Isotope effects in high-Tc cuprate superconductors: Ultimate proof for bipolaron theory of superconductivity

Developing a theory of high-temperature superconductivity in copper oxides is one of the outstanding problems in physics. Twenty-five years after its discovery, no consensus on the microscopic theory has been reached despite tremendous theoretical and experimental efforts. Attempts to understand this problem are hindered by the subtle interplay among a few mechanisms and the presence of several nearly degenerate and competing phases in these systems. Here we provide unified parameter-free explanation of the observed oxygen-isotope effects on the critical temperature, the magnetic-field penetration depth, and on the normal-state pseudogap for underdoped cuprate superconductors within the framework of the bipolaron theory compatible with the strong Coulomb and Froehlich interactions, and with many other independent observations in these highly polarizable doped insulators. Remarkably, we also quantitatively explain measured critical temperatures and magnitudes of the magnetic-field penetration depth. The present work thus represents an ultimate proof of the bipolaron theory of high-temperature superconductivity, which takes into account essential Coulomb and electron-phonon interactions.Comment: 8 pages, 2 figure

C-axis negative magnetoresistance and upper critical field of Bi2Sr2CaCu2O8

The out-of-plane resistance and the resistive upper critical field of BSCCO-2212 single crystals with Tc=91-93 K have been measured in magnetic fields up to 50 T over a wide temperature range. The results are characterised by a positive linear magnetoresistance in the superconducting state and a negative linear magnetoresistance in the normal state. The zero field normal state c-axis resistance, the negative linear normal state magnetoresistance, and the divergent upper critical field Hc2(T)are explained in the framework of the bipolaron theory of superconductivity.Comment: 4 pages (REVTeX), 4 figures, submitted to Physical Review Letters 6 April 1999, rejected in February 2000, accepted for publication in Europhysics Letters on 31 May 200

High Temperature Superconductivity: the explanation

Soon after the discovery of the first high temperature superconductor by Georg Bednorz and Alex Mueller in 1986 the late Sir Nevill Mott answering his own question "Is there an explanation?" [Nature v 327 (1987) 185] expressed a view that the Bose-Einstein condensation (BEC) of small bipolarons, predicted by us in 1981, could be the one. Several authors then contemplated BEC of real space tightly bound pairs, but with a purely electronic mechanism of pairing rather than with the electron-phonon interaction (EPI). However, a number of other researchers criticized the bipolaron (or any real-space pairing) scenario as incompatible with some angle-resolved photoemission spectra (ARPES), with experimentally determined effective masses of carriers and unconventional symmetry of the superconducting order parameter in cuprates. Since then the controversial issue of whether the electron-phonon interaction (EPI) is crucial for high-temperature superconductivity or weak and inessential has been one of the most challenging problems of contemporary condensed matter physics. Here I outline some developments in the bipolaron theory suggesting that the true origin of high-temperature superconductivity is found in a proper combination of strong electron-electron correlations with a significant finite-range (Froehlich) EPI, and that the theory is fully compatible with the key experiments.Comment: 8 pages, 2 figures, invited comment to Physica Script

Detection of QED vacuum nonlinearities in Maxwell's equations by the use of waveguides

We present a novel method for detecting nonlinearities, due to quantum electrodynamics through photon-photon scattering, in Maxwell's equation. The photon-photon scattering gives rise to self-interaction terms, which are similar to the nonlinearities due to the polarisation in nonlinear optics. These self-interaction terms vanish in the limit of parallel propagating waves, but if instead of parallel propagating waves the modes generated in wavesguides are used, there will be a non-zero total effect. Based on this idea, we calculate the nonlinear excitation of new modes and estimate the strength of this effect. Furthermore, we suggest a principal experimental setup.Comment: 4 pages, REVTeX3. To appear in Phys. Rev. Let

Electron-phonon coupling in crystalline organic semiconductors: Microscopic evidence for nonpolaronic charge carriers

We consider electron(hole)-phonon coupling in crystalline organic semiconductors, using naphthalene for our case study. Employing a first-principles approach, we compute the changes in the self-consistent Kohn-Sham potential corresponding to different phonon modes and go on to obtain the carrier-phonon coupling matrix elements (vertex functions). We then evaluate perturbatively the quasiparticle spectral residues for electrons at the bottom of the lowest-unoccupied- (LUMO) and holes at the top of the highest-occupied (HOMO) band, respectively obtaining $Z_e\approx 0.74$ and $Z_h\approx 0.78$. Along with the widely accepted notion that the carrier-phonon coupling strengths in polyacenes decrease with increasing molecular size, our results provide a strong microscopic evidence for the previously conjectured nonpolaronic nature of band-like carriers in these systems.Comment: final, published versio

Bose-Einstein condensation of strongly correlated electrons and phonons in cuprate superconductors

The long-range Froehlich electron-phonon interaction has been identified as the most essential for pairing in high-temperature superconductors owing to poor screening, as is now confirmed by optical, isotope substitution, recent photoemission and some other measurements. I argue that low energy physics in cuprate superconductors is that of superlight small bipolarons, which are real-space hole pairs dressed by phonons in doped charge-transfer Mott insulators. They are itinerant quasiparticles existing in the Bloch states at low temperatures as also confirmed by continuous-time quantum Monte-Carlo algorithm (CTQMC) fully taking into account realistic Coulomb and long-range Froehlich interactions. Here I suggest that a parameter-free evaluation of Tc, unusual upper critical fields, the normal state Nernst effect, diamagnetism, the Hall-Lorenz numbers and giant proximity effects strongly support the three-dimensional (3D) Bose-Einstein condensation of mobile small bipolarons with zero off-diagonal order parameter above the resistive critical temperature Tc at variance with phase fluctuation scenarios of cuprates.Comment: 35 pages, 10 figures, to appear in the special volume of Journal of Physics: Condensed Matte

INVESTIGATION OF NOOTROPIC DRUGS (PYRAMEM AND OROCETAM) FOR THEIR TERATOGENIC EFFECT

The teratogenic effect of the nootropic drugs Pyramem and Orocetam on the skeleton and internal organs of rats was investigated. Pregnant Wistar rats were treated orally by 200 mg/kg b. m. Pyramem and 200 mg/kg b. m. Orocetam during the period of organogenesis since the 7th until 15th day of pregnancy. The effect of these preparations on the fetuses in terms of malformations, skeletal fragility and abnormalities in the internal organs (lungs, liver, spleen and kidneys) close to delivery (day 20-21) was analyzed. The nootropics Orocetam and Pyramem failed to prove to be embryotoxic agents at all

Parameter-free expression for superconducting Tc in cuprates

A parameter-free expression for the superconducting critical temperature of layered cuprates is derived which allows us to express Tc in terms of experimentally measured parameters. It yields Tc values observed in about 30 lanthanum, yttrium and mercury-based samples for different levels of doping. This remarkable agreement with the experiment as well as the unusual critical behaviour and the normal-state gap indicate that many cuprates are close to the Bose-Einstein condensation regime.Comment: 5 pages, 2 figures. Will be published in Physical Review