Low-frequency Raman excitations in phase I of solid<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">H</mml:mi></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow><mml:mo>:</mml:mo></mml:math>Role of crystal fields

Goncharov, Alexander F.; Strzhemechny, M. A.; Mao, Ho‐kwang; Hemley, Russell J.

doi:10.1103/physrevb.63.064304

Cited by 22 publications

(31 citation statements)

References 25 publications

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“…These results are in accord with conclusions made by Goncharov et al in Ref. 24. The authors have measured low-frequency Raman spectra at low temperature for the pressure range up to the I-II phase transition.…”

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confidence: 82%

Molecular rotation in p-H2 and o-D2 in phase I under pressure

Freǐman

Tretyak

Goncharov

et al. 2011

Low Temperature Physics

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The orientational order parameter, rotational ground-state energy, and lattice distortion parameter (the deviation of the c / a ratio from the ideal hcp value 1.633) in hcp lattice of phase I of p-H 2 and o-D 2 are calculated using a semi-empirical approach. It is shown that the lattice distortion in these J-even species is small compared with that found in n-H 2 , and n-D 2 . The difference presumably is caused by the J-odd species.

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confidence: 82%

Molecular rotation in p-H2 and o-D2 in phase I under pressure

Freǐman

Tretyak

Goncharov

et al. 2011

Low Temperature Physics

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“…While understanding the orientational transition in ortho-para mixtures of quantum rotors in two and three dimensions is a problem of long-standing theoretical and experimental interest [13,14,15,16,17,18,19,20,21], less attention has been given at establishing the effects of ortho-para distinction. Exceptions [13,14,15,16] are the investigation of vibrons in ortho-para mixtures by Feldman et al [13] and a recent work by Goncharov et al [14], where ortho-D 2 mixed with small amounts of para-D 2 indicated that the possibility of an orientationally frustrated phase between phases I and II (phase II'). Phase II' persisted for a narrow pressure range (∼ 2GPa) for a thermally equilibrated ortho-para mixture.…”

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confidence: 99%

Theoretical Evidence for a Reentrant Phase Diagram in Ortho-Para Mixtures of SolidH2at High Pressure

2005

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We develop a multi order parameter mean-field formalism for systems of coupled quantum rotors. The scheme is developed to account for systems where ortho-para distinction is valid. We apply our formalism to solid H2 and D2. We find an anomalous reentrant orientational phase transition for both systems at thermal equilibrium. The correlation functions of the order parameter indicate short-range order at low temperatures. As temperature is increased the correlation increases along the phase boundary. We also find that even extremely small odd-J concentrations (1%) can trigger short-range orientational ordering.PACS numbers: 61.43.-j,64.70.-p,67.65.+z Quantum effects dominate the low temperature (T < 200K) phase diagram of solid molecular hydrogen in a wide range of pressures from ambient up to ∼ 100 GPa [1,2]. In this regime the coupling between molecules is smaller than the molecular rotational constant, so quantum effects are generally described by means of weakly coupled quantum-rotor models [3]. Homonuclear molecules (H 2 and D 2 ) can assume only even or odd values of the rotational quantum number J, depending on the parity of the nuclear spin. Important differences exist in the phase diagrams of even-J (para-H 2 and ortho-D 2 ), odd-J (ortho-H 2 and para-D 2 ) and all-J (HD) species. (For a summary of experimental results on the orientational ordering in H 2 , D 2 , and HD see Fig. 1b of reference 4.) At low pressure or high temperature, even-J species are found in a rotationally disordered free-rotor state (phase I). Increasing pressure causes an increase of the intermolecular coupling, and eventually leads to an orientationally ordered state (phase II). Odd-J systems on the other hand are orientationally ordered at low temperature and ambient pressure and remain ordered as pressure is increased. The stronger tendency of ortho-H 2 to order can be traced to the fact that its J = 1 lowest rotational state allows for a spherically asymmetric ground state, unlike the J = 0 ground state of even-J species. The pressure-temperature phase diagram of HD exhibits a peculiar reentrant shape [5]. Reentrance refers to phase diagrams where in some range of pressure the system reenters the disordered phase at ultra-low temperatures (see HD in Fig. 1). The zero-temperature orientationally disordered phase is characterized by an energy gap against J = 1 excitations. When this gap is sufficiently small and the temperature is finite, the thermally generated J = 1 excitations suffice to induce ordering, which is then reentrant, as also shown by mean-field the- * Present address

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“…In addition, our simulations do not account for nuclear exchange effects such as those that occur in ortho and para molecules, which are known to be important in a quantitative description of phase I and its transition to phase II [44][45][46][47] . Notwithstanding these limitations, it is clear that quantum nuclear effects and molecular rotations play a very important role in the transition between phases I and II and the transition is strongly quantum in nature even when nuclear exchange is neglected.…”

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confidence: 99%

Classical and quantum ordering of protons in cold solid hydrogen under megabar pressures

Walker

Probert

et al. 2013

J. Phys.: Condens. Matter

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A combination of state-of-the-art theoretical methods has been used to obtain an atomiclevel picture of classical and quantum ordering of protons in cold high-pressure solid hydrogen. We focus mostly on phases II and III of hydrogen, exploring the effects of quantum nuclear motion on certain features of these phases (through a number of ab initio path integral molecular dynamics (PIMD) simulations at particular points on the phase diagram).We also examine the importance of van der Waals forces in this system by performing calculations using the optB88-vdW density functional, which accounts for non-local correlations.Our calculations reveal that the transition between phases I and II is strongly quantum in nature, resulting from a competition between anisotropic inter-molecular interactions that restrict molecular rotation and thermal plus quantum fluctuations of the nuclear positions that facilitate it. The transition from phase II to III is more classical because quantum nuclear motion plays only a secondary role and the transition is determined primarily by the underlying potential energy surface. A structure of P 2 1 /c symmetry with 24 atoms in the primitive unit cell is found to be stable when anharmonic quantum nuclear vibrational motion is included at finite temperatures using the PIMD method. This structure gives a good account of the infra-red (IR) and Raman vibron frequencies of phase II. We find additional support for a C2/c structure as a strong candidate for phase III, since it remains transparent up to 300 GPa, even when quantum nuclear effects are included. Finally, we find that accounting for van der Waals forces improves the agreement between experiment and theory for the parts of the phase diagram considered, when compared to previous work which employed the widely-used Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional.arXiv:1302.0062v1 [cond-mat.mtrl-sci]

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Low-frequency Raman excitations in phase I of solidH2:Role of crystal fields

Cited by 22 publications

References 25 publications

Molecular rotation in p-H2 and o-D2 in phase I under pressure

Molecular rotation in p-H2 and o-D2 in phase I under pressure

Theoretical Evidence for a Reentrant Phase Diagram in Ortho-Para Mixtures of SolidH2at High Pressure

Classical and quantum ordering of protons in cold solid hydrogen under megabar pressures

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