Search results for "polaron"
showing 10 items of 79 documents
Luminescence in SrTiO3and LiNbO3Crystals Under High Density Pulsed Electron Excitation
2004
The time-resolved luminescence as well as stationary excited luminescence in SrTiO3 (undoped, Nb and Pb doped) and LiNbO3 (congruent and stoichiometric) single crystals was studied. The charge-transfer vibronic exciton in the regular oxy-anion site is suggested to be responsible for luminescence bands at 2.8 eV and 2.75 eV for SrTiO3 and LiNbO3 crystals respectively. It is suggested that luminescence band observed in SrTiO3 at ∼2.2-2.3 eV above 80 K possibly is due to some defect common for all SrTiO3 crystals studied.
First-principles and semi-empirical calculations for bound hole polarons in KNbO3
1999
The ab initio linear muffin-tin-orbital (LMTO) formalism and the semi-empirical method of the Intermediate Neglect of the Differential Overlap (INDO) based on the Hartree-Fock formalism are combined for the study of the hole polarons (a hole trapped nearby the cation vacancy) in a cubic phase of KNbO3 perovskite crystals. The 40-atom and 320-atom supercells were used, respectively. We predict existence of both, one-site and two-site (molecular) polarons with close optical absorption energies (0.9 eV and 0.95 eV). The relevant experimental data are discussed.
Polaronic-type excitons in ferroelectric oxides: Microscopic calculations and experimental manifestation
2002
We discuss the current experimental and theoretical understanding of new polaronic-type excitons in ferroelectric-oxides charge-transfer vibronic excitons ~CTVE’s!, which are pairs of strongly correlated electronic and hole polarons. It is shown that charge-transfer‐lattice distortion interactions are the driving forces for CTVE formation. Hartree-Fock-type calculations performed in the framework of the intermediate neglect of differential overlap ~INDO! method as well as photoluminescence, second-harmonic generation, and UVabsorption high-temperature studies performed for ABO3 ferroelectric oxides strongly support the CTVE existence. Both single CTVE and a phase of strongly correlated CTVE…
Fingerprints of spin-orbital polarons and of their disorder in the photoemission spectra of doped Mott insulators with orbital degeneracy
2017
We explore the effects of disordered charged defects on the electronic excitations observed in the photoemission spectra of doped transition metal oxides in the Mott insulating regime by the example of the $R_{1-x}$Ca$_x$VO$_3$ perovskites, where $R=$La,$\dots$,Lu. A fundamental characteristic of these vanadium $d^2$ compounds with partly filled $t_{2g}$ valence orbitals is the persistence of spin and orbital order up to high doping, in contrast to the loss of magnetic order in high-$T_c$ cuprates at low defect concentration. We demonstrate that the disordered electronic structure of doped Mott-Hubbard insulators can be obtained with high precision within the unrestricted Hartree-Fock appro…
Magnetopolaron in a weakly elliptical InAs/GaAs quantum dot
2003
We study theoretically the properties of a polaron formed in a shallow, weakly elliptical, disk-shaped InAs/GaAs quantum dot in the presence of a magnetic field by using the Davydov's canonical transformation. Special attention is paid to the energy-level splitting due to the Frohlich interaction of an electron in a quantum dot with optical phonons near resonance. The polaron relaxation rates, including the anharmonicity induced channel, are analyzed for various confinement energies and magnetic field magnitudes, taking into account coherent polaronic effects.
Quantum chemical modelling of "green" luminescence in ABO $ \mathsf {_3}$ perovskites
2002
The origin of the intrinsic excitonic (“green”) luminescence in ABO3 perovskites remains a hot topic over the last quarter of a century. We suggest as a theoretical interpretation for the “green” luminescence in these crystals, the recombination of electron and hole polarons forming a charge transfer vibronic exciton. In order to check quantitatively the proposed model, we performed quantum chemical calculations using the Intermediate Neglect of Differential Overlap (INDO) method combined with the periodic defect model. The luminescence energies calculated for four perovskite crystals are found to be in good agreement with experimental data.
Theory of bound polarons in oxide compounds
2001
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.…
Transient absorption of polarons in KNbO3
2002
Abstract Transient absorption spectra have been measured in a number of undoped KNbO 3 crystals following two kinds of electronic excitation: two-photon absorption of 200 fs laser pulses and 10 ns pulsed electron beam. A band peaking at 0.8 eV and broad, poorly resolved absorption in the range 1.3–3.3 eV were observed. Based on similarity to the 1 eV band in LiNbO 3 previously identified as the intrinsic electron polaron (electron self-localized on niobium in the regular lattice site), we suggest that the transient absorption band at 0.8 eV in KNbO 3 is also associated with the intrinsic electron polaron.
Relaxation and decoherence of orbital and spin degrees of freedom in quantum dots
2002
The phonon induced mechanisms of relaxation/decoherence in quantum dots are analysed. A non-perturbative technique - a modification of the Davydov transformation appropriate to the localised particles is applied for solving the electron-phonon eigenvalue problem in a quantum dot at magnetic field presence. The decay rates for polaron relaxation via the anharmonicity induced channel are analysed in details. In particular, it is indicated that previous, of perturbative type, estimations of the anharminicity induced relaxation rates were too severe and after including the coherence effects they are of, at least, one order longer. The process of exciton dressing with phonons is also analysed as…
Small-polaron transport inLa0.67Ca0.33MnO3thin films
1998
We present a detailed study of the activated resistivity of ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}{\mathrm{MnO}}_{3}$ films up to 600 K under the influence of high magnetic fields. Data in zero field can be explained by small polaron hopping as treated in the Friedman-Holstein theory. Based on the spin orientation of ferromagnetic clusters in a magnetic field, we develop a phenomenological model describing the temperature and field dependence of the resistivity with a minimum of free parameters. We find that the polarons have a magnetic contribution to their activation energy for hopping which depends on the variation of the spin order with increasing temperature and can be modified by a…