Link between the Superconducting Dome and Spin-Orbit Interaction in the (111) LaAlO_{3}/SrTiO_{3} Interface

Enhanced Nonlinear Response by Manipulating the Dirac Point at the (111) LaTiO_{3}/SrTiO_{3} Interface

Tunable spin-orbit interaction (SOI) is an important feature for future spin-based devices. In the presence of a magnetic field, SOI induces an asymmetry in the energy bands, which can produce nonlinear transport effects (V∼I^{2}). Here, we focus on such effects to study the role of SOI in the (111) LaTiO_{3}/SrTiO_{3} interface. This system is a convenient platform for understanding the role of SOI since it exhibits a single-band Hall response through the entire gate-voltage range studied. We report a pronounced rise in the nonlinear longitudinal resistance at a critical in-plane field H_{cr}. This rise disappears when a small out-of-plane field component is present. We explain these results by considering the location of the Dirac point formed at the crossing of the spin-split energy bands. An in-plane magnetic field pushes this point outside of the Fermi contour, and consequently changes the symmetry of the Fermi contours and intensifies the nonlinear transport. An out-of-plane magnetic field opens a gap at the Dirac point, thereby significantly diminishing the nonlinear effects. We propose that magnetoresistance effects previously reported in interfaces with SOI could be comprehended within our suggested scenario.

Giant Tunability of Rashba Splitting at Cation-Exchanged Polar Oxide Interfaces by Selective Orbital Hybridization

The 2D electron gas (2DEG) at oxide interfaces exhibits extraordinary properties, such as 2D superconductivity and ferromagnetism, coupled to strongly correlated electrons in narrow d-bands. In particular, 2DEGs in KTaO3 (KTO) with 5d t2g orbitals exhibit larger atomic spin-orbit coupling and crystal-facet-dependent superconductivity absent for 3d 2DEGs in SrTiO3 (STO). Herein, by tracing the interfacial chemistry, weak anti-localization magneto-transport behavior, and electronic structures of (001), (110), and (111) KTO 2DEGs, unambiguously cation exchange across KTO interfaces is discovered. Therefore, the origin of the 2DEGs at KTO-based interfaces is dramatically different from the electronic reconstruction observed at STO interfaces. More importantly, as the interface polarization grows with the higher order planes in the KTO case, the Rashba spin splitting becomes maximal for the superconducting (111) interfaces approximately twice that of the (001) interface. The larger Rashba spin splitting couples strongly to the asymmetric chiral texture of the orbital angular moment, and results mainly from the enhanced inter-orbital hopping of the t2g bands and more localized wave functions. This finding has profound implications for the search for topological superconductors, as well as the realization of efficient spin-charge interconversion for low-power spin-orbitronics based on (110) and (111) KTO interfaces.

Designing spin and orbital sources of Berry curvature at oxide interfaces

Quantum materials can display physical phenomena rooted in the geometry of electronic wavefunctions. The corresponding geometric tensor is characterized by an emergent field known as the Berry curvature (BC). Large BCs typically arise when electronic states with different spin, orbital or sublattice quantum numbers hybridize at finite crystal momentum. In all the materials known to date, the BC is triggered by the hybridization of a single type of quantum number. Here we report the discovery of the first material system having both spin- and orbital-sourced BC: LaAlO₃/SrTiO₃ interfaces grown along the [111] direction. We independently detect these two sources and probe the BC associated to the spin quantum number through the measurements of an anomalous planar Hall effect. The observation of a nonlinear Hall effect with time-reversal symmetry signals large orbital-mediated BC dipoles. The coexistence of different forms of BC enables the combination of spintronic and optoelectronic functionalities in a single material.

Light-Induced Giant Rashba Spin-Orbit Coupling at Superconducting KTaO3 (110) Heterointerfaces

The 2D electron system (2DES) at the KTaO₃ surface or heterointerface with 5d orbitals hosts extraordinary physical properties, including a stronger Rashba spin-orbit coupling (RSOC), higher superconducting transition temperature, and potential of topological superconductivity. Herein, a huge enhancement of RSOC under light illumination achieved at a superconducting amorphous-Hf_0.5 Zr_0.5 O₂ /KTaO₃ (110) heterointerfaces is reported. The superconducting transition is observed with T_c = 0.62 K and the temperature-dependent upper critical field reveals the interaction between spin-orbit scattering and superconductivity. A strong RSOC with B_so = 1.9 T is revealed by weak antilocalization in the normal state, which undergoes sevenfold enhancement under light illumination. Furthermore, RSOC strength develops a dome-shaped dependence of carrier density with the maximum of B_so = 12.6 T achieved near the Lifshitz transition point n_c ≈ 4.1 × 10¹³ cm^-2 . The highly tunable giant RSOC at KTaO₃ (110)-based superconducting interfaces show great potential for spintronics.

Field-induced boson insulating states in a 2D superconducting electron gas with strong spin-orbit scatterings

The phenomenon of field-induced superconductor-to-insulator transitions observed experimentally in electron-doped SrTiO₃/LaAlO₃interfaces, analyzed recently by menas of 2D superconducting fluctuations theory, is revisited with new insights associating it with the appearnace at low temperatures of field-induced boson insulating states. Within the framework of the time-dependent Ginzburg-Landau functional approach, we pinpoint the origin of these states in field-induced extreme softening of fluctuation modes over a large region in momentum space, upon diminishing temperature, which drives Cooper-pair fluctuations to condense into mesoscopic puddles in real space. Dynamical quantum tunneling of Cooper-pair fluctuations out of these puddles, introduced within a phenomenological approach, which break into mobile single-electron states, contains the high-field resistance onset predicted by the exclusive boson theory.

Manipulation of 2DEG at double-doped high-entropy heterointerfaces

Chemical doping is a dominating method for manipulating oxide two-dimensional electron gas (2DEG). However, enhancing the doping level while maintaining the metallic conduction remains a challenge, which limits detailed knowledge of 2DEG manipulation. Herein, we propose a concept of high-entropy heterointerface, which consists of a complex oxide (containing at least 5 elements) at either or both sides of the interface. By doubly doping Sr and Mn elements in the Nd and Al sites of NdAlO₃, we grow Nd_1-xSr_xAl_1-xMn_xO₃ (NSAMO) films onto SrTiO₃ (STO) substrates to fabricate NSAMO/STO high-entropy heterointerfaces with different thicknesses (2-30 nm) and a wide range of doping ratios x (0.14-0.56). The 2DEG conducting behavior is maintained until x = 0.42, which is higher compared with similar studies. The varying x results in the coexistence of rich properties like a weak anti-localization (0.14-0.42), abnormal Hall effect (0.28 & 0.42), Lifshitz transition (0.42) and stable structure. These results confirm the potential of this strategy to tailor 2DEG in all-oxide interfaces.

On the Superconducting Critical Temperature of Heavily Disordered Interfaces Hosting Multi-Gap Superconductivity

LaAlO3/SrTiO3 interfaces are a nice example of a two-dimensional electron gas, whose carrier density can be varied by top- and back-gating techniques. Due to the electron confinement near the interface, the two-dimensional band structure is split into sub-bands, and more than one sub-band can be filled when the carrier density increases. These interfaces also host superconductivity, and the interplay of two-dimensionality, multi-band character, with the possible occurrence of multi-gap superconductivity and disorder calls for a better understanding of finite-bandwidth effects on the superconducting critical temperature of heavily disordered multi-gap superconductors.

Cooperative evolution of polar distortion and nonpolar rotation of oxygen octahedra in oxide heterostructures

Polarity discontinuity across LaAlO₃/SrTiO₃ (LAO/STO) heterostructures induces electronic reconstruction involving the formation of two-dimensional electron gas (2DEG) and structural distortions characterized by antiferrodistortive (AFD) rotation and ferroelectric (FE) distortion. We show that AFD and FE modes are cooperatively coupled in LAO/STO (111) heterostructures; they coexist below the critical thickness (t _c) and disappear simultaneously above t _c with the formation of 2DEG. Electron energy-loss spectroscopy and density functional theory (DFT) calculations provide direct evidence of oxygen vacancy (V _O) formation at the LAO (111) surface, which acts as the source of 2DEG. Tracing the AFD rotation and FE distortion of LAO reveals that their evolution is strongly correlated with V _O distribution. The present study demonstrates that AFD and FE modes in oxide heterostructures emerge as a consequence of interplay between misfit strain and polar field, and further that their combination can be tuned to competitive or cooperative coupling by changing the interface orientation.

Zero-Bias Conductance Peaks Effectively Tuned by Gating-Controlled Rashba Spin-Orbit Coupling

Zero-bias conductance peaks (ZBCPs) can manifest a number of notable physical phenomena and thus provide critical characteristics to the underlying electronic systems. Here, we report observations of pronounced ZBCPs in hybrid junctions composed of an oxide heterostructure LaAlO_{3}/SrTiO_{3} and an elemental superconductor Nb, where the two-dimensional electron system (2DES) at the LaAlO_{3}/SrTiO_{3} interface is known to accommodate gate-tunable Rashba spin-orbit coupling (SOC). Remarkably, the ZBCPs exhibit a domelike dependence on the gate voltage, which correlates strongly with the nonmonotonic gate dependence of the Rashba SOC in the 2DES. The origin of the observed ZBCPs can be attributed to the reflectionless tunneling effect of electrons that undergo phase-coherent multiple Andreev reflection, and their gate dependence can be explained by the enhanced quantum coherence time of electrons in the 2DES with increased momentum separation due to SOC. We further demonstrate theoretically that, in the presence of a substantial proximity effect, the Rashba SOC can directly enhance the overall Andreev conductance in the 2DES-barrier-superconductor junctions. These findings not only highlight nontrivial interplay between electron spin and superconductivity revealed by ZBCPs, but also set forward the study of superconducting hybrid structures by means of controllable SOC, which has significant implications in various research fronts from superconducting spintronics to topological superconductivity.

Gap suppression at a Lifshitz transition in a multi-condensate superconductor

In multi-orbital materials, superconductivity can exhibit several coupled condensates. In this context, quantum confinement in two-dimensional superconducting oxide interfaces offers new degrees of freedom to engineer the band structure and selectively control the occupancy of 3d orbitals by electrostatic doping. Here, we use resonant microwave transport to extract the superfluid stiffness of the (110)-oriented LaAlO₃/SrTiO₃ interface in the entire phase diagram. We provide evidence of a transition from single-condensate to two-condensate superconductivity driven by continuous and reversible electrostatic doping, which we relate to the Lifshitz transition between 3d bands based on numerical simulations of the quantum well. We find that the superconducting gap is suppressed while the second band is populated, challenging Bardeen-Cooper-Schrieffer theory. We ascribe this behaviour to the existence of superconducting order parameters with opposite signs in the two condensates due to repulsive coupling. Our findings offer an innovative perspective on the possibility to tune and control multiple-orbital physics in superconducting interfaces.

Symmetry and Correlation Effects on Band Structure Explain the Anomalous Transport Properties of (111) LaAlO_{3}/SrTiO_{3}

The interface between the two insulating oxides SrTiO_{3} and LaAlO_{3} gives rise to a two-dimensional electron system with intriguing transport phenomena, including superconductivity, which are controllable by a gate. Previous measurements on the (001) interface have shown that the superconducting critical temperature, the Hall density, and the frequency of quantum oscillations, vary nonmonotonically and in a correlated fashion with the gate voltage. In this Letter we experimentally demonstrate that the (111) interface features a qualitatively distinct behavior, in which the frequency of Shubnikov-de Haas oscillations changes monotonically, while the variation of other properties is nonmonotonic albeit uncorrelated. We develop a theoretical model, incorporating the different symmetries of these interfaces as well as electronic-correlation-induced band competition. We show that the latter dominates at (001), leading to similar nonmonotonicity in all observables, while the former is more important at (111), giving rise to highly curved Fermi contours, and accounting for all its anomalous transport measurements.

Superconducting Tunneling Spectroscopy of Spin-Orbit Coupling and Orbital Depairing in Nb:SrTiO_{3}

We have examined the intrinsic spin-orbit coupling and orbital depairing in thin films of Nb-doped SrTiO_{3} by superconducting tunneling spectroscopy. The orbital depairing is geometrically suppressed in the two-dimensional limit, enabling a quantitative evaluation of the Fermi level spin-orbit scattering using Maki's theory. The response of the superconducting gap under in-plane magnetic fields demonstrates short spin-orbit scattering times τ_{so}≤1.1  ps. Analysis of the orbital depairing indicates that the heavy electron band contributes significantly to pairing. These results suggest that the intrinsic spin-orbit scattering time in SrTiO_{3} is comparable to those associated with Rashba effects in SrTiO_{3} interfacial conducting layers and can be considered significant in all forms of superconductivity in SrTiO_{3}.