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Conventional $s$-wave Superconductivity in LaRh$_2$As$_2$; the Analog without the 4$f$ Electrons of CeRh$_2$As$_2$

Shiki Ogata, Shunsaku Kitagawa, Kenji Ishida, Manuel Brando, Elena Hassinger, Christoph Geibel, Seunghyun Khim

Abstract

Superconductor LaRh$_2$As$_2$ has the same crystal structures as CeRh$_2$As$_2$, which exhibits superconducting (SC) multiphase in the $c$-axis magnetic field. Although the SC transition temperatures $T_c$ are similar, around 0.3 K, LaRh$_2$As$_2$ shows conventional type-II superconductivity with a small upper critical field $H_{c2}\sim$ 10 mT. At present, the SC properties of LaRh$_2$As$_2$ have not been clarified yet. We performed $^{75}$As-nuclear quadrupole resonance (NQR) measurements on LaRh$_2$As$_2$ to investigate the SC properties and gap structure. $1/T_1$ shows a clear coherence peak just below $T_c$ and an exponential decrease at lower temperatures, suggesting full-gap $s$-wave superconductivity. The numerical calculations based on an $s$-wave SC model reveal an SC gap size of $Δ(0)/k_{B}T_{c} \sim 1.48$, consistent with the weak-coupling $s$-wave superconductivity. These results suggest that the 4$f$ electrons in CeRh$_2$As$_2$ not only enhance the orbital limiting field but also contribute to the formation of unconventional superconductivity with SC multiphase.

Conventional $s$-wave Superconductivity in LaRh$_2$As$_2$; the Analog without the 4$f$ Electrons of CeRh$_2$As$_2$

Abstract

Superconductor LaRhAs has the same crystal structures as CeRhAs, which exhibits superconducting (SC) multiphase in the -axis magnetic field. Although the SC transition temperatures are similar, around 0.3 K, LaRhAs shows conventional type-II superconductivity with a small upper critical field 10 mT. At present, the SC properties of LaRhAs have not been clarified yet. We performed As-nuclear quadrupole resonance (NQR) measurements on LaRhAs to investigate the SC properties and gap structure. shows a clear coherence peak just below and an exponential decrease at lower temperatures, suggesting full-gap -wave superconductivity. The numerical calculations based on an -wave SC model reveal an SC gap size of , consistent with the weak-coupling -wave superconductivity. These results suggest that the 4 electrons in CeRhAs not only enhance the orbital limiting field but also contribute to the formation of unconventional superconductivity with SC multiphase.

Paper Structure

This paper contains 2 equations, 4 figures.

Figures (4)

  • Figure 1: (Color online) (a) The NMR spectrum of LaRh$_2$As$_2$ at 4.2 K in $H\parallel c$. (b) The field angular dependence of the central peak of the As(1) site at 3.84 T. (c) Temperature dependence of ac magnetic susceptibility $\chi_{\mathrm{AC}}$ measured at the frequency of 10 MHz under zero field. The black arrows indicate the two-step superconducting transition temperatures.
  • Figure 2: (Color online) (a) Temperature evolution of the NQR spectra of the As(2) site. (b) The NQR frequency distribution $(\nu - \nu_{\mathrm{Q}})/\nu_{\mathrm{Q}}$ at the As(2) site of LaRh$_2$As$_2$ and CeRh$_2$As$_2$Kibune.
  • Figure 3: (Color online) Temperature dependence of 1/$T_1$ measured at the peak frequencies of the As(2) site. The blue and green circles represent the results of the $c$-axis field NMR Kibune_normal and zero-field NQR measurements, respectively. The light blue triangles represent the NQR 1/$T_1$ of the As(2) site of CeRh$_2$As$_2$ as references Kibune. The arrows indicate $T_{\mathrm{c}}$s and $T_{\mathrm{0}}$.
  • Figure 4: (Color online) The 1/$T_{1}T$ at the As(2) site plotted against $T$. The calculation using BCS theory is also shown in the red solid curve [Eq. (2)]. The black arrows indicate the SC transition temperatures. The inset shows Arrhenius plot of $T_1$ against $T_{\mathrm{c}}$ /$T$ ($T_{\mathrm{c}}$ = 0.31 K).