Direct observation of nodal quasiparticles in the non-magnetic superconductors yttrium(lutetium)nickel-boron-carbon field-angle dependent heat capacity

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 YNi₂B ₂C and LuNi₂B₂C 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 a line of zeros normal to <100>. 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, borocarbides, etc.