Layered 1T-TaS_2–xSe_x ($0\le x\le 2$ ) single crystals have been systematically fabricated by a chemical vapor transport technique. Surprisingly, the superconductivity is induced by the Se doping. We demonstrate the appearance of a series of electronic states: the Mott phase melts into a nearly commensurate charge-density-wave (CDW) phase, superconductivity in a wide x range develops within the nearly commensurate CDW state, and finally commensurate CDW phase reproduces for heavy Se content. The rich phase diagram provides an ideal platform to investigate the interplay between electron correlation, CDW, and superconductivity, as well as the potential applications in electronic devices.

We have performed high-resolution angle-resolved photoemission spectroscopy of layered chalcogenide $1T\mathrm{\text{−}}{\mathrm{Fe}}_x{\mathrm{Ta}}_{1-x}{\mathrm{S}}_2$ which undergoes a superconducting transition in the nearly commensurate charge-density-wave phase (melted Mott phase). We found a single electron pocket at the Brillouin-zone center in the melted Mott phase, which is created by the backfolding of bands due to the superlattice potential of charge-density-wave. This electron pocket appears in the $x$ region where the samples show superconductivity, and is destroyed by the Mott- and Anderson-gap opening. Present results suggest that the melted Mott state and the superconductivity coexist in real space, providing a new insight into the interplay between electron correlation, charge order, and superconductivity.

Conventional PbTe-based semiconductors have been leading the developments in the thermoelectrics community due to their rich capabilities in optimizing the electronic and phonon transport properties. However, growing environmental concerns related to Pb toxicity is guiding thermoelectrics research toward the lead-free route. As an alternative analog of PbTe, SnTe has recently attracted increasing research interest for thermoelectric energy conversion. The key challenges for enhancing the thermoelectric performance of SnTe to be comparable with that of p-type PbTe include decreases in the carrier concentration, energy offset between the light valence band (L band) and heavy valence band (Σ band), and lattice thermal conductivity. In this review, the recent progresses made in SnTe alloys are first outlined. Based on the insights gained into the intrinsic nonstoichiometry as well as the electronic and phonon transport properties of SnTe materials, fundamental and successful strategies focusing on confronting the challenges faced by figure-of-merit zT enhancement in SnTe are discussed and highlighted, aiming at providing a straightforward overview on the underlying physics of these effective strategies. Eventually, future challenges and outlook are summarized for boosting advancements in environment-friendly SnTe thermoelectric materials.

We have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) of layered chalcogenide 1$T$-TaS${}_{2-x}$Se${}_x$ to elucidate the electronic states especially relevant to the occurrence of superconductivity. We found a direct evidence for a Ta-5$d$-derived electron pocket associated with the superconductivity, which is fragile against a Mott-gap opening observed in the insulating ground state for S-rich samples. In particular, a strong electron-electron interaction-induced Mott gap driven by a Ta 5$d$ orbital also exists in the metallic ground state for Se-rich samples, while finite ARPES intensity near the Fermi level likely originating from a Se 4$p$ orbital survives, indicative of the orbital-selective nature of the Mott transition. Present results suggest that effective electron correlation and $p$-$d$ hybridization play a crucial role to tune the superconductivity and Mott metal-insulator transition.