Session G13: Focus Session: Electrostatic Control of Phenomena at Complex Oxide Interfaces

Interrelation of polarity, screening, structure, and electronic states at nickelate interfaces

A basic property of metal oxides is ionicity: the metal (cation) and oxygen (anion) sites have different electron densities and thus different charge states. This then permits the existence of polarity whether (1) in static form deriving from the choice of the terminating atomic plane at an interface, or (2) in dynamic form through the existence of ferroelectricity which permits switching of the polarity at an interface. The polarity drives the accumulation of charge, modification of the atomic-scale structure, and alteration of the electronic states at an interface. Here we describe an interconnected body of recent collaborative work on nickelate interfaces where electronic transport is strongly modified by the choice of polarity and/or ferroelectric polarization [1,2,3]. Combining first principles theory and experiments allows one to unravel the microscopic origin of the interfacial phenomena. In the case of ultrathin LaNiO$_3$ films, there is a strong dependence of transport properties on the choice of surface termination and the resulting near-surface structural distortions, all of which take place within the screening length of the nickelate. In the case of LaNiO$_3$/Pb(Zr$_x$Ti$_{1-x}$)O$_3$ interfaces, we show that the striking dependence of measured transport properties on the ferroelectric polarization is based on a change of mobility, as opposed to carrier density, at the interface that derives from the creation of a new conducting channel in the (nominally insulating) interfacial region of the ferroelectric.\\[4pt] [1] D. P. Kumah, A. S. Disa, J. H. Ngai, H. Chen, A. Malashevich, J. W. Reiner, S. Ismail-Beigi, F. J. Walker, and C. H. Ahn, ``Tuning the Structure of Nickelates to Achieve Two-Dimensional Electron Conduction,'' Advanced Materials, vol. 26, 1935 (2014).\\[0pt] [2] D. P. Kumah, A. Malashevich, A. S. Disa, D. A. Arena, F. J. Walker, S. Ismail-Beigi, and C. H. Ahn, ``Effect of Surface Termination on the Electronic Properties of LaNiO3 Films,'' Phys. Rev. Applied, vol. 2, 054004 (2014).\\[0pt] [3] M. S. J. Marshall, A. Malashevich, A. S. Disa, M.-G. Han, H. Chen, Y. Zhu, S. Ismail-Beigi, F. J. Walker, and C. H. Ahn, ``Conduction at a Ferroelectric Interface,'' Phys. Rev. Applied, vol. 2, 051001 (2014).   

Electrolyte Gating of SrTiO$_{3}$ Nanostructures

We report low-temperature transport measurements of a two-dimensional electron system (2DES) at the surface of Strontium Titanate. We use electrolyte gating to create the 2DES, and nanopatterning techniques to create gate-tunable submicron constrictions. We observe universal conductance fluctuations, from which we extract an electron dephasing rate linear in temperature, characteristic of electron-electron interaction in a disordered conductor. Furthermore, the dephasing rate has a temperature-independent offset, suggestive of unscreened local magnetic moments in the sample. Finally, we demonstrate that protecting the Strontium Titanate with a thin layer of hexagonal Boron Nitride allows us to create a 2DES with dramatically increased mobility, while also preventing surface electrochemistry.   

A high-mobility electronic system at an electrolyte-gated oxide surface

Electrolyte gating is a powerful technique for accumulating large carrier densities in surface two-dimensional electron systems (2DES). Yet this approach suffers from significant sources of disorder: electrochemical reactions can damage or alter the surface of interest, and the ions of the electrolyte and various dissolved contaminants sit Angstroms from the 2DES. In this talk, we demonstrate that this disorder can be minimized by protecting the sample with a chemically inert, atomically smooth sheet of hexagonal boron nitride (BN). We illustrate our technique with electrolyte-gated strontium titanate, whose mobility improves more than tenfold when protected with BN. We find this improvement even for our thinnest BN, of measured thickness 6 Angstrom, with which we can accumulate electron densities nearing $10^{14}$ cm$^{-2}$. Our technique is portable to other materials, and should enable future studies where high carrier density modulation is required but electrochemical reactions and surface disorder must be minimized.   

LaAlO$_3$/SrTiO$_3$ field-effect nanodevices using in-situ-grown Au top gates

Conductive-atomic force microscope (c-AFM) lithography can create a wide range of nanostructures based on the LaAlO$_3$/SrTiO$_3$ system, including field effect transistors\footnote{C. Cen, S. Thiel, J. Mannhart, and J. Levy, Science \textbf{323}, 1026 (2009).} , single-electron transistors\footnote{G. L. Cheng, \textit{et al.}, Nature Nanotechnology \textbf{6}, 343 (2011).} and superconducting nanoelectronics\footnote{J. P. Veazey, \textit{et al.}, Nanotechnology \textbf{24}, 375201 (2013).}. However, the operating range of gated devices is often limited by tunneling through insulating barriers. Using in-situ Au deposited on top of LaAlO$_3$, we create vertical field-effect devices with significantly lower leakage due to the large bandgap of LaAlO$_3$. We describe the fabrication process for vertical field-effect nanodevices and show representative transport measurements both at room temperature and low temperatures.   

Large Thermopower of $\delta $-doped LaTiO$_{3}$/SrTiO$_{3}$ Interfaces and it's Field Dependence

We will present the magneto-thermopower (S(T, H)) of interfacial delta doped LaTiO$_{3}$/SrTiO$_{3}$ heterostructure by an iso-structural antiferromagnetic perovskite LaCrO$_{3}$. The thermoelectric power of 2-dimensional electron gas (2DEG) of pure LaTiO$_{3}$/SrTiO$_{3}$ at 300 K is $\approx $ 118 $\mu $V/K, but increases dramatically to 337 $\mu $V/K on inserting 5 uc LaCrO$_{3}$ at the interface. The negative sign of the thermoelectric power confirms the electron as major carriers in these interfaces. A linear temperature dependence of S(T) has been observed in the temperature range 100 K to 300 K which is in agreement with the theory of diffusion thermopower of 2DEG. The S(T) shows a distinct enhancement at temperature \textless 100 K, where a Kondo-type minimum has been observed in sheet resistance. We attribute this maximum in S(T) to Kondo scattering of conduction electron by localized impurity spin at the interface. The S in this temperature range is suppressed significantly ($\le $ 20{\%}) by moderate magnetic field ($\le $ 13 T) applied either perpendicular or parallel to the film surface. The isotropic nature of the suppression of S by magnetic field further strengthen the Kondo based interpretation of S(T, H).   

Contrasting electrostatic tuning of the superconducting LaAlO$_{3}$/SrTiO$_{3}$ interface by top and back gating

We report an experimental study of electrostatic tuning of the superconducting interface in dual gated LaAlO$_{3}$/SrTiO$_{3}$ controlled by the electric field effect. The dual gate devices are formed by simultaneously gating from the top of the epitaxial LaAlO$_{3}$ layer and the back of the SrTiO$_{3}$ substrate. Along with electrostatic doping to tune the carrier density, the quantum well profile of the interface electron system is modified due to the inversion asymmetry of the structure, resulting in the modulation of the effective disorder of the system (carrier mobility). We find therefore a strong contrast in the superconducting phase transition by top versus back gating. Simultaneous gating provides a unique opportunity to tune both the carrier density and disorder in this 2D superconducting system.   

Electric field control of thermoelectric effect in oxide interface LaAlO$_{3}$/SrTiO$_{3}$

Oxide interface LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) has been attracting huge interest and the origin of the magnetic-field-induced phase transition is yet to be understood. However, thermoelectric power studies, a powerful tool for detecting phase transitions, have been sparsely reported so far. In this study we measured the carrier density dependence of thermopower to understand the origin of this phase transition. Below critical carrier density, thermopower is found to increase dramatically while the interface is still conductive. These results may not only help in understanding the physics around the phase transition but also shed light on the significance of this system as a thermoelectric material.   

Link Between Mobile Band Population and Superconductivity in SrTiO3$\backslash$LaAlO3 Interface

The entire superconducting phase diagram of SrTiO3$\backslash$LaAlO3 is scanned using back gate voltage. The superconducting transition temperature Tc and critical field Hc are recorded along with the~Hall resistance and Shubnikov-de Haas (SdH) effect. The latter is sensitive only to the density of the mobile band while the former probes all bands. We find that Tc and Hc follow a similar non-monotonic gate voltage dependence as the SdH frequency and the low field Hall. This suggests that the mobile band is getting depopulated at high gate voltages resulting in the reduction of Tc on the overdoped side. We discuss a possible scenario for this peculiar behavior.   

External voltage control of LaAlO$_{3}$/SrTiO$_{3}$ interface structure

Physical properties of the LaAlO$_{3}$ (LAO)/ SrTiO$_{3}$ (STO) interface are controlled by external voltage such as voltage-induced metal-insulator transition [1] and superconductivity-insulator transition [2]. The modulated parameters by external voltage are not only the carrier density but also the Hall mobility [3]. In order to clarify local electric field and polarization around the interface, we have investigated the effect of external voltage on the interface structure at room temperature by means of a surface x-ray diffraction technique, crystal truncation rod (CTR) scattering. Our measurements were performed at BL-4C of the Photon Factory, KEK, Japan. A 5-unit-cell thick sample was prepared using the pulsed laser deposition technique. The scattered x-ray intensity profile shows slight external voltage dependence, which means that the interface structure is changed by the external voltage. We also found that the voltage dependence of the scattering intensity exhibits hysteresis. Least squares refinement revealed that main atomic displacement induced by the eternal voltage occur in the LAO. The displacements in STO at the interface are of the order of sub-pm, which is much larger than expected for bulk STO ($\sim$ 0.01pm). \\[4pt] [1] S. Thiel \textit{et al}. Science \textbf{313,} 1942 (2006).\\[0pt] [2] A. D. Caviglia \textit{et al}. Nature \textbf{456,} 624 (2008).\\[0pt] [3] C. Bell \textit{et al}. Phys. Rev. Lett. \textbf{103,} 226802 (2009).   

Superconducting properties of single and bilayer LaAlO$_3$/SrTiO$_3$ interfaces

The two-dimensional electron liquid present at the LaAlO$_3$/SrTiO$_3$ interface exhibits superconductivity and hosts a large spin-orbit interaction. Quite remarkably, both phenomena can be controlled by an electric field. In this work, we have mapped the evolution of the superconducting properties upon gate voltage tuning, revealing a surprising change in thickness of the superconducting layer across the phase diagram. Using a single LaAlO$_3$/SrTiO$_3$ interface, we have realized field effect transistors and estimated the characteristic lengths (the coherence length and the superconducting thickness) as a function of the gate voltage by measuring the critical magnetic fields in parallel and perpendicular geometry. We have also realized bilayer interfaces, where two superconducting liquids are separated by the LaAlO$_3$ layer. In such structures, we have investigated the possible coupling of the two superconducting sheets, tuning one of the two by electric field.   

Dominance of spin-orbit coupling in magnetotransport at the LaAlO$_3$/SrTiO$_3$ interface

Today, the planar confinement of electrons is no longer synonymous with semiconducting quantum wells. The mobile electrons at the metallic LAO/STO interface are better confined by at least one order of magnitude, in a regime closer to that achieved in graphene. Nonetheless, graphene and, for example, GaAs/AlGaAs quantum well have more in common with each other than with LAO/STO. In these two systems, spin-orbit coupling effects are for many purposes unimportant or negligibly weak, and neither system has ever shown superconductivity. In contrast, spin-orbit coupling effects are strong at the LAO/STO interface, and superconductivity is a well established, potentially quite exotic, feature. We measure the resistance of the LAO/STO interface in an in-plane magnetic field perpendicular to the current, for a wide range of applied fields B, temperatures, and carrier densities controlled by a back-gate voltage $V_g$. Our measurements show dramatic, fast and very large drops in magnetoresistance, either as a function of B or $V_g$, decrease slowly with temperature and seem suggestive of magnetic ordering. However, they compare very well with an alternative explanation that strongly suggests that the effect of spin-orbit coupling on the band structure alone can dominate magnetotransport.   

Modulating the Conductivity of LaAlO$_{3}$/SrTiO$_{3}$ interface by Ionic Liquid-assisted Electric Field Effect

The LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) interface exhibits various novel properties such as conductivity, superconductivity and magnetism, which are not observed in its bulk materials. Modulation of its conductivity could help to understand the origin of such properties and explore potential application. We demonstrate the modulation of electrical transport properties in LAO/STO interface by electric field effect using electric double layer transistor (EDLT) configuration. In initially metallic samples, reversible metallic-insulating phase transition, field-effect transistor operation and electron mobility enhancement were observed in liquid-gated LaAlO$_{3}$/SrTiO$_{3}$ interface. Due to enhancement of mobility, we can observe quantum oscillations of the conductance at liquid-gated LAO/STO interface.   

Gate-tunable superconducting 2DEG at the amorphous LaAlO3/SrTiO3 interface

We investigate superconductivity in Hall-bar devices patterned at the amorphous LaAlO$_{3}$/SrTiO$_{3}$ interface. We find a critical temperature of 360 mK, which is gate-tunable to up to 460 mK, higher than is usually found in its crystalline equivalent. We measure the phase diagram and investigated the 2DEG confinement and its gate-dependence by probing the critical parallel and perpendicular magnetic fields using a vector magnet system. We find that the Ginzburg-Landau coherence length is larger than the thickness of the layer of superconducting electrons, confirming its two-dimensional nature.