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Sign reversal of field-angle resolved heat capacity oscillations in a heavy fermion superconductor CeCoIn$_5$ and $d_{x^2-y^2}$ pairing symmetry

To identify the superconducting gap symmetry in CeCoIn5 (Tc=2.3 K), we performed angle-resolved specific heat (C_ϕ) measurements in a field rotated around the c-axis down to very low temperatures 0.05Tc and detailed theoretical calculations. In a field of 1 T, a sign reversal of the fourfold angular oscillation in C_ϕhas been observed at T ~ 0.1Tc on entering a quasiclassical regime where the maximum of C_ϕcorresponds to the antinodal direction, coinciding with the angle-resolved density of states (ADOS) calculation. The C_ϕbehavior, which exhibits minima along [110] directions, unambiguously allows us to conclude d_{x^2-y^2} 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).