Session Y37: Superconducting Diodes

Zero-field superconducting diode effect in twisted trilayer graphene

A zero-field superconducting diode effect is recently reported in twisted trilayer graphene, evidenced by the highly nonreciprocal I-V curve and directional dependence in the critical supercurrent [1]. This phenomenon points towards an electronic order in graphene moir'e systems that breaks both parity and time-reversal symmetry (PT-breaking). In this talk, I use a scheme of angle-resolved nonreciprocal transport response to identify the PT-breaking order in twisted trilayer graphene. By analyzing the evolution with varying magnetic field, moir'e band filling and current bias, I show that this PT-breaking order is distinct from orbital ferromagnetism. While the zero-field superconducting diode effect is observed over a wide range of twist angles, it is the most prominent when the twist angle is detuned from the magic-angle regime. To understand this unique twist angle dependence, I will discuss the subtle interplay between the moir'e flatband and Coulomb interaction, which gives rise to a new cascade order in the small twist angle regime.

[1] Jiang-Xiazi Lin, et al. "Zero-field superconducting diode effect in small-twist-angle trilayer graphene." Nature Physics 18, 10 (2022)   

Intrinsic superconducting diode effect

Nonreciprocal transport phenomena are attracting much attention as a new functionality of materials [1,2].  Recently, it has been observed in a Nb/V/Ta superlattice without an inversion center that the critical current for the phase transition between normal and superconducting states is different between the leftward and the rightward electric currents. Such a nonreciprocity of the critical current is called the superconducting diode effect (SDE) [3]. SDE might be applicable to dissipationless electric circuits and is an intriguing phenomenon manifesting the interplay between superconductivity and the inversion-symmetry breaking. It is an important issue to study how the SDE occurs to further promote the experimental/theoretical works.

In this study, we clarify that the critical current of noncentrosymmetric superconductors can become intrinsically nonriciprocal [4], by studying the nonreciprocity of the depairing critical current. We reveal the temperature scaling of the intrinsic SDE near the transition temperature as well as its significant enhancement at low temperatures. In particular, the low-temperature SDE is found to be closely related to the finite center-of-mass momentum Cooper pairs (called helical superconductivity). SDE could play an important role as a promising bulk probe of the helical superconductivity. We also discuss the relationship between the SDE and the nonreciprocity of the transition lines under supercurrent.   

Non-reciprocal superconductivity and the field free Josephson Diode

Nonreciprocal transport is incredibly important in technology; for example, asymmetry in the current-voltage response in semiconductor junctions has been the cornerstone of computing technology for half a century. The “diode” effect is a very basic demonstration of nonreciprocity. Nonreciprocal superconductivity, however, proved elusive, and only in 2020 was the superconducting diode effect (superconducting in one direction while normal conducting in the other) discovered for the first time in a bulk alloy of V/Nb/Ta. By breaking both inversion and time reversal symmetry (using an applied magnetic field), a difference in the critical superconducting current (I) for positive vs negative voltages (V) was seen. Recently, we demonstrated a Josephson diode (JD), created in a quantum material Josephson junction (QMJJ, a junction made up of two superconductors separated by a barrier comprised of a quantum material). A diodic effect was seen without an applied magnetic field; a puzzling result for theoretical physicists but an important advance for potential technological application as nanoscale field control and manipulation remains a challenge. Using an inversion symmetry breaking heterostructure of NbSe2/Nb3Br8/NbSe2, half-wave rectification of a square-wave excitation was achieved with low switching current density, high rectification ratio, and high robustness. Following that work, a plethora of nonreciprocal QMJJ and other architectures have been investigated and shown to demonstrate nonreciprocal superconductivity. Future directions for optimizations and novel explorations, as well as a broader impact of using other quantum materials will be discussed.

    

Theory of zero-field superconducting diode effect in twisted trilayer graphene

In a recent experiment (Lin et al 2022 Nat. Phys. 18, 1221–1227), the superconducting phase hosted by a heterostructure of mirror-symmetric twisted trilayer graphene and WSe2 was shown to exhibit significantly different critical currents in opposite directions in the absence of external magnetic fields. We here develop a microscopic theory and analyze necessary conditions for this zero-field superconducting diode effect. Taking into account the spin–orbit coupling induced in trilayer graphene via the proximity effect, we classify the pairing instabilities and normal-state orders and derive which combinations are consistent with the observed diode effect, in particular, its field trainability. We perform explicit calculations of the diode effect in several different models, including the full continuum model for the system, and illuminate the relation between the diode effect and finite-momentum pairing. Our theory also provides a natural explanation of the observed sign change of the current asymmetry with doping, which can be related to an approximate chiral symmetry of the system, and of the enhanced transverse resistance above the superconducting transition. Our findings not only elucidate the rich physics of trilayer graphene on WSe2, but also establish a means to distinguish between various candidate interaction-induced orders in spin-orbit-coupled graphene moiré systems, and could therefore serve as a guide for future experiments as well.