915 research outputs found
Theory of Dynamic Stripe Induced Superconductivity
Since the recently reported giant isotope effect on T* [1] could be
consistently explained within an anharmonic spin-charge-phonon interaction
model, we consider here the role played by stripe formation on the
superconducting properties within the same model. This is a two-component
scenario and we recast its basic elements into a BCS effective Hamiltonian. We
find that the stripe formation is vital to high-Tc superconductivity since it
provides the glue between the two components to enhance Tc to the unexpectedly
large values observed experimentally.Comment: 7 pages, 2 figure
Hybrid paramagnon phonon modes at elevated temperatures in EuTiO3
EuTiO3 (ETO) has recently experienced an enormous revival of interest because
of its possible multiferroic properties which are currently in the focus of
research. Unfortunately ETO is an unlikely candidate for enlarged
multifunctionality since the mode softening - typical for ferroelectrics -
remains incomplete, and the antiferromagnetic properties appear at 5.5K only.
However, a strong coupling between lattice and Eu spins exists and leads to the
appearance of a magnon-phonon-hybrid mode at elevated temperatures as evidenced
by electron paramagnetic resonance (EPR), muon spin rotation ({\mu}SR)
experiments and model predictions based on a coupled spin-polarizability
Hamiltonian. This novel finding supports the notion of strong
magneto-dielectric (MD) effects being realized in ETO and opens new strategies
in material design and technological applications.Comment: 9 pages, 4 figure
The polarizability model for ferroelectricity in perovskite oxides
This article reviews the polarizability model and its applications to
ferroelectric perovskite oxides. The motivation for the introduction of the
model is discussed and nonlinear oxygen ion polarizability effects and their
lattice dynamical implementation outlined. While a large part of this work is
dedicated to results obtained within the self-consistent-phonon approximation
(SPA), also nonlinear solutions of the model are handled which are of interest
to the physics of relaxor ferroelectrics, domain wall motions, incommensurate
phase transitions. The main emphasis is to compare the results of the model
with experimental data and to predict novel phenomena.Comment: 55 pages, 35 figure
Graphene on Si(111)7x7
We demonstrate that it is possible to mechanically exfoliate graphene under
ultra high vacuum conditions on the atomically well defined surface of single
crystalline silicon. The flakes are several hundred nanometers in lateral size
and their optical contrast is very faint in agreement with calculated data.
Single layer graphene is investigated by Raman mapping. The G and 2D peaks are
shifted and narrowed compared to undoped graphene. With spatially resolved
Kelvin probe measurements we show that this is due to p-type doping with hole
densities of n_h \simeq 6x10^{12} cm^{-2}. The in vacuo preparation technique
presented here should open up new possibilities to influence the properties of
graphene by introducing adsorbates in a controlled way.Comment: 8 pages, 7 figure
Correlation between oxygen isotope effects on the transition temperature and the magnetic penetration depth in high-temperature superconductors close to optimal doping
The oxygen-isotope (^{16}O/^{18}O) effect (OIE) on the in-plane magnetic
penetration depth \lambda_{ab}(0) in optimally-doped YBa_2Cu_3O_{7-\delta} and
La_{1.85}Sr_{0.15}CuO_4, and in slightly underdoped YBa_2Cu_4O_8 and
Y_{0.8}Pr_{0.2}Ba_2Cu_3O_{7-\delta} was studied by means of muon-spin rotation.
A substantial OIE on \lambda_{ab}(0) with an OIE exponent
\beta_O=-d\ln\lambda_{ab}(0)/d\ln M_O\approx - 0.2 (M_O is the mass of the
oxygen isotope), and a small OIE on the transition temperature T_c with an OIE
exponent \alpha_O=-d\ln T_{c}/d \ln M_O\simeq0.02 to 0.1 were observed. The
observation of a substantial isotope effect on \lambda_{ab}(0), even in
cuprates where the OIE on T_c is small, indicates that lattice effects play an
important role in cuprate HTS.Comment: 6 pages, 4 figure
Onset of dielectric modes at 110K and 60K due to local lattice distortions in non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals
We report the observation of two dielectric transitions at 110K and 60K in
the microwave response of non-superconducting YBa_{2}Cu_{3}O_{6.0} crystals.
The transitions are characterized by a change in polarizability and presence of
loss peaks, associated with overdamped dielectric modes. An explanation is
presented in terms of changes in polarizability of the apical O atoms in the
Ba-O layer, affected by lattice softening at 110K, due to change in buckling of
the Cu-O layer. The onset of another mode at 60K strongly suggests an
additional local lattice change at this temperature. Thus microwave dielectric
measurements are sensitive indicators of lattice softening which may be
relevant to superconductivity.Comment: 5 pages, 3 ps format figure
Electron-Phonon Coupling in High-Temperature Cuprate Superconductors Determined from Electron Relaxation Rates
We determined electronic relaxation times via pump-probe optical spectroscopy
using sub-15 fs pulses for the normal state of two different cuprate
superconductors.We show that the primary relaxation process is the
electron-phonon interaction and extract a measure of its strength, the second
moment of the Eliashberg function\lambda=800\pm200 meV^{2} for
La_{1.85}Sr_{0.15}CuO_{4} and \lambda=400\pm100 meV^{2} for
YBa_{2}Cu_{3}O_{6.5}. These values suggest a possible fundamental role of the
electron-phonon interaction in the superconducting pairing mechanism.Comment: As published in PR
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