Volume 40, Issue 6, June 2014

On the thermodynamics of quantum liquids
View Description Hide DescriptionThe thermodynamic characteristics of liquid ^{4}He and ^{3}He are determined in the temperature range where effects of the particle statistics can be neglected. Experimental data are used to determine the kinetic energies and of liquid ^{4}He and ^{3}He as functions of density n at T = 0 to an accuracy corresponding to small exchange effects. The potential energies and are indistinguishable to high accuracy and depend linearly on n. The kinetic energies K are inversely proportional to the atomic mass M, so that . A diffusion model of quantum nondegenerate liquids is proposed for liquid ^{4}He. A detailed comparison of theory and experiments reveals the existence of quantum scaling. With a suitable temperature scale, it is possible to obtain reduced thermodynamic characteristics of the quantum nondegenerate liquids that depend weakly on the density of the medium and the particle statistics.

Photonassisted scattering and magnetoconductivity oscillations in a strongly correlated 2D electron system formed on the surface of liquid helium
View Description Hide DescriptionThe influence of strong internal forces on photonassisted scattering and on the displacement mechanism of magnetoconductivity oscillations in a twodimensional (2D) electron gas is theoretically studied. The theory is applied to the highly correlated system of surface electrons on liquid helium under conditions that the microwave frequency is substantially different from intersubband resonance frequencies. A strong dependence of the amplitude of magnetoconductivity oscillations on the electron density is established. The possibility of experimental observation of such oscillations caused by photonassisted scattering is discussed.

Conductivity of singlecrystal Y_{1−} _{ y }Pr_{ y }Ba_{2}Cu_{3}O_{7−δ} over a wide range of temperatures and Pr concentrations
View Description Hide DescriptionThe inplane electrical resistance of Y1− y Pr y Ba2 Cu 3O7−δ single crystals with a high degree of perfection is studied for temperatures in the range TC 300 K, where TC ranges from 92 to 52 K, with varying praseodymium content. The experimental data are approximated by an expression that takes electron scattering on phonons and defects into account, as well as fluctuation conductivity in a 3D AslamazovLarkin model. According to the approximation data the Debye temperature ranges from 350–370 K, depending on the praseodymium content. The transverse coherence length is ∼1 Å.

Unusual microwave response and bulk conductivity of very thin FeSe_{0.3}Te_{0.7} films as a function of temperature
View Description Hide DescriptionResults of Xband microwave surface impedance measurements of FeSe1− x Te x very thin film are reported. The effective surface resistance shows appearance of peak at T ≤ Tc when plotted as a function of temperature. The authors suggests that the most wellreasoned explanation can be based on the idea of the changing orientation of the microwave magnetic field at a S–N phase transition near the surface of a very thin film. The magnetic penetration depth exhibits a powerlaw behavior of δλ L (T) ∝ CT ^{ n }, with an exponent n ≈ 2.4 at low temperatures, which is noticeably higher than in the published results on FeSe1 − x Te x single crystal. However the temperature dependence of the superfluid conductivity remains very different from the behavior described by the BCS theory. Experimental results are fitted very well by a twogap model with Δ1/kTc = 0.43 and Δ2/kTc = 1.22, thus supporting s ±wave symmetry. The rapid increase of the quasiparticle scattering time is obtained from the microwave impedance measurements.

Ground state and excitations of a BoseEinstein condensate of atoms and their diatomic bound states
View Description Hide DescriptionThis is a theoretical study of a manybody system of spinzero atoms and their diatomic bound states (molecules) which form a single BoseEinstein condensate at zero temperature. The GrossPitaevskii approach is used to analyze the equilibrium states and dynamics of this system. It is shown that, depending on the binding energy of the system, the system can exist in two phases: an atommolecule condensate and a molecular condensate. The basic thermodynamic characteristics and the criteria for their stability are found. Both phases have two branches of collective excitations, one of which is acoustic, while the other has an activation energy.

Magnetic resonance frequencies of ferromagnets with partially frozen orbital momentum
View Description Hide DescriptionThe dynamics of the angular momentum of ferromagnets is discussed with the crystal field and the spinorbital interaction taken systematically into account. The spectrum of the magnetic oscillations of this kind of magnet contains three branches of oscillations of different types, one of which is longitudinal oscillations in the magnitude of the orbital momentum while the other two are transverse branches corresponding to coupled oscillations of the spin and orbital momentum components.

The squarekagome quantum Heisenberg antiferromagnet at high magnetic fields: The localizedmagnon paradigm and beyond
View Description Hide DescriptionWe consider the spin1/2 antiferromagnetic Heisenberg model on the twodimensional squarekagome lattice with almost dispersionless lowest magnon band. For a general exchange coupling geometry we elaborate lowenergy effective Hamiltonians which emerge at high magnetic fields. The effective model to describe the lowenergy degrees of freedom of the initial frustrated quantum spin model is the (unfrustrated) squarelattice spin1/2 XXZ model in a zaligned magnetic field. For the effective model we perform quantum Monte Carlo simulations to discuss the lowtemperature properties of the squarekagome quantum Heisenberg antiferromagnet at high magnetic fields. We pay special attention to a magneticfield driven BerezinskiiKosterlitzThouless phase transition which occurs at low temperatures.

Universality of the magnetization irreversibility curve of systems with competing interactions (manganites, cobaltites, ferrites)
View Description Hide DescriptionExperimental data on the temperature dependences of the magnetization of manganites, cobaltites, and ferrites are analyzed. Their common feature is magnetization irreversibility: below a temperature T* for a fixed field strength, the magnetization is different for cooling in a field (MFC ) and in zero field (MZFC ). It is shown that the T*(H) curve is universal for all the dispersed oxides (nanocompound, nanopowder) and is consistent with the theoretical AlmeidaThouless curve (T* ∼ H ^{2/3}) over the entire range of magnetic fields. In the case of monolithic samples (single crystals, ceramics), in strong magnetic fields there is a transition to a quadratic dependence T* ∼ H ^{2} in agreement with the GabayToulousse theory. This phenomenon is interpreted in terms of spinglass polyamorphism.

Acoustopiezomagnetism and the elastic moduli of CoF_{2}
View Description Hide DescriptionThe piezomagnetic response of single crystal CoF2 is studied using acoustic oscillations at frequencies of ∼55 MHz. Without an external magnetic field the results reproduce the data from direct static measurements. A magnetic field opens up an additional piezomagnetic response channel with an amplitude that is linear in the field strength. The efficiencies of the two channels become equal in the antiferromagnetic state for H ∼ 3 T. Above the Neel temperature the mechanism for the first channel ceases to operate, while the piezomagnetic response associated with the new channel falls off approximately exponentially. The sound speeds for the different crystallographic directions are measured and the components of the elastic modulus tensor are calculated.

Influence of narrow inner barriers on the lowtemperature lateral conduction in quantum wells
View Description Hide DescriptionThe electron energy spectrum of a wide quantum well with a different number of narrow barriers inside the well region was calculated. It was shown that the sizequantization levels rise in energy upon the introduction of such barriers. When the maximum filling of the quantum well with narrow barriers was reached and the fragment of a shortperiod superlattice was formed, the envelopes of the electron wave functions on the sizequantization levels were similar in shape to those in a usual quantum well. In this case, the scattering by the surface roughness of the heterojunctions was significantly increased. The lowtemperature lateral conduction in a quantum well tunnelcoupled with a 10well shortperiod superlattice and, separately, in the superlattice alone was investigated experimentally. The obtained results agreed sufficiently well with the model calculations and demonstrated a new way to form parallel conducting channels with different electron mobility.

3ωresponse and energy relaxation times of the electronphonon system in a metal film
View Description Hide DescriptionAlternating current of frequency ω generates heat in a metal film oscillating with the frequency 2ω. Due to the temperature dependence of the resistance, the voltage across the film has a V 3ωcomponent oscillating with the frequency 3ω. In the present paper the amplitude and phase of the V 3ωcomponent are calculated on the basis of the equation for electron temperature. It is shown that for sufficiently long films on dielectric substrates with high thermal conductivity, both the average time of phonon escape from the film τ es and the time of electronphonon collisions τ e can be found from the frequency dependence of the amplitude and phase of the V 3ωcomponent.

Structural, electronic and magnetic properties of chiral nanotubes filled with a linear chain of iron
View Description Hide DescriptionDensityfunctional calculations of the electronic structure of (n,n/2) chiral carbon nanotubes filled with a linear chain of Fe atoms were conducted for the first time. It was found that upon encapsulating a chain of Fe atoms, the initially semiconducting nanotubes Fe5@(4,2), Fe5@(6,3) and Fe5@(8,4) became metallic. In the case of the Fe5@(8,4) structure, only the iron chain was conductive. All the considered nanotubes maintained large energy of the magnetic anisotropy, which is characteristic for a free linear chain of iron atoms. The magnetic moment on an iron atom was found to vary nonmonotonically with increasing the chiral index n; for the Fe5@(6,3) nanotube, the magnetic moment reached that of a free linear chain of iron atoms.

Highfrequency conductivity of multilayer graphene and graphite under the conditions of quantum cyclotron resonance
View Description Hide DescriptionThe conductivity tensor of a layered conductor with the Diractype energy spectrum of charge carriers placed in a quantizing magnetic field under the condition of normal skineffect is investigated using the method of quantum kinetic equation. It is shown that under the cyclotron resonance conditions there appear hightemperature quantum oscillations of conductivity, which are weakly sensitive to thermal broadening of the Fermi level. We present the expressions for the classical and hightemperature contributions to the conductivity tensor which determine the conductivity in the range of not too low temperatures where the Shubnikov–de Haas oscillations are vanishing.

Polaron Rashba effect in an asymmetric quantum dot
View Description Hide DescriptionWe study the influence of polaron Rashba effect in an asymmetric quantum dot. Using variational method, we derive the expression of the polaron ground state energy. We also discuss the dependencies of the ground state energy on the wave vector and the transverse (longitudinal) effective confinement length. It is found that the ground state energy splits into two branches due to the Rashba effect. The spin splitting energy is an increasing function of the wave vector and the Rashba SO parameter.

Singlecomponent singlepartial acoustic surface waves in cubic crystals with surface distortion taken into account
View Description Hide DescriptionThe surface waves and bulk acoustic bands were studied taking into account the interaction between the nearest and nextnearest neighbors in a cubic crystal. Expressions for the dispersion relations, the frequencies at which the surface waves split off the bulk spectrum, and the parameters of the amplitude attenuation have been obtained for the crystalline systems in which the surface waves are singlecomponent and singlepartial. The calculations were conducted taking into account the discrete nature of crystal lattice for arbitrary values of the twodimensional wave vector. The analysis has demonstrated that the results obtained in the longwavelength limit are in full agreement with those calculated in the framework of linear nonlocal elasticity theory. The influence of an adsorbed surface monolayer on the characteristics of the surface waves was studied.

Conformational equilibrium of 2deoxyadenosine molecules isolated in inert Ar matrices
View Description Hide DescriptionThe FTIR spectra of 2deoxyadenosine (dA) molecules isolated in lowtemperature Ar matrices were obtained in the range of 3800−180 cm^{−1} with a resolution of 0.3 cm^{−1}. The population analysis of the major structural isomers of dA was carried out by MP2 and DFT quantummechanical calculation methods. It was established that dA can sustain prolonged evaporation at 440 K without undergoing thermal decomposition. For the first time, it was shown that the populations of dA synconformers with the intramolecular hydrogen bond O5′H–N3 is close to (76 ± 5)% in the gas phase and matrix. Upon freezing in the matrix the synconformers with the intramolecular hydrogen bond O5′H–N3 and C3′endo structure of the deoxyribose ring were fully transformed into the subset of synconformers with the C2′endo structure of the deoxyribose ring. Only two structures from this subset with different rotation angles of the O3′Hgroup can be stabilized in the matrix. Unlike pyrimidine nucleosides, the dA conformers with the intramolecular hydrogen bonds O3′H–O5 or O5′H–O3 were not found.