Study on Unconventional Superconductors via Angle-resolved Specific Heat

TL;DR

This study demonstrates that angle-resolved heat capacity measurements reveal the nodal structure of unconventional superconductors, confirming theoretical predictions and uncovering disorder effects, thus providing a powerful method to probe their gap symmetry.

Contribution

The paper presents the first observation of fourfold oscillations in heat capacity with magnetic field angle in unconventional superconductors, aligning with theoretical models and exploring disorder effects.

Findings

Observation of fourfold oscillations in heat capacity confirming nodal quasiparticles.

Anomalous secondary minima indicating disorder effects and eightfold patterns.

Field-angle-dependent heat capacity as a tool for probing gap structure.

Abstract

The gap function in unconventional superconductors may vanish at points or lines in momentum space, permitting electronic excitations, termed nodal quasiparticles, to exist at temperatures well below the superconducting transition. In the vortex phase, the presence of nodal quasiparticles should be directly observable through the variation of the heat capacity with the angle between a magnetic field and the location of the zeroes of the gap. The heat capacity of candidate non-magnetic unconventional superconductors Lu(Y)Ni2B2C were found to exhibit fourfold oscillations with field angle, the first such observation. The observed angular variations are in quantitative agreement with theory, confirming that quasiparticles are created via Doppler shifts at nodes along <100>. Anomalous disorder effects have been also observed in the field-angle dependent heat capacity C_{p}(\alpha). In a slightly disordered sample, anomalous secondary minima along <110> appeared for \mu_{0}H > 1 T, leading to an eightfold pattern. The coexistence of an anisotropic superconducting gap and nonlocal effects is shown to drive the anomalous behavior. These results demonstrate that field-angle-dependent heat capacity can be a powerful tool in probing the momentum-space gap structure in unconventional superconductors such as high T_{c} cuprates, heavy fermions, borocarbides, etc.