Yogesh K. Vohra

Los Alamos National Laboratory, Los Alamos, California, United States

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Publications (334)720.97 Total impact

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    ABSTRACT: Nanocrystalline titanium (nc-Ti) metal was investigated up to 161 GPa at room temperature using a diamond anvil cell. X-ray diffraction and electrical resistance techniques were used to investigate the compressibility and structural phase stability. nc-Ti is observed to undergo three structural phase transitions at high pressures, starting with at 10 GPa and followed by at 127 GPa and at 140 GPa. The observed structural phase transitions, as well as compressibility, are consistent with previously reported values for coarse grained Ti (c-Ti). The high pressure experiments on nc-Ti samples do no show any significant variation of the transition pressure under varying non-hydrostatic conditions. This is in sharp contrast to c-Ti, where a significant decrease in the transition pressure is observed under increasing non-hydrostatic conditions. This would indicate that the decrease in grain size in nano grained titanium makes the phase transition less sensitive to shear stresses as compared to bulk or c-Ti.
    Materials Research Express. 09/2014; 1(3):035044.
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    ABSTRACT: Iridium-based 5d transition-metal oxides are attractive candidates for the study of correlated electronic states due to the interplay of enhanced crystal-field, Coulomb and spin-orbit interaction energies. At ambient pressure, these conditions promote a novel Jeff = 1/2 Mott-insulating state, characterized by a gap of the order of ~0.1 eV. We present high-pressure electrical resistivity measurements of single crystals of Sr2IrO4 and Sr3Ir2O7. While no indications of a pressure-induced metallic state up to 55 GPa were found in Sr2IrO4, a strong decrease of the gap energy and of the resistance of Sr3Ir2O7 between ambient pressure and 104 GPa confirm that this compound is in the proximity of a metal-insulator transition.
    Journal of Physics Condensed Matter 06/2014; 26(25):255603. · 2.22 Impact Factor
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    ABSTRACT: We have designed a portable pressure controller module to tune compression rates and maximum pressures attainable in a standard gas-membrane diamond anvil cell (DAC). During preliminary experiments, performed on zirconium (Zr) metal sample, pressure jumps of up to 80 GPa were systematically obtained in less than 0.2s (resulting in compression rate of few GPa/s up to more than 400 GPa/s). In-situ x-ray diffraction and electrical resistance measurements were performed simultaneously during this rapid pressure increase to provide the first time resolved data on α → ω → β structural evolution in Zr at high pressures. Direct control of compression rates and peak pressures, which can be held for prolonged time, allows for investigation of structural evolution and kinetics of structural phase transitions of materials under previously unexplored compression rate-pressure conditions that bridge traditional static and shock/dynamic experimental platforms.
    Journal of Physics Conference Series 05/2014; 500(3):032020.
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    ABSTRACT: 1,1-diamino-2,2-dinitroethylene (FOX-7) is a low sensitivity energetic material with performance comparable to commonly used secondary explosives such as RDX and HMX. At ambient pressure, FOX-7 exhibits complex polymorphism with at least three structurally distinct phases (α, β, and γ). In this study, we have investigated the high pressure-temperature stability of FOX-7 polymorphs using synchrotron mid-infrared (MIR) spectroscopy. At ambient pressure, our MIR spectra and corresponding differential scanning calorimetry (DSC) measurements confirmed the known α → β (~110 °C) and α → β (~160 °C) structural phase transitions; as well as, indicated an additional transition γ → (~210 °C), with the δ phase being stable up to ~251 °C prior to decomposition. In situ MIR spectra obtained during isobaric heating at 0.9 GPa, revealed a potential α → β transition that could occur as early as 180 °C, while β → β+δ phase transition shifted to ~300 °C with suppression of γ phase. Decomposition was observed slightly above 325 °C at 0.9 GPa.
    Journal of Physics Conference Series 05/2014; 500(5):052005.
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    ABSTRACT: Magnetic ordering temperatures in heavy rare earth metal dysprosium (Dy) have been studied using an ultrasensitive electrical transport measurement technique in a designer diamond anvil cell to a pressure of 69 GPa and a temperature of 10 K. Previous studies using magnetic susceptibility measurements at high pressures were able to track magnetic ordering temperature only till 7 GPa in the hexagonal close packed (hcp) phase of Dy. Our studies indicate that the magnetic ordering temperature shows an abrupt drop of 80 K at the hcp-Sm phase transition followed by a gradual decrease that continues till 17 GPa. This is followed by a rapid increase in the magnetic ordering temperatures in the double hcp phase and finally leveling off in the distorted face centered cubic phase of Dy. Our studies reaffirm that 4f-shell remains localized in Dy and there is no loss of magnetic moment or 4f-shell delocalization for pressures up to 69 GPa.
    High Pressure Research 01/2014; 34(2). · 0.90 Impact Factor
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    12/2013; 7(1).
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    ABSTRACT: High-pressure superconductivity in a rare-earth doped Ca0.86Pr0.14Fe2As2 single crystalline sample has been studied up to 12 GPa and temperatures down to 11 K using designer diamond anvil cell under a quasi-hydrostatic pressure medium. The electrical resistance measurements were complemented by high pressure and low temperature x-ray diffraction studies at a synchrotron source. The electrical resistance measurements show an intriguing observation of superconductivity under pressure, with Tc as high as ~51 K at 1.9 GPa, presenting the highest Tc reported in the intermetallic class of 1-2-2 iron-based superconductors. The resistive transition observed suggests a possible existence of two superconducting phases at low pressures of 0.5 GPa: one phase starting at Tc1 ~48 K, and the other starting at Tc2~16 K. The two superconducting transitions show distinct variations with increasing pressure. High pressure low temperature structural studies indicate that the superconducting phase is a collapsed tetragonal ThCr2Si2-type (122) crystal structure. Our high pressure studies indicate that high Tc state attributed to non-bulk superconductivity in rare-earth doped 1-2-2 iron-based superconductors is stable under compression over a broad pressure range.
    High Pressure Research 10/2013; 34(1). · 0.90 Impact Factor
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    ABSTRACT: Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi_{2}Se_{3} under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature T_{c} and upper critical field H_{c2} 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, T_{c} 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 T_{c} remains constant. Together with a quasilinear temperature dependence of H_{c2} that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of T_{c} points to an unconventional pressure-induced pairing state in Bi_{2}Se_{3} that is unique among the superconducting topological insulators.
    Physical Review Letters 08/2013; 111(8):087001. · 7.73 Impact Factor
  • Journal of Neurotrauma 08/2013; 30(15):A61-A61. · 4.30 Impact Factor
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    ABSTRACT: Neutron diffraction and electrical transport measurements have been carried out on the heavy rare-earth metal terbium at high pressures and low temperatures in order to elucidate the onset of ferromagnetic (FM) order as a function of pressure. The electrical resistance measurements show a change in slope as the temperature is lowered through the FM Curie temperature. The temperature of this FM transition decreases at a rate of−16.7 K/GPa up to a pressure of 3.6 GPa, at which point the onset of FM order is suppressed. The neutron diffraction measurements as a function of pressure at temperatures ranging from 90 to 290 K confirm that the change of slope in the resistance is associated with the FM ordering, since this occurs at pressures similar to those determined from the resistance results at these temperatures. A disappearance of FM ordering was observed as the pressure is increased above 3.6 GPa and is correlated with the phase transition from the ambient hexagonal close packed structure to an α-Sm-type structure at high pressures.
    High Pressure Research 08/2013; 33(3):555-562. · 0.90 Impact Factor
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    ABSTRACT: We have used X-ray diffraction to study the structural phase of CeCoIn5 in external pressure. Using high-pressure X-ray diffraction, we find that the crystalline phase is stable in the P4/mmm phase for pressures ≤51.2 GPa. From our measured equation of state, we find a bulk modulus given by B 0 = 72.8 ± 2.9 GPa and a first pressure derivative of B ′ = 5.1 ± 0.3. Measurement of the electrical resistivity of CeCoIn5 to pressures as high as 34.4 GPa shows the existence of a peak in resistivity at p ∗ = 8.2 ± 0.2 GPa.
    Physics of Condensed Matter 07/2013; 86(7). · 1.28 Impact Factor
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    ABSTRACT: Uranium sulfide belongs to a class of uranium monochalcogenides that crystallize in the rocksalt structure and exhibit ferromagnetism at low temperature. The magnetism is believed to play a role in the low-temperature rhombohedral distortion, possibly due to its large magnetic anisotropy. We have performed electrical and structural characterization along with density-functional theory calculations as functions of pressure to help understand the interplay between structure and magnetism in US. Theoretical calculations suggest that ferromagnetic order is responsible for the small distortion at ambient pressure and low temperature. Under pressure, the Curie temperature is reduced monotonically until it discontinuously disappears near a pressure-induced deformation of the crystal structure. This high-pressure distortion is identical to the one correlated with the onset of magnetic order, but with a larger change in the cell angle. Calculations imply a reduction in the electronic band energy as the driving force for the pressure-induced structure, but the loss of magnetic order associated with this distortion remains a mystery. The high-pressure electronic phase diagram may shed light on the magnetostructural free energy landscape of US.
    Physical review. B, Condensed matter 06/2013; 87(21). · 3.77 Impact Factor
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    ABSTRACT: High pressure electrical resistance and x-ray diffraction measurements have been performed on ruthenium-doped Ba(Fe0.9Ru0.1)2As2, up to pressures of 32 GPa and down to temperatures of 10 K, using designer diamond anvils under quasi-hydrostatic conditions. At 3.9 GPa, there is an evidence of pressure-induced superconductivity with Tc onset of 24 K and zero resistance at Tc zero of ~14.5 K. The superconducting transition temperature reaches maximum at ~5.5 GPa and then decreases gradually with increase in pressure before completely disappearing above 11.5 GPa. Upon increasing pressure at 200 K, an isostructural phase transition from a tetragonal (I4/mmm) phase to a collapsed tetragonal phase is observed at 14 GPa and the collapsed phase persists up to at least 30 GPa. The changes in the unit cell dimensions are highly anisotropic across the phase transition and are qualitatively similar to those observed in undoped BaFe2As2 parent.
    04/2013; 87(3).
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    ABSTRACT: We have investigated the insulator-to-metal transition in thin film vanadium dioxide as a function of pressure at ambient temperature using a designer diamond anvil cell (DAC). Four-point probe resistance measurements show a monotonic decrease over the entire pressure range studied with no significant discontinuity. High-pressure X-ray diffraction measurements observe an M1 (P21/c) phase at 0 GPa, an M2 (C2/m) phase from 0.8 GPa to 1.1 GPa, and a reentrant M1 phase from 1.1 GPa to 13.5 GPa. Crystal refinement above 1.1 GPa shows a monotonically decreasing a, b and c lattice constants and a minimum in the monoclinic angle, β, near 8.5±0.5 GPa. The atomic positions show that the first V-V nearest neighbor distance (d) decreases over the entire pressure range, the second nearest neighbor distance (s) increases until 5 GPa after which it is constant with s f 3.2 å. The next most closely spaced V-V distance (f), which corresponds to V atoms in different unit cells, is approximately constant across the entire pressure range measured.
    03/2013;
  • Sunil Karna, Yogesh Vohra
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    ABSTRACT: The growing demand of wide band semiconductors entice researcher to investigate electronic properties in diamond. The chemical vapor deposition (CVD) method has shown that various level of doping can be possible in diamond films. The purpose of this study was to investigate the growth morphology and quality of boron doped diamond film with deposition parameters. Various level of boron doped diamond films were synthesized epitaxially on synthetic (100) ib type diamond substrate using microwave plasma assisted CVD. The structural, optical and electrical characterizations were made to study effect of deposition parameters and pretreatment of substrates on surface morphology and growth quality. Raman spectra showed shape modification of the zone center optical phonon line and its downshift with the increasing boron content in the film. Additional bands were also observed in lower wavelength region below optical phonon line. Surface modification of films with increasing boron content has been observed in atomic force microscopy. High growth rate and high quality films were obtained with the addition of a few ppm of nitrogen in feed gas during deposition with little compromise on conductivity. Electrical measurement showed carriers have been transported via two different conduction mechanisms.
    03/2013;
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    ABSTRACT: 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 quasi-linear 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.
    Physical Review Letters 02/2013; 111(8). · 7.73 Impact Factor
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    S. K. Karna, Y. K. Vohra, P. Kung, S. T. Weir
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    ABSTRACT: Boron-doped single crystal diamond films were grown homoepitaxially on synthetic (100) Type Ib diamond substrates using microwave plasma assisted chemical vapor deposition. A modification in surface morphology of the film with increasing boron concentration in the plasma has been observed using atomic force microscopy. Use of nitrogen during boron doping has been found to improve the surface morphology and the growth rate of films but it lowers the electrical conductivity of the film. The Raman spectra indicated a zone center optical phonon mode along with a few additional bands at the lower wavenumber regions. The change in the peak profile of the zone center optical phonon mode and its downshift were observed with the increasing boron content in the film. However, shrinkage and upshift of Raman line was observed in the film that was grown in presence of nitrogen along with diborane in process gas.
    01/2013;
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    ABSTRACT: Nanoparticles are used in an increasing number of biomedical, industrial, and food applications, but their safety profiles in developing organisms, including the human fetus and infant, have not been evaluated. Titanium oxide (TiO(2)) nanoparticles which are commonly used in cosmetics, sunscreens, paints, and food, have been shown to induce emphysema and lung inflammation in adult mice. We hypothesized that exposure of newborn mice to TiO(2) would induce lung inflammation and inhibit lung development.C57BL/6 mice were exposed to TiO(2) (anatase; 8-10 nm) nanoparticles by intranasal instillation as a single dose on postnatal day 4 (P4) or as three doses on postnatal days 4, 7, and 10 (each dose = 1µg/g body weight). Measurements of lung function (compliance and resistance), development (morphometry), inflammation (histology; multiplex analysis of bronchoalveolar lavage fluid for cytokines; PCR array and multiplex analysis of lung homogenates for cytokines) was performed on postnatal day 14. It was observed that a single dose of TiO(2) nanoparticles led to inflammatory cell influx, and multiple doses led to increased inflammation and inhibition of lung development without significant effects on lung function. Macrophages were noted to take up the TiO(2) nanoparticles, followed by polymorphonuclear infiltrate. Multiple cytokines and matrix metalloproteinase-9 were increased in lung homogenates, and vascular endothelial growth factor (VEGF) was reduced. These results suggest that exposure of the developing lung to nanoparticles may lead to ineffective clearance by macrophages and persistent inflammation with resulting effects on lung development, and may possibly impact the risk of respiratory disorders in later life.
    AJP Lung Cellular and Molecular Physiology 12/2012; · 3.52 Impact Factor
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    ABSTRACT: The structural phase stability of 1,1-diamino-2,2-dinitroethylene (FOX-7) has been studied up to 10 GPa through isothermal compression at 100 °C and 200 °C using synchrotron mid- and far-infrared spectroscopy. During isothermal compression at 100 °C changes are observed in vibrational spectra with increase in pressure that are indicative of significant distortion to monoclinic α phase or a possible structural transformation to a high pressure α(') phase at 2.2 GPa and α(") phase at 6.1 GPa. At 200 °C, for the far- and mid-IR regimes, the similar changes were observed at 2.1 (2.0) GPa and 5.3 (5.5) GPa, respectively. The observed change is nearly isobaric, consistent with previously reported high pressure and room temperature values, up to the highest temperature of 200 °C reached in our experiments. Over the total P-T range investigated, up to ∼10 GPa and 200 °C, we observed no evidence of sample decomposition. The observed changes are partially reversible with only slight evidence of the high pressure distortion remaining upon complete decompression. Additional isobaric heating at 1.07 GPa was performed in the mid-IR regime, which clearly revealed an onset of decomposition at 360 °C. Further x-ray or neutron diffraction, which are needed to fully resolve the cause of observed changes above 2 and 5 GPa, are ongoing.
    The Journal of Chemical Physics 11/2012; 137(17):174304. · 3.12 Impact Factor
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    ABSTRACT: High-pressure electrical resistance measurements have been performed on single crystal Ba0.5Sr0.5Fe2As2 platelets to pressures of 16 GPa and temperatures down to 10 K using designer diamond anvils under quasi-hydrostatic conditions with an insulating steatite pressure medium. The resistance measurements show evidence of pressure-induced superconductivity with an onset transition temperature at ~31 K and zero resistance at ~22 K for a pressure of 3.3 GPa. The transition temperature decreases gradually with increasing in pressure before completely disappearing for pressures above 12 GPa. The present results provide experimental evidence that a solid solution of two 122-type materials, e.g., Ba1-x.SrxFe2As2 (0 < x <1), can also exhibit superconductivity under high pressure
    Journal of Physics Condensed Matter 10/2012; 24(49). · 2.22 Impact Factor

Publication Stats

3k Citations
720.97 Total Impact Points

Institutions

  • 2014
    • Los Alamos National Laboratory
      • Shock and Detonation Physics Group (DE-9)
      Los Alamos, California, United States
  • 1993–2014
    • University of Alabama at Birmingham
      • • Department of Physics
      • • Department of Materials Science and Engineering
      Birmingham, Alabama, United States
  • 1991–2014
    • Lawrence Livermore National Laboratory
      • Condensed Matter and Materials Division
      Livermore, California, United States
  • 2013
    • University of Maryland, College Park
      • Department of Physics
      College Park, MD, United States
  • 2011
    • University of California, San Diego
      • Department of Physics
      San Diego, CA, United States
  • 1985–2011
    • Cornell University
      • Department of Materials Science and Engineering
      Ithaca, New York, United States
  • 1977–2001
    • Bhabha Atomic Research Centre
      • Chemistry Division
      Mumbai, State of Maharashtra, India
  • 1982–2000
    • Universität Paderborn
      Paderborn, North Rhine-Westphalia, Germany
  • 1994–1995
    • University of Alabama
      Tuscaloosa, Alabama, United States