Volovik effect and Fermi-liquid behavior in thes-wave superconductorCaPd2As2: As75

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 75 As 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 75 As 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 Doppler shift effect, has been clearly evidenced by the observation of the suppression of the HS peak with applied magnetic field.

Superconductivity in 122-Type Pnictides without Iron

“…The spin-lattice relaxation rate 1/T 1 varies proportionately to temperature, following the Korringa relation 1/T 1 T = constant, resembling the behavior observed in the aforementioned SrPt 2 As 2 [65]. Besides, through detailed analysis on the 1/T 1 and Knight shift K, Ding et al came to the conclusion that spin correlations are absent in this compound [67]. This is in contrast to iron-based superconductors, in which AFM fluctuations are believed to be important for the occurrence of unconventional SC.…”

“…Furthermore, all three of them are believed to be conventional s-wave superconductors from transport property analysis [17,24,67]. Extra solid proof comes from the NMR measurements for CaPd 2 As 2 [67]. The spin-lattice relaxation rate 1/T 1 varies proportionately to temperature, following the Korringa relation 1/T 1 T = constant, resembling the behavior observed in the aforementioned SrPt 2 As 2 [65].…”

“…together with the parameters obtained (such as Sommefeld coefficient and the value of ∆C/γT c ), suggests a conventional s-wave pairing states in these compounds [17,22,52,65]. More direct proofs are derived from ARPES measurements for LaRu 2 P 2 , (Sr/Ba)Ni 2 As 2 and BaNi 2 P 2 ; NMR measurements for CaPd 2 As 2 and SrPt 2 As 2 ; field-dependent thermal conductivity measurements for BaNi 2 As 2 and SrNi 2 P 2 ; and de Haas-van Alphen measurements for BaNi 2 P 2 and BaIr 2 P 2 [34,47,52,55,59,62,63,65,67]. However, the relationship between 122-IFPSs and iron-based pnictides, especially the similarities of structure property and the variation of T c with doping or pressure effects, still needs more experimental and theoretical work to clarify.…”

“…Despite the difference in crystal structure between BaPd 2 As 2 and (Sr/Ca)Pd 2 As 2 , these three compounds show similar physical properties both in the normal and superconducting states, and the parameters defined from the measurements have small discrepancy (see Table 1). Furthermore, all three of them are believed to be conventional s-wave superconductors from transport property analysis [17,24,67]. Extra solid proof comes from the NMR measurements for CaPd 2 As 2 [67].…”

“…In the superconducting state, a clear coherence peak is observed in the 1/T 1 − T curve. Below the temperature when the peak shows up, 1/T 1 decreases exponentially as lowering temperatures, indicative of BCS SC [67]. SrPd 2 Sb 2 has two different kinds of polymorphs: a low-temperature phase with a tetragonal CaBe 2 Ge 2 -type structure and a high-temperature phase with ThCr 2 Si 2 -type structure [22].…”