Pressure-induced charge density wave phase in Ag2−δTe

Considerable excitement was generated by the observation of large and linear positive magnetoresistance in nonmagnetic silver chalcogenides. Renewed interest in these materials was kindled by the discovery that Ag2Te in particular is a topological insulator with gapless linear Dirac-type surface states. High-pressure x-ray-diffraction studies, combined with first-principles electronic structure calculations, have identified three phase transitions as the pressure is increased: an isostructural transition identified with an electronic topological transition followed by two structural phase transitions. These recent studies were carried out on nominally stoichiometric Ag2Te. For the present work we have prepared single-phase self-doped Ag2−δTe samples with a well-characterized silver deficit (δ=2.0×10−4) for structural and electrical transport measurements over extended ranges of pressure (0–43 GPa), temperature (2–300 K), and magnetic field (0–9 T). The temperature dependence of the resistivity exhibits anomalous behavior at 2.3 GPa, slightly above the isostructural transition, which we postulate is due to Fermi surface reconstruction associated with a charge density wave (CDW) phase. The anomaly is enhanced by the application of a 9-T magnetic field and shifted to higher temperature, implying that the electronic Zeeman energy is sufficient to alter the gapping of the Fermi surface. A peak in the pressure dependence of the resistivity and a sudden drop in the pressure dependence of the mobility, occurring at 2.3 GPa, provide additional evidence for a CDW phase at pressures slightly above the isostructural transition.