LETTER Tunneling-Spectroscopy Evidence for Two-Gap Superconductivity in a Binary Mo-Re Alloy Vladimir Tarenkov 1 & Alexander Dyachenko 2 & Vladimir Krivoruchko 1 & Andrii Shapovalov 3 & Mikhail Belogolovskii 2,4 Received: 24 August 2019 /Accepted: 8 September 2019 # Springer Science+Business Media, LLC, part of Springer Nature 2019 Abstract Expectation of fundamentally new phenomena owing to multiband superconductivity stimulates a search for novel materials with, at least, two distinct gaps in the superconducting state. Recent temperature-dependent electronic heat capacity and lower critical field measurements for molybdenum-rhenium alloys have revealed signatures of two-gap superconductivity. In this letter, we report a direct evidence of two superconducting energy gaps in a Mo 1-x Re x alloy films probed by single-electron tunneling spectroscopy. Earlier, it was found that the superconducting transition temperature T c in the alloys with very close atomic concentrations of Mo and Re is about an order of magnitude higher than T c ’ s of the constituent elements. Our observation of two distinct gaps in this system testifies in favor of the scenario that relates this enhancement to the interband interaction rather than to specific modifications of the molybdenium phonon spectrum after alloying with rhenium, i.e., a heavy mass defect. Keywords Two-gap superconductivity . Electron tunneling spectroscopy . Mo 1-x Re x alloy 1 Introduction In 1959, a two-band–two-gap model [1] generalizing the well- known BCS theory of superconductivity to the situation when the superconducting order parameter has two different magni- tudes in two distinct bands was proposed. In 2001, it became evident that the model may be directly applied to magnesium diboride, a superconducting material with an unexpectedly high critical temperature T c = 39 K. This statement was direct- ly proven by scanning tunneling spectroscopy measurements of the quasiparticle density of states [2–4]. In conventional BCS superconductors (SCs), the latter characteristic is gapped and peaked at the gap Δ value, whereas MgB 2 tunneling spectra exhibited two, instead of one, peaks indicating the presence of two distinct and weakly coupled Cooper-pair condensates. Generally, the presence of several electronic bands at the Fermi level is the attribute of most SCs, but due to sufficiently strong interband interaction, the energy gap is single although anisotropic, i.e., spreads continuously over a range of values. In MgB 2 , it was found an entirely distinct character of differ- ent bands opening on disconnected Fermi-surface parts, the Fermi sheets and linking via comparatively weak interband coupling. It is just the case of a two-band–two-gap system where, according to Ref. [1], the critical temperature T c should be higher than T c ’ s of separated bands. Even more, an in- creased number of degrees of freedom in the condensate wave function may generate totally novel phenomena concerning the response to external perturbations that are lacking in single-gap SCs. Such effects are that of relative phase and pair density fluctuations in the distinct condensates predicted by Leggett [5] or the appearance of interband phase-difference solitons [6]. Expectation of fundamentally new phenomena owing to multiband superconductivity stimulates the search for new materials clearly exhibiting the presence of, at least, two distinct gaps in the superconducting state. Nowadays, it is clear that magnesium diboride is not the only two-band–two-gap SC. Even in such simple and “trivial” metal as a bulk lead, the presence of two different gaps offset * Vladimir Tarenkov tarenko.donfti@gmail.com 1 O.O. Galkin Donetsk Institute for Physics and Engineering, National Academy of Sciences of Ukraine, Kyiv 03028, Ukraine 2 G.V. Kurdyumov Institute for Metal Physics, National Academy of Sciences of Ukraine, Kyiv 03142, Ukraine 3 V.N. Bakul Institute for Superhard Materials, National Academy of Sciences of Ukraine, Kyiv 04074, Ukraine 4 Vasyl’ Stus Donetsk National University, Vinnytsia 21021, Ukraine Journal of Superconductivity and Novel Magnetism https://doi.org/10.1007/s10948-019-05297-0