We investigated the pressure-dependence of electric transport and crystal structure of Ag-doped Bi 2 Se 3 . In the sample prepared by Ag-doping of Bi 2 Se 3 , the Bi atom was partially replaced by Ag, i.e., Ag 0.05 Bi 1.95 Se 3 . X-ray diffraction (XRD) patterns of Ag 0.05 Bi 1.95 Se 3 measured at 0 -30 GPa showed three different structural phases, with rhombohedral, monoclinic and tetragonal structures forming in turn as pressure increased, and structural phase transitions at 8.8 and 24 GPa. Ag 0.05 Bi 1.95 Se 3 showed no superconductivity down to 2.0 K at 0 GPa, but under pressure, superconductivity suddenly appeared at 11 GPa. The magnetic field (H) dependence of the superconducting transition temperature, T c , was measured at 11 and 20.5 GPa, in order to investigate whether the pressure-induced superconducting phase is explained by either p-wave polar model or s-wave model.
The temperature dependence of the resistivity ( ρ ) of Ag-doped Bi 2 Se 3 (Ag x Bi 2−x Se 3 ) shows insulating behavior above 35 K, but below 35 K, ρ suddenly decreases with decreasing temperature, in contrast to the metallic behavior for non-doped Bi 2 Se 3 at 1.5–300 K. This significant change in transport properties from metallic behavior clearly shows that the Ag doping of Bi 2 Se 3 can effectively tune the Fermi level downward. The Hall effect measurement shows that carrier is still electron in Ag x Bi 2−x Se 3 and the electron density changes with temperature to reasonably explain the transport properties. Furthermore, the positive gating of Ag x Bi 2−x Se 3 provides metallic behavior that is similar to that of non-doped Bi 2 Se 3 , indicating a successful upward tuning of the Fermi level.
We have studied the valence electronic structure of AgSnSe (x = 0.0, 0.1, 0.2, 0.25) and SnSe (x = 1.0) by a combined analysis of X-ray absorption spectroscopy (XAS) and X-ray photoemission spectroscopy (XPS) measurements. Both XAS and XPS reveal an increase in electron carriers in the system with x (i.e. excess Sn concentration) for 0 ≤ x ≤ 0.25. The core-level spectra (Sn 3d, Ag 3d and Se 3d) show that the charge state of Ag is almost 1+, while that of of Sn splits into Sn and Sn (providing clear evidence of valence skipping for the first time) with a concomitant splitting of Se into Se and Se states. The x dependence of the split components in Sn and Se together with the Se-K edge XAS reveals that the Se valence state may have an essential role in the transport properties of this system.
Employing high-pressure infrared spectroscopy we unveil the Weyl semimetal phase of elemental Te and its topological properties. The linear frequency dependence of the optical conductivity provides clear evidence for metallization of trigonal tellurium (Te-I) and the linear band dispersion above 3.0 GPa. This semimetallic Weyl phase can be tuned by increasing pressure further: a kink separates two linear regimes in the optical conductivity (at 3.7 GPa), a signature proposed for Type-II Weyl semimetals with tilted cones; this however reveals a different origin in trigonal tellurium. Our density-functional calculations do not reveal any significant tilting and suggest that Te-I remains in the Type-I Weyl phase, but with two valence bands in the vicinity of the Fermi level. Their interplay gives rise to the peculiar optical conductivity behavior with more than one linear regime. Pressure above 4.3 GPa stabilizes the more complex Te-II and Te-III polymorphs, which are robust metals.
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