Abstract
A theory of a pseudogap phase of high-temperature superconductors where current carriers are translation invariant bipolarons is developed. A temperature of a transition from a pseudogap phase to a normal one is calculated. For the temperature of a transition to the pseudogap phase, the isotope coefficient is found. It is shown that the results obtained, in particular, the possibility of negative values of the isotope coefficient, are consistent with the experiment. New experiments on the influence of the magnetic field on the isotope coefficient are proposed.
Highlights
Among the most amazing and mysterious phenomena of high-temperature superconductivity (HTSC) is the existence of a pseudogap phase at a temperature above the critical temperature of a superconducting (SC) transition [1,2,3,4,5]
The approach developed is based on the idea that in such HTSC as MgB2, Bi-2223, Bi-2212, YBaCuO, etc. at temperatures substantially above critical temperature Tc electron pairs in the form of translation invariant (TI)-bipolarons represent the noninteracting strongly bound bosons, demonstrating with a decreasing temperature a transition to the Bose–Einstein condensation
We have shown that the existence of the pseudogap state and non-standard behavior of the isotope coefficient in HTSC materials can be explained on the basis of the electron-phonon interaction without the involvement of any other scenarios [25,26,27,28,29]
Summary
Among the most amazing and mysterious phenomena of high-temperature superconductivity (HTSC) is the existence of a pseudogap phase at a temperature above the critical temperature of a superconducting (SC) transition [1,2,3,4,5]. Me = 2m, ∆k,0 = 1 for k = 0 and ∆k,0 = 0 for k = 0, where Ebp is the ground state energy of a TI bipolaron (reckoned from the Fermi level), ω0 is the frequency of an optical phonon (h = 1), m is a mass of a band electron (hole), k is a wave vector numbering excited states of a TI bipolaron This spectrum has a gap which in the isotropic case is equal to the frequency of an optical phonon ω0. The scenario of an SC based on the idea of a TI bipolaron as a fundamental boson responsible for superconducting properties explains many thermodynamic and spectroscopic properties of HTSC [13,14,15,16] For this reason, the problem of the temperature of a transition to the pseudogap state and its isotope dependence is of interest
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