  Field-angle dependent heat capacity of the non-magnetic borocarbide
superconductor YNi2B2C reveals a clear fourfold oscillation, the first
observation of its kind. The observed angular variations were analyzed as a
function of magnetic field angle, field intensity and temperature to provide
its origin. The quantitative agreement between experiment and theory strongly
suggests that we are directly observing nodal quasiparticles generated along
<100> by the Doppler effect. The results demonstrate that field-angle heat
capacity can be a powerful tool in probing the momentum-space gap structure in
unconventional superconductors such as high Tc cuprates, heavy fermion
superconductors, etc.
