54 research outputs found
Phase transition in the one-dimensional Kondo lattice model with attractive electron-electron interaction
The one-dimensional Kondo lattice model with attractive interaction among the
conduction electrons is analyzed in the case of half-filling. It is shown that
there are three distinct phases depending on the coupling constants of the
model. Two phases have a spin and charge gap. While one shows a clear
separation of the spin and charge excitation spectrum the other phase may be
characterized as a band insulator type where both excitations are due to
two-particle states. The third phase is gapless in both channels and has quasi
long-range order in the spin and charge density wave correlation. In this phase
the spin and charge excitations have again a clearly separated spectrum. For
the analysis we discuss first two limiting cases. Then a density matrix
renormalization group calculation on finite systems is applied to determine the
phase diagram and the correlation functions in the gapped and gapless phase for
general couplding constants.Comment: 9 pages, 7 Postscript figures, REVTe
Pinning quantum phase transition for a Luttinger liquid of strongly interacting bosons
One of the most remarkable results of quantum mechanics is the fact that
many-body quantum systems may exhibit phase transitions even at zero
temperature. Quantum fluctuations, deeply rooted in Heisenberg's uncertainty
principle, and not thermal fluctuations, drive the system from one phase to
another. Typically, the relative strength of two competing terms in the
system's Hamiltonian is changed across a finite critical value. A well-known
example is the Mott-Hubbard quantum phase transition from a superfluid to an
insulating phase, which has been observed for weakly interacting bosonic atomic
gases. However, for strongly interacting quantum systems confined to
lower-dimensional geometry a novel type of quantum phase transition may be
induced for which an arbitrarily weak perturbation to the Hamiltonian is
sufficient to drive the transition. Here, for a one-dimensional (1D) quantum
gas of bosonic caesium atoms with tunable interactions, we observe the
commensurate-incommensurate quantum phase transition from a superfluid
Luttinger liquid to a Mott-insulator. For sufficiently strong interactions, the
transition is induced by adding an arbitrarily weak optical lattice
commensurate with the atomic granularity, which leads to immediate pinning of
the atoms. We map out the phase diagram and find that our measurements in the
strongly interacting regime agree well with a quantum field description based
on the exactly solvable sine-Gordon model. We trace the phase boundary all the
way to the weakly interacting regime where we find good agreement with the
predictions of the 1D Bose-Hubbard model. Our results open up the experimental
study of quantum phase transitions, criticality, and transport phenomena beyond
Hubbard-type models in the context of ultracold gases
Crystallization and Preliminary Analysis of Crystals of the 24-Meric Hemocyanin of the Emperor Scorpion (Pandinus imperator)
Hemocyanins are giant oxygen transport proteins found in the hemolymph of several invertebrate phyla. They constitute giant multimeric molecules whose size range up to that of cell organelles such as ribosomes or even small viruses. Oxygen is reversibly bound by hemocyanins at binuclear copper centers. Subunit interactions within the multisubunit hemocyanin complex lead to diverse allosteric effects such as the highest cooperativity for oxygen binding found in nature. Crystal structures of a native hemocyanin oligomer larger than a hexameric substructure have not been published until now. We report for the first time growth and preliminary analysis of crystals of the 24-meric hemocyanin (MW = 1.8 MDa) of emperor scorpion (Pandinus imperator), which diffract to a resolution of 6.5 Å. The crystals are monoclinc with space group C 1 2 1 and cell dimensions a = 311.61 Å, b = 246.58 Å and c = 251.10 Å (α = 90.00°, β = 90.02°, γ = 90.00°). The asymmetric unit contains one molecule of the 24-meric hemocyanin and the solvent content of the crystals is 56%. A preliminary analysis of the hemocyanin structure reveals that emperor scorpion hemocyanin crystallizes in the same oxygenated conformation, which is also present in solution as previously shown by cryo-EM reconstruction and small angle x-ray scattering experiments
Production of Single W Bosons at LEP
We report on the observation of single W boson production in a data sample collected by the L3 detector at LEP2. The signal consists of large missing energy final states with a single energetic lepton or two hadronic jets. The cross-section is measured to be at the centre of mass energy \sqrt{s}=172 \GeV{}, consistent with the Standard Model expectation. From this measurement the following limits on the anomalous WW gauge couplings are derived at 95\% CL: and
Evidence for Gluon Interference in Hadronic Z-Decays
Contains fulltext :
26227.pdf (publisher's version ) (Open Access
One-Prong Tau-Decays with Neutral Kaons
Contains fulltext :
26231.pdf (publisher's version ) (Open Access
Principles for supporting school drug education.
Children and young people often find visitors to school a useful and informative part of their learning, but for visits to be successful there needs to be a shared understanding about the nature and content of the sessions.
This paper is intended to set out principles that visitors and schools can consider applying to ensure better outcomes from using external support in a school's delivery of drug education
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