R. Akis

Combined phonon-exciton mechanism in La2−xSrxCuO4

Abstract The possibility of a combined phonon-exciton mechanism is explored within Eliashberg theory. Measurements of 2Δ 0 k B T c and the isotope effect coefficient give a range of possible values for the ratio of exciton coupling to phonon coupling. We examine the possibilities for thermodynamic and electromagnetic properties, as well as the effect of the electron-exciton coupling on the I–V characteristic at low frequencies. It is found that a combined phonon-exciton mechanism improves the agreement between theory and experiment.

A large isotope effect is consistent with a mainly electronic mechanism in the oxides

Abstract Recently, large values and rapid variation upon doping of the oxygen isotope coefficient β 0 have been observed in the copper oxides. We show that such effects can be understood, qualitatively, in a model with a rapidly varying electronic density of states in which the pairing is mainly due to an electronic mechanism with only a relatively small, additional electron-phonon contribution. By contrast, a pure phonon model is inconsistent with the existing data.

DAMPING EFFECTS ON NMR IN SUPERCONDUCTORS

Abstract The logarithmic singularity in the nuclear magnetic resonance spin relaxation rate predicted in BCS theory is removed through quasiparticle damping effects. We have carried out full calculations of the NMR rate based on numerical solutions of the Eliashberg equations as a function of coupling strength. As the coupling strength is increased, the Hebel-Slichter peak is rapidly reduced and becomes nonexistent before the end of the conventional strong coupling range is reached. Thus, a peak is not a general feature of the superconducting state for conventional materials, and its absence is not limited to the oxides.

On the specific heat of strong coupling superconductors

Abstract We have calculated the specific heat difference between normal and superconducting state at all temperatures (T) below Tc as a function of coupling strength spanning weak, intermediate and very strong coupling. We establish a maximum for the slope at Tc as well as for its value normalized to the jump which applies to any Eliashberg superconductor independent of the details of the boson exchange mechanism involved. The maximum is tested against known values for the slope in conventional superconductors. A comparison of our results with a recent experiment in the oxides is given and found to be problematic.

Strong coupling effects on the low frequency conductivity of superconductors

Abstract When examining the conductivity at low frequencies in a dirty BCS superconductor, one expects to see a peak just below Tc due to the presence of coherence factors. In this paper, we study how strong coupling effects within the Eliashberg formalism influence this peak. We find that if the coupling strength is sufficient, the peak is eliminated. We have also studied the effect of varying the frequency and the impurity concentration have on the magnitude of the peak.

Quasiparticle damping, anisotropy and Fermi liquid corrections to NMR in superconductors

Abstract Quasiparticle damping removes the singularity in the nuclear magnetic resonance signal which, in BCS theory, occurs at the superconducting critical temperature. In the high- T c oxides, it is observed that no peak remains at T c . We have studied within an Eliashberg formulation which fully accounts for damping effects, the reduction in peak value at T c as a function of increasing coupling strength. Antiferromagnetic Fermi liquid corrections which also reduce the peak are included as in the work of Monien and Pines. In a separate study, anisotropy reflecting the planar nature of the copper oxides is also included and a comparison with experiment attempted.

Eliashberg theory in the very strong coupling regime

Abstract The effects of strong coupling in Eliashberg theory are well understood. In light of the high T c oxides, however, we consider the very strong coupling regime. Results are given for thermodynamic and electromagnetic properties. It is found that while the gap ratio continues to increase, other properties reverse their trend and a “BCS-like” result is possible in the very strong coupling regime. The behaviour of the tunneling density of states is anomalous.

Properties of a 96 K superconductor with a combined phonon-high energy boson mechanism

Abstract We have calculated several properties of a T c =96 K superconductor with a combined phonon and high energy pairing mechanism. The high energy mechanism is left unspecified and is modeled with a constant BCS pairing term. We have found that, even when the phonon contribution to T c is minimal, its influence can still have a profound effect in determining the physical properties of the superconductor.

Optimum spectra for superconducting properties

Abstract Upper or lower bounds exist on dimensionless superconducting properties for any superconductor which can be described by Eliashberg theory whatever the shape or size of electron-boson exchange kernel. We consider here the second upper critical magnetic field H c2 (T) at T = 0 and T c for variable Coulomb pseudopotential μ * and impurity concentration described by the scattering matrix t + .

Combined phonon-exciton mechanism for La1.85Sr0.15CuO4

Abstract The non-zero isotype effect measured for La 1.85 Sr 0.15 CuO 4 suggests that the electron-phonon interaction may play an important role in the superconductivity of this compound. We have calculated thermodynamic, electromagnetic, and critical magnetic field properties for a combined phonon-exciton mechanism. The measured isotope effect can decide what ratio of coupling strengths is needed, from which these properties follow. We make a comparison with experiment where possible, and some of the expected results of this model are delineated.