Multi-condensate lengths with degenerate excitation gaps in BaNi_2As_2 revealed by muon spin relaxation study

The recently discovered (Ba,Sr)Ni_2As_2 family provides an ideal platform for investigating the interaction between electronic nematicity and superconductivity. Here we report the muon spin relaxation (μSR) measurements on BaNi_2As_2. Transverse-field μSR experiments indicate that the temperature dependence of superfluid density is best fitted with a single-band s-wave model. On the other hand, the magnetic penetration depth λ shows magnetic field dependence, which contradicts with the single-band fully-gapped scenario. Zero-field μSR experiments indicate the absence of spontaneous magnetic field in the superconducting state, showing the preservation of time-reversal symmetry in the superconducting state. Our μSR experiments suggest that BaNi_2As_2 is a fully-gapped multiband superconductor. The superconducting gap amplitudes of each band are nearly the same while different bands exhibit different coherence lengths. The present work helps to elucidate the controversial superconducting property of this parent compound, paving the way for further research on doping the system with Sr to enhance superconductivity.