Investigating the superconducting state of 2$H$-NbS$_2$ as seen by the vortex lattice

TL;DR

This study uses small-angle neutron scattering to analyze the vortex lattice in 2H-NbS2, confirming anisotropic London theory and providing detailed measurements of superconducting parameters, revealing field-independent anisotropy and gap structure.

Contribution

First comprehensive validation of anisotropic London theory in 2H-NbS2 through vortex lattice analysis, with precise measurements of anisotropy and penetration depths.

Findings

Superconducting anisotropy Γ_ac ≈ 7.07, field independent

London penetration depths λ_ab ≈ 142 nm, λ_c ≈ 1 μm

Field response consistent with larger superconducting gap

Abstract

2$H$-NbS$_2$ is a classic example of an anisotropic multi-band superconductor, with significant recent work focussing on the interesting responses seen when high magnetic fields are applied precisely parallel to the hexagonal niobium planes. It is often contrasted with its sister compound 2$H$-NbSe$_2$ because they have similar onset temperatures for superconductivity, but 2$H$-NbS$_2$ has no charge density wave whereas in 2$H$-NbSe$_2$ the charge density wave order couples strongly to the superconductivity. Using small-angle neutron scattering, a bulk-sensitive probe, we have studied the vortex lattice and how it responds to the underlying superconducting anisotropy. This is done by controlling the orientation of the field with respect to the Nb planes. The superconducting anisotropy, $\Gamma_{ac} = 7.07 \pm 0.2$, is found to be field independent over the range measured (0.15 to 1.25 T), and the magnetic field distribution as a function of the applied magnetic field is found to be in excellent quantitative agreement with anisotropic London theory modified with a core-size cut-off correction, providing the first complete validation of this model. We find values of $\lambda_{ab} = 141.9 \pm 1.5 $ nm for the in-plane London penetration depth, and $\lambda_{c} \sim$ 1 \textmu m for the out-of-plane response. The field-independence indicates that we are primarily sampling the larger of the two gaps generating the superconductivity in this material.