Origin of the Unusual Temperature Dependence of the Upper Critical Field of Kagome Superconductor CsV3Sb5: Multiple Bands or van Hove Singularities?

TL;DR

This study investigates the unusual temperature dependence of the upper critical field in CsV3Sb5, attributing it mainly to Fermi velocity anisotropy caused by van Hove singularities, rather than multi-band effects.

Contribution

The paper introduces a single-band model that explains the anomalous H_c2(T) behavior through vHs-induced Fermi velocity anisotropy, supported by experimental irradiation data.

Findings

VHs cause upward curvature in H_c2(T)

Proton irradiation reduces anisotropy and normalizes H_c2(T)

Model aligns with experimental magneto-transport data

Abstract

Van Hove singularities (vHs) located close to the Fermi level in Kagome superconductors AV3Sb5 (A = K, Rb, Cs) have profound influence on their electronic and transport characteristics. Specifically, magneto-transport and susceptibility measurements on CsV3Sb5 reveal an anomalous temperature dependence of the upper critical field H_c2 (T), characterized by a pronounced upward curvature for both in-plane and c-axis magnetic fields, with zero-temperature H_c2 values of ~6.0 T and ~1.2 T, respectively. Our theoretical analysis, using a newly developed single-band model incorporating vHs and gap anisotropy, suggests that the observed upper critical field behavior is predominantly driven by the anisotropy of the Fermi velocity originating from vHs, instead of multi-band effects or gap anisotropy. Increased electron scattering introduced by proton irradiation defects smears out the vHs, reduces anisotropy, and recovers the conventional H_c2 (T) behavior, corroborating our proposed model.