Upper critical field of high Tc superconductors

The upper critical field H_c2 of the high T_c superconductors such as La-Sr-Cu-O and La-Ba-Cu-O systems has been calculated by using the modified Suhl's et al. two-band model. A modification to Gor'kov theory is found for the ratio of K₁(T) at T=0 and T=T_c. The results may explain the discrepancy between the BCS theory and experimental data.     

Nature of pairing in high Tc oxide superconductors

In connection with the unique structural features of the recently discovered high T_c YBa₂Cu₃O₇ and (La,Ba)₂CuO₄ we show that in such a multi-narrow-band system the opposite-spin interband pairs can function concurrently as both antiferromagnetic real pairs in x-space and Cooper pairs in k-space. So, contrary to the usually depairing Coulomb pseudotential μ¹ considered in traditional superconductors, the intra-atomic Coulomb repulsive interactions in a multiband system result in a positive contribution to Cooper pairing and enhance the conventional boson-mediated (electron-phonon and/or electron-exciton) superconductivity.     

Are the high Tc superconductors d-wave?

Although it has been widely accepted for several years that the normal state of high T_c superconductors is anomalous, only recently has there been growing evidence for the anomalous nature of the superconducting state. A number of recent experiments show clear evidence for the existence of low-lying excitations in the superconducting state. Moreover, the observation by Bonn, Hardy and coworkers of a linear temperature dependence in the microwave surface resistance and penetration depth of YBCO at low temperatures suggests d-wave pairing with line nodes. The evidence for and against unconventional superconductivity in the high T_c oxides is reviewed. Specific topics discussed include the different behaviours observed for the penetration depth in thin films and in single crystals, what is known about the absolute value of the penetration depth, the role of disorder and inelastic scattering, the relative merits of strongvs. weak correlation theories of d-wave superconductivity in the high T_c oxides, and most important, what have we actually learned about the penetration depth of high T_c superconductors fromμSR?     

A phenomenological model for high Tc superconductors

We propose a phenomenological model describing the high T_c superconductors. Adjacent Cu-O layers have negative Josephson couplings, forcing the order paramater to change sign from one layer to the next. The number of closely spaed layers determines T_c.     

Optical conductivity studies of high‐Tc superconductors

A study of the optical conductivity spectra of high‐T_c copper oxides elucidates the evolution of superconducting gap and magnetic penetration depth (superfluid density) with doping. It is complementary to the μSR study and provides microscopic basis for the μSR results. This revised version was published online in August 2006 with corrections to the Cover Date.     

Raman studies of the high pressure effects in high Tc superconductors

Abstract

The effect of a hydrostatic pressure (up to 6GPa) on the Raman spectra of YBa₂Cu₃O_x(x = 6.5 and overdoped) single crystals is investigated in the temperature range 77-300 K in connection with all available data in the literature for the other oxygen concentrations. All five strong modes (4A_g+1B_1g) are examined for both oxygen doping and compared with previously published results. From the systematic investigation of the phonon spectral characteristics with the application of pressure, the formation or disappearance of the phases is examined. The results provide a spectroscopic support for the changes induced by the hydrostatic pressure on the chain ordering and the corresponding phases.     

Correlation effects in the electronic structure of high Tc superconductors

We model Cu-O high T_c superconductors by a generalized Hubbard Hamiltonian. We obtain the ground state for finite linear chain systems exactly. This allows us to calculate physical quantities to show that: a) those systems can change from a highly ionic to a highly metallic state as holes are added. b) In the metallic state holes tend to form pairs in next nearest neighbour sites.     

Evidence for the strain critical point in high Tc superconductors

We report experimental evidence for the phase diagram of doped cuprate superconductors as a function of the micro-strain of the planar Cu-O bond length, measured by Cu K-edge EXAFS, and hole doping . The local lattice distortions are measured by EXAFS and the charge ordering is measured by synchrotron radiation diffuse X-ray diffraction. This phase diagram shows a QCP at P() where for charge-orbital-spin stripes and free carriers co-exist. The superconducting phase occurs in the region of critical fluctuations around this QCP. The function of two variables shows its maximum at the strain QCP. The critical fluctuations near this strain QCP give the self-organization of a metallic superlattice of quantum wires “superstripes" that favors the amplification of the critical temperature. Received 25 September 2000     

Dissipative flow of Josephson and Abrikosov fluxons in high Tc superconductors

A model is presented for the field dependent microwave absorption in high T_c superconductors; fluxon motion in Josephson juctions and in bulk material causes the absorption. Junction dissipation is demonstrated by microwave induced dc voltage across YBaCuO samples which has a magnetic field dependence.     

Simple algorithm in time-window extending for magnetic relaxation in high Tc superconductors

A one to one map is established between the magnetic-hysteresis-loop (MHL) measurements and the magnetization relaxation measurements performed at the same external field, noticeably keeping no uncertainties of such factors as the microscopic attempt frequency v ₀ and the hopping distance l _hop for pinned flux motion. By utilizing this map, we are able to construct and extend the magnetization relaxation data from the corresponding reported MHL measurements. The present algorithm proves to be reliable and simple for investigations of the U(J) behavior and relaxation effects.     

FTIR spectroscopic study for improving material preparation of high Tc superconductors

FTIR spectroscopic studies have been carried out to understand high Tc superconductor YBa₂Cu₃O_7−δ during growth process. Characteristic IR absorption bands of the material are compared with those of the constitutent compounds to determine the presence of unused components and the completion of solid state reactions. IR spectroscopy has been found to be very convenient in such investigations.     

Collective modes in superconductors near Tc

A phonon-like critical mode is obtained in superconductors near T_c as a result of screening due to electron-lattice ion interaction.     

Mode propagation in superconductors near Tc

The linearized Eliashberg theory for dirty superconductors in the vicinity of T_c is used to generate the dynamic Ginzburg-Landau (GL) equation for the order parameter and the coupled Boltzmann equation for the quasi-particle distribution function, with and without magnetic impurities and inelastic phonon scattering. The Schmid and Schön (SS) separation of the order parameter into longitudinal (real) and transverse (imaginary) components is shown to be exact to linear order. The former does not couple with the wave vector (k) and frequency (ω) dependent external field, while the latter does. Explicit expressions are exhibited for the current and charge densities to leading order [$\text{Δ}\text{T}$ in the gap regime, in which Δτ_s > 1 where τ_s is the conduction electron spin flip lifetime; and $\text{Δ}{}^2\text{T(ω + iDk}{}^2\text{)}$ in the gapless regime, Δτ_s < 1. D is the diffusion constant]. The continuity equation is shown to be obeyed to this order.Use of the Poisson equation in relation to the transverse component leads to a propagating mode, which, in the absence of magnetic impurities, reduces to the SS result. We find that the mode propagates only in the gap regime, whereas it is purely diffusive in the gapless regime and for gapless superconductors. In the presence of magnetic impurities, the solution reduces to the SS limit, but with reduced mode velocity and damping. The charge density appropriate to the propagating mode is shown to be zero to order $\text{ω}\text{ω}{}_{\mathrm{p}}{}^2\text{τ}{}_1$, where τ₁ is the impurity scattering time and θ_p the plasma frequency. The characteristic penetration length of an electric field into a superconductor is calculated, taking into account the phonon inelastic scattering time. The result reproduces the Waldram and SS expressions. Careful examination is made of the conditions under which these results pertain. The characteristic penetration length is also obtained in the gapless regime and for gapless superconductors for which T_cτ_s < 1.Finally, the longitudinal and transverse pair susceptibilities are calculated with and without magnetic impurities and in the gap and gapless regimes.     

Charge-transfer hole-lifetime broadening of the Cu Auger-electron spectra of high Tc superconductors

The satellite intensity in the Cu L₂₃-VV Auger-electron spectrum of the high T_c superconductor (YBa₂Cu₃O_7−x, 123) is much more enhanced as compared to that of CuO. This enhancement was previously interpreted by Ramaker et al. [D.E. Ramaker, N.H., Turner, F.L. Hutson, Phys. Rev. B 38 (1988) 11368] as a result of the mixing between the ddp and dpp states. Here, d is a hole in the Cu 3d band and p is a hole in the O 2p band. However, the dramatic Auger electron spectroscopy (AES) spectral lineshape change from CuO to 123 is not only in the charge-transfer (CT) satellite but also in the main-line width. The change arises from the transit of the “pairing” of two bound d holes in the ddp state to that of two bound p holes in the dpp state. As a result, in CuO there is no CT satellite but the dd state becomes a resonant state broadened by the CT hole-lifetime broadening, whereas in 123 the dd state becomes a mixture of a resonant-like state and nonresonant band states. The present many-body theory can explain the overall AES lineshape change from CuO to 123.     

Temperature dependence of the low field microwave absorption of high Tc superconductors

For YBa₂Cu₃O_7−x samples the temperature dependences of the low field microwave absorption spectra as well as of the magnetization curves are investigated and discussed. Combination of these data shows that strong microwave absorption can be observed under conditions excluding the possibility of flux trapping by the sample.     

On the prediction of the Tc dependence on the non-stoichiometry and of the critical-field anisotropy in high Tc superconductors

The critical temperature dependency versus the non-stoichiometry in high temperature superconductors has been derived from a modified Landau-Ginzburg calculation. A qualitative agreement has been obtained with experiments. We have also studied the anisotropy of the critical field (H_c2) which is in agreement with experiment and shows that dimensionality controls superconductivity in YBa₂Cu₃O_7−x.     

Possibility of Kosterlitz-Thouless effect at the resistive transition of high Tc oxide superconductors

The intrinsic resistive transition of an oxide superconductor is found to follow the relation of a Kosterlitz-Thouless transition of binding-unbinding of vortex pairs.