arXiv subjects

Eugene Gregoryanz

Publications and source records attributed to Eugene Gregoryanz.

At least 19 recordsLinked to original sources

arXiv2026

Observation of ΔJ=0 Rotational Excitation in Dense Hydrogens

Raman measurements performed on dense H2, D2 and H2+D2 in a wide pressure-temperature range reveal the presence of the ΔJ=0 rotational excitation. In the gas/fluid state this excitation has zero Raman shift, but in the solid, the crystal field drive s it away from the zero value e.g. 75 cm-1 at around 50 GPa and 10 K for both isotopes and their mixture. In the case of deuterium, the ΔJ=0 mode splits upon entering phase II suggesting a very complex molecular environment of the broken symmetry phase (BSP). In the fluid state and phases I and II the frequencies (energies) of the ΔJ=0 transition for H2 and D2 do not scale either as rotational (by factor of 2) nor vibrational (by square 2) modes and appear to be completely isotope independent. This independence on mass marks this transition as unique and a fundamentally different type of excitation from the commonly considered harmonic oscillator and quantum rotor.

quant-ph↗

arXiv2023

Magnetic detection under high pressures using designed silicon vacancy centers in silicon carbide

Pressure-induced magnetic phase transition is attracting interest due to its ability to detect superconducting behaviour at high pressures in diamond anvil cells. However, detection of the local sample magnetic properties is a great challenge due to the small sample chamber volume. Recently, optically detected magnetic resonance (ODMR) of nitrogen vacancy (NV) centers in diamond have been used for in-situ pressure-induced phase transition detection. However, owing to their four orientation axes and temperature-dependent zero-field-splitting, interpreting the observed ODMR spectra of NV centers remain challenging. Here, we study the optical and spin properties of implanted silicon vacancy defects in 4H-SiC, which is single-axis and temperature-independent zero-field-splitting. Using this technique, we observe the magnetic phase transition of Nd2Fe14B at about 7 GPa and map the critical temperature-pressure phase diagram of the superconductor YBa2Cu3O6.6. These results highlight the potential of silicon vacancy-based quantum sensors for in-situ magnetic detection at high pressures.

physics.app-ph↗

arXiv2021

High-Pressure Structural Evolution of Disordered Polymeric CS$_2$

Carbon disulfide, CS$_2$, is an archetypal double-bonded molecular system belonging to the rich class of group IV-group VI, AB$_2$ compounds. It is widely and since long time believed that upon compression at several GPa a polymeric chain of type (-(C=S)-S-)$_n$ named Bridgman's black polymer will form. By combining optical spectroscopy and synchrotron X-ray diffraction data with ab initio simulations, we demonstrate that the structure of the Bridgman's black polymer is remarkably different. Solid molecular CS$_2$ undergoes a pressure-induced structural transformation at around 10-11 GPa, developing a disordered polymeric system. The polymer consists of 3-fold and 4-fold coordinated carbon atoms with an average carbon coordination continuously increasing upon further compression to 40 GPa. Polymerization also gives rise to some C=C double bonds. Upon decompression, the structural changes are partially reverted, a very small amount of molecular CS$_2$ is recovered, while the sample undergoes partial chemical disproportionation. Our work uncovers the non-trivial high-pressure structural evolution in one of the simplest molecular systems exhibiting molecular as well as polymeric phases.

cond-mat.other↗

arXiv2020

The raman signal of a hindered rotor

We present a method for calculation of Raman modes of the quantum solid phase I solid hydrogen and deuterium. We use the mean-field assumption that the quantised excitations are localized on one molecule. This is done by explicit solution of the time-dependent Schroedinger equation in an angle-dependent potential, and direct calculation of the polarisation. We show that in the free-rotor limit, the H$_2$ and D$_2$ frequencies differ by a factor of 2, which evolves toward $\sqrt{2}$ as the modes acquire librational character due to stronger interactions. The ratio overshoots $\sqrt{2}$ if anharmonic terms weaken the harmonic potential. We also use density functional theory and molecular dynamics to calculate the E$_{2_g}$ optical phonon frequency and the Raman linewidths. The molecular dynamics shows that the molecules are not free rotors except at very low pressure and high temperature, and become like oscillators as phase II is approached. We fit the interaction strengths to experimental frequencies, but good agreement for intensities requires us to also include strong preferred-orientation and stimulated Raman effects between S$_0$(1) and S$_0$(0) contributions. The experimental Raman spectrum for phase II cannot be reproduced, showing that the mean-field assumption is invalid in that case.

cond-mat.mtrl-sci↗

arXiv2019

Molecular and polymeric amorphous forms in dense SO$_2$

We report here a study of reversible pressure-induced structural transformation between two amorphous forms of SO$_2$: molecular at pressures below 26 GPa and polymeric above this pressure, at temperatures of 77 - 300 K. The transformation was observed by Raman spectroscopy and x-ray diffraction in a diamond anvil cell. The same phenomenon was also observed in ab initio molecular dynamics simulations where both forward and reverse transitions were detected, allowing to analyze in detail the atomic structure of both phases. The high-pressure polymeric amorphous form was found to consist mainly of disordered polymeric chains made of 3-coordinated sulfur atoms connected via oxygen atoms, and few residual intact molecules. The simulation results are in good agreement with experimental data. Our observations suggest a possible existence of molecular liquid - polymeric liquid transition in SO$_2$ and show a case example of polyamorphism in system consisting of simple molecules with multiple bonds.

cond-mat.dis-nn↗

arXiv2018

On the Reactivity of Hydrogen-Helium and Hydrogen-Nitrogen at High Pressures

Through a series of Raman spectroscopy studies, we investigate the behaviour of hydrogen-helium and hydrogen-nitrogen mixtures at high pressure across wide ranging concentrations. We find that there is no evidence of chemical association, miscibility, nor any demixing of hydrogen and helium in the solid state up to pressures of 250 GPa at 300 K. In contrast, we observe the formation of concentration-dependent N$_2$-H$_2$ van der Waals solids, which react to form N-H bonded compounds above 50 GPa. Through this combined study, we can demonstrate that the recently claimed chemical association of H$_2$-He can be attributed to significant N$_2$ contamination and subsequent formation of N$_2$-H$_2$ compounds.

cond-mat.mtrl-sci↗

arXiv2018

Structure and metallicity of phase V of hydrogen

A new phase V of hydrogen was recently claimed in experiments above 325 GPa and 300 K. Due to the extremely small sample size at such record pressures the measurements were limited to Raman spectroscopy. The experimental data on increase of pressure shows decreasing Raman activity and darkening of the sample, which suggests band-gap closure and impending molecular dissociation, but no definite conclusions could be reached. Furthermore, the available data is insufficient to determine the structure of phase V, which remains unknown. Introducing saddle-point $ab$ $initio$ random structure searching (sp-AIRSS), we find several new structural candidates of hydrogen which could describe the observed properties of phase V. We investigate hydrogen metallisation in the proposed candidate structures, and demonstrate that smaller band gaps are associated with longer bond lengths. We conclude that phase V is a stepping stone towards metallisation.

cond-mat.mtrl-sci↗

arXiv2017

Comment on "Observation of the Wigner-Huntington transition to metallic hydrogen"

Dias and Silvera (Letters, p. 715, 2017) claim the observation of the Wigner-Huntington transition to metallic hydrogen at 495 GPa. We show that neither the claims of the record pressure or the phase transition to a metallic state are supported by any data and contradict the authors' own unconfirmed previous results.

cond-mat.mtrl-sci↗

arXiv2016

Hexagonal structure of phase III of solid hydrogen

A hexagonal structure of solid molecular hydrogen with $P6_122$ symmetry is calculated to be more stable below about 200 GPa than the monoclinic $C2/c$ structure identified previously as the best candidate for phase III. We find that the effects of nuclear quantum and thermal vibrations play a central role in the stabilization of $P6_122$. The $P6_122$ and $C2/c$ structures are very similar and their Raman and infra-red data are in good agreement with experiment. However, our calculations show that the hexagonal $P6_122$ structure provides better agreement with the available x-ray diffraction data than the $C2/c$ structure at pressures below about 200 GPa. We suggest that two phase-III-like structures may be formed at high pressures, hexagonal $P6_122$ below about 200 GPa and monoclinic $C2/c$ at higher pressures.

cond-mat.mtrl-sci↗

arXiv2015

Elastic anisotropy and Poisson's ratio of solid helium under pressure

The elastic moduli, elastic anisotropy coefficients, sound velocities and Poisson's ratio of hcp solid helium have been calculated using density functional theory in generalized gradient approximation (up to $30$ TPa), and pair+triple semi-empirical potentials (up to 100 GPa). Zero-point vibrations have been treated in the Debye approximation assuming $^4$He isotope (we exclude the quantum-crystal region at very low pressures from consideration). Both methods give a reasonable agreement with the available experimental data. Our calculations predict significant elastic anisotropy of helium ($\triangle P \approx 1.14$, $\triangle S_1 \approx 1.7$, $\triangle S_2 \approx 0.93$ at low pressures). Under terapascal pressures helium becomes more elastically isotropic. At the metallization point there is a sharp feature in the elastic modulus $C_S$, which is the stiffness with respect to the isochoric change of the $c/a$ ratio. This is connected with the previously obtained sharp minimum of the $c/a$ ratio at the metallization point. Our calculations confirm the previously measured decrease of the Poisson's ratio with increasing pressure. This is not a quantum effect, as the same sign of the pressure effect was obtained when we disregarded zero-point vibrations. At TPa pressures Poisson's ratio reaches the value of $0.31$ at the theoretical metallization point ($V_{mol}=0.228$ cm$^3$/mol, $p=17.48$ TPa) and $0.29$ at 30 TPa. For $p=0$ we predict a Poisson's ratio of $0.38$ which is in excellent agreement with the low-$p$-low-$T$ experimental data.

cond-mat.other↗

arXiv2015

Poisson's ratio in cryocrystals under pressure

We present results of lattice dynamics calculations of Poisson's ratio (PR) for solid hydrogen and rare gas solids (He, Ne, Ar, Kr and Xe) under pressure. Using two complementary approaches - the semi-empirical many-body calculations and the first-principle density-functional theory calculations we found three different types of pressure dependencies of PR. While for solid helium PR monotonically decreases with rising pressure, for Ar, Kr, and Xe it monotonically increases with pressure. For solid hydrogen and Ne the pressure dependencies of PR are non-monotonic displaying rather deep minimums. The role of the intermolecular potentials in this diversity of patterns is discussed.

cond-mat.mtrl-sci↗

arXiv2012

Bonding, structures, and band gap closure of hydrogen at high pressures

We have studied dense hydrogen and deuterium experimentally up to 320 GPa and using ab initio molecular dynamic (MD) simulations up to 370 GPa between 250 and 300 K. Raman and optical absorption spectra show significant anharmonic and quantum effects in mixed atomic and molecular dense phase IV of hydrogen. In agreement with these observations, ab initio MD simulations near 300 K show extremely large atomic motions, which include molecular rotations, hopping and even pair fluctuations suggesting that phase IV may not have a well-defined crystalline structure. The structurally diverse layers (molecular and graphene-like) are strongly coupled thus opening an indirect band gap; moreover, at 300 GPa we find fast synchronized intralayer structural fluctuations. At 370 GPa the mixed structure collapses to form a metallic molecular Cmca-4 phase, which exhibit a new interstitial valence charge bonding scheme.

cond-mat.mtrl-sci↗

arXiv2009

Graphene under hydrostatic pressure

In-situ high pressure Raman spectroscopy is used to study monolayer, bilayer and few-layer graphene samples supported on silicon in a diamond anvil cell to 3.5 GPa. The results show that monolayer graphene adheres to the silicon substrate under compressive stress. A clear trend in this behaviour as a function of graphene sample thickness is observed. We also study unsupported graphene samples in a diamond anvil cell to 8 GPa, and show that the properties of graphene under compression are intrinsically similar to graphite. Our results demonstrate the differing effects of uniaxial and biaxial strain on the electronic bandstructure.

cond-mat.mtrl-sci↗

arXiv2009

Formation of transition metal hydrides at high pressures

Silane (SiH4) is found to (partially) decompose at pressures above 50 GPa at room temperature into pure Si and H2. The released hydrogen reacts with surrounding metals in the diamond anvil cell to form metal hydrides. A formation of rhenium hydride is observed after the decomposition of silane. From the data of a previous experimental report (Eremets et al., Science 319, 1506 (2008)), the claimed high-pressure metallic and superconducting phase of silane is identified as platinum hydride, that forms after the decomposition of silane. These observations show the importance of taking into account possible chemical reactions that are often neglected in high-pressure experiments.

cond-mat.mtrl-sci↗

arXiv2005

Crystal structures of superconducting phases of S and Se

Compressed S and Se are studied by x-ray diffraction with synchrotron radiation up to 160 GPa. The S-IV phase is shown to have a body-centered monoclinic structure and to be stable between 83 and 150 GPa on pressure increase, where it transform to S-V with a primitive rhombohedral $β$-Po structure. Observation of the modulation reflections in S-IV up to 135 GPa shows that its crystal structure is incommensurately modulated, as recently reported for Se and Te. The modulated body-centered monoclinic phase of Se, Se-IV, is shown to transform to the $β$-Po phase Se-V at around 80 GPa. Se-V transforms to a body-centered cubic phase at 140 GPa in accordance with previous studies. Pressure dependence of the structural parameters of these high-pressure phases in S and Se are discussed in relation to their superconducting behaviour.

cond-mat.other↗

arXiv2002

New Methods for Investigating Superconductivity at Very High Pressures

We review recent advances in diamond anvil cell development, focusing on the details of magnetic susceptibility techniques. We have developed and implemented highly sensitive technique that allowed observations of superconductivity using magnetic susceptibility at megabar pressures, including the observation of a Tc in sulfur to above 230 GPa. The advancement of the technique should enable the most demanding diamond anvil cell applications. The Tc(P) data for simple s-p metals (Se,S, MgB_2) are presented and analyzed. The possibility for a phonon-assisted electronic topological transition (ETT) in MgB_2 at high pressure is discussed.

cond-mat.supr-con↗

arXiv2001

Raman Scattering of Metals to Very High Pressures

We review recent development in Raman techniques and diamond anvil cell technology, that allow measurements of Raman of metals in the megabar pressure range. The Raman technique includes holographic transmission optics and single-stage spectrographs with CCD array detectors. Use of synthetic high-purity diamonds and anglular excitation geometry reduces spurious background scattering substantially, which makes observable very weak Raman peaks in metals. We give several examples of studies including the hexagonal close packed (hcp) $ε$-Fe, Fe$_{(1-x)}$Ni${_x}$, and Re and the recently discovered high-temperature superconductor MgB$_2$. The shift of phonon modes with pressure is used for determination of the mode-Grüneisen parameters, estimation of shear elastic moduli and provide information on structural and electronic transitions.

cond-mat.mtrl-sci↗

arXiv2001

New Phases of Solid Nitrogen

We report the discovery of a new class of molecular phases of solid nitrogen at high pressures and temperatures by Raman and infrared spectroscopy and powder x-ray diffraction. Unlike the molecular phases consisting of disk- and sphere-like molecular disorder ($δ$-, $ε$- and $ζ$-N$_2$) and reportedly stable over a wide P-T range, one of the new phases ($ι$) is diatomic with disk-like molecules. A second new phase (the higher pressure $θ$-phase) is characterized by strong intermolecular interactions and infrared vibron absorption. Both phases exhibit wide P-T ranges of stability and metastability.

cond-mat.mtrl-sci↗