Probing the QCD Critical Point with Higher Moments of Net-proton Multiplicity Distributions

Higher moments of event-by-event net-proton multiplicity distributions are applied to search for the QCD critical point in the heavy ion collisions. It has been demonstrated that higher moments as well as moment products are sensitive to the correlation length and directly connected to the thermodynamic susceptibilities computed in the Lattice QCD and Hadron Resonance Gas (HRG) model. In this paper, we will present measurements for kurtosis ($\kappa$), skewness ($S$) and variance ($\sigma^{2}$) of net-proton multiplicity distributions at the mid-rapidity ($|y|<0.5$) and $0.4<p_{T}<0.8$ GeV/$c$ for Au+Au collisions at $\sqrt{s_{NN}}$=19.6, 39, 62.4, 130 and 200 GeV, Cu+Cu collisions at $\sqrt{s_{NN}}$=22.4, 62.4 and 200 GeV, d+Au collisions at $\sqrt{s_{NN}}$=200 GeV and p+p collisions at $\sqrt{s_{NN}}$=62.4 and 200 GeV. The moment products $\kappa \sigma^{2}$ and $S \sigma$ of net-proton distributions, which are related to volume independent baryon number susceptibility ratio, are compared to the Lattice QCD and HRG model calculations. The $\kappa \sigma^{2}$ and $S \sigma$ of net-proton distributions are consistent with Lattice QCD and HRG model calculations at high energy, which support the thermalization of the colliding system. Deviations of $\kappa \sigma^{2}$ and $S \sigma$ for the Au+Au collisions at low energies from HRG model calculations are also observed.Comment: 10 pages, 8 figures, Proceedings of 27th Winter Workshon on Nuclear Dynamics. Feb. 6-13 (2011

Enhanced electron correlations in the new binary stannide PdSn4: a homologue of the Dirac nodal arc semimetal PtSn4

The advent of nodal-line semi-metals, i.e. systems in which the conduction and valence bands cross each other along a closed trajectory (line or loop) inside the Brillouin zone, has opened up a new arena for the exploration of topological condensed matter in which, due to a vanishing density of states near the Fermi level, electron correlation effects may also play an important role. In spite of this conceptual richness however, material realization of nodal-line (loop) fermions is rare, with PbTaSe2, ZrSiS and PtSn4 the only promising known candidates. Here we report the synthesis and physical properties of a new compound PdSn4 that is isostructural with PtSn4 yet possesses quasiparticles with significantly enhanced effective masses. In addition, PdSn4 displays an unusual polar angular magnetoresistance which at a certain field orientation, varies linearly with field up to 55 Tesla. Our study suggests that, in association with its homologue PtSn4 whose low-lying excitations were recently claimed to possess Dirac node arcs, PdSn4 may be a promising candidate in the search for novel topological states with enhanced correlation effects.Comment: 6 figures, 1 tabl

van der Waals Stacking-Induced Topological Phase Transition in Layered Ternary Transition Metal Chalcogenides

Novel materials with nontrivial electronic and photonic band topology are crucial for realizing novel devices with low power consumption and heat dissipation and quantum computing free of decoherence. Here, we theoretically predict a novel class of ternary transition metal chalcogenides that exhibit dual topological characteristics, quantum spin Hall insulators (QSHIs) in their two-dimensional (2D) monolayers and topological Weyl semimetals in their 3D noncentrosymmetric crystals upon van der Waals (vdW) stacking. Remarkably, we find that one can create and annihilate Weyl fermions and realize the transition between Type-I and Type-II Weyl fermions by tuning vdW interlayer spacing, providing the missing physical picture of the evolution from 2D QSHIs to 3D Weyl semimetals. Our results also show that these materials possess excellent thermodynamic stability and weak interlayer binding; some of them were synthesized two decades ago, implying their great potentials for experimental synthesis, characterization, and vdW heterostacking. Moreover, their ternary nature will offer more tunability for electronic structure by controlling different stoichiometry and valence charges. Our findings provide an ideal materials platform for realizing QSH effect and exploring fundamental topological phase transition and will open up a variety of new opportunities for two-dimensional materials and topological materials research.National Science Foundation (U.S.). Materials Research Science and Engineering Centers (Program) (Award DMR-1419807)United States. Department of Energy. Division of Materials Sciences and Engineering (Award DE-SC0010526

Predictive coupled-cluster isomer orderings for some Sin{}_nCm{}_m (m,n≤12m, n\le 12) clusters; A pragmatic comparison between DFT and complete basis limit coupled-cluster benchmarks

The accurate determination of the preferred ${\rm Si}_{12}{\rm C}_{12}$ isomer is important to guide experimental efforts directed towards synthesizing SiC nano-wires and related polymer structures which are anticipated to be highly efficient exciton materials for opto-electronic devices. In order to definitively identify preferred isomeric structures for silicon carbon nano-clusters, highly accurate geometries, energies and harmonic zero point energies have been computed using coupled-cluster theory with systematic extrapolation to the complete basis limit for set of silicon carbon clusters ranging in size from SiC$_3$ to ${\rm Si}_{12}{\rm C}_{12}$. It is found that post-MBPT(2) correlation energy plays a significant role in obtaining converged relative isomer energies, suggesting that predictions using low rung density functional methods will not have adequate accuracy. Utilizing the best composite coupled-cluster energy that is still computationally feasible, entailing a 3-4 SCF and CCSD extrapolation with triple-$\zeta$ (T) correlation, the {\it closo} ${\rm Si}_{12}{\rm C}_{12}$ isomer is identified to be the preferred isomer in support of previous calculations [J. Chem. Phys. 2015, 142, 034303]. Additionally we have investigated more pragmatic approaches to obtaining accurate silicon carbide isomer energies, including the use of frozen natural orbital coupled-cluster theory and several rungs of standard and double-hybrid density functional theory. Frozen natural orbitals as a way to compute post MBPT(2) correlation energy is found to be an excellent balance between efficiency and accuracy

Search for the QCD Critical Point: Higher Moments of Net-proton Multiplicity Distributions

Higher moments of event-by-event net-proton multiplicity distributions have been applied to search for the QCD critical point. Model results are used to provide a baseline for this search. The measured moment products, $\kappa\sigma^2$ and $S\sigma$ of net-proton distributions, which are directly connected to the thermodynamical baryon number susceptibility ratio in Lattice QCD and Hadron Resonance Gas (HRG) model, are compared to the transport and thermal model results. We argue that a non-monotonic dependence of $\kappa \sigma^2$ and $S \sigma$ as a function of beam energy can be used to search for the QCD critical point.Comment: 7 pages, 3 figures. CPOD 2010 Proceeding

In vitro transposition of ISY100, a bacterial insertion sequence belonging to the Tc1/mariner family

The Synechocystis sp. PCC6803 insertion sequence ISY100 (ISTcSa) belongs to the Tc1/mariner/IS630 family of transposable elements. ISY100 transposase was purified and shown to promote transposition in vitro. Transposase binds specifically to ISY100 terminal inverted repeat sequences via an N-terminal DNA-binding domain containing two helix–turn–helix motifs. Transposase is the only protein required for excision and integration of ISY100. Transposase made double-strand breaks on a supercoiled DNA molecule containing a mini-ISY100 transposon, cleaving exactly at the transposon 3′ ends and two nucleotides inside the 5′ ends. Cleavage of short linear substrates containing a single transposon end was less precise. Transposase also catalysed strand transfer, covalently joining the transposon 3′ end to the target DNA. When a donor plasmid carrying a mini-ISY100 was incubated with a target plasmid and transposase, the most common products were insertions of one transposon end into the target DNA, but insertions of both ends at a single target site could be recovered after transformation into Escherichia coli. Insertions were almost exclusively into TA dinucleotides, and the target TA was duplicated on insertion. Our results demonstrate that there are no fundamental differences between the transposition mechanisms of IS630 family elements in bacteria and Tc1/mariner elements in higher eukaryotes