171 research outputs found
Bond-Stretching-Phonon Anomalies in Stripe-Ordered La(1.69)Sr(0.31)NiO(4)
We report a neutron scattering study of bond-stretching phonons in
La(1.69)Sr(0.31)NiO(4), a doped antiferromagnet in which the added holes order
in diagonal stripes at 45 deg to the Ni-O bonds. For the highest-energy
longitudinal optical mode along the bonds, a softening of 20% is observed
between the Brillouin zone center and zone boundary. At 45 deg to the bonds, a
splitting of the same magnitude is found across much of the zone. Surprisingly,
the charge-ordering wave vector plays no apparent role in the anomalous
dispersions. The implications for related anomalies in the cuprates are
discussed.Comment: 4 two-col pages, including 4 figures (2 in color); references added
and updated Final version, accepted for publication in PR
Apparent electron-phonon interaction in strongly correlated systems
We study the interaction of electrons with phonons in strongly correlated
solids, having high-T_c cuprates in mind. Using sum-rules, we show that the
apparent strength of this interaction strongly depends on the property studied.
If the solid has a small fraction (doping) delta of charge carriers, the
influence of the interaction on the phonon self-energy is reduced by a factor
delta, while there is no corresponding reduction of the coupling seen in the
electron self-energy. This supports the interpretation of recent photoemission
experiments, assuming a strong coupling to phonons.Comment: 4 pages, RevTeX, 2 eps figure
The Dimer Model for k-phase Organic Superconductors
We prove that the upper electronic bands of k-phase BEDT-TTF salts are
adequately modeled by an half-filled tight-binding lattice with one site per
cell. The band parameters are derived from recent ab-initio calculations,
getting a very simple but extremely accurate one-electron picture. This picture
allows us to solve the BCS gap equation adopting a real-space pairing
potential. Comparison of the calculated superconducting properties with the
experimental data points to isotropic s_0-pairing. Residual many-body or
phonon-mediated interactions offer a plausible explanation of the large variety
of physical properties observed in k-phase BEDT-TTF salts.Comment: 8 pages, 6 PostScript figures, uses RevTe
Electron-phonon anomaly related to charge stripes: static stripe phase versus optimally-doped superconducting La1.85Sr0.15CuO4
Inelastic neutron scattering was used to study the Cu-O bond-stretching
vibrations in optimally doped La1.85Sr0.15CuO4 (Tc = 35 K) and in two other
cuprates showing static stripe order at low temperatures, i.e.
La1.48Nd0.4Sr0.12CuO4 and La1.875Ba0.125CuO4. All three compounds exhibit a
very similar phonon anomaly, which is not predicted by conventional band
theory. It is argued that the phonon anomaly reflects a coupling to charge
inhomogeneities in the form of stripes, which remain dynamic in superconducting
La1.85Sr0.15CuO4 down to the lowest temperatures. These results show that the
phonon effect indicating stripe formation is not restricted to a narrow region
of the phase diagram around the so-called 1/8 anomaly but occurs in optimally
doped samples as well.Comment: to appear in J. Low Temp. Phy
Electron-phonon interaction in the t-J model
We derive a t-J model with electron-phonon coupling from the three-band
model, considering modulation of both hopping and Coulomb integrals by phonons.
While the modulation of the hopping integrals dominates, the modulation of the
Coulomb integrals cannot be neglected. The model explains the experimentally
observed anomalous softening of the half-breathing mode upon doping and a
weaker softening of the breathing mode. It is shown that other phonons are not
strongly influenced, and, in particular, the coupling to a buckling mode is not
strong in this model.Comment: 4 pages, RevTeX, 3 eps figures; final version with minor correction
Phase diagram of a Bose gas near a wide Feshbach resonance
In this paper, we study the phase diagram of a homogeneous Bose gas with a
repulsive interaction near a wide Feshbach resonance at zero temperature. The
Bose-Einstein-condensation (BEC) state of atoms is a metastable state. When the
scattering length exceeds a critical value depending on the atom density
, , the molecular excitation energy is imaginary and the atomic
BEC state is dynamically unstable against molecule formation. The BEC state of
diatomic molecules has lower energy, where the atomic excitation is gapped and
the molecular excitation is gapless. However when the scattering length is
above another critical value, , the molecular BEC state becomes a
unstable coherent mixture of atoms and molecules. In both BEC states, the
binding energy of diatomic molecules is reduced due to the many-body effect.Comment: 5 pages, 4 figure
Oxygen Phonon Branches in Detwinned YBa2Cu3O7
We report results of inelastic neutron scattering measurements of phonon
dispersions on a detwinned sample of YBaCu3O7 and compare them with model
calculations. Plane oxygen bond stretching phonon branches disperse steeply
downwards from the zone center in both the a and the b direction indicating a
strong electron-phonon coupling. Half way to the zone boundary, the phonon
peaks become ill-defined but we see no need to invoke unit cell doubling or
charge stripe formation: lattice dynamical shell model calculations predict
such behavior as a result of branch anticrossings. There were no observable
superconductivity-related temperature effects on selected plane oxygen bond
stretching modes measured on a twinned sample.Comment: 5 pages, 3 figures, To appear in Journal of Low Temperature Physics
(Proceedings of MOS2002; Revised version (1) with many changes throughout the
tex
Polaronic behavior of undoped high-Tc cuprates
We present angle-resolved photoemission spectroscopy (ARPES) data on undoped
La2CuO4, indicating polaronic coupling between bosons and charge carriers.
Using a shell model, we calculate the electron-phonon coupling and find that it
is strong enough to give polarons. We develop an efficient method for
calculating ARPES spectra in undoped systems. Using the calculated couplings,
we find the width of the phonon side band in good agreement with experiment. We
analyze reasons for the observed dependence of the width on the binding energy.Comment: 4 pages, RevTeX, 5 eps figures, more material available at
http://www.fkf.mpg.de/andersen/phonons
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