2,340 research outputs found
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
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
Diamagnetism of real-space pairs above Tc in hole doped cuprates
The nonlinear normal state diamagnetism reported by Lu Li et al. [Phys. Rev.
B 81, 054510 (2010)] is shown to be incompatible with an acclaimed Cooper
pairing and vortex liquid above the resistive critical temperature. Instead it
is perfectly compatible with the normal state Landau diamagnetism of real-space
composed bosons, which describes the nonlinear magnetization curves in less
anisotropic cuprates La-Sr-Cu-O (LSCO) and Y-Ba-Cu-O (YBCO) as well as in
strongly anisotropic bismuth-based cuprates in the whole range of available
magnetic fields.Comment: 4 pages, 4 figure
Polaron and bipolaron transport in a charge segregated state of doped strongly correlated 2D semiconductor
The 2D lattice gas model with competing short and long range interactions is
appliedused for calculation of the incoherent charge transport in the classical
strongly-correlated charge segregated polaronic state. We show, by means of
Monte-Carlo simulations, that at high temperature the transport is dominated by
hopping of the dissociated correlated polarons, where with thetheir mobility is
inversely proportional to the temperature. At the temperatures below the
clustering transition temperature the bipolaron transport becomes dominant. The
energy barrier for the bipolaron hopping is determined by the Coulomb effects
and is found to be lower than the barrier for the single-polaron hopping. This
leads to drastically different temperature dependencies of mobilities for
polarons and bipolarons at low temperatures
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
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
Hall effect and resistivity in underdoped cuprates
The behaviour of the Hall ratio as a function of temperature is
one of the most intriguing normal state properties of cuprate superconductors.
One feature of all the data is a maximum of in the normal state that
broadens and shifts to temperatures well above with decreasing doping. We
show that a model of preformed pairs-bipolarons provides a selfconsistent
quantitative description of together with in-plane resistivity and
uniform magnetic susceptibility for a wide range of doping.Comment: 4 pages, 2 figures, the model and fits were refine
Thermodynamics of 2D string theory
We calculate the free energy, energy and entropy in the matrix quantum
mechanical formulation of 2D string theory in a background strongly perturbed
by tachyons with the imaginary Minkowskian momentum
(``Sine-Liouville'' theory). The system shows a thermodynamical behaviour
corresponding to the temperature . We show that the
microscopically calculated energy of the system satisfies the usual
thermodynamical relations and leads to a non-zero entropy.Comment: 13 pages, lanlmac; typos correcte
Superlight small bipolarons
Recent angle-resolved photoemission spectroscopy (ARPES) has identified that
a finite-range Fr\"ohlich electron-phonon interaction (EPI) with c-axis
polarized optical phonons is important in cuprate superconductors, in agreement
with an earlier proposal by Alexandrov and Kornilovitch. The estimated
unscreened EPI is so strong that it could easily transform doped holes into
mobile lattice bipolarons in narrow-band Mott insulators such as cuprates.
Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the
total energy, effective mass, pair radius, number of phonons and isotope
exponent of lattice bipolarons in the region of parameters where any
approximation might fail taking into account the Coulomb repulsion and the
finite-range EPI. The effects of modifying the interaction range and different
lattice geometries are discussed with regards to analytical
strong-coupling/non-adiabatic results. We demonstrate that bipolarons can be
simultaneously small and light, provided suitable conditions on the
electron-phonon and electron-electron interaction are satisfied. Such light
small bipolarons are a necessary precursor to high-temperature Bose-Einstein
condensation in solids. The light bipolaron mass is shown to be universal in
systems made of triangular plaquettes, due to a novel crab-like motion. Another
surprising result is that the triplet-singlet exchange energy is of the first
order in the hopping integral and triplet bipolarons are heavier than singlets
in certain lattice structures at variance with intuitive expectations. Finally,
we identify a range of lattices where superlight small bipolarons may be
formed, and give estimates for their masses in the anti-adiabatic
approximation.Comment: 31 pages. To appear in J. Phys.: Condens. Matter, Special Issue
'Mott's Physics
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