Sign reversal of field-angle resolved heat capacity oscillations in a heavy fermion superconductor CeCoIn5 and dx2-y2 pairing symmetry

To identify the superconducting gap symmetry in CeCoIn5 (Tc=2.3K), we measured the angle-resolved specific heat (Cφ) in a field rotated around the c axis down to a very low temperature, 0.05Tc, and made detailed theoretical calculations. In a field of 1 T, a sign reversal of the fourfold angular oscillation in Cφ was observed at T≃0.1Tc upon entering a quasiclassical regime where the maximum of Cφ corresponds to the antinodal direction, coinciding with the angle-resolved density of states (ADOS) calculation. The Cφ behavior, which exhibits minima along the [110] directions, unambiguously allows us to conclude dx2-y2 symmetry of this system. The ADOS-quasiclassical region is confined to a narrow T and H domain within T/Tc∼0.1 and 1.5 T (0.13Hc2).