2,767 research outputs found
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
Action functionals for strings in four dimensions
All possible action functionals on the space of surfaces in 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
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
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