Equation of state of bismuth to 222 GPa and comparison of gold and platinum pressure scales to 145 GPa

The equation-of-state (EoS) of bcc-bismuth was determined using the Pt pressure scale. Unit cell volumes of Bi, Pt, and Au were also measured simultaneously to megabar pressures by X-ray powder diffraction using a diamond anvil cell and a synchronus radiation source. The results suggest that Au pressure scale gave lower pressure than the Pt pressure scale

Double carrier transport in electron doped region in black phosphorus FET

The double carrier transport has been observed in thin film black phosphorus (BP) field effect transistor (FET) devices in highly electron doped region. BP thin films with typical thickness of 15 nm were encapsulated by hexagonal boron nitride (h-BN) thin films to avoid degradation by air exposure. Their Hall mobility has reached 5300 cm2/Vs and 5400 cm2/Vs at 4.2 K in the hole and electron doped regions, respectively. The gate voltage dependence of conductivity exhibits an anomalous shoulder structure in electron doped region. In addition, at gate voltages above the shoulder, the magnetoresistance changes to positive, and there appears an additional slow Shubnikov-de Haas oscillation. These results strongly suggest the appearance of the second carriers, which originate from the second subband with localized band edge.Comment: 16 pages, 4 figure

Anomalous Quantum Transport Properties in Semimetallic Black Phosphorus

Magnetoresistance in single crystals of black phosphorus is studied at ambient and hydrostatic pressures. In the semiconducting states at pressures below 0.71 GPa, the magnetoresistance shows periodic oscillations, which can be ascribed to the magneto-phonon resonance that is characteristic of high mobility semiconductors. In the metallic state above 1.64 GPa, the both transverse and longitudinal magnetoresistance show titanic increase with exhibiting superposed Shubnikov-de Haas oscillations. The observed small Fermi surfaces, high mobilities and light effective masses of carriers in semimetallic black phosphorus are comparable to those in the representative elemental semimetals of bismuth and graphite.Comment: 11 pages, 5 figures, to be published in J. Phys. Soc. Jp

Comparison of structural transformations and superconductivity in compressed Sulfur and Selenium

Density-functional calculations are presented for high-pressure structural phases of S and Se. The structural phase diagrams, phonon spectra, electron-phonon coupling, and superconducting properties of the isovalent elements are compared. We find that with increasing pressure, Se adopts a sequence of ever more closely packed structures (beta-Po, bcc, fcc), while S favors more open structures (beta-Po, simple cubic, bcc). These differences are shown to be attributable to differences in the S and Se core states. All the compressed phases of S and Se considered are calculated to have weak to moderate electron-phonon coupling strengths consistent with superconducting transition temperatures in the range of 1 to 20 K. Our results compare well with experimental data on the beta-Po --> bcc transition pressure in Se and on the superconducting transition temperature in beta-Po S. Further experiments are suggested to search for the other structural phases predicted at higher pressures and to test theoretical results on the electron-phonon interaction and superconducting properties

Structural and superconducting transition in selenium under high pressures

First-principles calculations are performed for electronic structures of two high pressure phases of solid selenium, $\beta$-Po and bcc. Our calculation reproduces well the pressure-induced phase transition from $\beta$-Po to bcc observed in selenium. The calculated transition pressure is 30 GPa lower than the observed one, but the calculated pressure dependence of the lattice parameters agrees fairly well with the observations in a wide range of pressure. We estimate the superconducting transition temperature $T_{\rm c}$ of both the $\beta$-Po and the bcc phases by calculating the phonon dispersion and the electron-phonon interaction on the basis of density-functional perturbation theory. The calculated $T_{\rm c}$ shows a characteristic pressure dependence, i.e. it is rather pressure independent in the $\beta$-Po phase, shows a discontinuous jump at the transition from $\beta$-Po to bcc, and then decreases rapidly with increasing pressure in the bcc phase.Comment: 8 pages, 11 figure

Pressure-induced alpha-to-omega transition in titanium metal: A systematic study of the effects of uniaxial stress

We investigated the effects of uniaxial stress on the pressure-induced alpha-to-omega transition in pure titanium (Ti) by means of angle dispersive x-ray diffraction in a diamond-anvil cell. Experiments under four different pressure environments reveal that: (1) the onset of the transition depends on the pressure medium used, going from 4.9 GPa (no pressure medium) to 10.5 GPa (argon pressure medium); (2) the a and w phases coexist over a rather large pressure range, which depends on the pressure medium employed; (3) the hysteresis and quenchability of the w phase is affected by differences in the sample pressure environment; and (4) a short term laser-heating of Ti lowers the alpha-to-omega transition pressure. Possible transition mechanisms are discussed in the light of the present results, which clearly demonstrated the influence of uniaxial stress in the alpha-to-omega transition.Comment: 16 pages, 6 figures, 1 tabl

Spiral Chain O4 Form of Dense Oxygen

Oxygen is in many ways a unique element: the only known diatomic molecular magnet and the capability of stabilization of the hitherto unexpected O8 cluster structure in its solid form at high pressure. Molecular dissociations upon compression as one of the fundamental problems were reported for other diatomic solids (e.g., H2, I2, Br2, and N2), but it remains elusive for solid oxygen, making oxygen an intractable system. We here report the theoretical prediction on the dissociation of molecular oxygen into a polymeric spiral chain O4 structure (\theta-O4) by using first-principles calypso method on crystal structure prediction. The \theta-O4 stabilizes above 2 TPa and has been observed as the third high pressure phase of sulfur (S-III). We find that the molecular O8 phase remains extremely stable in a large pressure range of 0.008 - 2 TPa, whose breakdown is driven by the pressure-induced instability of a transverse acoustic phonon mode at zone boundary, leading to the ultimate formation of \theta-O4. Remarkably, stabilization of \theta-O4 turns oxygen from a superconductor into an insulator with a wide band gap (approximately 5.9 eV) originating from the sp3-like hybridized orbitals of oxygen and the localization of valence electrons. (This is a pre-print version of the following article: Li Zhu et al, Spiral chain O4 form of dense oxygen, Proc. Natl. Acad. Sci. U.S.A. (2011), doi: 10.1073/pnas.1119375109, which has been published online at http://www.pnas.org/content/early/2011/12/27/1119375109 .)Comment: 13 apages, 3 figure

Crystal structure of solid Oxygen at high pressure and low temperature

Results of X-ray diffraction experiments on solid oxygen at low temperature and at pressures up to 10 GPa are presented.A careful sample preparation and annealing around 240 K allowed to obtain very good diffraction patterns in the orthorhombic delta-phase. This phase is stable at low temperature, in contrast to some recent data [Y. Akahama et al., Phys. Rev. B64, 054105 (2001)], and transforms with decreasing pressure into a monoclinic phase, which is identified as the low pressure alpha-phase. The discontinuous change of the lattice parameters, and the observed metastability of the alpha-phase increasing pressure suggest that the transition is of the first order.Comment: 4 pages with three figure

Thermal Stabilization of the HCP Phase in Titanium

We have used a tight-binding model that is fit to first-principles electronic-structure calculations for titanium to calculate quasi-harmonic phonons and the Gibbs free energy of the hexagonal close-packed (hcp) and omega crystal structures. We show that the true zero-temperature ground-state is the omega structure, although this has never been observed experimentally at normal pressure, and that it is the entropy from the thermal population of phonon states which stabilizes the hcp structure at room temperature. We present the first completely theoretical prediction of the temperature- and pressure-dependence of the hcp-omega phase transformation and show that it is in good agreement with experiment. The quasi-harmonic approximation fails to adequately treat the bcc phase because the zero-temperature phonons of this structure are not all stable