237 research outputs found
High-order correlation effects in the two-dimensional Hubbard model
The electronic states of the two-dimensional Hubbard model are investigated
by means of a 4-pole approximation within the Composite Operator Method. In
addition to the conventional Hubbard operators, we consider other two operators
which come from the hierarchy of the equations of motion and carry information
regarding nearest-neighbor spin and charge configurations. By means of this
operatorial basis, we can study the physics related to the energy scale of
J=4t^2/U in addition to the one of U. Present results show relevant physical
features, well beyond those previously obtained by means of a 2-pole
approximation, such as a four-band structure with shadow bands and a
quasi-particle peak at the Fermi level. The Fermi level stays pinned to the
band flatness located at (pi,0)-point within a wide range of hole-doping (0 <=
delta <= 0.15). A comprehensive analysis of double occupancy, internal energy,
specific heat and entropy features have been also performed. All reported
results are in excellent agreement with the data of numerical simulations.Comment: 13 pages, 8 figure
On the lower bound on the exchange-correlation energy in two dimensions
We study the properties of the lower bound on the exchange-correlation energy
in two dimensions. First we review the derivation of the bound and show how it
can be written in a simple density-functional form. This form allows an
explicit determination of the prefactor of the bound and testing its tightness.
Next we focus on finite two-dimensional systems and examine how their distance
from the bound depends on the system geometry. The results for the high-density
limit suggest that a finite system that comes as close as possible to the
ultimate bound on the exchange-correlation energy has circular geometry and a
weak confining potential with a negative curvature
Effects of two-site composite excitations in the Hubbard model
The electronic states of the Hubbard model are investigated by use of the
Composite Operator Method. In addition to the Hubbard operators, two other
operators related with two-site composite excitations are included in the
basis. Within the present formulation, higher-order composite excitations are
reduced to the chosen operatorial basis by means of a procedure preserving the
particle-hole symmetry. The positive comparison with numerical simulations for
the double occupancy indicates that such approximation improves over the
two-pole approximation.Comment: 2 pages, 1 figur
Induction of tumours by administration of N-dibutylnitrosamine and derivatives to infant mice.
Pulse doses of N-dibutylnitrosamine(DBN), N-butyl-N-(4-hydroxybutyl)nitrosamine(BBN) and N-butyl-N-(3carboxypropyl)nitrosamine(BCPN) suspended in 1% gelatin, were administered s.c. to infant CDF1 mice, and the experiment terminated at one year of age. Tumours were induced in lungs and liver. The incidences of lung adenomas were 73-95% in all treated mice, with no sex differences. Hepatocellular adenomas and a carcinoma were found with an incidence of 81% (21/26) in DBN, 59% (13/22) in BBN, and 32% (9/28) in BCPN-treated males and the incidence was 23% (5/22) in DBN-treated females. Only one papilloma of the fore-stomach was induced in mice treated with DBN. These results indicated that the s.c. administration of DBN, BBN, and BCPN induced tumours of the lung and liver, but no tumours of the urinary bladder, under these experimental conditions. The carcinogenic effect on mice at the treated dose level was DBN greater than BBN greater than BCPN
Effect of nonmagnetic impurities on stripes in high-Tc cuprates
We perform the numerically exact diagonalization study of the t-J model with
nonmagnetic impurities to clarify the relation between Zn impurities and the
stripes. By examining the hole-hole correlation function for a two-hole
\sqrt{18}x\sqrt{18} cluster with a single impurity, we find that the impurity
has a tendency to stabilize vertical charge stripes. This tendency is caused by
the gain of the kinetic energy of holes moving along the stripes that are
formed avoiding the impurity.Comment: 3 pages including 2 figures. Proceedings for ISS2000 (Tokyo, October
2000). To be published in Physica
Tightened Lieb-Oxford bound for systems of fixed particle number
The Lieb-Oxford bound is a constraint upon approximate exchange-correlation
functionals. We explore a non-empirical tightening of that bound in both
universal and electron-number-dependent form. The test functional is PBE.
Regarding both atomization energies (slightly worsened) and bond lengths
(slightly bettered), we find the PBE functional to be remarkably insensitive to
the value of the Lieb-Oxford bound. This both rationalizes the use of the
original Lieb-Oxford constant in PBE and suggests that enhancement factors more
sensitive to sharpened constraints await discovery.Comment: six figures (color
Two-photon interference and coherent control of single InAs quantum dot emissions in an Ag-embedded structure
We have recently reported the successful fabrication of bright single-photon
sources based on Ag-embedded nanocone structures that incorporate InAs quantum
dots. The source had a photon collection efficiency as high as 24.6%. Here we
show the results of various types of photonic characterizations of the
Ag-embedded nanocone structures that confirm their versatility as regards a
broad range of quantum optical applications. We measure the first-order
autocorrelation function to evaluate the coherence time of emitted photons, and
the second-order correlation function, which reveals the strong suppression of
multiple photon generation. The high indistinguishability of emitted photons is
shown by the Hong-Ou-Mandel-type two-photon interference. With quasi-resonant
excitation, coherent population flopping is demonstrated through Rabi
oscillations. Extremely high single-photon purity with a (0) value of
0.008 is achieved with -pulse quasi-resonant excitation.Comment: 15 pages, 6 figure
Systematic investigation of a family of gradient-dependent functionals for solids
Eleven density functionals are compared with regard to their performance for
the lattice constants of solids. We consider standard functionals, such as the
local-density approximation and the Perdew-Burke-Ernzerhof (PBE)
generalized-gradient approximation (GGA), as well as variations of PBE GGA,
such as PBEsol and similar functionals, PBE-type functionals employing a
tighter Lieb-Oxford bound, and combinations thereof. Several of these
variations are proposed here for the first time. On a test set of 60 solids we
perform a system-by-system analysis for selected functionals and a full
statistical analysis for all of them. The impact of restoring the gradient
expansion and of tightening the Lieb-Oxford bound is discussed, and confronted
with previous results obtained from other codes, functionals or test sets. No
functional is uniformly good for all investigated systems, but surprisingly,
and pleasingly, the simplest possible modifications to PBE turn out to have the
most beneficial effect on its performance. The atomization energy of molecules
was also considered and on a testing set of six molecules, we found that the
PBE functional is clearly the best, the others leading to strong overbinding
Stage-dependent differential gene expression profiles of cranial neural crest-like cells derived from mouse-induced pluripotent stem cells.
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