Why non-superconducting metallic elements become superconducting under high pressure

We predict that simple metals and early transition metals that become superconducting under high pressures will show a change in sign of their Hall coefficient from negative to positive under pressure. If verified, this will strongly suggest that hole carriers play a fundamental role in `conventional' superconductivity, as predicted by the theory of hole superconductivity.Comment: Submitted to M2S-IX Tokyo 200

ac-Field-Controlled Anderson Localization in Disordered Semiconductor Superlattices

An ac field, tuned exactly to resonance with the Stark ladder in an ideal tight binding lattice under strong dc bias, counteracts Wannier-Stark localization and leads to the emergence of extended Floquet states. If there is random disorder, these states localize. The localization lengths depend non-monotonically on the ac field amplitude and become essentially zero at certain parameters. This effect is of possible relevance for characterizing the quality of superlattice samples, and for performing experiments on Anderson localization in systems with well-defined disorder.Comment: 10 pages, Latex; figures available on request from [email protected]

Dielectric response of charge induced correlated state in the quasi-one-dimensional conductor (TMTTF)2PF6

Conductivity and permittivity of the quasi-one-dimensionsional organic transfer salt (TMTTF)2PF6 have been measured at low frequencies (10^3-10^7 Hz) between room temperature down to below the temperature of transition into the spin-Peierls state. We interpret the huge real part of the dielectric permittivity (up to 10^6) in the localized state as the realization in this compound of a charge ordered state of Wigner crystal type due to long range Coulomb interaction.Comment: 11 pages, 3 .eps figure

A quantum Monte Carlo study of the one-dimensional ionic Hubbard model

Quantum Monte Carlo methods are used to study a quantum phase transition in a 1D Hubbard model with a staggered ionic potential (D). Using recently formulated methods, the electronic polarization and localization are determined directly from the correlated ground state wavefunction and compared to results of previous work using exact diagonalization and Hartree-Fock. We find that the model undergoes a thermodynamic transition from a band insulator (BI) to a broken-symmetry bond ordered (BO) phase as the ratio of U/D is increased. Since it is known that at D = 0 the usual Hubbard model is a Mott insulator (MI) with no long-range order, we have searched for a second transition to this state by (i) increasing U at fixed ionic potential (D) and (ii) decreasing D at fixed U. We find no transition from the BO to MI state, and we propose that the MI state in 1D is unstable to bond ordering under the addition of any finite ionic potential. In real 1D systems the symmetric MI phase is never stable and the transition is from a symmetric BI phase to a dimerized BO phase, with a metallic point at the transition

Symmetry breaking in the Hubbard model at weak coupling

The phase diagram of the Hubbard model is studied at weak coupling in two and three spatial dimensions. It is shown that the Neel temperature and the order parameter in d=3 are smaller than the Hartree-Fock predictions by a factor of q=0.2599. For d=2 we show that the self-consistent (sc) perturbation series bears no relevance to the behavior of the exact solution of the Hubbard model in the symmetry-broken phase. We also investigate an anisotropic model and show that the coupling between planes is essential for the validity of mean-field-type order parameters

Performance of Braford steers grazing on cultivated pastures and fed or not fed an energy supplement

This experiment evaluated the performance of 84 Braford steers grazing on summer and winter cultivated pastures fed or not fed an energy-protein supplement. Steers were 10 months old and weighed, on average, 165 kg at the beginning of the trial. Steers grazed on cultivated winter pasture, consisting of black oats (Avena strigosa) and ryegrass (Lolium multiflorum Lam.), and then on cultivated summer pasture, consisting of Napier grass (Pennisetum purpureum). The following treatments were applied: SS - supplement was fed during both seasons; SN - supplement was fed only during the winter; NS - supplement was fed only during the summer; and NN - steers were not fed any supplement. Supplementation resulted in higher body weight and better body condition score (SS: 369.2 and 4.52; SN: 335.2 and 4.01; NS: 352.5 and 4.49; SS: 322.5 and 3.83). Longissimus dorsi area was larger in supplemented steers, which also presented thicker backfat when compared with those not fed any supplement (3.67 and 2.29 mm, respectively)

Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV

A search for a Higgs boson decaying into two photons is described. The analysis is performed using a dataset recorded by the CMS experiment at the LHC from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross section of the standard model Higgs boson decaying to two photons. The expected exclusion limit at 95% confidence level is between 1.4 and 2.4 times the standard model cross section in the mass range between 110 and 150 GeV. The analysis of the data excludes, at 95% confidence level, the standard model Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The largest excess of events above the expected standard model background is observed for a Higgs boson mass hypothesis of 124 GeV with a local significance of 3.1 sigma. The global significance of observing an excess with a local significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is estimated to be 1.8 sigma. More data are required to ascertain the origin of this excess.Comment: Submitted to Physics Letters

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 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