Ubiquitous V-shape density of states in mixed state of clean limit type II superconductors; STM experiment and theory

TL;DR

This paper demonstrates both theoretically and experimentally that the low energy density of states in clean type II superconductors exhibits a universal V-shape form in the vortex state, regardless of the gap structure, with implications for interpreting STM experiments.

Contribution

The paper introduces the discovery of a universal linear V-shape in the density of states of clean superconductors in vortex states, supported by theory and STM experiments, regardless of gap anisotropy.

Findings

V-shape density of states observed in experiments

The linear term in DOS is theoretically derived

Universal behavior across different superconductor types

Abstract

It is demonstrated theoretically and experimentally that the low energy density of states $N(E)$ is described by a singular V-shape form $N(E)=N_0(H)+\alpha |E|+O(E^2)$ for all clean superconductors in a vortex state, irrespective of the underlying gap structure. The linear term $\alpha |E|$ which has been not recognized so far is obtained by exactly evaluating the vortex contribution. Based on microscopic Eilenberger theory $N(E)$ is evaluated for isotropic gap, line and point node gaps to yield a V-shape $N(E)$. STS-STM experiments on NbSe$_2$ and YNi$_2$B$_2$C give direct evidence for this. We provide arguments on significance of this finding and on relevance to other experiments.