Radostina K. Koleva

Medical University of Sofia, Ulpia Serdica, Sofia-Capital, Bulgaria

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Publications (6)5.16 Total impact

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    ABSTRACT: Four atom states Cu3dx2− y2, Cu4s, Oa2pxare involved in a tight-binding model for the superconducting CuO2plane. The orthorhombic distortion is taken into account by the differences of Cu–O hopping amplitudes and single-site oxygen energies εaand εbof two oxygen positions in the elementary cell as well. An effective ‘oxygen’ Hamiltonian including only the electron amplitudes at the oxygen ions is derived. Simple expressions for the constant energy contours and the Fermi surface are obtained and they qualitatively describe the photoemission spectra. Extended saddle points nearp=(π,0) andp=(0,π) are observed in qualitative agreement with the ARPES data. The van Hove singularities of the density of states (DOS) related to the extended saddle points are calculated by a Monte Carlo method. It is found that the splitting of the singularity of the DOS at the bottom of the conduction band is created by the energy difference εa− εb=2δ.
    Superlattices and Microstructures 01/1998; 24(5):381-384. · 1.56 Impact Factor
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    ABSTRACT: The oxygen O2p_sigma and copper Cu4s and Cu3d_x^2-y^2 orbitals are involved in a simple LCAO model for determination of the conduction band and the oxygen-oxygen hopping is considered as a small parameter with respect to the transition amplitude between nearest neighbours. The traditional Cooper pairing is obtained by taking into account the double-electron exchange between the nearest neighbours within the two-dimensional CuO_2 plane. The equation for the superconducting gap is obtained as a result of the standard BCS treatment. It is shown that the order parameter could have either s-type or d-type symmetry depending on the ratio between the transition amplitudes. This model allows understanding the experiments reporting a pi-shift of the Josephson phase indicative for a d-type gap symmetry as well as the observed s-type in the case of strongly irradiated samples.
    Bulgarian Journal of Physics. 01/1997; 24(3-4):114.
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    ABSTRACT: Simple analytical expressions for the electron band dispersion ϵpare obtained within the framework of a tight-binding model. Formulae for the Fermi surfacepz=pz(px,py;EF) are derived as well. Opσstates of the perovskite anion andsstate of small cation Bi6sare taken into account. The hopping amplitude between next-nearest neighbours is considered as a small parameter of the theory and an effective oxygen Hamiltonian is derived. Band dispersions and density of states are graphically presented. Isoenergy surfaces of cubic bismuthates for the particular caseEF/wcd=0.38 (wcdis the band width) are shown and discussed as well. The adopted model can be used as a realistic one-electron part of the tight-binding models of superconducting perovskites.
    Superlattices and Microstructures 01/1997; 21(3):471-475. · 1.56 Impact Factor
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    ABSTRACT: Electron spectra and wave functions of CuO4 cluster presented in the conducting CuO2 plane of high-T c superconductors areab initio calculated using linear combination of Gaussian orbitals method. The tight-binding fit to such obtained spectrum gives the next-nearest neighbour O-O hopping amplitudet. The adopted small cluster approximation gives a reasonable estimation fort in agreement with the state-of-the-art ARPES data for layered cuprates. It is briefly considered also why the solid state calculations of the bandwidth afiasco undergo.
    Czechoslovak Journal of Physics 01/1996; 46:2645-2646. · 0.42 Impact Factor
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    ABSTRACT: A simple tight-binding model for the conduction band of Sr2Ru04 is presented. The Ru4de and 02pp orbitals of RuU2 conduction plane are taken into account. In momentum representation the LCAO Hamiltonian has a block structure, containing hopping amplitudes between the in-plane orbitals Oa2py, Ob2px, Ru4dxy, and those vertical to RuO2 plane — Ru4dyz, Ru4dzx, Oa2pz and Ob2pz. This simple structure allows an analytical derivation of electron band dispersions e(px,Py) and constant energy contours e(px,py) = const. The energy surfaces e(px,Py) (numerically calculated) are also related to angle-resolved photoemission spectra. Based on the analytical results a fitting philosophy for both the available experimental data and ab initio calculations is discussed. The tight-binding parameters calculated within the adopted model are found to be in acceptable agreement with the first principles calculations.
    Journal of Low Temperature Physics 01/1996; 105(5):1611-1616. · 1.18 Impact Factor
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    ABSTRACT: The Fermi surface of Sr2RuO4 mapped out recently by Angle-Resolved Photoemission Spectroscopy [D. H. Luet al., Fermi surface and extended van Hove singularity in non-cuprate superconductor Sr2RuO4, submitted to Phys. Rev. Lett.] is fitted by a simple tight-binding model for conducting RuO2 plane. The results obtained are in agreement with LDA calculations performed by Oguchi and Singh. The Fermi surface is analytically expressed by tight-binding parameters.
    Czechoslovak Journal of Physics 01/1996; 46:953-954. · 0.42 Impact Factor