Tran Minh-Tien

Charge ordered ferromagnetic phase in manganites

A mechanism for charge ordered ferromagnetic phase in manganites is proposed. The mechanism is based on the double exchange in the presence of diagonal disorder. It is modeled by a combination of the Ising double-exchange and the Falicov-Kimball model. Within the dynamical mean-field theory the charge and spin correlation function are explicitly calculated. It is shown that the system exhibits two successive phase transitions. The first one is the ferromagnetic phase transition, and the second one is a charge ordering. As a result a charge ordered ferromagnetic phase is stabilized at low temperature.

Correlated hopping in the Falicov-Kimball model: a dynamical mean-field study

We study the effects of correlated hopping in the two-dimensional (2D) Falicov-Kimball model by means of an extension of the dynamical mean-field approximation (DMFA). The extension is based on the projection technique and the DMFA, in which nonlocal correlations are taken into account through static quantities of a relevant subspace. The effect of the correlated hopping is to introduce nonlocal self-energy components which remain even at D→∞. We show that the sum rules of the spectral function and its moments are preserved. The spectral function obtained reveals significant nonlocal contributions which are absent in the DMFA.

Optical conductivity and resistivity in the two-band Emery model

Abstract The temperature- and frequency-dependent conductivity due to the scattering of oxygen holes by antiferromagnetic spin fluctuations of the copper spins in the two-band Emergy model is calculated. Using the dynamic spin susceptibility obtained in the mean-field Schwinger boson approach, the resistivity obeys a near linear temperature dependence at high temperatures, whereas at low temperatures a quadratic behaviour holds. At the same time, the optical conductivity contains the Drude peak around zero frequency, whereas the scattering rate of quasiparticle appears to be proportional to frequency. Our results are essentially in agreement with experiments, at least qualitatively, and support the conclusion that the normal-state basal-plane resistivity and optical conductivity of high- T c superconductors may be explained by two-dimensional spin-fluctuation scattering in the Fermi-liquid picture.

Magnetic interactions in the Emery model

Abstract The Emery model for CuO 2 planes in high- T c superconductors is considered. Treating this model by the cell-perturbation method, a singlet-triplet model with effective spin interactions is derived. Both superexchange (SE) and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions are calculated. It is found that with increasing carrier concentration the SE interaction decreases, while the RKKY interaction increases. With increasing oxygen transfer both SE and RKKY interactions increase. The SE interaction is dominant, except for a small region of large hole doping where the SE interaction changes its sign. The suppression of the exchange interaction with hole doping is found and it may explain the drop in magnetic order with hole doping. The magnetic frustrations are also calculated and they are found even for zero bandwidth and for the half-filling insulator case.