Coherent state and superconductivity in the free carrier-bipolaron interacting system

In this paper a model to describe the free carrier-bipolaron interacting system is proposed. Effective hopping of the bipolaron is studied in the slave-boson approach, and a characteristic temperature T¹ is obtained, below which the system enters a coherent state. The density of states in the normal state and the superconductivity of the system are discussed in a quasiparticle picture. The results show that the mixing between the free carrier and the bipolaron results in an enhancement of the effective mass of the quasiparticle and meanwhile the renormalized coupling interaction, arising from the negative correlation energy in the bipolaron region, enhances the effective superconducting coupling interaction. Under the most favourable conditions, the superconducting transition temperature T_c ∼ ω_c, where ω_c is the Debye frequency related with local electron-phonon coupling. In general we have T¹ > T_c ⪢ T_c0 (T_c0 is the superconducting transition temperature of a usual superconductor). Therefore the system will firstly enter a coherent state before becoming a high-T_c superconductor.     

Surface superconductivity and twinning-plane superconductivity in aluminum

The critical supercooling field H _sc is measured in aluminum single crystals and twinned bicrystals in a temperature range slightly below T _c0 (T _c0 ? 0.055 K < T < T _c0), where T _c0 is the critical superconducting transition temperature. It is found that, even in this small temperature range, the H _sc(H _c) dependence, which is considered to be identical to the H _c3(H _c) dependence for single crystals, is substantially nonlinear. The H _sc(H _c) dependences of the twinned bicrystals and single crystals are shown to be significantly different. The qualitative features of the phase diagram of the twinned aluminum bicrystals coincide with those of the phase diagram of twinning-plane superconductivity obtained earlier for tin in [1]. These findings allow the conclusion that the phenomenon of twinning-plane superconductivity also exists in face-centered cubic crystal lattices.     

Pressure dependence of the Kondo resistance anomaly and the pair breaking effect in La-Ce alloys

The pressure dependence of the parr breaking effect and of the resistance anomaly was measured in LaCe alloys. The results indicate that the maximum in the pressure dependent pair breaking effect is due to a monotonic shift of the Kondo temperatureT _k with pressure from valuesT _k > ?T _c0 toT _k ?T _c0, whereT _c0 is the superconducting transition temperature of pure lanthanum.     

Electrical investigations in the normal and incommensurate phases of [N(CH3)4]2ZnCl4 single crystals

The dielectric permittivity (?) of TMA-ZC single crystals was measured along the mean crystallographic axes a, b and c, in a temperature range from 273 to 340 K. The ?-T relationship exhibited peak values at T_i=296 K and T_c1=279 K for the three axes. These peaks are attributed to the contribution of discommensurations. The d.c. and a.c. electrical conductivity showed anomalous variation at the same transition temperatures, with a remarkable change in the value of the activation energy around the transition temperatures. The j-E characteristic indicates different types of electrical conduction. The mechanism of the phase transition and the electrical process were discussed on the basis of Shottky and Frenkel conduction mechanisms.     

Influence of an adsorbing gas on the layering transitions in crystal growth

The influence of an adsorbing gas on the layering transition of a film is studied. In agreement with an earlier publication, we find that the adsorbate lowers the critical temperature T_c(n) for the nth layer. We also find that a system which is dry near T = 0 may undergo an apparent wetting transition due to the adsorbate. Indirect evidence is given that the sequence T_c(n) ends at T_R, the roughening temperature, as conjectured by de Oliviera and Griffiths. Re-entrant behavior in the layering transitions is observed for sufficiently strong adsorption energy; similar behavior also appears in the roughening transition, for which there exists some experimental evidence.     

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.     

Compatibility of BCS pairing and the peierls distortion in two-band systems

The effect of the Peierls transition on superconductivity in a two-band system, where one of the bands is flat, is examined theoretically. We find that superconductivity appears at the background of the insulating phase (T_P >; T_c). At T_c > T_P only the superconducting transition is possible.     

Ultrasonic attenuation near the cholesteric → smectic A transition

Ultrasonic attenuation at 2 MHz has been measured through the cholesteric → smectic A transition in cholestryl nonanoate. Very near the transition, 0.1 < (T ? T_c) < 0.6°C, the excess attenuation varies as (T ? T_c)ⁿ, where n for four separate runs varies between 0.30 and 0.34. These values are very close to the exponent of 0.33 predicted using a scaling laws and observed for the divergence of the twist viscosity in the nematic phase as the smectic A transition is approached.     

Superconducting phase transitions in ErcY1−cRh4B4

The superconducting transition temperature T_c1 and the reentering temperature T_c2 to the normal ferromagnetically ordered state in the system Er_cY_1?cRh₄B₄ were determined as a function of the concentration of Er. Comparing our results with the recent theory of Maekawa et al., the exchange interactions J′ among local spins and I between local spins and superconducting electrons were estimated as J′ ? 0.30 K and I²N(0) ? 0.046 K, where N(0) is the density of states. A sudden decrease of H_c2 at T_c2 was observed in a high concentration region suggesting the first order transition.     

Tricritical point in inhomogeneous superconductors

We show that in large period modulated alloys the tricritical points curve begins at α_c = 1/(1 + δ, where δ is the modulation amplitude. In the domain of the 2nd order transition, which appears smaller than in the homogeneous case, H_c2¹(T) and H_c3¹(T) are calculated.     

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.     

One-dimensional spin glass with oscillating long-range interaction

In this paper a long-range interacgion approximation for spin glasses is proposed as an alternative to the Sherrington-Kirkpatrick model. The one-dimensional model of Ising spins with the interaction κV _O cosQ x exp (?κ|x|), where κ?c?Q (c is the spin concentration) is studied in detail. The long-range approximation enables one to describe the spin configuration in terms of slowly varying in space fields of the type of amplitude (ρ) and phase (ψ); the ψ-dependent part of the Hamiltonian is analogous to the Hamiltonian, describing the weak pinning of the charge density waves by impurities. As a result, the phase variable apears to be gaples in equilibrium thermodynamics and parametrizes different metastable states under quasiequilibrium conditions. In the mean field approximation (MFA) (κ»0) in the vicinity of the transition pointT _c =cV ₀, there is a symmetric cusp of the magnetic susceptibility ξ; at low temperatures the heat capacity is proportional toT, whereas the susceptibility does not depend on temperature. The MFA cannot be applied in the close vicinity ofT _c (|τ?(κ/c)^2/3) and at very low temperaturesT?κV ₀ when a gap appears in the distribution of the molecular fielsh ath≈0.     

Dynamical properties of the Peierls-Fröhlich state on the many-phonon-coupling model

The amplitude and phase phonons and the frequency dependent conductivity below the mean-field Peierls-Fröhlich transition temperature T_c, and the Kohn anomaly and fluctuation induced charge-density-wave conductivity above T_c, are discussed on the basis of the many-phonon-coupling model recently introduced by Rice, Duke and Lipari. For dominant intramolecular phonon coupling an isotope effect in T_c is related to the isotopic shift in the small polaron binding energy.     

Exploiting the flexibility of a family of models for taxation and redistribution

In this paper we present and discuss results of Monte Carlo numerical simulations of the two-dimensional Ising ferromagnet in contact with a heat bath that intrinsically has a thermal gradient. The extremes of the magnet are at temperatures T ₁?<?T _c ?<?T ₂, where T _c is the Onsager critical temperature. In this way one can observe a phase transition between an ordered phase (T?<?T _c ) and a disordered one (T?>?T _c ) by means of a single simulation. By starting the simulations with fully disordered initial configurations with magnetization m????0 corresponding to T?=???, which are then suddenly annealed to a preset thermal gradient, we study the short-time critical dynamic behavior of the system. Also, by setting a small initial magnetization m?=?m ₀, we study the critical initial increase of the order parameter. Furthermore, by starting the simulations from fully ordered configurations, which correspond to the ground state at T?=?0 and are subsequently quenched to a preset gradient, we study the critical relaxation dynamics of the system. Additionally, we perform stationary measurements (t??????) that are discussed in terms of the standard finite-size scaling theory. We conclude that our numerical simulation results of the Ising magnet in a thermal gradient, which are rationalized in terms of both dynamic and standard scaling arguments, are fully consistent with well established results obtained under equilibrium conditions.     

Dynamic melting of driven vortex matter below 1 K

We report on measurements of a mode-locking resonance in amorphous Mo_xGe_1?x films at different temperatures T down to 0.8 K, which is well below the superconducting transition (6 K). We observe dynamic ordering of driven vortex matter for all the temperatures studied. As the field exceeds a certain critical field B_c,dyn at fixed T, moving vortices do not exhibit dynamic ordering. At high T, this field B_c,dyn(T), so-called dynamic melting, nearly coincides with a characteristic field B_c(T) where the linear resistivity vanishes. At low T, however, B_c,dyn(T) is significantly suppressed compared to B_c(T), suggesting intrinsic quantum melting in the absence of pinning.     

Disorder effects in the BCS-BEC crossover region of the attractive Hubbard model

We study the disorder effects upon superconducting transition temperature T _c and the number of local pairs within the attractive Hubbard model in the combined Nozieres-Schmitt-Rink and DMFT + Σ approximations. We analyze the wide range of attractive interaction U, from the weak coupling region, where instability of the normal phase and superconductivity are well described by the BCS model, to the limit of strong coupling, where superconducting transition is determined by Bose-Einstein condensation of compact Cooper pairs, forming at temperatures much higher than superconducting transition temperature. It is shown that disorder can either suppress T _c in the weak coupling limit, or significantly enhance T _c in the case of strong coupling. However, in all cases we actually prove the validity of generalized Anderson theorem, so that all changes in T _c are related to change in the effective bandwidth due to disorder. Similarly, disorder effects on the number of local pairs are only due to these band-broadening effects.     

Electron transport in diborides: Observation of superconductivity in ZrB2

We report on syntheses and electron transport properties of polycrystalline samples of diborides (AB₂) with different transition metals atoms (A=Zr, Nb, Ta). The temperature dependence of resistivity, ρ(T), and ac susceptibility of these samples reveal a superconducting transition of ZrB₂ with T _c =5.5 K, while NbB₂ and TaB₂ have been observed to be nonsuperconducting up to 0.37K. H _c2(T) is linear in temperature below T _c , leading to a rather low H _c2(0)=0.1 T. At T close to T _c , H _c2(T) demonstrates a downward curvature. We conclude that these diborides, as well as MgB₂ samples, behave like simple metals in the normal state with usual Bloch-Grüneisen temperature dependence of resistivity and with Debye temperatures 280, 460, and 440 K for ZrB₂, NbB₂, and MgB₂, respectively, rather than T ² and T ³, as previously reported for MgB₂.     

Temperature dependence of the gap in the density of states near the Fermi level in a hole doped manganite

In this paper, we report a model-based quantitative analysis of temperature dependent scanning tunneling spectroscopy (STS) data taken on epitaxial thin films of the hole doped manganite La_0.7Ca_0.3MnO₃. The film, grown on lattice matched NdGaO₃ substrate, has a ferromagnetic transition temperature T_c=268 K. The analysis allows us to evaluate how the tunneling curve evolves across the transition temperature. We find that there is a gap Δ in the density of states (DOS), which peaks at T≈T_c. The gap closes in the ferromagnetic state following the evolution of the magnetization. The gap closing is gradual and not sudden at T=T_c. Above T_c the gap reduces from the peak value and reaches a limiting value of ≈75 meV for T/T_c≥1.1 which is close to the value of 60 meV seen from transport experiments.     

Superconductivity in the intermetallic compound YRhAl

The intermetallic compound, YRhAl, has been prepared and is found to be isomorphic with RRhAl (R=Pr, Nd, Gd, Ho and Tm) compounds crystallizing in the orthorhombic TiNiSi-type structure (space group Pnma). Heat capacity and electrical resistivity measurements in the He-3 temperature range reveal that this compound is superconducting with a transition temperature, T_c, of 0.9 K. The electronic specific heat coefficient, γ, and the Debye temperature are found to be 6.1 mJ/mol K and 197 K, respectively. The specific heat jump at the superconducting transition is found to be consistent with the BCS weak-coupling limit. This combined with the earlier observation of superconductivity in LaRhAl (T_c=2.4 K) having a different structure than that of YRhAl, suggests that the underlying structure is not very crucial for the occurrence of superconductivity in RRhAl series of compounds.