Volume 35, Issue 10, October 2009
 QUANTUM LIQUIDS AND QUANTUM CRYSTALS


Equation of state for , including a regular and a scalar part
View Description Hide DescriptionA new unified equation of state is proposed which describes the data of with an error with respect to pressure of about in the interval of reduced densities from to and reduced temperatures from to . The unified equation, which for the first time is written in explicit functions of density and temperature , includes a regular equation of state for approximating the data outside the critical region, a nonparametric scaling equation of state that adequately represents the data near the critical point of vaporization, and a crossover function that joins the two different equations of state. The crossover function that is proposed is a classical damping function for the density and temperature fluctuations characteristic of the critical region. The regular part of the unified equation consists of a universal sevenconstant Kaplun–Meshalkin equation of state and a new, fiveconstant cubic equation. The unified equation of state obeys the condition that the first and second derivatives of the pressure with respect to the density vanish at the critical point; there are a binodal and a spinodal.

On the possible surface nature of the electrical activity in He II^{a)}
View Description Hide DescriptionA simple phenomenological model is proposed which can explain qualitatively the generation of electric fields in He II by second sound and vibrations of the capacitor walls. The model is based on the assumption that a bound state of He atoms exists on a heliumwetted surface, forming a dipole moment (dipole layer). Furthermore, it is assumed that the the character of the wetting changes upon the transition of the helium in the bulk to the superfluid state, and the change of the wetting is a secondorder phase transition due to the onset of additional condensation of atoms from the bulk into the dipole layer and the transition of that layer to the superfluid state. This transition is described using Landau theory with an order parameter . It follows from the theory that the value of the ratio of the oscillations of the potential difference to the temperature difference on the capacitor plates generating the second sound is independent of temperature and that the potential difference generated by vibrations of the wall of a torsion pendulum is proportional to the square of its velocity.

The features of the collective modes in aerogels filled with superfluid helium
View Description Hide DescriptionThe velocity of fast and slow collective modes of 90, 94, and 98% porosity aerogels filled with superfluidhelium are measured by a lowfrequency resonant technique at temperatures . The temperature dependences of the velocities of the two modes are compared with the hydrodynamic theory modified to take into account the mobility of the aerogel matrix, porosity of the media, and tortuosity of the acoustic path. It is found that the fast and slow modes in an aerogel are coupled much more strongly than the first and second sounds in bulk He II.

Nonlinear surface electron transport over liquid helium
View Description Hide DescriptionWe present experimental data and a theoretical analysis of the nonequilibrium mobility of surface electrons in liquid helium. The experiments are carried out in the temperature range where electron mobility is limited by electron scattering on surface excitations of the liquid helium(ripplons). Holding and driving electric fields of wide ranges are used in the measurements. Special attention is paid to the condition of strong holding fields, under which hot electrons are confined to the ground surface level. Depending on the relation between the momentum relaxation rate and electronelectron collision frequency, different theoretical approaches are used to describe the nonlinear mobility of surface electrons. The results obtained allow one to estimate the range of physical parameters in which experimental data can be described by the theory of nonlinear electron transport within the ground surface level.

Features of the transport of quasionedimensional surface electrons in dense gaseous helium
View Description Hide DescriptionThe results of an experimental investigation of the mobility of surfaceelectrons localized in quasitwodimensional conducting channels in liquid helium on a profiled substrate are presented. The electron mobility is measured in a temperature region corresponding to scattering of particles on helium atoms in the vapor phase. It is shown that with increasing temperature the mobility decreases monotonically in proportion to the change in helium vapor density, then suffers a sharp drop that the authors attribute to the possible formation of a surface anion—electron polaron formation in the dense helium vapor. The results are in agreement with the results on the mobility of twodimensional and threedimensional electron systems in helium vapor.
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 BOSE EINSTEIN CONDENSATION


On the theory of Bose–condensate fluctuations in systems of finite size
View Description Hide DescriptionAn asymptotic expansion for the partition function of an ideal Bose gas is obtained for a canonical ensemble with arbitrary number of particles. It is shown that the expressions found are valid at all temperatures, including the critical region. A comparison of the asymptotic formulas for fluctuations of the Bose condensate with the exact expressions is carried out and their quantitative agreement is established.
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 LOWTEMPERATURE MAGNETISM


Features of diagrams of the solid solution
View Description Hide DescriptionExperimental investigations of reversible and irreversible magnetic field induced firstorder phase transitions under pressure in the sample of the system have made it possible to construct a magnetic phase diagram in which a wide (of the order of ) metastable region of firstorder phase transitions between the ferromagnetic,paramagnetic, and lowtemperature phases is present. It is shown that the ferromagnetic phase can be expected to appear spontaneously at pressure of the order of . At atmospheric pressure the lowtemperature phase and the irreversibly induced ferromagnetic phase are metastable and vanish at temperatures 49 and , respectively. It is conjectured that the lowtemperature region of the diagram and its boundary features are formed under the influence of competition between interacting order parameters, one of which is the ferromagnetic order parameter.

Magnetic and magnetocaloric properties of the alloys
View Description Hide DescriptionThe results of investigations of the magnetic and magnetocaloric properties of alloys from the system are presented. The magnetization measurements are performed in the temperature interval in magnetic fields up to . The entropy changes with the magnetic field changing from 0 to 2, 4, 5, and are determined from the magnetization isotherms obtained near temperatures of the spontaneous appearance of the ferromagnetic state , and the curves are constructed. It is found that and decrease monotonically with increasing manganese concentration and that the ferromagnetic phase is completely suppressed in . It is found that the concentration dependences of the maximum entropy jump (and the corresponding coldstorage capacity) and the magnitudes of the ferromagnetic moment of the unit cell with maxima for and 0.8 show extremal behavior. The data obtained are compared with the ferromagnetic moments calculated from first principles by the KorringaKohnRostoker method using the coherentpotential approximation (KKRCPA)—the discrepancy for is attributed to the appearance of an antiferromagnetic component of the magnetic structure. It is concluded that the alloys have promise for use in magnetic refrigerators operating at room temperature.

MonteCarlo investigation of an anisotropic Ising model
View Description Hide DescriptionThe Monte Carlo method is used to study longperiod modulated structures in an anisotropic Ising model with competing interactions (ANNNI model). The character, particularities, and dependence of the modulated structures on the temperature and ratio of the exchange interaction constants between the nearest and nexttonearest neighbors are determined. The phase diagram is constructed.

Interaction of solitons with a localized rf field
View Description Hide DescriptionThe nonlinear Schrödinger equation is used to study the propagation of smallamplitude nonlinear magnetic pulses (solitons) in an actively dissipative medium (easyaxis ferromagnet) in which an external, circularly polarized, rf magnetic field is applied in spatially localized regions. The interaction of solitons with nonlinearly excited regions of the medium which are coupled with rf pumping centers is studied in the adiabatic approximation. The change of the parameters of solitons during such an interaction is calculated and the possibility of amplification of soliton pulses is examined.
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 ELECTRONIC PROPERTIES OF CONDUCTING SYSTEMS


Heat capacity studies of the magnetic phase transition in sodiumrich
View Description Hide DescriptionSpecific heatmeasurements in the temperature region from in magnetic field up to , oriented parallel and perpendicular to the layers, are carried out on a series of highquality single crystals of (, 0.76, 0.77, 0.78 and 0.87). Surprisingly, a sharp lambdatype anomaly was observed only for the concentration at temperature, while for all the remaining doping levels a rounded anomaly in the experimental data was visible at a temperature of , indicating a smeared magnetic phase transition. While a magnetic field oriented perpendicular to the layers shifts the temperature of this anomaly to lower values, a parallel magnetic field has no influence on it; this indirectly supports the idea of Atype antiferromagnetic ordering in the systems studied.

Thermoelectric effect in layered conductors at low temperatures
View Description Hide DescriptionThe linear response of the electronic system of a layered conductor to the presence of a temperature gradient is investigated theoretically. The dependence of the thermoelectric power on the temperature and external magnetic field is found at temperatures below the Debye temperature. Experimental investigation of this dependence will make it possible to study different relaxation mechanisms in a system of conduction electrons and to determine the structure of the electronic energy spectrum.
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 QUANTUM EFFECTS IN SEMICONDUCTORS AND DIELECTRICS


Ab initio calculations of the phonon frequencies and related properties of crystalline Ne under pressure
View Description Hide DescriptionAb initiotheoretical investigations of the dynamics and thermodynamics of a compressed Ne crystal performed using a model that explicitly takes account of the deformation of the electronic shells in the dipole approximation are presented. A dynamical matrix based on a nonempirical shortrange repulsive potential and integration over the principalvalue points of the Brillouin zone are used to calculate the specific heat of compressed Ne, the Debye temperature, and the meansquare displacement in the harmonic approximation. The temperature dependences obtained are in good agreement with an experiment performed at zero pressure.
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 SHORT NOTES


Nonequilibrium effects during tunneling into manganites
View Description Hide DescriptionThe currentvoltage characteristics of Ag–I–LaCaMnO tunneling junctions have shown an anomalously sharp increase of current near voltages , irrespective of the polarity of the applied potential and the resistance of the sample. The anomalous growth of current in the IVCs of the metalinsulatormanganite contacts is attributed to the presence of resonance levels in the barrier of the tunneling contact and a redistribution of oxygen vacancies under the action of a current of “hot” electrons, resulting in partial metallization of the barrier surface.

Fluctuation conductivity and pseudogap in
View Description Hide DescriptionThe temperature dependences of the fluctuation conductivity and pseudogap in are determined from resistivity measurements. It is shown that the temperature dependence of the pseudogap contains a number of features that substantially distinguish it from that of HTSCcuprates.
