Session M13: Focus Session: Complex Oxide Interfaces - LaAlO3/SrTiO3 I

Emergent Phenomena in Oxide Superlattices from DFT+DMFT

Density functional theory plus dynamical mean-field theory (DFT+DMFT) has proven to be an instrumental tool in describing transition metal-oxides, and DFT+DMFT is well poised to search the rich phase space of oxide superlattices for emergent phenomena. In this talk, we focus on double-perovskite heterostructures of the form $AA^{\prime}BB^{\prime}O_{6}$. Our two separate goals are to design new ferroelectric Mott insulators and high temperature ferromagnetic Mott insulators. In the context of ferroelectric Mott insulators, we demonstrate the ability to design in-plane ferroelectrics based upon transition metal charge transfer and A/A' cation size mismatch, harnessing displacements of both the A-site and B-site. In the context of ferromagnetic Mott insulators, we detail the different mechanisms for obtaining high temperature ferromagnetic Mott states, and we present new candidates for high temperature ferromagnetic insulators. Finally, we discuss the prospect of coupling magnetic and ferroelectric states in addition to potential applications.   

Structural, chemical and electronic changes at LaAlO$_{3}$/SrTiO$_{3}$ interfaces near the critical thickness

The formation of two-dimensional electron gas (2DEG) at the n-type LaAlO$_{3}$/SrTiO$_{3}$(LAO/STO) heterostructure is a paradigmatic example for emergent functionalities at oxide heterointerfaces. However, the origin of this 2DEG at the LAO/STO interface still remains unclear. In this work, we use scanning transmission electron microscopy and electron energy loss spectroscopy (STEM/EELS) to study a series of (LAO)$_{n}$/(STO) films, both below ($n=$3) and above ($n=$5,7) the critical thickness for the formation of the 2DEG ($n$\textgreater 3). From STEM imaging, we observe dramatic changes in the polar distortion of the AlO$_{2}$ layers in response to the built-in electrostatic potential in the three films. We also observe changes in the octahedral tilts across the interface for different LAO thicknesses. We obtain information about cation intermixing across the heterointerface from EELS. We combine these results with density functional theory calculations to discuss the effect of each of these different structural and chemical changes on the electronic property of the interfaces. Overall, our results present a detailed understanding of the role of electronic and atomic reconstructions and defects on the emergence of 2DEG at these interfaces.   

Surface charging in LaAlO$_3$/SrTiO$_3$ heterostructures

The two-dimensional electron gas (2DEG) observed at the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) is known to depend on its proximity to the LAO surface (thickness of LAO) and the conditions to which the surface is exposed. It has been proposed that electrons from the 2DEG can leak to surface states, resulting in a charged surface. Using first-principles calculations, we determine the electronic structure of the LAO surface and the coupling between the surface of LAO and the interface 2DEG. We develop a methodology for treating charged dielectric surfaces and apply it to LAO. This allows for the determination of the 2DEG density as well as the stable surface terminations and reconstructions of LAO on STO as a function of LAO thickness, under any given experimental conditions. Under oxygen-rich conditions, we find an increase in critical thickness required to form the 2DEG, and under hydrogen-rich environment the critical thickness decreases.   

Understanding enhanced UV photo-response of Pd nanoparticle-coated LaAlO$_{3}$/SrTiO$_{3}$ using ambient control Kelvin probe force microscopy

Significant enhancement in photoresponse of Pd nanoparticle-coated LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) heterstructure is observed under UV illumination compared with a bare LAO/STO sample. Clear ambient dependence suggests crucial roles of gas adsorption/desorption and resulting carrier transfer in the enhanced UV response. In this work, we have measured surface work function of the samples with and without UV light using Kelvin probe force microscopy (KPFM). We perform both transport and the surface potential measurements at the same time. Such simultaneous measurements can help us to improve our understanding regarding the light sensing and strong ambient dependence of the LAO/STO heterointerface.   

Increasing the capacitance beyond the classical limits in capacitors with free-electron like electrodes

Capacitors are ubiquitous in solid state devices like metal oxide semiconductors field effect transistors (MOSFET). For energy efficiency, the MOSFET should operate at small gate voltages. In this regime, to increase the channel conductivity and performance, the capacitance of the capacitor between the gate and the channel should be made as large as possible. Recently, an enhancement of the capacitance of upto 40 \% with respect this classical limit has been reported in two-dimensional electron gases formed at the interface between two oxides, SrTiO$_{3}$ and LaAlO$_{3}$. A first theoretical analysis pointed to the quantum exchange-energy in the electron-electron interactions as the driving force to explain the anomalous behavior. The exchange-interaction would produce a lowering of the chemical potential of the electron system as the electronic density increases (the negative compressibility effect). Here we test the validity of the approach, carrying out self-consistent calculations on a capacitor where the metallic plates are simulated by a jellium. We study the effect of the thickness of the metallic electrode and the density of the free electron, on the spillage of the electronic clouds into the dielectric and the variation of the chemical potential.   

2D charge confinement in oxide heterostructures: fundaments from First-Principles

The accurate description of the fundamental properties of complex oxides by First-Principles (FP) is tantamount with the possibility of achieving reliable theoretical design. Here we use the variational pseudo-self-interaction corrected density functional theory (VPSIC) to investigate structural and electronic properties of several exceptionally interesting heterostructures (the STO/LAO interface, the Nb-doped STO superlattice, the SrTiO3/SrZrO3 superlattice, the SrTiO3 surface). The key features which are at the fundament of the 2DEG formation (film polarity, band disalignment, doping, structural and electronic screening, chemistry of 3d orbitals) are described and discussed. Furthermore, we propose a combined VPSIC plus Bloch-Boltzmann approach as reliable and efficient method, applicable to generic doped oxides, to obtain thermoelectric and transport properties in satisfying agreement with the available experimental data. Our theoretical analysis consents to relate the calculated transport properties to the specific features of the band structure at the interface, e.g. on-site and inter-site band splitting, which govern the confinement process. This is instrumental to achieve guidelines for the design of interfaces with enhanced transport and thermoelectric functionalities.   

Role of Oxygen in the Resistance Upturn of LaAlO$_{3}$ /SrTiO$_{3}$ Heterostructures

Among the phenomena exhibited by LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures, the appearance of a low-temperature resistance upturn has attracted much recent debate [1-4]. This phenomenon has been observed to co-occur with both nonlinear Hall effect and negative magnetoresistance, and attributed to either the Kondo effect, multiband conduction, or weak localization. More importantly, it is primarily seen in samples grown by pulsed laser deposition, and is sensitive to film layer thickness, growth condition and electrostatic gating, all of which could affect the oxygen content. In this work, we study the occurrence of resistance upturn in samples grown by molecular beam epitaxy, and how the sheet resistance and Hall conductance are affected by post-growth oxygenation. X-ray photoelectron spectroscopy is used to monitor the cation valences, and correlate them with the conduction and magnetic properties. Our results are analyzed in terms of oxygen vacancies in the presence of polar charge transfer, and the effect of these vacancies on the resistance upturn in LAO/STO heterostructures.\\[4pt] [1] V. K. Guduru et al., PRB 88 (2013).\\[0pt] [2] Z. Huang et al., PRB 88 (2013).\\[0pt] [3] C. Bell et al., APL 94 (2009).\\[0pt] [4] A. D. Caviglia et al., Nature 456 (2008).   

The Role of Lattice Mismatch and Polar Fluctuation at the Conducting Oxide Interfaces

Two key questions have been raised since the discovery of two-dimensional electron gas (2DEG) at the interface between insulating oxides LaAlO3 and SrTiO3. One is the origin for such 2DEG, and the other is how to improve carrier mobility. By replacing LaAlO3 with the cubic polar perovskite insulator (La0.3Sr0.7)(Al0.65Ta0.35)O3 (LSAT), carrier mobility can be increased by 30 times and reach 35,000 cm2V-1s-1 at 2 K, accompanied by reducing lattice mismatch from 2.98{\%} to 0.96{\%}. Moreover, two critical thicknesses for LSAT/SrTiO3(001) interface are found: one is 5 unit cells for 2DEG appearance, and the other is around 12 unit cells for carrier mobility optimization. This phenomenon can be explained in terms of polar fluctuation in LSAT(001), while the conducting (110)- and (111)-orientated LSAT/STO interfaces without such polar fluctuation shows less thickness-dependent carrier mobility.   

Interface orientation dependent carrier distribution in LaAlO$_{3}$/SrTiO$_{3}$ heterostructures

Carrier distribution in widely studied LaAlO$_{3}$/SrTiO$_{3}$ heterostructures depends upon the directionality of the interface. From the first-principles calculations, we find that, for the (001) interface, most carriers are localized at the interface and have the $d_{xy}$ character, which decays away from the interface on the scale of 1 nm. The $d_{yz} $and$ d_{zx}$ carrier concentration increases, peaks at about three unit cells (1.15 nm), and then decays very slowly away from the interface. For the (111) interface, we find that the total carrier distribution is similar to that of $d_{yz}$ and $ d_{zx}$ in the (001) heterostructure. It has a maximum that lies at about four-bilayers (0.89 nm) away from interface and then decays very slowly producing a long-ranged tail. This difference in the carrier distribution is controlled by the $d$-orbital splitting which depends on the interface orientation. For the (001) interface, the tetragonal distortion brings the in-plane $d_{xy}$ orbital to about 0.5eV below the Fermi energy, creating a deep localized state, whereas the $d_{yz}$ and $ d_{zx}$ orbitals are much shallower and thence less localized. For the (111) interface, the rhombohedral distortion splits the $d-$orbitals by about 0.1 eV so that they are all located in vicinity of the Fermi energy and therefore less localized at the interface.   

Scanning Tunneling Microscopy of 2DEG in Complex Oxide Interfaces and Heterostructures

New experimental data clarifying the origin of 2DEG at LaAlO3 (LAO) - SrTiO3 (STO) interfaces will be presented. The materials that have been studied include (a) 4 unit cells of LAO on STO and (b) symmetric n-type 2DEG bilayer heterostructures prepared by atomic layer engineering method [1]. All studied materials are grown by pulsed laser deposition (PLD). UHV STM results obtained on 4LAO/STO samples do not confirm the prediction of electronic reconstruction model [2]. On the other hand, UHV STM results obtained on symmetric n-type 2DEG bilayer heterostructures indicate that the formation of n-type 2DEG is caused by complex interplay of oxygen vacancy doping and interfacial band bending at LaO-TiO2 interfaces. The physical mechanisms of interface induced charge density multiplication effect will be discussed.\\[4pt] [1] H. W. Jang et al., Science 331, 886 (2011);\\[0pt] [2] Z. Ristic et al., Europhysics Letters 93, 17004 (2011).   

The role of La displacement in Titanium $d_{xy}$ ferromagnetism at the LaAlO$_3$/SrTiO$_3$-- double-exchange like mediation

In this talk, we argue that La site polar distortion is the key for magnetism without oxygen vacancy, whereby coupling between La $d_{z^2}$ and O $2p_{x,y}$ offer a pathway for double-exchange like interaction to support Ti $d_{xy}$ ferromagnetism. While ferromagnetism found in the $n$-type LAO/STO has been mainly attributed to oxygen vacancy near the interface, the mechanism we propose a possibility of ferromagnetism without any defect. We show that there is a phase separation between charge-order and half-metallicity that can be characterized by La displacement - this can be tailored through adsorption on the outer AlO$_2$ surface or electrostatic bias.   

Importance of the surface in interfacial conductivity in LaAlO$_3$ thin films on SrTiO$_3$

Careful growth of LaAlO$_3$ thin films on SrTiO$_3$ by molecular beam epitaxy has shown that the La/Al ratio of the film is key to the formation of a two-dimensional electron liquid at the interface---metallic conductivity is only observed in Al-rich films. The interfacial electron liquid forms due to the polar catastrophe, the diverging potential caused by the atomic layer arrangement at the interface when polar LaAlO$_3$ is grown on TiO$_2$-terminated non-polar SrTiO$_3$. The system eventually reconstructs, moving negative charges to the interface to screen the diverging potential. I will present density functional calculations showing the defects that form in the film depend on the stoichiometry, and these defects lead to electronic reconstruction in Al-rich films and ionic reconstruction in La-rich films. The stoichiometry of the surface differs from that the bulk of the film, and I will show that the LaAlO$_3$ surface is unstable to phase separation. The interaction between surface and bulk leads to a scenario where the insulating La-rich films have an Al-rich surface.   

Effects of structural phase transitions on the interface of perovskite oxides

Conductivity at the interface of perovskite oxides such as LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ is a wildly growing area of research. These interfaces are rich in phenomena including superconductivity, large negative in-plane magnetoresistance, giant persistent photoconductivity, and ferromagnetism. Although this field has seen rapid growth in the recent decade, there is yet to be a systematic study on what effect structural phase transitions in strontium titanate play on the interface. It is well understood that strontium titanate transitions from a cubic to tetragonal phase most notably at 105K, but how this effects oxide interface conductivity has yet to be reported on. Through transport measurements, we observe evidence of structural phase transitions in LaAlO3/SrTiO3 and Al2O3/SrTiO3 interfaces at 80K and 200K. These effects are enhanced when the scale of the devices is reduced to a few microns.