7,152 research outputs found
Exact Solution of Noncommutative Field Theory in Background Magnetic Fields
We obtain the exact non-perturbative solution of a scalar field theory
defined on a space with noncommuting position and momentum coordinates. The
model describes non-locally interacting charged particles in a background
magnetic field. It is an exactly solvable quantum field theory which has
non-trivial interactions only when it is defined with a finite ultraviolet
cutoff. We propose that small perturbations of this theory can produce solvable
models with renormalizable interactions.Comment: 9 Pages AMSTeX; Typos correcte
Quiver Gauge Theory of Nonabelian Vortices and Noncommutative Instantons in Higher Dimensions
We construct explicit BPS and non-BPS solutions of the Yang-Mills equations
on the noncommutative space R^{2n}_\theta x S^2 which have manifest spherical
symmetry. Using SU(2)-equivariant dimensional reduction techniques, we show
that the solutions imply an equivalence between instantons on R^{2n}_\theta x
S^2 and nonabelian vortices on R^{2n}_\theta, which can be interpreted as a
blowing-up of a chain of D0-branes on R^{2n}_\theta into a chain of spherical
D2-branes on R^{2n} x S^2. The low-energy dynamics of these configurations is
described by a quiver gauge theory which can be formulated in terms of new
geometrical objects generalizing superconnections. This formalism enables the
explicit assignment of D0-brane charges in equivariant K-theory to the
instanton solutions.Comment: 45 pages, 4 figures; v2: minor correction
Electronic structure of amorphous germanium disulfide via density functional molecular dynamics simulations
Using density functional molecular dynamics simulations we study the
electronic properties of glassy g-GeS. We compute the electronic density of
states, which compares very well with XPS measurements, as well as the partial
EDOS and the inverse participation ratio. We show the electronic contour plots
corresponding to different structural environments, in order to determine the
nature of the covalent bonds between the atoms. We finally study the local
atomic charges, and analyze the impact of the local environment on the charge
transfers between the atoms. The broken chemical order inherent to amorphous
systems leads to locally charged zones when integrating the atomic charges up
to nearest-neighbor distances.Comment: 13 pages, 9 figures; to appear in Phys. Rev.
Edge states in graphene quantum dots: Fractional quantum Hall effect analogies and differences at zero magnetic field
We investigate the way that the degenerate manifold of midgap edge states in
quasicircular graphene quantum dots with zig-zag boundaries supports, under
free-magnetic-field conditions, strongly correlated many-body behavior
analogous to the fractional quantum Hall effect (FQHE), familiar from the case
of semiconductor heterostructures in high magnetic fields. Systematic
exact-diagonalization (EXD) numerical studies are presented for the first time
for 5 <= N <= 8 fully spin-polarized electrons and for total angular momenta in
the range of N(N-1)/2 <= L <= 150. We present a derivation of a
rotating-electron-molecule (REM) type wave function based on the methodology
introduced earlier [C. Yannouleas and U. Landman, Phys. Rev. B 66, 115315
(2002)] in the context of the FQHE in two-dimensional semiconductor quantum
dots. The EXD wave functions are compared with FQHE trial functions of the
Laughlin and the derived REM types. It is found that a variational extension of
the REM offers a better description for all fractional fillings compared with
that of the Laughlin functions (including total energies and overlaps), a fact
that reflects the strong azimuthal localization of the edge electrons. In
contrast with the multiring arrangements of electrons in circular semiconductor
quantum dots, the graphene REMs exhibit in all instances a single (0,N)
polygonal-ring molecular (crystalline) structure, with all the electrons
localized on the edge. Disruptions in the zig-zag boundary condition along the
circular edge act effectively as impurities that pin the electron molecule,
yielding single-particle densities with broken rotational symmetry that portray
directly the azimuthal localization of the edge electrons.Comment: Revtex. 14 pages with 13 figures and 2 tables. Physical Review B, in
press. For related papers, see http://www.prism.gatech.edu/~ph274cy
The QCD phase diagram from analytic continuation
We present the crossover line between the quark gluon plasma and the hadron
gas phases for small real chemical potentials. First we determine the effect of
imaginary values of the chemical potential on the transition temperature using
lattice QCD simulations. Then we use various formulas to perform an analytic
continuation to real values of the baryo-chemical potential. Our data set
maintains strangeness neutrality to match the conditions of heavy ion physics.
The systematic errors are under control up to MeV. For the
curvature of the transition line we find that there is an approximate agreement
between values from three different observables: the chiral susceptibility,
chiral condensate and strange quark susceptibility. The continuum extrapolation
is based on 10, 12 and 16 lattices. By combining the analysis for these
three observables we find, for the curvature, the value .Comment: 14 pages, 4 figures, revised versio
The effect of quantization on the FCIQMC sign problem
The sign problem in Full Configuration Interaction Quantum Monte Carlo
(FCIQMC) without annihilation can be understood as an instability of the
psi-particle population to the ground state of the matrix obtained by making
all off-diagonal elements of the Hamiltonian negative. Such a matrix, and hence
the sign problem, is basis dependent. In this paper we discuss the properties
of a physically important basis choice: first versus second quantization. For a
given choice of single-particle orbitals, we identify the conditions under
which the fermion sign problem in the second quantized basis of antisymmetric
Slater determinants is identical to the sign problem in the first quantized
basis of unsymmetrized Hartree products. We also show that, when the two
differ, the fermion sign problem is always less severe in the second quantized
basis. This supports the idea that FCIQMC, even in the absence of annihilation,
improves the sign problem relative to first quantized methods. Finally, we
point out some theoretically interesting classes of Hamiltonians where first
and second quantized sign problems differ, and others where they do not.Comment: 4 pages w/ 2 page appendix, 2 figures, 1 tabl
Electron-vibration interaction in transport through atomic gold wires
We calculate the effect of electron-vibration coupling on conduction through
atomic gold wires, which was measured in the experiments of Agra\"it et al.
[Phys. Rev. Lett. 88, 216803 (2002)]. The vibrational modes, the coupling
constants, and the inelastic transport are all calculated using a tight-binding
parametrization and the non-equilibrium Green function formalism. The
electron-vibration coupling gives rise to small drops in the conductance at
voltages corresponding to energies of some of the vibrational modes. We study
systematically how the position and height of these steps vary as a linear wire
is stretched and more atoms are added to it, and find a good agreement with the
experiments. We also consider two different types of geometries, which are
found to yield qualitatively similar results. In contrast to previous
calculations, we find that typically there are several close-lying drops due to
different longitudinal modes. In the experiments, only a single drop is usually
visible, but its width is too large to be accounted for by temperature.
Therefore, to explain the experimental results, we find it necessary to
introduce a finite broadening to the vibrational modes, which makes the
separate drops merge into a single, wide one. In addition, we predict how the
signatures of vibrational modes in the conductance curves differ between linear
and zigzag-type wires.Comment: 19 pages, 12 figure
Photometry of SN 2002bo with template image subtraction
VRI photometry of the type Ia supernova 2002bo is presented. This SN exploded
in a dusty region of the host galaxy NGC 3190, thus, subtraction of a template
frame was necessary to obtain reliable photometry. We used a template frame of
NGC 3190 taken during the course of our galaxy imaging project, fortunately,
just a few days before SN 2002bo was discovered. The aim of this project is to
collect template frames of nearby galaxies that are potential hosts of bright
SNe. Subtraction of pre-SN images helped us to exclude the background light
contamination of the host galaxy. The maximum occurred at JD 2452346, with
maximal V brightness of 13.58. MLCS analysis led to T0(B)=JD 2452346.1 pm 0.8
(fiducial B-maximum), E(B-V)=0.24 pm 0.02, mu0=32.46 pm 0.06, Delta=-0.14 pm
0.04. E(B-V)=0.24(2) indicates a significant extinction in the host galaxy as
the galactic reddening is negligible toward NGC 3190. The accepted value of
Delta indicates that SN 2002bo was a slightly overluminous SN by about 0.14
relative to fiducial SN Type Ia. The distance turned out to be 31.0 pm 3 Mpc.
In addition, the heavily obscured SN 2002cv was also detected on the I frame
taken on JD 2452434 (June 8, 2002), and a variable star is found in the field,
very close to the host galaxy.Comment: accepted by Astronomy and Astrophysic
Towards the QCD phase diagram from analytical continuation
We calculate the QCD cross-over temperature, the equation of state and
fluctuations of conserved charges at finite density by analytical continuation
from imaginary to real chemical potentials. Our calculations are based on new
continuum extrapolated lattice simulations using the 4stout staggered actions
with a lattice resolution up to . The simulation parameters are tuned
such that the strangeness neutrality is maintained, as it is in heavy ion
collisions.Comment: 4 pages, 2 figures, Proceedings of the Quark Matter 2015 conference,
Kobe, Japa
Axion cosmology, lattice QCD and the dilute instanton gas
Axions are one of the most attractive dark matter candidates. The evolution
of their number density in the early universe can be determined by calculating
the topological susceptibility of QCD as a function of the
temperature. Lattice QCD provides an ab initio technique to carry out such a
calculation. A full result needs two ingredients: physical quark masses and a
controlled continuum extrapolation from non-vanishing to zero lattice spacings.
We determine in the quenched framework (infinitely large quark
masses) and extrapolate its values to the continuum limit. The results are
compared with the prediction of the dilute instanton gas approximation (DIGA).
A nice agreement is found for the temperature dependence, whereas the overall
normalization of the DIGA result still differs from the non-perturbative
continuum extrapolated lattice results by a factor of order ten. We discuss the
consequences of our findings for the prediction of the amount of axion dark
matter.Comment: 9 pages, 7 figure
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