Study on Unconventional Superconductors via Angle-resolved Specific Heat.pdf

a r X i v : c o n d - m a t / 0 4 1 1 0 0 1 v 1 [ c o n d - m a t .s u p r - c o n ] 2 9 O c t 2 0 0 4 Study on Unconventional Superconductors via Angle-resolved Specific Heat Tuson Park Los Alamos National Laboratory, Los Alamos, New Mexico 87545 M. B. Salamon Department of Physics and Materials Research Laboratory, University of Illinois at Urbana–Champaign, Urbana, Illinois 61801 (Dated: February 2, 2008) 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 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. I. INTRODUCTION Most superconductors behave conventionally; elec- tronic excitations are suppressed by the BCS gap causing the electronic heat capacity, for example, to be exponen- tially small at temperatures well below t