arXiv subjects

N. M. Plakida

Publications and source records attributed to N. M. Plakida.

24 records · Page 2Linked to original sources

arXiv2000

Charge and magnetic order in two-orbital double-exchange model for manganites

Phase diagram of half-doped perovskite manganites is studied within the extended double-exchange model. To demonstrate the role of orbital degrees of freedom both one- and two-orbital models are examined. A rich phase diagram is obtained in the mean-field theory at zero temperature as a function of J (antiferromagnetic (AFM) superexchange interaction) and V (intersite Coulomb repulsion). For the one-orbital model a charge-ordered (CO) state appears at any value of V with different types of magnetic order which changes with increasing J from ferromagnetic (F) to AFM ones of the types A, C and G. The orbital degeneracy results in appearance of a new CE-type spin order that is favorable due to opening of the "dimerization" gap at the Fermi surface. In addition, the CO state appears only for V>V_c for F and CE states while C-type AFM state disappears and A-type AFM state is observed only at small values of V as a charge disordered one. The relevance of our results to the experimental data are dicussed.

cond-mat.str-el↗

arXiv2000

s+d pairing in orthorhombic phase of copper-oxides

A microscopical theory of electronic spectrum and superconductivity is formulated within the two-dimensional anisotropic t-J model with t_{x}\neq t_{y} and J_{x}\neq J_{y}. Renormalization of electronic spectrum and superconductivity mediated by spin-fluctuations are investigated within the Eliashberg equation in the weak coupling approximation. The gap function has d+s symmetry with the extended s-wave component being proportional to the asymmetry t_{y} - t_{x}. Some experimental consequences of the obtained results are discussed.

cond-mat↗

arXiv1999

Dynamic spin susceptibility in the t-J model

Dynamic spin susceptibility is calculated for the t-J model in the paramagnetic phase by applying the memory function method in terms of the Hubbard operators. A self-consistent system of equations for the memory function is obtained within the mode coupling approximation. Both itinerant hole excitations and localized spin fluctuations give contributions to the memory function. Spin dynamics have a diffusive character in the hydrodynamic limit; spin-wave-like excitations are regained in the high-frequency region.

cond-mat.str-el↗

arXiv1999

Charge dynamics and optical conductivity of the t-J model

The dynamic charge susceptibility and the optical conductivity are calculated in the planar t-J model within the memory function method, working directly in terms of Hubbard operators. The density fluctuation spectrum consists of a damped sound-like mode for small wave vectors and a broad high energy peak ($\sim t$) for large momenta. The study of the optical conductivity shows that electron scattering from spin fluctuations leads to the Drude-frequency dependent relaxation rate which exhibits a crossover from $ω^{3/2}$ behavior at low frequencies ($ω<2|μ|$), to a linear $ω$-dependence for frequencies larger than $2|μ|$. Due to the spin-polaron nature of charge carriers, extra absorbtions arise starting at a frequency $ω\agt J$. The obtained results are in a good agreement with exact diagonalization studies.

cond-mat.str-el↗

arXiv1999

Spin dynamics in the generalized ferromagnetic Kondo model for manganites

Dynamical spin susceptibility is calculated for the generalized ferromagnetic Kondo model which describes itinerant $e_{g}$ electrons interacting with localized $t_{2g}$ electrons with antiferromagnetic coupling. The calculations done in the mean field approximation show that the spin-wave spectrum of the system in ferromagnetic state has two branches, acoustic and optic ones. Self-energy corrections to the spectrum are considered and the acoustic spin-wave damping is evaluated.

cond-mat.str-el↗

arXiv1995

Superconducting pairing of spin polarons in the t-J model

The pairing of quasiparticles in a CuO$_2$ plane is studied within a spin polaron formulation of the $t$-$t^{'}$-$J$ model. Our numerical solution of the Eliashberg equations unambiguously shows d-wave pairing between spin polarons on different sublattices mediated by the exchange of spin-fluctuations, and a strong doping dependence of the quasiparticle bandwidth. The transition temperature $T_c$ is an increasing function of $J/t$ and crosses a maximum at an optimal doping concentration $δ_{opt}$. For the t-J model with $J/t=0.4$ we obtain $T_c\simeq 0.013 t$ at $δ_{opt}\simeq 0.2$.

cond-mat↗