The role of the Coulomb interaction in the formation of superconducting and pseudogap states in cuprate metal-oxides

It is shown that the key role in the mechanism of high-Tc superconductivity in the layered cuprate metal-oxides with anisotropic quasi-two-dimensional electronic spectrum and d-wave symmetry of the superconducting order parameter is played by the retarded screened Coulomb interaction and many-body correlations. We argue that the pseudogap observed in these materials is the anisotropic dielectric gap, which appears due to the electron-hole pairing on the flat portions of the Fermi surface in the vicinity of the extended saddle points in the quasiparticle spectrum. This gap coexists with the superconducting gap and is partially suppressed by scattering of current carriers on the charged point defects. The suppression of dielectric gap is analogous to the suppression of superconducting gap by magnetic impurities in «gapless» superconductors. The complete destruction of the pseudogap by charged impurities is assumed to increase Tc significantly

Gap symmetry and charge density excitations in high-Tc superconductors with extended saddle points in electron spectrum

It is shown that the strong anisotropy of the one-particle electron spectrum, due to the presence of extended saddle-point features (ESPF) close to the Fermi level in the hole-type cuprates YBCO and BSCCO, leads to the occurrence of a low-frequency peak in the spectral function of the charge density fluctuations due to the presence of acoustic plasmon branch in the collective electron spectrum. The retarded anisotropic electron-plasmon interaction leads to the suppression of the static screened Coulomb repulsion for small transferred momenta and, consequently, to the effective attraction between electrons in the dx²-y²-wave channel of the Cooper pairing of current carriers. Breaking of C₄v symmetry in YBCO crystals leads to a possibility of a change of dx²-y²-wave symmetry of the gap to a mixed s − d gap symmetry for singlet Cooper pairs or to a p-wave gap symmetry for triplet pairs.Показано, що сильна анізотропія одночасткового електронного спектру веде, завдяки наявності подовжених сідлових особливостей біля рівня Фермі у купратах YBCO та BSCCO, до появи низькочастотного піку у спектральній функції флуктуацій зарядової густини, що є наслідком присутності гілки акустичних плазмонів у колективному електронному спектрі. Електрон-плазмонна взаємодія веде до значного зменшення статичного кулонівського відштовхування в області малих переданих імпульсів та, як наслідок, до ефективного притягнення між електронами у dx²-y²-хвильовому каналі куперівського спарювання носіїв струму. Порушення C₄v симетрії у кристалах YBCO призводить до можливості заміни dx²-y²-хвильової симетрії надпровідної щілини на змішану s − d симертрію для сінглетних куперівських пар або на p -хвильову симетрію щілини для триплетних пар

Electronic Collective Modes and Superconductivity in Layered Conductors

A distinctive feature of layered conductors is the presence of low-energy electronic collective modes of the conduction electrons. This affects the dynamic screening properties of the Coulomb interaction in a layered material. We study the consequences of the existence of these collective modes for superconductivity. General equations for the superconducting order parameter are derived within the strong-coupling phonon-plasmon scheme that account for the screened Coulomb interaction. Specifically, we calculate the superconducting critical temperature Tc taking into account the full temperature, frequency and wave-vector dependence of the dielectric function. We show that low-energy plasmons may contribute constructively to superconductivity. Three classes of layered superconductors are discussed within our model: metal-intercalated halide nitrides, layered organic materials and high-Tc oxides. In particular, we demonstrate that the plasmon contribution (electronic mechanism) is dominant in the first class of layered materials. The theory shows that the description of so-called ``quasi-two-dimensional superconductors'' cannot be reduced to a purely 2D model, as commonly assumed. While the transport properties are strongly anisotropic, it remains essential to take into account the screened interlayer Coulomb interaction to describe the superconducting state of layered materials.Comment: Final version (minor changes) 14 pages, 6 figure

Epitaxial YBa2Cu3O7-8 films : Crucial role of growth-induced linear defects in microwave properties

A significant contribution to high-frequency electromagnetic response of epitaxial YBCO films is supposed to be accounted for growth-induced linear defects. There are two major types of linear defects in the YBCO films : out-of-plane and in-plane edge dislocations. The out-of-plane edge dislocations are shown to play a remarkable role in the surface resistance due to their extended elastic strain fields around a normal core. In-plane edge dislocations do not contribute observably into HF losses, since strain fields around their cores are negligible. The area of suppressed order parameter per one dislocation line was estimated to be 8.10-13) cm2 at 77 K. When out-of- plane edge dislocation density is 1011 lines/cm2, the normal phase fraction in the film volume can reach approximately 10%. The heat instability induced by the linear defects is assumed to enhance the remarkable difference between microwave properties of YBCO single crystals and thin films and results in a nonlinear response

Resonance Microwave Absorption in He II.

Microwave (MW) absorption in liquid 4He is investigated in the frequency range of 40-200 GHz at T = 1.4 - 2.5 K. Whispering gallery of waves was generated by a dielectric disc resonator immersed into the liquid. Resonance absorption of MWs was detected at 180.3 GHz, which corresponds to the roton minimum of the liquid helium excitation spectrum. The creation of a single roton is possible because of the presence of the resonator wall which absorbs an extra momentum. The resonance frequency is shown to decreases with temperature in an excellent agreement with the temperature dependence of the roton gap obtained previously in the neutron scattering experiment. The temperature dependence of the MW absorption data indicates the anomalous behavior near the lambda-point and displays the hysteretic character.Comment: 10pages, 8 figure