Showing papers in "Low Temperature Physics in 2002"
TL;DR: In this paper, the authors investigated the fluctuation conductivity in YBa2Cu3O7−y (YBCO) films with T≈80 K. Unlike the optimally doped samples, these films exhibit a transition from a Maki-Thompson (MT) mechanism for scattering of fluctuational pairs to a Lawrence-Doniach (LD) mechanism and, as the temperature approaches Tc, to an Aslamazov-Larkin (AL) mechanism.
Abstract: The fluctuation conductivity in YBa2Cu3O7−y (YBCO) films with T≈80 K is investigated for the first time. Unlike the optimally doped samples, these films exhibit a transition from a Maki–Thompson (MT) mechanism for the scattering of fluctuational pairs to a Lawrence–Doniach (LD) mechanism and, as the temperature approaches Tc, to an Aslamazov–Larkin (AL) mechanism. It is shown that the coherence length ξc(0) along the c axis and the phase relaxation time τφ(100 K) of the fluctuational pairs are determined by the temperature of this second transition. The features observed on both the fluctuation conductivity and resistive behavior are sharply enhanced as Tc approaches 80 K, probably because of a significant increase in the intensity of the magnetic interaction in high-Tc superconductors at these temperatures. In spite of this, for two samples a value τφ(100 K)=(3.35±0.01)×10−13 s is measured, i.e., the same value as for optimally doped YBCO films. It is shown that the dependence of ξc(0) on Tc obeys the standard theory of superconductivity. The mechanisms for the scattering of charge carriers and the superconducting pairing in YBCO are analyzed.
58 citations
TL;DR: In this article, a review is given of the experimental and theoretical research on the magnetic properties and magnetic phase diagrams of the intermetallic compounds RMn2Ge2, including measurements in high and ultrahigh magnetic fields.
Abstract: The magnetic and other physical properties of the ternary intermetallic compounds RMn2Ge2 (R is a rare earth) are of great interest on account of effects due to the coexistence of the 3d (manganese) and 4f (rare-earth) magnetic subsystems. The layered structure, the high sensitivity of the exchange parameters to the interatomic distances, the antiferromagnetic exchange interaction in the manganese subsystem in intermetallides containing heavy rare earths, and also the appreciable crystal-field effects in the rare-earth subsystem make for complex magnetic phase diagrams in these compounds. A review is given of the experimental and theoretical research on the magnetic properties and magnetic phase diagrams of the intermetallic compounds RMn2Ge2, including measurements in high and ultrahigh magnetic fields. A theoretical model is proposed which takes into account the features of the crystal structure and the hierarchy of exchange interactions in intermetallides containing heavy rare earths; this model is in ...
53 citations
TL;DR: In this article, the complete Raman spectra of a single crystal of LiNiPO4 for a wide temperature range are reported, and the spectra are successfully analyzed in terms of internal modes of the (PO4)3− group and external modes.
Abstract: The complete Raman spectra of a single crystal of LiNiPO4 for a wide temperature range are reported. Of the 36 Raman-active modes predicted by group theory, 33 have been detected. The spectra are successfully analyzed in terms of internal modes of the (PO4)3− group and external modes. Multiphonon Raman scattering is also discussed. Low-frequency lines, observed in the antiferromagnetic phase, are assigned to magnon scattering and are discussed briefly.
49 citations
TL;DR: In this paper, the magnetic field and temperature dependence of the critical current density jc in epitaxial thin films of HTSCs were investigated, and it was shown that an approximation of the give...
Abstract: The magnetic-field dependence (in a field H applied along the normal to the film plane z) and temperature dependence of the critical current density jc are investigated in high-quality biaxially oriented epitaxial films of YBa2Cu3O7−δ, with maximum values of jc reaching 2×106 A/cm2 in zero magnetic field at T=77 K. It is found that the value of jc(Hz,T), which is independent of Hz in the low-field region Hz Hm by the function jc(Hz,T)/jc(0,T)=α ln(H*/Hz) over a rather wide region 0.95>jc(Hz,T)/jc(0,T)>0.3. Here H*=Hme1/α is proportional to τ=1−T/Tc, at least in the temperature region where the measurements are made, and the parameter α is nearly independent of temperature. For the sample with the highest value of the critical current density for Hz→0 an anomalously sharp transition from the low-field plateau to the logarithmic behavior is observed. Some published data on jc(Hz,T) in epitaxial thin films of HTSCs are analyzed, and it is shown that an approximation of the give...
33 citations
TL;DR: In this article, the microscopic mechanisms for single-particle core pinning of quantum vortices on the insulating cores of isolated dislocations are investigated for both parallel and mutually tilted orientations of the vortex and dislocation.
Abstract: The microscopic mechanisms for single-particle core pinning of quantum vortices on the insulating cores of isolated dislocations are investigated for both parallel and mutually tilted orientations of the vortex and dislocation and also for pinning on an infinite periodic chain of edge dislocations. The theoretical results are consistent with experiment in regard to the temperature and orientation dependences of the depinning critical current in low magnetic fields, when the interaction between vortices is exponentially small, and they also predict strong anisotropy of the pinning force along and transverse to the chains of edge dislocations.
32 citations
TL;DR: In this paper, the temperature dependence of the four Raman-active phonons in NiF2 was investigated at temperatures above and below the antiferromagnetic ordering temperature of TN=73 K.
Abstract: The temperature dependence of the four Raman-active phonons in NiF2 is investigated at temperatures above and below the antiferromagnetic ordering temperature of TN=73 K All four modes exhibit distinct anomalies in their intensities and frequencies near TN due to spin–phonon coupling The phonon linewidths also exhibit weak anomalies From the temperature dependences of the phonon frequencies, estimates are made of the spin–phonon coupling coefficients The B1g phonon exhibits anomalous mode softening with decreasing temperature from 300 K to TN
32 citations
TL;DR: In this article, the causes of the formation of domain walls with a vortex structure, their energy, and their static properties are analyzed as functions of the film parameters, and the influence of the surface anisotropy and external fields on these properties is elucidated.
Abstract: The causes of the formation of domain walls with a vortex structure, their energy, and their static properties are analyzed as functions of the film parameters. The influence of the surface anisotropy and external fields on these properties is elucidated. It is it is shown that that both one-and two-vortex symmetric and asymmetric walls can be stable. The experimental evidence for the existence of asymmetric walls is presented. A review is given of the dynamic properties of walls with a vortex structure. The fields at which catastrophic changes of their structure occur with changing field (the bifurcation fields) are indicated. The mechanisms of dynamic transformation of the internal structure of the walls, in particular, the mechanisms involving the creation and annihilation of vortexlike wall formations, are examined, and the tunneling of these formations through the center of the wall is discussed. The transformations of the wall due to the onset of subperiodic magnetization oscillations are described....
30 citations
TL;DR: In this article, an analysis of Cr3+ exciton absorption in RCrO3 (R=Tb, Dy, and Ho) was performed and it was shown that these compounds exhibit an anomalous spin reorientation in a magnetic field along the b axis, where the weak ferromagnetic moment of the R3+ spins rotates in the ac plane perpendicular to the baxis.
Abstract: Various kinds of magneto-optical properties are investigated in rare earth orthochromites. From an analysis of Cr3+ exciton absorption in RCrO3 (R=Tb, Dy, and Ho), it is unferred that these compounds exhibit an anomalous spin-reorientation in a magnetic field along the b axis, where the weak ferromagnetic moment of the Cr3+ spins rotates in the ac plane perpendicular to the b axis. In these compounds, when the R3+ spin configuration is disordered, an anomalous satellite band appears on the lower-energy side of the Cr3+ exciton absorption, which is associated with the breakdown of the k=0 selection rule due to the disorder of the R3+ spin configuration. In YbCrO3, various kinds of cooperative excitations, such as a Cr3+ exciton coupled with an Yb3+ magnon and a Cr3+−Yb3+ exciton molecule, which are induced by the antisymmetric exchange interaction between the Cr3+ and Yb3+ spins, appear in the visible region. The propagation of these cooperative excitations depends strongly on the spin structure and the ex...
25 citations
TL;DR: In this paper, the low-temperature plasticity and fracture of polycrystals of coarse-grained (CG) and nanostructural (NS) technical-grade titanium of two structural modifications with grain size 0.3 and 0.1 μm, which were prepared by equal channel angular pressing (ECAP) with additional thermomechanical treatment are studied.
Abstract: The low-temperature plasticity and fracture of polycrystals of coarse-grained (CG) and nanostructural (NS) technical-grade titanium of two structural modifications with grain size 0.3 and 0.1 μm, which were prepared by equal channel angular pressing (ECAP) with additional thermomechanical treatment are studied. The measurements are performed at temperatures 300, 77, and 4.2 K with uniaxial compression at deformation rate 4×10−4 s−1. The “stress-plastic deformation” hardening curves are obtained, the macroscopic yield stress, and the ultimate plasticity are measured for samples with compression axis orientations parallel and transverse to the ECAP axis. It is found that the yield stress for NS titanium is 1.5–2 times higher than for CG titanium and the yield stress on cooling from 300 to 4.2 K. Plasticity anisotropy is also observed in NS titanium—the yield stress is 1.2–1.5 times greater when the compression axis is oriented perpendicular to the ESAP axis than for parallel orientation. The ultimate plasti...
22 citations
TL;DR: In this article, the fluctuation-induced conductivity measured in YBa2Cu3O7−δ (YBCO) films grown on 10° miscut SrTiO3 (001) substrates is analyzed using various theoretical models describing weak fluctuations in high-Tc superconductors and considering both Aslamazov-Larkin and Maki-Thompson fluctuation contributions in the clean limit approach.
Abstract: The fluctuation-induced conductivity (paraconductivity) measured in YBa2Cu3O7−δ (YBCO) films grown on 10° miscut SrTiO3 (001) substrates is analyzed using various theoretical models describing weak fluctuations in high-Tc superconductors and considering both Aslamazov-Larkin and Maki-Thompson fluctuation contributions in the clean limit approach. The analysis reveals a highly anisotropic pair-breaking caused by structural defects produced. This result is in favor of an idea that pseudogap in high-Tc oxydes is mainly governed by the fluctuating pairing.
18 citations
TL;DR: In this paper, the following sequence of phase transformations were observed in the Nd0.6Ca0.4(Mn1−xCrx)O3 system: antiferromagnet (x=0), mixed magnetic state (0.015⩽x⦽0.04), ferrimagnet, spin glass (x = 1), spin glass, and spin glass.
Abstract: The following sequence of phase transformations is observed in the Nd0.6Ca0.4(Mn1−xCrx)O3 system: antiferromagnet (x=0), mixed magnetic state (0.015⩽x⩽0.04), ferrimagnet (0.04⩽x⩽0.8), and spin glass (x=1). At chromium concentrations near x∼0.5 the Neel temperature increases sharply, and the contribution from the Nd sublattice increases substantially. Since the chromium and manganese ions have a tendency to order, these results can be interpreted in a model of superexchange interactions.
TL;DR: In this paper, the sound velocity is measured in polycrystalline MgB2 synthesized from the constituent elements, and the bulk modulus in compression and the Debye temperature are calculated.
Abstract: The sound velocity is measured in polycrystalline MgB2 synthesized from the constituent elements, and the bulk modulus in compression and the Debye temperature are calculated. The conversion of an elastic wave into electromagnetic radiation is investigated in the mixed state. The dynamic parameters of the vortex lattice are estimated.
TL;DR: In this article, the results of a symmetry analysis and of modeling by the method of the phenomenological thermodynamic potential are presented, and the magnetic structure of the crystal in various temperature intervals of the magnetic ordering region is discussed.
Abstract: The results of experimental research on the magnetic and resonance properties, heat capacity, muon spin relaxation, and neutron scattering of single-crystal copper metaborate CuB2O4 are reviewed. The results of a symmetry analysis and of modeling by the method of the phenomenological thermodynamic potential are presented. The magnetic structure of the crystal in various temperature intervals of the magnetic ordering region is discussed.
TL;DR: In this article, the temperature dependence of the magnitude of the weak ferromagnetic moment is measured in the many-sublattice antiferromagnetic magnetoelectric crystal LiCoPO4.
Abstract: The temperature dependence of the magnitude of the weak ferromagnetic moment is measured in the many-sublattice antiferromagnetic magnetoelectric crystal LiCoPO4. The dependence is found to be nonmonotonic—as the temperature is decreased below the Neel temperature TN, the ferromagnetic moment initially increases and then decreases markedly. It reaches its highest value, close to 0.12 G, at a temperature of around 10.5 K, which is roughly TN/2. The possible mechanisms for the appearance of weak ferromagnetism in LiCoPO4 and the causes of the nonmonotonic temperature dependence of the spontaneous magnetic moment are discussed. Among the possible mechanisms, there is a preference for one in which the weak ferromagnetism is due to a nonuniform antiferromagnetic structure which is proposed for this crystal.
TL;DR: In this article, the structure of a Zr−Ti−Cu−Ni−Be bulk metallic glass was investigated by using high-resolution field emission microscopy (FEM).
Abstract: The cluster structure of a Zr–Ti–Cu–Ni–Be bulk metallic glass is investigated by the methods of high-resolution field emission microscopy. Internal interfaces are revealed, having a density of ∼106 cm−1 and a width of approximately 1 nm. It is shown that the binding energy of the atoms at the intercluster boundaries is lower by 0.13–0.43 eV than in the interior of a cluster. Subcluster heterogeneities of the structure of the bulk amorphous alloy are observed, with a characteristic scale of the short-range order equal to 1.5–3 nm, attesting to the diversity of the short-range order in glassy melts.
TL;DR: In this paper, the second and third harmonic optical spectra are studied in epitaxial magnetic thin films in the spectral ranges 1.7-3.2 eV and 2.4-4.2eV, respectively.
Abstract: Second and third harmonic optical spectra are studied in epitaxialmagnetic thin films in the spectral ranges 1.7–3.2 eV and 2.4–4.2 eV, respectively. No significant increase of the intensity of the nonlinear spectra is found above the bandgap near 3.2 eV, where the linear absorption increases by two orders of magnitude. Large magnetic contributions to the second harmonic spectra and magnetic contrast as high as 100% are observed at selected photon energies. Contrary to that, no magnetic contribution to the third harmonic spectra is found.
TL;DR: Theoretical results on lattice excitations in solid oxygen at equilibrium vapor pressure are reviewed with the emphasis on the behavior of phonons, librons, and magnons in the orientationally and magnetically ordered α phase as mentioned in this paper.
Abstract: Theoretical results on lattice excitations in solid oxygen at equilibrium vapor pressure are reviewed with the emphasis on the behavior of phonons, librons, and magnons in the orientationally and magnetically ordered α phase. Results on the magnetooptics of solid oxygen and their impact on the advancement of magnetic studies of solid oxygen are briefly reviewed. Some results under discussion are new: among these are theoretical results on the magnon heat capacity, the analysis of the behavior of librons at the α–β transition, and anisotropy of the magons spectrum.
TL;DR: In this paper, tensile test diagrams of commercial-grade polycrystalline titanium foil are obtained in the temperature range 10-373 K, and corresponding work hardening curves are plotted as stress versus plastic deformation, σ(ep).
Abstract: Tensile test diagrams of commercial-grade polycrystalline titanium foil are obtained in the temperature range 10–373 K, and the corresponding work hardening curves are plotted as stress versus plastic deformation, σ(ep). It is shown that the hardening curves consist of two (at low temperatures) or three (at high temperatures) fragments or stages, which are approximated by the equation σ=const+hepn with constant rheological parameters n and h within each stage, and the empirical values of these parameters are determined over the investigated temperature range. The behavior of the hardening curves is compared with the features of the substructure evolution of the polycrystalline titanium at different stages of the plastic deformation, which are revealed by methods of electron and optical microscopy. The presence of correlations between the individual stages of the hardening and the structural states formed in the samples in the process of deformation is established. The effectiveness of the different types ...
TL;DR: In this paper, the possibility of realization of a metallic ferromagnetic state in a generalized Hubbard model with correlated hopping and exchange interaction integrals is investigated, and an expression for the Curie temperature is found, and the behavior of the temperature dependences of the magnetization and paramagnetic susceptibility is analyzed.
Abstract: The possibility of realization of a metallic ferromagnetic state in a generalized Hubbard model with correlated hopping and exchange interaction integrals is investigated. The single-electron energy spectrum recently obtained by means of the mean-field approximation is applied for the description of ground-state and finite-temperature properties of the system. An expression for the Curie temperature is found, and the behavior of the temperature dependences of the magnetization and paramagnetic susceptibility is analyzed. Taking the correlated hopping into account allows one to explain certain peculiarities of the ferromagnetic behavior of transition metals and their compounds.
TL;DR: In this paper, a study of the formation and annihilation of high-energy phonons in anisotropic phonon systems in superfluid helium was made, and the role of each of these processes in the formation of the distribution function of h phonons was determined.
Abstract: A study is made of relaxation processes in anisotropic phonon systems in superfluid helium. The rates of creation and annihilation of high-energy (h) phonons in all possible processes in an anisotropic phonon system are obtained as functions of momentum, temperature, and the anisotropy parameter. The physical causes of the asymmetry in the creation and annihilation of h phonons are elucidated. All of the processes of interaction of h phonons with low-energy phonons (l phonons) and h phonons with each other are investigated, and the role of each of these processes in the formation of the distribution function of h phonons is determined. All of the processes that do not conserve the total number of h phonons are taken into account in constructing an equation describing the change in energy density of the h phonons in the main beam, and a quasistationary solution of this equation is found. The solution of this equation can be used to find the energy density of the created h phonons at any point in space at a...
TL;DR: In this paper, a reinforcement of pinning in high-Tc superconducting materials by surface magnetic microparticles obtained by ion implantation is observed, and measurements are made by the method of magnetically modulated microwave absorption.
Abstract: A reinforcement of pinning in high-Tc superconducting materials by surface magnetic microparticles obtained by ion implantation is observed. Measurements are made by the method of magnetically modulated microwave absorption. In single crystals of Bi2Sr2CaCu2O8 an ensemble of magnetic particles is formed directly in a subsurface layer of the superconductor by the implantation of Fe+ ions followed by annealing. In experiments with YBa2Cu3O7 the magnetic microparticles of iron were formed individually in SiO2 films which were then applied to the surface of a thin film of YBa2Cu3O7, and measurements were made on the composite structure. In both cases reinforcement of the pinning is observed. In the limits of the chosen implantation regimes the pinning effect becomes stronger with increasing fluence of implanted iron atoms.
TL;DR: The magnetic properties of a LaMn0.46Co0.54O3 single crystal, grown by electrochemical deposition, are studied in a wide range of fields and temperatures as mentioned in this paper.
Abstract: The magnetic properties of a LaMn0.46Co0.54O3 single crystal, grown by electrochemical deposition, are studied in a wide range of fields and temperatures. The two-phase magnetic state model of the crystal is used to analyze the results.
TL;DR: Theoretical and experimental results on the magnetic properties of insulating Van Vleck paramagnets at high magnetic fields are presented in this paper, where the Zeeman energy of an ion becomes comparable to the characteristic energies of the Stark splittings.
Abstract: Theoretical and experimental results are presented on the magnetic properties of insulating Van Vleck paramagnets at high magnetic fields, where the Zeeman energy of an ion becomes comparable to the characteristic energies of the Stark splittings.
TL;DR: The possible mechanisms of superconductivity and of the increase in the critical temperature Tc of the phase transition into the superconducting state in doped cubic and hexagonal C60 fullerine crystals are analyzed in this paper.
Abstract: The possible mechanisms of superconductivity and of the increase in the critical temperature Tc of the phase transition into the superconducting state in doped cubic and hexagonal C60 fullerine crystals are analyzed. It is shown that the intramolecular Jahn–Teller vibrations (which are due to the high symmetry of C60 molecules and fullerene crystals and arise as a result of a change in the charge state of fullerene in an electron-phonon interaction), the degeneracy (multivalley nature) of the structure of narrow bands with a high density of states, and multiparticle Coulomb correlations such as local field effects (which increase the coupling constant in Cooper pariing of current carriers) all play a key role in the mechanism of superconductivity in this molecular metal.
TL;DR: In this article, a theory of the propagation of acoustic waves in a porous medium filled with superfluid helium is developed, and the elastic coefficients in the system of equations are expressed in terms of physically measurable quantities.
Abstract: A theory of the propagation of acoustic waves in a porous medium filled with superfluid helium is developed. The elastic coefficients in the system of equations are expressed in terms of physically measurable quantities. The equations obtained describe all volume modes which propagate in a porous medium saturated with superfluid helium. The propagation velocities of longitudinal and transverse waves are calculated in the limit of high and low oscillation frequencies of the thermodynamic quantities.
TL;DR: In this paper, the spectrum of two-point correlators and two-wave boson fields were analyzed in a quantum bar system with a short-range interwire interaction, and the standard bosonization procedure was shown to be valid, within the two wave approximation.
Abstract: The spectrum of boson fields and two-point correlators are analyzed in a quantum bar system (a superlattice formed by two crossed interacting arrays of quantum wires), with a short-range interwire interaction. The standard bosonization procedure is shown to be valid, within the two-wave approximation. The system behaves as a sliding Luttinger liquid in the vicinity of the Γ point, but its spectral and correlation characteristics have either 1D or 2D nature depending on the direction of the wave vector in the rest of the Brillouin zone. Due to the interwire interaction, unperturbed states propagating along the two arrays of wires are always mixed, and the transverse components of the correlation functions do not vanish. This mixing is especially strong around the diagonals of the Brillouin zone, where the transverse correlators have the same order of magnitude as the longitudinal ones.
TL;DR: In this article, the electrical response of granular Sn-Ge thin films below the superconducting transition temperature was measured and the addition of an external noise to the magnetic field applied to the sample was found to increase the sample voltage response to a small externally applied ac signal.
Abstract: Measurements of the electrical response of granular Sn-Ge thin films below the superconducting transition temperature are reported. The addition of an external noise to the magnetic field applied to the sample is found to increase the sample voltage response to a small externally applied ac signal. The gain coefficient for this signal and the signal-to-noise ratio display clear maxima at particular noise levels. We interpret these observations as a stochastic resonance in the percolative Josephson media which occurs close to the percolation threshold.
TL;DR: The temperature dependence of the amplitude and phase of the electric potential arising at a plane boundary of a conductor when a longitudinal acoustic wave is incident normally on it is investigated theoretically and experimentally as discussed by the authors.
Abstract: The temperature dependence of the amplitude and phase of the electric potential arising at a plane boundary of a conductor when a longitudinal acoustic wave is incident normally on it is investigated theoretically and experimentally. The surface potential is formed by two contributions, one of which is spatially periodic inside the sample, with the period of the acoustic field; the second is aperiodic and arises as a result of an additional nonuniformity of the electron distribution in a surface layer of the metal. In the nonlocal of parameters region the second contribution is dominant. The phases of these contributions are shifted by approximately π/2. For metals found in the normal state the experiment is in qualitative agreement with the theory. The superconducting transition is accompanied by catastrophically rapid vanishing of the electric potential, in sharp contrast to the theoretical estimates, which predict behavior similar to the BCS dependence of the attenuation coefficient for longitudinal sound.
TL;DR: In this paper, it was shown that the occupation of the nonequivalent cation sites of the crystal lattice by the activator ions is irregular and that the quenching of the luminescence of the Pr3+ ions is due to the formation of dimers of these ions.
Abstract: It is shown on the basis of studies of the optical spectra and luminescence decay of nonequivalent Pr3+ optical centers in the Y2SiO5 crystal that the occupation of the nonequivalent cation sites of the crystal lattice by the activator ions is irregular. For excitation in the region of the optical transitions 3H4→1D2in the temperature interval 6–80 K there is no transfer of electronic excitation energy between Pr3+ ions localized in different cation sites. The luminescence decay law of the Pr3+ optical centers is determined by the concentration of activator ions and has a complicated non-single-exponential character; the quenching of the luminescence of the Pr3+ ions is due to the formation of dimers of these ions. © 2002 American Institute of Physics.
TL;DR: In this paper, a model of the dynamics of collective excitations of two-dimensional close-packed atomic crystal lattices (atomic monolayers on substrates) is proposed.
Abstract: A model is proposed which gives an analytical description of the dynamics of collective excitations of two-dimensional close-packed atomic crystal lattices (atomic monolayers on substrates). The model takes into account both the interaction between atoms of the layer and the interaction of the layer with the substrate. The phonon spectra are found for an ideal triangular lattice and for a triangular lattice with a uniform distortion along one of the close-packed directions in the plane of the layer. The temperature dependence of the heat capacity is constructed for crystalline structures of both the commensurate and incommensurate types (in relation to the substrate). The theoretical results obtained are used for a detailed discussion and interpretation of the published experimental data on the spectra of lattice excitations and the heat capacity of monolayers of rare gases, including 3He and 4He, on various types of substrates.