Temperature-induced first-order electronic topological transition in β -Ag2Se

Abstract

β-Ag2Se is a promising material for room temperature thermoelectric applications and magneto-resistive sensors. However, no attention was paid earlier to the hysteresis in the temperature dependence of resistivity [ρ(T)]. Here, we show that a broad hysteresis above 35 K is observed not only in ρ(T), but also in other electronic properties such as Hall coefficient [RH(T)], Seebeck coefficient, thermal conductivity, and ultraviolet photoelectron spectra (UPS). We also show that the hysteresis is not associated with a structural transition. The ρ(T) and RH(T) show that β-Ag2Se is semiconducting above 300 K, but metallicity is retained below 300 K. While electronic states are absent in the energy range from the Fermi level (EF) to 0.4 eV below the EF at 300 K, a distinct Fermi edge is observed in the UPS at 15 K suggesting that the β-Ag2Se undergoes an electronic topological transition from a high-temperature semiconducting state to a low-temperature metallic state. Our study reveals that a constant and moderately high thermoelectric figure of merit in the range 300–395 K is observed due to the broad semiconductor to metal transition in β-Ag2Se.