Electronic correlations in the Hubbard model on a bi-partite lattice

In this work we study the Hubbard model on a bi-partite lattice using the coupled-cluster method (CCM). We first investigate what, within this approach, allows us to reproduce the zero order parameter in the 1D model, as predicted by the exact solution. We show that we need a critical correlation in some of the coupled-cluster model coefficients to reproduce this result, as can be obtained by applying some very accurate results using the CCM for the Heisenberg model. Using the same approach we then tackle the 2D Hubbard model on a square and a honeycomb lattice, both of which can be thought to represent 2D materials. We also analyse the charge and spin excitations, with reasonable results.Comment: Major update to previous versio

Cosmological Perturbations in Ho\v{r}ava-Lifshitz Gravity

We study cosmological perturbations in Ho\v{r}ava-Lifshitz Gravity. We consider scalar metric fluctuations about a homogeneous and isotropic space-time. Starting from the most general metric, we work out the complete second order action for the perturbations. We then make use of the residual gauge invariance and of the constraint equations to reduce the number of dynamical degrees of freedom. After introducing the Sasaki-Mukhanov variable, the combination of spatial metric fluctuation and matter inhomogeneity for which the action in General Relativity has canonical form, we find that this variable has the standard time derivative term in the second order action, and that the extra degree of freedom is non-dynamical. The limit $\lambda \to 1$ is well-behaved, unlike what is obtained when performing incomplete analyses of cosmological fluctuations. Thus, there is no strong coupling problem for Ho\v{r}ava-Lifshitz gravity when considering cosmological solutions. We also compute the spectrum of cosmological perturbations. If the potential in the action is taken to be of "detailed balance" form, we find a cancelation of the highest derivative terms in the action for the curvature fluctuations. As a consequence, the initial spectrum of perturbations will not be scale-invariant in a general spacetime background. As an application, we consider fluctuations in an inflationary background and draw connections with the "trans-Planckian problem" for cosmological perturbations. In the special case in which the potential term in the action is of detailed balance form and in which $\lambda = 1$, the equation of motion for cosmological perturbations in the far UV takes the same form as in GR. However, in general the equation of motion is characterized by a modified dispersion relation.Comment: 17 pages, 2 eps figure

Two-dimensional Poisson Trees converge to the Brownian web

The Brownian web can be roughly described as a family of coalescing one-dimensional Brownian motions starting at all times in $\R$ and at all points of $\R$. It was introduced by Arratia; a variant was then studied by Toth and Werner; another variant was analyzed recently by Fontes, Isopi, Newman and Ravishankar. The two-dimensional \emph{Poisson tree} is a family of continuous time one-dimensional random walks with uniform jumps in a bounded interval. The walks start at the space-time points of a homogeneous Poisson process in $\R^2$ and are in fact constructed as a function of the point process. This tree was introduced by Ferrari, Landim and Thorisson. By verifying criteria derived by Fontes, Isopi, Newman and Ravishankar, we show that, when properly rescaled, and under the topology introduced by those authors, Poisson trees converge weakly to the Brownian web.Comment: 22 pages, 1 figure. This version corrects an error in the previous proof. The results are the sam

Flash flood simulations for an Egyptian city - mitigation measures and impact of infiltration

Within this work, the impact of mitigation measures and infiltration on flash floods is investigated by using a 2D robust shallow water model including infiltration with the Green-Ampt model. The results show the combined effects of infiltration and mitigation measures as well as the effectiveness of bypass channels in addition to retention basins. Retention basins at appropriate locations could reduce the maximum water depth at critical locations by 23%, while the additional implementation of drainage channels lead to a reduction of 75%, considering also infiltration lead to a further reduction of 97%. If infiltration was considered without mitigation measures, the peak water depth was reduced by 67%. For an exceptional extreme event the measures lead to a reduction of 73% at some locations, while at other locations the overflow from retention basins due to overstraining generated even higher inundations with an increase of 58%

The signalling hypothesis revisited: Evidence from foreign IPOs

While the signalling hypothesis has played a prominent role as the economic rationale associated with the initial public offering (IPO) underpricing puzzle (Welch, 1989), the empirical evidence on it has been mixed at best (Jegadeesh, Weinstein and Welch, 1993; Michaely and Shaw, 1994). This paper revisits the issue from the vantage point of close to two decades of additional experience by examining a sample of foreign IPOs – firms from both financially integrated and segmented markets – in US markets. The evidence indicates that signalling does matter in determining IPO underpricing, especially for firms domiciled in countries with segmented markets, which as a result face higher information asymmetry and lack access to external capital markets. We find a significant positive and robust relationship between the degree of IPO underpricing and segmented-market firms’ seasoned equity offering activities. For firms from integrated markets, in contrast, the analyst-coverage-purchase hypothesis appears to matter more in explaining IPO underpricing and the aftermarket price appreciation explains these firms’ seasoned equity offering activities. The evidence, therefore, clearly supports the notion that some firms are willing to leave money on the table voluntarily to get a more favorable price at seasoned offerings when they are substantially wealth constrained, a prediction embedded in the signalling hypothesis.IPO underpricing; seasoned equity offering; cross-listing; signalling hypothesis; financial market integration; market-feedback hypothesis

Ballistic Thermal Rectification in Asymmetric Three-Terminal Mesoscopic Dielectric Systems

By coupling the asymmetric three-terminal mesoscopic dielectric system with a temperature probe, at low temperature, the ballistic heat flux flow through the other two asymmetric terminals in the nonlinear response regime is studied based on the Landauer formulation of transport theory. The thermal rectification is attained at the quantum regime. It is a purely quantum effect and is determined by the dependence of the ratio $\tau_{RC}(\omega)/\tau_{RL}(\omega)$ on $\omega$, the phonon's frequency. Where $\tau_{RC}(\omega)$ and $\tau_{RL}(\omega)$ are respectively the transmission coefficients from two asymmetric terminals to the temperature probe, which are determined by the inelastic scattering of ballistic phonons in the temperature probe. Our results are confirmed by extensive numerical simulations.Comment: 10 pages, 4 figure

Latest Observational Constraints to the Ghost Dark Energy Model by Using Markov Chain Monte Carlo Approach

Recently, the vacuum energy of the QCD ghost in a time-dependent background is proposed as a kind of dark energy candidate to explain the acceleration of the universe. In this model, the energy density of the dark energy is proportional to the Hubble parameter $H$, which is the Hawking temperature on the Hubble horizon of the Friedmann-Robertson-Walker (FRW) universe. In this paper, we perform a constraint on the ghost dark energy model with and without bulk viscosity, by using the Markov Chain Monte Carlo (MCMC) method and the combined latest observational data from the type Ia supernova compilations including Union2.1(580) and Union2(557), cosmic microwave background, baryon acoustic oscillation, and the observational Hubble parameter data.Comment: 12 pages, 4 figure

Iron(III) bromide catalyzed bromination of 2-tert-butylpyrene and corresponding position-dependent aryl-functionalized pyrene derivatives

The present work probes the bromination mechanism of 2-tert-butylpyrene (1), which regioselectively affords mono-, di-, tri- and tetra-bromopyrenes, by theoretical calculation and detailed experimental methods. The bromine atom may be directed to the K-region (positions 5- and 9-) instead of the more reactive 6- and 8-positions in the presence of iron powder. In this process, FeBr₃ plays a significant role to release steric hindrance or lower the activation energy of the rearrangement. The intermediate bromopyrene derivatives were isolated and confirmed by ¹H NMR spectrometry, mass spectroscopy and elemental analysis. Further evidence on substitution position originated from a series of aryl substituted pyrene derivatives, which were obtained from the corresponding bromopyrenes on reaction with 4-methoxy-phenylboronic acid by a Suzuki–Miyaura cross-coupling reaction. All position-dependent aryl-functionalized pyrene derivatives are characterized by single X-ray diffraction, ¹H/¹³C NMR, FT-IR and MS, and offered straightforward evidence to support our conclusion. Furthermore, the photophysical properties of a series of compounds were confirmed by fluorescence and absorption, as well as by fluorescence lifetime measurements

Thermodynamic of the Ghost Dark Energy Universe

Recently, the vacuum energy of the QCD ghost in a time-dependent background is proposed as a kind of dark energy candidate to explain the acceleration of the Universe. In this model, the energy density of the dark energy is proportional to the Hubble parameter $H$, which is the Hawking temperature on the Hubble horizon of the Friedmann-Robertson-Walker (FRW) Universe. In this paper, we generalized this model and choice the Hawking temperature on the so-called trapping horizon, which will coincides with the Hubble temperature in the context of flat FRW Universe dominated by the dark energy component. We study the thermodynamics of Universe with this kind of dark energy and find that the entropy-area relation is modified, namely, there is an another new term besides the area term.Comment: 8 pages, no figure