Theory of bound polarons in oxide compounds

We present a multilateral theoretical study of bound polarons in oxide compounds MgO and \alpha-Al_2O_3 (corundum). A continuum theory at arbitrary electron-phonon coupling is used for calculation of the energies of thermal dissociation, photoionization (optically induced release of an electron (hole) from the ground self-consistent state), as well as optical absorption to the non-relaxed excited states. Unlike the case of free strong-coupling polarons, where the ratio \kappa of the photoionization energy to the thermal dissociation energy was shown to be always equal to 3, here this ratio depends on the Froehlich coupling constant \alpha and the screened Coulomb interaction strength \beta. Reasonable variation of these two parameters has demonstrated that the magnitude of \kappa remains usually in the narrow interval from 1 to 2.5. This is in agreement with atomistic calculations and experimental data for hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation energy for the ground self-consistent state and the energy of the optically induced charge transfer process (hops of a hole between O^{2-} ions) have been calculated using the quantum-chemical method INDO. Results obtained within the two approaches for hole O$^-$ polarons bound by the cation vacancies (V^-) in MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to experimental data and to each other. We discuss a surprising closeness of the results obtained on the basis of independent models and their agreement with experiment.Comment: 13 pages, 2 figures, 2 tables, E-mail addresses: [email protected], [email protected]

Thermal conductivity in the soliton bearing ferromagnetic system CHAB

The thermal conductivity of [C6H11NH3] CuBr3 (CHAB) has been measured in the region 1.5 K <T <10 K and B <7 T. Comparison of the data with a model describing the heat conduction by magnons and solitons shows a systematic deviation, most likely due to the neglect of heat transport by the phonon system

Thermal conductivity of some soliton-bearing magnetic systems

The thermal conductivity ¿ of [C6H11NH3]Cu Br3 (CHAB) has been measured in the region 1.5

Comparison between experiments and numerical calculations on S = 1/2 Heisenberg-XY ferromagnetic chains

Heat capacity measurements in fields 0 = B = 3 T were performed on [C6H11NH3] CuBr3 (CHAB), an excellent ferromagnetic S = 1/2 chain system with 5 % XY anisotropy. A comparison of the data on ¿C = C (B) - C (0) with extrapolations of numerical results for finite chains reveals systematic deviations at low temperatures, which are attributed to the limited chain lengths (up to 11 spins) used in the computations