a first principles study of their influence on electronic structure has been lacking. We have conducted a study of spin-orbit effects in pseudo-cubic t$_{2g}$ perovskite systems in which inversion symmetry has been broken by the presence of an external electric field. In this talk, we discuss our results and compare with available magneto transport studies on LAO/STO and related systems. [Preview Abstract] |
Tuesday, March 19, 2013 10:24AM - 10:36AM |
F12.00011: Theory of spin-orbit coupling at LaAlO3/SrTiO3 interfaces and SrTiO3 surfaces Zhicheng Zhong, Anna Toth, Karsten Held A full theoretical understanding of the spin-orbit coupling (SOC) effects at LaAlO$_{3}$/SrTiO$_{3}$ interfaces and SrTiO$_{3}$ surfaces is still needed. We perform first-principles density-functional-theory calculations and derive from these a simple tight-binding Hamiltonian, through a Wannier function projection and group theoretical analysis. We find striking differences to the standard Rashba theory for spin-orbit coupling in semiconductor heterostructures, because the relevant $t_{2g}$ orbitals are very different from nearly free electrons. The key ingredients to the spin splitting are the atomic SOC and the interface asymmetry, which enters via asymmetric $t_{2g}$ orbital lobes. ArXiv:1209.4705 by Zhicheng Zhong, Anna Toth, Karsten Held [Preview Abstract] |
Tuesday, March 19, 2013 10:36AM - 10:48AM |
F12.00012: Transport through oxide interfaces - The case of SrTiO$_3$ based hetero-structures M. Ben Shalom, E. Flekser, Y. Dagan, M. Kim, C. Bell, Y. Hikita, H.Y. Hwang Sharp interfaces can host phenomena that are absent in their constituting materials. By depositing a thin layer of LaAlO$_3$ on top of SrTiO$_3$, the interface between these two band-insulator is highly conducting. Conductivity emerges only for TiO$_2$ termination and above a critical LaAlO$_3$ thickness of 4 unit cells, pointing to the importance of the polar structure. The transition, from insulating to high mobility electron gas, can be controlled continuously by gate voltage, thus enabling a careful study of the dependence of system properties on charge density. Carrier-controlled two-dimensional superconductivity, and magnetic hysteresis were observed between the two non-magnetic oxides. We have found anisotropic magnetoresistance (AMR) in our samples, an outcome of magnetic scattering, which affect the transport through the spin orbit (SO) interaction, and coexists with superconductivity. Gate bias enables tuning the SO energy, which dominates the magnetotransport properties. The exceptionally large amplitude and sign of the AMR suggests a Rashba-type SO coupling. The different AMR characteristics for Nb doped SrTiO$_3$, a symmetric non-polar with similar resistivity and carrier density, demonstrates the significant role of interface polarity for its magnetic properties. [Preview Abstract] |
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