Unconventional superconductivity from magnetism in transition-metal dichalcogenides

Abstract

We investigate proximity-induced superconductivity in monolayers of transition metal dichalcogenides (TMDs) in the presence of an externally generated exchange field. A variety of superconducting order parameters is found to emerge from the interplay of magnetism and superconductivity, covering the entire spectrum of possibilities to be symmetric or antisymmetric with respect to the valley and spin degrees of freedom, as well as even or odd in frequency. More specifically, when a conventional s-wave superconductor with singlet Cooper pairs is tunnel-coupled to the TMD layer, both spin-singlet and triplet pairings between electrons from the same and opposite valleys arise due to the combined effects of intrinsic spin-orbit coupling and a magnetic-substrate-induced exchange field. As a key finding, we reveal the existence of an exotic even-frequency triplet pairing between equal-spin electrons from different valleys, which arises whenever the spin orientations in the two valleys are noncollinear. All types of superconducting order turn out to be highly tunable via straightforward manipulation of the external exchange field.

Van Der Waals Heterostructures with Spin‐Orbit Coupling

“…In this case, the magnitudes of the odd‐frequency correlations dominate over the even‐frequency amplitudes at discrete energy levels coinciding exactly with peaks in the local density of states, indicating a relationship between these odd‐frequency pair amplitudes and McMillan–Rowell oscillations as well as midgap Andreev resonances . Similar phenomena have also been predicted to arise in layered 2D systems, like vdW heterostructures, in which one of the components is superconducting . Given the growing number of vdW systems available, the high level of tunability of their properties, and the presence of a 2D surface accessible for interrogation by experiments, vdW systems are ideal candidates for studying odd‐frequency superconducting states.…”

“…[175][176][177] Similar phenomena have also been predicted to arise in layered 2D systems, like vdW heterostructures, in which one of the components is superconducting. [16,[178][179][180][181][182][183][184][185] Given the growing number of vdW systems available, the high level of tunability of their properties, and the presence of a 2D surface accessible for interrogation by experiments, vdW systems are ideal candidates for studying oddfrequency superconducting states.…”

Proximity-induced mixed odd- and even-frequency pairing in monolayerNbSe2

“…The Ising SOC lifts the spin degeneracy of fermions in momentum space, which could lead to an alternative strategy for pairing possibilities 39,40 and spin-polarized currents 41,42 in TMDs. More recently, discovering intriguing phenomena such as unconventional superconductivity 37,[43][44][45] , coexistence of SC and charge density wave [46][47][48] and Bosemetal phase 49 at the ultrathin films of TMDs is presently generating much interest. Experimental studies have demonstrated the existence of an unconventional SC unlike s-wave SC, in single layer TMDs such as NbSe 2 38,50 and MoS 2 37,39 in critical upper in-plane magnetic field several times higher than the Pauli paramagnetic limit.…”

Controlling magnetism through Ising superconductivity in magnetic van der Waals heterostructures

“…In particular, NbSe 2 develops a so-called Ising superconducting state [24,25] as a direct consequence of the interplay of spin-singlet superconductivity and Ising spin-orbit coupling [18], leading to a dramatic enhancement of the in-plane critical field [25][26][27][28][29]. As a result, NbSe 2 provides a suggestive platform to explore the novel interplay between magnetism, superconductivity, and Ising spin-orbit coupling [14,30].…”