57 research outputs found
Exact bounds on the ground-state energy of the infinite-U Hubbard model
We give upper and lower bounds for the ground-state energy of the infinite-U
Hubbard model. In two dimensions, using these bounds we are able to rule out
the possibility of phase separation between the undoped-insulating state and an
hole-rich state.Comment: 2 pages, 1 figure, to appear in Phys. Rev.
Estimation of the hydraulic parameters of unsaturated samples by electrical resistivity tomography
In situ and laboratory experiments have shown that electrical resistivity tomography (ERT) is an effective tool to image transient phenomena in soils. However, its application in quantifying soil hydraulic parameters has been limited. In this study, experiments of water inflow in unsaturated soil samples were conducted in an oedometer equipped to perform three-dimensional electrical measurements. Reconstructions of the electrical conductivity at different times confirmed the usefulness of ERT for monitoring the evolution of water content. The tomographic reconstructions were subsequently used in conjunction with a finite-element simulation to infer the water retention curve and the unsaturated hydraulic conductivity. The parameters estimated with ERT agree satisfactorily with those determined using established techniques, hence the proposed approach shows good potential for relatively fast characterisations. Similar experiments could be carried out on site to study the hydraulic behaviour of the entire soil deposi
Spatially homogeneous ground state of the two-dimensional Hubbard model
We investigate the stability with respect to phase separation or charge
density-wave formation of the two-dimensional Hubbard model for various values
of the local Coulomb repulsion and electron densities using Green-function
Monte Carlo techniques. The well known sign problem is particularly serious in
the relevant region of small hole doping. We show that the difference in
accuracy for different doping makes it very difficult to probe the phase
separation instability using only energy calculations, even in the
weak-coupling limit () where reliable results are available. By contrast,
the knowledge of the charge correlation functions allows us to provide clear
evidence of a spatially homogeneous ground state up to .Comment: 7 pages and 5 figures. Phys. Rev. B, to appear 200
Two Superconducting Phases in the d=3 Hubbard Model: Phase Diagram and Specific Heat from Renormalization-Group Calculations
The phase diagram of the d=3 Hubbard model is calculated as a function of
temperature and electron density n_i, in the full range of densities between 0
and 2 electrons per site, using renormalization-group theory. An
antiferromagnetic phase occurs at lower temperatures, at and near the
half-filling density of = 1. The antiferromagnetic phase is unstable to
hole or electron doping of at most 15%, yielding to two distinct "tau" phases:
for large coupling U/t, one such phase occurs between 30-35% hole or electron
doping, and for small to intermediate coupling U/t another such phase occurs
between 10-18% doping. Both tau phases are distinguished by non-zero hole or
electron hopping expectation values at all length scales. Under further doping,
the tau phases yield to hole- or electron-rich disordered phases. We have
calculated the specific heat over the entire phase diagram. The low-temperature
specific heat of the weak-coupling tau phase shows a BCS-type exponential
decay, indicating a gap in the excitation spectrum, and a cusp singularity at
the phase boundary. The strong-coupling tau phase, on the other hand, has
characteristics of BEC-type superconductivity, including a critical exponent
alpha approximately equal to -1, and an additional peak in the specific heat
above the transition temperature indicating pair formation. In the limit of
large Coulomb repulsion, the phase diagram of the tJ model is recovered.Comment: 16 pages, 10 figures; typos in Fig. 2 correcte
Screening, Coulomb pseudopotential, and superconductivity in alkali-doped Fullerenes
We study the static screening in a Hubbard-like model using quantum Monte
Carlo. We find that the random phase approximation is surprisingly accurate
almost up to the Mott transition. We argue that in alkali-doped Fullerenes the
Coulomb pseudopotential is not very much reduced by retardation
effects. Therefore efficient screening is important in reducing
sufficiently to allow for an electron-phonon driven superconductivity. In this
way the Fullerides differ from the conventional picture, where retardation
effects play a major role in reducing the electron-electron repulsion.Comment: 4 pages RevTeX with 2 eps figures, additional material available at
http://www.mpi-stuttgart.mpg.de/docs/ANDERSEN/fullerene
Phase separation and enhanced charge-spin coupling near magnetic transitions
The generic changes of the electronic compressibility in systems which show
magnetic instabilities is studied. It is shown that, when going into the
ordered phase, the compressibility is reduced by an amount comparable to the
its original value, making charge instabilities also possible. We discuss,
within this framework, the tendency towards phase separation of the double
exchange systems, the pyrochlores, and other magnetic materials
Identical HLA antigens in two sisters with MEN IIA syndrome. J Endocrinol Invest. 1994 Mar;17(3):205-6.
Increased CD5+CD19+ B lymphocytes at the onset of type 1 diabetes in children.
The aim of this study was to determine whether the proportion of circulating B cells expressing the differentiative antigen CD5 was increased in children affected by type 1 diabetes, and whether the number of these cells was correlated with the presence of anti-islet cell autoantibodies. Sixteen children affected by insulin-dependent diabetes mellitus (type 1) were investigated for the presence of B lymphocytes bearing the CD5 surface molecule, T-cell-specific activation markers, organ- and nonorgan-specific autoantibodies. The number of CD5+CD19+ cells was higher in type 1 children with a very recent onset of the disease, as compared with patients on insulin therapy for more than 30 days and controls (P < 0.05). No correlation was found between the number of CD5+CD19+ cells and the presence of either organ- or nonorgan-specific autoantibodies. Our results indicate that CD5+CD19+ cells are involved in the pathogenesis of type 1 diabetes in children. A potential immunoregulatory role of this B cell population is discussed
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