650 research outputs found
The Hubbard model on a complete graph: Exact Analytical results
We derive the analytical expression of the ground state of the Hubbard model
with unconstrained hopping at half filling and for arbitrary lattice sites.Comment: Email:[email protected]
Frustration of antiferromagnetism in the t-t'-Hubbard model at weak coupling
The perfect-nesting instability towards antiferromagnetism of the Hubbard
model is suppressed by next-nearest neighbor hopping t'. The exact asymptotic
behavior of the critical coupling U_c(t') at small t' is calculated in
dimensions d=2,3 and infinity using Hartree theory, and the order of the
transition is determined. A region of stability of a metallic antiferromagnetic
phase in d=3 is identified.Comment: 5 pages revtex, 5 eps figures; typos corrected and a discussion about
the applicability to two-dimensional organic superconductors adde
Cost-effectiveness of exercise therapy in the treatment of non-specific neck pain and low back pain:a systematic review with meta-analysis
OBJECTIVE: To investigate the cost-effectiveness of exercise therapy in the treatment of patients with non-specific neck pain and low back pain. DESIGN: Systematic review of economic evaluations. DATA SOURCES: The search was performed in 5 clinical and 3 economic electronic databases. ELIGIBILITY CRITERIA FOR SELECTING STUDIES: We included economic evaluations performed alongside randomised controlled trials. Differences in costs and effects were pooled in a meta-analysis, if possible, and incremental cost-utility ratios (ICUR) were descriptively analysed. RESULTS: Twenty-two studies were included. On average, exercise therapy was associated with lower costs and larger effects for quality-adjusted life-year (QALY) in comparison with usual care for subacute and chronic low back pain from a healthcare perspective (based on ICUR). Exercise therapy had similar costs and effect for QALY in comparison with other interventions for neck pain from a societal perspective, and subacute and chronic low back pain from a healthcare perspective. There was limited or inconsistent evidence on the cost-effectiveness of exercise therapy compared with usual care for neck pain and acute low back pain, other interventions for acute low back pain and different types of exercise therapy for neck pain and low back pain. CONCLUSIONS: Exercise therapy seems to be cost-effective compared with usual care for subacute and chronic low back pain. Exercise therapy was not (more) cost-effective compared with other interventions for neck pain and low back pain. The cost-utility estimates are rather uncertain, indicating that more economic evaluations are needed. REGISTRATION: PROSPERO, CRD42017059025
Charge-order transition in the extended Hubbard model on a two-leg ladder
We investigate the charge-order transition at zero temperature in a two-leg
Hubbard ladder with additional nearest-neighbor Coulomb repulsion V using the
Density Matrix Renormalization Group technique. We consider electron densities
between quarter and half filling. For quarter filling and U=8t, we find
evidence for a continuous phase transition between a homogeneous state at small
V and a broken-symmetry state with "checkerboard" [wavevector Q=(pi,pi)] charge
order at large V. This transition to a checkerboard charge-ordered state
remains present at all larger fillings, but becomes discontinuous at
sufficiently large filling. We discuss the influence of U/t on the transition
and estimate the position of the tricritical points.Comment: 4 pages, 5 figs, minor changes, accepted for publication in PRB R
Dynamic optimization of open-loop input signals for ramp-up current profiles in tokamak plasmas
Establishing a good current spatial profile in tokamak fusion reactors is crucial to effective steady-state operation. The evolution of the current spatial profile is related to the evolution of the poloidal magnetic flux, which can be modeled in the normalized cylindrical coordinates using a parabolic partial differential equation (PDE) called the magnetic diffusion equation. In this paper, we consider the dynamic optimization problem of attaining the best possible current spatial profile during the ramp-up phase of the tokamak. We first use the Galerkin method to obtain a finite-dimensional ordinary differential equation (ODE) model based on the original magnetic diffusion PDE. Then, we combine the control parameterization method with a novel time-scaling transformation to obtain an approximate optimal parameter selection problem, which can be solved using gradient-based optimization techniques such as sequential quadratic programming (SQP). This control parameterization approach involves approximating the tokamak input signals by piecewise-linear functions whose slopes and break-points are decision variables to be optimized. We show that the gradient of the objective function with respect to the decision variables can be computed by solving an auxiliary dynamic system governing the state sensitivity matrix. Finally, we conclude the paper with simulation results for an example problem based on experimental data from the DIII-D tokamak in San Diego, California
miR-146a is a significant brake on autoimmunity, myeloproliferation, and cancer in mice
Excessive or inappropriate activation of the immune system can be deleterious to the organism, warranting multiple molecular mechanisms to control and properly terminate immune responses. MicroRNAs (miRNAs), ~22-nt-long noncoding RNAs, have recently emerged as key posttranscriptional regulators, controlling diverse biological processes, including responses to non-self. In this study, we examine the biological role of miR-146a using genetically engineered mice and show that targeted deletion of this gene, whose expression is strongly up-regulated after immune cell maturation and/or activation, results in several immune defects. Collectively, our findings suggest that miR-146a plays a key role as a molecular brake on inflammation, myeloid cell proliferation, and oncogenic transformation
Tricritical Behavior in the Extended Hubbard Chains
Phase diagrams of the one-dimensional extended Hubbard model (including
nearest-neighbor interaction ) at half- and quarter-filling are studied by
observing level crossings of excitation spectra using the exact
diagonalization. This method is based on the Tomonaga-Luttinger liquid theory
including logarithmic corrections which stem from the renormalization of the
Umklapp- and the backward-scattering effects.
Using this approach, the phase boundaries are determined with high accuracy,
and then the structure of the phase diagram is clarified. At half-filling, the
phase diagram consists of two
Berezinskii-Kosterlitz-Thouless (BKT) transition lines and one Gaussian
transition line in the charge sector, and one spin-gap transition line.
This structure reflects the U(1) SU(2) symmetry of the electron
system. Near the line, the Gaussian and the spin-gap transitions take
place independently from the weak- to the intermediate-coupling region, but
these two transition lines are coupled in the strong-coupling region. This
result demonstrates existence of a tricritical point and a
bond-charge-density-wave (BCDW) phase between charge- and spin-density-wave
(CDW, SDW) phases. To clarify this mechanism of the transition, we also
investigate effect of a correlated hopping term which plays a role to enlarge
BCDW and bond-spin-density-wave (BSDW) phases. At quarter-filling, a similar
crossover phenomenon also takes place in the large- region involving
spin-gap and BKT-type metal-insulator transitions.Comment: 18 pages(REVTeX), 17 figures(EPS(color)), 3 tables, Detailed paper of
JPSJ 68 (1999) 3123 (cond-mat/9903227), see also cond-mat/000341
Modular analysis of the probabilistic genetic interaction network
Motivation: Epistatic Miniarray Profiles (EMAP) has enabled the mapping of large-scale genetic interaction networks; however, the quantitative information gained from EMAP cannot be fully exploited since the data are usually interpreted as a discrete network based on an arbitrary hard threshold. To address such limitations, we adopted a mixture modeling procedure to construct a probabilistic genetic interaction network and then implemented a Bayesian approach to identify densely interacting modules in the probabilistic network
Further insights into the operation of the Chinese number system: Competing effects of Arabic and Mandarin number formats
Here we report the results of a speeded relative quantity task with Chinese participants. On each trial a single numeral (the probe) was presented and the instructions were to respond as to whether it signified a quantity less than or greater than five (the standard). In separate blocks of trials, the numerals were either presented in Mandarin or in Arabic number formats. In addition to the standard influence of numerical distance, a significant predictor of performance was the degree of physical similarity between the probe and the standard as depicted in Mandarin. Additionally, competing effects of physical similarity, defined in terms of the Arabic number format, were also found. Critically the size of these different effects of physical similarity varied systematically across individuals such that larger effects of one compensated for smaller effects of the other. It is argued that the data favor accounts of processing that assume that different number formats access different format-specific representations of quantities. Moreover, for Chinese participants the default is to translate numerals into a Mandarin format prior to accessing quantity information. The efficacy of this translation process is itself influenced by a competing tendency to carry out a translation into Arabic format
The Hubbard model within the equations of motion approach
The Hubbard model has a special role in Condensed Matter Theory as it is
considered as the simplest Hamiltonian model one can write in order to describe
anomalous physical properties of some class of real materials. Unfortunately,
this model is not exactly solved except for some limits and therefore one
should resort to analytical methods, like the Equations of Motion Approach, or
to numerical techniques in order to attain a description of its relevant
features in the whole range of physical parameters (interaction, filling and
temperature). In this manuscript, the Composite Operator Method, which exploits
the above mentioned analytical technique, is presented and systematically
applied in order to get information about the behavior of all relevant
properties of the model (local, thermodynamic, single- and two- particle ones)
in comparison with many other analytical techniques, the above cited known
limits and numerical simulations. Within this approach, the Hubbard model is
shown to be also capable to describe some anomalous behaviors of the cuprate
superconductors.Comment: 232 pages, more than 300 figures, more than 500 reference
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