Polarons in axial transport in single-layer high-T<Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> superconductors

The temperatureT dependencies ρ(T) of normal state electric resistivitiesρ _c (axial) andρ _ab (in plane) of single-layer high-T _c superconductors show common trends: AsT is raised, the resistivity first drops steeply before it starts rising αT above an apparent semiconductor-to-metal crossoverT _cross . To analyze ρ(T) we plottT/ρ againstT at various dopingsx for bothρ _c andρ _ab .T/ρ is inversely proportional to the traversal time across a potential barrier as an ionic particle drifts in an electric field. We findT/ρ in good agreement with theT dependence of the quantum rate of migrating particles: AsT is raised, a zero-point rate at the lowestT is extended to a nearly flat plateau before a thermally activated branch sets in. We also find evidence for the admixture of 1- & 2-phonon absorptions below the Arrhenius range. These features shape the semiconductor-like branch below T _cross . AboveT _cross a metallic-like branch sets in, its αT character deriving from the field coupling of the migrating particle. Our analysis suggests that metal physics may not suffice if ionic features play a role in transport. We attribute our conclusions to the drift of strong-coupling polarons along Cu−O bonds. These “bond polarons” originate from carrier scattering by double-well potentials associated with the bonds. A bond polaron dissociates to a free hole as it passes onto a neighboring O-O link.     

Magnetorefractive effect in La<Subscript>0.7</Subscript>Ca<Subscript>0.3</Subscript>MnO<Subscript>3</Subscript> in the infrared spectral range

The reflection and magnetic reflection spectra, magnetic resistance, electrical properties, and equatorial Kerr effect in La_0.7Ca_0.3MnO₃ crystals have been complexly investigated. The measurements have been performed in wide temperature and spectral ranges in magnetic fields up to 3.5 kOe. It has been found that magnetic reflection is a high-frequency response in the infrared spectral range to the colossal magnetore-sistance near the Curie temperature. Correlation between the field and temperature dependences of the magnetic reflection and colossal magnetoresistance has been revealed. The previously developed theory of the magnetorefractive effect for metallic systems makes it possible to explain the experimental data at the qualitative level. Both demerits of the theory of the magnetorefractive effect in application to the magnets and possible additional mechanisms responsible for the magnetic reflection are discussed.     

Polarization processes in BaBi<Subscript>2</Subscript>Nb<Subscript>2</Subscript>O<Subscript>9</Subscript> and SrBi<Subscript>2</Subscript>Ta<Subscript>2</Subscript>O<Subscript>9</Subscript> ceramics in infralow-frequency fields

The dielectric nonlinearity in BaBi₂Nb₂O₉ and SrBi₂Ta₂O₉ layered ceramics was studied by measuring their polarization loops and reverse dependences of permittivity. It was shown that the features of the dielectric response of BaBi₂Nb₂O₉ and SrBi₂Ta₂O₉ in strong fields can be explained by glass-like properties and the contribution of the domain structure of the ferroelectric material to repolarization processes, respectively.     

Josephson effect in S<Subscript>F</Subscript>XS<Subscript>F</Subscript> junctions

We investigate the Josephson effect in S_FXS_F junctions, where S_F is a superconducting material with a ferromagnetic exchange field, and X is a weak link. The critical current I_c increases with the (antiparallel) exchange fields, if the distribution of transmission eigenvalues of the X layer has its maximum weight at small values. This exchange-field enhancement of the supercurrent does not exist if X is a diffusive normal metal. At low temperatures, there is a correspondence between the critical current in an S_FIS_F junction with collinear orientations of the two exchange fields, and the AC supercurrent amplitude in an SIS tunnel junction. The difference in the exchange fields h₁-h₂ in an S_FIS_F junction corresponds to the potential difference V₁-V₂ in an SIS junction; i.e., the singularity in I_c [in an S_FIS_F junction] at |h₁-h₂|=Δ₁+Δ₂ is the analogue of the Riedel peak. We also discuss the AC Josephson effect in S_FIS_F junctions.     

ρ-ω mixing in J/ø→VP decays

The study of ρ-ω mixing has mainly focused on vector meson decays with isospin I=1, namely the ρ(ω)→π⁺π^- process. In this paper, we present a study of ρ-ω mixing in ρ(ω)→π⁺π^-π⁰ (I=0) using a flavor parameterization model for the J/ø→ VP process. By fitting a theoretical framework to PDG data, we obtain the SU(3)-breaking effect parameters s_V=0.03±0.12, s_P=0.17±0.17 and the ρ-ω mixing polarization operator ∏_ρω=(0.006±0.011) GeV². New values are found for the branching ratios when the mixing effect is incorporated: Br(J/ψ→ ωπ⁰) = (3.64 ±0.37)×10^-4, Br(J/ψ→ ωη) = (1.48 ±0.17)×10^-3, Br(J/ψ→ ωη') = (1.55±0.56)×10^-4, these are different from the corresponding PDG2012 values by 19%, 15% and 15%, respectively.     

Phonons in ZrB<Subscript>12</Subscript>

The first results of measuring dispersion curves of acoustic phonons in the ZrB₁₂ superconductor are reported. The measurements have been conducted by the inelastic neutron scattering method on an ATOS three-axis spectrometer (IR-8, Russian Research Centre “Kurchatov Institute,” Moscow) at room temperature with a single crystal grown using a 99.5%-enriched ¹¹B isotope. The results obtained have been analyzed in terms of the lattice dynamics model proposed earlier for rare-earth dodecaborides. In this model, the constants of the force interaction of Zr atoms with one another and with the boron framework have been determined.     

Phase transition in YbAl<Subscript>3</Subscript>C<Subscript>3</Subscript>

¹⁷⁰Yb M?ssbauer spectroscopy, temperature dependent X-ray, magnetisation and specific heat data are presented in the hexagonal intermetallic YbAl₃C₃, in order to shed light on the isostructural transition occurring near 80 K and to investigate the electronic state of the Yb ion above and below the transition. In the low temperature phase, we find that there occurs an atomic rearrangement in the hexagonal unit cell, leading to a strong symmetry lowering at the Yb site. We show that no magnetic ordering of the Yb³⁺ moments occurs down to 0.04 K, and we discuss this finding in terms of 4f-conduction electron hybridisation and geometric frustration.     

Magnetic phases in UNi<Subscript>2</Subscript>Si<Subscript>2</Subscript>

The tetragonal compound UNi₂Si₂ exhibits in zero magnetic field three different antiferromagnetic phases belowT _N =124 K. They are formed by ferromagnetic basal planes, which are antiferromagnetically coupled along thec-axis with the propagation vectorq=(0, 0, q _z ). Two additional order-order magnetic phase transitions are observed below T _N , namely atT ₁=108 K and T ₂=40 K in zero magnetic field. All three phases exhibit strong uniaxial anisotropy confining the U moments to a direction parallel to the c-axis. UNi₂Si₂ single crystals were studied in detail by measuring bulk thermodynamic properties, such as thermal expansion, resistivity, susceptibility, and specific heat. A microscopic study using neutron diffraction was performed in magnetic fields up to 14.5 T parallel to the c-axis, and a complex magnetic phase diagram has been determined. Here, we present the analysis of specific-heat data measured in magnetic fields up to 14 T compared with the results of the neutron-diffraction study and with other thermodynamic properties of UNi₂Si₂.     

Many-body effects in hyperfine interactions in <Superscript>205</Superscript>Pb<Superscript>+</Superscript>

Ab initio calculations have been carried out to study the magnetic dipole and electric quadrupole hyperfine structure constants of ²⁰⁵Pb⁺. Many-body effects have been considered to all orders using the relativistic coupled-cluster theory in the singles, doubles and partial triples approximation. The trends of these effects are found to be different from atomic systems that have been studied earlier.     

Manifestation of a ferroelectric phase transition in Li<Subscript>0.12</Subscript>Na<Subscript>0.88</Subscript>Ta<Subscript>0.2</Subscript> Nb<Subscript>0.8</Subscript>O<Subscript>3</Subscript> in the Raman spectrum

The ferroelectric-antiferroelectric phase transition in a ceramic solid solution Li_0.12Na_0.88Ta_0.2Nb_0.8O₃ at 350°C was studied using Raman spectroscopy. A considerable broadening of the lines referred to the translational vibrations of the cations in octahedral and cuboctahedral voids and to the vibrations of the oxygen framework, along with a decrease to zero of the intensity of the line corresponding to the bridge stretching mode of the oxygen atoms from the octahedral anion BO₆, was found to take place as the temperature of the solid solution approaches the transition point from below. It is shown that, during the transition, the solid solution loses its ferroelectric properties, probably owing to the preferential increase in the anharmonicity of the vibrations of the cations in the octahedral voids.     

Cluster formation in LiNi<Subscript>0.4</Subscript>Fe<Subscript>0.6</Subscript>O<Subscript>2</Subscript>

The structure of an LiNi_0.4Fe_0.6O₂ cubic solid solution is determined using magnetic measurements and electron diffraction. It is found that this solid solution has a microinhomogeneous structure due to the formation of superparamagnetic clusters. The electron diffraction analysis of LiNi_0.4Fe_0.6O₂ samples has revealed diffuse scattering characteristic of the substitutional short-range order in ordered solid solutions with a B1-type structure. It is shown that the short-range order is associated with the LiNiO₂-type rhombohedral superstructure (space group \(R\bar 3m\)), i.e., with the redistribution of lithium and nickel atoms in the (111)_B1 alternating planes. The short-range order is observed in regions with a nickel content higher than the mean nickel content corresponding to the macroscopic composition.     

Manifestation of a ferroelectric-antiferroelectric phase transition in Li<Subscript>0.12</Subscript>Na<Subscript>0.88</Subscript>Ta<Subscript>0.4</Subscript>Nb<Subscript>0.6</Subscript>O<Subscript>3</Subscript> in its Raman scattering spectra

The ferroelectric-antiferroelectric phase transition in the Li_0.12Na_0.88Ta_0.4Nb_0.6O₃ ceramic solid solution has been studied by the Raman scattering technique. As the temperature approached the transition point from below, we observed an appreciable broadening of the lines associated with the vibrations of the cations occupying octahedral and cubooctahedral cavities of the structure and with the oxygen network vibrations (which implies a substantial increase in disorder on the cation sublattices), as well as a decrease to zero intensity of the 875-cm^?1 line corresponding to stretch vibrations of the bridging oxygen in the BO₆ octahedral anion in the vicinity of the transition. The temperature dependence of the 875-cm_?1 line intensity near the transition was used to study the behavior of the phase transition order parameter η. The behavior of η was found to disagree markedly with the Landau theory of second-order phase transitions. It is shown that discrepancies originate from the increase in disorder in the niobium and tantalum sublattices in the Li_0.12Na_0.88Ta _y Nb_1-y O₃ solid solution system with increasing y. The order of the transition is lowered.     

Resonance Frequency and NMR Relaxation Times in Two Inequivalent <Superscript>133</Superscript>Cs in Cs<Subscript>2</Subscript>CuBr<Subscript>4</Subscript> and Cs<Subscript>2</Subscript>ZnBr<Subscript>4</Subscript> Single Crystals

The resonance frequencies and relaxation mechanisms of Cs₂CuBr₄ and Cs₂ZnBr₄ were examined by static nuclear magnetic resonance (NMR) method. Here, the two inequivalent Cs(1) and Cs(2) sites surrounded by Br ions were distinguished. The saturation recovery traces for ¹³³Cs nuclei in Cs₂CuBr₄ with the paramagnetic ions, and those in Cs₂ZnBr₄ without the paramagnetic ions were each fitted by four exponential functions. From these results, the spin–lattice relaxation times T₁ in the laboratory frame of ¹³³Cs nuclei in the two crystals were obtained, and Cs(1) surrounded by 11 bromide ions has a longer relaxation time than Cs(2) surrounded by 9 bromide ions.     

Current-phase relation in SFS Josephson junctions in the presence of <Emphasis Type="Italic">s</Emphasis>-<Emphasis Type="Italic">d</Emphasis> scattering

The analytical equations for the current-phase relation (CPR) coefficients for Josephson junctions with a ferromagnetic interlayer in the model where the scattering from s- to d-band is considered to be the main type scattering of charge carrier in a ferromagnet are obtained. It is shown that the magnitude of the coefficients oscillates and decays with an increase of the interlayer thickness. Both the oscillation period and the decay length for the first and the second harmonics differ by two times. The temperature dependence of the critical current demonstrating 0-π temperature transitions is obtained. The possibility of design a flux qubit based on such Josephson junctions is demonstrated.     

Magnetotransmission in a Magnetostrictive CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Crystal in the Faraday Geometry

An analytical approach that makes it possible to reconstruct the current–phase relation (CPR) in Josephson structures included in one of the arms of a two-junction superconducting quantum interference device (SQUID), where the second junction has a significantly higher critical current and a known (sinusoidal) CPR, has been developed. The developed methods of analytical and numerical studies of current–flow transformations in two-junction SQUIDs make it possible to reconstruct the CPR of a junction with a low critical current taking into account both the existence of the self-inductance of the interferometer contour and a possible asymmetry in the supply current system. The efficiency of this approach has been confirmed by the experimental study of niobium–aluminum/aluminum oxide–niobium test structures with the known CPR.     

On the problem of nucleation (cell formation) in self-organization of protein nanostructures <Emphasis Type="Italic">in vitro</Emphasis> and <Emphasis Type="Italic">in vivo</Emphasis>

No convincing theory or hypothesis concerning the origin of biological cells exists today. Insight into the problem is difficult, because an empiric model of cell origination and division at the crucial phase of life, self-organization of protein nanostructures, is lacking. It has been shown experimentally that protein nanostructures exhibit signs of self-organization when an open far-from-equilibrium protein-water system condenses in vitro. In other words, to be active, protein must be in the nonequilibrium state. Such a form of self-organization is accompanied by nucleation and the formation of defects, which divide the protein film into domains (“cells”) with nuclei. This type of structuring in the nonequilibrium (active) protein may be viewed as a crude empiric model of protein nucleation, since it includes the formation and division (self-organization) of biological cells, the origination of which, in turn, is intimately related to the self-organization of protein at the nanolevel. The reason for the similarity of the basic processes is identical conditions of protein condensation in vitro and in vivo. In both cases, when water evaporates rapidly from an open water-protein system that is far from thermodynamic equilibrium, the conditions necessary for protein nonequilibrium nanostructures be self-organized with nucleation in the form of nucleus-containing “cells” are set.     

Isotropic positive magnetoresistance in Co-Al<Subscript>2</Subscript>O<Subscript>n</Subscript> nanocomposites

The magnetotransport properties of Co_x(Al₂O_n)_{100 ? x} nanocomposites were studied in a wide concentration range (34 ≤ x ≤ 74 at %). Negative tunnel magnetoresistance reaching 6.5% in a field of 10 kOe was established. In addition to the negative magnetoresistance, the Co_x(Al₂O_n)_{100 ? x} composites were found to exhibit positive magnetoresistance reaching 1.5% in fields of 10 kOe over the concentration range corresponding to the percolation threshold (54 ≤ x ≤ 67 at %). The positive magnetoresistance is assumed to be due to the simultaneous existence in the composite structure of clusters and individual nanoparticles characterized by different values of the magnetic anisotropy and due to the dipole-dipole interaction between the clusters and nearest neighbor particles.     

Electron momentum density in Cu<Subscript>0.9</Subscript>Al<Subscript>0.1</Subscript>

A reconstruction technique based on the solution of the Radon transform in terms of Jacobi polynomials is used to obtain the 3D electron momentum density, ϱ(p), from nine high-resolution Compton profiles (CPs) for a Cu_0.9Al_0.1 disordered alloy single crystal. The method was also applied to theoretical CPs computed within the Korringa–Kohn–Rostoker coherent potential approximation (KKR-CPA) first-principles scheme for the same nine orientations of the crystal. The experimental ϱ(p) is in satisfactory agreement with the theoretical ϱ(p), shows most details of the Fermi surface (FS) and exhibits electron correlation effects. We comment on the map of the FS obtained by folding the reconstructed ϱ(p) into the first Brillouin zone, which yields the occupation number density, ϱ(k). A test of the validity of data via a consistency condition (within our reconstruction algorithm) as well as the propagation of experimental noise in the reconstruction of both ϱ(p) and ϱ(k) are investigated. Received: 24 October 2001 / Accepted: 20 January 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +48-71/344-10-29, E-mail: samsel@int.pan.wroc.pl     

Low-temperature thermal-expansion anomaly in Bi<Subscript>2</Subscript>Sr<Subscript>2</Subscript>CuO<Subscript>6</Subscript>

Single-crystal samples of the Bi_{2 + x}Sr_{2 ? x ? y}Cu_{1 + y}O_{6 + δ} system revealed anomalous (negative) thermal expansion in the temperature range 10–20 K. Magnetic fields of 1–3 T were found to strongly affect the position and width of the anomaly region. A thermal-expansion singularity was detected at temperatures T≈30–50 K, which may be related to the formation of a pseudogap.     

Symmetry-dependent Mn-magnetism in Al<Subscript>69.8</Subscript>Pd<Subscript>12.1</Subscript>Mn<Subscript>18.1</Subscript>

We investigated the stability of magnetic moments in Al_69.8Pd_12.1Mn_18.1. This alloy exists in both, the icosahedral (i) and the decagonal (d) quasicrystalline form. The transition from the i- to the d-phase is achieved by a simple heat treatment. We present the results of measurements of the ²⁷Al NMR-response, the dc magnetic susceptibility, and the low-temperature specific heat of both phases. In the icosahedral compound, the majority of the Mn ions carries a magnetic moment. Their number is reduced by approximately a factor of two by transforming the alloy to its decagonal variety. For both compounds, we have indications for two different local environments of the Al nuclei. The first reflects a low density of states of conduction electrons and a weak coupling of the Al nuclei to the Mn-moments. The second type of environment implies a large d-electron density of states at the Fermi level and a strong coupling to the magnetic Mn moments. Spin-glass freezing transitions are observed at T^deca_f=12 K for the decagonal, and T^ico_f=19 K for the icosahedral phase.