Polarons In Crystals Research Articles

Vibronic polarons: Self‐trapping, local rotation, and band features

AbstractWe revisit basic theoretical concepts of local and itinerant vibronic polarons in crystals. The following results may be regarded as novel: (1) The electron self‐trapping rate to a small polaron is calculated via the reaction rate method; subsequently, self‐trapped on‐center small polarons relax to an off‐center vibronic polaron state. (2) The general vibronic Hamiltonian is redefined so as to incorporate both local and itinerant behavior and pairing into bipolarons or Cooper pairs. (3) The planar rotation and diametral tunneling of an off‐center polaron around and across its centrosymmetrical site are dealt with to adiabatic approximation. (4) Variational calculations are made for vibronic polarons itinerant along 1‐D chains by means of a two‐band extension of Merrifield's ansatz. This investigation of vibronic polarons is undertaken in view of their presumed role in high‐temperature superconductivity and colossal magnetoresistance. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

Semi-empirical supercell calculations for free- and bound-hole polarons in crystal

Two different parametrizations of the semi-empirical method of the intermediate neglect of the differential overlap (INDO) are applied to the calculations of the small-radius hole polarons in the corundum crystal. The 80-atom supercell has been used for the study of the atomic and electronic structure of a free small-radius hole polaron (the self-trapped hole, STH) and a hole polaron bound by a Mg impurity (the so-called centre), respectively. Both parametrizations indicate that the two-site (quasi-molecular) configurations of both kinds of polaron have the lowest energy (which does not exclude the existence of one-site polarons also characterized by considerable relaxation energies). For centres the lower energy is calculated for the defect configuration where the Mg-ion substitution is nearest to the hole polaron. Experimental ENDOR data on defects are discussed in the light of the calculations.

Oxygen vacancies in BaTiO3.

Using electron paramagnetic resonance two types of ${\mathrm{Ti}}^{3+}$-related centers are identified in BaTi${\mathrm{O}}_{3}$, which are attributed to ${\mathrm{Ti}}^{3+}$-${V}_{\mathrm{O}}$ and to ${\mathrm{Ti}}^{3+}$-${V}_{\mathrm{O}}$ associated with Na or K, respectively. This assignment is based on the chemical and illumination conditions leading to the detection of the defects. It is furthermore supported by the Ti hyperfine structure, resolved for ${\mathrm{Ti}}^{3+}$-${V}_{\mathrm{O}}$-Na(K). The symmetry of the centers is broken in the sense that an electron is localized near one of the two equivalent ${\mathrm{Ti}}^{4+}$ ions next to ${V}_{\mathrm{O}}$. Also the observed Ti hyperfine structure points to electron capture at one Ti site. The ${V}_{\mathrm{O}}$-type centers are contrasted to isolated ${\mathrm{Ti}}^{3+}$, representing a free small polaron in crystals containing ${\mathrm{Nb}}^{5+}$ and no oxygen vacancies. The participation of the ${V}_{\mathrm{O}}$ centers in photochromic processes in BaTi${\mathrm{O}}_{3}$ is studied. In crystals doped with Na or K an optical absorption precursor of the fundamental absorption is identified and attributed to the creation of an exciton bound to the ${V}_{\mathrm{O}}$-Na(K) complex.