Double Exchange Models: Self Consistent Renormalisation

We propose a scheme for constructing classical spin Hamiltonians from Hunds coupled spin-fermion models in the limit J_H/t \to \infinity. The strong coupling between fermions and the core spins requires self-consistent calculation of the effective exchange in the model, either in the presence of inhomogeneities or with changing temperature. In this paper we establish the formalism and discuss results mainly on the ``clean'' double exchange model, with self consistently renormalised couplings, and compare our results with exact simulations. Our method allows access to system sizes much beyond the reach of exact simulations, and we can study transport and optical properties of the model without artificial broadening. The method discussed here forms the foundation of our papers Phys. Rev. Lett. 91, 246602 (2003), and Phys. Rev. Lett. 92, 126602 (2004).Comment: 11 pages revtex. Final version, to appear in EPJ

The phase structure of lattice QCD with two flavours of Wilson quarks and renormalization group improved gluons

The effect of changing the lattice action for the gluon field on the recently observed [1] first order phase transition near zero quark mass is investigated by replacing the Wilson plaquette action by the DBW2 action. The lattice action for quarks is unchanged: it is in both cases the original Wilson action. It turns out that Wilson fermions with the DBW2 gauge action have a phase structure where the minimal pion mass and the jump of the average plaquette are decreased, when compared to Wilson fermions with Wilson plaquette action at similar values of the lattice spacing. Taking the DBW2 gauge action is advantageous also from the point of view of the computational costs of numerical simulations.Comment: 34 pages, 3 tables, 14 figures; revised version to appear in EPJ

Radial solitons in armchair carbon nanotubes

Radial solitons are investigated in armchair carbon nanotubes using a generalized Lennard-Jones potential. The radial solitons are found in terms of moving kink defects whose velocity obeys a dispersion relation. Effects of lattice discreteness on the shape of kink defects are examined by estimating the Peierls stress. Results suggest that the typical size for an unpinned kink phase is of the order of a lattice spacing.Comment: 11 pages, 3(eps) figure

Anisotropic field dependence of the magnetic transition in Cu2Te2O5Br2

We present the results of measurements of the thermal conductivity of Cu2Te2O5Br2, a compound where tetrahedra of Cu^{2+} ions carrying S=1/2 spins form chains along the c-axis of the tetragonal crystal structure. The thermal conductivity kappa was measured along both the c- and the a-direction as a function of temperature between 3 and 300 K and in external magnetic fields H up to 69 kOe, oriented both parallel and perpendicular to the c-axis. Distinct features of kappa(T) were observed in the vicinity of T_N=11.4 K in zero magnetic field. These features are unaltered in external fields which are parallel to the c-axis, but are more pronounced when a field is applied perpendicularly to the c-axis. The transition temperature increases upon enhancing the external field, but only if the field is oriented along the a-axis.Comment: 5 pages, 3 figure

Josephson oscillation of a superfluid Fermi gas

Using the complete numerical solution of a time-dependent three-dimensional mean-field model we study the Josephson oscillation of a superfluid Fermi gas (SFG) at zero temperature formed in a combined axially-symmetric harmonic plus one-dimensional periodic optical-lattice (OL) potentials after displacing the harmonic trap along the axial OL axis. We study the dependence of Josephson frequency on the strength of the OL potential. The Josephson frequency decreases with increasing strength as found in the experiment of Cataliotti et al. [Science 293 (2001) 843] for a Bose-Einstein condensate and of the experiment of Pezze et al. [Phys. Rev. Lett. 93 (2004) 120401] for an ideal Fermi gas. We demonstrate a breakdown of Josephson oscillation in the SFG for a large displacement of the harmonic trap. These features of Josephson oscillation of a SFG can be tested experimentally.Comment: 7 pages, 10 figure

Bragg spectroscopy of discrete axial quasiparticle modes in a cigar-shaped degenerate Bose gas

We propose an experiment in which long wavelength discrete axial quasiparticle modes can be imprinted in a 3D cigar-shaped Bose-Einstein condensate by using two-photon Bragg scattering experiments, similar to the experiment at the Weizmann Institute [J. Steinhauer {\em et al.}, Phys. Rev. Lett. {\bf 90}, 060404 (2003)] where short wavelength axial phonons with different number of radial modes have been observed. We provide values of the momentum, energy and time duration of the two-photon Bragg pulse and also the two-body interaction strength which are needed in the Bragg scattering experiments in order to observe the long wavelength discrete axial modes. These discrete axial modes can be observed when the system is dilute and the time duration of the Bragg pulse is long enough.Comment: 5 pages, 3 figures, title, abstract, results changed, references added. to appear in The European Physical Journal

Exclusive Semileptonic Rare Decays B→K(∗)l+l−B \to K^{(*)} l^+ l^- in a SUSY SO(10) GUT

In the SUSY SO(10) GUT context, we study the exclusive processes $B \to K^{(*)} l^+l^-(l=\mu,\tau)$. Using the Wilson coefficients of relevant operators including the new operators $Q_{1,2}^{(\prime)}$ which are induced by neutral Higgs boson (NHB) penguins, we evaluate some possible observables associated with these processes like, the invariant mass spectrum (IMS), lepton pair forward backward asymmetry (FBA), lepton polarization asymmetries etc. In this model the contributions from Wilson coefficients $C_{Q_{1,2}}^\prime$, among new contributions, are dominant. Our results show that the NHB effects are sensitive to the FBA, $dL/d\hat{s}$, and $dT/d\hat{s}$ of $B \to K^{(*)} \tau^+ \tau^-$ decay, which are expected to be measured in B factories, and the average of the normal polarization $dN/d\hat{s}$ can reach several percent for $B \to K \mu^+ \mu^-$ and it is 0.05 or so for $B\to K \tau^+\tau^-$, which could be measured in the future super B factories and provide a useful information to probe new physics and discriminate different models.Comment: 16 pages,7 figure

J/ψ\psi suppression at s=\sqrt{{\bf s}} = 200 GeV in the comovers interaction model

The yield of $J/\psi$ per binary nucleon-nucleon collision in $AuAu$ and $CuCu$ collisions at $\sqrt{s} = 200$ GeV is computed in the framework of the dual parton model, supplemented with final state interaction (comovers interaction). For the latter we use the same value of the cross-section, $\sigma_{co} = 0.65$ mb, which describes the anomalous $J/\psi$ suppression observed at CERN-SPS energies. Several possibilities for the value of the absorptive cross-section are considered. Shadowing is introduced in both the comovers and the $J/\psi$ yields. A comparison with the results at CERN-SPS, including a prediction for $InIn$ collisions, is also presented.Comment: 15 pages, 3 figure

Hard scattering and jets--from p-p collisions in the 1970's to Au+Au collisions at RHIC

Hard scattering in p-p collisions, discovered at the CERN ISR in 1972 by the method of leading particles, proved that the partons of Deeply Inelastic Scattering strongly interacted with each other. Further ISR measurements utilizing inclusive single or pairs of hadrons established that high pT particles are produced from states with two roughly back-to-back jets which are the result of scattering of constituents of the nucleons as described by Quantum Chromodynamics (QCD), which was developed during the course of these measurements. These techniques, which are the only practical method to study hard-scattering and jet phenomena in Au+Au central collisions, are reviewed, with application to measurements at RHIC.Comment: 4 pages, 5 figures, Proceedings of Hard Probes 2004, International Conference on Hard and Electromagnetic Probes of High Energy Nuclear Collisions, Nov 4-10, 2004, to appear in EPJ

The 4-particle hydrogen-antihydrogen system revisited: twofold Hamiltonian symmetry and natural atom antihydrogen

Modern ab initio treatments of H-Hbar systems are inconsistent with the logic behind algebraic Hamiltonians H(+-)=H(0)+/-deltaH for charge-symmetrical and charge-asymmetrical 4 unit charge systems like H(2) and HHbar. Since these 2 Hamiltonians are mutually exclusive, only the attractive one can apply for stable natural molecular H(2). A wrong choice leads to problems with antiatom Hbar. In line with earlier results on band and line spectra, we now prove that HL chose the wrong Hamiltonian for H(2). Their theory explains the stability of attractive system H(2) with a repulsive Hamiltonian instead of with the attractive one, representative for charge-asymmetrical system HHbar. A new second order symmetry effect is detected. Repulsive HL Hamiltonian H(+) applies at long range but at the critical distance, attractive charge-inverted Hamiltonian H(-)takes over and leads to bond H(2) but in reality, HHbar, for which we give an analytical proof. Another wrong asymptote choice in the past also applies for atomic antihydrogen Hbar, which has hidden the Mexican hat potential for natural hydrogen. This generic solution removes most problems, physicists and chemists experience with atomic Hbar and molecular HHbar, including the problem with antimatter in the Universe.Comment: at the instituional UGent archive, 37 pag, 10 fig, tabb, version as submitted, abstract shortene