Sum rules for four-spinon dynamic structure factor in XXX model

In the context of the antiferromagnetic spin 1/2 Heisenberg quantum spin chain (XXX model), we estimate the contribution of the exact four-spinon dynamic structure factor $S_4$ by calculating a number of sum rules the total dynamic structure factor $S$ is known to satisfy exactly. These sum rules are: the static susceptibility, the integrated intensity, the total integrated intensity, the first frequency moment and the nearest-neighbor correlation function. We find that the contribution of $S_4$ is between 1% and 2.5%, depending on the sum rule, whereas the contribution of the exact two-spinon dynamic structure factor $S_2$ is between 70% and 75%. This is consistent with the expected scattering weight of states from outside the spin-wave continuum. The calculations are numerical and Monte Carlo based. Good statistics are obtained.Comment: 21 pages, Revtex, 02 figure

Strong Correlations and Magnetic Frustration in the High Tc Iron Pnictides

We consider the iron pnictides in terms of a proximity to a Mott insulator. The superexchange interactions contain competing nearest-neighbor and next-nearest-neighbor components. In the undoped parent compound, these frustrated interactions lead to a two-sublattice collinear antiferromagnet (each sublattice forming a Neel ordering), with a reduced magnitude for the ordered moment. Electron or hole doping, together with the frustration effect, suppresses the magnetic ordering and allows a superconducting state. The exchange interactions favor a d-wave superconducting order parameter; in the notation appropriate for the Fe square lattice, its orbital symmetry is $d_{xy}$. A number of existing and future experiments are discussed in light of the theoretical considerations.Comment: (v2) 4+ pages, 4 figures, discussions on several points expanded; references added. To appear in Phys. Rev. Let

A Critical Review of Recent Progress on Negative Capacitance Field-Effect Transistors

The elegant simplicity of the device concept and the urgent need for a new "transistor" at the twilight of Moore's law have inspired many researchers in industry and academia to explore the physics and technology of negative capacitance field effect transistor (NC-FET). Although hundreds of papers have been published, the validity of quasi-static NC and the frequency-reliability limits of NC-FET are still being debated. The concept of NC - if conclusively demonstrated - will have broad impacts on device physics and technology development. Here, the authors provide a critical review of recent progress on NC-FETs research and some starting points for a coherent discussion.Comment: 19 pages, 2 figure

Investigation of Top quark spin correlations at hadron collider

We report on our results about hadronic $t\bar t$ production at NLO QCD including $t, \bar t$ spin effects, especially on $t\bar t$ spin correlations.Comment: talk given at the 32nd International Conference on High Energy Physics (ICHEP'04), Beijing, China, 16-22 Aug. 200

Quantum criticality of the sub-ohmic spin-boson model

We revisit the critical behavior of the sub-ohmic spin-boson model. Analysis of both the leading and subleading terms in the temperature dependence of the inverse static local spin susceptibility at the quantum critical point, calculated using a numerical renormalization-group method, provides evidence that the quantum critical point is interacting in cases where the quantum-to-classical mapping would predict mean-field behavior. The subleading term is shown to be consistent with an w/T scaling of the local dynamical susceptibility, as is the leading term. The frequency and temperature dependences of the local spin susceptibility in the strong-coupling (delocalized) regime are also presented. We attribute the violation of the quantum-to-classical mapping to a Berry-phase term in a continuum path-integral representation of the model. This effect connects the behavior discussed here with its counterparts in models with continuous spin symmetry.Comment: 9 pages, 10 figure

On the convergence of the maximum likelihood estimator for the transition rate under a 2-state symmetric model

Maximum likelihood estimators are used extensively to estimate unknown parameters of stochastic trait evolution models on phylogenetic trees. Although the MLE has been proven to converge to the true value in the independent-sample case, we cannot appeal to this result because trait values of different species are correlated due to shared evolutionary history. In this paper, we consider a $2$-state symmetric model for a single binary trait and investigate the theoretical properties of the MLE for the transition rate in the large-tree limit. Here, the large-tree limit is a theoretical scenario where the number of taxa increases to infinity and we can observe the trait values for all species. Specifically, we prove that the MLE converges to the true value under some regularity conditions. These conditions ensure that the tree shape is not too irregular, and holds for many practical scenarios such as trees with bounded edges, trees generated from the Yule (pure birth) process, and trees generated from the coalescent point process. Our result also provides an upper bound for the distance between the MLE and the true value

Top quark spin correlations at hadron colliders: Predictions at next-to-leading order QCD

The collider experiments at the Tevatron and the LHC will allow for detailed investigations of the properties of the top quark. This requires precise predictions of the hadronic production of $t\bar t$ pairs and of their subsequent decays. In this Letter we present for the reactions $p {\bar p}, p p \to t{\bar t} + X \to \ell^+\ell'^-X$ the first calculation of the dilepton angular distribution at next-to-leading order (NLO) in the QCD coupling, keeping the full dependence on the spins of the intermediate $t\bar{t}$ state. The angular distribution reflects the degree of correlation of the $t$ and $\bar t$ spins which we determine for different choices of $t$ and $\bar t$ spin bases. In the case of the Tevatron, the QCD corrections are sizeable, and the distribution is quite sensitive to the parton content of the proton.Comment: 9 pages, 2 figure

Consistency and convergence rate of phylogenetic inference via regularization

It is common in phylogenetics to have some, perhaps partial, information about the overall evolutionary tree of a group of organisms and wish to find an evolutionary tree of a specific gene for those organisms. There may not be enough information in the gene sequences alone to accurately reconstruct the correct "gene tree." Although the gene tree may deviate from the "species tree" due to a variety of genetic processes, in the absence of evidence to the contrary it is parsimonious to assume that they agree. A common statistical approach in these situations is to develop a likelihood penalty to incorporate such additional information. Recent studies using simulation and empirical data suggest that a likelihood penalty quantifying concordance with a species tree can significantly improve the accuracy of gene tree reconstruction compared to using sequence data alone. However, the consistency of such an approach has not yet been established, nor have convergence rates been bounded. Because phylogenetics is a non-standard inference problem, the standard theory does not apply. In this paper, we propose a penalized maximum likelihood estimator for gene tree reconstruction, where the penalty is the square of the Billera-Holmes-Vogtmann geodesic distance from the gene tree to the species tree. We prove that this method is consistent, and derive its convergence rate for estimating the discrete gene tree structure and continuous edge lengths (representing the amount of evolution that has occurred on that branch) simultaneously. We find that the regularized estimator is "adaptive fast converging," meaning that it can reconstruct all edges of length greater than any given threshold from gene sequences of polynomial length. Our method does not require the species tree to be known exactly; in fact, our asymptotic theory holds for any such guide tree.Comment: 34 pages, 5 figures. To appear on The Annals of Statistic

Top Quark Pair Production and Decay including Spin Effects at Hadron Colliders: Predictions at NLO QCD

Top quark-antiquark ($t\bar t$) pairs will be produced copiously at the Tevatron collider and in huge numbers at the LHC. This will make possible detailed investigations of the properties and interactions of this quark flavor. The analysis and interpretation of future data requires precise predictions of the hadronic production of $t\bar t$ pairs and of their subsequent decays. In this talk the reactions $p {\bar p}, p p \to t{\bar t} + X \to l^+ l'^- + X$ are considered and results are presented of our calculation of the dilepton angular distribution at next-to-leading order QCD, keeping the full dependence on the spins of the intermediate $t\bar{t}$ state. The angular distribution is determined for different choices of reference axes that can be identified with the $t$ and $\bar t$ spin axes. While the QCD corrections to the leading-order distribution turn out to be small in the case of the LHC, we find them to be sizeable in the case of the Tevatron and find, moreover, the angular distribution to be sensitive to the parton content of the proton.Comment: Talk given at 3rd Circum-Pan-Pacific Symposium on "High Energy Spin Physics", Beijing, China, 8-13, 200

Semiclassical Analysis of Extended Dynamical Mean Field Equations

The extended Dynamical Mean Field Equations (EDMFT) are analyzed using semiclassical methods for a model describing an interacting fermi-bose system. We compare the semiclassical approach with the exact QMC (Quantum Montecarlo) method. We found the transition to an ordered state to be of the first order for any dimension below four.Comment: RevTex, 39 pages, 16 figures; Appendix C added, typos correcte