Model Of High-temperature Superconductivity Research Articles

Comment on the linear resistivity of high-temperature superconducting compounds

Abstract We report calculations of the phonon-induced electrical resistivity ρ(T) of YBa2Cu3O7 using Allen's expression for inverse transport relaxation time τph −1. ρ(T) is found to be a quasi-linear function of T(ρ ∞ T 0.98) for T ≥ 90 K, i.e. for T ≥ T c. The calculation does not take into account Umklapp scattering, which is likely to be important in the high-temperature superconductors (HTSCs) because of the low-lying optical phonon branches found in these compounds. Therefore, linear T behaviour of ρ(T) might well extend substantially below the transition temperatures of most HTSCs. Results for ρ (T) for a Debye-Einstein model of HTSCs (which gives realistic results for cu (T) and θDebye(T)) show linear behaviour of ρ(T) starting at even lower tempertures (T ≥ 30 K). We conclude from these results that linear T behaviour of the resistivity of HTSCs is fully compatible with the Bloch-Grineisen theory of resistivity in ordinary metals, and is, in itself, not an indication that a non-phonon mechanism m...

On the dependence of the critical temperature on pressure in the bisoliton model of high temperature superconductors

Abstract In the framework of the bisoliton model we have studied the critical temperature T c, as a function of the pressure P and of the hole concentration δ for the high temperature superconductors YBa2Cu3Ox and (La1-xMx)2CuO4. Our results for δ ln Tc/δ ln V as a function of T c describe quite satisfactorily the general trend of the experimental data. Furthermore we show that in the bisoliton model the energy gap δ (in units of Jg3/3, where J is the nearest-neighbour exchange integral and g is the nonlinearity parameter) is an universal function of δ/g. An analogous property is valid for T c By fitting the maximum value of T c we are able also to reproduce the experimental data for T c(δ).

Electronic susceptibility in double-layered high-temperature superconductors: Plasmons and renormalization of infrared-active phonons

We investigate the electronic susceptibility of a periodic double-layer system serving as a model for high-temperature superconductors in the normal and in the superconducting state. Allowing for a hopping of charge carriers between the planes of each double layer yields a strong influence on the acoustic plasmon branch in both states. Frequency shifts and linewidth-effects observed experimentally on infrared-active phonons at the superconducting transition are explained consistently within our model.

Boson linewidth in high-temperature superconductors.

We have considered boson exchange models of high-temperature superconductors which use Eliashberg theory and in which the bare bosons have sharp spectral features. In particular, we have calculated the boson linewidth due to the interaction with the charge carriers. We find for a recent model of Arnold, Mueller, and Swihart that the width of the 10-meV peak in their {alpha}{sup 2}{ital F} is consistent with a broadened boson peak. However, for a weak-coupling model with the boson peak in the eV range, the interaction causes a broadening in the boson peak that is comparable to or larger than the energy of the peak.

LOCALIZED (POLARONIC) CHARGE-TRANSFER EXCITATIONS IN CuO2 LAYERS

We show that the recently found polarons and excitons of charge-transfer origin in the 1D p−d model of high temperature superconductors can also occur in 2D. We use a 2D spinless model which is expected to qualitatively describe the charge degrees of freedom of the p−d model in the limit of infinite on-site Coulomb repulsion on both Cu and O. The Cu-O repulsion Upd is treated by an unrestricted Hartree-Fock scheme. We found that in addition to polarons and excitons, other nonlinear excitations like bipolarons and clustering of carriers (phase separation) arise as well. Since the energies of all these excitations are very close to each other at the mean field level, no definite statement can be made about the true ground state.

Charge-transfer polarons and excitons.

We show that polarons and excitons of charge-transfer origin arise quite naturally in the p-d model of high-temperature superconductors. We consider the limit in which on-site Coulomb repulsion on both Cu and O is taken to be infinite and treat the Cu-O repulsion ${\mathit{U}}_{\mathit{p}\mathit{d}}$ in the Hartree-Fock approximation. We find that the uniform Hartree-Fock solution is unstable (the compressibility is negative) for small enough doping. Using an unrestricted Hartree-Fock scheme we find in one dimension (1D) that one particle added to the half-filled case forms a self-trapped polaron state of charge-transfer origin. We also find that there is no phase separation. Within the same formalism we subsequently study exciton states. In 2D we obtain similar compressibility results, suggesting that the ground state is also polaronic. Our results are also relevant for one-dimensional charge-transfer systems.

Chern-Simons superconductivity at finite temperature

A simple gauge theory discussed recently in the literature as a model of high-temperature superconductors is examined. The model contains a Maxwell field and a Chern-Simons field coupled to fermions in (2+1)-dimensional spacetime. This model has been shown to exhibit a kind of Meissner effect at zero temperature, which originates in the 1-loop mixing between the two gauge fields. We use a euclidean effective action formulation to show that the effect persists at finite temperatures. Although a long-range magnetic type interaction arises at non-zero temperatures, in competition with the finite range forces which dominate the zero temperature interaction, the effect varies smoothly with temperature.

A model for high temperature superconductors in a system of two coupled energy bands

A zero-parameter model for a superconductor is constructed, where the pairing force arises from the virtual excitation of hole pairs to the upper Hubbard level in oxide superconductors. This virtual excitation is assumed to be due to the Coulomb interaction between the two holes. The essential ingredient is the modification of the basic two-body interaction in the presence of a high lying resonant level. The predictions of the model agree very well with experiment in the recently discovered high temperature copper oxide superconductors. A possible explanation for the small isotope effect in YBa2Cu3O7 superconductor is also discussed.

Ground state instability and attractive interactions in the extended Hubbard model

The extended Hubbard model for high temperature superconductors is studied for infinite systems in the strong coupling limit. The attractive interaction between added holes that has been noted for nearly degenerate copper and oxygen energy levels arises because the ground state is near an instability. Adding holes nucleates small regions of this unstable, infinitely compressible phase. Three distinct bound states of two holes occur in the strong coupling limit when double occupation is forbidden. In this limit, the attractive interaction results in phase separation rather than pairing when many holes are present. Perturbative calculations suggest that a pairing state is stable in a narrow parameter range as t increases.

Enhancement of the Debye frequency due to a charged-boson exchange model of high temperature superconductors

A simple model of 1-2-3 superconductors in which electrons (holes) in CuO 2 planes interact via exchange with two kinds of bosons is considered. Namely, via one-phonon exchange (weak coupling-Cooper pairing), and via paired holes on oxygen O 0 from Cu-O chains. The mechanisms of paired holes exchange (“charged bosons”-“O 0” exchange) considered here in strong coupling leads to the enhancement of the Fröhlich constant g f ( g 2 F→ Kg 2 F), and as a consequence to the enhancement of the Debye frequency ω D K = f K ω D , f K ≫ 1. In the proposed model the exact expression for the constant K is derived.

Quasiparticles in the copper-oxygen planes of high-Tc superconductors: An exact solution for a ferromagnetic background.

A model for a mobile hole in the copper oxide planes of high-temperature superconductors is solved exactly. The hole moves on the oxygen atoms through a lattice of spins localized on the copper atoms. In order to obtain a solvable problem, it is assumed that the copper atoms provide a ferromagnetic background. The resulting quasiparticles have both charge and spin in contrast to the Cu-O singlets occurring in proposed effective single-band Hubbard models derived from the Cu-O network. Thus these two models of high-temperature superconductors may have different low-energy physics.

Polarizability mechanisms and high temperature superconductivity

We describe a lattice dynamical model for high temperature superconductors which represents an extension of a polarizability model for displacive type ferroelectrics. The model is based on the instability of the oxygen ion O 2-, dynamically described by an on-site nonlinear electron-ion coupling, and on the valence instability of the copper ion, involving Cu(I), Cu(III), represented by a nonlinear electron-electron interaction which provides a pairwise coupling of the electrons. The transformation of the classical Hamiltonian to its quantum mechanical equivalent yields a combination of different Peierls-Hubbard-like Hamiltonians.

Antiferromagnetism and superconductivity

Some well established features found in magnetic insulating crystals are used in a possible model of high-temperature superconductors. They include electron-transfer processes to account for both conduction and antiferromagnetism, and an electron-phonon coupling similar, but of different symmetry, to that invoked in the Jahn-Teller effect. It is shown that the electron-phonon system can be decoupled by a transformation, which results in an electron-electron interaction containing BCS-like terms with a negative sign and no isotopic oxygen effect.