Fluctuations of order parameters in the highT c superconductors

Recently we have proposed a phenomenological approach in terms of two coexisting macroscopic order parameters corresponding to the superconducting and insulating states and have discussed the electrodynamical responses of the superconducting ceramics. In this paper we discuss the fluctuations of the order parameters both in static and dynamical situations in the mean field approach and obtain results for the electrical conductivity which possesses anomalies as in granular materials.     

Dimensional crossover and finite-size scaling belowT c

Using the formalism developed in earlier work, dimensional crossover on ad-dimensional layered Ising-type system satisfying periodic boundary conditions and of sizeL is considered belowT _c (L), T _c (L) being the critical temperature of the finite-size system. Effective critical exponents _eff and _eff are shown explicitly to crossover between theird- and (d–1)-dimensional values for _L in the limitsL/ _L andL/ _L 0, respectively, _L , being the correlation length in the layers. Using anL-dependent renormalization group, the effective exponents are shown to satisfy natural generalizations of the standard scaling laws. In addition,L-dependent global scaling fields which span the entire crossover are defined and a scaling form of the equation of state in terms of them derived. All the above assertions are verified explicitly to one loop in perturbation theory, in particular effective exponents and a universal crossover equation of state are obtained and shown in the above asymptotic limits to be in good agreement with known results.     

Fluctuations in high field superconductors

We present calculations for the influence of fluctuations in high field superconductors where the critical field is limited by Pauli paramagnetism. Due to the fact that the critical field at the second order phase transition point as function of temperature may have a maximum atT≠0 the additional conductivity due to fluctuations may have a nonmonotonic temperature dependence. This way we can account for recent experimental findings by Tedrow, Meservey and Schwartz. We also calculate the additional tunneling density of states due to fluctuations. Under proper conditions it exhibits a maximum at zero frequency like in the gapless regime. Finally we show that our findings of a nonmonotonic resistivity should also apply to superconductors containing magnetic impurities such as La_3-x Gd _x In in an external field.     

Abrupt increase in thermal diffusion belowT c in YBa2Cu3O7−δ

We report the variation of the thermal diffusivity in the high temperature superconductor YBa₂Cu₃O_7−δ below the transition temperature measured using photo-acoustic technique α is found to increase sharply belowT _c. Qualitative explanations are given.     

Fluctuations and defects in the spiral state of magnetic superconductors

We want to point out three properties of a magnetic superconductor: (i) The absence of true long-range order in the spiral state leads to the structure functions behaving like (q₆ ? q₀)^η?2 and (q²_⊥)^η?2 for q_⊥=0 and q₆=0, respectively, where q₀ is the preferred momentum. The indices η are measured via Bragg-like neutron scattering. (ii) The state is perforated by line-like defects. (iii) Above some critical temperature the defect lines proliferate, thereby destroying the spiral quasi-order.     

Flux diffusion in high-T c superconductors

In type-II superconductors in the flux flow (J J _c ), flux creep (J _c J _c ), and thermally activated flux flow (TAFF) (J J _c ) regimes the inductionB(r,t), averaged over several penetration depths , in general follows from a nonlinear equation of motion into which enter the nonlinear resistivities (B, J ,T) caused by flux motion and (B, J ,T) caused by other dissipative processes.J andJ are the current densities perpendicular and parallel toB,B=|B|, andT is the temperature. For flux flow and TAFF in isotropic superconductors with weak relative spatial variation ofB, this equation reduces to the diffusion equation plus a correction term which vanishes whenJ =0 (this means B××B=0) or when – = 0 (isotropic normal conductor). When this diffusion equation holds the material anisotropy may be accounted for by a tensorial . The response of a superconductor to an applied current or to a change of the applied magnetic field is considered for various geometries. Such perturbations affect only a surface layer of thickness where a shielding current flows which pulls at the flux lines; the resulting deformation of the vortex lattice diffuses into the interior until a new equilibrium or a new stationary state is reached. The a.c. response, in particular the frequency with maximum damping, depends thus on the geometry and size of the superconductor.     

Strong-coupling effects in high-T c superconductors

It is conceivable that the high-T _c superconducting perovskites are conventional electronphonon superconductors. In this case one expects significant strong-coupling effects because of the unusually high ratiok _B T _c / of the order 0.1 and greater. We use a set of reasonable models for the Eliashberg function ² F() (which takes into account available information on the phonon spectra and which fit the measuredT _c 's) and calculate strong-coupling effects in the specific heatc _s (T)/T _c , the ratio ₀/k _B T _c , the critical fieldsH _c (T) andH _c2 (T) including Pauli limiting, and other measurable quantities. Strongcoupling corrections turn out to be in the range of 0 to about 100%, depending on the quantity of interest. We discuss the perspectives of using strong-coupling effects as indicators for conventional electron-phonon superconductivity in the new materials.     

Photoemission on highT c superconductors

Ultrathin-layer (AlAs) _m (GaAs) _m superlattices withm = 1, 2, and 3 were grown by molecular beam epitaxy and characterized by x-ray diffraction and photoluminescence measurements. The appearance of distinct satellite peaks around the Bragg reflections demonstrate the formation of high-quality layered crystals. The observed luminescence shows a maximum at 2.033 eV form = 3, and the emission energy decreases form = 2 andm = 1 as well as for them = 4 superlattice. This result for the monolayer superlattice is in good agreement with recent theoretical calculations, and it shows that the (AlAs)₁(GaAs)₁ superlattice represents a new artificial semiconductor material with novel electronic properties.On leave from Mitsubishi Electric Corp., Central Research Laboratory, Amagasaki, Hyogo 661, JapanOn leave from Institute of Semiconductors, Academia Sinica, Beijing, PR China     

Magnetization of high-T c superconductors

We have tried to understand the field dependence of magnetization of high temperature superconductors in the light of phenomenological theory. Especially, the field dependence of dM/d lnB of polycrystalline Bi(2212) is understood by incorporating the overlap of vortices in the London theory.     

Electron-phonon induced superconductivity in highT c superconductors

It is proposed that phonon induced mechanism is capable of explaining the observed range ofT _c for the newly discovered high temperature superconductor within the Allen and Dynes strong coupling theory.     

Possible double-pairing effect in high-T c superconductors

Based on a reexamination of experimental facts a simple model called Double-Pairing Superconductivity is set up to describe the high temperature layered superconductors (HTLS). A mean-field scenario is carried out to investigate the model Hamiltonian. Within our model some of the exotic properties of HTLS may be understood, at least qualitatively.     

Double phase transition in highT c superconductors

Experiments have revealed that some samples of high-T _c ceramic superconductors possess a double peak structure in the specific heat. We shall show that within a BCS formalism it is possible to explain this double peak as a pure superconduction phenomenon. An important feature of the model is the onsite pairing interaction term which turns out to determine the difference between the two peaks.     

Magnetostriction of several high-T c superconductors

We report on magnetostriction measurements of superconducting and nonsuperconducting YBa₂Cu₃O_7– and of two Bi samples with the nominal compositions Bi₂CaSr₂Cu₂O _x and Bi_0.8Pb_0.2Sr_0.8CaCu_1.5O _x . Both types of high temperature superconductors show a nearly identical strongly hysteretic field dependence of the magnetostriction (MS) (l/l+(2–6)*10^–6 at 5 T and 1.5 K). In both cases the MS is a linear function of field in the region 1–5 T, which we explain by striction due to surface currents. Between 1.5 and 35 K the hysteresis of the MS decreases strongly with increasing temperature, which implies a decrease of the pinning force. The MS of quenched nonsuperconducting YBa₂Cu₃O_7– is at least two orders of magnitude smaller than that of the superconductor.     

Fermi-condensate quantum phase transition in high-T c superconductors

The effect of a quantum phase transition associated with the appearance of fermionic condensation in an electron liquid on the properties of superconductors is considered. It is shown that the electron system in both superconducting and normal states exhibits characteristic features of a quantum protectorate after the point of this Fermi-condensate quantum phase transition. The single-particle spectrum of a superconductor can be represented by two straight lines corresponding to two effective masses M _FC * and M _L *. The M _FC * mass characterizes the spectrum up to the binding energy E ₀ , which is of the order of the superconducting gap in magnitude, and determines the spectrum at higher binding energies. Both effective masses are retained in the normal state; however, E ₀ ?4 T. These results are used to explain some remarkable properties of high-T _c superconductors and are in good agreement with recent experimental data.     

On the pairing interaction in highT c superconductors

It is argued, from the dependence of the critical current density and ofT _c on applied magnetic field, that the pairing interaction in highT _c superconductors must be strongly field-dependent.