Abstract
Two-dimensional SrTiO3-based interfaces stand out among non-centrosymmetric superconductors due to their intricate interplay of gate-tunable Rashba spin-orbit coupling and multi-orbital electronic occupations, whose combination theoretically prefigures various forms of non-standard superconductivity. By employing superconducting transport measurements in nano-devices we present strong experimental indications of unconventional superconductivity in the LaAlO3/SrTiO3 interface. The central observations are the substantial anomalous enhancement of the critical current by small magnetic fields applied perpendicularly to the plane of electron motion, and the asymmetric response with respect to the magnetic field direction. These features cannot be accommodated within a scenario of canonical spin-singlet superconductivity. We demonstrate that the experimental observations can be described by a theoretical model based on the coexistence of Josephson channels with intrinsic phase shifts. Our results exclude a time-reversal symmetry breaking scenario and suggest the presence of anomalous pairing components that are compatible with inversion symmetry breaking and multi-orbital physics.
Highlights
Accessing the fundamental structure of Cooper pairs in unconventional superconductors[1] and the mechanisms behind electron pairing are among the most notable challenges in condensed matter physics
The combination of inversion symmetry breaking and multiple orbital degrees of freedom can yield a superconducting order parameter that goes beyond the canonical singlet-triplet mixed parity, with an inter-band antiphase pairing (e.g., s+−)[9] or pure even-parity inter-orbital spintriplet pairs[8]
The overall scenario underlines fundamental challenges not yet fully settled about the unconventional nature of the superconducting state in LaAlO3/ SrTiO3 (LAO/STO) interface, its interrelation with the multi-orbital degrees of freedom, the survival in a strongly inhomogeneous environment, as well as the eventual occurrence of extra symmetry breaking
Summary
Accessing the fundamental structure of Cooper pairs in unconventional superconductors[1] and the mechanisms behind electron pairing are among the most notable challenges in condensed matter physics. The superconducting phase is likely to be marked by nonuniform superconducting islands[28,30,31] While these strong nonmagnetic inhomogeneities, at first sight, can be incompatible with an unconventional type of pairing, it has been recently figured out that extra internal degrees of freedom such as orbitals, sublattices, or valleys can protect Cooper pairs from strong scattering[32]. The overall scenario underlines fundamental challenges not yet fully settled about the unconventional nature of the superconducting state in LAO/STO interface, its interrelation with the multi-orbital degrees of freedom, the survival in a strongly inhomogeneous environment, as well as the eventual occurrence of extra symmetry breaking. The effect is significantly enhanced at positive gate voltages corresponding to the regime of multiband occupation This anomalous magnetic field dependence of critical current cannot be explained by classical models of s-wave superconductivity.
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