Yogesh K. Vohra

Miniaturization techniques for obtaining static pressures comparable to the pressure at the center of the earth: X‐ray diffraction at 416 GPa

X‐ray diffraction studies on tungsten and molybdenum were performed, using a 4‐μm‐diam x‐ray beam, to very high pressures, with the pressures being obtained from the measured lattice parameters and isothermal equations of state of tungsten and molybdenum deduced from shock data. The bcc structure persists to the highest pressure, 378 GPa in tungsten, and 416 GPa in molybdenum. The static pressures generated and measured here exceed the pressure of 361 GPa at the center of the earth, the first time that this has been achieved and measured with a calibrated pressure scale. The details of the pressure profile at 335 GPa are shown and are of great use in designing anvils for future research. It is noted that the maximum pressures attained by x‐ray diffraction with beveled anvils varies linearly with D−1/2 where D is the diameter of the flat suggesting that, perhaps, even higher pressures are possible with further miniaturization. A scaling law is used to calculate the minimum correction due to the presence of...

Pressure-induced unconventional superconducting phase in the topological insulator Bi2Se3.

Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi2Se3 under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature Tc and upper critical field Hc2 that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, Tc remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where Tc remains constant. Together with a quasilinear temperature dependence of Hc2 that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc points to an unconventional pressure-induced pairing state in Bi2Se3 that is unique among the superconducting topological insulators.

The closing diamond anvil optical window in multimegabar research

The tetragonal distortion of a diamond anvil supporting a sample pressure of over 4 Mbars is such that the cubic crystal becomes elastically distorted to a tetragonal crystal with c/a ■0.69. These large distortions in the anvil greatly change its optical properties. The decrease of the band gap of diamond with pressure is described in terms of a dielectric model and in terms of experimental data to 4.05 Mbars. It is shown how this band gap decrease makes it impossible to excite ruby fluorescence using argon or He‐Cd lasers above about 250 GPa or so (depending on the wavelength). The radiation cannot get through the diamond anvil to the ruby. There is a very strong stress‐induced luminescence in Type Ia diamond in the red at pressures above 2 Mbars and in infrared above 2.5 Mbar. This latter fluorescence, if assumed to be due to ruby R1 fluorescence (no ruby is present) suggests that the pressure is 5.6 Mbars.

Microcollimated energy-dispersive x-ray diffraction apparatus for studies at megabar pressures with a synchrotron source

A new high-pressure energy-dispersive x-ray diffraction apparatus is described. Two major features distinguish this system from previous ones: the ability to collect diffraction data from small sample areas subjected to pressures over 1 Mbar and to scan across the sample. This system also has the ability to calibrate the diffraction angle without making any assumptions about the lattice spacings of the sample.

Crystal Structures at Megabar Pressures Determined by Use of the Cornell Synchrotron Source

X-ray diffraction studies have been carried out on alkali halide samples 10 micrometers in diameter (volume 10-9 cubic centimeter) subjected to megabar pressures in the diamond anvil cell. Energy-dispersive techniques and a synchrotron source were used. These measurements can be used to detect crystallographic phase transitions. Cesium iodide was subjected to pressures of 95 gigapascals (fractional volume of 46 percent) and rubidium iodide to pressures of 89 gigapascals (fractional volume of 39 percent). Cesium iodide showed a transformation from the cubic B2 phase (cesium chloride structure) to a tetragonal phase and then to an orthorhombic phase, which was stable to 95 gigapascals. Rubidium iodide showed only a transition from the low-pressure cubic B1 phase (sodium chloride structure) to the B2 phase, which was stable up to 89 gigapascals.

Optical properties of diamond at pressures of the center of Earth

The optical properties of type Ia natural diamond were investigated to 364±9 GPa in a diamond anvil cell. The pressures were measured by x‐ray diffraction on tungsten sample using a synchrotron x‐ray source. The secondary absorption edge of diamond decreased from its ambient value of 3.7 to 2.5 eV at 364 GPa. The visible diamond luminescence at pressures above 300 GPa is different from the characteristic red luminescence of type Ib synthetic diamond. These are the first optical and structural observations at calibrated static pressures exceeding the pressure at the center of Earth.

OPTICAL REFLECTIVITY AND AMORPHIZATION OF GAAS DURING DECOMPRESSION FROM MEGABAR PRESSURES

Polycrystalline GaAs was studied in a diamond anvil cell by optical reflection spectroscopy and energy‐dispersive x‐ray diffraction to pressures of 115 GPa (1.15 Mbar). Complete amorphization was observed at ambient conditions after decompression from 115 GPa, and, subsequent compression caused crystallization around 27 GPa to an orthorhombic phase. The results are compared with other group IV and III‐V semiconductor materials and implantation‐amorphized GaAs.

A new ultra-high pressure phase in samarium

Abstract Structural changes in samarium under pressure were studied in a diamond-anvil cell (DAC) to 189 GPa. A number of phase transformations were observed between room pressure and 75GPa. At about 91 GPa samarium transforms to a body-centered tetragonal structure, and the unit cell parameters at 189 GPa are a=2.402(4) A and c=4.231(17) A , V=24.40(12) A 3 and Z=2. Identification of a body-centered tetragonal phase in Sm at about 90 GPa, which is similar to that reported in Ce, suggests that this phase could appear in other rare-earth elements too.

Pressure profiles at multimegabar pressures in a diamond anvil cell using x‐ray diffraction

Pressure distributions in a diamond anvil cell with a rhenium gasket have been measured at various pressures up to 212±6 GPa using energy dispersive x‐ray diffraction with a synchrotron source. Three sets of type IA yellow diamonds were used with bevels of 5°, 7°, and 10°. For the 7°‐beveled tips, a 5‐μm‐diam collimated beam was used to a pressure of 206±6 GPa. In the other experiments, collimators of 10–30 μm were used. In the region of the 50‐μm central flat, the pressure was essentially uniform. The effect of finite collimator size on the measurement of pressure profiles is also analyzed.

The effect of nonhydrostaticity on measuring the pressure in metals by energy dispersive X-ray diffraction in the diamond anvil cell

Abstract Using the Chua-Ruoff scaling relation for the pressure dependence of the compressive flow stress, the effect of the presence of a finite yield strength on the measurement of pressure in a diamond anvil cell by energy dispersive X-ray diffraction is calculated. In experiments in which the pressure was found by X-ray diffraction to be 364 GPa (exceeding the pressure at the center of the earth) without this correction, it is calculated that the actual pressure lies in the range 378-395 GPa with the correction for nonhydrostaticity being 5-9%.