High pressure transport properties of the topological insulator Bi2Se3

We report x-ray diffraction, electrical resistivity, and magnetoresistance measurements on Bi2Se3 under high pressure and low temperature conditions. Pressure induces profound changes in both the room temperature value of the electrical resistivity as well as the temperature dependence of the resistivity. Initially, pressure drives Bi2Se3 towards increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance that is positive at low fields and becomes negative at higher fields. Our results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3

High pressure study on LaFeAsO with different Tc

We report studies on pressure dependence of superconducting transition temperature (Tc) of LaFeAsO, LaFeAs(O0.5F0.5) and LaFeAs(O0.89F0.11) samples. In-situ resistance measurements under high pressure showed that the Tc of these three compounds increases with pressure initially, reaches a maximum value and then decreases with further increasing pressure, although the Tc at ambient pressure are different. The onset Tc of LaFeAsO is ~50 K at 1.5 GPa, which is the highest record in La-based oxypnicited system. The significant change in Tc induced by pressure is attributed to the orbital degeneracy and the electron density of state at the Fermi level.Comment: 13 pages, 4 figure

Elektronische Struktur und Supraleitung von divalenten Metallen unter sehr hohem Druck

A single crystal, high-pressure diamond cell has been developed for the study of superconductors under pressures to over 50 GPa. The pressure distribution in the cell (200 $\mu$m in diameter) is obtained by the ruby fluorescence technique, where the exciting laser beam can be focussed into a spot of 6 Am in diameter. A highsensitivity AC-SQUID magnetometer has been employed to detect the diamagnetic response of the very small samples at T$_{c}$. The T$_{c}$(p)-dependence of the lead-manometer has been calibrated against the ruby-pressure-scale up to pressures of 30 GPa. In spite of the well-known fcc/hcp-transition at 13 GPa lead shows a smooth T$_{c}$(p)-behaviour and thus is a very suitable manometer. Band structure calculations for the alkaline earth metals indicate an appreciable s-to-d transfer with increasing pressure. In fact, superconductivity was previously observed in the pressure induced d-transition metals Sr and Ba (however not yet in Ca). For the first time we present a quantitative investigation of T$_{c}$ as a function of p up to 50 GPa. Both elements turn out to be "good" superconductors featuring T$_{c}$'s of about 7 K. The possibility of a generalized phase diagram for the alkaline earth metals will be critically discussed. At any rate, the occurrence of such high T$_{c}$'s is rather strong evidence for a substantial d-transition metal character at high p. Investigations of very dilute $\underline{Ba}$Eu-alloys up to 45 GPareveal a strong, monotonic increase of $\Delta T_{c} = T_{c}^{Ba}-T_{c}^{BaEu}$. The strong increase of the pairbreaking behaviour in the magnetic Eu-ions may hint at a valence instability of the 4f$^{7}$-configuration towards 4f$^{6}$ under high pressure

Elektronische Struktur und Supraleitung von divalenten Metallen unter sehr hohem Druck

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