Plasma excitations in the superconducting state of two-band layered superconductors

We investigate the low-frequency plasma modes polarized in the c-direction in two-band layered superconductors. The dynamic dielectric function (ω,q) is derived at T=0, using the generalized random-phase-approximation which is consistent with the Hartree–Fock single-particle states. It is shown that the dielectric function has two zero-points which correspond to the longitudinal plasma modes in two-band layered superconductors.     

The c-axis fluctuation conductivity in layered superconductors in a strong electric field under a magnetic field

The effect of a high electric field on the c-axis fluctuation conductivity in layered superconductors near the superconducting transition is investigated by the time-dependent Ginzburg-Landau equation. The c-axis fluctuation conductivity is calculated in self-consistent Gaussian approximation for an arbitrarily strong electric field and a magnetic field perpendicular to the layers. Our results include all Landau levels and have refined analytical form. The results in linear response are in good agreement with the experimental data in a wide region around T _c in high T _c superconductor. We also show that high electric fields can be effectively used to suppress the c-axis fluctuation conductivity in high-temperature superconductors.     

The effect of collective spin-1 excitations on electronic spectra in high- Tc superconductors

We review recent experimental and theoretical results on the interaction between single-particle excitations and collective spin excitations in the superconducting state of high-T_c cuprates. We concentrate on the traces that sharpen features in the magnetic-excitation spectrum (measured by inelastic neutron scattering) and imprint in the spectra of single-particle excitations (measured, e.g. by angle-resolved photoemission spectroscopy, tunnelling spectroscopy, and indirectly also by optical spectroscopy). The ideal object to obtain a quantitative picture for these interaction effects is a spin-1 excitation around 40?meV, termed ‘resonance mode’. Although the total weight of this spin-1 excitation is small, the confinement of its weight to a rather narrow momentum region around the antiferromagnetic wavevector makes it possible to observe strong self-energy effects in parts of the electronic Brillouin zone. Notably, the sharpness of the magnetic excitation in energy has allowed these self-energy effects to be traced in the single-particle spectrum rather precisely. Namely, the doping and temperature dependence together with the characteristic energy and momentum behaviour of the resonance mode has been used as a tool to examine the corresponding self-energy effects in the dispersion and in the spectral line-shape of the single-particle spectra, and to separate them from similar effects due to the electron–phonon interaction. This leads to the unique possibility to single out the self-energy effects due to the spin–fermion interaction and to directly determine the strength of this interaction in high-T_c cuprate superconductors. The knowledge of this interaction is important for the interpretation of other experimental results as well as for the quest for the still unknown pairing mechanism in these interesting superconducting materials.

The effect of collective spin-1 excitations on electronic spectra in high- T_c superconductors

All authors

Matthias Eschrig

https://doi.org/10.1080/00018730600645636

Published online:

01 February 2007

Table     

Anomalous c-axis transport in layered metals

Transport in metals with a strongly anisotropic single-particle spectrum is studied. Coherent band transport in all directions, described by the standard Boltzmann equation, is shown to withstand both elastic and inelastic scattering as long as EFtau > 1. A model of phonon-assisted tunneling via resonant states located in between the layers is suggested to explain a nonmonotonic temperature dependence of the c-axis resistivity observed in experiments.     

Collective excitations in superconductors

The excitations connected with quantum condensation of a fermion gas are calculated quantitatively along the lines of a previous paper. We find exact agreement with earlier results: excitations of the phonon type in the absence of Coulomb interaction, strong indications for their nonexistence in the presence of this interaction.     

Macroscopic excitations in superconductors

The dynamical density functional theory (DDFT) as an alternative approach to the mode coupling theory (MCT) for supercooled liquids and glass transitions was recast in the path integral form, which can then be analyzed by the methods of equilibrium statistical mechanics. The hope is to avoid drastic approximations entering the original MCT such as factorization of the multiparticle correlation function. The renormalized perturbation theory is developed and the freezing is interpreted as a result of the life-time renormalization. The multicomponent extension of the theory is proposed, which could be treated exactly in the limit of infinite number of the components.     

Bound-pair excitations in superconductors

Properties of bound-pair excitations as correlated states of two Fermi quasiparticles in superconductors are calculated within the BCS theory of superconductivity. The partition function of the interacting electron system is expressed as a product of the partition function of a free Fermi gas and the partition function of a free Bose gas. The Bose gas partition function determines energy levels of the bound-pair excitations and their degeneracies. The calculated properties of the bound-pair excitations perfectly respect generally proven theorems concerning the BCS theory of superconductivity. This work received financial support from the Grant Agency of the Slovak Republic, Grant No V2 F20-G and 2/4109/97.     

Spin excitations in nickelate superconductors

Applying a three-band model and the random phase approximation, we theoretically study the spin excitations in nickelate superconductors, which have been newly discovered. The spin excitations were found to be incommensurate in the low energy region.The spin resonance phenomenon emerged as the excitation energy increased. The intensity can be maximized at the incommensurate or commensurate momentum, depending on the out-of-plane momentum. The spin excitations reverted to incommensurate at higher energies. We also discuss the similarities and differences in the spin excitations of nickelate and cuprate superconductors.Our predicted results can be later validated in inelastic neutron scattering experiments.     

Raman-active c-axis plasma modes in multilayer high-Tc cuprate superconductors

The spectacular changes of the A(1g)-polarized Raman spectra of trilayer and four-layer high-T(c) cuprate superconductors below T(c) observed in recent experiments can be explained in terms of a new type of excitation that occurs in these systems, a Raman active c-axis plasmon. Its frequency, intensity, resonance properties, polarization properties, and coupling to phonon modes involving vibrations of the planar oxygens are estimated and shown to agree with those of the observed A(1g) superconductivity-induced electronic peak.     

Atomic collective excitations in liquid lead

The atomic dynamics of liquid lead at the temperature T = 600 K has been simulated on the basis of the embedded atom model potential (the “embedded” atom model making it possible to effectively take into account the many-particle interactions) in order to study the mechanisms of formation of the atomic collective excitations. Spectra of the dynamic structure factor S(k, ω) and the spectral densities of the time correlation functions of the longitudinal \(\tilde C_L\) (k, ω) and transverse \(\tilde C_T\) (k, ω) currents have been calculated for the wavenumber region 0.11 Å^?1 ≤ k ≤ 2.01 Å^?1. It has been established that the dynamics of density fluctuations is characterized by two dispersion “acoustic-like” branches of the longitudinal and transverse polarization.     

Josephson pancakes in layered superconductors

We consider a pancake vortex in a layered superconductor with linear defects in the superconducting planes. We treat these defects as Josephson junctions. We show that the tunneling Josephson current through these junctions results in a pancake with a superconducting core. We find the mobility of a Josephson pancake.