Structure of the Abrikosov vortex lattice in a thin superconducting film in a parallel magnetic field

The structure of a vortex lattice in thin (d<λ, where d is the film thickness and λ is the London penetration depth) superconducting films is investigated in a magnetic field parallel to the film surface. It is shown that the stable configuration has the form of discrete vortex rows whose number changes discretely with an increase in the applied magnetic field. The entry fields H _c1 ^(N) (d) for vortex rows are calculated for N=1, 2. It is shown that the structural transition in the vortex ensemble is a second-order phase transition. A simpler method (as compared to the Monte Carlo technique) is proposed for calculating the vortex lattice parameters.     

Monte Carlo investigation of magnetic properties of a two-dimensional plane-rotator model

The equilibrium properties of a simple quadratic lattice of plane rotators with nearest-neighbor isotropic interactions have been examined by the Monte Carlo method. It was found that the 2-d plane-rotator model shows much the same magnetic properties as those of the 2-d Heisenberg model if temperature is scaled in units of T_M, the transition temperature predicted by the mean-field theory. It is pointed out that the present results are closely related to the prediction of Zittartz of a phase transition of continuous order.     

Cuprate superconductors in the vicinity of a Pomeranchuk instability

We propose that cuprate superconductors are in the vicinity of a spontaneous d-wave type Fermi surface symmetry breaking, often called a d-wave Pomeranchuk instability. This idea is explored by means of a comprehensive study of magnetic excitations within the slave-boson mean-field theory of the t-J model. We can naturally understand the pronounced xy anisotropy of magnetic excitations in untwinned YBa₂Cu₃O_y and the sizable change of incommensurability of magnetic excitations at the transition temperature to the low-temperature tetragonal lattice structure in La_2-xBa_xCuO₄. In addition, the present theoretical framework allows the understanding of the similarities and differences of magnetic excitations in Y-based and La-based cuprates.     

Kinetics of a mixed spin-1/2 and spin-3/2 Ising ferrimagnetic model

We present a study, within a mean-field approach, of the kinetics of a mixed ferrimagnetic model on a square lattice in which two interpenetrating square sublattices have spins that can take two values, , alternated with spins that can take the four values, . We use the Glauber-type stochastic dynamics to describe the time evolution of the system with a crystal-field interaction in the presence of a time-dependent oscillating external magnetic field. The nature (continuous and discontinuous) of transition is characterized by studying the thermal behaviors of average order parameters in a period. The dynamic phase transition points are obtained and the phase diagrams are presented in the reduced magnetic field amplitude (h) and reduced temperature (T) plane, and in the reduced temperature and interaction parameter planes, namely in the (h, T) and (d, T) planes, d is the reduced crystal-field interaction. The phase diagrams always exhibit a tricritical point in (h, T) plane, but do not exhibit in the (d, T) plane for low values of h. The dynamic multicritical point or dynamic critical end point exist in the (d, T) plane for low values of h. Moreover, phase diagrams contain paramagnetic (p), ferromagnetic (f), ferrimagnetic (i) phases, two coexistence or mixed phase regions, (f+p) and (i+p), that strongly depend on interaction parameters.     

Josephson vortex lattice melting in Bi-2212

The B-T diagram of Josephson vortex lattice melting in Bi-2212 is analyzed (B is magnetic induction parallel to the layers, T is temperature). It is shown that the Josephson vortex lattice melting at B > B* = 0.6–0.7 T is associated with Berezinsky-Kosterlitz-Thouless transition in individual Bi-2212 superconducting layers and is a second-order phase transition.     

Correlation between the Hall coefficient,penetration depth,transition temperature,gap anisotropy and hole concentration in layered high-temperature superconductors

We explore the correlation between the Hall coefficient, penetration depth, transition temperature, gap anisotropy and hole concentration in a tight binding model for layered high-temperature superconductors. By adopting the BCS strategy, nearest neighbor intralayer singlet and extendeds-wave pairing cut off at the hole Fermi energy, remarkable agreement with generic experimental facts is obtained. Thus, a cylindrical hole Fermi surface and intralayer extendeds-wave singlet pairing appear to be generic features of the cuprate semiconductors. For high doping levels, however, intralayerd-wave singlet pairing appears to be the stable phase.     

The competition and coexistence of antiferromagnetism and <Emphasis Type="Italic">d</Emphasis>-wave superconductivity: probing coupled thermal fluctuations in a two dimensional minimal model

We consider the coexistence of antiferromagnetism and d-wave superconductivity, motivated by what one observes in the quasi-two dimensional organic salts. We study an electronic model that approximates some features of the Hubbard model, e.g., a repulsion that promotes local moments and Neel order, and an attractive intersite density–density coupling that promotes d-wave superconductivity. Staying at half-filling and a fixed attractive interaction we probe the effect of varying repulsion, using mean field theory for the ground state but retaining the full O(3) × U(1) spectrum of classical fluctuations at finite temperature. The ground state is superconducting at weak repulsion, a Neel ordered insulator at large repulsion, and a coexistence of the two orders in the intermediate regime. We observe four distinct kinds of thermal behaviour depending on the strength of repulsion. Starting with weak repulsion these are, first, a d-wave superconductor renormalised by magnetic fluctuations, second, a d-wave state transiting to an antiferromagnetic insulator and then to the normal state, third, a coexistent state transiting to the antiferromagnetic insulator and then the normal state, and, fourth, a Neel ordered insulator with weak pairing fluctuations. The low temperature state is either “nodal” or gapped, due to long range order, and the low energy spectral weight generally increases monotonically with temperature. At intermediate repulsion, however, the transition from the d-wave state to Neel antiferromagnet causes a loss of low energy weight which is gradually regained only at high temperature.     

Size effects in electrical and magnetic properties of quasi-one-dimensional tin wires in asbestos

Bulk composites have been prepared based on one-dimensional fibers of natural chrisothil-asbestos with various internal diameters (d = 6–2.5 nm) filled with tin. The electrical and magnetic properties of quasi-one-dimensional Sn wires have been studied at low temperatures. The electrical properties have been measured at T = 300 K at a pressure P = 10 kbar. It has been found that the superconducting (SC) characteristics of the nanocomposites (critical temperature T_c and critical magnetic field H_c) increase as the Sn filament diameter decreases. The temperature spreading of the resistive SC transition also increases as the Sn filament diameter decreases, which is explained by the SC order parameter fluctuations. The size effects (the increase in critical temperature T_c and transition width ΔT_c) in Sn nanofilaments are well described by the independent Aslamazov–Larkin and Langer–Ambegaokara fluctuation theories, which makes it possible to find the dependence of T_c of the diffuse SC transition on the nanowire diameter. Using the temperature and magnetic-field dependences of the magnetic moment M(T, H), it has been found that the superconductor–normal metal phase diagram of the Sn–asbestos nanocomposite has a wider region of the SC state in T and H as compared to the data for bulk Sn. The magnetic properties of chrisotil-asbestos fibers unfilled with Sn have been studied. It has been found that the Curie law is fulfilled and that the superparamagnetism is absent in such samples. The obtained results indicate the absence of magnetically ordered impurities (magnetite) in the chrisotil-asbestos matrix, which allowed one to not consider the problem of the interaction of the magnetic subsystem of the asbestos matrix and the superconducting subsystem of Sn nanowires.     

Phase structure of a U(1) lattice gauge theory with dual gauge fields

We introduce a U(1) lattice gauge theory with dual gauge fields and study its phase structure. This system is partly motivated by unconventional superconductors like extended s-wave and d  -wave superconductors in the strongly-correlated electron systems and also studies of the t–J$t\text{–}J$ model in the slave-particle representation. In this theory, the “Cooper-pair” (or RVB spinon-pair) field is put on links of a cubic lattice due to strong on-site repulsion between original electrons in contrast to the ordinary s  -wave pair field on sites. This pair field behaves as a gauge field dual to the U(1) gauge field coupled with the hopping of electrons or quasi-particles of the t–J$t\text{–}J$ model, holons and spinons. By Monte Carlo simulations we study this lattice gauge model and find a first-order phase transition from the normal state to the Higgs (superconducting) phase. Each gauge field works as a Higgs field for the other gauge field. This mechanism requires no scalar fields in contrast to the ordinary Higgs mechanism. An explicit microscopic model is introduced, the low-energy effective theory of which is viewed as a special case of the present model.     

Phase transition and ferroelastic property studied by using the Cs nuclear magnetic resonance in a CsHSO4 single crystal

The ¹³³Cs spin-lattice relaxation time in a CsHSO₄ single crystal was measured in the temperature range from 300 to 450 K. The changes in the ¹³³Cs spin-lattice relaxation rate near T_c1 (=333 K) and T_c2 (=415 K) correspond to phase transitions in the crystal. The small change in the spin-lattice relaxation time across the phase transition from II to III is due to the fact that during the phase transition, the crystal lattice does not change very much; thus, this transition is a second-order phase transition. The abrupt change of T₁ around T_c2 (II-I phase transition) is due to a structural phase transition from the monoclinic to the tetragonal phase; this transition is a first-order transition. The temperature dependences of the relaxation rates in phases I, II, and III are indicative of a single-phonon process and can be represented by T₁⁻¹=A+BT. In addition, from the stress-strain hysteresis loop and the ¹³³Cs nuclear magnetic resonance, we know that the CsHSO₄ crystal has ferroelastic characteristics in phases II and III.     

Josephson current in ferromagnetic d-wave superconductor/ferromagnetic d-wave superconductor junction

We solve a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, study the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor (S)/insulating layer (I)/d-wave S junction are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction depend to a great extent on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field can under certain conditions enhance the Josephson critical current in a d-wave S/I/d-wave S junction.     

Zeeman effects on d-wave superconductor and the coherent tunneling spectrum in normal-metal/d-wave superconductor/normal-metal tunnel junctions

We solve a self-consistent equation for the d-wave superconducting gap and the magnetization in the mean-field approximation, study the Zeeman effects on the thermodynamic potential of d-wave superconductor (S) and coherent quantum transport in normal-metal (N)/d-wave S/N double tunnel junctions. Taking simultaneously into account the electron-injected current from one N electrode and the hole-injected current from the other N electrode, we derive a general formula for the differential conductance in a N/d-wave S/N system under a Zeeman magnetic field on the d-wave S. It is found that oscillations of all quasiparticle transport coefficients and differential conductance with the bias voltage and the thickness of the d-wave S depend to a great extent on the crystal orientation of the d-wave S. In the N/d-wave S/N junctions, the Zeeman magnetic field can lead to the Zeeman splitting of conductance peaks, and the temperature can reduce the coherent effect.     

Structural phase transition in the mixed state of superconducting films in a parallel magnetic field

A thin film of a second-kind superconductor in a magnetic field parallel to the surface of the film is considered in the London approximation. It has been shown that if bulk pinning is absent and the suppression of super-conductivity by the magnetic field is negligible, the splitting of a vortex chain in the film occurs as a structural phase transition either of the first or second order, depending on the ratio of the thickness of the film d to the penetration depth of the magnetic field λ. The ratio d/λ, and thereby the character of the transition in the vortex lattice, can be changed by varying the temperature. The corresponding critical thicknesses of films and field ranges in which this effect can be observed experimentally have been calculated.     

Magnetostructural phase transitions in manganese arsenide single crystals

The effect of an external magnetic field with a strength up to 140 kOe on the phase transitions in manganese arsenide single crystals has been investigated. The existence of unstable magnetic and crystal structures at temperatures above the Curie temperature T _C = 308 K has been established. The displacements of manganese and arsenic atoms during the magnetostructural phase transition and the shift in the temperature of the first-order magnetostructural phase transition in a magnetic field have been determined. It has been shown that the magnetocaloric effect in a magnetic field of 140 kOe near the Curie temperature T _C is equal to ??T ?? 13 K. A model of the superparamagnetic state in MnAs above the temperature T _C has been proposed using the data on the magnetic properties and structural transformation in the region of the first-order magnetostructural phase transition. It has been demonstrated that, at temperatures close to T _C, apart from the contribution to the change in the entropy from the change in the magnetization there is a significant contribution from the transformation of the crystal lattice due to the magnetostructural phase transition.     

Effects of A-site disorder on magnetic,electrical and thermal properties of La0.5−xLnxCa0.5−ySryMnO3manganites

The magnetic, electrical and thermal properties in the La_0.5?xLn_xCa_0.5?ySr_yMnO₃ (Ln=Pr, Nd, Sm) bulk system were investigated. Detailed dc magnetization and linear ac susceptibility measurements reveal that the samples first undergo phase transition from paramagnetic to ferromagnetic phase and then to an antiferromagnetic phase upon further cooling. It is found that both the Curie and Neel temperatures decrease systematically with increasing A-site disorder in these manganites. The electrical resistivity exhibits semiconducting behavior throughout the temperature range investigated and the electronic conduction mechanism can be conveniently described within the framework of the variable range hopping model above T=150 K. The Seebeck coefficient (S) in the magnetically ordered regime infers that the complicated temperature dependence of S is an indication of electron–magnon scattering. Specific heat measurements depict a broadened hump in the vicinity of T_C, indicating the existence of magnetic ordering and magnetic inhomogeneity in the samples. The temperature dependence of thermal conductivity, κ(T), reveals a positive dκ/dT in the paramagnetic region, which may be related to the local anharmonic lattice distortions associated with small polarons.     

Low-temperature structural phase transition in a monoclinic KDy(WO4)2 crystal

The low-temperature thermal and magnetic-resonance properties of a monoclinic KDy(WO₄)₂ single crystal are investigated. It is established that a structural phase transition takes place at T _c=6.38 K. The field dependence of the critical temperature is determined for a magnetic field oriented along the crystallographic a and c axes. The initial part of the H-T phase diagram is plotted for H∥a. The prominent features of the structural phase transition are typical of a second-order Jahn-Teller transition, which is not accompanied by any change in the symmetry of the crystal lattice in the low-temperature phase. The behavior of C(T) in a magnetic field shows that the transition goes to an antiferrodistortion phase. An anomalous increase in the relaxation time (by almost an order of magnitude) following a thermal pulse is observed at T>T _c(H), owing to the structural instability of the lattice. A theoretical model is proposed for the structural phase transition in a magnetic field, and the magnetic-field dependence of T _c is investigated for various directions of the field. Fiz. Tverd. Tela (St. Petersburg) 40, 750–758 (April 1998)     

On the theory of superconductivity in the extended Hubbard model

A microscopic theory of superconductivity in the extended Hubbard model which takes into account the intersite Coulomb repulsion and electron-phonon interaction is developed in the limit of strong correlations. The Dyson equation for normal and pair Green functions expressed in terms of the Hubbard operators is derived. The self-energy is obtained in the noncrossing approximation. In the normal state, antiferromagnetic short-range correlations result in the electronic spectrum with a narrow bandwidth. We calculate superconducting T _c by taking into account the pairing mediated by charge and spin fluctuations and phonons. We found the d-wave pairing with high-T _c mediated by spin fluctuations induced by the strong kinematic interaction for the Hubbard operators. Contributions to the d-wave pairing coming from the intersite Coulomb repulsion and phonons turned out to be small.     

Paraconductivity for a d-wave superconductor in short-wavelength fluctuation regime

A theoretical study of the fluctuation conductivity above T_c (paraconductivity) is reported for a d-wave superconductor with resonant scattering impurities. A d-wave system is modeled by tight-binding electrons in the two-dimensional squared lattice, and the impurity scattering is treated in the T-matrix approximation in a unitary limit. In calculating the Aslamazov–Larkin (AL) and the Maki–Thompson (MT) terms, we also consider effects of a short-wavelength cutoff in the fluctuation spectrum. The d-wave character in the AL and MT terms manifests itself to renormalization effects on the fluctuation amplitude and reduced temperature, whereas an anomalous-MT term is absent. The present calculations can describe fairly well experiments on the paraconductivity in zinc-doped cuprate superconductors provided that effects of a total-energy cutoff are taken into account.