Specific Heat of YBa$_2$Cu$_3$O$_{7 - {\rm \delta}}$ Single Crystals: Implications for the Vortex Structure

TL;DR

This study investigates the anisotropic specific heat of YBa₂Cu₃O₇₋δ single crystals under magnetic fields, revealing quasiparticle excitations throughout vortices and supporting the presence of gap nodes, with implications for vortex structure understanding.

Contribution

It provides experimental evidence of field-dependent quasiparticle excitations consistent with gap nodes, advancing understanding of vortex structure in high-temperature superconductors.

Findings

Specific heat includes a linear-T term proportional to √H.

Quasiparticle excitations exist throughout the vortex, not just in the core.

Field dependence supports the presence of gap nodes in the superconductor.

Abstract

The anisotropy of the magnetic field dependence of the specific heat of YBa$_2$Cu$_3$O$_{7 - {\rm \delta}}$ can be used to identify different low-energy excitations, which include phonons, spin-$ \frac{1}{2}$ particles, and electronic contributions. With a magnetic field H applied perpendicular to the copper oxide planes, we find that the specific heat includes a linear-T term proportional to $\sqrt{H}$. The nonlinear field dependence of the density of states at the Fermi level suggests that there are quasiparticle excitations throughout the entire vortex, not just in the vortex core. The $\sqrt{H} T$ term agrees quantitatively with G. Volovik's prediction for a superconductor with lines of nodes in the gap. A similar, but much smaller, effect is predicted for fields parallel to the planes, and sensitive measurements of the in-plane anisotropic magnetic field dependence of the specific heat could be used to map out the nodes.