11,180 research outputs found
Locality with staggered fermions
We address the locality problem arising in simulations, which take the square
root of the staggered fermion determinant as a Boltzmann weight to reduce the
number of dynamical quark tastes. A definition of such a theory necessitates an
underlying local fermion operator with the same determinant and the
corresponding Green's functions to establish causality and unitarity. We
illustrate this point by studying analytically and numerically the square root
of the staggered fermion operator. Although it has the correct weight, this
operator is non-local in the continuum limit. Our work serves as a warning that
fundamental properties of field theories might be violated when employing
blindly the square root trick. The question, whether a local operator
reproducing the square root of the staggered fermion determinant exists, is
left open.Comment: 24 pages, 7 figures, few remarks added for clarity, accepted for
publication in Nucl. Phys.
Magnetic structure of the edge-sharing copper oxide chain compound NaCu2O2
Single-crystal neutron diffraction has been used to determine the
incommensurate magnetic structure of NaCu2O2, a compound built up of chains of
edge-sharing CuO4 plaquettes. Magnetic structures compatible with the lattice
symmetry were identified by a group-theoretical analysis, and their magnetic
structure factors were compared to the experimentally observed Bragg
intensities. In conjunction with other experimental data, this analysis yields
an elliptical helix structure in which both the helicity and the polarization
plane alternate among copper-oxide chains. This magnetic ground state is
discussed in the context of the recently reported multiferroic properties of
other copper-oxide chain compounds
Magnetic properties of PdAs2O6: a dilute spin system with an unusually high N\'eel temperature
The crystal structure and magnetic ordering pattern of PdAs2O6 were
investigated by neutron powder diffraction. While the magnetic structure of
PdAs2O6 is identical to the one of its isostructural 3d-homologue NiAs2O6, its
N\'{e}el temperature (140 K) is much higher than the one of NiAs2O6 (30 K).
This is surprising in view of the long distance and indirect exchange path
between the magnetic Pd ions. Density functional calculations yield
insight into the electronic structure and the geometry of the exchange-bond
network of both PdAs2O6 and NiAs2O6, and provide a semi-quantitative
explanation of the large amplitude difference between their primary exchange
interaction parameters
InAs-AlSb quantum wells in tilted magnetic fields
InAs-AlSb quantum wells are investigated by transport experiments in magnetic
fields tilted with respect to the sample normal. Using the coincidence method
we find for magnetic fields up to 28 T that the spin splitting can be as large
as 5 times the Landau splitting. We find a value of the g-factor of about 13.
For small even-integer filling factors the corresponding minima in the
Shubnikov-de Haas oscillations cannot be tuned into maxima for arbitrary tilt
angles. This indicates the anti-crossing of neighboring Landau and spin levels.
Furthermore we find for particular tilt angles a crossover from even-integer
dominated Shubnikov-de Haas minima to odd-integer minima as a function of
magnetic field
The eta' meson from lattice QCD
We study the flavour singlet pseudoscalar mesons from first principles using
lattice QCD. With N_f=2 flavours of light quark, this is the so-called eta_2
meson and we discuss the phenomenological status of this. Using maximally
twisted-mass lattice QCD, we extract the mass of the eta_2 meson at two values
of the lattice spacing for lighter quarks than previously discussed in the
literature. We are able to estimate the mass value in the limit of light quarks
with their physical masses.Comment: 16 pages: version accepted for publicatio
Helicoidal magnetic order in a clean copper oxide spin chain compound
We report susceptibility, specific heat, and neutron diffraction measurements
on NaCuO, a spin-1/2 chain compound isostructural to LiCuO,
which has been extensively investigated. Below 13 K, we find a long-range
ordered, incommensurate magnetic helix state with a propagation vector similar
to that of LiCuO. In contrast to the Li analogue, substitutional
disorder is negligible in NaCuO. We can thus rule out that the helix is
induced by impurities, as was claimed on the basis of prior work on
LiCuO. A spin Hamiltonian with frustrated longer-range exchange
interactions provides a good description of both the ordered state and the
paramagnetic susceptibility.Comment: 4 pages, 4 figures Improved Fig.1 and 4. Minor rephrasing. Reference
adde
The locality problem for two tastes of staggered fermions
We address the locality problem arising in simulations, which take the square
root of the staggered fermion determinant as a Boltzmann weight to reduce the
number of dynamical quark tastes from four to two. We study analytically and
numerically the square root of the staggered fermion operator as a candidate to
define a two taste theory from first principles. Although it has the correct
weight, this operator is non-local in the continuum limit. Our work serves as a
warning that fundamental properties of field theories might be violated when
employing blindly the square root trick. The question, whether a local operator
reproducing the square root of the staggered fermion determinant exists, is
left open.Comment: Talk presented at Lattice2004(theory), Fermilab, June 21-26, 200
Magnetic excitations and phonons in the spin-chain compound NaCu2O2
We report an inelastic light scattering study of single-crystalline
NaCuO, a spin-chain compound known to exhibit a phase with helical
magnetic order at low temperatures. Phonon excitations were studied as a
function of temperature and light polarization, and the phonon frequencies are
compared to the results of ab-initio lattice dynamical calculations, which are
also reported here. The good agreement between the observed and calculated
modes allows an assignment of the phonon eigenvectors. Two distinct high-energy
two-magnon features as well as a sharp low-energy one-magnon peak were also
observed. These features are discussed in terms of the magnon modes expected in
a helically ordered state. Their polarization dependence provides evidence of
substantial exchange interactions between two closely spaced spin chains within
a unit cell. At high temperatures, the spectral features attributable to
magnetic excitations are replaced by a broad, quasielastic mode due to
overdamped spin excitations
A new simulation algorithm for lattice QCD with dynamical quarks
A previously introduced multi-boson technique for the simulation of QCD with
dynamical quarks is described and some results of first test runs on a
lattice with Wilson quarks and gauge group SU(2) are reported.Comment: 7 pages, postscript file (166 KB
Four-dimensional Simulation of the Hot Electroweak Phase Transition with the SU(2) Gauge-Higgs Model
We study the finite-temperature phase transition of the four-dimensional
SU(2) gauge-Higgs model for intermediate values of the Higgs boson mass in the
range 50 \lsim m_H \lsim 100GeV on a lattice with the temporal lattice size
. The order of the transition is systematically examined using finite
size scaling methods. Behavior of the interface tension and the latent heat for
an increasing Higgs boson mass is also investigated.Comment: Talk presented at LATTICE96(electroweak), 3 pages of LaTeX, 4
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