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.     

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.     

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.     

Collective excitations in superconductors with structural transition

In this paper we have found new low-frequency collective excitations in superconductors with structural transformation. The connection between our results and the experiments on Raman scattering in 2H-NbSe₂ cDw- superconductor is discussed.     

Spin-wave excitations of paramagnetic impurities in superconductors

The existence of spin-wave excitations of paramagnetic impurities in a superconductor in an external magnetic field due to the long-range exchange interactions is predicted. The damping has been calculated and possible manifestations of this collective excitations in ESR experiments are analysed.     

Variety of c-axis collective excitations in layered multigap superconductors

We present a dynamical theory for the phase differences along a stacked direction of intrinsic Josephson junctions (IJJ's) in layered multigap superconductors, motivated by the discovery of highly anisotropic iron-based superconductors with thick perovskite-type blocking layers. The dynamical equations describing ac and dc intrinsic Josephson effects peculiar to multigap IJJ's are derived, and collective Leggett mode excitations in addition to the Josephson plasma established in single-gap IJJ's are predicted. The dispersion relations of their collective modes are explicitly displayed, and the remarkable peculiarity of the Leggett mode is demonstrated.     

Low-lying excitations induced by non-magnetic impurities in d-wave superconductors

We have performed numerical calculations for low-lying excitations induced by a single non-magnetic impurity in a d-wave superconductor on the basis of two different frameworks. One is the Bogoliubov–de Gennes theory in real coordinates, while the other is the Born approximation in wave-number coordinates. This paper shows that these results for low-lying excitations are identical by using Fourier transformations.     

On the energy spectrum of excitations in type-II superconductors

The quasi-classical approximation has been applied in order to study the energy spectrum of the bound excitations with the radial quantum number n≠0 in the isolated vortex line core in a pure type-II superconductor. The qualitative estimations of the number of levels and the numeric calculations of the spectrum for the special form of potentials A_θ(r) and Δ(r) have been made.     

Spectroscopy of magnetic excitations in magnetic superconductors using vortex motion

In magnetic superconductors a moving vortex lattice is accompanied by an ac magnetic field which leads to the generation of spin waves. At resonance conditions the dynamics of vortices in magnetic superconductors changes drastically, resulting in strong peaks in the dc I-V characteristics at voltages at which the washboard frequency of the vortex lattice matches the spin wave frequency omegaS(g), where g are the reciprocal vortex lattice vectors. We show that if the washboard frequency lies above the magnetic gap, measurement of the I-V characteristics provides a new method to obtain information on the spectrum of magnetic excitations in borocarbides and cuprate layered magnetic superconductors.     

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.     

Spin excitations in fluctuating stripe phases of doped cuprate superconductors

Using a phenomenological lattice model of coupled spin and charge modes, we determine the spin susceptibility in the presence of fluctuating stripe charge order. We assume the charge fluctuations to be slow compared to those of the spins, and combine Monte Carlo simulations for the charge order parameter with exact diagonalization of the spin sector. Our calculations unify the spin dynamics of both static and fluctuating stripe phases and support the notion of a universal spin excitation spectrum in doped cuprate superconductors.     

The vortex-like excitations detected by Nernst signals in high-Tc superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

The vortex-like excitations detected by Nernst signals in high-T c superconductors

The nature of the pseudogap state and its relation to the d-wave superconductivity in high-T _c superconductors is still an open issue. The vortex-like excitations detected by the Nernst effect measurements exist in a certain temperature range above superconducting transition temperature T _c, which strongly support that the pseudogap phase is characterized by finite pairing amplitude with strong phase fluctuations and imply that the phase transition at T _c is driven by the loss of long-range phase coherence. We first briefly introduce the electronic phase diagram and pseudogap state of high-T _c superconductors, and then review the results of Nernst effect for different high-T _c superconductors. Related theoretical models are also discussed.     

Two-mode variational Monte Carlo study of quasiparticle excitations in cuprate superconductors

Recent measurements of quasiparticles in hole-doped cuprates revealed highly unusual features: (i) the doping-independent Fermi velocity, (ii) two energy scales in the quasiparticle spectral function, and (iii) a suppression of the low-energy spectral weight near the zone center. We explain these important facts by a novel two-mode variational Monte Carlo (VMC) study of the t-J model, which resolves a long-standing issue of the sum rule for quasiparticle spectral weights in VMC studies. The electron-doped case is also discussed.     

Competing orders and the doping and momentum dependent quasiparticle excitations in cuprate superconductors

The low-energy quasiparticle excitations in hole- and electron-type cuprate superconductors are investigated via both experimental and theoretical means. It is found that the doping and momentum dependence of the empirical low-energy quasiparticle excitations is consistent with a scenario of coexisting competing orders and superconductivity in the ground state of the cuprates. This finding, based on zero-field quasiparticle spectra, is further corrobarated by the spatially resolved vortex-state scanning tunneling spectroscopy, which reveals pseudogap-like features consistent with a remaining competing order inside the vortex core upon the suppression of superconductivity. The competing orders compatible with empirical observations include the charge-density wave and spin-density wave. In contrast, spectral characteristics derived from incorporating the d-density wave as a competing order appear unfavorable in comparison with experiments.     

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

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