Volume 36, Issue 3, March 2010
 SUPERCONDUCTIVITY, INCLUDING HIGHTEMPERATURE SUPERCONDUCTIVITY


Nonlinear Wood anomalies in the reflectivity of layered superconductors
View Description Hide DescriptionThe excitation of nonlinear surface Josephsonplasma waves in slabs of layered superconductors is studied theoretically by means of incident plane waves using the prism method in the Otto configuration. The bilateral excitation symmetric with respect to the magnetic field is considered. The excitation leads to the resonant suppression of the specular reflection, i.e. to the Wood anomalies. Due to the nonlinearity the reflectivity resonantly depends not only on the wave frequency and the incident angle but on the wave amplitude as well. It is shown that total suppression of reflectivity can be attained by an appropriate choice of , , , and other parameters of the problem. The results of the analytical calculations are supported by numerical simulations.

Charge and spin currents in a ballistic SNS Josephson junction between wave superconductors
View Description Hide DescriptionThe charge and spin transport properties of a clean TS–N–TS Josephson junction (triplet superconductornormal metaltriplet superconductor) are studied using the quasiclassical Eilenberger equation for the Green’s function. The effects of the thickness of normal layer between superconductors on the spin and chargecurrents are investigated. The effect of misorientation between the triplet superconductors which creates the spin current is the main subject of this paper. It is shown that for some values of the phase difference between the superconductors a spin current exists in the absence of chargecurrent and vice versa.
 Top

 LOWTEMPERATURE MAGNETISM


The evidence of a new magnetic anomaly in Zndoped LSCO cuprates
View Description Hide DescriptionThe lowtemperature heat capacity in pure and Zndoped samples ( and 0.06) have been performed in the temperature interval by the method of highprecision pulsed differential calorimetry, providing measurements under the equilibrium conditions in contrast to commonly used differential scanning calorimeters. For these systems a new heat capacity anomaly was observed in the nonsuperconducting state, which is related with Zn impurities and has the form of single wide peak. The anomaly does not show a phonon character as it strongly shifts towards higher temperatures with increasing Zn content, as is characteristic for a magnetic anomaly. The anomaly increases almost linearly with the impurity concentration.

Effect of the dipole–dipole interaction on the lowtemperature magnetism of linear spin chains
View Description Hide DescriptionThe magnetic dipole–dipole interaction in a linear chain of spins with uniaxial exchange anisotropy gives raise to evolution of the system of energy levels. As the magnetic dipole interaction increases, the fastest decrease of the energy of the system is observed for a level with the maximum modulus of the spin of the system. For sufficiently large ratio of the dipole–dipole contribution to the exchange a transition occurs between the spingap states—antiferromagnetic and ferromagnetic. An extremely sharp change of the meansquare spin of the system at low temperatures followed by a “plateau” in the temperature dependence is characteristic for a narrow neighborhood of the point at which the type of ground state changes.

Magnetic and magnetoresistive properties of sodiumsubstituted lanthanum manganites
View Description Hide DescriptionThe magnetic, electric, and magnetoresistive properties of bulk samples of are studied. It is shown that at low temperatures all samples are ferromagnetic and the temperature of the transition from the paramagnetic into the ferromagnetic state increases with increasing sodium content. It is found that the saturationmagnetization reaches its maximum value, close to that computed theoretically, in samples with . A deviation of from 0.12 decreases the saturationmagnetization and broadens the magnetic transition. Analysis of the behavior of the magnetoresistance shows that there exists, together with the ferromagnetic conducting phase, a disordered weakly conducting phase, the volume fraction of the latter increasing as deviates away from 0.12. The characteristic behavior of the magnetic and magnetoresistive properties of samples are tied to the particulars of the evolution of the chemical composition and defectiveness of the samples as a function of the sodium concentration.

Magnetic state of nanocrystals of Znsubstituted magnetite
View Description Hide DescriptionA system of ultrasmall nanocrystals of zincsubstituted magnetite has been synthesized by chemical condensation. Indications of a transition of the particles into a superparamagnetic state have been found by two methods—1) measurements of the temperature dependence of the magnetization in the temperature interval for fixed values of the external magnetic field from and 2) the major hysteresis loop at . The dependence of the blocking temperature on the magnetic field strength is obtained.

Magnetic properties and electronic structure of
View Description Hide DescriptionThe magnetic properties of the compound were investigated by measurements of the dc magnetization for different values of the magnetic field, 1.0 and in the temperature range . Superconducting behavior was found below , whereas a distinct peculiarity in the lowfield dependence of the magnetic susceptibility was clearly observed at , which resembles a weak ferromagnetic(FM) response with saturation magnetic moment of about per formula unit at . The transition at is presumably not governed by magnetic impurities but rather correlated with the antiferromagnetic(AFM) transition in undoped LaFeAsO at about the same temperature. We suggest that the observed magnetic properties of the sample are due to an interplay of FM and AFM transitions, and are presumably related with an intrinsic feature of a small portion of the undoped LaFeAsO phase inherent in our sample. In order to shed light on the problem of magnetic instability of the LaFeAsO, ab initio DFT calculations of the electronic structure and paramagneticsusceptibility were performed within the local spin density approximation. It is shown that a Vshaped peculiarity in the density of electronic states close to the Fermi level can govern magnetic properties of LaFeAsO with fluorine doping and/or oxygen deficiency.
 Top

 LOWDIMENSIONAL AND DISORDERED SYSTEMS


Dynamical equation for an electronhole pair condensate in a system of two graphene layers
View Description Hide DescriptionIn bilayer graphene systems the electronhole pairs with spatially separated components at low temperaturestransition into a superfluid state. A microscopic derivation of the dynamical equation for the wave function of the electronhole pair condensate in a strong perpendicular magnetic field is given for low pair density. The temperature of the transition into a superfluid state is calculated in a wide interval of interlayer distances and magnetic fields on the basis of the elementary excitation spectrum.

Monte Carlo simulation of a twodimensional electron gas on a disordered host lattice
View Description Hide DescriptionThe lowtemperature thermodynamic properties of a twodimensional electron gas on a disordered host lattice are studied in the limit of low electron concentration. A novel Monte Carlo simulation algorithm making it possible to study the properties of this system effectively is proposed. Nonzero residual entropy per particle is found and its value is determined. It is shown on the basis of the proposed model that a cusp characteristic for spinglass systems is present in the lowtemperature dependence of the dielectric susceptibility as a function of the external electric field.
 Top

 NANOSTRUCTURES AT LOW TEMPERATURES


Quantum oscillations of resistivity in bismuth nanowires
View Description Hide DescriptionThe influence of uniaxial deformation on the transport properties of bismuthwires is studied in a wide range of temperatures. Measurements of the resistance of bismuthnanowires with several diameters and different quality reveal oscillations in the strain dependence of the resistance for uniaxial strain at . The amplitude of oscillations is significant (38%) at helium temperature and becomes smeared at . The oscillations observed are due to the quantum size effect. By evaluating the period of oscillations we are able to identify the groups of carriers involved in transport. The computed periods of 42.2 and are approximatively in the ratio 2:1 for two experimentally observed sets of oscillations from light and heavy electrons.
 Top

 PHYSICAL PROPERTIES OF CRYOCRYSTALS


Lattice parameters of cryoalloys
View Description Hide DescriptionThe structure of the cryoalloys has been studied in the entire range of relative concentrations by means of transmission electron diffraction. Gaseous samples were deposited from a tank at room temperature onto substrates cooled to . The filling channel was thermally insulated from the cryogenic screens. An anomalous dependence of the lattice parameter on the molar content of krypton in the initial gas mixture was observed; specifically, the function contains a section which can be regarded as a minimum to within the limits of error. The maximum solubility of the krypton in the carbon dioxide crystal is , and the solubility of in a Kr crystal is low. The experimental data taken together give us a basis for supposing that the phase composition as a function of has the following regions: 1) —regular solution of Kr in ; —mixture of a regular solution with critical concentration and nano inclusions of almost pure Kr; 3) —mixture of almost pure Kr with nano inclusions of a solution with . A theoretical explanation is given for the anomalous behavior of the function for ; the explanation is based on a cluster model and takes account of three mechanisms of interaction of Kr clusters with the crystalline environment of .

Meanfield theory of orientational ordering in rigid rotor models with identical atoms: spin conversion and thermal equilibration
View Description Hide DescriptionIn coupled rotor models which describe identical rotating nuclei the nuclear spin states restrict the possible angular momenta of each molecule. There are two meanfield approaches to determining the orientational phase diagrams in such systems. In one the nuclear spin conversion times are assumed to be instantaneous and in the other infinite. In this paper the intermediate case, when the spin conversion times are significantly slower than those of rotational time scales but are not infinite on the timescale of the experiment, is investigated. Via incorporation of the configurational degeneracy it is shown that in the thermodynamic limit the meanfield approach in the intermediate case is identical to the instantaneous spin conversion time approximation. The total entropy can be split into configurational and rotational terms. The meanfieldphase diagram of a model of coupled rotors of threefold symmetry is also calculated in the two approximations. It is shown that the configurational entropy has a maximum as a function of temperature which shifts to lower temperatures with increasing order.

Effect of impurity oxygen molecules on the structural and thermodynamic properties of fullerite
View Description Hide DescriptionIntercalation of fullerite with oxygen molecules is performed by two methods—under ordinary conditions of saturation at room temperature and gas pressure as well as at elevated temperature and pressure. The time dependences of the lattice parameter of fullerite during intercalation are determined. This made it possible to evaluate the activation energy and the coefficient of diffusion of oxygen molecules in fullerite. It is shown that increasing the temperature and pressure of the gas increases the diffusion coefficient almost fivefold: from at atmospheric pressure and room temperature to for pressure and temperature. The oxygen concentration in octahedral voids and the temperatures of phase transitions in solutions are determined on the basis of an analysis of the results obtained together with published data on the effect of atomic and molecular impurities on the properties of . The effect of oxygen impurity on the properties of an orientational phase transition and the temperature dependence of the lattice parameter is investigated.
 Top

 LOWTEMPERATURE PHYSICS OF PLASTICITY AND STRENGTH


Effect of Cd content on the kinetics of lowtemperature structural transformation in In–Cd alloy
View Description Hide DescriptionThe effect of the Cd concentration on the kinetics of a spontaneous lowtemperature structural phase transformation previously observed in the martensite phase of the solid solution In–Cd is investigated. Acoustic, resistive, and calorimetric measurements are performed in the temperature interval on samples containing 4.3, 5.5, and Cd. It is established that the transformation rate increases considerably with increasing Cd concentration. The empirical activation parameters of the transformation are obtained: activation energy and period of attempts . It is shown that this transformation has the characteristic features of a firstorder phase transition. It is suggested that the lowtemperature isothermal instability of the macroscopic properties of the alloy is due to the decomposition of the Inbased solid solution. The substantial increase of the decomposition rate with increasing Cd concentration could be due to an increase of the driving force of this process.
 Top

 SHORT NOTES


Lowtemperature phase transitions in the rareearth ferroborate
View Description Hide DescriptionThe lowtemperature behavior of the elasticcharacteristics of a single crystal has been studied. Features are found in the temperature and magneticfield dependences of the velocity and absorption of transverse sound. These features are interpreted as being a manifestation of magnetic phase transitions in the compound studied. The phase diagram is constructed.
