1,086 research outputs found
Lattice QCD study of a five-quark hadronic molecule
We compute the ground-state energies of a heavy-light K-Lambda like system as
a function of the relative distance r of the hadrons. The heavy quarks, one in
each hadron, are treated as static. Then, the energies give rise to an
adiabatic potential Va(r) which we use to study the structure of the five-quark
system. The simulation is based on an anisotropic and asymmetric lattice with
Wilson fermions. Energies are extracted from spectral density functions
obtained with the maximum entropy method. Our results are meant to give
qualitative insight: Using the resulting adiabatic potential in a Schroedinger
equation produces bound state wave functions which indicate that the ground
state of the five-quark system resembles a hadronic molecule, whereas the first
excited state, having a very small rms radius, is probably better described as
a five-quark cluster, or a pentaquark. We hypothesize that an all light-quark
pentaquark may not exist, but in the heavy-quark sector it might, albeit only
as an excited state.Comment: 11 pages, 15 figures, 4 table
General Algorithm For Improved Lattice Actions on Parallel Computing Architectures
Quantum field theories underlie all of our understanding of the fundamental
forces of nature. The are relatively few first principles approaches to the
study of quantum field theories [such as quantum chromodynamics (QCD) relevant
to the strong interaction] away from the perturbative (i.e., weak-coupling)
regime. Currently the most common method is the use of Monte Carlo methods on a
hypercubic space-time lattice. These methods consume enormous computing power
for large lattices and it is essential that increasingly efficient algorithms
be developed to perform standard tasks in these lattice calculations. Here we
present a general algorithm for QCD that allows one to put any planar improved
gluonic lattice action onto a parallel computing architecture. High performance
masks for specific actions (including non-planar actions) are also presented.
These algorithms have been successfully employed by us in a variety of lattice
QCD calculations using improved lattice actions on a 128 node Thinking Machines
CM-5.
{\underline{Keywords}}: quantum field theory; quantum chromodynamics;
improved actions; parallel computing algorithms
Spin-Electric Coupling in Molecular Magnets
We study the triangular antiferromagnet Cu in external electric fields,
using symmetry group arguments and a Hubbard model approach. We identify a
spin-electric coupling caused by an interplay between spin exchange, spin-orbit
interaction, and the chirality of the underlying spin texture of the molecular
magnet. This coupling allows for the electric control of the spin (qubit)
states, e.g. by using an STM tip or a microwave cavity. We propose an
experimental test for identifying molecular magnets exhibiting spin-electric
effects.Comment: 5 pages, 3 figure
Symmetry Analysis of Second Harmonic Generation at Surfaces of Antiferromagnets
Using group theory we classify the nonlinear magneto-optical response at
low-index surfaces of fcc antiferromagnets, such as NiO. Structures consisting
of one atomic layer are discussed in detail. We find that optical second
harmonic generation is sensitive to surface antiferromagnetism in many cases.
We discuss the influence of a second type of magnetic atoms, and also of a
possible oxygen sublattice distortion on the output signal. Finally, our
symmetry analysis yields the possibility of antiferromagnetic surface domain
imaging even in the presence of magnetic unit-cell doubling.Comment: 23 pages, 10 figures incorporated. Accepted to Phys. Rev. B,
scheduled for July'9
Exotic Meson Decay Widths using Lattice QCD
A decay width calculation for a hybrid exotic meson h, with JPC=1-+, is
presented for the channel h->pi+a1. This quenched lattice QCD simulation
employs Luescher's finite box method. Operators coupling to the h and pi+a1
states are used at various levels of smearing and fuzzing, and at four quark
masses. Eigenvalues of the corresponding correlation matrices yield energy
spectra that determine scattering phase shifts for a discrete set of relative
pi+a1 momenta. Although the phase shift data is sparse, fits to a Breit-Wigner
model are attempted, resulting in a decay width of about 60 MeV when averaged
over two lattice sizes.Comment: 9 pages, 8 figures, RevTex4, minor change to Fig.
Residual meson-meson interaction from lattice gauge simulation in a simple QED model
The residual interaction for a meson-meson system is computed utilizing the
cumulant, or cluster, expansion of the momentum-space time correlation matrix.
The cumulant expansion serves to define asymptotic, or free, meson-meson
operators. The definition of an effective interaction is then based on a
comparison of the full (interacting) and the free (noninteracting) time
correlation matrices. The proposed method, which may straightforwardly be
transcribed to other hadron-hadron systems, here is applied to a simple 2+1
dimensional U(1) lattice gauge model tuned such that it is confining. Fermions
are treated in the staggered scheme. The effective interaction exhibits a
repulsive core and attraction at intermediate relative distances. These
findings are consistent with an earlier study of the same model utilizing
L\"{u}scher's method where scattering phase shifts are obtained directly.Comment: 28 pages, compressed postscript fil
The complex multiferroic phase diagram of MnCoWO
The complete magnetic and multiferroic phase diagram of
MnCoWO single crystals is investigated by means of magnetic,
heat capacity, and polarization experiments. We show that the ferroelectric
polarization in the multiferroic state abruptly changes
its direction twice upon increasing Co content, x. At x=0.075,
rotates from the axis into the plane and at
x=0.15 it flips back to the axis. The origin of the multiple
polarization flops is identified as an effect of the Co anisotropy on the
orientation and shape of the spin helix leading to thermodynamic instabilities
caused by the decrease of the magnitude of the polarization in the
corresponding phases. A qualitative description of the ferroelectric
polarization is derived by taking into account the intrachain (axis) as
well as the interchain (axis) exchange pathways connecting the magnetic
ions. In a narrow Co concentration range (0.1x0.15), an
intermediate phase, sandwiched between the collinear high-temperature and the
helical low-temperature phases, is discovered. The new phase exhibits a
collinear and commensurate spin modulation similar to the low-temperature
magnetic structure of MnWO.Comment: 18 pages, 6 figure
- …