32 research outputs found
Softening of Cu-O bond stretching phonon in tetragonal HgBaCuO
Phonons in nearly optimally doped HgBaCuO were studied by
inelastic X-ray scattering. The dispersion of the low energy modes is well
described by a shell model, while the Cu-O bond stretching mode at high energy
shows strong softening towards the zone boundary, which deviates strongly from
the model. This seems to be common in the hole-doped high-
superconducting cuprates, and, based on this work, not related to a lattice
distortion specific to each material.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let
Charge fluctuations and electron-phonon interaction in the finite- Hubbard model
In this paper we employ a gaussian expansion within the finite-
slave-bosons formalism to investigate the momentum structure of the
electron-phonon vertex function in the Hubbard model as function of and
. The suppression of large momentum scattering and the onset a small- peak structure, parametrized by a cut-off , are shown to be
essentially ruled by the band narrowing factor due to the
electronic correlation. A phase diagram of and in the whole
- space is presented. Our results are in more than qualitative agreement
with a recent numerical analysis and permit to understand some anomalous
features of the Quantum Monte Carlo data.Comment: 4 pages, eps figures include
Phonon dispersion and lifetimes in MgB2
We measure phonon dispersion and linewidth in a single crystal of MgB_2 along
the Gamma-A, Gamma-M and A-L directions using inelastic X-Ray scattering. We
use Density Functional Theory to compute the effect of both electron-phonon
coupling and anharmonicity on the linewidth, obtaining excellent agreement with
experiment. Anomalous broadening of the E_2g phonon mode is found all along
Gamma-A. The dominant contribution to the linewidth is always the
electron-phonon coupling.Comment: 4 pages, 3 figure
Inelastic X-ray Scattering by Electronic Excitations in Solids at High Pressure
Investigating electronic structure and excitations under extreme conditions
gives access to a rich variety of phenomena. High pressure typically induces
behavior such as magnetic collapse and the insulator-metal transition in 3d
transition metals compounds, valence fluctuations or Kondo-like characteristics
in -electron systems, and coordination and bonding changes in molecular
solids and glasses. This article reviews research concerning electronic
excitations in materials under extreme conditions using inelastic x-ray
scattering (IXS). IXS is a spectroscopic probe of choice for this study because
of its chemical and orbital selectivity and the richness of information it
provides. Being an all-photon technique, IXS has a penetration depth compatible
with high pressure requirements. Electronic transitions under pressure in 3d
transition metals compounds and -electron systems, most of them strongly
correlated, are reviewed. Implications for geophysics are mentioned. Since the
incident X-ray energy can easily be tuned to absorption edges, resonant IXS,
often employed, is discussed at length. Finally studies involving local
structure changes and electronic transitions under pressure in materials
containing light elements are briefly reviewed.Comment: submitted to Rev. Mod. Phy
Phonon anomalies at the valence transition of SmS : An inelasticX-ray scattering study under pressure
The phonon dispersion curve of SmS under pressure was studied by inelastic
x-ray scattering around the pressure-induced valence transition. A significant
softening of the longitudinal acoustic modes propagating along the [111]
direction was observed spanning a wide region from
() up to the zone boundary as
SmS becomes metallic. The largest softening occurs at the zone boundary and
stays stable up to the highest measured pressure of 80 kbar while a gradual
hardening of the low modes simultaneously appears. This phonon spectrum
indicates favorable conditions for the emergence of pressure-induced
superconductivity in SmS.Comment: 4 pages, 3 figure
Superconductivity of bulk CaC6
We have obtained bulk samples of the graphite intercalation compound, CaC6,
by a novel method of synthesis from highly oriented pyrolytic graphite. The
crystal structure has been completely determined showing that it is the only
member of the MC6, metal-graphite compounds, which has rhombohedral symmetry.
We have clearly shown the occurrence of superconductivity in the bulk sample at
11.5K, using magnetization measurements.Comment: 8 pages of text + 4 figures = 12 page
Sharp optical phonon softening close to optimal doping in LaBaCuO
We report a direct observation of a sharp Kohn-like anomaly in the doubly
degenerate copper-oxygen bond-stretching phonon mode occurring at
in LaBaCuO with
, thanks to the high resolution of
inelastic x-ray scattering. This anomaly is clearly seen when the inelastic
signal is analysed using a single mode but is also consistent with a two mode
hypothesis possibly due to a splitting of the degenerate modes due to symmetry
breaking stripes. Our observation shows that the effect persists at the stripe
propagation vector in a superconducting system close to optimal doping
Observation of Umklapp processes in non-crystalline materials
Umklapp processes are known to exist in cristalline materials, where they
control important properties such as thermal conductivity, heat capacity and
electrical conductivity. In this work we report the provocative observation of
Umklapp processes in a non-periodical system, namely liquid Lithium. The lack
of a well defined periodicity seems then not to prevent the existence of these
scattering processes mechanisms provided that the local order of the systems
i.e. the maxima of the static structure factor supply the equivalent of a
reciprocal lattice vector in the case of cristalline materials.Comment: 13 pages P
Poor screening and nonadiabatic superconductivity in correlated systems
In this paper we investigate the role of the electronic correlation on the
hole doping dependence of electron-phonon and superconducting properties of
cuprates. We introduce a simple analytical expression for the one-particle
Green's function in the presence of electronic correlation and we evaluate the
reduction of the screening properties as the electronic correlation increases
by approaching half-filling. The poor screening properties play an important
role within the context of the nonadiabatic theory of superconductivity. We
show that a consistent inclusion of the reduced screening properties in the
nonadiabatic theory can account in a natural way for the - phase
diagram of cuprates. Experimental evidences are also discussed.Comment: 12 Pages, 6 Figures, Accepted on Physical Review