Superconducting gap within a modified interlayer tunneling model

A modified version of the interlayer tunneling model, including interlayer single particle hopping (ISPH), is considered as a phenomenological model to describe cuprate superconductors. The effective ISPH (t_\perp^{eff}) is taken along with a probability factor P, that involves the normal state pseudogap (E_g). This makes t_\perp^{eff} to mimic experimental observations that, ISPH is small in the underdoped regime and increases towards overdoping. Within the modified model, we establish the absence of bilayer splitting as observed in case of layered cuprates. Transition temperature (T_c) and the superconducting gap are calculated. A match, to the T-dependent superconducting gap data from experiment, is obtained and high values of the ratio of the superconducting gap to T_c are recovered. Depending on the values of E_g, T_c as a function of interlayer coupling shows mixed behaviour. This is a prediction and can be checked further.Comment: Latex file, 14 pages, 4 figures (postscript files) include

Effects of c-axis Hopping in the Interlayer Tunneling Model of High-Tc Layered Cuprates

We consider the interlayer pair-tunneling model for layered cuprates, including an effective single particle hopping along the c-axis. A phenomenological suppression of the c-axis hopping matrix element, by the pseudogap in cuprate superconductors, is incorporated. At optimal doping, quantities characteristic to the superconducting state, such as the transition temperature and the superconducting gap are calculated. Results from our calculations are consistent with the experimental observations with the noteworthy point that, the superconducting gap as a function of temperature shows excellent match to the experimental data. Predictions within the model, regarding T_c variation with interlayer coupling, are natural outcomes which could be tested further.Comment: Latex file, 18 pages, 4 figures (postscript files included), to appear in Int. J. Mod. Phys.

An extended pair tunneling model: studies on bilayer splitting and some superconducting state properties

We consider an extended version of the pair tunneling model including interlayer single particle hopping (ISPH) as a complementary process to pair tunneling. The normal state gap, as found in cuprates, is taken to suppress the effective ISPH in conformity with the experimental observations, and this in turn enhances the pair tunneling process. The effective ISPH involves a probability factor P for which we consider two choices and provide phenomenological arguments in favour of them. We address the issue of bilayer splitting by calculating the spectral density function and corresponding photoemission intensity curves and show that our calculations conform with the absence of bilayer splitting observed in ARPES experiments on Bi2212. We have also studied the temperature variation of the superconducting gap and ratio of the superconducting gap to T_c. Our results, obtained for both the choices of P, are reasonably in good agreement with those from experiments on cuprate superconductors. A linear T-dependent choice of P, however, yields a precise match to the experimantal data of the temperature varying superconducting gap.Comment: Latex file, 17 pages, 5 figures (postscript files) include

Impurity Substitution in Bismuth and Thallium Cuprates: Suppression of T_c and Estimation of Pseudogap

Suppression of T_c in bilayer bismuth and thallium cuprates, by substitution of Co impurities at Cu sites, are taken for examination. T_c suppression data on differently doped Bi2212 and Tl2212 are analysed within the unitary pair-breaking formalism due to Abrikosov and Gorkov, by fitting data points to a phenomenological relation valid for weak coupling d-wave superconductors. Values of the pseudogap magnitude at each doping are thereby estimated within a "fermi-level density of states suppression" picture. Pseudogap magnitude from our estimation is observed to have a correspondence with a related characteristic temperature T^\star obtained by thermoelectric power measurements. Effects of pseudogap, on the density of states, is studied by calculating the susceptibility which shows a broad peak at high temperature. This peak feature in susceptibility is indicative of an unusual metallic state which could further be explored by systematic other measurements.Comment: Latex file, 14 pages, 3 figures (ps files included). To appear in Physica

d-Wave Order Parameter in Bi2212 from a Phenomenological Model of High Tc_c Cuprates

A phenomenological lattice model of high $T_c$ cuprates including order parameter phase fluctuations is considered within the BCS approximation, to interpret the experimental data from ARPES measurements on Bi2212 samples. A Kosterlitz-Thouless (KT) transition temperature $T_c^{KT}$ is estimated below the mean field transition $T_c^{MF}$, phase boundaries between competing order parameters of different symmetries are obtained and best model parameters, fitting the ARPES gap of $d_{x^2-y^2}$ symmetry, are determined. Variation of $T_c^{KT}$, as a function of the dopant concentration $\delta$, is in qualitative agreement with experiments.Comment: Latex file, 11 output pages, 5 figures (available from the author on request

Phase Diagram of the Half-Filled Extended Hubbard Model in Two Dimensions

We consider an extended Hubbard model of interacting fermions on a lattice. The fermion kinetic energy corresponds to a tight binding Hamiltonian with nearest neighbour (-t) and next nearest neighbour (t') hopping matrix elements. In addition to the onsite Hubbard interaction (U) we also consider a nearest neighbour repulsion (V). We obtain the zero temperature phase diagram of our model within the Hartree-Fock approximation. We consider ground states having charge and spin density wave ordering as well as states with orbital antiferromagnetism or spin nematic order. The latter two states correspond to particle-hole binding with $d_{x^2-y^2}$ symmetry in the charge and spin channels respectively. For $t' = 0$, only the charge density wave and spin density wave states are energetically stable. For non-zero t', we find that orbital antiferromagnetism (or spin nematic) order is stable over a finite portion of the phase diagram at weak coupling. This region of stability is seen to grow with increasing values of t'.Comment: Latex file, 10 output pages, 3 Figures (available on request to [email protected]), to appear in Phys. Rev. B (BR