nan Ajay

Role of interlayer interactions on transition temperature in high-Tc cuprate superconductors

Abstract The effect of interlayer interactions on transition temperature in high- T c layered superconductors has been studied using a one-band two-layer tight binding Hamiltonian with intra- and interlayer attractive interactions. The double-time Greens function technique is employed and higher-order Greens functions are linearized by using a mean field decoupling approximation. The expressions for the transition temperature ( T c ) and excitonic type interlayer correlation ( γ c ) are obtained. It is shown that interlayer interactions play an important role in the enhancement of T c in high- T c cuprates. We have also explained the relevance of our results in high- T c layered superconductors having two layers per unit cell under externally applied pressure.

Thermodynamic properties of bilayer cuprate superconductors

The present paper deals with thermodynamic properties of bilayer cuprate superconductors using an extended Hubbard model that includes the various intraplanar and interplaner contributions. It is shown that the thermodynamic properties like transition temperature, density of states (DOS) and electronic contribution to specific heat depend on the intrabilayer interactions as well as on doping level in an essential way. The theoretically evaluated nature of these properties are found to be in qualitative agreement with the existing experimental observations.

Temperature dependence of magnetization and optical magnon gap in bilayer antiferromagnetic YBa2Cu3O6

We study the magnetization and spin excitations of anisotropic bilayer antiferromagnets. We consider a model Hamiltonian which incorporates the inplane, the intrabilayer and the interlayer exchange couplings for a cuprate which has two CuO 2 layers per unit cell. The expression for the sublattice magnetization is obtained by employing the Greens function technique and we have used the Callen decoupling approximation. The numeral results, obtained using the parameters appropriate for YBa 2 Cu 3 O 6 , a bilayer antiferromagnet, show that Callen decoupling procedure gives a value of the Neel temperature which is in reasonable agreement with the measured value of 450 K for YBa 2 Cu 3 O 6 . We have discussed the effects of anisotropy in exchange coupling constants on the Neel temperature of the system and results for the 3D case are also presented. Further, we obtain energy values for the spin excitations (magnons) of the bilayer systems. The excitation spectrum consists of two branches namely the acoustic branch and the optic branch. These branches are separated by an energy gap and the value of this gap at zero wave vector is known as optical magnon gap E G . This gap is found to be dependent on temperature as well as on the anisotropy in the values of the exchange coupling parameters of the system. We have numerically evaluated the anisotropy and the temperature dependence of E G for YBa 2 Cu 3 O 6 .

Interplay of single particle and Cooper pair tunnelings on the superconducting state of layered high-Tc cuprates

Abstract In the present paper we report the interplay of single particle tunneling and Cooper pair tunneling on the low temperature superconducting state of layered high- T c cuprate superconductors. For this a microscopic model Hamiltonian has been considered that incorporates the intra-planar interactions along with contributions corresponding to the coupling between the planes in the form of single particle tunneling as well as Josephson Cooper pair tunneling. The expressions of the superconducting order parameter and the out-of-plane correlation parameter which describes the hopping of a particle from one layer to another layer in the superconducting state, are obtained as a function of various parameters of the model Hamiltonian within the BCS formalism using the Green’s function technique for a bilayer cuprate. It is found that the superconducting order parameter at zero temperature is robust against single particle tunneling while the Cooper pair tunneling between the layers enhances superconducting order parameter and provides favorable conditions for the formation of Cooper pairs within the planes. We have also calculated the out-of-plane contributions to the superconducting condensation energy which is in agreement with the existing calculations on the condensation energy of bilayer cuprates from the specific heat and the c -axis penetration depth measurements.

Role of interlayer coupling in the superconducting state of layered cuprate superconductors

Abstract The role of interlayer coupling in the superconducting state of high- T c layered cuprate systems has been studied using one-band two-layer tight binding Hamiltonian with intra- and interlayer attractive interactions. The expressions of charge carrier (hole) density and momentum dependent energy-gap function are derived on the basis of BCS-type pairing in narrow band within Hubbard-III like decoupling approximations. The hole density and generalized gap-parameter are found to be dependent on the interlayer interactions. It is found that for a two layer per unit cell cuprate systems there is a contribution in the hole density due to interlayer interactions and numerical analysis shows that change in the holes between the cuprate planes depends on the magnitude of interlayer hopping element and superconducting transition temperature.

Temperature dependence of the supercurrent density in bilayer cuprate superconductors

The present work deals with the study of the supercurrent density as a function of temperature and various microscopic interactions that exist in bilayer high temperature cuprate superconducting materials. For this purpose a tight binding bilayer Hubbard Hamiltonian has been considered that includes the in-plane (within CuO2 plane) and out-ofplane interactions. The in-plane component of the model Hamiltonian is just in the BCS form with in-plane hopping energy and effective attractive interaction that comprises on-site and intersite interactions and is responsible to form pairing of the charge carriers within the plane. The out-of-plane part of the Hamiltonian includes single particle tunneling along with intrinsic Josephson like coupling between the planes within the unit cell which is responsible for transfer of single particle as well as Cooper pair from one layer to another and vice-versa, in the superconducting state. The situation for bilayered cuprates considered here is equivalent to a Josephson’s coupled SIS junction. To derive the expressions of the superconducting order parameter, carrier density and supercurrent density, we rely on the Green’s function equations of motion approach within the simple BCS formalism. The numerical analysis shows that in bilayer cuprates the supercurrent density depend on various in-plane and out-of-plane contributions as well as on temperature in an essential way. Finally, we have compared our theoretical results on supercurrent density with that of recent experimental results on temperature and pressure dependence of supercurrent density in bilayer superconducting cuprate materials and found to be in qualitative agreement. 2002 Elsevier Science B.V. All rights reserved.

Role of Cu d-d inter-orbital electron correlation on the out-of-plane conduction in cuprates

Abstract In the present paper we study a model which is relevant to an analysis of the effects of the Cu d–d inter-orbital electron correlation on the motion of charge carriers along c -axis in high- T c cuprates. For this a microscopic model Hamiltonian for the three atom cluster CuO 2 which incorporates the essential features of the basic unit of high- T c cuprates has been considered. The model Hamiltonian for this three atom cluster includes various in-plane and out-of-plane orbital energies, their intra- and inter-orbital Coulomb interactions relevant for the electrons in the cluster. The out-of-plane correlation which appears when we consider the hopping of a hole from the Cu 3d 3 z 2 − r 2 to apical O 2p z orbitals has been calculated using the Greens function technique. The equation of motion for the relevant Greens function contains higher order Greens functions and we evaluate the correlation parameter relevant to motion for a hole along the c -axis by using suitable decoupling approximations. It has been found through numerical calculations that the out-of-plane correlations depend on the intra- and inter-orbital Coulomb interactions, the out-of-plane orbital energies, hole occupancy and on temperature. Finally, the relevance of the out-of-plane correlation parameter for present three atom CuO 2 cluster model to the c -axis conductivity of the bulk high- T c cuprate systems has been pointed out.

Role of dipole-dipole interaction on the magnetic dynamics of anisotropic layered cuprate antiferromagnets

In the present paper, we report the role of dipole-dipole interaction on the magnetic dynamics of single layer antiferromagnets. For this, the model Hamiltonian includes the exchange Heisenberg Hamiltonian as well as dipole-dipole interactions. Within the linear spin-wave theory, we employ the double time Greens function technique to obtain expressions for the spin wave dispersion, sublattice magnetization and the magnetic contribution to specific heat as a function of various parameters of the model Hamiltonian. We observe through numerical calculations that in the absence of anisotropy in exchange couplings the dipole-dipole interaction works as an anisotropy and sustains the magnetization even in a pure 2D system.

Study of doping-dependent shift in the chemical potential of high Tc cuprates by t–t′–J model

Abstract A t – t ′– J model has been employed to study the variation of the chemical potential shift with the change in the density of doped holes in high- T c cuprates. The model Hamiltonian incorporates the nearest neighbour hopping, the next nearest-neighbour hopping and a fictitious Coulomb interaction ( U ′). The fictitious Coulomb interaction is treated within the Hubbard self-energy approximation and U ′→∞ limit is taken to avoid double occupancy. The Hamiltonian also includes the antiferromagnetic (AFM) exchange energy ( J ). This term is treated within the mean-field approximation. The density of states (DOS) is calculated and it depends upon t , t , J and hole density n H . It is found that the t – t ′– J model is successful in explaining the doping dependent DOS and the shift of the chemical potential with n H only in the overdoped regime of the La 2− x Sr x CuO 4 system. For low and moderate dopings the agreement is not good. Reasons for the failure of the model for low doping regime are mentioned.

Spin wave contribution to the thermal expansion of high-Tc cuprate superconductors

Abstract At low temperatures the spin wave excitations are of great importance. In the present paper, we represent the contribution of spin wave modes to the thermal expansion of high- T c cuprate systems in their normal state. The numerical results have been plotted for YBa 2 Cu 3 O 7 .