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

We report a study of NiBi3 single crystals by means of electrical-resistivity, magnetization, and muon-spin rotation and relaxation (μSR) measurements. As a single crystal, NiBi3 adopts a needlelike shape and exhibits bulk superconductivity with Tc≈4.1 K. By applying magnetic fields parallel and perpendicular to the b axis of NiBi3, 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 μSR, reveals a fully gapped superconductivity in NiBi3, with a shared superconducting gap Δ0=2.1kBTc and magnetic penetration depths λ0 = 223 and 210 nm for Hb and Hb, respectively. The lack of spontaneous fields below Tc indicates that time-reversal symmetry is preserved in NiBi3. The absence of a fast muon-spin relaxation and/or precession in the zero-field μSR spectra definitely rules out any type of magnetic ordering in NiBi3 single crystals. Overall, our investigation suggests that NiBi3 behaves as a conventional s-type superconductor.