The effect of band Jahn-Teller distortion on the magnetoresistivity of manganites: a model study.

We present a model study of magnetoresistance through the interplay of magnetisation, structural distortion and external magnetic field for the manganite systems. The manganite system is described by the Hamiltonian which consists of the s-d type double exchange interaction, Heisenberg spin-spin interaction among the core electrons, and the static and dynamic band Jahn-Teller (JT) interaction in the e(g) band. The relaxation time of the e(g) electron is found from the imaginary part of the Green's function using the total Hamiltonian consisting of the interactions due to the electron and phonon. The calculated resistivity exhibits a peak in the pure JT distorted insulating phase separating the low temperature metallic ferromagnetic phase and the high temperature paramagnetic phase. The resistivity is suppressed with the increase of the external magnetic field. The e(g) electron band splitting and its effect on magnetoresistivity is reported here.

Theoretical study of the Raman active CDW gap mode in manganites.

We report here the microscopic theory of the Raman spectra of the colossal magnetoresistive (CMR) manganite systems. The system is described by a model Hamiltonian consisting of the double exchange interaction in addition to the charge ordering interaction in the e(g) band and spin-spin interaction among the t(2g) core electrons. Further the phonon coupling to the conduction electron density is incorporated in the model for phonons in the harmonic approximation. The spectral density function for the Raman spectra is calculated from the imaginary part of the phonon Green's function. The calculated spectra display the Raman active bare phonon peak along with the charge ordering peak. The magnetic field and temperature dependence of the charge ordering peak agrees with the 480 cm(-1) JT mode observed in the experiments. The evolution of this mode is investigated in the report.

Theory of photo-magnetization of an interacting particle system: application to Hg(1-x)Mn(x)Te.

We derive a theory of light-induced magnetization of an interacting particle system, which describes a model diluted magnetic semiconductor. We set up a many-body Hamiltonian that includes electron and hole kinetic energies, carrier-photon and carrier-local-moment interactions and all the Coulomb interactions. Using Heisenberg equations of motion and a mean-field approximation, we derive expressions for the carrier spin density and excitonic amplitudes. Non-equilibrium relaxation is included phenomenologically. The interdependence of carrier spin density and the local-moment magnetization is calculated self-consistently. We apply our theory of photo-magnetization to Hg(1-x)Mn(x)Te, choosing parameters of the model appropriate to this system. Our results for the photo-magnetization versus the input laser power agree qualitatively with the observed trends in this system. We also study the photo-magnetization as a function of temperature, manganese concentration and photon energy and the results obtained are along expected lines.