Competition between orbital effects, Pauli limiting, and Fulde-Ferrell-Larkin-Ovchinnikov states in 2D transition metal dichalcogenide superconductors

TL;DR

This study investigates how orbital effects, Pauli limiting, and FFLO states influence the upper critical field in 2D transition metal dichalcogenide superconductors NbSe2 and NbS2, revealing different behaviors related to their dimensionality.

Contribution

It provides a comparative analysis of the upper critical fields in NbSe2 and NbS2, highlighting the role of orbital effects and the emergence of FFLO states in these layered superconductors.

Findings

NbS2 exceeds Pauli limit and forms FFLO state

Orbital effects suppress superconductivity in NbSe2 below Pauli limit

NbSe2 has larger out-of-plane coherence length than interlayer distance

Abstract

We compare the upper critical field of bulk single-crystalline samples of the two intrinsic transition metal dichalcogenide (TMD) superconductors, 2H-NbSe2 and 2H-NbS2, in high magnetic fields where their layer structure is aligned strictly parallel and perpendicular to the field, using magnetic torque experiments and a high-precision piezo-rotary positioner. While both superconductors show that orbital effects still have a significant impact when the layer structure is aligned parallel to the field, the upper critical field of NbS2 rises above the Pauli limiting field and forms a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, while orbital effects suppress superconductivity in NbSe2 just below the Pauli limit. From the out-of-plane anisotropies, the coherence length perpendicular to the layers of 31 {\AA} in NbSe2 is much larger than the interlayer distance, leading to a significant orbital effect suppressing superconductivity before the Pauli limit is reached, in contrast to the more 2D NbS2.