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Volume 37, Issue 4, April 2011

Preparation of CdBa_{2}Cu_{3}O_{7−δ} superconducting ceramic and a study of its electrical properties
View Description Hide DescriptionA new superconductingceramic material CdBa_{2}Cu_{3}O_{7−δ} is obtained. It is found that the superconducting transition (at 86 K) is retained when yttrium is fully replaced by cadmium in the YBaCuO system. Xray diffraction studies show that the crystalline structure is unchanged and the identified peaks coincide when Y is replaced by Cd in YBa_{2}Cu_{3}O_{7−δ}. Besides a stable superconducting transition, a phase transition near 275 K was seen in the temperature dependence of the specific resistivity which shifted toward lower temperatures (252 K) and then disappeared completely over repeated measurement cycles. The thermal conductivity k of YBa_{2}Cu_{3}O_{7−δ} and CdBa_{2}Cu_{3}O_{7−δ} was found to vary little with temperature up to the superconducting transition. Substituting Cd for Y in the initial HTSC material led to a substantial increase in the lattice thermal conductivity. Measurements of the thermal emf showed that it is positive in YBa_{2}Cu_{3}O_{7−δ} and negative in CdBa_{2}Cu_{3}O_{7−δ}.

Superconductivity of a quantum cylinder
View Description Hide DescriptionThe thermodynamic potential of a superconducting quantum cylinder is calculated. The dependence of the critical temperature and heat capacity of the superconducting system on the surface electron density and nanotube radius is studied. It is shown that the critical temperature oscillates with variations in a parameter equal to the ratio of the Fermi energy to the dimensional confinement energy.

Andreev reflection spectroscopy of the new Febased superconductor EuAsFeO_{0.85}F_{0.15}: Evidence of strong anisotropy in the order parameter
View Description Hide DescriptionAndreev reflection spectra have been measured in a new superconductor EuAsFeO_{0.85}F_{0.15} with an unexpectedly low superconducting transition temperature T_{c} ≈ 11.3 K compared to related FeAs compounds based on Sm and Gd surrounding Eu in lanthanide series. The nearly fivefold lower T_{c} , compared to the expected value, is attributed to the divalent properties of Eu ions; the weakly magnetic Eu^{3+} ions may be present and the highly magnetic Eu^{2+} ions are strong suppressors of superconductivity. Most of the measured spectra have features corresponding to two energy gaps whose values varied, from contact to contact, over 2Δ _{s} /kT_{c} = 2.2 −4.7 and 2Δ_{1}/kT_{c} = 5.1−11.7 for small and large gaps, respectively. The corresponding variations for singlegap spectra are 2Δ/kTc = 2.6−6.4. The relatively large crystallite sizes (at least ∼25 μm) and the large number of measured contacts (several tens) suggest, with a high degree of probability, that these spectra account quite fully for the gap distribution in essentially all the crystallographic directions. These data and the absence of zero gaps in the measured spectra indicate anisotropics− or s±symmetry of the order parameter in EuAsFeO_{0.85}F_{0.15} that has been found in other, similar compounds with higher T_{c} . Thus, the character of the gap function Δ(k) in this compound is inconsistent with the dwave superconductivity observed in some lowT_{c}pnictides.

Selfsimilar magnetic structures during the vortexglass to vortexliquid transition of type II superconductors
View Description Hide DescriptionWe examine the response to an external magnetic field by a multilayer superconductor with an electrical resistance ρ_{ff}(b)αb^{σ}, where b is the dimensionless magnetic induction and σ is a parameter characterizing the ratio of the pinning activation energy to the energy of thermal fluctuations. When σ > 1 the sample is in the vortexglass phase, when 0 < σ < 1, it is in the vortexliquid phase, and a vortexglass to vortexliquidphase transition takes place at σ = 1. In the vortexglass phase, the magnetic field penetrates into the superconductor in the form of a selfsimilar wave. At all times it penetrates to a finite depth and its front moves at a finite velocity which depends on the parameters of the problem, such as the rate of pumping by the external magnetic field. In the vortexliquid phase the magnetic field penetrates to an infinite depth. Thus, the magnetic field penetrates to an infinite depth in the superconductor during a transition from the vortexglass phase into the vortexliquid phase.

Estimating the coherence length in the electrondoped superconductor Nd_{2−x }Ce _{x} CuO_{4+δ}
View Description Hide DescriptionThe lowtemperature upper critical field is measured for single crystal Nd_{2−x }Ce _{x} CuO_{4+δ} with different values of x and nonstoichiometric disorder (δ). The coherence length for pair correlation ξ and the product k_{F} ξ, where k_{F} is the Fermi wave vector, are estimated. It is shown that the parameter for these single crystals, so that, phenomenologically, the NdCeCuOsystem is in the domain of Cooperpairbased (BCS)superconductivity.

Distorted diamond IsingHubbard chain
View Description Hide DescriptionThe ground state and thermodynamics of distorted IsingHubbard chains are studied with onsite Coulomb repulsion taken into account. A decorationiteration transformation method is used to obtain exact results for the free energy, entropy,specific heat, magnetization of the Ising and Hubbard subsystems, and magnetic susceptibility. The effect of Coulomb repulsion on the ground state, field and temperature dependences of the magnetization, magnetic susceptibility, and specific heat is studied for the case of a geometrically frustrated system. Strong repulsion leads to formation of an additional hightemperature peak in the specific heat. Independently of any repulsion, the temperature dependence of the specific heat can have two lowtemperature peaks.

Origin of an enhanced colossal magnetoresistance effect in epitaxial Nd_{0.52}Sr_{0.48}MnO_{3} thin films
View Description Hide DescriptionNd_{0.52}Sr_{0.48}MnO_{3}films of various thicknesses have been prepared by dc magnetron sputtering on single crystal LaAlO_{3} (001) substrates. Reducing the film thickness leads to a significant suppression of ferromagnetic (FM) ordering and the Curie point falls below the antiferromagnetic(AFM)transitiontemperature. When this occurs, a huge rise of the magnetoresistance ratio from 400 to 60 000% is observed in an applied magnetic field of 5 T. We surmise that this new kind of the enhanced colossal magnetoresistance effect originates in the FM/AFM competition and the collapse of the chargeordered state at high magnetic fields, rather than in the regular doubleexchange mechanism.

Magnetic orderorder phase transitions in itinerant magnets: Fe_{2−} _{x} Mn _{x} As
View Description Hide DescriptionMagneticfield induced magnetic orderorder phase transitions in Fe_{0.69}Mn_{1.31}As are studied using first principles calculations of the electronic structure of Fe_{2−x}Mn_{x}As system alloys and a twoband model of itinerant magnets. It is shown that, within this model, the lowtemperature ground and magneticfield induced states can be described in terms of a coexistence of uniform ferromagnetic and periodic antiferromagnetic components of the total magnetic moment of the dband. The effect of hydrostatic pressure on orderorder transitions arises from a structural renormalization of the electron density of states when the material is compressed. The model predicts that collinear ferromagnetism in the Fe_{2−x}Mn_{x}As system is a metastable state, transitions to which can only occur in ultrahigh magnetic fields.

Mixed 1D2D quantum electron transport in percolating gold film
View Description Hide DescriptionA goldfilm (mean thickness ≈ 3.5 nm) was deposited in high vacuum on a singlecrystal sapphire substrate at a temperature of 70 K. The transport properties of the film at low temperature reveal simultaneous 1D and 2D quantum interference effects of weak localization and electronelectron interaction. This behavior is found to be determined by inhomogeneous electron transport at the threshold of a thicknesscontrolled metalinsulator transition.

Lowtemperature specific heat of single crystal bismuth oxyhalides
View Description Hide DescriptionThe lowtemperature specific heat of layered tetragonal bismuth oxyhalide crystals, prepared by the method of chemical gas transport reactions, is measured. The specific heat is found to have Debye behavior at temperatures below 4 K. The Debye temperatures θ_{D} of BiOCl (205 K), BiOBr (168 K), and BiOI (146 K) single crystals are determined. The entropy and Debye temperatures of these compounds are found to depend linearly on the corresponding correlations parameters. The low values of the Debye temperatures with the rather high values of the upper boundaries of the phonon spectrum of these compounds may be caused by a high density of phonon states at low frequencies and a substantial deviation of their phonon spectrum from a Debye spectrum at frequencies above 10 cm^{−1}.

Acoustic oscillations of spherical metallic nanoparticles in dielectric media driven by ultrashort laser pulses
View Description Hide DescriptionA theory is developed for the acoustic oscillations of smallsized spherical metallic particles in a dielectric medium driven by ultrashort laser pulses. Analytic expressions are obtained for the amplitude and power of the longitudinal spherical acoustic oscillations as functions of the density and elastic properties of the medium, the laser pulse duration, electron temperature, particle radii, and electronphonon coupling constant. The factors affecting the dynamics of the damping of the power of these waves are studied in detail and illustrated for the example of Au, Ag, and Cu particles embedded in a plexiglas matrix.

Theoretical analysis of telescopic oscillations in multiwalled carbon nanotubes
View Description Hide DescriptionA simplified theory of telescopic oscillations in multiwalled carbon nanotubes is developed. Explicit expressions are derived for the telescopic force constants (longitudinal rigidity) and the frequencies of telescopic oscillations. The contribution of smallamplitude telescopic oscillations to the lowtemperature specific heat of nanotubes is estimated.

The effect of O_{2} impurities on the lowtemperature radial thermal expansion of bundles of closed singlewalled carbon nanotubes
View Description Hide DescriptionThe effect of oxygen impurities on the radial thermal expansion α _{r} of bundles of closed, singlewalled carbon nanotubes has been investigated at temperatures of 2.248 K by a dilatometric method. Saturation of bundles of nanotubes with oxygen causes an increase in the positive α _{r} values over the entire range of temperatures studied here. Also, several peaks appeared in the α _{r} (T) curve above 20 K. Lowtemperature desorption of oxygen from powders consisting of bundles of singlewalled nanotubes with open and closed ends has been investigated.

Anomalies in the temperature dependence of the contribution to the speed of sound from hybridized electronic states of transition element impurities
View Description Hide DescriptionThe temperature dependence of the speed of sound in crystalline mercury selenide with low concentrations of iron impurities is studied. Experiments are conducted in the ranges of concentration and temperature where hybridized electronic states in iron impurities have been observed previously. It is found that at temperatures below 10 K the speed of slow transverse ultrasonicwaves has an anomalous nonmonotonic segment of its temperature variation that is related to the influence of the impurities and reflects the existence of hybridized states. The observed anomalies in the sound speed are described in terms of a theory for the electron contribution to the elastic moduli that includes hybridization of impurity states and electronelectron interactions. Fits of the theoretical dependences to the experimental data yield quantitative information on the parameters of the hybridized states and of the Fermiliquid interaction.

Lowtemperature anomalies in the dynamic elastic moduli of cubic A^{II}B^{VI} crystals with 3dtransition metal impurities
View Description Hide DescriptionA negative paramagnetic contribution to the dynamic elastic moduli is identified in A^{II}B^{VI}:3d wide bandgap compounds for the first time. It appears as a paramagneticelastic, or, briefly, paraelastic, susceptibility. These compounds are found to have a linear temperature dependence for the inverse paraelastic susceptibility. This is explained by a contribution from the diagonal matrix elements of the orbitlattice interaction operators in the energy of the spinorbital states of the 3dion as a function of applied stress (by analogy with the Curie contribution to the magnetic susceptibility). The inverse paraelastic susceptibility of A^{II}B^{VI} crystals containing nonKramers 3dions is found to deviate from linearity with decreasing temperature and reaches saturation. This effect is explained by a contribution from nondiagonal matrix elements (analogous to the well known van Vleck contribution to the magnetic susceptibility of paramagnets).

Infrared spectroscopy of solid normal hydrogen doped with CH_{3}F and O_{2} at 4.2 K: CH_{3}F:O_{2} complex and CH_{3}F migration
View Description Hide DescriptionDouble doping of solid normal hydrogen with CH_{3}F and O_{2} at about 4.2 K gives evidence of (orthoH_{2})n:CH_{3}F clusters and of O_{2}:CH_{3}F complex formation. FTIRanalysis of the time evolution of the spectra in the region of the v_{3} CF stretching mode indicates that these clusters behave very differently from (orthoH_{2})n:H_{2}O clusters. The main point is the observed migration of CH_{3}F molecules in solid paraH_{2} at 4.2 K which differs from that of H_{2}O under identical experimental conditions. This is confirmed by an increase over time of the integrated intensity of the CH_{3}F:O_{2} complex with a rate constant K = 2.7(2) · 10^{−4} s^{−1}.