Correlations in inhomogeneous Ising models

Using general methods developed in a previous treatment we study correlations in inhomogeneous Ising models on a square lattice. The nearest neighbour couplings can vary both in strength and sign such that the coupling distribution is translationally invariant in diagonal direction. We calculate correlations parallel to the layering in the diagonally layered model with periodv=2, the so-called “general square lattice” model (GS). If the model has a finite critical temperature,T _c>0, we have a spontaneous magnetization belowT _c vanishing atT _c with the Ising exponent β=1/8. AtT _c correlations decay algebraically with critical exponnet η=1/4 and exponentially forT>T _c. In the frustrated case we have oscillatory behaviour superposed on the exponential decay where the wavevector of the oscillations changes at some “disorder temperature”T _D(>T _c) from commensurate to temperature-dependent in commensurate periods. If the critical temperature vanishes,T _c=0 we always have exponential decay at finite temperatures, while atT=T _c=0 we encounter either long-range order or algebraic decay with critical index η=1/2, i.e.T=0 is thus a critical point.     

Heat capacity and lattice dynamics of yttrium diboride in the temperature range 5–300 K

This paper reports an experimental study of the heat capacity and crystal lattice parameters of a polycrystalline sample of yttrium diboride prepared by high-temperature synthesis from elements. The electronic and lattice contributions to the heat capacity are isolated. The temperature dependences of the characteristic temperature, the linear thermal expansion coefficients α_a(T) and α _c (T), the bulk thermal expansion coefficient β(T), and the Grüneisen coefficient are calculated. A region of negative values of α _c (T) and β(T) is revealed. Anharmonicity is found to exert only a minor effect on the YB₂ lattice dynamics over a larger part of the temperature range covered.     

An effect of the uniaxial strain on the temperature of Bose–Einstein condensation of the intersite bipolarons

We have studied an effect of uniaxial strain to the temperature of Bose–Einstein condensation of intersite bipolarons within the framework of extended Holstein–Hubbard model. Uniaxial lattice strains are taken into an account by introducing a generalized density–displacement type force for electron–lattice interaction. Associating the superconducting critical temperature T_c with the temperature of Bose–Einstein condensation T_BEC of intersite bipolarons we have calculated strain derivatives of T_BEC and satisfactorily explained the results of the experiments on La-based high-T_c films.     

Sign reversal of the magnetoplastic effect in C60 single crystals during the sc-fcc phase transition

It is found that the magnetoplastic effect in C₆₀ single crystals in a pulsed magnetic field with induction larger than 10 T changes its sign in the vicinity of the phase transition at T _c =250–260 K: crystal strengthening is observed for T<T _c , and softening occurs for T>T _c . This indicates a change in the crystal lattice structure in the magnetic field.     

Upper critical field in the heavy fermion superconductors

The upper critical field H_c2 (T) and its temperature derivative at T_c in the heavy fermion superconductors have been calculated by using the simplified Kondo lattice model.     

LaAg under hydrostatic pressure: Superconductivity and phase transformation

The electrical resistivity of LaAg has been measured from 1–300 K at hydrostatic pressures to 12.4 kbar and the superconducting transition temperature T_c determined inductively to 23 kbar. For P ? 6.2 kbar a lattice transformation is observed at a temperature T_M which increases rapidly with pressure. T_c shows an oscillatory pressure dependence, increasing initially from T_c (0) = 1.062 K. There is no obvious correlation between the pressure dependence of T_M and T_c.     

Monte Carlo study of surface roughening in the three-dimensional Ising model

Above a roughening temperature of about 0.56T _c the thickness of the two-dimensional interface separating domain on a simple cubic lattice is found to increase roughly logarithmically with system size. Also, the interface tension is determined for temperatures belowT _c.     

A search for extra lattice distortions associated with the superconductivity and the charge inhomogeneities in the high-Tc La1.85Sr0.15CuO4 system

Precise measurements of the temperature-dependent lattice parameters in a single crystal of La_1.85Sr_0.15CuO₄ are made by X-ray bond method and analyzed comparing the normal thermal expansion term, proportional to T⁴. A spontaneous lattice contraction of about 2.3×10⁻⁵ Å was found in the ab-plane, while along the c-axis a small expansion of about 1.5×10⁻⁵ Å was observed below the superconducting transition temperature of the system, showing some correlation between the structure and the high-T_c superconductivity. A careful search has been made for existence of any extra lattice distortion above the T_c, that might be associated with the stripe inhomogeneities in the title system, however, within the experimental sensitivity we could not find such distortions.     

Specific features in the behavior of the central peak in Raman spectra of lithium tantalate

Low-frequency Raman spectra of a LiTaO₃ ferroelectric crystal are studied in the temperature range 300–1273 K. The central peak associated with the relaxation susceptibility of the crystal lattice is recorded throughout the temperature range covered, including the temperatures both substantially lower and considerably higher than the Curie point (T _c = 900 K). The critical slowing down of the relaxation response time predicted by the Ginzburg-Landau-Devonshire theory is observed in the temperature range from 0.9T _c to 1.1T _c . In contrast to the critical slowing down, the width of the central peak γ_R increases below 0.9T _c and decreases above 1.1T _c with increasing temperature.     

Hydrogen absorption and its effect on the magnetic properties of rare-earth iron intermetallics

X-ray diffraction studies of the hydrogen absorption in several YFe and CeFe intermetallic compounds showed that no structural changes occur upon hydrogen absorption in Y₆Fe₂₃, YFe₃, YFe₂. The lattice constants of the hydrides were found to be appreciably larger than those of the pure intermetallic compounds. The magnetic properties of the hydrides were determined and compared with the original compounds. In all cases the magnetic moment per Fe atom proved to be much larger in the hydride phases. Hydrogen absorption can lead to a decrease as well as to an increase of the magnetic ordering temperature (T_c). These changes in T_c could adequately be explained in terms of the observed increases in lattice constant and the data available for the pressure derivative of T_c of these compounds.     

Specific features of the central peak in the Raman spectra of a lithium niobate crystal

The low-frequency Raman scattering (RS) spectra of a LiNbO₃ ferroelectric crystal are studied in the temperature range 300–1423 K. The central peak characterizing the relaxation susceptibility of the crystal lattice is observed over the entire temperature range studied, including at temperatures much lower than the Curie temperature (T _c = 1470 K). Far from T _c, the characteristics of the central peak are shown to be unlike those expected in the framework of the standard approaches. (i) The central-peak width γ_R increases as the temperature increases to T ～ 1300 K, and the critical slowing down (γ_R ∝ T _c ? T) occurs only above this temperature. (ii) A central peak arises in the RS geometry where the scattering by “nonferroelectric” E phonons is allowed. The experimental results are interpreted with allowance for relaxation dynamics in local regions of the crystal lattice.     

NMR investigations of dynamical processes in orientationally ordered and disordered NaCN,RbCN and CsCN

The nuclear spin lattice relaxation timeT ₁ of the²³Na,⁸⁵Rb,⁸⁷Rb,¹³³Cs,¹⁴N nuclei is measured in NaCN, RbCN and CsCN as a function of temperature below and above the ferroelastic phase transition temperatureT _c. BelowT _c the behaviour ofT ₁ of the alkali nuclei renders possible to determine the flip frequency of the CN molecules and its temperature dependence. AboveT _c from the¹⁴NT ₁ the correlation time τ_c of the rotational motions of the CN molecules and its temperature dependence is determined. An empirical rule is verified demonstrating that atT _c the correlation times take nearly the same values for all cyanides. For the high and low temperature phases one obtains atT _c about τ_c=5·10^?13s and τ_c=5·10^?11s, respectively. The results are discussed with respect to the mechanism of the phase transition.     

Vortex lattice in Bi2Sr2CaCu2O8+δ well above the first-order phase-transition boundary

Measurements of non-local in-plane resistance originating from transverse vortex-vortex correlations have been performed on a Bi₂Sr₂CaCu₂O_8+δ high-T_c superconductor in a magnetic field up to 9 T applied along the crystal c-axis. Our results demonstrate that a rigid vortex lattice does exist over a broad portion of the magnetic field-temperature (H-T) phase diagram, well above the first-order transition (FOT) boundary H_FOT(T). The results also provide evidence for the vortex lattice melting and vortex liquid decoupling phase transitions, occurring above the H_FOT(T).     

Metal atom-induced diffuse scattering from short-range order among deuterium atoms in single crystal α-phase V2D

X-ray scattering has been studied above the critical ordering temperature, T_c (133°C), in b.c.c. α-phase V₂D. At 140 and 200°C quite weak diffuse intensity was observed centered on the reciprocal lattice points of the ordered superlattice. This intensity is due to short-range order among deuterium atoms which induces a local or short-range modulation of the parent vanadium lattice and suggests the possibility of local octahedral D—D fluctuations building up well above T_c.     

Magnetoresistance anomalies at the Curie point of some FeCr spinels

Magnetoresistance measurements on three sintered FeCr spinels up to H = 14.5 koe show clear peaks at the Curie points T_c. It is suggested that this effect is associated with Jahn-Teller lattice distortions and possibly with a non Néel spin configuration leading to large spin disorder at T_c.     

Simulation studies of gas-liquid transitions in two dimensions via a subsystem-block-density distribution analysis

The finite-size scaling analysis of the density distribution function of subsystems of a system studied at constant total density is studied by a comparative investigation of two models: (i) the nearest-neighbor lattice gas model on the square lattice, choosing a total lattice size of 64×64 sites. (ii) The two-dimensional off-lattice Lennard-Jones system (truncated at a distance of 2.5 σ, σ being the range parameter of the interaction) withN=4096 particles, applying the NVT ensemble. In both models, the density distribution functionP _L (ρ) is obtained forL×L subsystems for a wide range of temperaturesT, subblock linear dimensionsL and average densities <ρ>. Particular attention is paid to the question whether accurate estimates of critical temperatureT _c and critical density ρ _c can be obtained. In the lattice gas model these critical parameters are known exactly and the limitations of the approach can thus be definitively asserted. The final estimates for the Lennard Jones problem areT _c =0.47±0.01 (in units of the Lennard Jones energy ε) and ρ _c (in units of σ²), a comparison with previous estimates is made.     

Re-entry in ferromagnetic superconductors

The re-entry phenomenon in magnetic superconductors is studied using the generalized Ginzburg-Landau free energy introduced by Blount and Varma. The re-entry temperature T_c2 is simply that temperature at which the magnetization acts as a source of induction strong enough to destroy superconductivity. Above T_c2 ferromagnetism and superconductivity coexist. The structure is an Abrikosov vortex lattice, with ferromagnetic magnetization spreading widely around the vortex cores. Within our approximations, the phase transition at T_c2 is of second order.     

Correlation between structural and superconducting properties of nano-granular disordered Nb thin films

We report on low temperature transport measurements on nano-granular Nb thin films deposited on Si (1 0 0) substrates using DC magnetron sputtering. The superconducting transition temperature (T_c) is found to decrease monotonically with the increase of the lattice parameter (a) irrespective of its thickness and grain size. The superconducting transition temperature is found to depend only on the lattice parameter whereas the normal state resistivity depends both on lattice parameter and the details of the sample morphology. We have modeled this T_c variation with lattice expansion in terms of Debye temperature reduction using Morse potential as the interatomic potential in Nb.     

QCD equation of state for heavy ion collisions

In this work, we calculate the equation of state(EoS) of quark gluon-plasma(QGP) using the CornwallJackiw-Tomboulis(CJT) effective action. We get the quark propagator by using the rank-1 separable model within the framework of the Dyson-Schwinger equations(DSEs). The results from CJT effective action are compared with lattice QCD data. We find that, when μ is small, our results generally fit the lattice QCD data when TT_c,but show deviations at and below T_c. It can be concluded that the EoS of CJT is reliable when TT_c. Then,by adopting the hydrodynamic code UVH2+1, we compare the CJT results of the multiplicity and elliptic flow v2 with the PHENIX data and the results from the original EoS in UVH2+1. While the CJT results of multiplicities generally match the original UVH2+1 results and fit the experimental data, the CJT results of v2 are slightly larger than the original UVH2+1 results for centralities smaller than 40% and smaller than the original UVH2+1 results for higher centralities.     

Stoichiometry of the chevrel phase SnMo6S8

The phase field of the Chevrel phase Sn_xMo_yS₈ was investigated by metallographic methods. A shift of the phase field away from the “ideal” compound SnMo₆S₈ towards the molybdenum-rich side of the phase diagram was observed. The degree of off-stoichiometry could not yet be determined, however, the c/a ratio of the hexagonal lattice parameters can be used to characterize the samples. Measurements of the superconducting transition temperature indicate two superconducting “phases”, which differ in the c/a ratio of the lattice parameters. While phase A showed a linear variation of T^a_c with c/a in the region from 11.0 to 13.0 K phase B was independent of the c/a ratio with a T^b_c = 10.8 ± 0.2 K.