Teet Örd

Critical and Non-Critical Coherence Lengths in a Two-Band Superconductor

We study the peculiarities of coherency in a two-gap superconductor. Both intraband couplings, inducing superconductivity in the independent bands, and interband pair-transfer interaction have been taken into account. On the basis of the Ginzburg–Landau equations derived from the Bogoliubov–de Gennes equations and the relevant self-consistency conditions for a two-gap system, we find critical and non-critical coherence lengths in the spatial behaviour of the fluctuations of order parameters. The character of the temperature dependencies of these length scales is determined by the relative contributions from intra- and interband interaction channels.

Modeling MgB2 two-gap superconductivity

Abstract A two-band model for the description of MgB 2 two-gap superconductivity is developed. Debye-layer electron–phonon σ intraband coupling with unbounded σ intraband and σ–π interband Coulombic interactions are incorporated. An adjustable parameter set is determined which reproduces consistently T c , the specific heat jump and the gap ratio. Absolute values of the gaps and the isotope effect exponent containing no further adjustable parameters agree with the experiment. T c vs doping and gaps vs temperature curves are given.

Two relaxation times and the high-Tc superconductivity two-component scenario

Abstract The relaxation times for a two-band superconductor with two-component order parameter driven by interband scattering of intraband pairs are calculated. In agreement with a recent experiment [J. Demsar, B. Podobnik, V.V. Kabanov, Th. Wolf, D. Mihailovic, Phys. Rev. Lett. 82 (1999) 4918.], a critical and noncritical relaxation channel with different time scales have been found. This circumstance supports the applicability of two-band models to the high- T c phenomenon.

Diffusion and current of Brownian particles in tilted piecewise linear potentials: Amplification and coherence

Overdamped motion of Brownian particles in tilted piecewise linear periodic potentials is considered. Explicit algebraic expressions for the diffusion coefficient, current, and coherence level of Brownian transport are derived. Their dependencies on temperature, tilting force, and the shape of the potential are analyzed. The necessary and sufficient conditions for the nonmonotonic behavior of the diffusion coefficient as a function of temperature are determined. The diffusion coefficient and coherence level are found to be extremely sensitive to the asymmetry of the potential. It is established that at the values of the external force, for which the enhancement of diffusion is most rapid, the level of coherence has a wide plateau at low temperatures with the value of the Péclet factor 2. An interpretation of the amplification of diffusion in comparison with free thermal diffusion in terms of probability distribution is proposed.

Paired carrier effective mass isotope effect in the two-band model

By modelling the two-component scenario of high-T c superconductivity the isotope effect (β) in the supercarrier effective mass is calculated by a two-band scheme with interband pair scattering. The behaviour of T c , α and β in the underdoped region is illustrated. The repulsive interband interaction contains only a small electron-phonon contribution. The dependence of β (also of T c , α) on doping is as observed [7], where β is larger.

Two-gap superconductivity: interband interaction in the role of an external field

We show that weak interband pair-transfer interaction affects the weaker-superconductivity component of a two-gap system as an external field associated with the order parameter, similarly to an external magnetic field for ferromagnetics or an external electric field for ferroelectrics.

Band filling effects on coherence lengths and penetration depth in the two-orbital negative-U Hubbard model of superconductivity

Abstract The two-orbital superconducting state is modeled by on-site intra-orbital negative- U Hubbard correlations together with inter-orbital pair-transfer interactions. The critical temperature is mainly governed by intra-orbital attractive interactions and it can pass through an additional maximum as a function of band filling. For the certain number of electrons the clear interband proximity effect is observable in the superconducting state of the band with a smaller gap. The influence of band fillings and orbital site energies on the temperature dependencies of two-component superconductivity coherence lengths and magnetic field penetration depth is analyzed. The presence of proximity effect is probably reflected in the relative temperature behaviour of characteristic lengths.

Correlation functions and coherence lengths in a two-gap superconductor

We derive analytically the spatial correlation functions for gap fluctuations in two-band scenario with intraand interband pair-transfer interactions. These functions demonstrate the changes in spatial functionality due to the presence of two channels of coherency described by the divergent and finite correlation lengths. Even at the phase transition point both channels are essential for two-band superconductivity. Generally their relative contributions depend on the temperature and system parameters.

Correlation in the velocity of a Brownian particle induced by frictional anisotropy and magnetic field

We study the motion of charged Brownian particles in an external magnetic field. It is found that a correlation appears between the components of particle velocity in the case of anisotropic friction, approaching asymptotically zero in the stationary limit. If magnetic field is smaller compared to the critical value, determined by frictional anisotropy, the relaxation of the correlation is non-oscillating in time. However, in a larger magnetic field this relaxation becomes oscillating. The phenomenon is related to the statistical dependence of the components of transformed random force caused by the simultaneous influence of magnetic field and anisotropic dissipation.

On the fluctuation-induced superconductivity pseudogap

Abstract The influence of spatially correlated order parameter fluctuations on the elementary excitations in the normal phase of a superconductor has been investigated. As a result of fluctuations, a pseudogap on the Fermi surface is present. The magnitude of the pseudogap decreases with temperature rising and it is essentially scaled with the parameter α = ϵ F / kT c . This explains why the pseudogap has been observed in high- T c systems and not in the conventional superconductors and its suppression in the overdoped high- T c systems.