Quantum Treatment of the Anderson-Hasegawa Model -- Effects of Superexchange and Polarons

We revisit the Anderson-Hasegawa double-exchange model and critically examine its exact solution when the core spins are treated quantum mechanically.We show that the quantum effects, in the presence of an additional superexchange interaction between the core spins, yield a term, the significance of which has been hitherto ignored. The quantum considerations further lead to new results when polaronic effects, believed to be ubiquitous in manganites due to electron-phonon coupling, are included. The consequence of these results for the magnetic phase diagrams and the thermal heat capacity is also carefully analysed.Comment: 18 pages, Revtex, 7 postscript figure

Magnetic transition and polaron crossover in a two-site single polaron model including double exchange interaction

A two-site double exchange model with a single polaron is studied using a perturbation expansion based on the modified Lang-Firsov transformation. The antiferromagnetic to ferromagnetic transition and the crossover from small to large polaron are investigated for different values of the antiferromagnetic interaction ($J$) between the core spins and the hopping ($t$) of the itinerant electron. Effect of the external magnetic field on the small to large polaron crossover and on the polaronic kinetic energy are studied. When the magnetic transition and the small to large polaron crossover coincide for some suitable range of $J/t$, the magnetic field has very pronounced effect on the transport.Comment: REVTEX and ps files, accepted in the European Physical Jour.

Effect of electron corelation on superconducting pairing symmetry

The role of electron correlation on different pairing symmetries are discussed in details where the electron correlation has been treated within the slave boson formalism. It is shown that for a pure $s$ or pure $d$ wave pairing symmetry, the electronic correlation suppresses the $s$ wave gap magnitude (as well as the $T_c$) at a faster rate than that for the $d$ wave gap. On the otherhand, a complex order parameter of the form ($s+id$) shows anomalous temperature dependence. For example, if the temperature ($T_{c}^d$) at which the $d$ wave component of the complex order parameter vanishes happens to be larger than that for the $s$ wave component ($T_{c}^s$) then the growth of the $d$ wave component is arrested with the onset of the $s$ wave component of the order parameter. In this mixed phase however, we find that the suppression in different components of the gap as well as the corresponding $T_c$ due to coulomb correlation are very sensitive to the relative pairing strengths of $s$ and $d$ channels as well as the underlying lattice. Interestingly enough, in such a scenario (for a case of $T_{c}^s > T_{c}^d$) the gap magnitude of the $d$ wave component increases with electron correlation but not $T_{c}^d$ for certain values of electron correlation. However, this never happens in case of the $s$ wave component. We also calculate the temperature dependence of the superconducting gap along both the high symmetry directions ($\Gamma$ - M and $\Gamma$ - X) in a mixed $s+id$ symmetry pairing state and the thermal variation of the gap anisotropy ($\frac{\Delta_{\Gamma - M}}{\Delta_{\Gamma - X}}$) with electron correlation. The results are discussed with reference to experimental observations.Comment: 22 pages, latex, 12 figures (attached in ps /eps) Journal : Accepted for publication in Euro. J. Phys

Coexistence of Band Jahn Teller Distortion and superconductivity in correlated systems

The co-existence of band Jahn-Teller (BJT) effect with superconductivity (SC) is studied for correlated systems, with orbitally degenerate bands using a simple model. The Hubbard model for a doubly degenerate orbital with the on-site intraorbital Coulomb repulsion treated in the slave boson formalism and the interorbital Coulomb repulsion treated in the Hartree-Fock mean field approximation, describes the correlated system. The model further incorporates the BJT interaction and a pairing term to account for the lattice distortion and superconductivity respectively. It is found that structural distortion tends to suppress superconductivity and when SC sets in at low temperatures, the growth of the lattice distortion is arrested. The phase diagram comprising of the SC and structural transition temperatures $T_c$ and $T_s$ versus the dopant concentration $\delta$ reveals that the highest obtainable $T_c$ for an optimum doping is limited by structural transition. The dependence of the occupation probabilities of the different bands as well as the density of states (DOS) in the distorted-superconducting phase, on electron correlation has been discussed.Comment: RevTex, 4 pages, 4 figuers (postscript files attached) Journal Reference : Phys. Rev. B (accepted for publication

Polaronic Heat Capacity in The Anderson - Hasegawa Model

An exact treatment of the Anderson - Hasegawa two - site model, incorporating the presence of superexchange and polarons, is used to compute the heat capacity. The calculated results point to the dominance of the lattice contribution, especially in the ferromagnetic regime. This behavior is in qualitative agreement with experimental findings.Comment: 9 pages, Revtex, 4 postscript figure