Volume 40, Issue 6, June 2014
Index of content:
40(2014); http://dx.doi.org/10.1063/1.4881196View Description Hide Description
The thermodynamic characteristics of liquid 4He and 3He 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 4He and 3He 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 4He. 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.
Photon-assisted scattering and magnetoconductivity oscillations in a strongly correlated 2D electron system formed on the surface of liquid helium40(2014); http://dx.doi.org/10.1063/1.4881177View Description Hide Description
The influence of strong internal forces on photon-assisted scattering and on the displacement mechanism of magnetoconductivity oscillations in a two-dimensional (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 inter-subband 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 photon-assisted scattering is discussed.
Conductivity of single-crystal Y1− y Pr y Ba2Cu3O7−δ over a wide range of temperatures and Pr concentrations40(2014); http://dx.doi.org/10.1063/1.4881197View Description Hide Description
The in-plane 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 Aslamazov-Larkin 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 FeSe0.3Te0.7 films as a function of temperature40(2014); http://dx.doi.org/10.1063/1.4881178View Description Hide Description
Results of X-band 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 well-reasoned 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 power-law 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 two-gap 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.
40(2014); http://dx.doi.org/10.1063/1.4883893View Description Hide Description
This is a theoretical study of a many-body system of spin-zero atoms and their diatomic bound states (molecules) which form a single Bose-Einstein condensate at zero temperature. The Gross-Pitaevskii 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 atom-molecule 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.
40(2014); http://dx.doi.org/10.1063/1.4883895View Description Hide Description
The dynamics of the angular momentum of ferromagnets is discussed with the crystal field and the spin-orbital 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 square-kagome quantum Heisenberg antiferromagnet at high magnetic fields: The localized-magnon paradigm and beyond40(2014); http://dx.doi.org/10.1063/1.4881184View Description Hide Description
We consider the spin-1/2 antiferromagnetic Heisenberg model on the two-dimensional square-kagome lattice with almost dispersionless lowest magnon band. For a general exchange coupling geometry we elaborate low-energy effective Hamiltonians which emerge at high magnetic fields. The effective model to describe the low-energy degrees of freedom of the initial frustrated quantum spin model is the (unfrustrated) square-lattice spin-1/2 XXZ model in a z-aligned magnetic field. For the effective model we perform quantum Monte Carlo simulations to discuss the low-temperature properties of the square-kagome quantum Heisenberg antiferromagnet at high magnetic fields. We pay special attention to a magnetic-field driven Berezinskii-Kosterlitz-Thouless phase transition which occurs at low temperatures.
Universality of the magnetization irreversibility curve of systems with competing interactions (manganites, cobaltites, ferrites)40(2014); http://dx.doi.org/10.1063/1.4883896View Description Hide Description
Experimental 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 Almeida-Thouless 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 Gabay-Toulousse theory. This phenomenon is interpreted in terms of spin-glass polyamorphism.
40(2014); http://dx.doi.org/10.1063/1.4883897View Description Hide Description
The 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.
40(2014); http://dx.doi.org/10.1063/1.4884522View Description Hide Description
The 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 size-quantization 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 short-period superlattice was formed, the envelopes of the electron wave functions on the size-quantization 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 low-temperature lateral conduction in a quantum well tunnel-coupled with a 10-well short-period 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.
40(2014); http://dx.doi.org/10.1063/1.4885062View Description Hide Description
Alternating 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 3ω-component oscillating with the frequency 3ω. In the present paper the amplitude and phase of the V 3ω-component 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 electron-phonon collisions τ e can be found from the frequency dependence of the amplitude and phase of the V 3ω-component.
Structural, electronic and magnetic properties of chiral nanotubes filled with a linear chain of iron40(2014); http://dx.doi.org/10.1063/1.4883884View Description Hide Description
Density-functional 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.
High-frequency conductivity of multilayer graphene and graphite under the conditions of quantum cyclotron resonance40(2014); http://dx.doi.org/10.1063/1.4884523View Description Hide Description
The conductivity tensor of a layered conductor with the Dirac-type energy spectrum of charge carriers placed in a quantizing magnetic field under the condition of normal skin-effect is investigated using the method of quantum kinetic equation. It is shown that under the cyclotron resonance conditions there appear high-temperature quantum oscillations of conductivity, which are weakly sensitive to thermal broadening of the Fermi level. We present the expressions for the classical and high-temperature 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.
40(2014); http://dx.doi.org/10.1063/1.4881185View Description Hide Description
We 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.
Single-component single-partial acoustic surface waves in cubic crystals with surface distortion taken into account40(2014); http://dx.doi.org/10.1063/1.4885575View Description Hide Description
The surface waves and bulk acoustic bands were studied taking into account the interaction between the nearest and next-nearest 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 single-component and single-partial. The calculations were conducted taking into account the discrete nature of crystal lattice for arbitrary values of the two-dimensional wave vector. The analysis has demonstrated that the results obtained in the long-wavelength 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.
40(2014); http://dx.doi.org/10.1063/1.4883891View Description Hide Description
The FTIR spectra of 2-deoxyadenosine (dA) molecules isolated in low-temperature 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 quantum-mechanical 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 syn-conformers 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 syn-conformers with the intramolecular hydrogen bond O5′H–N3 and C3′-endo structure of the deoxyribose ring were fully transformed into the subset of syn-conformers with the C2′-endo structure of the deoxyribose ring. Only two structures from this subset with different rotation angles of the O3′H-group 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.