Spectroscopic signature of two superconducting gaps and their unusual field dependence in RuB₂

Recently RuB₂was shown to be a possible two-gap, type-I superconductor. Temperature dependent heat capacity measurements revealed a two-gap superconducting ground state, while magnetic field dependent magnetization measurements indicated surprizing type-I superconductivity with a very low experimental critical field (H_c) ∼120 Oe. In this paper, we report direct spectroscopic evidence of two superconducting energy gaps in RuB₂. We have measured scanning tunnelling spectra exhibiting signature of two gaps on different grains of polycrystalline RuB₂, possibly originating from multiple bands. Analysis of the temperature dependent tunnelling spectra revealed that the gaps from different bands evolve differently with temperature before disappearing simultaneously at a singleT_c. Interestingly, our experiments also reveal that the gaps in quasiparticle density of states survive up to magnetic fields much higher than the bulkH_cand they evolve smoothly with field, unlike what is expected for a type-I superconductor, indicating the existence of a 'mixed state'.