Search

We use the Mott-Littleton approach to evaluate polarisation energies in LaMnO$_3$ lattice associated with holes localized on both Mn$^{3+}$ cation and O$^{2-}$ anion. The full (electronic and ionic) lattice relaxation energy for a hole localized at the O-site is estimated as 2.4 eV which is appreciably greater than that of 0.8 eV for a hole localized at the Mn-site, indicating on the strong electron-phonon interaction in the former case. Using a Born-Haber cycle we examine thermal and optical energies of the hole formation associated with electron ionization from Mn$^{3+}$, O$^{2-}$ and La$^{3+}$ ions in LaMnO$_3$ lattice. For these calculations we derive a phenomenological value for the second electron affinity of oxygen in LaMnO$_3$ lattice by matching the optical energies of La$^{4+}$ and O$^-$ hole formation with maxima of binding energies in the experimental photoemission spectra. The calculated thermal energies predict that the electronic hole is marginally more stable in the Mn$^{4+}$ state in LaMnO$_3$ host lattice, but the energy of a hole in the O$^-$ state is only higher by a small amount, 0.75 eV, rather suggesting that both possibilities should be treated seriously. We examine the energies of a number of fundamental optical transitions, as well as those involving self-trapped holes of Mn$^{4+}$ and O$^-$ in LaMnO$_3$ lattice. The reasonable agreement with experiment of our predicted energies, linewidths and oscillator strengths leads us to plausible assignments of the optical bands observed. We deduce that the optical band near 5 eV is associated with O(2p) - Mn(3d) transition of charge-transfer character, whereas the band near 2.3 eV is rather associated with the presence of Mn$^{4+}$ and/or O$^-$ self-trapped holes in non-stoichiometric LaMnO$_3$ compound., Comment: 18 pages, 6 figures, it was presented partially at SCES-2001 conference in Ann Arbor, Michigan

Quantum dots show a range of time–dependent behaviours. We show that the polarity of II–VI nanoparticles has important dynamical implications for electronic, vibrational and other phenomena. Polarity–dependent phenomena are found even for nearly spherical stoichiometric clusters of ZnO and ZnS in studies based on interatomic potentials or on a plane–wave density–functional approach. We find a substantial dipole moment for free nanoparticles, whether of the zinc blende or wurtzite structure. This dipole causes a highly non–uniform spin–density distribution on electronic excitation or after a change in the dot9s electronic charge state. The spin density of the triplet exciton shows that the dipole aligns so as to reduce the dipole moment in the electronically excited state. The polarity of II–VI dots also affects their vibrational properties. High– and low–frequency tails of the vibration density of states arise from modes strongly localized at surface atoms, near the poles of the dipole. These features, first noted for free clusters, also hold for particles embedded in a wide–gap dielectric, a –SiO 2 . We present the results of molecular dynamics of the ZnS particle embedded into the silica glass, and consider the role played by the soft modes in energy–dissipation processes such as dephasing during non–radiative recombination of excitons, and energy transfer from the dot to the matrix.