Heat capacity of the spinel superconductors CuRh2S4 and CuRh2Se4

We report low-temperature heat capacity measurements on CuRh2S4 and CuRh2Se4. These compounds undergo a superconducting transition at T c = 4.72 and 3.38 K, respectively, with a fully opened superconducting gap. With increasing magnetic field H , the Sommerfeld coefficient γ ( H ) shows a crossover from linear to sublinear H dependence that is consistent with the predictions of recent theoretical calculations. Compared to CuRh2Se4, CuRh2S4 exhibits a smaller region of the linear- H dependence in γ ( H ) and pronounced positive curvature in the temperature dependence of the upper critical field.