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Observation of a pronounced Hebel-Slichter peak in the spin-lattice relaxation rate and implications for gap and pairing symmetry in LaNiGa$_2$

P. Sherpa, R. Hingorani, A. Menon, I. Vinograd, C. Chaffey, A. P. Dioguardi, R. Yamamoto, M. Hirata, F. Ronning, J. R. Badger, P. Klavins, R. R. P. Singh, V. Taufour, N. J. Curro

Abstract

We report a pronounced Hebel-Slichter coherence peak in the zero field nuclear quadrupolar resonance (NQR) spin-lattice relaxation rate of the topological crystalline superconductor LaNiGa$_2$ in the superconducting state. Previously, a two-band internally antisymmetric non-unitary triplet pairing (INT) state was proposed for this system, with equal spin-pairing and two distinct gaps associated with different spins. A detailed examination of the temperature dependence of the NQR data shows that the data best fit an INT model if the two gaps are equal and the model is unitary. Even a tiny non-unitarity with two unequal gaps causes the coherence peak to diminish rapidly and deviate from the data. On the other hand, the data are well-fit by a two-band singlet BCS-like pairing with two distinct gaps consistent with previous measurements. This raises doubts on the identification of non-unitary triplet-pairing with time-reversal symmetry breaking in this material.

Observation of a pronounced Hebel-Slichter peak in the spin-lattice relaxation rate and implications for gap and pairing symmetry in LaNiGa$_2$

Abstract

We report a pronounced Hebel-Slichter coherence peak in the zero field nuclear quadrupolar resonance (NQR) spin-lattice relaxation rate of the topological crystalline superconductor LaNiGa in the superconducting state. Previously, a two-band internally antisymmetric non-unitary triplet pairing (INT) state was proposed for this system, with equal spin-pairing and two distinct gaps associated with different spins. A detailed examination of the temperature dependence of the NQR data shows that the data best fit an INT model if the two gaps are equal and the model is unitary. Even a tiny non-unitarity with two unequal gaps causes the coherence peak to diminish rapidly and deviate from the data. On the other hand, the data are well-fit by a two-band singlet BCS-like pairing with two distinct gaps consistent with previous measurements. This raises doubts on the identification of non-unitary triplet-pairing with time-reversal symmetry breaking in this material.

Paper Structure

This paper contains 6 equations, 3 figures, 2 tables.

Figures (3)

  • Figure 1: $^{139}$La $T_1^{-1}$ versus inverse temperature in the superconducting state of LaNiGa$_2$ (with $T_c = 1.83$ K). The solid and dashed lines are calculations as discussed in the text, and the gap values, ($\Delta_1$, $\Delta_2$) and nuclear resonance frequency, $\omega_N$, are given in units of $k_BT_c$. The data are well fit by a two-gap singlet-pairing model as well as by an INT triplet-pairing model with two equal gaps, but not with the INT model with two different gaps. The inset shows the unit cell of LaNiGa$_2$.
  • Figure 2: $^{139}$La $T_1^{-1}$ versus inverse temperature in the superconducting state. The solid and dashed curves are calculated values as discussed in the text, and the gap values are given in units of $k_BT_c$.
  • Figure 3: Normalized $(T_1T)^{-1}$ versus reduced temperature for several different BCS superconductors, including: LaNiGa$_2$, PrRu$_4$As$_{12}$Shimizu2007, CaPd$_2$As$_2$Ding2016, SrOs$_4$As$_{12}$Ding2019, PtSb Yamada2025, LaFe$_4$P$_{12}$IshidaLaFe4P12NMR, MgB$_2$MgB2NMR, CaSb$_2$Takahashi2021, and Al masudaredfield.