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

Phase coexistence and resistivity near the ferromagnetic transition of manganites

Pairing of oxygen holes into heavy bipolarons in the paramagnetic phase and their magnetic pair-breaking in the ferromagnetic phase [the so-called current-carrier density collapse (CCDC)] has accounted for the first-order ferromagnetic phase transition, colossal magnetoresistance (CMR), isotope effect, and pseudogap in doped manganites. Here we propose an explanation of the phase coexistence and describe the magnetization and resistivity of manganites near the ferromagnetic transition in the framework of CCDC. The present quantitative description of resistivity is obtained without any fitting parameters by using the experimental resistivities far away from the transition and the experimental magnetization, and essentially model independent.Comment: 10 pages, 3 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

Superlight small bipolarons from realistic long-range Coulomb and Fr\"ohlich interactions

We report analytical and numerical results on the two-particle states of the polaronic t-Jp model derived recently with realistic Coulomb and electron-phonon (Frohlich) interactions in doped polar insulators. Eigenstates and eigenvalues are calculated for two different geometries. Our results show that the ground state is a bipolaronic singlet, made up of two polarons. The bipolaron size increases with increasing ratio of the polaron hopping integral t to the exchange interaction Jp but remains small compared to the system size in the whole range 0<t/Jp<1. Furthermore, the model exhibits a phase transition to a superconducting state with a critical temperature well in excess of 100K. In the range t/Jp<1, there are distinct charge and spin gaps opening in the density of states, specific heat, and magnetic susceptibility well above Tc.Comment: Calculation section and discussion of gap have been updated. Revised calculations now enhance the predicted T_c in our model to over 200 K at large hoppin

Vortex matter in the charged Bose liquid at absolute zero

The Gross-Pitaevskii-type equation is solved for the charge Bose liquid in the external magnetic field at zero temperature. There is a vortex lattice with locally broken charge neutrality. The boson density is modulated in real space and each vortex is charged. Remarkably, there is no upper critical field at zero temperature, so the density of single flux-quantum vortices monotonously increases with the magnetic field up to B=infinity and no indication of a phase transition. The size of each vortex core decreases as about 1/sqrt(B) keeping the system globally charge neutral. If bosons are composed of two fermions, a phase transition to a spin-polarized Fermi liquid at some magnetic field larger than the pair-breaking field is predicted.Comment: 4 pages, 4 figures, references update

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

Action functionals for strings in four dimensions

All possible action functionals on the space of surfaces in ${\bf R}^4$ that depend only on first and second derivatives of the functions, entering the equation of the surface, and satisfy the condition of invariance with respect to rigid motions are described.Comment: 9 pages, LaTeX, 7 figure