Cooper pairs as low-energy excitations in the normal state

We discuss the normal state of a fermionic system in an idealized PSEUDOGAP REGIME, $k_B T_c < k_B T << |\Delta| << E_F$. Stable Cooper pairs induce a pseudogap of width $|\Delta|$ in the fermion energy spectrum. Near two dimensions, we find a Bose-like condensation temperature in this predominantly fermionic system.Comment: 2 pages, LaTeX, espcrc2.sty file included. An outline of a presentation at the Beijing conference M2S-HTSC-V. To be published in Physica

Knight Shift, Magnetic Susceptibility and Electrical Resistivity of Pure Gallium and Gallium-Indium Eutectic Alloy in the Normal and the Supercooled Liquid State

Knight shift, magnetic susceptibility, electrical resistivity and density of pure gallium and gallium-indium eutectic alloy were measured over a wide temperature range above and below the melting point in order to study the properties of the supercooled state. From the results of the measurements of Knight shift and electronic magnetic susceptibility, a difference in the temperature dependence was detected between the supercooled liquid and the normal liquid of gallium and gallium-indium eutectic alloy. On the other hand, the measurements of electrical resistivity and density did not indicate any discontinuous change. The temperature dependences of effective mass ratio and average probability density at the nucleus for the s-electrons on the Fermi surface were calculated by using the observed results of Knight shift, magnetic susceptibility and density for liquid gallium. From these results a structural change may be expected between the supercooled and the normal liquid gallium

Heat transport of electron-doped Cobaltates

Within the t-J model, the heat transport of electron-doped cobaltates is studied based on the fermion-spin theory. It is shown that the temperature dependent thermal conductivity is characterized by the low temperature peak located at a finite temperature. The thermal conductivity increases monotonously with increasing temperature at low temperatures T $<$ 0.1$J$, and then decreases with increasing temperature for higher temperatures T $>$ 0.1$J$, in qualitative agreement with experimental result observed from Na$_{x}$CoO$_{2}$ .Comment: 4 pages, 1 fig, corrected typos, accepted for publication in Commun. Theor. Phy

Thermodynamics of Crossover from Weak- to Strong-Coupling Superconductivity

In this paper we study an evolution of low-temperature thermodynamical quantities for an electron gas with a $\delta$-function attraction as the system crosses over from weak-coupling (BCS-type) to strong-coupling (Bose-type) superconductivity in three and two dimensions.Comment: Replaced with journal version. Insignificant presentation changes. Links to related papers are also available at the author home page http://www.teorfys.uu.se/PEOPLE/egor

Bose-Einstein condensation of nonzero-center-of-mass-momentum Cooper pairs

Cooper pair (CP) binding with both zero and nonzero center-of-mass momenta (CMM) is studied with a set of renormalized equations assuming a short-ranged (attractive) pairwise interfermion interaction. Expanding the associated dispersion relation in 2D in powers of the CMM, in weak-to-moderate coupling a term {\it linear} in the CMM dominates the pair excitation energy, while the quadratic behavior usually assumed in Bose-Einstein (BE)-condensation studies prevails for any coupling {\it only} in the limit of zero Fermi velocity when the Fermi sea disappears, i.e., in vacuum. In 3D this same behavior is observed numerically. The linear term, moreover, exhibits CP breakup beyond a threshold CMM value which vanishes with coupling. This makes all the excited (nonzero-CMM) BE levels with preformed CPs collapse into a single ground level so that a BCS condensate (where only zero CMM CPs are usually allowed) appears in zero coupling to be a special case in either 2D or 3D of the BE condensate of linear-dispersion-relation CPs.Comment: Four pages including four figures. To be published in Physica

BCS-Bose Crossover in Color Superconductivity

It is shown that the onset of the color superconducting phase occurs in the BCS-BE crossover region.Comment: 5 pages, LaTeX, references adde

Nonadiabatic Superconductivity and Vertex Corrections in Uncorrelated Systems

We investigate the issue of the nonadiabatic superconductivity in uncorrelated systems. A local approximation is employed coherently with the weak dependence on the involved momenta. Our results show that nonadiabatic vertex corrections are never negligible, but lead to a strong suppression of $T_c$ with respect to the conventional theory. This feature is understood in terms of the momentum-frequency dependence of the vertex function. In contrast to strongly correlated systems, where the small ${\bf q}$-selection probes the positive part of vertex function, vertex corrections in uncorrelated systems are essentially negative resulting in an effective reduction of the superconducting pairing. Our analysis shows that vertex corrections in nonadiabatic regime can be never disregarded independently of the degree of electronic correlation in the system.Comment: 4 pages, 3 eps fig

Linear to quadratic crossover of Cooper pair dispersion relation

Cooper pairing is studied in three dimensions to determine its binding energy for all coupling using a general separable interfermion interaction. Also considered are Cooper pairs (CPs) with nonzero center-of-mass momentum (CMM). A coupling-independent {\it linear} term in the CMM dominates the pair excitation energy in weak coupling and/or high fermion density, while the more familiar quadratic term prevails only in the extreme low-density (i.e., vacuum) limit for any nonzero coupling. The linear-to-quadratic crossover of the CP dispersion relation is analyzed numerically, and is expected to play a central role in a model of superconductivity (and superfluidity) simultaneously accommodating a BCS condensate as well as a Bose-Einstein condensate of CP bosons.Comment: 13 pages plus 2 figure