Effective interactions and superconductivity in the t-J model in the large-N limit

The feasibility of a perturbation expansion for Green's functions of the t-J model directly in terms of X-operators is demonstrated using the Baym- Kadanoff functional method. As an application we derive explicit expressions for the kernel of the linearized equation for the superconducting order parameter in leading order of a 1/N expansion. The linearized equation is solved numerically on a square lattice. We find that a reasonably strong instability occurs only for even frequency pairing with d-wavelike symmetry. Results for the transition temperature and the effective interaction are given as a function of doping.Comment: 31 pages, 11 figure

ING4 (inhibitor of growth family, member 4)

Review on ING4 (inhibitor of growth family, member 4), with data on DNA, on the protein encoded, and where the gene is implicated

C-Axis Tunneling Spectra in High-Tc_c Superconductors in the Presence of a d Charge-Density Wave

The optimally doped and underdoped region of the $t-J$ model at large N (N is the number of spin components) is governed by the competition of d-wave superconductivity (SC) and a d Charge-Density Wave (d-CDW).The partial destruction of the Fermi surface by the d-CDW and the resulting density of states are discussed. Furthermore, c-axis conductances for incoherent and coherent tunneling are calculated, considering both an isotropic and an anisotropic in-plane momentum dependence of the hopping matrix element between the planes. The influence of self-energy effects on the conductances is also considered using a model where the electrons interact with a dispersionless, low-lying branch of bosons. We show that available tunneling spectra from break-junctions are best explained by assuming that they result from incoherent tunneling with a strongly anisotropic hopping matrix element of the form suggested by band structure calculations. The conductance spectra are then characterized by one single peak which evolves continuously from the superconducting to the d-CDW state with decreasing doping. The intrinsic c-axis tunneling spectra are, on the other hand, best explained by coherent tunneling. Calculated spectra show at low temperatures two peaks due to SC and d-CDW. With increasing temperature the BCS-like peak moves to zero voltage and vanishes at T$_c$,exactly as in experiment.Our results thus can explain why break junction and intrinsic tunneling spectra are different from each other. Moreover, they support a scenario of two competing order parameters in the underdoped region of high-T$_c$ superconductors.Comment: 12 pages, 16 figure

Superconductivity, d Charge-Density Wave and Electronic Raman Scattering in High-Tc_c Superconductors

The competition of superconductivity and a d charge-density wave (CDW) is studied in the t-J model as a function of temperature at large N where N is the number of spin components. Applying the theory to electronic Raman scattering the temperature dependence of the $B_{1g}$ and the $A_{1g}$ spectra are discussed for a slightly underdoped case.Comment: 2 pages, 3 figures, Proc. M2S-HTSC-VII, to appear in Physica

Electronic correlations, electron-phonon interaction, and isotope effect in high-Tc cuprates

Using a large-N expansion we present and solve the linearized equation for the superconducting gap for a generalized t-J model which also contains phonons within a Holstein model. The leading Tc has d-wave symmetry with phonons giving a positive contribution to Tc. The corresponding isotope coefficient is very small at optimal doping and increases towards the classical value 1/2 with increasing dopings similar as in many cuprates.Comment: 14 pages, 7 figure

Collective excitations in unconventional charge-density wave systems

The excitation spectrum of the t-J model is studied on a square lattice in the large $N$ limit in a doping range where a $d$-$density$-$wave$ (DDW) forms below a transition temperature $T^\star$. Characteristic features of the DDW ground state are circulating currents which fluctuate above and condense into a staggered flux state below $T^\star$ and density fluctuations where the electron and the hole are localized at different sites. General expressions for the density response are given both above and below $T^\star$ and applied to Raman, X-ray, and neutron scattering. Numerical results show that the density response is mainly collective in nature consisting of broad, dispersive structures which transform into well-defined peaks mainly at small momentum transfers. One way to detect these excitations is by inelastic neutron scattering at small momentum transfers where the cross section (typically a few per cents of that for spin scattering) is substantially enhanced, exhibits a strong dependence on the direction of the transferred momentum and a well-pronounced peak somewhat below twice the DDW gap. Scattering from the DDW-induced Bragg peak is found to be weaker by two orders of magnitude compared with the momentum-integrated inelastic part.Comment: 10 pages, 8 figure

Influence of collective effects and the d-CDW on electronic Raman scattering in high-Tc_c superconductors

Electronic Raman scattering in high-T$_c$ superconductors is studied within the t-J model. It is shown that the A$_{1g}$ and B$_{1g}$ spectra are dominated by amplitude fluctuations of the superconducting and the d-wave CDW order parameters, respectively. The B$_{2g}$ spectrum contains no collective effects and its broad peak reflects vaguely the doping dependence of T$_c$, similarly to the pronounced peak in the A$_{1g}$ spectrum. The agreement of our theory with the experiment supports the picture of two different, competing order parameters in the underdoped regime of high-T$_c$ superconductors.Comment: 4 pages, 4 figures, will appear in PR