The spin-polaron theory of high-T_csuperconductivity

Abstract

An outline is given of the model for some high-temperature superconductors which assumes that the carriers are holes in the (hybridized) oxygen 2p band and form 'spin polarons' with the moments on the copper atoms. A comparison is made with observations of spin polarons in Gda-xvxS4 and with the properties of Lal_xSr~VOa in relation to those of La2_xSrxCuO4. It is assumed, following several authors, that in the superconductors the polarons form bipolarons, which are bosons, and a comparison is made with some other treatments of this hypothesis. It is proposed that in many such superconductors the boson, essentially a pair of these holes, moves in an impurity band, and that normally all the polarons (fermions) form bipolarons; the fermions repel each other on the same site (positive Hubbard U) but attract when on adjacent sites; the critical temperature T~ is then that at which the Bose gas becomes non-degenerate. In such materials a nondegenerate gas of bosons would carry the current above T~ as first suggested by Alexandrov et al. (1986). The linear increase in the resistivity above T~ is explained on this hypothesis. The effective mass of the bipolaron is, we believe, large (~20-30me). The copper 3d 9 moments in the superconducting range resonate between their two orientations as a consequence of the motion of the carriers, as they do in the description by Brinkman and Rice (1970) of highly correlated metals. Spin polarons, we believe, form only when this is so, but not in the antiferromagnetic range of x. A discussion is given of the resistivity above T~, thermopower above T~, and of the nature of the superconducting gap as shown by tunnelling. We confine our discussion to the materials containing copper, excluding for instance cubic Bal_xKxBiO3, and possibly any superconductor containing bismuth, where the bosons may be Bi 3+. Contents PAGE

Spin Susceptibility Scaling in High-Temperature Superconductors

“…The method is thus sensitive to subtle differences in the equilibrium distribution of soluble Al, an outcome that has not been achieved with other methods for the analysis of Al. Stability constants for aqueous aluminum fluoride and aluminum sulfate complexes measured by the morin technique compare favorably to published values (4).…”

“…3. At the vector Q the susceptibility becomes [tanh (9) + tanh ( $11 (4) When p = 0, this function is singular at w = 0 because the electron velocity at the comers of the corresponding square orbit vanishes and thus generates a logarithmic singularity in the density of states N(w). For p = 0 the system is unstable (2 1, 22) toward formation of a CDW or SDW for arbitrarily weak coupling.…”

“…morin (Table 1) (4). We measured concentrations of A13+ and total silicatebound A1 (Al-Si) by comparing the fluorescence signal to that of solutions lacking Si (4). Because the Al-morin complexes give a large fluorescence signal, the fraction of total soluble A1 responsible for the fluorescence signal can be maintained at a low value (typically less than 10%) in the analysis.…”

Low-frequency kinetics of high-T c metal oxides

“…As an example, bosons may be spin-bipolarons, as was suggested by Mott,19 or short-range RVB. 2~ The latter possibility differs from our model not only by the type of the interaction, but also by the bosonic excitations, which have charge e (holons) and not 2e as bipolarons.…”

Polarons in axial transport in single-layer high-Tc superconductors

“…Boltzmann statistics had led Sir Nevill to deduce a related temperature dependence for the carrier mobility µ σ , and thereby conductivity σ σ , from a temperature-independent diffusion coefficient D σ of spin polarons using the Nernst-Einstein relation [30]. From µ l in Section 2, our dielectric bond-polarons have a diffusion coefficient…”