Critical temperature changes in many-layer films between plates

Two distinct mechanisms produce two separate terms in the change of the critical temperature T_c in an n-layer (n ↗ 1) film. The interface interaction induces a change ΔT_c, ΔT_c/T_c ⩽ O(2/n). The finite thickness of a film gives another term in the change of T_c, which has the exponent 1/ν≈1.56. Comparison with experiments is made and further experiments are discussed.     

Critical point wetting: The effect of hydrodynamical and steric interfacial interactions

It is shown for the first time that the capillary waves and hydrodynamic correlations in an adsorbed film of a simple fluid provide incomplete wetting in some vicinity of the critical point T_c. On approaching T_c the film thickness grows infinitely and the wetting angle tends to zero due to the steric repulsion between the film-bulk fluid interface and the substrate.     

Interface superconductivity – Possible origin of high critical temperature in layered superconductors

Superconducting transition temperatures T_c of the YBCO film interfaces were measured inductively. It was observed that the interface-T_c is always higher then the surface-T_c probably because of the substrate influence. Deposition of the silver over-layer on the film surface enhances also its critical temperature. In the annealed YBCO/Ag bilayers magnetic properties of the interface were observed. This two dimensional structure reminds the layered microstructure of the high-temperature superconductors and one can suppose that enhanced superconductivity of the interfaces is the origin of the high critical temperatures in layered superconductors.     

Surface electromagnetic waves at a superconductor-dielectric interface

This paper reports on the results of investigating the frequency dependence, the structure, and the polarization-and energy-related characteristics of surface electromagnetic waves propagating along a superconductor-dielectric interface. An expression for the complex permittivity of a superconductor is derived in the approximation of a two-component plasma containing “normal” and “superconducting” electrons. Basic relations are obtained in the general case at temperatures T≤T _c (where T _c is the critical temperature) and in the limiting case at T?T _c, when the contribution from normal electrons to the permittivity of the superconductor can be disregarded.     

表面与界面的绝缘铁磁理论

采用Green函数技术,我们讨论了温度从T=0直到T=T_c范围内铁磁体表面和界面的磁化强度随温度的变化情况,所得到表面磁化强度的结果与其他作者的结果一致;而在界面计算中,我们发现界面的T_c与体内的不同,并且找到了一个关系式:T_cc^A,T_c^B)。 关键词：     

Dynamic and spatial behavior of a corrugated interface in the driven lattice gas model

The spatiotemporal behavior of an initially corrugated interface in the two-dimensional driven lattice gas (DLG) model with attractive nearest-neighbors interactions is investigated via Monte Carlo simulations. By setting the system in the ordered phase, with periodic boundary conditions along the external field axis. i.e. horizontal, and open along the vertical directions respectively, an initial interface was imposed, that consists in a series of sinusoidal profiles with amplitude A₀ and wavelength λ set parallel to the applied driving field axis. We studied the dynamic behavior of its statistical width or roughness W(t), defined as the root mean square of the interface position. We found that W(t) decays exponentially for all λ and lattice longitudinal sizes L_x, i.e., the lattice side that runs along the axis of the external field. We determined its relaxation time τ, and found that depends on λ as a power law τ∝λ^p, where p depends on the temperature and L_x. At low T’s (T?T_c(E)) and large L_x, p approaches to p=3/2. At intermediate T’s (T<T_c(E)), p decreases up to p≈1, and is free of finite effects. This indicates that the interface stabilizes faster than in the equilibrium model, i. e. the Ising lattice gas (E=0) where p=3. At higher T’s p increases for T?T_c(E), and the finite size dependence is recovered. Also, if T is fixed, p increases with L_x until it saturates at large values of it, while this regime is vanishing at T?T_c(E). In this way, the dynamic relaxation process of a sinusoidal interface is improved by the external driving field with respect to its equilibrium counterpart, if the system is set in an intermediate temperature stage far from T_c(E) and in a lattice with a sufficiently large longitudinal side. The behavior of τ was also investigated as a function of E and in the intermediate stage T<T_c(E). It was found that τ decreases exponentially with E in the interval 0<E?1, while for higher fields it remains constant. The exponential decay depends on the wavelength of the initial profile.In order to study the spatial evolution of the profiles, we evaluated the structure factor of the interface, and the Fourier coefficients corresponding to the same wave vector of the initial profile. The obtained results allowed us to conclude that the spatial evolution of the profile maintains its initial wavelength, does not travel along the external field axis, and its shape is preserved over all the relaxation process.     

Superconductivity and Peierls instability in coupled linear chain systems

We study the superconducting transition temperature (T_c) and the Peierls instability temperature (T_p) using Eliashberg type equations for both T_c and T_p self consistently with finite interchain coupling. We show that T_c > T_p below a critical electron-phonon coupling constant which depends on the bare phonon frequency. This determines an upper bound on T_c so that for higher transition temperatures T_p > T_c and superconductivity is unlikely. Higher values of T_c are possible if the interchain coupling is increased above a critical value where the Peierls instability is suppressed.     

Possible mechanism of superconductivity in the CuO-Cu interface

Apparently, a two-dimensional CuO lattice is formed on the surface of copper oxide in the CuO-Cu interface. This lattice consists of Cu²⁺ and O^1? ions, which form a narrow, partially filled two-dimensional band. In this case, local electron pairs (LEPs) can form in the oxygen subsystem as a result of the fulfillment of the Shubin-Vonsovskii conditions. A crude estimate of the formation temperature of LEPs gives T* ～ 10 4 K. At the concentration in the interface layer n～1.6×10²⁰ cm^?3 and the effective mass of carriers m* ～ m _e, the onset temperature of Bose-Einstein condensation may take a value of T _c ～ 1000 K. The estimate obtained for the temperature T _c corresponds to the experimental value by an order of magnitude.     

Crystallization and melting of spin-polarized<Superscript>3</Superscript>He

The melting and growth of³He crystals, spin-polarized by an external magnetic field, are different in nature depending on whether the temperature is higher or lower than the characteristic ordering temperatures in the crystal (the Neel temperatureT _N ) and in the liquid (the superfluid transition temperatureT _c ). In the high-temperature region (T≥T _N ,T _c ) the liquid which appears upon melting has a high nonequilibrium spin density. In the low-temperature region (T?T _N ,T _c ) the melting and growth are accompanied by spin supercurrents both in the liquid and in the crystal in addition to mass supercurrents in the liquid. The crystallization waves at the liquid-solid interface should exist in the low-temperature region. With increasing magnetic field the waves change in nature, because the spin currents begin to play a dominant role. The wave spectrum becomes linear with a velocity inversely proportional to the magnetic field. The attenuation of the waves at low enough temperatures is mainly due to the interaction of the moving crystal-liquid interface with thermal spin waves in the crystal. The waves could be weakly damped at temperatures below a few hundreds microkelvins.     

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.     

Repulsive effect of low frequency phonons on superconductivity

The Eliashberg integral equations are investigated to determine the effect of low frequency phonons on the superconducting transition temperature T_c. It is found that phonons of frequency less than T_c are repulsive (diminish T_c) while phonons of higher frequency have a diminished attraction unless the frequency is substantially above T_c. Various implications are discussed concerning observed values of T_c and predicted mechanisms for raising T_c.     

Model of superconducting domains––a unified theory of high- and low-Tc superconductors

A unified model of the superconducting mechanism has been put forward. The model suits not only to high-T_c but also to low-T_c superconductors. It is found that there are superconducting domains (SD) in crystal. When T⩽T_c, all the SD’s in the whole crystal are connected with one another. We have obtained the formula of T_c. On the basis of the formula and theory of quantum mechanics, the different behaviours of isotopic effects in low- and high-T_c superconductors as well as C₆₀, the triangular peak of T_c of transition metals, Matthias rules, and other effects are explained. New superconductors with higher T_c are predicted.     

High pressure study of BaPb0.7Bi0.3O3 films

The resistance R, the superconducting transition temperature T_c and the energy gap Δ(T) have been measured on the BaPb_0.7Bi_0.3O₃ films up to 14 kbar. We have found that up to 14 kbar: (1) pressure suppresses T_c and Δ(T) while enhances R, (2) the value of 2Δ(0)/kT_c is 3.8±0.1, independent of pressure, and (3) the Δ(T)/Δ(0) varies with T/T_c in a BCS fashion but only for T/T_c<0.75 and independent of pressure. The results show that BaPb_1?xBi_xO₃ is a weak-coupling superconductor, but fail to provide information about the cause for the high T_c of the compound.     

Low-temperature x-ray diffraction studies of the crystallographic parameters and thermal expansion of (CH3)2NH2Al(SO4)2 · 6H2O crystals

The a, b, c, and β crystallographic parameters of the (CH₃)₂NH₂Al(SO₄)₂ · 6H₂O crystal (DMAAS) have been measured by x-ray diffraction in the 90–300-K temperature range. The thermal expansion coefficients along the principal crystallographic axes α_a, α_b, and α_c have been determined. It was shown that, as the temperature is increased, the parameter α decreases and b increases, whereas c decreases for T<T _c (where T _c is the transition temperature) and increases for T>T _c, so that one observes a minimum in the c=f(T) curve in the region of the phase transition (PT) temperature T _c ～ 152 K. The thermal expansion coefficients α_a, α_b, and α_c vary in a complicated manner with increasing temperature, more specifically, α_a and α_c assume negative values at low temperatures, and the α_a=f(T), α_b=f(T), and α_c=f(T) curves exhibit anomalies at the PT point. The crystal has been found to be substantially anisotropic in thermal expansion.     

The Nernst-Ettingshausen coefficient in hole-doped manganites

A systematic study of the Nernst-Ettingshausen coefficient (NEC) in hole-doped manganites of the LaMnO₃ and SmMnO₃ systems was carried out at temperatures both above and below the point of transition to the magnetically ordered state (T _c). The results obtained for T > T _c suggest that conduction is mediated here by small-radius polarons. For all the compositions studied, the Nernst mobilities at T = 300 K are small (of the order of 0.1–2 cm²/V s) and the carrier relaxation time at T > T _c increases with carrier energy. At temperatures below T _c, the NEC exhibits an anomalous behavior. The giant NEC effect was observed, which consists in a strong dependence of the NEC on applied magnetic field and the presence of a peak in the temperature dependence of the NEC at T ≈ T _c. Near the transition to the magnetically ordered state, the NEC follows a behavior similar to that of colossal magnetoresistance and giant magnetothermoelectric power. A possible origin of the anomalous NEC behavior at T < T _c is discussed.     

The zero-distribution of the great partition function in the complex fugacity plane of gases of point particles with logarithmic interaction

The equation of state for gases of point particles with logarithmic interaction is derived. The system exhibits a phase transition at a critical temperature T_c. The critical temperature is a function of the dimension of the system. A hard core must be added below T_c to prevent the system from collapsing. The specific heat diverges on both sides as |T_c - T|^-2 in any dimension.For T≤T_c there are no zeros of the grand partition function in the complex fugacity plane, for T >T_c all zeros occupy the whole negative real axis. The density of zeros will be calculated.     

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.     

Properties of disordered Mo3Ge thin films with A15 structure

Heavy ion irradiation of A15 Mo₃Ge with the low transition temperature T_c = 1.45 K raises T_c to ? 6 K. For the first time in A15 compounds a T_c degradation (≈ 3 K) after having passed through a maximum is observed until near 3.5 K the saturation value is reached. The effects on T_c are interpreted by variations of the deduced (H'_c2, ?) density of states at the Fermi level. This picture consistently explains the T_c degradation of the high-T_c A15 compounds.     

Validity of model-potential approach for the expanded liquid alkali metals

The liquid density along the saturation line is calculated for Na, K, Rb and Cs. Good results are obtained in sodium in the whole experimental range T ? 0.85 T_c (T_c is the critical temperature). The observed discrepancy in the heavier alkali metals for T ? 0.75 T_c is tentatively related to recent speculations.