Session L64: Two-Dimensional Electron gas in STO-based Heterostructures

Probing inhomogeneous superconductivity and magnetism in complex oxide heterostructures with scanning SQUID

Complex oxide heterostructures offer unique opportunities for engineering electronic and magnetic systems, combining functionalities of their constituent materials and, in some cases, exhibiting emergent properties. The reduced dimensionality and broken inversion symmetry at interfaces can make them particularly sensitive to local variation in the crystal lattice. A key problem is to disentangle the intrinsic properties of heterostructures from those of ordered states stabilized by defects or other sources of inhomogeneity. I will discuss two studies which used scanning superconducting quantum interference device (SQUID) microscopy, a local magnetic probe, to investigate the effects of intrinsic or controlled inhomogeneity in complex oxide heterostructures. In the first, we imaged superconductivity in lanthanum aluminate/strontium titanate (LAO/STO) and delta-doped STO heterostructures. Spatial motifs in our measurements demonstrate that different orientations of structural domains with respect to the symmetry-breaking interface result in different superconducting transition temperatures. While LAO and STO are both nonmagnetic in the bulk, many experimental probes have found signatures of magnetism in LAO/STO heterostructures, with local measurements showing that magnetism, when present, is inhomogeneous. In our second study, we investigated oxygen vacancies as a proposed mechanism for the magnetism by measuring samples with differing oxygen content, placing tight limits on magnetism, even in the most strongly reduced samples.   

Cryogenic Piezoforce Microscopy of Sketched LaAlO3/SrTiO3 Nanostructures

It is possible to reveal otherwise hidden electronic properties in 2D oxide interfaces, namely LaAlO₃/SrTiO₃, by using local probes instead of traditional bulk measurements. For example, scanning single electron transistor microscopy has shown enhanced transport along ferroelastic domain boundaries [1]. Furthermore, Frenkel et al. have demonstrated that the local application of pressure to the interface of LaAlO₃/SrTiO₃ can reveal ferroelastic domain boundaries [2]. Here we detail our efforts to reveal electron transport in “sketched” LaAlO₃/SrTiO₃ nanostructures [3] using a modified vacuum AFM at room and low (T < 25 K) temperatures.
[1] M. Honig et al., Nature Materials 12, 1112 (2013).
[2] Y. Frenkel et al., Nature Materials 16, 1203 (2017).
[3] C. Cen et al., Nature Materials 7, 298 (2008).   

Dependence of LaAlO3/SrTiO3 Electron Pairing Strength on Crystallographic Orientation

Recently, it has been shown that superconductivity at the LaAlO₃/SrTiO₃ interface is essentially 1D in nature, and that the pairing mechanism appears to be linked to the boundaries of naturally formed X, Y and Z ferroelastic domains in SrTiO₃ [1]. Here, we investigate the nature of the pairing mechanism in LaAlO₃/SrTiO₃ nanostructures by examining the electron pairing strength with respect to the crystallographic direction. We write 1D, cross-shaped electron waveguides or “nanocrosses” [2] at the LaAlO₃/SrTiO₃ interface using conductive atomic force microscope (c-AFM) lithography. The c-AFM lithography and device geometry together defines the X, Y and Z domain boundaries across the nanocross at low temperatures. A sinusoidal dependence is observed as the nanocross is rotated with respect to the crystallographic direction. This characteristic angular dependence helps to constrain and sharpen our understanding of the pairing mechanism at the interface and the role of ferroelastic domain structure.

[1] Y. Pai et al., Physical review letters 120 (14), 147001 (2018)
[2] A. Nethwewala et al., Nanoscale Horizons 4, 1194-1201 (2019)   

Transport properties of non-reciprocal 1D quantum channels at the LaAlO3/SrTiO3 interface

Abstract: Complex oxide heterostructures exhibit many interesting phenomena [1] that can be controlled on the nanoscale using a conductive atomic force microscope (c-AFM) lithography technique [2]. We create chiral 1D superlattices at the LaAlO₃/SrTiO₃ interface by adding a periodic modulation to an electron waveguide device. These nanostructures support quantized ballistic transport of electrons and electron pairs, as well as oscillatory transport behavior indicating an engineered spin orbit interaction [3]. These experiments represent a first step toward engineering properties in 1D quantum wires and can be regarded as a building block for more complex quantum systems.

[1] Y-Y. Pai, et al., Reports on Progress in Physics 81 (3), 036503 (2018).
[2] C. Cen, et al., Nature Materials 7, 298 (2008).
[3] A. Annadi, et al., Nano Letter 18, 4473-4481 (2018).   

Nanoscale control of the metal-insulator transition in free-standing LaAlO3/SrTiO3 membranes

By combining 2D materials such as graphene into layered van der Waals heterostructures, new types of designer materials with unique and emergent properties have been realized [1]. We describe a method for achieving free-standing single-crystal LaAlO₃/SrTiO₃ membranes by etching away a sacrificial SrRuO₃ layer underneath SrTiO₃. LaAlO₃/SrTiO₃ membranes are transferred to a sapphire substrate, and we use conductive AFM lithography [2] to achieve nanoscale control the insulator to metal transition at the LaAlO₃/SrTiO₃ interface. Nanoscale conductive channels created at room temperature remain conductive at two Kelvin. These results demonstrate the capacity to create LaAlO₃/SrTiO₃ membranes that can be integrated with other materials and reversibly patterned at nanoscale dimensions.

[1] A. K. Geim & I. V. Grigorieva, Nature 499, 419 (2013).

[2] C. Cen, et al., Nature Materials 7, 298 (2008).   

Influence of Temperature on LaAlO3/SrTiO3 Nanowire Lifetimes Under Ambient Conditions

The discovery of a tunable 2DEG at the interface of LaAlO₃/SrTiO₃ has spurred interest in LaAlO₃/SrTiO₃, which exhibits numerous interesting properties. Using a conductive AFM lithography technique, we can create novel devices at the interface of LaAlO₃/SrTiO₃ by locally confining the 2DEG to zero or one dimension [1]. However, in ambient conditions, an appreciable degradation of device lifetime occurs [2]. Previously, the decay rate of such devices was characterized as a function of humidity and pressure; however, its dependence on temperature has yet to be determined. Here we describe experiments detailing efforts to quantify the device lifetimes as a function of temperature under otherwise ambient conditions, which will aid in the creation of new, more complex devices.

[1] C. Cen et al., Science 323, 1026-1030 (2009).
[2] F. Bi et al., Appl. Phys. Lett. 97, 173110 (2010).   

Surface Acoustic Wave Generation and Detection on LaAlO3/SrTiO3

We aim to generate and detect surface acoustic waves (SAW) in LaAlO₃/SrTiO₃ heterostructures. Using a well-developed conductive-AFM lithography technique ^[1], we “sketch” interdigitated transducers (IDT) on LaAlO₃/SrTiO₃ “canvases”, which convert electronic signals into SAW and vice-versa. Two sets of IDTs are written on the structure to function as a generator and detector. SAW can be used to generate dynamic potentials based on piezoelectric properties of LaAlO₃/SrTiO₃, and have the potential to drive electrons through nanostructures, a property that could be useful for quantum information applications.

[1] C. Cen, et al., Nature Materials 7, 298 (2008).   

Creating nano-scale “vias” in LaAlO3/SrTiO3 for integration of nanostructures

The 2D electron gas (2DEG) at the LaAlO₃/SrTiO₃ interface exhibits a wide range of gate-tunable properties like superconductivity, magnetism, and spin orbit coupling [1]. Furthermore, the interface can be patterned with nanoscale dimensions using c-AFM lithography [2] and integrated with other 2D materials such as graphene [3]. While the surface of LaAlO₃/SrTiO₃ is only 1.2 nm from the interface, improved coupling of particles on the surface with the interface requires closer proximity. Here, we discuss methods to create nanoscale holes or “nano vias” in LaAlO₃/SrTiO₃ with the goal of integrating nanomaterials directly to interface while maintaining interfacial conductivity. These nano vias will allow us to study the effect of local coupling between the 2DEG at LaAlO₃/SrTiO₃ interface and a wide variety of interesting nanomaterials such as colloidal quantum dots or graphene nanoribbons.
[1] Yun-Yi P., et al., Rep. Prog. Phys. 81, 036503 (2018)
[2] C. Cen, et al., Nature Materials 7, 298 (2008).
[3] Huang, M. et al., APL Mat. 3, 062502 (2015).   

Interface band engineering in LaAlO3/SrTiO3 heterostructures

Novel two-dimensional electron systems at the interfaces of oxide heterostructures such as LaAlO₃ /SrTiO₃ recently have attracted much attention as they display intriguing properties which may be exploited in future electronic devices. A key requirement for such applications is the controllability of the electronic interface properties such as the valence band offset. We demonstrate that these properties can be effectively engineered in LaAlO₃ /SrTiO₃ by virtue of the oxygen vacancy concentration. By angle-dependent hard x-ray photoelectron spectroscopy, we derive a complete band diagram of the heterostructure dependent on the oxygen vacancy concentration which is adjusted during the photoemission experiments by means of synchrotron light irradiation and simultaneous oxygen dosing. The dielectric constant of the SrTiO₃ substrates and its strong non-linear dependence on the electric field prove to be essential for the band arrangement of the LaAlO₃/SrTiO₃ heterostructures. The comprehensive analysis of the band situation at the LaAlO₃/SrTiO₃ heterointerface as a function of the oxygen vacancy concentration can reconcile the contradicting results of previous photoemission studies.   

Character of the insulator-metal transition at the LaAlO3/SrTiO3 interface

Under a film of lanthanum aluminate (LAO) on strontium titanate (STO) a two dimensional metal appears after a critical thickness, as a response to the electrostatic energy build-up originated by the polar discontinuity at the interface. The character of the transition has not been much explored given the discrete character of the controlling parameter, the film thickness. However, an applied electric field across the film for a thickness close to the transition will drive the transition as well, the character of the transition becoming relevant, not only fundamentally but for possible applications, too. A phenomenological mean-field theory predicts a continuous transition, while a saturating discontinuous one has been assumed in other contexts. We will present a theoretical analysis showing a richer phenomenology than expected so far, with the possibility of either continuous or discontinuous transitions, including a situation in which there is a second discontinuous transition with a jump in carrier concentration after an earlier continuous metal-insulator one.   

Giant nonreciprocal charge transport in noncentrosymmetric LaAlO3/SrTiO3 interfaces

Electrons confined at an interfacial quantum well of a LaAlO₃/SrTiO₃ associated with broken inversion symmetry show various exotic condensed matter phases and rich spin-orbitronic functionalities. This two-dimensional polar conductor may exhibit directional propagation of itinerant electrons, i.e. the leftward and rightward currents differ from each other, when time-reversal symmetry is further broken. This potential rectification effect generally was displayed to be very weak due to the fact that kinetic energy is much higher than energies related to symmetry breakings producing weak perturbation. Here, we present giant gate-tunable nonreciprocal charge transport in the LaAlO₃/SrTiO₃ conductive oxide interface, where the electrons are confined at two-dimension with low Fermi energy. The coefficient γ indicating the magnitude of nonreciprocal response, was estimated to be as high as ~ 10² T^-1A^-1, which is about 3 order of magnitude higher than those reported for noncentrosymmetric conductors. The observed behavior of nonreciprocal response in LaAlO₃/SrTiO₃ is related to comparable energy scales among kinetic energy, magnetic field and spin-orbit interaction, which opens a promising route to improve nonreciprocal response and its functionalities in the emerging spin-orbitronics.   

High-mobility two-dimensional hole gas at the SrTiO3 interface formed by depositing an ultrathin metal film at room temperature

Despite intensive studies on the two-dimensional electron gas (2DEG) at the SrTiO₃ (STO) interface [1], forming a 2D hole gas (2DHG) at the STO interface is extremely difficult [2], although both are essential for the realization of high-speed oxide-based electronics. Here, we demonstrate a very simple method to realize a 2DHG with an ultrahigh mobility of 24,000 cm²V^–1s^–1 at an STO interface. The 2DHG is obtained by depositing a sub-nm-thick Fe layer (thickness t ≦ 0.2 nm) on STO substrates at room temperature in an ultrahigh vacuum chamber. The Fe layer is oxidized and becomes insulating amorphous FeO_x. Magnetotransport measurements reveal the existence of high-mobility carriers in the STO side, and the carrier type changes from a pure p-type (t ≦ 0.2 nm) to n-type (t > 0.3 nm) by varying the Fe thickness. In a p-type sample (t = 0.1 nm), the Shubnikov - de Haas oscillation is clearly observed in out-of-plane magnetic field but disappears in in-plane field. These results clearly demonstrate the 2D nature of the high-mobility hole carriers.
[1] A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004). [2] H. Lee et al., Nat. Mater. 17, 231 (2018).   

Shubnikov-de Hass oscillations to the quantum limit in (111) oriented SrTiO3 interface 2D electron gas

The (111) surface of perovskite oxide SrTiO₃ has a hexagonal structure, similar to topological insulator Bi₂Se₃. Existence of topological states has been predicted in this system and high mobility two-dimensional (2D) electron gases at interfaces provide a promising platform to study these properties. We created a 2D electron gas at SrTiO3 (111) interfaces with high mobility over 15000 cm²V^-1s^-1 at low temperatures. Utilizing interface engineering, we have achieved relatively low carrier density of 1.3x10¹³ cm^-2. Longitudinal magnetoresistance shows clear Shubnikov-de Hass (SdH) oscillations at low temperatures and high magnetic fields at both 35 T (DC field) and 60 T (pulsed field). In the low carrier density sample, we have observed the lowest Landau level at magnetic field around 20 T, which shifts to higher fields with higher carrier densities. More strikingly, a linear magnetoresistance (LMR) behavior appears at high magnetic field beyond the quantum limit. Temperature and angular dependence of the quantum oscillations as well as the LMR will be discussed.