On temperature versus doping phase diagram of high critiical temperature superconductors

The attempt to describe the bell-shape dependence of the critical temperature of high-$T_{c}$ superconductors on charge carriers density is made. Its linear increase in the region of small densities (underdoped regime) is proposed to explain by the role of the order parameter phase 2D fluctuations which become less at this density growth. The critical temperature suppression in the region of large carrier densities (overdoped regime) is connected with the appearance (because of doping) of the essential damping of long-wave bosons which in the frame of the model proposed define the mechanism of indirect inter-fermion attraction.Comment: 15 pages, 3 figures, EMTE

Spectral moment sum rules for strongly correlated electrons in time-dependent electric fields

We derive exact operator average expressions for the first two spectral moments of nonequilibrium Green's functions for the Falicov-Kimball model and the Hubbard model in the presence of a spatially uniform, time-dependent electric field. The moments are similar to the well-known moments in equilibrium, but we extend those results to systems in arbitrary time-dependent electric fields. Moment sum rules can be employed to estimate the accuracy of numerical calculations; we compare our theoretical results to numerical calculations for the nonequilibrium dynamical mean-field theory solution of the Falicov-Kimball model at half-filling.Comment: (16 pages, submitted to Phys. Rev. B

Fluctuating order parameter in doped cuprate superconductors

We discuss static fluctuations of the d-wave superconducting order parameter $\Delta$ in CuO$_2$ planes, due to quasiparticle scattering by charged dopants. The analysis of two-particle anomalous Green functions at $T = 0$ permits to estimate the mean-square fluctuation $\delta^2 = - ^2$, averaged in random dopant configurations, to the lowest order in doping level $c$. Since $\Delta$ is found to saturate with growing doping level while $\delta$ remains to grow, this can explain the collapse of $T_c$ at overdoping. Also we consider the spatial correlations $$ for order parameter in different points of the plane.Comment: RevTex4, 3 pages, to be published in Proceedings of New3SC-4 International Conference, San Diego, California, January 15-21, 200

Superconducting properties of a boson-exchange model of doped graphene

We study the superconducting properties of a doped one-layer graphene by using a model in which the interparticle attraction is caused by a boson (phonon-like) excitations. We study the dependence of the superconducting gap Delta and the mean-field critical temperature T-c(MF) on the carrier density, attraction strength, and characteristic (Debye) bosonic frequency. In addition, we study the temperature-carrier density phase diagram of the model by taking into account the thermal fluctuations of the order parameter. We show that the fluctuations result in a significant suppression of T-c(MF), such that the real (Berezinskii-Kosterlitz-Thouless) critical temperature T-c is such lower than T-c(MF). The region T-c \u3c T \u3c (MF)(c) is characterized by a finite density of states at the Fermi level (the pseudogap phase). We show that the width of the temperature interval of the pseudogap phase depends strongly on the model parameters-carrier concentration, attraction amplitude, and boson frequency

Superconducting properties of a two-dimensional doped semiconductor

This is a study of the superconducting properties of a two-dimensional model with an additional (insulating) gap Delta(ins) that depends on temperature and doping. In particular, we study the doping dependence of the Berezinskii-Kosterlitz-Thouless critical temperature T(c) and the superconducting pseudogap temperature T(c)(MF) for different values of Delta(ins) by taking hydrodynamic fluctuations of the superconducting order parameter into account. We show that the gap Delta(ins) affects the values of the superconducting gap and the temperatures T(c) and T(c)(MF) within the range of carrier densities where the Delta(ins) approaches zero. In particular, the derivatives of these quantities have a jump in this region. We discuss the possible relevance of these results to high-temperature superconductors

Nonequilibrium sum rules for the retarded self-energy of strongly correlated electrons

We derive the first two moment sum rules of the conduction electron retarded self-energy for both the Falicov-Kimball model and the Hubbard model coupled to an external spatially uniform and time-dependent electric field (this derivation also extends the known nonequilibrium moment sum rules for the Green's functions to the third moment). These sum rules are used to further test the accuracy of nonequilibrium solutions to the many-body problem; for example, we illustrate how well the self-energy sum rules are satisfied for the Falicov-Kimball model in infinite dimensions and placed in a uniform electric field turned on at time t=0. In general, the self-energy sum rules are satisfied to a significantly higher accuracy than the Green's functions sum rules

Superconductivity and superconducting order parameter phase fluctuations in a weakly doped antiferromagnet

The superconducting properties of a recently proposed phenomenological model for a weakly doped antiferromagnet are analyzed, taking into account fluctuations of the phase of the order parameter. In this model, we assume that the doped charge carriers can't move out of the antiferromagnetic sublattice they were introduced. This case corresponds to the free carrier spectra with the maximum at ${\bf k}=(\pm \pi /2 ,\pm \pi /2)$, as it was observed in ARPES experiments in some of the cuprates in the insulating state [1]. The doping dependence of the superconducting gap and the temperature-carrier density phase diagram of the model are studied in the case of the $d_{x^{2}-y^{2}}$ pairing symmetry and different values of the effective coupling. A possible relevance of the results to the experiments on high-temperature superconductors is discussed.Comment: 16 pages, 4 figure