Index of content:
Volume 37, Issue 7, July 2011
37(2011); http://dx.doi.org/10.1063/1.3626842View Description Hide Description
The temperature dependencies of drag coefficient for quartztuning forks of various geometric dimensions, immersed in the He II, were determined experimentally in the temperature range 0.1–3 K. It is identified, that these dependencies are similar, but the values of drag coefficient are different for tuning forks with different geometric dimensions. It is shown, that the obtained specific drag coefficient depends only on the temperature and frequency of vibrations, when the value of drag coefficient is normalized to the surface area of moving tuning-fork prong. The temperature dependencies of normalized drag coefficient for the tuning forks of various dimensions, wire, and microsphere, oscillating in the Не II, are compared. It is shown, that in the ballistic regime of scattering of quasiparticles, these dependencies are identical and have a slope proportional to T 4, which is determined by the density of thermal excitations. In the hydrodynamic regime at T > 0.5 K, the behavior of the temperature dependence of specific drag coefficient is affected by the size and frequency of vibrating body. The empirical relation, which allows to describe the behavior of specific drag coefficient for vibrating tuning forks, microsphere, and wire everywhere over the temperature region and at various frequencies, is proposed.
37(2011); http://dx.doi.org/10.1063/1.3630135View Description Hide Description
We report the analysis of pseudogap Δ* derived from resistivity experiments in FeAs-based superconductor SmFeAsO0.85, having a critical temperature Tc = 55 K. Rather specific dependence Δ*(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corresponding to the structural transition in SmFeAsO, Δ*(T) decreases linearly down to the temperature TAFM ≈ 133 K. This last peculiarity can likely be attributed to the antiferromagnetic(AFM) ordering of Fe spins. It is believed that the found behavior can be explained in terms of Machida, Nokura, and Matsubara theory developed for the AFMsuperconductors.
37(2011); http://dx.doi.org/10.1063/1.3633685View Description Hide Description
The elementary excitation spectrum of a two-component Bose–Einstein condensate is obtained by Green’s function method. It is found to have two branches. In the long-wave limit, the two branches of the excitation spectrum are reduced to one phonon excitation and one single-particle excitation. With the obtained excitation spectrum and the Green’s functions, the depletion of the condensate and the ground stateenergy have also been calculated in this paper.
Low-frequency ferromagnetic resonance in nanocomposite manganites p-La0.78Mn0.99O3.5 and p-La0.80Mn1.04O3.537(2011); http://dx.doi.org/10.1063/1.3626834View Description Hide Description
The low-frequency ferromagnetic resonance (FMR) spectra of nanocomposite manganites p-La0.78Mn0.99O3.5 and p-La0.80Mn1.04O3.5 with the cubic perovskite structure and hole conductivity were studied. The samples were prepared under pressure of 7.5 GPa in oxygen atmosphere. It was found that the temperature dependence of FMR parameters in the vicinity of the Curie point is similar to that of magnetization. The characteristic time of decay of metastable magnetic phase above T* (metal-semiconductor transition) and the time of its formation below T* were determined.
Natural media with negative index of refraction: Perspectives of complex transition metal oxides (Review Article)37(2011); http://dx.doi.org/10.1063/1.3626835View Description Hide Description
The capabilities of perovskite-like compounds with the effect of colossal magnetoresistance(CMR) and some other complex oxides to have a negative index of refraction (NIR) are considered. Physical properties of these compounds are also analyzed from the standpoint of designing tunable metamaterials on their base. Of particular interest are temperature and magnetic field driven first-order transformations in oxides with perovskite structure and in spinels. These transformations give rise to nanophase separated states, using which the properties of negative refraction can be affected. The magnetic-field controlled metamaterials with CMR oxides as a boundary NIR media for a photonic crystal are discussed.
37(2011); http://dx.doi.org/10.1063/1.3638141View Description Hide Description
In bilayer systems, electron-hole (e–h) pairs with spatially separated components can exhibit a transition to a superfluid state. An influence of random inhomogeneities on superfluid properties of bilayer systems is considered. It is shown that ionized impurities and distortions of conducting layers reduce the density of superfluidcurrent of e–h-pairs. In the case when the interlayer distance is less than or of the order of magnetic length, the fluctuations of the interlayer distance lead to a significant decrease of the superfluid transition temperature.
Hydrogen sorption and radial thermal expansion of bundles of single-walled nanotubes irradiated by γ-rays in hydrogen atmosphere37(2011); http://dx.doi.org/10.1063/1.3643271View Description Hide Description
The influence of radiation exposure in hydrogen atmosphere on the radial thermal expansion of single-walled carbon nanotubes and on their sorption of hydrogen is studied. The irradiation was carried out with cobalt-60 γ-rays (with an energy of 1.2 MeV and an irradiation dose of 1.5·107 rad) in the normal hydrogen atmosphere under pressure 1 atm and at temperature of 300 K. The sorption and desorption of hydrogen by samples of nanotubes were investigated over the temperature range 15–1170 K before and after irradiation. It was found that the irradiation of carbon nanotubes(CNTs) in hydrogen atmosphere leads to considerable increasing an amount of hydrogen sorbed by the sample. The irradiation in hydrogen atmosphere resulted in increasing absolute values of radial thermal expansion of CNTs bundles over the temperature range investigated (2–120 K). The effects of hydrogen physically and chemically sorbed in bundles of CNTs on the radial thermal expansion of the irradiated samples were distinguished.
37(2011); http://dx.doi.org/10.1063/1.3645008View Description Hide Description
The equations of statistical crystal theory with the pair interaction by Lennard-Jones are applied to the calculation of a thermodynamic stability region of hypothetical bcc phase discussed in the literature for crystals of a type similar to Ar. It is shown that this region is completely within a similar region previously calculated by authors for a real fcc phase of such crystals. This result makes improbable the realization of the bcc modification, predicted by a group of authors in computer “experiments” on Xe at high temperatures and pressures, and provoked a critical discussion in the literature. The quantum corrections to the thermodynamic functions of “classical” noble-gas crystals were also taken into consideration and the deviation of the spinodal line of Ne from the “universal” curve is calculated. A good quantitative agreement of obtained results with the data of numerical “experiments” on model (with the Lennard-Jones interaction) “crystals” of Ar, Kr, Xe, as well as of Ne in the “exotic” region of negative pressures is found. Although, at present, the indications of the polymorphic transformation of neon to a modification different than fcc have not been experimentally observed, the discovery of this phenomenon in the predicted range of temperatures T < 70 K and pressures > 0.3 GPa would be of principal interest.
Matrix isolation study of the formation of methanol cluster structures in the spectral region of C–O and O–H stretch vibrations37(2011); http://dx.doi.org/10.1063/1.3643482View Description Hide Description
Infrared absorption spectra of methyl alcohol isolated in an argon matrix are recorded experimentally. The transformation of the structure of the spectral bands with rising matrix temperature is analyzed in the spectral region of the C–O and O–H stretch deformation vibrations (1000–1100 and 3000–3800 cm−1). The experimental data are interpreted using quantum-chemical modelling of an optimal spatial structure and of the vibrational spectra of different nano-sized methyl alcohol clusters. The structure of the clusters changes gradually as the temperature of the argon matrix is raised from 10 to 50 K.
Strain hardening and microstructure evolution during uniaxial compression of ultrafine grained zirconium at temperatures of 4.2–300 K37(2011); http://dx.doi.org/10.1063/1.3645014View Description Hide Description
The mechanical properties of ultrafine grained (UFG) zirconium (grain size 200 nm) obtained by a combination of extrusion, drawing, and annealing, are studied experimentally under uniaxial compression at temperatures of 4.2–300 K and compared with the mechanical properties of coarse grained (CG) Zr. The evolution of the texture and microstructure of Zr during strain is studied by x-ray diffraction and transmission electron microscopy. The volume fractions of twinned material are determined for UFG and CG Zr. It is found that at room temperature and below, twinning activity is lower in coarse grained zirconium, but at the very lowest temperatures (4.2 K) the opposite effect is observed, i.e., increased twinning activity with decreasing grain size. The influence of internal thermal anisotropy stresses on twinning in CG and UFG zirconium is discussed. The effect of twinning on the mechanical properties of UFG Zr is analyzed.
Low-amplitude abrupt deformation of Pb–27 at.% In alloy in superconducting and normal states at temperatures of 1.65–4.2 K37(2011); http://dx.doi.org/10.1063/1.3645024View Description Hide Description
This is a study of plasticflow instability in polycrystalline Pb–27 at.% In alloy in the form of an abrupt (jump) deformation stress of variable amplitude at temperatures of 1.65–4.2 K and for different states of the electronic system. At temperatures of 4.2–2 K, in the superconducting state of the alloy the ranges of variation in the amplitudes and concentrations of the jumps are δσdr = (1–20)·104 Pa and n dr = 10–250, respectively. At lower temperatures, 2.2–1.65 K, the low-amplitude instability shows up in the normal state of the alloy with jumps with δσdr = (1–20)·104 Pa and n dr = 10–250 against the background of which jumps of δσdr = 0.5–2 MPa occur. Mechanisms for the abrupt deformation of the alloy within the different temperature ranges are discussed.
Statistical analysis of the low-temperature internal friction dislocation peak (Bordoni peak) in nanostructured copper37(2011); http://dx.doi.org/10.1063/1.3645026View Description Hide Description
The frequency-temperature relations for internal friction in nanostructured samples of Cu and of fiber composite Cu-32 vol.% Nb with structural fragment sizes of ∼200 nm are analyzed. Data from earlier experiments are used in which a Bordoni peak characteristic of highly deformedcopper was found to be localized near a temperature of 90 K in the temperature dependence of the damping decrement for the oscillations (frequencies 73–350 kHz). This peak is caused by a resonance interaction of sound with a system of thermally activated relaxation oscillators, but its width is substantially greater than the width of the standard internal friction peak with a single relaxation time. The peak is analyzed statistically under the assumption that the broadening is caused by the random spread in the activation energy of the relaxation oscillators owing to strong distortions of the crystalline structure of the copper. Good agreement is obtained between the experimental data and the theory of Seeger in which the relaxation oscillators for the Bordoni peak are assumed to be thermally activated kink pairs in rectilinear segments of dislocation lines located in valleys of the Peierls potential relief. It is shown that the experimentally observed height of the peak corresponds to the presence, on the average, of one dislocation segment within a copper crystallite of size 200 nm. Empirical estimates of σ P ≈ 2·107 Pa for the Peierls critical stress and ρ d ≈ 1013 m−2 for the integrated density of intragrain dislocations are obtained. Nb fibers in the Cu-Nb composite facilitate the formation of nanostructured copper, but have no significant effect on the Bordoni peak.