Supercurrent diode effect and finite momentum superconductivity (2106.01909v3)

Abstract: When both inversion and time-reversal symmetries are broken, the critical current of a superconductor can be nonreciprocal. In this work we show that in certain classes of two-dimensional superconductors with antisymmetric spin-orbit coupling, Cooper pairs acquire a finite momentum upon the application of an in-plane magnetic field, and as a result, critical currents in the direction parallel and antiparallel to the Cooper pair momentum become unequal. This supercurrent diode effect is also manifested in the polarity-dependence of in-plane critical fields induced by a supercurrent. These nonreciprocal effects may be found in polar SrTiO$_3$ film, few-layer MoTe$_2$ in the $T_d$ phase, and twisted bilayer graphene in which the valley degrees of freedom plays the role analogous to spin.

• The paper demonstrates that broken inversion symmetry induces finite Cooper pair momentum, leading to direction-dependent supercurrents.
• The paper employs numerical simulations and Ginzburg-Landau theory to model helical superconducting states under varying magnetic fields.
• The paper predicts skewed superconducting phase boundaries in the B-J plane, aligning with recent experimental observations of nonreciprocal transport.

Supercurrent Diode Effect and Finite Momentum Superconductivity

The paper under review explores a fascinating phenomenon in condensed matter physics: the supercurrent diode effect and finite momentum superconductivity in noncentrosymmetric two-dimensional (2D) superconductors. Specifically, the authors investigate conditions under which superconducting states with broken inversion and time-reversal symmetries exhibit nonreciprocal critical currents, dependent on the intrinsic momentum of the Cooper pairs—a haLLMark of finite momentum superconductivity.

This work builds on the theoretical foundation laid by Bardeen-Cooper-Schrieffer (BCS) theory and its extensions, such as the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, characterized by a spatially varying superconducting order parameter. The presence of antisymmetric spin-orbit coupling (SOC) in 2D superconductors, such as those found in polar SrTiO$_3$ films and few-layer MoTe$_2$, induces finite Cooper pair momentum when an in-plane magnetic field is applied. This results in critical currents that differ by their direction relative to the Cooper pair momentum, thus creating a supercurrent diode effect.

Key Findings

1. Helical Superconductivity: The research identifies helical superconducting states where the presence of SOC and a magnetic field cause Cooper pairs to acquire finite momentum. The magnitude of this momentum is linked to the Rashba SOC strength and the applied magnetic field, leading to an observable nonreciprocity in critical currents.
2. Numerical and Theoretical Analysis: Detailed numerical simulations accompanied by Ginzburg-Landau theory elucidate the conditions under which this effect is maximized. The authors derive expressions for the critical current and demonstrate its dependence on temperature, magnetic field, and SOC parameters.
3. Phase Diagrams and Predictions: The work predicts a skewed superconductive phase in the $B$-$J$ (magnetic field versus current) plane, defining boundaries of superconductivity. These predictions align with recent experimental observations of nonreciprocal transport phenomena, such as those documented in artificial metal films under magnetic fields.
4. Implications for Material Systems: Theoretical predictions extend to materials with nontrivial spin textures, such as polar SrTiO$_3$, twisted bilayer graphene, and transition metal dichalcogenides. These findings suggest that future experimental work could directly demonstrate finite momentum superconductivity.

Implications and Future Directions

This paper's implications are profound for both fundamental physics and potential applications in superconducting devices. The detailed understanding of nonreciprocal critical currents could pave the way for novel superconducting circuits and superconducting logic devices, where direction-dependent behavior is a desired property. Additionally, the findings offer a new perspective on detecting hidden orders in unconventional superconductors where direct observation of the Cooper pair momentum remains challenging.

As research in superconductivity progresses, exploring other 2D systems with strong SOC, such as novel heterostructures or new quantum materials, may yield further insights into the interplay between symmetry breaking and superconducting properties. Theoretical advancements and experimental techniques, such as angle-resolved photoemission spectroscopy (ARPES), could together enhance our understanding of the supercurrent diode effect, strengthening the connection between theory and experimental observation.

In conclusion, this paper contributes a significant advancement to our understanding of finite momentum superconductivity and supercurrent diode effects, providing a foundation for future exploration in both theoretical and applied superconductivity research.