Charge-density-wave phase and magnetoresitance in Ag_2-δTe

Considerable excitement was generated by the observation of large and linear positive magnetoresistance (MR) in non-magnetic 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 Ag_2-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 9T 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.