Evidence for two-dimensional Ising superconductivity in gated MoS ₂

Abstract

The Zeeman effect, which is usually detrimental to superconductivity, can be strongly protective when an effective Zeeman field from intrinsic spin-orbit coupling locks the spins of Cooper pairs in a direction orthogonal to an external magnetic field. We performed magnetotransport experiments with ionic-gated molybdenum disulfide transistors, in which gating prepared individual superconducting states with different carrier dopings, and measured an in-plane critical field B(c2) far beyond the Pauli paramagnetic limit, consistent with Zeeman-protected superconductivity. The gating-enhanced B(c2) is more than an order of magnitude larger than it is in the bulk superconducting phases, where the effective Zeeman field is weakened by interlayer coupling. Our study provides experimental evidence of an Ising superconductor, in which spins of the pairing electrons are strongly pinned by an effective Zeeman field.

Ising superconductivity and Majorana fermions in transition-metal dichalcogenides

“…In gated MoS 2 , the experimental data are well explained by solving the self-consistent gap equation which includes the Ising SOC fields of about 100T ( ≈ 6 meV in Zeeman energy) [19,20]. In NbSe 2 , the Ising SOC estimated is even larger at about 660T due to the stronger SOC in the hole bands [22].…”

“…[19,20,22], the Ising SOC have opposite directions in opposite valleys so that it preserves time-reversal symmetry and is compatible with superconductivity. Due to the strong pinning of electron spins to the out-of-plane directions by Ising SOC, external inplane magnetic fields are much less effective in aligning electron spins.…”

“…It was first shown recently that gated MoS 2 thin films, with conducting electrons trapped in a single layer, exhibit superconductivity at about 10K with optimal gating [16,17]. Importantly, the in-plane upper critical field H c2 of the system can be several times larger than the Pauli limit and an order of magnitude larger than the H c2 of bulk superconducting samples where inversion symmetry is restored [19,20]. In recent experiments, monolayers of NbSe 2 have been successfully fabricated, which are superconducting [21] and the in-plane H c2 exhibits strong enhancement similar to gated MoS 2 [22].…”

“…As a result, H c2 is strongly enhanced. We refer to this special type of non-centrosymmetric superconductor as Ising superconductor [19,22].…”

“…Even though the normal states of monolayer TMDs have been studied extensively in recent years [15], the experimental [16][17][18][19][20][21][22] and theoretical [23][24][25][26] studies of the superconducting monolayer TMDs have only just started. It was first shown recently that gated MoS 2 thin films, with conducting electrons trapped in a single layer, exhibit superconductivity at about 10K with optimal gating [16,17].…”