Giant nonreciprocal transport effect in noncentrosymmetric superconductors is studied both theoretically and experimentally.

Polar conductors/superconductors with Rashba-type spin-orbit interaction are potential material platforms for quantum transport and spintronic functionalities. One of their inherent properties is the nonreciprocal transport, where the rightward and leftward currents become inequivalent, reflecting spatial inversion/time-reversal symmetry breaking. Such a rectification effect originating from the polar symmetry has been recently observed at interfaces or bulk Rashba semiconductors, while its mechanism in a polar superconductor remains elusive. Here, we report the nonreciprocal transport in gate-induced two-dimensional superconductor SrTiO 3 , which is a Rashba superconductor candidate. In addition to the gigantic enhancement of nonreciprocal signals in the superconducting fluctuation region, we found kink and sharp peak structures around critical temperatures, which reflect the crossover behavior from the paraconductivity origin to the vortex origin, based on a microscopic theory. The present result proves that the nonreciprocal transport is a powerful tool for investigating the interfacial/polar superconductors without inversion symmetry, where rich exotic features are theoretically prognosticated.

Two-dimensional (2D) crystals are attracting growing interest in various research fields such as engineering, physics, chemistry, pharmacy and biology owing to their low dimensionality and dramatic change of properties compared to the bulk counterparts. Among the various techniques used to manufacture 2D crystals, mechanical exfoliation has been essential to practical applications and fundamental research. However, mechanically exfoliated crystals on substrates contain relatively thick flakes that must be found and removed manually, limiting high-throughput manufacturing of atomic 2D crystals and van der Waals heterostructures. Here we present a deep learning-based method to segment and identify the thickness of atomic layer flakes from optical microscopy images. Through carefully designing a neural network based on U-Net, we found that our neural network based on U-net trained only with the data based on 24 images successfully distinguish monolayer and bilayer MoS2 with a success rate of 70%, which is a practical value in the first screening process for choosing monolayer and bilayer flakes of MoS2 of all flakes on substrates without human eye. The remarkable results highlight the possibility that a large fraction of manual laboratory work can be replaced by AI-based systems, boosting productivity.

Dynamical behavior of vortices plays central roles in the quantum phenomena of twodimensional (2D) superconductors. Quantum metallic state, for example, showing an anomalous temperature-independent resistive state down to low-temperatures, has been a common subject in recently developed 2D crystalline superconductors, whose microscopic origin is still under debate. Here, we unveil a new aspect of the vortex dynamics in a noncentrosymmetric 2D crystalline superconductor of MoS2 through the nonreciprocal transport measurement. The second harmonic resistance R 2 at low temperature with high current indicates the classical vortex flow accompanying the ratchet motion. Furthermore, we found that R 2 is substantially suppressed in the quantum metallic state with low current region, allowing identification of the quantum and classical ratchet motions of vortices by the magnitude of the second harmonic generation. This suggests that nonreciprocal transport measurement can be a powerful tool to probe the vortex dynamics in noncentrosymmetric 2D superconductors.A variety of two-dimensional (2D) superconductors have emerged in the past decade, providing a novel materials' platform of unique physical phenomena 1-5 . Among them, quantum phases and their transitions in the vortex states are of particular interest [6][7][8] . In conventional amorphous metallic films, a superconductor-insulator transition occurring at a single critical point in the zero temperature limit is one of the well-known quantum phenomena 9,10 . Such quantum critical behavior is predominantly controlled by the disorder of films, which are inevitably enhanced by the reduction of film thickness, and hence well described by the scaling theory in the framework of the dirty boson picture 10 .On the other hand, a new route has been desired to approach the quantum phase transition with minimal disorder, for the comprehensive understanding of 2D superconductors. The recently emerging 2D superconductors based on single crystals 1,2 are one of such candidate materials, and in fact, some of them reveal a completely different physical picture from the dirty boson model 9 . Instead of superconducting and insulating states separated by the quantum critical point, gate-induced and exfoliated 2D superconductors display a broad metallic state dominating a magnetic field and temperature (B-T) phase diagram 6,7 . In this state, once the outof-plane magnetic field is applied, the zero-resistance state is immediately destroyed into the temperature-independent finite resistive state even at low temperatures. Such a resistive state is observed not only in the 2D crystalline superconductors but also in amorphous thin film superconductors with weak pinning effect 8,11,12 , though its origin is still under debate. In addition to the extrinsic heating by the environmental noise 13 , several intrinsic mechanisms such as quantum collective creep of vortices 11,14 and Bose metal 7,15,16 are being discussed. However, it is technically difficult to distinguish such various vortex stat...

Recent discoveries of two-dimensional (2D) superconductors have uncovered various new aspects of physical properties including vortex matter. In this paper, we report transport properties and a dynamical phase diagram at zero magnetic field in ion-gated MoS2. In addition to the universal jump in the current-voltage characteristic showing unambiguous evidence of the Berezinskii-Kosterlitz-Thouless (BKT) transition, we observed multiple peaks in the temperature-and current-derivative of the electrical resistance, based on which a dynamical phase diagram in the current-temperature plane was constructed. We found current-induced dynamical states of vortex-antivortex pairs, containing that with the phase slip line. Also, we present a global phase diagram of vortices in gated MoS2 which captures the nature of vortex matter of clean 2D superconductors.

Nonreciprocal or even-order nonlinear responses in symmetry-broken systems are powerful probes of emergent properties in quantum materials, including superconductors, magnets, and topological materials. Recently, vortex matter has been recognized as a key ingredient of giant nonlinear responses in superconductors with broken inversion symmetry. However, nonlinear effects have been probed as excess voltage only under broken time-reversal symmetry. In this study, we report second harmonic transport under time-reversal symmetry in the noncentrosymmetric trigonal superconductor PbTaSe2. The magnitude of anomalous nonlinear transport is two orders of magnitude larger than those in the normal state, and the directional dependence of nonlinear signals are fully consistent with crystal symmetry. The enhanced nonlinearity is semiquantitatively explained by the asymmetric Hall effect of vortex-antivortex string pairs in noncentrosymmetric systems. This study enriches the literature on nonlinear phenomena by elucidating quantum transport in noncentrosymmetric superconductors.

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