Valence fluctuation driven superconductivity in orthorhombic lead telluride

Chemical doping or elemental substitution and applying pressure are considered to be two effective tools to create superconductivity from a parent compound. The latter is more powerful than the former for contracting the lattice but does not bring about impurities, defects, or disorders compared with the former, thus serving as a good platform to examine or test any theoretical models for the born superconductivity. Among the various theories proposed for superconductivity, the electron-phonon interactions (or phonon mediated), spin fluctuations (or spin mediated), and valence fluctuations (or valence mediated) are the three major mechanisms. The first two have received extensive support from many elements and compounds over the last century, and only a few examples are believed to be a result of valence fluctuations. Lead telluride (PbTe), a Group IV-VI compound, is being examined as a promising candidate whose superconductivity is mediated by the valence fluctuations. The superconductivity has been reported in doped PbTe with the rocksalt NaCl structure at ambient pressure or parent PbTe with the body-centered cubic CsCl structure at high pressure. The pressure-driven intermediate orthorhombic phase in between has been known as a semiconductor with a large band gap and no expectation for the appearance of superconductivity. By doping I in the Te site to adding more carriers and partially substituting Pb by In with the expected valence change, however, we show that the co-doped PbTe exhibits superconductivity upon compression in the orthorhombic phase besides the expected one with the CsCl structure. Through the systematic studies of the structural, vibrational, electrical, and superconducting properties, we suggest that the observed superconductivity in the orthorhombic phase is mediated by the valence fluctuations associated with mixed valency on the In sites. This work thus not only offers an example for valence-mediated superconductivity but also fills the gap for the superconducting phase of PbTe in between the NaCl and CsCl structure.