Volume 37, Issue 4, April 2011
Index of content:
37(2011); http://dx.doi.org/10.1063/1.3590752View Description Hide Description
A new superconductingceramic material CdBa2Cu3O7−δ is obtained. It is found that the superconducting transition (at 86 K) is retained when yttrium is fully replaced by cadmium in the Y-Ba-Cu-O system. X-ray diffraction studies show that the crystalline structure is unchanged and the identified peaks coincide when Y is replaced by Cd in YBa2Cu3O7−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 YBa2Cu3O7−δ and CdBa2Cu3O7−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 YBa2Cu3O7−δ and negative in CdBa2Cu3O7−δ.
37(2011); http://dx.doi.org/10.1063/1.3590749View Description Hide Description
The 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 Fe-based superconductor EuAsFeO0.85F0.15: Evidence of strong anisotropy in the order parameter37(2011); http://dx.doi.org/10.1063/1.3590753View Description Hide Description
Andreev reflection spectra have been measured in a new superconductor EuAsFeO0.85F0.15 with an unexpectedly low superconducting transition temperature Tc ≈ 11.3 K compared to related FeAs compounds based on Sm and Gd surrounding Eu in lanthanide series. The nearly fivefold lower Tc , compared to the expected value, is attributed to the divalent properties of Eu ions; the weakly magnetic Eu3+ ions may be present and the highly magnetic Eu2+ 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 /kTc = 2.2 −4.7 and 2Δ1/kTc = 5.1−11.7 for small and large gaps, respectively. The corresponding variations for single-gap 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 EuAsFeO0.85F0.15 that has been found in other, similar compounds with higher Tc . Thus, the character of the gap function Δ(k) in this compound is inconsistent with the d-wave superconductivity observed in some low-Tcpnictides.
Self-similar magnetic structures during the vortex-glass to vortex-liquid transition of type II superconductors37(2011); http://dx.doi.org/10.1063/1.3592212View Description Hide Description
We examine the response to an external magnetic field by a multi-layer superconductor with an electrical resistance ρff(b)αbσ, 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 self-similar 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.
37(2011); http://dx.doi.org/10.1063/1.3592218View Description Hide Description
The low-temperature upper critical field is measured for single crystal Nd2−x Ce x CuO4+δ with different values of x and nonstoichiometric disorder (δ). The coherence length for pair correlation ξ and the product kF ξ, where kF is the Fermi wave vector, are estimated. It is shown that the parameter for these single crystals, so that, phenomenologically, the NdCeCuO-system is in the domain of Cooper-pair-based (BCS)superconductivity.
37(2011); http://dx.doi.org/10.1063/1.3592221View Description Hide Description
The ground state and thermodynamics of distorted Ising-Hubbard chains are studied with on-site Coulomb repulsion taken into account. A decoration-iteration 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 high-temperature peak in the specific heat. Independently of any repulsion, the temperature dependence of the specific heat can have two low-temperature peaks.
37(2011); http://dx.doi.org/10.1063/1.3592229View Description Hide Description
Nd0.52Sr0.48MnO3films of various thicknesses have been prepared by dc magnetron sputtering on single crystal LaAlO3 (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 charge-ordered state at high magnetic fields, rather than in the regular double-exchange mechanism.
37(2011); http://dx.doi.org/10.1063/1.3592528View Description Hide Description
Magnetic-field induced magnetic order-order phase transitions in Fe0.69Mn1.31As are studied using first principles calculations of the electronic structure of Fe2−xMnxAs system alloys and a two-band model of itinerant magnets. It is shown that, within this model, the low-temperature ground and magnetic-field induced states can be described in terms of a coexistence of uniform ferromagnetic and periodic antiferromagnetic components of the total magnetic moment of the d-band. The effect of hydrostatic pressure on order-order 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 Fe2−xMnxAs system is a metastable state, transitions to which can only occur in ultrahigh magnetic fields.
37(2011); http://dx.doi.org/10.1063/1.3592529View Description Hide Description
A goldfilm (mean thickness ≈ 3.5 nm) was deposited in high vacuum on a single-crystal 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 electron-electron interaction. This behavior is found to be determined by inhomogeneous electron transport at the threshold of a thickness-controlled metal-insulator transition.
37(2011); http://dx.doi.org/10.1063/1.3592677View Description Hide Description
The low-temperature 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 θD 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 pulses37(2011); http://dx.doi.org/10.1063/1.3592679View Description Hide Description
A theory is developed for the acoustic oscillations of small-sized 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 electron-phonon 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.
37(2011); http://dx.doi.org/10.1063/1.3592692View Description Hide Description
A simplified theory of telescopic oscillations in multi-walled carbon nanotubes is developed. Explicit expressions are derived for the telescopic force constants (longitudinal rigidity) and the frequencies of telescopic oscillations. The contribution of small-amplitude telescopic oscillations to the low-temperature specific heat of nanotubes is estimated.
The effect of O2 impurities on the low-temperature radial thermal expansion of bundles of closed single-walled carbon nanotubes37(2011); http://dx.doi.org/10.1063/1.3592703View Description Hide Description
The effect of oxygen impurities on the radial thermal expansion α r of bundles of closed, single-walled carbon nanotubes has been investigated at temperatures of 2.2-48 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. Low-temperature desorption of oxygen from powders consisting of bundles of single-walled 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 impurities37(2011); http://dx.doi.org/10.1063/1.3593273View Description Hide Description
The 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 electron-electron interactions. Fits of the theoretical dependences to the experimental data yield quantitative information on the parameters of the hybridized states and of the Fermi-liquid interaction.
Low-temperature anomalies in the dynamic elastic moduli of cubic AIIBVI crystals with 3d-transition metal impurities37(2011); http://dx.doi.org/10.1063/1.3593580View Description Hide Description
A negative paramagnetic contribution to the dynamic elastic moduli is identified in AIIBVI:3d wide band-gap 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 orbit-lattice interaction operators in the energy of the spin-orbital states of the 3d-ion as a function of applied stress (by analogy with the Curie contribution to the magnetic susceptibility). The inverse paraelastic susceptibility of AIIBVI crystals containing non-Kramers 3d-ions 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 CH3F and O2 at 4.2 K: CH3F:O2 complex and CH3F migration37(2011); http://dx.doi.org/10.1063/1.3593297View Description Hide Description
Double doping of solid normal hydrogen with CH3F and O2 at about 4.2 K gives evidence of (ortho-H2)n:CH3F clusters and of O2:CH3F complex formation. FTIRanalysis of the time evolution of the spectra in the region of the v3 C-F stretching mode indicates that these clusters behave very differently from (ortho-H2)n:H2O clusters. The main point is the observed migration of CH3F molecules in solid para-H2 at 4.2 K which differs from that of H2O under identical experimental conditions. This is confirmed by an increase over time of the integrated intensity of the CH3F:O2 complex with a rate constant K = 2.7(2) · 10−4 s−1.