2,223,460 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
Soliton Wall Superlattice in Quasi-One-Dimensional Conductor (Per)2Pt(mnt)2
We suggest a model to explain the appearance of a high resistance high
magnetic field charge-density-wave (CDW) phase, discovered by D. Graf et al.
[Phys. Rev. Lett. v. 93, 076406 (2004)] in (Per)2Pt(mnt)2. In particular, we
show that the Pauli spin-splitting effects improve the nesting properties of a
realistic quasi-one-dimensional electron spectrum and, therefore, a high
resistance Peierls CDW phase is stabilized in high magnetic fields. In low and
very high magnetic fields, a periodic soliton wall superlattice (SWS) phase is
found to be a ground state. We suggest experimental studies of the predicted
phase transitions between the Peierls and SWS CDW phases in (Per)2Pt(mnt)2 to
discover a unique SWS phase.Comment: 10 pages, 3 figures. Submitted to Physical Review Letters (February
19, 2007
Green's and spectral functions of the small Frolich polaron
According to recent Quantum Monte Carlo simulations the small polaron theory
is practically exact in a wide range of the long-range (Frohlich)
electron-phonon coupling and adiabatic ratio. We apply the Lang-Firsov
transformation to convert the strong-coupling term in the Hamiltonian into the
form of an effective hopping integral and derive the single-particle Green's
function describing propagation of the small Frohlich polaron. One and two
dimensional spectral functions are studied by expanding the Green's function
perturbatively. Numerical calculations of the spectral functions are produced.
Remarkably, the coherent spectral weight (Z) and effective mass (Z')
renormalisation exponents are found to be different with Z'>>Z, which can
explain a small coherent spectral weight and a relatively moderate mass
enhancement in oxides.Comment: RevTeX, 5 pages, 2 postscript figures, LaTeX processing problems
correcte
Key pairing interaction in layered doped ionic insulators
A controversial issue on whether the electron-phonon interaction (EPI) is
crucial for high-temperature superconductivity or it is weak and inessential
has remained one of the most challenging problems of contemporary condensed
matter physics. We employ a continuum RPA approximation for the dielectric
response function allowing for a selfconsistent semi-analytical evaluation of
the EPI strength, electron-electron attractions, and the carrier mass
renormalisation in layered high-temperature superconductors. We show that the
Fr\"{o}hlich EPI with high-frequency optical phonons in doped ionic lattices is
the key pairing interaction, which is beyond the BCS-Migdal-Eliashberg
approximation in underdoped superconductors, and it remains a significant
player in overdoped compounds.Comment: 4 pages, 4 figure
Reflection High-Energy Electron Diffraction oscillations during epitaxial growth of artificially layered films of (BaCuOx)m /(CaCuO2)n
Pulsed Laser Deposition in molecular-beam epitaxy environment (Laser-MBE) has
been used to grow high quality BaCuOx/CaCuO2 superlattices. In situ Reflection
High Energy Electron Diffraction (RHEED) shows that the growth mechanism is
2-dimensional. Furthermore, weak but reproducible RHEED intensity oscillations
have been monitored during the growth. Ex-situ x-ray diffraction spectra
confirmed the growth rate deduced from RHEED oscillations. Such results
demonstrate that RHEED oscillations can be used, even for (BaCuOx)2/(CaCuO2)2
superlattices, for phase locking of the growth.Comment: 9 pages, 5 figures. Corresponding author: Dr. A. Tebano:
[email protected]
- …