273 research outputs found
Operator projection method applied to the single-particle Green's function in the Hubbard model
A new non-perturbative framework for many-body correlated systems is
formulated by extending the operator projection method (OPM). This method
offers a systematic expansion which enables us to project into the low-energy
structure after extracting the higher-energy hierarchy. This method also opens
a way to systematically take into account the effects of collective
excitations. The Mott-Hubbard metal-insulator transition in the Hubbard model
is studied by means of this projection beyond the second order by taking into
account magnetic and charge fluctuations in the presence of the high-energy
Mott-Hubbard structure. At half filling, the Mott-Hubbard gap is correctly
eproduced between the separated two bands. Near half filling, a strongly
renormalized low-energy single-particle excitations coexisting with the
Mott-Hubbard bands are shown to appear. Signifcance of momentum-dependent
self-energy in the results is stressed.Comment: 6 pages, final version to appear in J. Phys. Soc. Jp
Anomalous low doping phase of the Hubbard model
We present results of a systematic Quantum-Monte-Carlo study for the
single-band Hubbard model. Thereby we evaluated single-particle spectra (PES &
IPES), two-particle spectra (spin & density correlation functions), and the
dynamical correlation function of suitably defined diagnostic operators, all as
a function of temperature and hole doping. The results allow to identify
different physical regimes. Near half-filling we find an anomalous `Hubbard-I
phase', where the band structure is, up to some minor modifications, consistent
with the Hubbard-I predictions. At lower temperatures, where the spin response
becomes sharp, additional dispersionless `bands' emerge due to the dressing of
electrons/holes with spin excitatons. We present a simple phenomenological fit
which reproduces the band structure of the insulator quantitatively. The Fermi
surface volume in the low doping phase, as derived from the single-particle
spectral function, is not consistent with the Luttinger theorem, but
qualitatively in agreement with the predictions of the Hubbard-I approximation.
The anomalous phase extends up to a hole concentration of 15%, i.e. the
underdoped region in the phase diagram of high-T_c superconductors. We also
investigate the nature of the magnetic ordering transition in the single
particle spectra. We show that the transition to an SDW-like band structure is
not accomplished by the formation of any resolvable `precursor bands', but
rather by a (spectroscopically invisible) band of spin 3/2 quasiparticles. We
discuss implications for the `remnant Fermi surface' in insulating cuprate
compounds and the shadow bands in the doped materials.Comment: RevTex-file, 20 PRB pages, 16 figures included partially as gif. A
full ps-version including ps-figures can be found at
http://theorie.physik.uni-wuerzburg.de/~eder/condmat.ps.gz Hardcopies of
figures (or the entire manuscript) can also be obtained by e-mail request to:
[email protected]
Two-site dynamical mean-field theory
It is shown that a minimum realization of the dynamical mean-field theory
(DMFT) can be achieved by mapping a correlated lattice model onto an impurity
model in which the impurity is coupled to an uncorrelated bath that consists of
a single site only. The two-site impurity model can be solved exactly. The
mapping is approximate. The self-consistency conditions are constructed in a
way that the resulting ``two-site DMFT'' reduces to the previously discussed
linearized DMFT for the Mott transition. It is demonstrated that a reasonable
description of the mean-field physics is possible with a minimum computational
effort. This qualifies the simple two-site DMFT for a systematic study of more
complex lattice models which cannot be treated by the full DMFT in a feasible
way. To show the strengths and limitations of the new approach, the single-band
Hubbard model is investigated in detail. The predictions of the two-site DMFT
are compared with results of the full DMFT. Internal consistency checks are
performed which concern the Luttinger sum rule, other Fermi-liquid relations
and thermodynamic consistency.Comment: LaTeX, 14 pages, 8 eps figures included, Phys. Rev. B (in press
Synergistic action of serine- and metallo-proteases from Fusarium oxysporum f. sp. lycopersici cleaves chitin-binding tomato chitinases, reduces their antifungal activity and enhances fungal virulence
As part of their defence strategy against fungal pathogens, plants secrete chitinases that degrade chitin, the major structural component of fungal cell walls. Some fungi are not sensitive to plant chitinases because they secrete chitin-binding effector proteins that protect their cell wall against these enzymes. However, it is not known how fungal pathogens that lack chitin-binding effectors overcome this plant defence barrier. Here, we investigated the ability of fungal tomato pathogens to cleave chitin-binding domain (CBD)-containing chitinases and its effect on fungal virulence. Four tomato CBD-chitinases were produced in Pichia pastoris and incubated with secreted proteins isolated from seven fungal tomato pathogens. Of these, Fusarium oxysporum f. sp. lycopersici, Verticillium dahliae and Botrytis cinerea were able to cleave the extracellular tomato chitinases SlChi1 and SlChi13. Cleavage by F. oxysporum removed the CBD from the N-terminus, as shown by mass spectrometry, and significantly reduced the chitinase and antifungal activity of both chitinases. Both secreted metallo-protease FoMep1 and serine protease FoSep1 were responsible for this cleavage. Double deletion mutants of FoMep1 and FoSep1 of F. oxysporum lacked chitinase cleavage activity on SlChi1 and SlChi13 and showed reduced virulence on tomato. These results demonstrate the importance of plant chitinase cleavage in fungal virulence
Bacterial flagellar motors and osmoelectric molecular rotation by an axially transmembrane well and turnstile mechanism
AbstractBacterial ion-driven flagellar motors are the smallest known rotatory mechanical devices, natural or artificial, their overall diameter being only about 25 nm or one millionth of an inch. They are unique in the fields of biology and engineering. This paper develops a possible osmoelectric or local electrokinetic mechanism of molecular rotatory motion in bilayer membranes, which may help to explain how bacterial flagellar motors work, and may incidentally encourage new developments in the bioenergetics and biomechanics of enzyme, osmoenzyme and porter action
High rates of clinically relevant incidental findings by total-body CT scanning in trauma patients; results of the REACT-2 trial
To determine whether there is a difference in frequency and clinical relevance of incidental findings detected by total-body computed tomography scanning (TBCT) compared to those by the standard work-up (STWU) with selective computed tomography (CT) scanning. Trauma patients from five trauma centres were randomized between April 2011 and January 2014 to TBCT imaging or STWU consisting of conventional imaging with selective CT scanning. Incidental findings were divided into three categories: 1) major finding, may cause mortality; 2) moderate finding, may cause morbidity; and 3) minor finding, hardly relevant. Generalized estimating equations were applied to assess differences in incidental findings. In total, 1083 patients were enrolled, of which 541 patients (49.9 %) were randomized for TBCT and 542 patients (50.1 %) for STWU. Major findings were detected in 23 patients (4.3 %) in the TBCT group compared to 9 patients (1.7 %) in the STWU group (adjusted rate ratio 2.851; 95%CI 1.337-6.077; p <0.007). Findings of moderate relevance were detected in 120 patients (22.2 %) in the TBCT group compared to 86 patients (15.9 %) in the STWU group (adjusted rate ratio 1.421; 95%CI 1.088-1.854; p <0.010). Compared to selective CT scanning, more patients with clinically relevant incidental findings can be expected by TBCT scanning. aEuro cent Total-body CT scanning in trauma results in 1.5 times more incidental findings. aEuro cent Evaluation by TBCT in trauma results in more patients with incidental findings. aEuro cent In every category of clinical relevance, TBCT detects more incidental findings
Interpolating self-energy of the infinite-dimensional Hubbard model: Modifying the iterative perturbation theory
We develop an analytical expression for the self-energy of the
infinite-dimensional Hubbard model that is correct in a number of different
limits. The approach represents a generalization of the iterative perturbation
theory to arbitrary fillings. In the weak-coupling regime perturbation theory
to second order in the interaction U is recovered. The theory is exact in the
atomic limit. The high-energy behavior of the self-energy up to order (1/E)**2
and thereby the first four moments of the spectral density are reproduced
correctly. Referring to a standard strong-coupling moment method, we analyze
the limit of strong U. Different modifications of the approach are discussed
and tested by comparing with the results of an exact diagonalization study.Comment: LaTeX, 14 pages, 5 ps figures included, title changed, references
updated, minor change
Superconductivity in the two dimensional Hubbard Model.
Quasiparticle bands of the two-dimensional Hubbard model are calculated using
the Roth two-pole approximation to the one particle Green's function. Excellent
agreement is obtained with recent Monte Carlo calculations, including an
anomalous volume of the Fermi surface near half-filling, which can possibly be
explained in terms of a breakdown of Fermi liquid theory. The calculated bands
are very flat around the (pi,0) points of the Brillouin zone in agreement with
photoemission measurements of cuprate superconductors. With doping there is a
shift in spectral weight from the upper band to the lower band. The Roth method
is extended to deal with superconductivity within a four-pole approximation
allowing electron-hole mixing. It is shown that triplet p-wave pairing never
occurs. Singlet d_{x^2-y^2}-wave pairing is strongly favoured and optimal
doping occurs when the van Hove singularity, corresponding to the flat band
part, lies at the Fermi level. Nearest neighbour antiferromagnetic correlations
play an important role in flattening the bands near the Fermi level and in
favouring superconductivity. However the mechanism for superconductivity is a
local one, in contrast to spin fluctuation exchange models. For reasonable
values of the hopping parameter the transition temperature T_c is in the range
10-100K. The optimum doping delta_c lies between 0.14 and 0.25, depending on
the ratio U/t. The gap equation has a BCS-like form and (2*Delta_{max})/(kT_c)
~ 4.Comment: REVTeX, 35 pages, including 19 PostScript figures numbered 1a to 11.
Uses epsf.sty (included). Everything in uuencoded gz-compressed .tar file,
(self-unpacking, see header). Submitted to Phys. Rev. B (24-2-95
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