One target in the field of superconductivity is to understand the mechanisms which control the superconducting transition temperature Tc. Against this background, Varma pointed out the possibility of enhanced superconductivity in systems containing valence-skipping elements due to charge fluctuations [1]. One example is Indium: It skips its 2+ state and usually appears as In 1+ or In 3+ . Here we focus on In-doped GeTe and SnTe, in which In formally takes its energetically unstable 2+ state.

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... h starting compounds are semiconductors, which exhibit metallic resistivity and superconduct below ~350 mK due to unintentionally self-doped holes [2]. Doping In destroys these superconducting phases. However, upon further doping, new superconducting phases form in both Ge1-xInxTe and Sn1-xInxTe with one-order-ofmagnitude larger Tc values [3-6] leading to the question 'what is the role of the valence-skipping nature of In?'. Interestingly, both solid solutions exhibit unexpected and remarkable differences manifested in distinct phase diagrams, see Fig. 1 and Refs. [5, 6] . As for Ge1-xInxTe, x = 0.12 is a critical doping concentration where various properties change concurrently [6] including the formation of new superconductivity with monotonously enhanced Tc values [Fig. 1(a) ]. Simultaneously we observe a crossover of the In valence state from 3+ to 1+. This subtle correlation strongly suggests that the superconducting phase in Ge1-xInxTe is a direct consequence of the valence instability of In. By contrast, the superconductivity in Sn1-xInxTe is not closely correlated to additional doping-induced features. Moreover, Sn1-xInxTe exhibits a two-dome phase diagram with a very sharp suppression of the superconductivity around x = 0.58 [Fig. 1(b) and 5 ]. The latter may hint towards the existence of competing orders, such as a charge density wave, due to the anticipated In valence fluctuations [1]. In this presentation, we will comprehensively discuss Ge1-xInxTe and Sn1-xInxTe and comment on the role of valence-skipping In.