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  Ding, Q.; Wiecki, P.; Anand, V.; Sangeetha, N.; Lee, Y.; Johnston, D.; Furukawa, Y.: Volovik effect and Fermi-liquid behavior in the s-wave superconductor CaPd₂As₂: ⁷⁵As NMR-NQR measurements. Physical Review B 93 (2016), p. 140502/1-5

10.1103/PhysRevB.93.140502
Open Access Version

Abstract:
The electronic and magnetic properties of the collapsed-tetragonal CaPd2As2 superconductor (SC) with a transition temperature of 1.27 K have been investigated by 75As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements. The temperature (T) dependence of the nuclear spin lattice relaxation rates (1/T1) and the Knight shifts indicate the absence of magnetic correlations in the normal state. In the SC state, 1/T1 measured by 75As NQR shows a clear Hebel-Slichter (HS) peak just below Tc and decreases exponentially at lower T, confirming a conventional s-wave SC. In addition, the Volovik effect, also known as the Doppler shift effect, has been clearly evidenced by the observation of the suppression of the HS peak with applied magnetic field.