6,971 research outputs found
Valence Bond Solids and Their Quantum Melting in Hard-Core Bosons on the Kagome Lattice
Using large scale quantum Monte Carlo simulations and dual vortex theory we
analyze the ground state phase diagram of hard-core bosons on the kagome
lattice with nearest neighbor repulsion. In contrast to the case of a
triangular lattice, no supersolid emerges for strong interactions. While a
uniform superfluid prevails at half-filling, two novel solid phases emerge at
densities and . These solids exhibit an only partial
ordering of the bosonic density, allowing for local resonances on a subset of
hexagons of the kagome lattice. We provide evidence for a weakly first-order
phase transition at the quantum melting point between these solid phases and
the superfluid.Comment: 4 pages, 7 figure
Electrochemical Quartz Crystal Microbalance Study of Corrosion of Phases in AA2024
The electrochemical quartz crystal microbalance (EQCM) was used to directly measure the dissolution rate at cathodic potentials, and thus the cathodic corrosion rate, of thin-film analogs of phases in AA2024. Thin films of pure Al, Al-4% Cu, and Al2Cu were studied in 0.1 M NaCl containing 0, 10^-4, or 10^-2 M Cr2O7 . A range of cathodic potentials was studied for each material. The true cathodic current density was calculated from the difference of the net current density and the dissolution rate, which was determined by the EQCM. For pure Al and Al-4Cu, the cathodic corrosion rate was large relative to the net current density, so the true cathodic current density was considerably larger than the measured net current density. The cathodic current density was almost identical to the net current density for Al2Cu because the dissolution rate was very small compared to the cathodic reaction rate. Various potentials in the limiting oxygen reduction reaction region were examined, but the effect of the applied potential was small. The presence of dichromate in solution decreased both the cathodic corrosion rate and the cathodic current density on these thin-film analogs. In particular, it decreased more effectively the cathodic reaction rate on Al2Cu, which can support faster cathodic reaction rates.This work was supported by the United States Air Force Office of Scientific Research Grant no. F49620-96-1-0479 under the guidance of Dr. Paul Trulove
Resummation of Large Endpoint Corrections to Color-Octet J/psi Photoproduction
An unresolved problem in J/psi phenomenology is a systematic understanding of
the differential photoproduction cross section, dsigma/dz [gamma + p -> J/psi +
X], where z= E_psi/E_gamma in the proton rest frame. In the non-relativistic
QCD (NRQCD) factorization formalism, fixed-order perturbative calculations of
color-octet mechanisms suffer from large perturbative and nonperturbative
corrections that grow rapidly in the endpoint region, z -> 1. In this paper,
NRQCD and soft collinear effective theory are combined to resum these large
corrections to the color-octet photoproduction cross section. We derive a
factorization theorem for the endpoint differential cross section involving the
parton distribution function and the color-octet J/psi shape functions. A one
loop matching calculation explicitly confirms our factorization theorem at
next-to-leading order. Large perturbative corrections are resummed using the
renormalization group. The calculation of the color-octet contribution to
dsigma/dz is in qualitative agreement with data. Quantitative tests of the
universality of color-octet matrix elements require improved knowledge of shape
functions entering these calculations as well as resummation of the
color-singlet contribution which accounts for much of the total cross section
and also peaks near the endpoint.Comment: 30 pages, 6 figure
B_{s,d} -> l^+ l^- and K_L -> l^+ l^- in SUSY models with non-minimal sources of flavour mixing
We present a general analysis of B_{s,d}-> l^+ l^- and K_L -> l^+ l^- decays
in supersymmetric models with non-minimal sources of flavour mixing. In spite
of the existing constraints on off-diagonal squark mass terms, these modes
could still receive sizeable corrections, mainly because of Higgs-mediated
FCNCs arising at large tan(beta). The severe limits on scenarios with large
tan(beta) and non-negligible {tilde d}^i_{R(L)}-{d-tilde}^j_{R(L)} mixing
imposed by the present experimental bounds on these modes and Delta B=2
observables are discussed in detail. In particular, we show that scalar-current
contributions to K_L -> l^+ l^- and B-{bar B} mixing set non-trivial
constraints on the possibility that B_s -> l^+ l^- and B_d -> l^+ l^- receive
large corrections.Comment: 18 pages, 4 figures (v2: minor changes, published version
Nonequilibrium quantum criticality in bilayer itinerant ferromagnets
We present a theory of nonequilibrium quantum criticality in a coupled
bilayer system of itinerant electron magnets. The model studied consists of the
first layer subjected to an inplane current and open to an external substrate.
The second layer is closed and subject to no direct external drive, but couples
to the first layer via short-ranged spin exchange interaction. No particle
exchange is assumed between the layers. Starting from a microscopic fermionic
model, we derive an effective action in terms of two coupled bosonic fields
which are related to the magnetization fluctuations of the two layers. When
there is no interlayer coupling, the two bosonic modes possess different
dynamical critical exponents z with z=2 (z=3) for the first (second) layer.
This results in multi-scale quantum criticality in the coupled system. It is
shown that the linear coupling between the two fields leads to a low energy
fixed point characterized by the larger dynamical critical exponent z=3. The
perturbative renormalization group is used to compute the correlation length in
the quantum disordered and quantum critical regimes. We also derive the
stochastic dynamics obeyed by the critical fluctuations in the quantum critical
regime. Comparing the nonequilibrium situation to the thermal equilibrium
scenario, where the whole system is at a temperature T, we find that the
nonequilibrium drive does not always play the role of temperature.Comment: 20+ pages, 3 figures; Revised version as accepted by PRB, added
figure of mean field phase diagra
NMR and Relaxation in Superconductor
NMR and nuclear spin-lattice relaxation rate (NSLR) are reported at
7.2 Tesla and 1.4 Tesla in powder samples of the intermetallic compound
with superconducting transition temperature in zero field = 39.2 K. From
the first order quadrupole perturbed NMR specrum a quadrupole coupling
frequency of 835 5 kHz is obtained. The Knight shift is very small and it
decreases to zero in the superconducting phase. The NSLR follows a linear law
with = 165 10 (sec K) . The results in the normal phase indicate a
negligible -character of the wave function of the conduction electrons at
the Fermi level. Below the NSLR is strongly field dependent indicating
the presence of an important contribution related to the density and the
thermal motion of flux lines. No coherence peak is observed at the lower field
investigated (1.4 T)
Fully supersymmetric CP violations in the kaon system
We show that, on the contrary to the usual claims, fully supersymmetric CP
violations in the kaon system are possible through the gluino mediated flavor
changing interactions. Both and can be accommodated for relatively large without any
fine tunings or contradictions to the FCNC and EDM constraints.Comment: Contribution to the Proceedings of ICHEP2000, Osaka, 200
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