Influence of superconducting gap structure on the quasiparticle spectrum in the vortex state

TL;DR

This paper investigates how the structure of the superconducting gap affects quasiparticle excitations in the vortex state of layered superconductors with line nodes, using microscopic calculations and analyzing experimental implications.

Contribution

It provides a detailed analysis of the vortex state quasiparticle spectrum with rotating magnetic fields, revealing a crossover in zero-energy density of states behavior near the upper critical field.

Findings

Zero-energy density of states larger when magnetic field aligns with nodes near Hc2

Below Hc2, larger ZEDOS occurs when field is in anti-nodal direction

Analysis suggests Sr2RuO4 has vertical line nodes along a- and b-axes

Abstract

We study the vortex state of a layered superconductor with vertical line nodes on its Fermi surface when a magnetic field is applied in the ab-plane direction. We rotate the magnetic field within the plane, and analyze the change of low-energy excitation spectrum. Our analysis is based on the microscopic Bogoliubov-de Gennes equation and a convenient approximate analytical method developped by Pesch and Dahm. Both methods give a consistent result. Near the upper critical field H$_{c2}$, we observe a larger zero-energy density of states(ZEDOS) when the magnetic field is applied in the nodal direction, while much below H$_{c2}$, larger ZEDOS is observed under a field in the anti-nodal direction. We give a natural interpretation to this crossover behavior in terms of momentum distribution of low-energy quasiparticles. We examine the recent field angle variation experiments of thermal conductivity and specific heat. Comparison with our results suggest that special care should be taken to derive the position of line nodes from the experimental data. Combining the experimental data of the specific heat and our analyses, we conclude that Sr$_2$RuO$_4$ has vertical line nodes in the direction of the a-axis and the b-axis.