Fully gapped superconductivity with preserved time-reversal symmetry in NiBi3 single crystals

Abstract

We report a study of ${\mathrm{NiBi}}_{3}$ single crystals by means of electrical-resistivity, magnetization, and muon-spin rotation and relaxation ($\ensuremath{\mu}\mathrm{SR}$) measurements. As a single crystal, ${\mathrm{NiBi}}_{3}$ adopts a needlelike shape and exhibits bulk superconductivity with ${T}_{c}\ensuremath{\approx}4.1$ K. By applying magnetic fields parallel and perpendicular to the $b$ axis of ${\mathrm{NiBi}}_{3}$, we establish that its lower and upper critical fields as well as the magnetic penetration depths show slightly different values, suggesting a weakly anisotropic superconductivity. In both cases, the zero-temperature upper critical fields are much smaller than the Pauli-limit value, indicating that the superconducting state is constrained by the orbital pair breaking. The temperature evolution of the superfluid density, obtained from transverse-field $\ensuremath{\mu}\mathrm{SR}$, reveals a fully gapped superconductivity in ${\mathrm{NiBi}}_{3}$, with a shared superconducting gap ${\mathrm{\ensuremath{\Delta}}}_{0}=2.1{k}_{\mathrm{B}}{T}_{c}$ and magnetic penetration depths ${\ensuremath{\lambda}}_{0}$ = 223 and 210 nm for $H\ensuremath{\parallel}b$ and $H\ensuremath{\perp}b$, respectively. The lack of spontaneous fields below ${T}_{c}$ indicates that time-reversal symmetry is preserved in ${\mathrm{NiBi}}_{3}$. The absence of a fast muon-spin relaxation and/or precession in the zero-field $\ensuremath{\mu}\mathrm{SR}$ spectra definitely rules out any type of magnetic ordering in ${\mathrm{NiBi}}_{3}$ single crystals. Overall, our investigation suggests that ${\mathrm{NiBi}}_{3}$ behaves as a conventional $s$-type superconductor.