STUDY ON UNCONVENTIONAL SUPERCONDUCTORS VIA ANGLE-RESOLVED SPECIFIC HEAT

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 nonmagnetic unconventional superconductors Lu ( Y ) Ni 2 B 2 C 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 Cp(α). In a slightly disordered sample, anomalous secondary minima along <110> appeared for μ0H>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-Tc cuprates, heavy fermions, borocarbides, etc.