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Spectroscopic signature of two superconducting gaps and their unusual field dependence in RuB$_2$

Recently RuB$_2$ was 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 surprising type-I superconductivity with a very low experimental critical field ($H_c$) $\sim$ 120 Oe. In this paper, we report direct spectroscopic evidence of two superconducting energy gaps in RuB$_2$. We have measured scanning tunnelling spectra exhibiting signature of two gaps on different grains of polycrystalline RuB$_2$, 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 single $T_c$. Interestingly, our experiments also reveal that the gaps in quasiparticle density of states survive up to magnetic fields much higher than the bulk $H_c$ and they evolve smoothly with field, unlike what is expected for a type-I superconductor, indicating the existence of a ``mixed state".