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April 2009

Volume 35, Issue 4,  pp. 249-347

Seventh International Conference on Cryocrystals and Quantum Crystals Wroclaw, Poland, July 31st–August 5th, 2008

Low Temp. Phys. 35, 249 (2009) (2 pages)

Online Publication Date: 3 June 2009

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01.30.Cc, 67.80.-s, 99.10.Np

Investigation of dynamic glass transitions and structural transformations in cryovacuum condensates of ethanol

A. Aldiyarov, M. Aryutkina, A. Drobyshev, M. Kaikanov, and V. Kurnosov

Low Temp. Phys. 35, 251 (2009) (5 pages)

Online Publication Date: 3 June 2009

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An IR spectrometric investigation of the dynamic glass transition of ethanol from the rotationally disordered crystal to the orientationally disordered crystal is carried out. The samples considered are thin films formed from the gas phase at a substrate temperature of T=16  K. The measurements are performed using the experimental apparatus which has been described in detail in our recent work. The sample thickness was d=2  µm, and the typical rate of annealing is approximately 10  K/min. The results are compared with the phase diagram of solid ethanol proposed by M.A. Ramos et al. We observe good agreement between the temperature intervals of existence of the amorphous and crystalline states. The low-temperature amorphous phase (12–70  K) is described by the present authors as amorphous solid ethanol by analogy with the amorphous solid water.
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64.70.kj, 81.30.Hd, 78.30.Ly, 81.40.Gh, 81.30.Dz

Phase transitions in clusters

R. S. Berry and B. M. Smirnov

Low Temp. Phys. 35, 256 (2009) (9 pages)

Online Publication Date: 3 June 2009

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General concepts of cluster phase transitions are reviewed as well as the cluster behavior near the melting point. Configuration excitation determines the nature of the cluster phase transitions, but a significant contribution to the entropy jump is given by thermal motion of atoms that allows one to characterize the phase transition through thermal atom motion in the Lindemann and other criteria. Phase coexistence near the melting point is a peculiarity of non-large clusters. The void concept of phase transitions with a void as an elementary configuration excitation allows one to describe the phase transition for clusters and macroscopic atomic systems. Phase transitions in metal clusters resemble those in clusters with pairwise atomic interactions, but their numerical parameters are different because of a large number of isomers and an additional electron degree of freedom. Cluster models are convenient for the analysis of macroscopic atomic systems. They allow us to understand the nature of glassy transitions and the reason for the absence of a stable infinite crystal lattice for gases at zero temperature and high pressure.
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64.70.kd, 64.60.-i, 64.70.Q-, 61.46.Bc

Photoelectron emission from solid Ne tested by impurity adsorption

Yu. A. Dmitriev

Low Temp. Phys. 35, 265 (2009) (4 pages)

Online Publication Date: 3 June 2009

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Electron emission is obtained from a solid Ne sample growing from the gas phase on a low temperature substrate. The surface of the sample is irradiated by the light of an open-source microwave discharge running in the gaseous Ne. A second gas flow of CH4 is simultaneously passed onto the substrate, avoiding the discharge zone. Free electrons ejected into a vacuum chamber during the sample growth are detected by means of the electron cyclotron resonance (ECR) technique. The electron yield is found to decrease with increasing CH4 flow. Fitting curves to the experimental data show that the surface CH4 impurities play the major role in emission quenching. A temperature effect was observed in which a 4.2  K sample was much more sensitive to CH4 doping than a 1.6  K one. Based on the experimental results, a model is proposed for the surface sites where electrons escape the solid.
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79.60.-i, 67.80.-s, 68.43.Mn, 61.80.-x, 52.80.Pi, 76.40.+b

On kinetic features of photo- or gamma-induced polymerization in p-diethynylbenzene crystals in the temperature range 4.2–300  K

D. A. Gordon and A. I. Mikhaylov

Low Temp. Phys. 35, 269 (2009) (6 pages)

Online Publication Date: 3 June 2009

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The stereoregular radical polymerization in photo- or gamma-irradiated crystals of p-diethynylbenzene (DEB) has been found to proceed in the temperature range of 4.2–300  K [J. Low Temp. Phys. 139, 675 (2005)]. We investigate the kinetics of this process. The polymerization of acetylene monomers due to the system of conjugated bonds formation results in the shift of a crystal absorption band from UV to visible. When compared with gravimetrical data on the polymer yield, it allows the direct detection of polymerization process in a crystal. The monomer radicals initiating the polymerization process, as well as propagating macroradicals, are detected by the ESR method. The rates of both radical formation and chemical reaction of polymerization are retarded, as is inherent to solid-phase processes, already at low yields. Thus we are applying a mechanism that pertains to the first stages of the processes only. It is proved that both photo- and gamma-induced polymerization have chain character in the temperature range 77–300  K but it is most probably not chained at 4.2  K and, for photo-polymerization, every act of monomer addition to the polymer needs an extra quantum of light. The kinetic chain length (the number of added monomer molecules per radical) turn out to be around of 200 at 300  K and of 20 at 77  K. It is interesting to note that polymer just formed is able to be modified—the radicals newly created in a polymer chain are able to add monomer molecules, forming, in such a way, a branched polymer. This process, of course, is unchained, too.
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82.35.Cd, 61.41.+e, 61.80.Ed

Melting and thermodynamic properties of rare gas nanocrystals

A. I. Karasevskii and V. V. Lubashenko

Low Temp. Phys. 35, 275 (2009) (7 pages)

Online Publication Date: 3 June 2009

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A self-consistent statistical method [Phys. Rev. B 66, 054302 (2002)] is used to describe the thermodynamic properties of free rare gas nanocrystals, using thin plates as examples. It is shown that the influence of size on the thermodynamic properties of nanocrystals is due to size-dependent quantization of the vibration spectrum affecting the parameters of a statistical distribution function of atomic displacements and, thus, governing the size dependence of the average values of energetic contributions to the Gibbs free energy of the system. For Xe nanocrystals, we present calculated size dependences of the Debye temperature, heat capacity, interatomic distance, melting temperature, etc.
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65.80.+n, 64.70.dj, 63.70.+h, 63.22.Kn

On the problem of observation of two-level tunneling states in supercooled liquids, glass-forming polymers, orientational glasses, and metallic glasses via configurational entropy

V. B. Kokshenev

Low Temp. Phys. 35, 282 (2009) (4 pages)

Online Publication Date: 3 June 2009

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From general perspectives, there is no conceptual gap between the structural and orientational glasses. Both kinds of glasses expose universal features during primary relaxation [Low Temp. Phys. 33, 617 (2007)]. However, in spite of much effort made to observe generic two-level systems (TLS) in polymers, organic liquids, and plastic crystals via thermodynamic measurements, no similarity unifying glass formers has been established. Re-analyzing a number of experimental studies, it is revealed that no renormalization conditions imposed on occupation numbers of structural units, relaxing to the glass state, were taken into consideration by the experimentalists. In this study, the effective-cluster approach is applied to configurational (excess liquid-over-solid) entropy measured in both supercooled liquids and crystals through the heat capacity. As the result, new relationships between the observable thermodynamic and dynamic characteristics are found for molecular liquids, polymers, and networks on the basis of data available in the literature. Thereby, new constraints of structure relaxation are shown to give strong evidence for the existence of TLS-type embryos of glassy structure in all studied glass formers.
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64.70.pm, 64.70.pj, 64.70.pe, 65.40.Ba, 65.40.gd, 65.20.Jk

Heat transfer in solid halogenated methanes: trifluoromethane

V. A. Konstantinov, V. P. Revyakin, and V. V. Sagan

Low Temp. Phys. 35, 286 (2009) (4 pages)

Online Publication Date: 3 June 2009

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The isochoric thermal conductivity of solid trifluoromethane is investigated for three samples of different densities in the interval from 75  K to the onset of melting. The isochoric thermal conductivity first decreases with increasing temperature, passes through a minimum at T~100  K, and then starts to increase slowly. The results obtained are compared with the thermal conductivities of other freons of the methane series. The correlation between the temperature dependence of isochoric thermal conductivity and the character of the rotational molecular motion is discussed.
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66.70.Lm, 64.70.dj

Heat transfer in solid methyl alcohol

O. A. Korolyuk, A. I. Krivchikov, I. V. Sharapova, and O. O. Romantsova

Low Temp. Phys. 35, 290 (2009) (4 pages)

Online Publication Date: 3 June 2009

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The thermal conductivity coefficient kappa(T) is measured under equilibrium vapor pressure for two crystalline phases of pure methanol (orientationally ordered and orientationally disordered) at temperatures from 2  K to the melting temperature Tm and also for a CH3OH+6.6% H2O glass from 2  K to the glass transition temperature Tg and in the supercooled liquid from Tg to 120  K. The dependence kappa(T) is described approximately as a sum of two contributions: kappaI(T), describing heat transport by acoustic phonons, and kappaII(T)—by localized high-frequency excitations. The temperature dependences of the thermal conductivity of primary monoatomic alcohols CH3OH, C2H5OH, and C3H7OH in the glass state are compared. Different mechanisms of phonon scattering in the crystalline phases and glass are analyzed. The kappaII(T) contribution is calculated within the Cahill–Pohl model. There is an anomaly of the thermal conductivity of the glass state near Tg (a smeared minimum on the kappa(T) curve).
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66.70.-f, 64.70.dj, 63.20.-e, 64.70.P-, 47.27.te

Evidence for high saturation of porous amorphous carbon films by noble gases

N. V. Krainyukova

Low Temp. Phys. 35, 294 (2009) (5 pages)

Online Publication Date: 3 June 2009

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Results of the electron diffraction study of Kr and Xe deposits on amorphous porous carbon substrates are reported. We have observed mixtures of crystallites typical of condensates formed at such substrates at low deposition temperatures. However, in the warming process at temperatures about three to five degrees below the sublimation point characteristic of flat substrates, the diffraction patterns demonstrate that large crystallites gradually disappear and a highly disordered matter forms. Such transformed samples are kept inside substrates several dozens degrees above the sublimation points which are typical of these substances on flat (e.g., metallic) substrates. We ascribe these features to specificity of composites formed from noble gases strongly bonded inside porous carbon matrices due to capillary filling at temperatures close to the sublimation points.
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61.43.Gt, 81.05.Rm, 64.70.Hz, 68.55.A-, 81.15.-z

Atomic bubbles in impurity-stabilized solid 4He

V. Lebedev, P. Moroshkin, and A. Weis

Low Temp. Phys. 35, 299 (2009) (4 pages)

Online Publication Date: 3 June 2009

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The optical absorption and fluorescence spectra of alkali atoms isolated in liquid and solid He matrices depend on specific macroscopic matrix properties, such as their molar volume and (anisotropic) elasticity constants, and provide thus information about the quantum matrix. We apply optical spectroscopy to investigate the properties of a recently discovered impurity-stabilized doped 4He solid that exists in equilibrium with pressurized superfluid helium close to the solidification/melting point of pure helium. The difference between the local He density around the implanted atoms obtained in the present experiment and the average density measured earlier suggests that the impurity-stabilized solid He is in fact a porous structure filled with liquid helium.
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67.80.dj, 67.80.bf, 67.25.de, 62.20.D-

Localized electrons in dense heavy noble gases

S. Nazin and V. Shikin

Low Temp. Phys. 35, 303 (2009) (4 pages)

Online Publication Date: 3 June 2009

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The paper addresses counterintuitive behavior of electrons injected into dense cryogenic media with negative scattering length a0. Instead of the expected polaronic effect (formation of density enhancement clusters) which should substantially reduce the electron mobility, an opposite picture is observed: with increasing |a0| (the trend taking place for inert gases with the growth of atomic number) and the medium density, the electrons remain practically free. An explanation of this behavior is provided based on consistent accounting for the nonlinearity of electron interaction with the gaseous medium in the gas atom number density.
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51.50.+v, 71.38.-k

The thermal conductivity jump at crystal-liquid phase transition in CHCl3, C6H6, and CCl4: the action of rotational molecular motion

I. O. Pursky, V. A. Konstantinov, and V. V. Bulakh

Low Temp. Phys. 35, 307 (2009) (4 pages)

Online Publication Date: 3 June 2009

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The thermal conductivity of liquid CHCl3, C6H6, and CCl4 is measured by a steady-state method under saturated vapor pressure in the temperature regions corresponding to pre-crystallization temperatures. The experimental results obtained are used to investigate the isobaric thermal conductivity jump DeltaLambdap at the crystal-liquid phase transition in CHCl3, C6H6, and CCl4. The contributions of the phonon-phonon and phonon-rotational interaction to the total thermal resistance in solid and liquid state are specified using a modified method of reduced coordinates. The decrease in the thermal conductivity at the crystal-liquid phase transition, DeltaLambdap, is explained by a combined effect of variations in positional distribution of molecules and in the form of rotational molecular motion.
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66.25.+g, 64.70.D-, 63.20.kg

Zero-phonon emission bands of solid hydrogen at 6–12  µm wavelength. An astrophysical phenomenon

J. Schaefer

Low Temp. Phys. 35, 311 (2009) (7 pages)

Online Publication Date: 3 June 2009

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Infrared emission bands in the wavelength range of 6–12  µm observed in the ISO-SWS mission are assigned to rotational zero-phonon bands of solid parahydrogen by using Van Kranendonk's approximate rigid-lattice method. This method is based on superposed electric quadrupole pair interactions and superposed quadrupole induced dipole moments of pairs in the hcp crystal. Accordingly, the approximate formalism uses zero-order H2 pair wave functions. Symmetry effects of the hcp crystal require preference of rotational pair transitions. The interaction potential of the pairs is confined to the electric quadrupole-quadrupole interaction. Zero-phonon emission bands of H2 pair transitions fitted to the spectrum contain at least one delocalized j=2 state initially and/or finally because of their significantly enhanced emission rates. They also yield the characteristic bandwidths which fit nicely to the widths of the observed features. The frequency positions of the seven pure parahydrogen pair transitions used, obtained from experimentally determined rotational solid hydrogen energy levels, are in perfect agreement with the observed features, whereas the three mixed ortho-parapair transitions need a presently unknown frequency correction, caused by the migration of the ortho-H2 molecules into the parahydrogen crystal prior to emission, the so-called initial excess binding energies. The astrophysical setup of the observed source is discussed in the end of the paper.
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63.20.K-, 71.70.Ch, 71.70.Jp, 78.30.Hv, 95.30.Dr, 95.30.Ft

Pathways to metallic hydrogen

I. F. Silvera and S. Deemyad

Low Temp. Phys. 35, 318 (2009) (8 pages)

Online Publication Date: 3 June 2009

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The traditional pathway that researchers have used in the goal of producing atomic metallic hydrogen is to compress samples with megabar pressures at low temperature. A number of phases have been observed in solid hydrogen and its isotopes, but all are in the insulating phase. The results of experiment and theory for this pathway are reviewed. In recent years a new pathway has become the focus of this challenge of producing metallic hydrogen, namely a path along the melting line. It has been predicted that the hydrogen melt line will have a peak and with increasing pressure the melt line may descend to zero Kelvin so that high pressure metallic hydrogen may be a quantum liquid. Even at lower pressures hydrogen may melt from a molecular solid to an atomic liquid. Earlier attempts to observe the peak in the melting line were thwarted by diffusion of hydrogen into the pressure cell components and other problems. In the second part of this paper we present a detailed description of our recent successful demonstration of a peak in the melting line of hydrogen.
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67.80.ff, 67.80.dj, 62.50.-p, 64.70.dj, 71.30.+h

Amorphous polymeric nitrogen—toward an equation of state

L. Yakub

Low Temp. Phys. 35, 326 (2009) (3 pages)

Online Publication Date: 3 June 2009

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The thermodynamic properties of amorphous polymeric nytrogen are analyzed theoretically on the basis of a free-energy expansion. An approach allowing estimation of the Helmholtz free energy of the high-pressure disordered condensed phases of nitrogen is proposed. The approach is based on expansion of the excess entropy in interatomic correlations and is tested on existing Monte Carlo simulation data. Some specific problems arising on this path and their possible solutions are discussed.
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62.50.-p, 64.30.-t, 65.60.+a, 61.43.Er, 61.43.Bn

Cryocrystal phase transitions applied as temperature standards

A. Szmyrka-Grzebyk and A. Kowal

Low Temp. Phys. 35, 329 (2009) (6 pages)

Online Publication Date: 3 June 2009

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International temperature scales define several fixed points, the temperature values of which are determined with a very high precision. Some of them, which lie in a low-temperature range, are called gaseous fixed points because the reference substances under normal conditions—in a room-temperature range—are in a gaseous state. When occurring in a solid state at low temperature these reference substances are called cryocrystals or quantum crystals by physicists. These substances play an important role in a standard thermometry. The triple point of argon, oxygen, neon, and hydrogen are primary fixed points of the International Temperature Scale of 1990 (ITS-90) [H. Preston-Thomas, Metrologia 27, 3 (1990)].1 The triple points of nitrogen, methane, and carbon dioxide are secondary fixed points of the scale [R. E. Bedford, G. Bonnier, H. Maas, and F. Pavese, Metrologia 33, 133 (1996)].2 Also, the solid-solid phase transitions in the cryocrystals are secondary fixed points of the scale. The highest-accuracy methods of measurements applied in standard thermometry can be used to determine, with a very small uncertainty, the phase transition temperatures and their reproducibility and stability. The reproducibility and stability of the transition temperature are the most important factors deciding about usefulness of these points in thermometry. Using the highest-accuracy methods, better than 1  mK, which at present are used only in metrology, can give new information about the nature and properties of the investigated substances, also of importance for physicists.
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64.70.K-, 67.80.-s, 07.20.Dt

Electron traps in solid Xe

I. V. Khyzhniy, S. A. Uyutnov, E. V. Savchenko, G. B. Gumenchuk, A. N. Ponomaryov, and V. E. Bondybey

Low Temp. Phys. 35, 335 (2009) (4 pages)

Online Publication Date: 3 June 2009

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Correlated real-time measurements of thermally stimulated luminescence and exoelectron emission from solid Xe pre-irradiated with an electron beam are performed. The study enables us to distinguish between surface and bulk traps in solid Xe and to identify a peak related to electronically induced defects. The activation energy corresponding to annihilation of these defects is estimated by the following methods: the method of different heating rates, the initial-rise method, and the curve cleaning technique with fitting of the thermally stimulated luminescence glow curve.
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71.55.Ht, 79.75.+g, 78.60.Kn, 61.80.Fe

Structure of nitrous oxide-carbon dioxide alloys

A. A. Solodovnik

Low Temp. Phys. 35, 339 (2009) (4 pages)

Online Publication Date: 3 June 2009

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The structure of N2O–CO2 solid mixtures is investigated by transmission electron diffraction over the entire range of their mutual concentrations. In the temperature range 5–65  K the experimental data indicate no phase separation in these alloys. The average structure of the solutions belongs to the Pa3 symmetry. The behavior of the concentration dependence of the lattice parameter has a nonmonotonic character, displaying a sharp step, the positions of which on the concentration scale depend on the deposition temperature. A possible explanation of this phenomenon is suggested.
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61.66.Fn, 61.50.Ah, 64.75.Nx

The influence of the disordered dipole subsystem on the thermal conductivity of solid CO at low temperatures

V. Sumarokov, A. Jezowski, and P. Stachowiak

Low Temp. Phys. 35, 343 (2009) (5 pages)

Online Publication Date: 3 June 2009

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The thermal conductivity of solid CO is investigated in the temperature range 1–20  K. The experimental temperature dependence of thermal conductivity of solid CO is described using the time-relaxation method within the Debye model. The comparison of the experimental temperature dependences of the thermal conductivity of N2 and CO shows that in the case of CO there is an additional large phonon scattering at temperatures near the maximum. Analysis of the experimental data indicates that this scattering is caused by the frozen disordered dipole subsystem, similar to a dipole glass. The scattering is described by resonant phonon scattering on tunneling states and on low-energy quasi-harmonic oscillations within the soft potential model.
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66.70.Lm, 63.50.-x, 63.20.D-