Tight-binding g-Factor Calculations of CdSe Nanostructures

The Lande g-factors for CdSe quantum dots and rods are investigated within the framework of the semiempirical tight-binding method. We describe methods for treating both the n-doped and neutral nanostructures, and then apply these to a selection of nanocrystals of variable size and shape, focusing on approximately spherical dots and rods of differing aspect ratio. For the negatively charged n-doped systems, we observe that the g-factors for near-spherical CdSe dots are approximately independent of size, but show strong shape dependence as one axis of the quantum dot is extended to form rod-like structures. In particular, there is a discontinuity in the magnitude of g-factor and a transition from anisotropic to isotropic g-factor tensor at aspect ratio ~1.3. For the neutral systems, we analyze the electron g-factor of both the conduction and valence band electrons. We find that the behavior of the electron g-factor in the neutral nanocrystals is generally similar to that in the n-doped case, showing the same strong shape dependence and discontinuity in magnitude and anisotropy. In smaller systems the g-factor value is dependent on the details of the surface model. Comparison with recent measurements of g-factors for CdSe nanocrystals suggests that the shape dependent transition may be responsible for the observations of anomalous numbers of g-factors at certain nanocrystal sizes.Comment: 15 pages, 6 figures. Fixed typos to match published versio

Evidence for Shape Co-existence at medium spin in 76Rb

Four previously known rotational bands in 76Rb have been extended to moderate spins using the Gammasphere and Microball gamma ray and charged particle detector arrays and the 40Ca(40Ca,3pn) reaction at a beam energy of 165 MeV. The properties of two of the negative-parity bands can only readily be interpreted in terms of the highly successful Cranked Nilsson-Strutinsky model calculations if they have the same configuration in terms of the number of g9/2 particles, but they result from different nuclear shapes (one near-oblate and the other near-prolate). These data appear to constitute a unique example of shape co-existing structures at medium spins.Comment: Accepted for publication in Physics Letters

First principles electronic structure of spinel LiCr2O4: A possible half-metal?

We have employed first-principles electronic structure calculations to examine the hypothetical (but plausible) oxide spinel, LiCr2O4 with the d^{2.5} electronic configuration. The cell (cubic) and internal (oxygen position) structural parameters have been obtained for this compound through structural relaxation in the first-principles framework. Within the one-electron band picture, we find that LiCr2O4 is magnetic, and a candidate half-metal. The electronic structure is substantially different from the closely related and well known rutile half-metal CrO2. In particular, we find a smaller conduction band width in the spinel compound, perhaps as a result of the distinct topology of the spinel crystal structure, and the reduced oxidation state. The magnetism and half-metallicity of LiCr2O4 has been mapped in the parameter space of its cubic crystal structure. Comparisons with superconducting LiTi2O4 (d^{0.5}), heavy-fermion LiV2O4 (d^{1.5}) and charge-ordering LiMn2O4 (d^{3.5}) suggest the effectiveness of a nearly-rigid band picture involving simple shifts of the position of E_F in these very different materials. Comparisons are also made with the electronic structure of ZnV2O4 (d^{2}), a correlated insulator that undergoes a structural and antiferromagnetic phase transition.Comment: 9 pages, 7 Figures, version as published in PR

Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV

Results are presented from a search for a W' boson using a dataset corresponding to 5.0 inverse femtobarns of integrated luminosity collected during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV. The W' boson is modeled as a heavy W boson, but different scenarios for the couplings to fermions are considered, involving both left-handed and right-handed chiral projections of the fermions, as well as an arbitrary mixture of the two. The search is performed in the decay channel W' to t b, leading to a final state signature with a single lepton (e, mu), missing transverse energy, and jets, at least one of which is tagged as a b-jet. A W' boson that couples to fermions with the same coupling constant as the W, but to the right-handed rather than left-handed chiral projections, is excluded for masses below 1.85 TeV at the 95% confidence level. For the first time using LHC data, constraints on the W' gauge coupling for a set of left- and right-handed coupling combinations have been placed. These results represent a significant improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe

Measurement of the Lambda(b) cross section and the anti-Lambda(b) to Lambda(b) ratio with Lambda(b) to J/Psi Lambda decays in pp collisions at sqrt(s) = 7 TeV

The Lambda(b) differential production cross section and the cross section ratio anti-Lambda(b)/Lambda(b) are measured as functions of transverse momentum pt(Lambda(b)) and rapidity abs(y(Lambda(b))) in pp collisions at sqrt(s) = 7 TeV using data collected by the CMS experiment at the LHC. The measurements are based on Lambda(b) decays reconstructed in the exclusive final state J/Psi Lambda, with the subsequent decays J/Psi to an opposite-sign muon pair and Lambda to proton pion, using a data sample corresponding to an integrated luminosity of 1.9 inverse femtobarns. The product of the cross section times the branching ratio for Lambda(b) to J/Psi Lambda versus pt(Lambda(b)) falls faster than that of b mesons. The measured value of the cross section times the branching ratio for pt(Lambda(b)) > 10 GeV and abs(y(Lambda(b))) < 2.0 is 1.06 +/- 0.06 +/- 0.12 nb, and the integrated cross section ratio for anti-Lambda(b)/Lambda(b) is 1.02 +/- 0.07 +/- 0.09, where the uncertainties are statistical and systematic, respectively.Comment: Submitted to Physics Letters

Search for new physics in events with opposite-sign leptons, jets, and missing transverse energy in pp collisions at sqrt(s) = 7 TeV

A search is presented for physics beyond the standard model (BSM) in final states with a pair of opposite-sign isolated leptons accompanied by jets and missing transverse energy. The search uses LHC data recorded at a center-of-mass energy sqrt(s) = 7 TeV with the CMS detector, corresponding to an integrated luminosity of approximately 5 inverse femtobarns. Two complementary search strategies are employed. The first probes models with a specific dilepton production mechanism that leads to a characteristic kinematic edge in the dilepton mass distribution. The second strategy probes models of dilepton production with heavy, colored objects that decay to final states including invisible particles, leading to very large hadronic activity and missing transverse energy. No evidence for an event yield in excess of the standard model expectations is found. Upper limits on the BSM contributions to the signal regions are deduced from the results, which are used to exclude a region of the parameter space of the constrained minimal supersymmetric extension of the standard model. Additional information related to detector efficiencies and response is provided to allow testing specific models of BSM physics not considered in this paper.Comment: Replaced with published version. Added journal reference and DO

Measurement of isolated photon production in pp and PbPb collisions at sqrt(sNN) = 2.76 TeV

Isolated photon production is measured in proton-proton and lead-lead collisions at nucleon-nucleon centre-of-mass energies of 2.76 TeV in the pseudorapidity range |eta|<1.44 and transverse energies ET between 20 and 80 GeV with the CMS detector at the LHC. The measured ET spectra are found to be in good agreement with next-to-leading-order perturbative QCD predictions. The ratio of PbPb to pp isolated photon ET-differential yields, scaled by the number of incoherent nucleon-nucleon collisions, is consistent with unity for all PbPb reaction centralities.Comment: Submitted to Physics Letters

Characteristics of the nuclear (18S, 5.8S, 28S and 5S) and mitochondrial (12S and 16S) rRNA genes of Apis mellifera (Insecta: Hymenoptera): structure, organization, and retrotransposable elements

As an accompanying manuscript to the release of the honey bee genome, we report the entire sequence of the nuclear (18S, 5.8S, 28S and 5S) and mitochondrial (12S and 16S) ribosomal RNA (rRNA)-encoding gene sequences (rDNA) and related internally and externally transcribed spacer regions of Apis mellifera (Insecta: Hymenoptera: Apocrita). Additionally, we predict secondary structures for the mature rRNA molecules based on comparative sequence analyses with other arthropod taxa and reference to recently published crystal structures of the ribosome. In general, the structures of honey bee rRNAs are in agreement with previously predicted rRNA models from other arthropods in core regions of the rRNA, with little additional expansion in non-conserved regions. Our multiple sequence alignments are made available on several public databases and provide a preliminary establishment of a global structural model of all rRNAs from the insects. Additionally, we provide conserved stretches of sequences flanking the rDNA cistrons that comprise the externally transcribed spacer regions (ETS) and part of the intergenic spacer region (IGS), including several repetitive motifs. Finally, we report the occurrence of retrotransposition in the nuclear large subunit rDNA, as R2 elements are present in the usual insertion points found in other arthropods. Interestingly, functional R1 elements usually present in the genomes of insects were not detected in the honey bee rRNA genes. The reverse transcriptase products of the R2 elements are deduced from their putative open reading frames and structurally aligned with those from another hymenopteran insect, the jewel wasp Nasonia (Pteromalidae). Stretches of conserved amino acids shared between Apis and Nasonia are illustrated and serve as potential sites for primer design, as target amplicons within these R2 elements may serve as novel phylogenetic markers for Hymenoptera. Given the impending completion of the sequencing of the Nasonia genome, we expect our report eventually to shed light on the evolution of the hymenopteran genome within higher insects, particularly regarding the relative maintenance of conserved rDNA genes, related variable spacer regions and retrotransposable elements

Studies of jet quenching using isolated-photon + jet correlations in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV

Results from the first study of isolated-photon + jet correlations in relativistic heavy ion collisions are reported. The analysis uses data from PbPb collisions at a centre-of-mass energy of 2.76 TeV per nucleon pair corresponding to an integrated luminosity of 150 inverse microbarns recorded by the CMS experiment at the LHC. For events containing an isolated photon with transverse momentum pt(gamma) > 60 GeV and an associated jet with pt(Jet) > 30 GeV, the photon + jet pt imbalance is studied as a function of collision centrality and compared to pp data and PYTHIA calculations at the same collision energy. Using the pt(gamma) of the isolated photon as an estimate of the momentum of the associated parton at production, this measurement allows an unbiased characterisation of the in-medium parton energy loss. For more central PbPb collisions, a significant decrease in the ratio pt(Jet)/pt(gamma) relative to that in the PYTHIA reference is observed. Furthermore, significantly more pt(gamma) > 60 GeV photons in PbPb are observed not to have an associated pt(Jet) > 30 GeV jet, compared to the reference. However, no significant broadening of the photon + jet azimuthal correlation is observed.Comment: Submitted to Physics Letters