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Top 20 Most Read Articles
April 2012
The 20 articles with the most full-text downloads during the month, in descending order.
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Low Temp. Phys. 28, 17 (2002); http://dx.doi.org/10.1063/1.1449180 (8 pages)
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Fluctuation conductivity in YBa2Cu3O7−y (YBCO) films with different oxygen concentrations is investigated. All of the samples exhibit a clear transition from the Maki–Thompson mechanism of scattering of fluctuational pairs to the Aslamazov–Larkin mechanism as the temperature approaches Tc. The values of the transition temperature are used to determine the coherence length ξc(0) along the c axis and the phase relaxation time τφ(100K) of the fluctuational pairs. Despite the decrease of Tc from 87.4 to 54.2 K with decreasing oxygen content in the samples studied, a value τφ(100K) = (3.35±0.01)×10−13 s is obtained for all the samples. It is shown that the dependence of ξc(0) on Tc conforms to the standard theory of superconductivity. The mechanisms for the scattering of charge carriers and for superconducting pairing in high-Tc superconductors are analyzed. © 2002 American Institute of Physics. |
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Macroscopic thermal entanglement in a spin chain caused by the magnetic field: Inhomogeneity effect Low Temp. Phys. 38, 210 (2012); http://dx.doi.org/10.1063/1.3691529 (6 pages)
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The influence of the inhomogeneity on the macroscopic thermal pairwise entanglement for the system of coupled spins 1/2 (qubits) has been studied. The most important effect of the inhomogeneity on the thermal entanglement is in the new role of the external potential (magnetic field), which can produce nonzero entanglement for qubits, situated not far from the inhomogeneity.
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Low Temp. Phys. 38, 89 (2012); http://dx.doi.org/10.1063/1.3683408 (65 pages)
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Fermi surface properties in rare earth (R) and actinide (An) compounds of RX3 (X: Al, Ga, In, Si, Ge, Sn, Pb), AnX3, RTIn5 (T: transition metal), and AnTGa5 are clarified as a function of the number of valence electrons. The electronic state can be changed by applying pressure and magnetic field. A change of Fermi surface properties associated with the metamagnetic behavior and the quantum critical point is also clarified precisely, together with the split Fermi surfaces in the non-centrosymmetric crystal structure, and the de Haas–van Alphen oscillations in the superconducting mixed state. In the quantum critical point region, the conduction electrons, or quasi-particles, are of an f-electron character, with heavy effective masses of 10–100 m0 (m0 is the rest mass of an electron) and condense into the unconventional (anisotropic) superconducting state.
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Josephson plasma oscillations in confined layered superconductors Low Temp. Phys. 38, 193 (2012); http://dx.doi.org/10.1063/1.3691528 (6 pages)
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Intrinsic electromagnetic oscillations were investigated in layered superconductors of finite dimensions, filling a rectangular resonator. Spectra of both ordinary and extraordinary eigenmodes were obtained. A nonlinear effect of decreasing eigenfrequencies of extraordinary modes was analyzed and generation of the third harmonic of oscillations was studied. A nonlinearity of the system is related to a nonlinear dependence of the Josephson current density across superconducting layers on interlayer phase difference of the order parameter. Josephson plasma waves running along a waveguide filled with a layered superconductor were investigated as well as nonlinear effects appearing during propagations of these waves. In addition, in the work, an effect of the slowing down of terahertz waves in waveguides, which is caused by a mutual effect of nonlinearity and damping of waves is predicted.
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Phase transitions in TbMnO3 manganites Low Temp. Phys. 38, 216 (2012); http://dx.doi.org/10.1063/1.3691530 (5 pages)
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X-ray diffraction and magnetic measurements of polycrystalline and nanosize TbMnO3 manganites were performed. All the compounds studied crystallize in the orthorhombic crystal structure (space group Pnma) at room temperature. Nanosize manganites were synthesized via the sol-gel method at different (800 and 900 °C) temperatures. The average size of the synthesized nanoparticles (from 45 to 70 nm) was estimated by x-ray diffraction and low-temperature adsorption of argon. Information about the evolution of properties of TbMnO3 with changing grain size, temperature, and magnetic field was obtained. Crystal structure parameters of nano-samples change slightly with changing nanoparticle size. Peculiarities of magnetic ordering in polycrystalline and nanosize TbMnO3 were compared. Magnetization and the Nèel temperature corresponding to antiferromagnetic ordering of the Tb3+ sublattice decrease as the particle size is reduced. Inverse magnetic susceptibility of the nanoparticle samples deviates from the Curie–Weiss law below 50 K; this is related to the magnetic ordering of Mn3+ moments. Some peculiar anomalities related to the magnetic ordering of Tb3+ and Mn3+ sublattices were noticed in specific heat values of the nanosize samples.
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Wave functions and energies of bound electron states over liquid helium Low Temp. Phys. 38, 185 (2012); http://dx.doi.org/10.1063/1.3691524 (8 pages)
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Energy eigenvalues and wave functions for two surface states of electrons over liquid helium in the range of pressing electric fields 0 V/cm < E⊥ < 3000 V/cm were found solving wave differential equations by means of expansion method. Values of wave functions at the liquid-vapor interface were obtained using a model potential for image forces and taking into account finiteness of potential barrier for electron penetration into liquid. A comparison between the results obtained and approximate values found using wave functions determined by variational method shows the reliability of this method for calculating matrix elements of electron-ripplon scattering.
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Polaritons in the ferromagnetic material TbMnO3 at a boundary with a metal in a magnetic field Low Temp. Phys. 38, 175 (2012); http://dx.doi.org/10.1063/1.3684278 (6 pages)
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The spectrum of surface polaritons in an antiferromagnetic material with a spiral magnetic structure and spin-induced electrical polarization at a boundary with a metal in a magnetic field is analyzed. The manganite TbMnO3 in its ferroelectromagnetic and paraelectric phases is considered as an example. The resulting spectrum is characterized by a strong nonreciprocity and depends substantially on the directions of the magnetic field and of the spontaneous electrical polarization, as well as on the ratio of these quantities. Estimates for TbMnO3 show that a strong nonreciprocity, such that polaritons with a given frequency propagate only in one direction, can be created by a magnetic field of a few kOe.
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Quantum bits with Josephson junctions (Review Article) Low Temp. Phys. 33, 724 (2007); http://dx.doi.org/10.1063/1.2780165 (21 pages)
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Recent demonstrations of macroscopic quantum coherence in Josephson junction based electronic circuits have opened an entirely new dimension for research and applications in the established field of Josephson electronics. In this article we discuss basic Josephson circuits for qubit applications, methods of quantum description of these circuits, and circuit solutions for qubit couplings. Principles of manipulation and readout of superconducting qubits are reviewed and illustrated with recent experiments using various qubit types.
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Magnetic phase segregation and glass nature of Pr0.4Bi0.3Ca0.3MnO3 manganite Low Temp. Phys. 38, 206 (2012); http://dx.doi.org/10.1063/1.3693578 (4 pages)
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The results of detailed investigations of magnetic properties of Pr0.4Bi0.3Ca0.3MnO3 manganites at low temperatures are presented. The performed investigations of temperature, field and frequency dependences of magnetization and susceptibility as well as the magnetization relaxation processes indicate that below TC = 44 K the manganite Pr0.4Bi0.3Ca0.3MnO3 is in the magnetic phase-segregated state with ferromagnetic inclusions forming a cluster glass.
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Magnetoelectricity in the ferrimagnetic Cu2OSeO3: symmetry analysis and Raman scattering study Low Temp. Phys. 36, 550 (2010); http://dx.doi.org/10.1063/1.3455808 (8 pages)
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sublattices ferrimagnet Cu2OSeO3 with a cubic symmetry and a linear magnetoelectric effect. There is no spectroscopic evidence for structural lattice distortions below TC = 60 K, which are expected due to magnetoelectric coupling. Using symmetry arguments we explain this observation by considering a special type of ferrimagnetic ground state which does not generate a spontaneous electric polarization. Interestingly, Raman scattering shows a strong increase of electric polarization of media through a dynamic magnetoelectric effect as a remarkable enhancement of the scattering intensity below TC. New lines of purely magnetic origin have been detected in the magnetically ordered state. A part of them are attributed as scattering on exchange magnons. Using this observation and further symmetry considerations we argue for strong Dzyaloshinskii–Moriya interaction existing in the Cu2OSeO3.
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Low Temp. Phys. 38, 199 (2012); http://dx.doi.org/10.1063/1.3693577 (7 pages)
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An influence of demagnetizing field on a process of equilibrium magnetization of nanogranular films with perpendicular anisotropy in a tilted magnetic field is studied theoretically. An effect of the canting and flipping of granules magnetic moments on magnetization curves is described. It is shown that in a tilted field the transition from an inhomogeneous state to the state with a uniform orientation of magnetic moments takes place in a critical way.
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Macroscopic quantum phenomena in Josephson structures Low Temp. Phys. 36, 876 (2010); http://dx.doi.org/10.1063/1.3517171 (8 pages)
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The Josephson effect is a probe with unparalleled capabilities for the study of a variety of macroscopic quantum phenomena. This is a survey of important achievements and challenging trends, in particular macroscopic quantum tunneling and energy level quantization. We focus on high-TC superconducting structures and recent research on nanostructures.
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Magnetic structure and resonance properties of a hexagonal lattice of antidots Low Temp. Phys. 38, 157 (2012); http://dx.doi.org/10.1063/1.3684279 (11 pages)
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Static and resonance properties of ferromagnetic films with a hexagonal lattice of antidots (pores in the film) were studied. The description of the system is based on micromagnetic modeling and analytical solutions of the Landau–Lifshitz equation. The dependences of ferromagnetic resonance spectra on the in-plane direction of applied magnetic field and on the lattice parameters were investigated. The nature of the dependences of a dynamic system response on the frequency at fixed magnetic fields and on the field at fixed frequency when the field changes were explored. They cause the static magnetic order to change. It was found that the specific peculiarities of the system dynamics remain unchanged for both of these experimental conditions. Namely, for low damping the resonance spectra contain three quasi-homogeneous modes which are due to the resonance of different regions (domains) of the antidot lattice cell. It is shown that the angular field dependences of each mode are characterized by a twofold symmetry, and the related easy axes are mutually rotated by 60 °. As a result, a hexagonal symmetry of the system’s static and dynamic magnetic characteristics is realized. The existence in the resonance spectrum of several quasi-homogeneous modes related to different regions of the unit cell could be fundamental for the function of the working elements of magnonics devices.
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Edge states in quantum Hall effect in graphene (Review Article) Low Temp. Phys. 34, 778 (2008); http://dx.doi.org/10.1063/1.2981387 (12 pages)
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We review recent results concerning the spectrum of edge states in the quantum Hall effect in graphene. In particular, special attention is paid to the derivation of the conditions under which gapless edge states exist in the spectrum of graphene with “zigzag” and “armchair” edges. It is found that in the case of a half-plane or a ribbon with zigzag edges, there are gapless edge states only when a spin gap dominates over a Dirac mass gap. In the case of a half-plane with an armchair edge, the existence of the gapless edge states depends on the specific type of Dirac mass gaps. The implications of these results for the dynamics in the quantum Hall effect in graphene are discussed.
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Low-temperature specific heat of single crystal bismuth oxyhalides Low Temp. Phys. 37, 326 (2011); http://dx.doi.org/10.1063/1.3592677 (3 pages)
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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.
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Theory of oscillations in STM conductance caused by subsurface defects (Review Article) Low Temp. Phys. 36, 849 (2010); http://dx.doi.org/10.1063/1.3514417 (16 pages)
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In this review we discuss recent theoretical studies of single subsurface defects by means of a scanning tunneling microscope (STM). These investigations are based on quantum interference effects between the electron partial waves that are directly transmitted through the contact and the partial waves scattered by a defect. In particular, we demonstrate the feasibility of imaging the position of a defect below a metal surface by means of STM. Different types of subsurface defects are discussed: point-like magnetic and nonmagnetic defects, magnetic clusters in a nonmagnetic host metal, and nonmagnetic defects in an s-wave superconductor. The effect of Fermi surface anisotropy is analyzed. Studies of the effect of high magnetic fields on the STM conductance of tunnel point contacts in the presence of a single defect are also discussed.
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Excess conductivity of Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ superconductors Low Temp. Phys. 38, 22 (2012); http://dx.doi.org/10.1063/1.3677234 (9 pages)
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Oxide high-Tc superconductors (HTSC) are anisotropic in character since the charge carriers have free moment in the conducting CuO2 planes [A. L. Solov’ev and V. M. Dmitriev, Fiz. Nizk. Temp. 35, 227 (2009) [Low Temp. Phys. 35, 169 (2009)]] whereas their motion is impeded by insulating/partially insulating MBa2O4−δ (M = Y, Bi, Hg, Tl, CuTl, etc.) charge reservoir layers. In the transport process the charge carriers have to tunnel across insulating/partially insulating barriers along the c-axis and across the grain boundaries, which promote a fluctuation in the order parameter and in turn to the conductivity of the carriers. The studies of such fluctuation conductivity (FIC) may help in understanding the intrinsic mechanism of superconductivity. Here the electrical resistivity ρ(T) versus temperature data of as-prepared and oxygen post-annealed Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ
(y = 0, 0.5, 1.5, 2.5) samples is studied for FIC analyses in the temperature regime well above the critical temperature; such analyses have been carried out by employing Lawrence and Doniach (LD) and Maki–Thompson (MT) models. The coherence length, inter-plane coupling, exponent, dimensionality of fluctuations and the phase relaxation time of the carriers are determined from such analyses. It is observed that the crossover temperature associated with two distinct exponents fits very well with the two-dimensional (2D) and three-dimensional (3D) LD equations. The crossover temperature T0 is shifted to higher temperatures with enhanced Zn doping. The 3D LD region is shifted to higher temperature with the increased Zn doping. We have elucidated from these analyses that lower Tl content in the final compound may increase the charge carrier’s doping efficiency of MBa2O4−δ charge reservoir layer, resulting into an increase in the coherence length along the c-axis and superconductivity parameters. A small decrease in the coherence length along the c-axis ξc(0) is observed in the samples with Zn doping of y = 1.5 whereas ξc(0) increases in the samples y = 0.5, 2.5. In comparison with as-prepared samples, the ξc(0) decreases after post-annealing in oxygen atmosphere. It is most likely that a decrease in the density of charge carrier’s is promoted by oxygen diffusion in the unit cell may suppress the ξc(0). The increase oxygen diffusion is evidenced from the softening of phonon modes after post-annealing in oxygen atmosphere. The decreased population of small spins of Cu atoms induced by doping of Zn is viewed in the terms of suppression of spin gap and hence the pseudo-gap in Cu0.5Tl0.5Ba2Ca3Cu4−yZnyO12−δ (y = 0, 0.5, 1.5, 2.5) samples.
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Low Temp. Phys. 36, 489 (2010); http://dx.doi.org/10.1063/1.3455721 (22 pages)
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The results of theoretical and experimental studies of charge transfer (CT) transitions in multiferroic manganites, ferrites, and related insulators are reviewed. Starting with a simple cluster model approach one-center p-d and two-center d-d CT transitions, their polarization properties, the role played by structural parameters, orbital mixing, and spin degree of freedom are all addressed. Optical ellipsometry data in the spectral range of 0.6–5.8 eV in perovskite and hexagonal rare-earth manganites RMnO3 and in orthorhombic manganites RMn2O5 are analyzed. Two groups of iron oxides, where Fe3+ ions occupy either only octahedral positions (BiFeO3, orhoferrites RFeO3 et al.) and materials with Fe3+ ions both in octahedral and tetrahedral positions (hematite α-Fe2O3, garnets RFe5O12, lithium ferrite LiFe5O8, Ca2Fe2O5 et al.), are discussed.
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Superparamagnetic resonance of single-domain nanoparticles of LaSrMnO3 Low Temp. Phys. 33, 433 (2007); http://dx.doi.org/10.1063/1.2737553 (6 pages)
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The microwave absorption of an ensemble of single-domain nanoparticles of La0.7Sr0.3MnO3 is investigated in the temperature range 5–300 K. At low temperatures the resonance spectra demonstrate the appreciable increase in linewidth and downward shift of the resonance field which are typical for superparamagnetic resonance. With increasing temperature the line shape changes, and at T>TB (TB ∼ 100 K) one observes a narrowing of the resonance curve and saturation of the resonance field. The line shape of an individual nanoparticle is described by the dynamic Landau–Lifshitz equations with damping. Modeling of the magnetic dynamics of the system is based on the assumption of a random distribution of the directions of the magnetic moments and of the thermal fluctuations of the direction of the anisotropy axis of the particles. The theory takes into account the dependence of the value of the resonance field on the linewidth.
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Micromechanical properties of C70 single crystals in the temperature range 77–350 K Low Temp. Phys. 38, 227 (2012); http://dx.doi.org/10.1063/1.3693580 (8 pages)
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Hexagonal single crystals of C70 down to 1–2 mm in size were grown, which allowed the investigation of their low-temperature mechanical properties for the first time. Morphology, microplasticity anisotropy, and the temperature dependence of Vickers microhardness HV (T) of the C70 crystals involving all known phase transitions were studied with the aid of optical microscopy and microindentation in the temperature range 77–350 K. The association of the features of HV (T) dependence with orientation phase transformations was analyzed. It is suggested that microplasticity anisotropy of the C70 crystals correlates with the active slip systems.
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