Browsing by Author "Ivanov, V. A."
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Item Open Access Exactly soluble coherent state path integral with non-polynomial action(Institute of Physics Publishing Ltd., 1999) Ivanov, V. A.; Zhuravlev, M. Ye.; Yarunin, V. S.; Hakioǧlu T.We present an example of an exactly soluble bosonic coherent state path integral with non-polynomial action.Item Open Access Isotope Effect and Phonon Softening in Superconducting Borocarbides and Boronitrides(1998) Hakioǧlu, Tuğrul; Ivanov, V. A.The isotope effect in the recently disvovered class o superconductors LuNi2B2C and La3Ni2B2N3 is investigated in the context of electron-squezed phonon interaction renormalizing the Ni-d electron-electron correlations. Squeezed phonon mode originates from the anharmonic character of the tetragonal Ni-B structure and is polarized in the vortical direction to the Ni layers. The isotope effect arises as a result of the zero point motion of the Ni-Ni d-electron hopping amplitude dominantly due to this vertical phonon mode. Within this model the isotope exponent is calculated to be αB ≤ 0.20 as compared to the recently found experimental value αexp B=0.27∓0.10. Finally, the phonon frequency softening predicted by our model eletron-phonon interaction is discussed within the context of recent experiments on the relevant boron A1g softening.Item Open Access Phonon squeezing in superconducting borocarbides(Elsevier BV * North-Holland, 1994) Hakioglu, T.; Ivanov, V. A.; Shumovsky, A. S.; Tanatar, BilalThe recently discovered superconductor LuNi2B2C is investigated in the context of strong electron-electron correlations modulated by a squeezed phonon mode propagating in the perpendicular direction to the layers with longitudinal polarization. The squeezed phonons arise in the anharmonic lattice expansion since the linear electron-phonon interaction vanishes due to the structure of the NiB4 tetrahedra. The observed weak isotope effect and small dependence of Tc on pressure is qualitatively understood within the framework of this model. © 1994.Item Open Access Phonon squeezing via correlations in the superconducting electron-phonon interaction(American Physical Society, 1995) Hakioğlu, T.; Ivanov, V. A.; Shumovsky, A. S.; Tanatar, BilalSuperconductivity in the conventional BCS model with correlated squeezed phonons is discussed. It is shown that the energy gap and the critical temperature are maximally enhanced in an optimum and finite range of squeezed coupling. For finite-squeezed coupling the ratio 2Δ/Tc becomes coupling-constant dependent and increases beyond the BCS value of 3.53. Ion-mass dependence of the squeezed coupling constant can yield variations of the isotope exponent from its conventional BCS value of 0.5. © 1995 The American Physical Society.Item Open Access SU(2) coherent state path integral investigation of the Holstein dimer(2000) Hakioğlu, A. T.; Ivanov, V. A.; Zhuravlev, M. Ye.The SU(2) coherent state path integral is used to investigate the partition function of the Holstein dimer. This approach naturally takes into account the dynamical symmetry of the model. The ground-state energy and the number of the phonons are calculated as functions of the parameters of the Hamiltonian. The renormalizations of the phonon frequency and electron hopping for bonding and antibonding states are considered. The destruction of quasiclassical mean field solution is discussed.Item Open Access SU(2)-path integral investigation of Holstein dimer(Elsevier Science Publishers B.V., Amsterdam, Netherlands, 2000) Hakioğlu, A. T.; Ivanov, V. A.; Zhuravlev, M. Ye.The SU(2) coherent state path integral is used to investigate the partition function of the Holstein dimer. This approach naturally takes into account the symmetry of the model. The ground-state energy and the number of the phonons are calculated as functions of the parameters of the Hamiltonian. The renormalizations of the phonon frequency and electron orbital energies are considered. The destruction of quasiclassical mean-field solution is discussed.Item Open Access The tight-binding approach to the corundum-structure d compounds(Institute of Physics Publishing Ltd., 1994) Ivanov, V. A.The analysis of electronic structures has been carried out for the transition-metal compounds showing the corundum-type crystal symmetry using the suggested tight-binding method for interacting bands. With the self-consistent field approximation, the branches of the electronic spectra and energy gaps have been analytically calculated. The role of the electron correlations was found to be decisive for the dielectrization of spectra for which no additional assumptions, e.g. the existence of spin- or charge-density waves, was necessary. The data obtained provide an explanation for the appearance of the insulator state in such compounds as Ti2O3, V2O 3, Cr2O3, alpha -Mn2O3 and alpha -Fe2O3. The calculated values of band gaps agree reasonably with the experimental data available. The Peierls problem is solved for the corundum-structure d compounds.