610 research outputs found
A unique parametrization of the shapes of secondary dilepton spectra observed in central heavy-ion collisions at CERN-SPS energies
A unique parametrization of secondary (thermal) dilepton yields in heavy-ion
experiments at CERN-SPS is proposed. This parametrization resembles a thermal
annihilation rate. This is inspired by the observation that lepton
pair production rates are quantitatively similar, whether expressed in a
hadronic or partonic basis. Adding the thermal yield and the background
contributions (hadronic cocktail, Drell-Yan, correlated semileptonic decays of
open charm) the spectral shapes of the CERES/NA45, NA38, NA50 and HELIOS/3 data
from experiments with lead and sulfur beams can be well described.Comment: 23 pages including figures (new version: only new output format
Systematic Effective Field Theory Investigation of Spiral Phases in Hole-Doped Antiferromagnets on the Honeycomb Lattice
Motivated by possible applications to the antiferromagnetic precursor of the
high-temperature superconductor NaCoOyHO, we use a systematic
low-energy effective field theory for magnons and holes to study different
phases of doped antiferromagnets on the honeycomb lattice. The effective action
contains a leading single-derivative term, similar to the Shraiman-Siggia term
in the square lattice case, which gives rise to spirals in the staggered
magnetization. Depending on the values of the low-energy parameters, either a
homogeneous phase with four or a spiral phase with two filled hole pockets is
energetically favored. Unlike in the square lattice case, at leading order the
effective action has an accidental continuous spatial rotation symmetry.
Consequently, the spiral may point in any direction and is not necessarily
aligned with a lattice direction.Comment: 10 pages, 6 figure
Spiral phases and two-particle bound states from a systematic low-energy effective theory for magnons, electrons, and holes in an antiferromagnet
We have constructed a systematic low-energy effective theory for hole- and
electron-doped antiferromagnets, where holes reside in momentum space pockets
centered at and where electrons live in
pockets centered at or . The effective
theory is used to investigate the magnon-mediated binding between two holes or
two electrons in an otherwise undoped system. We derive the one-magnon exchange
potential from the effective theory and then solve the corresponding
two-quasiparticle Schr\"odinger equation. As a result, we find bound state wave
functions that resemble -like or -like symmetry. We also
study possible ground states of lightly doped antiferromagnets.Comment: 2 Pages; Proc. of SCES'07, Housto
Microscopic Model versus Systematic Low-Energy Effective Field Theory for a Doped Quantum Ferromagnet
We consider a microscopic model for a doped quantum ferromagnet as a test
case for the systematic low-energy effective field theory for magnons and
holes, which is constructed in complete analogy to the case of quantum
antiferromagnets. In contrast to antiferromagnets, for which the effective
field theory approach can be tested only numerically, in the ferromagnetic case
both the microscopic and the effective theory can be solved analytically. In
this way the low-energy parameters of the effective theory are determined
exactly by matching to the underlying microscopic model. The low-energy
behavior at half-filling as well as in the single- and two-hole sectors is
described exactly by the systematic low-energy effective field theory. In
particular, for weakly bound two-hole states the effective field theory even
works beyond perturbation theory. This lends strong support to the quantitative
success of the systematic low-energy effective field theory method not only in
the ferromagnetic but also in the physically most interesting antiferromagnetic
case.Comment: 34 pages, 1 figur
Water vapor transport in the lower mesosphere of the subtropics: a trajectory analysis
The Institute of Applied Physics operates an airborne microwave radiometer AMSOS that measures the rotational transition line of water vapor at 183.3 GHz. Water vapor profiles are retrieved for the altitude range from 15 to 75 km along the flight track. We report on a water vapor enhancement in the lower mesosphere above India and the Arabian Sea. The measurements took place on our flight from Switzerland to Australia and back in November 2005 conducted during EC- project SCOUT-O3. We find an enhancement of up to 25% in the lower mesospheric H<sub>2</sub>O volume mixing ratio measured on the return flight one week after the outward flight. The origin of the air is traced back by means of a trajectory model in the lower mesosphere and wind fields from ECMWF. During the outward flight the air came from the Atlantic Ocean around 25 N and 40 W. On the return flight the air came from northern India and Nepal around 25 N and 90 E. Mesospheric H<sub>2</sub>O measurements from Aura/MLS confirm the transport processes of H<sub>2</sub>O derived by trajectory analysis of the AMSOS data. Thus the large variability of H<sub>2</sub>O VMR during our flight is explained by a change of the winds in the lower mesosphere. This study shows that trajectory analysis can be applied in the mesosphere and is a powerful tool to understand the large variability in mesospheric H<sub>2</sub>O
Magnon-mediated binding between holes in an antiferromagnet
Abstract.: The long-range forces between holes in an antiferromagnet are due to magnon exchange. The one-magnon exchange potential between two holes is proportional to cos(2 ϕ)/r 2 where r is the distance vector of the holes and ϕ is the angle between r and an axis of the square crystal lattice. One-magnon exchange leads to bound states of holes with antiparallel spins resembling d-wave symmetr
Homogeneous versus Spiral Phases of Hole-doped Antiferromagnets: A Systematic Effective Field Theory Investigation
Using the low-energy effective field theory for magnons and holes -- the
condensed matter analog of baryon chiral perturbation theory for pions and
nucleons in QCD -- we study different phases of doped antiferromagnets. We
systematically investigate configurations of the staggered magnetization that
provide a constant background field for doped holes. The most general
configuration of this type is either constant itself or it represents a spiral
in the staggered magnetization. Depending on the values of the low-energy
parameters, a homogeneous phase, a spiral phase, or an inhomogeneous phase is
energetically favored. The reduction of the staggered magnetization upon doping
is also investigated.Comment: 35 pages, 5 figure
Systematic Low-Energy Effective Field Theory for Magnons and Holes in an Antiferromagnet on the Honeycomb Lattice
Based on a symmetry analysis of the microscopic Hubbard and t-J models, a
systematic low-energy effective field theory is constructed for hole-doped
antiferromagnets on the honeycomb lattice. In the antiferromagnetic phase,
doped holes are massive due to the spontaneous breakdown of the
symmetry, just as nucleons in QCD pick up their mass from spontaneous chiral
symmetry breaking. In the broken phase the effective action contains a
single-derivative term, similar to the Shraiman-Siggia term in the square
lattice case. Interestingly, an accidental continuous spatial rotation symmetry
arises at leading order. As an application of the effective field theory we
consider one-magnon exchange between two holes and the formation of two-hole
bound states. As an unambiguous prediction of the effective theory, the wave
function for the ground state of two holes bound by magnon exchange exhibits
-wave symmetry.Comment: 33 pages, 6 figure
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