Effect of oxygen deficiency (δ) on transition temperature of yttrium cuprate superconductors (original) (raw)
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Oxygen deficiency dependence of transition temperature in (Sm, Er) Ba2Cu3O7−δ superconductors
Bulletin of Materials Science, 1997
We have investigated the effects of oxygen deficiency (6) on the transition temperature (To) of (Sin, Er)Ba2CU3OT_ ~ superconductors by incorporating the effects of the two dimensional (2I)) acoustic phonons and plasmons in the framework of strong coupling theory. The proposed approach for yttrium cuprates properly takes care of the double CuO 2 plane in a unit cell and has been found earlier to be successful in describing the pairing mechanism as well as the variation of T c with 6 in YBa2CU3OT_ 6 system. The coupling strength (2), the screening parameter (/~*) and the two dimensional acoustic phonon (plasmon) energy hco (oJ +) as a function of oxygen deficiency is worked out. Finally, the transition temperature is evaluated and is found to be consistent with the earlier experimental data on yttrium euprates. Thus, coupled phonon-plasmon mechanism is adequate to understand the nature of pairing mechanism and oxygen deficiency dependence of transition temperature in 90 K (Sm, Er) Ba 2 Cu 3 0 7-6 superconductors.
Effect of oxygen deficiency (δ) on normal state resistivity of YBa2Cu3O7-δ superconductors
Bulletin of Materials Science, 1998
We have investigated theoretically the effect of oxygen deficiency (6) on normal state resistivity ~0) as well as its temperature dependence in YBa2CU3OT_ ~ superconductors. This has been based on a potential which incorporates the structure factors and various interactions for double two-dimensional (2-D) conducting CuO 2 plane. Using the Coulomb and electron-phonon terms of the interaction potential, we have then worked out the coupling strength (~) for neighbouring electrons linked via 2-D acoustic phonons (hw_). Furthermore, the scattering time (Z'e_ph) due to electron-phonon interaction is deduced. The variations in "Ke_ph and Pe-ph are studied with oxygen deficiency (¢~) which is in the range of 0-0 < 6 < 1.0, and the results thus obtained are found to be consistent with the earlier reported data. The residual resistivity P0 obtained by extrapolation from experimental data together with Pe-ph will predict the nearly-linear behaviour of normal state resistivity at temperatures (T) [90 < T<300 K] in YBa2Cu307_ ~ superconductors.
CNDO studies of Y-Ba-Cu-O superconductors II. Simple models of YBa2Cu3Ox
Czechoslovak Journal of Physics, 1992
The electronic structure of YBa2Cu3Oz (x = 6 and 7) is investigated using the CNDO molecular orbital method. Electronic structures of model clusters [Cu3010] -15, [BasCu3010] +1, [Y8Cu3010] +9 of the non-superconducting (x ----6) and [Cu3012]-17,[Ba8Cu3012] -1, [Y8Cu3012] +7 of the superconducting (x = 7) phases are compared. Y and Ba layers cause a considerable electron density transfer from the central Cu(1) region.
Proceedings of the National Academy of Sciences, 2020
Significance The Cu extended X-ray absorption fine structure of YSr 2 Cu 2.75 Mo 0.25 O 7.54 (with superconducting critical temperature, T c , = 84 K) and Sr 2 CuO 3.3 (T c = 95 K) through their superconducting transitions demonstrates that the common factor in superconductivity in cuprates, including those prepared by high-pressure oxygenation, is an internal quantum tunneling polaron in its dynamical structure. In addition, Sr 2 CuO 3.3 is the first material to show a concomitant transformation in this structure involving atom displacements >1 Å that would be expected to modify its Fermi surface, which would complicate the transition beyond a purely electronic one consisting of the pairing of electrons of opposite momentum across fixed electronic states.
High Temperature Superconductivity in Cuprates: a model
A model is proposed such that quasi-particles (electrons or holes) residing in the CuO 2 planes of cuprates may interact leading to metallic or superconducting behaviors. The metallic phase is obtained when the quasi-particles are treating as having classical kinetic energies and the superconducting phase occurs when the quasi-particles are taken as extremely relativistic objects. The interaction between both kinds of particles is provided by a force dependent-on-velocity. In the case of the superconducting behavior, the motion of apical oxygen ions provides the glue to establish the "Cooper pair". The model furnishes explicit relations for the Fermi velocity, the perpendicular and the in-plane coherence lengths, the zero-temperature energy gap, the critical current density, the critical parallel and perpendicular magnetic fields. All these mentioned quantities are expressed in terms of fundamental physical constants as: charge and mass of the electron, light velocity in vacuum, Planck constant, electric permittivity of the vacuum. Numerical evaluation of these quantities show that their values are close those found for the superconducting YBaCuO, leading to think the model as being a possible scenario to explain superconductivity in cuprates. 1-INTRODUCTION Since the discovery of the high temperature superconductivity in copper oxides (cuprates) by Bednorz and Müller [1], a great amount of theoretical work has been dedicated to understand the mechanism behind this phenomenon. One of the first trying to elucidate this puzzle was proposed by Anderson through the resonant-valence-bond model [2]. Another model [3], due to Emery, assumes that the charge carriers are holes in the O(2p) states and the pairing is mediated by strong coupling to local spin configurations in Cu sites. Emery [3] used an extended Hubbard model in order to describe the main features of this mechanism. Meanwhile Plakida et al [4] explained the high-temperature transition in perovskite-type oxides within the framework of the non-harmonic model for superconductors with structurally unstable lattices. In the model of Plakida and collaborators [4] the highly non-harmonic motion is written in terms of a pseudo-spin representation through a Transverse Ising Model and the interaction of the electrons with the non-harmonic ions vibrations is also described in terms of this pseudo-spin representation. Two opposite views of the superconductivity in cuprates have been disputed by Anderson and Schrieffer. Anderson [2] attributes the novel phenomenology present on cuprates materials to a second kind of metallic state, namely, the Luttinger liquid. Schrieffer [5] has pursued the interplay between anti-ferromagnetism and superconductivity, extending the BCS pairing theory beyond the Fermi-liquid regime in terms of spin polarons or "bags". According to Cox and Maple [6] superconductivity in heavy-fermion materials and high-Tc cuprates may involve electron pairing with unconventional symmetries and mechanisms.
Physica C: Superconductivity and its Applications, 2007
We have measured the near-normal reflectance of Tl 2 Ba 2 CaCu 2 O 8 (Tl2212) for energies from 0.1 to 4.0 eV at room temperature and used a Kramers-Kronig analysis to find the complex, frequency dependent dielectric function, !(") from which the optical conductivity #(") was determined. Using Thermal-Difference-Reflectance (TDR) Spectroscopy the reflectance of the sample in the normal state just above the superconducting transition, and in the superconducting state were then obtained. From these data we determined the ratio of the superconducting-to normal-state optical conductivities, # S (")/# N ("). Mattis and Bardeen had calculated this function within the BCS theory, where the gap is a fixed energy-independent quantity. Taking into account the retarded nature of the electron-phonon coupling results in a complex, energy dependent gap $(") causing deviations from the Mattis-Bardeen plot at energies where the phonon coupling function % 2 (")F("), is large. We find a typical deviation near the phonon energies in Tl2212, and in addition, at 1.2 and 1.7eV. The phonon, and these electronic terms can each be described by a coupling constant & i. None of which by itself gives rise to a high transition temperature, but the combination does. Using Resonant Inelastic X-Ray Scattering (RIXS) we find that the d-to-d excitations of the cuprate ion in Tl2212 fall at the same energies as the dips in the Mattis-Bardeen plot. We conclude that the high superconducting transition temperature of the cuprates is due to the sum of the phonon interaction, and interactions with the Cu-ion d-shell.
Physica C: Superconductivity, 1990
The infrared-active phonon spectra of YBa2 ( CoxCu ~ _ x ) 307 _ ~ ( 0 < x < 0.07 ) have been studied between 35 K and room temperature. The mode near 580 cm-~ depends sensitively on Co doping and renormalizes due to the metal-superconductor phase transition. The excess phonon frequency and absorption cross-section scale as the square of the order parameter in the superconducting phase. This temperature evolution is in accordance with Landau-Ginzburg theory including saturation effects, which is numerically similar to BCS behaviour. Phonon contributions to the specific heat as a function of Co content correlate closely with observed anomalies in calorimetric measurements, indicating that phonons contribute intrinsically to the transition mechanism, e.g. via Bose condensation of structural bipolarons. The growth of a phonon doublet at ~ 657 cm-~ is attributed to effects of Co doping and discussed in view of the orthorhombic to tetragonal structural phase transitions at x~0.025 and J~0.5 in the YBa2Cu307_,~ end-member, ~ Notation as used in refs. [4,10]. 0921-4534/90/$03.50 © 1990 -Elsevier Science Publishers B.V. (North-Holland)