Volume 40, Issue 8, August 2014
Index of content:
40(2014); http://dx.doi.org/10.1063/1.4892642View Description Hide Description
The orbital Knight shifts and g factors for the tetragonal 63 Cu 2+ site in HgBa2Ca2 Cu 3O8+δ at 133 and 115 K are theoretically investigated based on the high-order perturbation formulae of these quantities for a 3d 9 ion situated into tetragonally elongated octahedra. The theoretical results reveal good agreement with the observed values. The significant anisotropies of the Knight shifts are illustrated as the considerable local tetragonal elongation distortions of the five-coordinated Cu 2+ sites. The results at different temperatures are also discussed in view of the local structure of the Cu 2+ sites.
Low-temperature dynamics of matrix isolated methane molecules in fullerite C60: The heat capacity, isotope effects40(2014); http://dx.doi.org/10.1063/1.4892643View Description Hide Description
The heat capacity of the interstitial solid solution (CH4)0.4C60 has been investigated in the temperature interval 1.4–120 K. The contribution of CH4 molecules to the heat capacity of the solution has been separated. The contributions of CH4 and CD4 molecules to the heat capacity of the solutions (CH4)0.40C60 and (CD4)0.40C60 have been compared. It is found that above 90 K the character of the rotational motion of CH4 and CD4 molecules changes from libration to hindered rotation. In the interval 14–35 K the heat capacities of CH4 and CD4 molecules are satisfactorily described by contributions of the translational and libration vibrations, as well as the tunnel rotation for the equilibrium distribution of the nuclear spin species. The isotope effect is due to mainly, the difference in the frequencies of local translational and libration vibrations of molecules CH4 and CD4. The contribution of the tunnel rotation of the CH4 and CD4 molecules to the heat capacity is dominant below 8 K. The isotopic effect is caused by the difference between both the conversion rates and the rotational spectra of the nuclear spin species of CH4 and CD4 molecules. The conversion rate of CH4 molecules is several times lower than that of CD4 ones. Weak features observed in the curves of temperature dependencies of the heat capacity of CH4 and CD4 molecules near 6 and 8 K, respectively, are most likely a manifestation of first-order polyamorphic phase transitions in the orientational glasses of these solutions.
Effect of nitrogen sorption mechanisms on the properties of fullerite C60 over a wide range of temperatures40(2014); http://dx.doi.org/10.1063/1.4894316View Description Hide Description
X-ray diffractometry is used to study the effect of the adsorption of nitrogen at a pressure of 30 atm and temperatures of 200–550 °C on the structural and thermodynamic properties of fullerite C60. The sorption kinetics of nitrogen at different temperatures are studied, and the lattice parameter is plotted as a function of the time for fullerite to saturate with nitrogen. The sorption mechanism is found to change with increasing saturation temperature. The diffusive filling of lattice octahedral voids by nitrogen atoms at temperatures below 450 °C is supplanted at higher temperatures by a chemical interaction of nitrogen with fullerite molecules leading to the formation of a new molecular compound, fullerite nitride C60N x . The transition from physisorption to chemisorption of nitrogen by fullerite (the adsorption crossover) takes place at saturation temperatures of 450 > T > 400 °C. When C60 molecules are nitrogenated, the volume of the cubic cell increases dramatically, while the intensity of the x-ray reflections decreases and their width increases rapidly. The limiting distortions of the fcc lattice are determined, as well as the characteristic times for diffusional filling of the lattice voids and for nitrogenation, respectively, during physical and chemical sorption of nitrogen. Nitrogenation of fullerite molecules leads to a significant reduction in thermal expansion of the crystalline material, and suppresses both the orientational phase transition and formation of the glassy state.
40(2014); http://dx.doi.org/10.1063/1.4894319View Description Hide Description
The absorption spectrum of thin film CsPbCl3 in the 2–6 eV range is studied at temperatures of 90–500 K. Sudden changes show up in the temperature dependences of the parameters of the long-wavelength exciton band (spectral position Em (T), half width Γ(T), and oscillator strength f(T)) at the first order phase transitions at 310 and 320. No phase transitions in Em (T), Γ(T), and f(T) are detected at low temperatures. The exciton excitations in CsPbCl3 are found to have a three-dimensional character.
Effect of dynamic plastic deformation on the fluctuation stage of creep in single-crystal β-tin at 1.6 K40(2014); http://dx.doi.org/10.1063/1.4894320View Description Hide Description
Plastic deformation is studied in single-crystal β-tin oriented for plastic slip in the (100) 〈010〉 system. Cases are examined in which the logarithmic low-temperature creep curves are preceded by dynamic movement of dislocations. For this purpose an attachment was developed for the deformation machine that made it possible to change the external load in a standard way over identical times on the order of 0.1 s. A computer was used to record the creep process with a time resolution of 0.04 s. The logarithmic creep coefficient α is found to decrease dramatically when the preceding dynamic component of the deformation increment is increased. Thus, correct study of low-temperature logarithmic creep and its mechanisms requires a correct choice of the experimental conditions that excludes the dynamic stage when possible.
Effect of pressure on the critical temperature of single-crystal Y0.95Pr0.05Ba2Cu3O7–δ with a specified planar defect geometry40(2014); http://dx.doi.org/10.1063/1.4894321View Description Hide Description
The effect of high hydrostatic pressures up to 17 kbar on the conductivity in the basal ab-plane of single crystal samples of Y1– x Pr x Ba2 Cu 3O7–δ with a system of unidirectional twin boundaries and lightly doped with praseodymium (x = 0.05) is studied. It is found that, in contrast with undoped YBa2 Cu 3O7-δ samples with an optimum oxygen content, high pressure leads to a double increase in the pressure derivative dTc /dP. Possible mechanisms for the effect of high pressure on Tc are discussed taking the existence of singularities in the carrier electronic spectrum into account.
Effect of deformation-thermal processing using equal-channel multiangle pressing and low-temperature drawing on the superconducting properties of niobium-titanium alloys40(2014); http://dx.doi.org/10.1063/1.4894322View Description Hide Description
The effect of combined deformation, including equal-channel multiangle pressing (ECMAP), low-temperature (77 K) drawing, and thermal processing, on the critical current density of superconducting wires based on Nb-Ti alloys is investigated. Reasonable parameters are established for deformation-thermal processing of NT-50 alloy using ECMAP that will ensure improved functionality of superconductors based on this alloy. Vortex pinning on structural inhomogeneities of these alloys is analyzed qualitatively.
Suppression of transparency of thin modulated slabs of layered superconductors in the terahertz frequency range40(2014); http://dx.doi.org/10.1063/1.4894323View Description Hide Description
A reduction in the transmission of terahertz radiation through a periodically modulated slab of layered superconductor owing to diffraction of the incident wave and resonance excitation of eigenmodes is predicted and studied theoretically. The slab thickness is assumed to be much less than the skin depth and when there is no modulation the slab transmission is close to unity.
Excitation of magnetostatic spin waves in anisotropic ferromagnetic films, magnetized in arbitrary direction40(2014); http://dx.doi.org/10.1063/1.4892644View Description Hide Description
Exact analytical expressions for propagator of small-amplitude linear magnetostatic waves in ferromagnetic thin film between two antennae and their corresponding mutual impedance are obtained by solving the linearized torque equation of spin dynamics (Landau–Lifshitz equation) in magnetostatic approximation. This is done for the case of arbitrary orientation of uniform static magnetization of the film and full account for arbitrary magnetic anisotropy. The result also contains full description of the magnetostatic spin-wave spectrum.
Structure, phase transitions, 55Mn NMR and magnetoresistive properties of Pr0.6− x Nd x Sr0.3Mn1.1O3−δ (x = 0−0.6)40(2014); http://dx.doi.org/10.1063/1.4894317View Description Hide Description
Ceramic samples of Pr0.6− x Nd x Sr0.3Mn1.1O3−δ (x = 0−0.6) were studied by x-ray diffraction, resistive, magnetic (χ ac and 55Mn NMR), magnetoresistive and electron microscopy methods. It was shown that with increasing the concentration x, the type of unit cell distortion changes from orthorhombic (x = 0–0.2) to pseudo-cubic (x = 0.4–0.6), and the imperfection of the structure, which contains anion and cation vacancies, is increased. A decrease in the temperatures of metal–semiconductor (Tms ) and ferromagnetic–paramagnetic (TC ) phase transitions and an increase in the resistivity and activation energy with increasing x was explained by an increase in the concentration of vacancies, which weakens high-frequency electronic double-exchange Mn3+ ↔ Mn4+. It was found that the compositions with a higher content of neodymium exhibit a transition to the antiferromagnetic state at temperatures below 130 K. Two types of magnetoresistive effects were observed. The magnitude of the first effect, which occurs near the phase transition temperatures Tms and TC , increases with concentration x. The magnitude of the second effect, which was observed at low temperatures, exceeds that of the first one. The magnetic phase diagrams which describe strong correlations between the composition, structure defects, phase transitions, and functional characteristics, including magnetoresistive effect, were constructed.
40(2014); http://dx.doi.org/10.1063/1.4894315View Description Hide Description
Based on the assumption of a negative volume dependence of random exchange integrals, it is possible to switch to a compressible Sherrington–Kirkpatrick spin-glass model. Within the proposed model, temperature–pressure phase diagrams were calculated and pressure- and magnetic-field-induced first-order phase transitions from the initial paramagnetic and spin-glass states to the ferromagnetic state were predicted. It was shown that the application of pressure in the spin-glass state not only increases and shifts magnetic susceptibility, but also reduces the critical magnetic fields of irreversible induced phase transitions from the spin-glass to the ferromagnetic state. The obtained results are used to describe the spin-glass state in (Sm1– x Gd x )0.55Sr0.45MnO3.
40(2014); http://dx.doi.org/10.1063/1.4894324View Description Hide Description
The EPR spectra of impurity Dy3+ ions in YAl3(BO3)4 and EuAl3(BO3)4 aluminoborates were investigated. The signals from the ground and excited doublets were observed and the g-factors and the hyperfine interaction constants were determined. From the temperature dependence of the signal intensity of the excited doublet, the energy distances between the ground and excited doublets were obtained as (3.26 ± 0.13) cm–1 and (2.54 ± 0.16) cm–1 for YAl3(BO3)4 and EuAl3(BO3)4, respectively. A broadening of the absorption line with increasing temperature is related with the strong spin-phonon interaction and is described by the Orbach–Aminov process via the excited doublet. It was found that the decrease in g-factor is caused by the spin-phonon interaction.
40(2014); http://dx.doi.org/10.1063/1.4892645View Description Hide Description
High-pressure magnetic susceptibility experiments can provide insights into the changes in magnetic behavior and electric properties which can accompany extreme compressions of material. Instrumentation plays an important role in the experimental work in this field since 1990s. Here we present a comprehensive review of the high-pressure instrumentation development for magnetic measurement from the engineering perspective in the last 20 years. Suitable nonmagnetic materials for high pressure cell are introduced initially. Then we focus on the existing cells developed for magnetic property measurement system (MPMS®) SQUID magnetometer from Quantum Design (USA). Two categories of high pressure cells for this system are discussed in detail respectively. Some high pressure cells with built-in magnetic measurement system are also reviewed.
40(2014); http://dx.doi.org/10.1063/1.4892646View Description Hide Description
Temperature dependent positron annihilation lifetime spectroscopy (PALS) measurements in the range of 50–300 K are carried out to study positronium formation in 40 KeV B+-ion implanted polymethylmethacrylate (B:PMMA) with two ion doses of 3.13 × 1015 and 3.75 × 1016 ions/cm2. The investigated samples show the various temperature trends of ortho-positronium (o-Ps) lifetime τ 3 and intensity I 3 in PMMA before and after ion implantation. Two transitions in the vicinity of ∼150 and ∼250 K, ascribed to γ and β transitions, respectively, are observed in the PMMA and B:PMMA samples in consistent with reference data for pristine sample. The obtained results are compared with room temperature PALS study of PMMA with different molecular weight (Mw ) which known from literature. It is found that B+-ion implantation leads to decreasing Mw in PMMA at lower ion dose. At higher ion dose the local destruction of polymeric structure follows to broadening of lifetime distribution (hole size distribution).
40(2014); http://dx.doi.org/10.1063/1.4892647View Description Hide Description
The MgB2-based samples were synthesized at 2 GPa at 800 and 1050 °C for 1 h with and without Ti and SiC. X-ray, SEM and Auger structural studies showed that with increasing of manufacturing temperature grain boundary pinning transforms into point pinning, which is well correlated with the transformation of discontinuous oxygen enriched layers into separately located Mg–B–O inclusions in MgB2. Ti and SiC additions can influence the oxygen and boron distribution, but cannot change the type of pinning at relatively low temperatures.
Optical and magnetoresistive properties of polycrystalline La0.7Sr0.3MnO3 films on crystalline Al2O3 and Gd3Ga5O12 substrates40(2014); http://dx.doi.org/10.1063/1.4894318View Description Hide Description
The influence of thickness and heat treatment on the optical, magnetic and magnetoresistive properties of polycrystalline La0.7Sr0.3MnO3 (LSMO) films deposited onto Al2O3 (012) and Gd3Ga5O12 (111) crystalline substrates was investigated. It was shown that the optical properties of the annealed films are similar in the investigated range of photon energies 1–6 eV, independently of their thickness and substrate material. The optical conductivity spectra of the annealed films exhibit the same structure as epitaxial LSMO films but shifted to higher energies by ∼0.5 eV. The temperatures of broadened metal–insulator and ferromagnetic–paramagnetic transitions in the LSMO films are size-dependent and considerably lower as compared with the epitaxial films. The magnetoresistive effect is ∼15% in the field of 10 kOe and at the temperature of metal-insulator transition. These features of the films under study are explained by the presence of grain boundaries and the influence of the film-substrate interface.
Exciton density pattern formation in laser irradiated quantum wells under electrodes of various shapes40(2014); http://dx.doi.org/10.1063/1.4892648View Description Hide Description
(Submitted December 18, 2013)
The condensation of indirect excitons in double quantum wells is studied in an electric field created by electrodes of different shapes. The finite value of the exciton lifetime, the pumping and nonuniformity of the electric field under the electrode are taken into account. It is shown that islands of exciton condensed phase emerge under electrodes when the pumping exceeds a certain threshold value. They appear first under the rim where the potential energy of excitons has a dip. Calculations predict a complicated evolution of the exciton density distribution: from the gaseous phase at low laser intensities to the condensed phase in the whole area under the electrode at larger intensities. Therefore, the configurations of the exciton condensed phase may be manipulated by choosing the setups with conductive electrodes of different shapes via forming specific potentials of the electrical field and controlled by the level of the laser irradiation.
Spatial confinement, self-polarization and exciton–phonon interaction effect on the location of exciton line in lead iodide nanofilms40(2014); http://dx.doi.org/10.1063/1.4892649View Description Hide Description
Theoretical investigation of the spatial confinement, self-polarization and exciton–phonon interaction influence on the exciton state in plane double nanoheterostructure (nanofilm)–lead iodide in polymeric matrix is performed within the effective mass approximation for the electron and dielectric continuum for the phonons in the framework of infinitely deep single quantum well. It is shown that spatial confinement is the dominating feature determining the energy of the bottom of exciton ground band and its binding energy. The relationship of two others depends on nanofilm thickness: in ultrathin films the influence of self-polarization effect is essentially bigger than the role of exciton–phonon interaction.