715 research outputs found
Dominant particle-hole contributions to the phonon dynamics in the spinless one-dimensional Holstein model
In the spinless Holstein model at half-filling the coupling of electrons to
phonons is responsible for a phase transition from a metallic state at small
coupling to a Peierls distorted insulated state when the electron-phonon
coupling exceeds a critical value. For the adiabatic case of small phonon
frequencies, the transition is accompanied by a phonon softening at the
Brillouin zone boundary whereas a hardening of the phonon mode occurs in the
anti-adiabatic case. The phonon dynamics studied in this letter do not only
reveal the expected renormalization of the phonon modes but also show
remarkable additional contributions due to electronic particle-hole
excitations.Comment: 7 pages, 4 figures and 1 table included; v2: discussion of Luttinger
liquid parameters adde
Optical absorption of non-interacting tight-binding electrons in a Peierls-distorted chain at half band-filling
In this first of three articles on the optical absorption of electrons in
half-filled Peierls-distorted chains we present analytical results for
non-interacting tight-binding electrons. We carefully derive explicit
expressions for the current operator, the dipole transition matrix elements,
and the optical absorption for electrons with a cosine dispersion relation of
band width and dimerization parameter . New correction
(``''-)terms to the current operator are identified. A broad band-to-band
transition is found in the frequency range whose shape
is determined by the joint density of states for the upper and lower Peierls
subbands and the strong momentum dependence of the transition matrix elements.Comment: 17 pages REVTEX 3.0, 2 postscript figures; hardcopy versions before
May 96 are obsolete; accepted for publication in The Philosophical Magazine
Perturbation theory for optical excitations in the one-dimensional extended Peierls--Hubbard model
For the one-dimensional, extended Peierls--Hubbard model we calculate
analytically the ground-state energy and the single-particle gap to second
order in the Coulomb interaction for a given lattice dimerization. The
comparison with numerically exact data from the Density-Matrix Renormalization
Group shows that the ground-state energy is quantitatively reliable for Coulomb
parameters as large as the band width. The single-particle gap can almost
triple from its bare Peierls value before substantial deviations appear. For
the calculation of the dominant optical excitations, we follow two approaches.
In Wannier theory, we perturb the Wannier exciton states to second order. In
two-step perturbation theory, similar in spirit to the GW-BSE approach, we form
excitons from dressed electron-hole excitations. We find the Wannier approach
to be superior to the two-step perturbation theory. For singlet excitons,
Wannier theory is applicable up to Coulomb parameters as large as half band
width. For triplet excitons, second-order perturbation theory quickly fails
completely.Comment: 32 pages, 12 figures, submtted to JSTA
Localization Properties of the Periodic Random Anderson Model
We consider diagonal disordered one-dimensional Anderson models with an
underlying periodicity. We assume the simplest periodicity, i.e., we have
essentially two lattices, one that is composed of the random potentials and the
other of non-random potentials. Due to the periodicity special resonance
energies appear, which are related to the lattice constant of the non-random
lattice. Further on two different types of behaviors are observed at the
resonance energies. When a random site is surrounded by non-random sites, this
model exhibits extended states at the resonance energies, whereas otherwise all
states are localized with, however, an increase of the localization length at
these resonance energies. We study these resonance energies and evaluate the
localization length and the density of states around these energies.Comment: 4 page
Transarterial chemoembolisation: effect of selectivity on tolerance, tumour response and survival
Aims To compare selective and non-selective TACE techniques in the treatment of HCC with a special emphasis on clinical and liver tolerance, tumour response and survival. Methods 184 patients with advanced HCC were retrospectively included. Three different TACE techniques were compared: non selective lipiodol-chemotherapy + non selective embolisation (TACE-technique group 1), non selective lipiodol-chemotherapy + selective embolisation (group 2), and selective lipiodol-chemotherapy + selective embolisation (group 3). Results In multivariate analysis TACE-technique group is an independently significant prognostic factor for poor clinical tolerance, poor liver tolerance and tumour response. The rate of patients with poor clinical tolerance was lower in group 3 (27.0%) than in groups 1 (64.1%, p < 10−3) or 2 (66.7%, p < 10−3). The rate of patients with poor liver tolerance was higher in group 2 (34.0%) than in groups 1 (17.6%, p = 0.050) or 3 (6.9%, p = 0.011). The rate of patients with tumour response was higher when embolisation was selective versus non-selective, i.e., group 2 + 3 (78.7%) versus group 1 (62.5%, p = 0.054). Overall survival was not significantly different between the three groups (p = 0.383). Conclusion Both selective techniques resulted in better tumour response. As for improving tolerance, our study suggests that the main technical factor is the use of selective lipiodol-chemotherapy injection
Exact results for the optical absorption of strongly correlated electrons in a half-filled Peierls-distorted chain
In this second of three articles on the optical absorption of electrons in a
half-filled Peierls-distorted chain we present exact results for strongly
correlated tight-binding electrons. In the limit of a strong on-site
interaction we map the Hubbard model onto the Harris-Lange model which can
be solved exactly in one dimension in terms of spinless fermions for the charge
excitations. The exact solution allows for an interpretation of the charge
dynamics in terms of parallel Hubbard bands with a free-electron dispersion of
band-width , separated by the Hubbard interaction . The spin degrees of
freedom enter the expressions for the optical absorption only via a momentum
dependent but static ground state expectation value. The remaining spin problem
can be traced out exactly since the eigenstates of the Harris-Lange model are
spin-degenerate. This corresponds to the Hubbard model at temperatures large
compared to the spin exchange energy. Explicit results are given for the
optical absorption in the presence of a lattice distortion and a
nearest-neighbor interaction . We find that the optical absorption for
is dominated by a peak at and broad but weak absorption bands for . For an appreciable nearest-neighbor interaction, ,
almost all spectral weight is transferred to Simpson's exciton band which is
eventually Peierls-split.Comment: 50 pages REVTEX 3.0, 6 postscript figures; hardcopy versions before
May 96 are obsolete; accepted for publication in The Philosophical Magazine
Molecular dynamics simulations of lead clusters
Molecular dynamics simulations of nanometer-sized lead clusters have been
performed using the Lim, Ong and Ercolessi glue potential (Surf. Sci. {\bf
269/270}, 1109 (1992)). The binding energies of clusters forming crystalline
(fcc), decahedron and icosahedron structures are compared, showing that fcc
cuboctahedra are the most energetically favoured of these polyhedral model
structures. However, simulations of the freezing of liquid droplets produced a
characteristic form of ``shaved'' icosahedron, in which atoms are absent at the
edges and apexes of the polyhedron. This arrangement is energetically favoured
for 600-4000 atom clusters. Larger clusters favour crystalline structures.
Indeed, simulated freezing of a 6525-atom liquid droplet produced an imperfect
fcc Wulff particle, containing a number of parallel stacking faults. The
effects of temperature on the preferred structure of crystalline clusters below
the melting point have been considered. The implications of these results for
the interpretation of experimental data is discussed.Comment: 11 pages, 18 figues, new section added and one figure added, other
minor changes for publicatio
Adequate symptom relief justifies hepatic resection for benign disease
BACKGROUND: The purpose of this study was to evaluate the long-term results of partial liver resection for benign liver lesions. METHODS: All patients operated on for benign liver lesions from 1991 to 2002 were included. Information was retrieved from medical records, the hospital registration system and by a telephonic questionnaire. RESULTS: Twenty-eight patients with a median age of 41 years (17–71) were operated on (M/F ratio 5/23). The diagnosis was haemangioma in 8 patients, FNH in 6, HCA in 13 and angiomyolipoma in 1. Eight patients were known to have relevant co-morbidity. Median operating time was 207 minutes (45–360). The morbidity rate was 25% and no postoperative mortality was observed. Twenty-two patients (79%) had symptoms (mainly abdominal pain) prior to surgery. Twenty-five patients were reached for a questionnaire. The median follow up was 55 months (4–150). In 89% of patients preoperative symptoms had decreased or disappeared after surgery. Four patients developed late complications. CONCLUSION: Long-term follow up after liver surgery for benign liver lesions shows considerable symptom relief and patient satisfaction. In addition to a correct indication these results justify major surgery with associated morbidity and mortality
Unbiased Global Optimization of Lennard-Jones Clusters for N <= 201 by Conformational Space Annealing Method
We apply the conformational space annealing (CSA) method to the Lennard-Jones
clusters and find all known lowest energy configurations up to 201 atoms,
without using extra information of the problem such as the structures of the
known global energy minima. In addition, the robustness of the algorithm with
respect to the randomness of initial conditions of the problem is demonstrated
by ten successful independent runs up to 183 atoms. Our results indicate that
the CSA method is a general and yet efficient global optimization algorithm
applicable to many systems.Comment: revtex, 4 pages, 2 figures. Physical Review Letters, in pres
Explosions of water clusters in intense laser fields
Energetic, highly-charged oxygen ions, (), are copiously
produced upon laser field-induced disassembly of highly-charged water clusters,
and , 60, that are formed by seeding high-pressure
helium or argon with water vapor. clusters (n40000) formed under
similar experimental conditions are found undergo disassembly in the Coulomb
explosion regime, with the energies of ions showing a
dependence. Water clusters, which are argued to be considerably smaller in
size, should also disassemble in the same regime, but the energies of fragment
O ions are found to depend linearly on which, according to
prevailing wisdom, ought to be a signature of hydrodynamic expansion that is
expected of much larger clusters. The implication of these observations on our
understanding of the two cluster explosion regimes, Coulomb explosion and
hydrodynamic expansion, is discussed. Our results indicate that charge state
dependences of ion energy do not constitute an unambiguous experimental
signature of cluster explosion regime.Comment: Submitted to Phys. Rev.
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