6,450 research outputs found
A BCS-BEC crossover in the extended Falicov-Kimball model: Variational cluster approach
We study the spontaneous symmetry breaking of the excitonic insulator state
induced by the Coulomb interaction in the two-dimensional extended
Falicov-Kimball model. Using the variational cluster approximation (VCA) and
Hartree-Fock approximation (HFA), we evaluate the order parameter,
single-particle excitation gap, momentum distribution functions, coherence
length of excitons, and single-particle and anomalous excitation spectra, as a
function of at zero temperature. We find that in the weak-to-intermediate
coupling regime, the Fermi surface plays an essential role and calculated
results can be understood in close correspondence with the BCS theory, whereas
in the strong-coupling regime, the Fermi surface plays no role and results are
consistent with the picture of BEC. Moreover, we find that HFA works well both
in the weak- and strong-coupling regime, and that the difference between the
results of VCA and HFA mostly appears in the intermediate-coupling regime. The
reason for this is discussed from a viewpoint of the self-energy. We thereby
clarify the excitonic insulator state that typifies either a BCS condensate of
electron-hole pairs (weak-coupling regime) or a Bose-Einstein condensate of
preformed excitons (strong-coupling regime).Comment: 11 pages, 9 figure
Theory of the waterfall phenomenon in cuprate superconductors
Based on exact diagonalization and variational cluster approximation
calculations we study the relationship between charge transfer models and the
corresponding single band Hubbard models. We present an explanation for the
waterfall phenomenon observed in angle resolved photoemission spectroscopy
(ARPES) on cuprate superconductors. The phenomenon is due to the destructive
interference between the phases of the O2p orbitals belonging to a given
Zhang-Rice singlet and the Bloch phases of the photohole which occurs in
certain regions of k-space. It therefore may be viewed as a direct experimental
visualisation of the Zhang-Rice construction of an effective single band model
for the CuO2 plane.Comment: 11 pages, 9 Postscript figure
Mott transition and ferrimagnetism in the Hubbard model on the anisotropic kagom\'e lattice
Mott transition and ferrimagnetism are studied in the Hubbard model on the
anisotropic kagom\'e lattice using the variational cluster approximation and
the phase diagram at zero temperature and half-filling is analyzed. The
ferrimagnetic phase rapidly grows as the geometric frustration is relaxed, and
the Mott insulator phase disappears in moderately frustrated region, showing
that the ferrimagnetic fluctuations stemming from the relaxation of the
geometric frustration is enhanced by the electron correlations. In metallic
phase, heavy fermion behavior is observed and mass enhancement factor is
computed. Enhancement of effective spatial anisotropy by the electron
correlations is also confirmed in moderately frustrated region, and its effect
on heavy fermion behavior is examined.Comment: 5 pages, 6 figure
Self-energy and Fermi surface of the 2-dimensional Hubbard model
We present an exact diagonalization study of the self-energy of the
two-dimensional Hubbard model. To increase the range of available cluster sizes
we use a corrected t-J model to compute approximate Greens functions for the
Hubbard model. This allows to obtain spectra for clusters with 18 and 20 sites.
The self-energy has several `bands' of poles with strong dispersion and
extended incoherent continua with k-dependent intensity. We fit the self-energy
by a minimal model and use this to extrapolate the cluster results to the
infinite lattice. The resulting Fermi surface shows a transition from hole
pockets in the underdoped regime to a large Fermi surface in the overdoped
regime. We demonstrate that hole pockets can be completely consistent with the
Luttinger theorem. Introduction of next-nearest neighbor hopping changes the
self-energy stronlgy and the spectral function with nonvanishing
next-nearest-neighbor hopping in the underdoped region is in good agreement
with angle resolved photoelectron spectroscopy.Comment: 17 pages, 18 figure
A Potent CD1d-binding Glycolipid for iNKT-Cell-based Therapy Against Human Breast Cancer
Background/Aim: Invariant natural killer T-cells (iNKT) stimulated by CD1d-binding glycolipids have been shown to exert antitumor effects by a number of studies in a mouse model. Breast cancer is a devastating disease, with different types of breast cancer recurring locally or distant as metastatic/advanced disease following initial treatment. The aim of this study was to examine the tumoricidal effect of a CD1d-binding glycolipid, called 7DW8-5, against a highly invasive human breast cancer cell line both in vitro and in vivo. Materials and Methods: Parental MDA-MB-231 cells and MDA-MB-231 cells transduced with human CD1d were labeled with carboxyfluorescein diacetate succinimidyl ester (CFSE), followed by loading with glycolipids. After co-culturing with human iNKT cells, the cells were permeabilized and stained with Alexa Flour 647-conjugated antibody to active caspase-3, and analyzed using a BD LSR II. For the in vivo tumoricidal effect, MDA-MB-231 cells transduced with human CD1d and luciferase genes were injected into the mammary fat pad of female NOD/SCID/IL2rγnull (NSG) mice, followed by the injection of human iNKT cells with or without 7DW8-5, and the levels of luminescence were analyzed with whole-body imaging. Results: Human iNKT cells could kill CD1d-expressing human breast cancer cells in vitro in the presence of 7DW8-5, but not α-GalCer. As for in vivo, the adoptive transfer of human iNKT cells into tumor-challenged NSG mice significantly inhibited the growth of CD1d+ MDA-MB-231 human breast cancer cells in the presence of 7DW8-5. Conclusion: CD1d-binding, glycolipid-based iNKT-cell therapy is suggested as a potent and effective treatment against breast cancer in humans
Monoid automata for displacement context-free languages
In 2007 Kambites presented an algebraic interpretation of
Chomsky-Schutzenberger theorem for context-free languages. We give an
interpretation of the corresponding theorem for the class of displacement
context-free languages which are equivalent to well-nested multiple
context-free languages. We also obtain a characterization of k-displacement
context-free languages in terms of monoid automata and show how such automata
can be simulated on two stacks. We introduce the simultaneous two-stack
automata and compare different variants of its definition. All the definitions
considered are shown to be equivalent basing on the geometric interpretation of
memory operations of these automata.Comment: Revised version for ESSLLI Student Session 2013 selected paper
Nuclear Matter on a Lattice
We investigate nuclear matter on a cubic lattice. An exact thermal formalism
is applied to nucleons with a Hamiltonian that accommodates on-site and
next-neighbor parts of the central, spin- and isospin-exchange interactions. We
describe the nuclear matter Monte Carlo methods which contain elements from
shell model Monte Carlo methods and from numerical simulations of the Hubbard
model. We show that energy and basic saturation properties of nuclear matter
can be reproduced. Evidence of a first-order phase transition from an
uncorrelated Fermi gas to a clustered system is observed by computing
mechanical and thermodynamical quantities such as compressibility, heat
capacity, entropy and grand potential. We compare symmetry energy and first
sound velocities with literature and find reasonable agreement.Comment: 23 pages, 8 figures (some in color), to be submitted to Phys. Rev.
- …