Browsing by Subject "High temperature superconductivity."

Now showing 1 - 4 of 4

Open Access
High Tc superconductivity
(1989) Gedik, M. Zafer
critical summary of the recent developments in high temperature superconductivity is given. Physical properties of the new materials are summarized. The new theories proposed to explain high Tc are reviewed. In the case of a special type anisotropic gap, the currents and conductivity for single particle tunneling in high Tc superconductor junctions are calculated. Tunneling is assumed to be specular. The position of the peak in the conductivity curve, is found to be determined by the shape of the Fermi surface. Two geometries are considered: tunneling paxallel and perpendicular to CuO^ planes. The order of the peaks for these two configurations turns out to be opposite to experimental results. It is concluded that, conventional one band theory of superconductivity is not enough to explain observed tunneling spectra of high Tc superconductors.
Open Access
Impurity effects and symmetry of the order parameter in high-temperature oxide superconductors
(1997) Bayındır, Mehmet
Identification of the symmetry of the order parameter (OP) of high-l'c cuprates is important because it helps to understand possible mechanism that govern the physics of these materials. However, up to now, there is no consensus on the symmetry of the OP. On the other hand, nonmagnetic impurity substitutions would test the symmetry of OP. Present theoretical calculations overestimate suppression of the critical temperature (by a factor of 2 or more) in comparison to the experimental data. So far, differences between zinc [Zn) and nickel {Ni) substitutions have not been well understood. Considering the above arguments, effects of nonmagnetic impurities on the high-temperature cuprates are investigated by solving the Bogoliulpv-de Gennes (BdG) equations on two-dimensional square lattice. The critical impurity concentration is found to be very close to the experimental values. Possibility of extracting the symmetry of OP from the obtained results is discussed. Finiteranged impurity potentials and different potential strengths for different impurities are proposed to explain differences between Zn and Ni substitutions. Finally, it is concluded that our results are in favor of d-wave symmetry for tetragonal and s+d-wave symmetry for orthorhombic phase, and explain quite well the effects of nonmagnetic impurity substitutions in the high-7'c oxide superconductors. Beside these, the physical properties of the high-temperature oxide superconductors, the BdG equations and effects of nonmagnetic impurities on isotropic and anisotropic superconductors are reviewed.
Open Access
Mean-field renormalization group theory of the t-J model
(2002) Şen, Cengiz
The quantum nature of the high temperature superconductivity models makes analytical approaches to these systems almost impossible to implement. In this thesis, a computational study of the one and two dimensional t − J models that combines mean-field treatments with renormalization group techniques will be presented. This allows one to deal with the noncommutations of the operators at two consecutive sites of the lattices on which these models are defined. The resulting phase diagram for the 1D t − J model reveals an antiferromagnetic ground state, which may, upon doping with increasing temperature, show striped formation that is seen in the high-Tc cuprates. The qualitative features of the phase diagram of the 2D case is also presented, which reveals a phase transition between the disordered and antiferomagnetically ordered phases.
Open Access
Supercurrents and persistent currents in strongly correlated electron systems
(1995) Boyacı, Hüseyin
The lull understanding· ol’the solution Гог the 1-tl Hubbard model is of interest in its own right, and may provide clues to the understanding of higher dimensioned systiMiis. We have found tin? exact solution of tlie model for tw