5,017 research outputs found
Generalized mean-field approach to simulate large dissipative spin ensembles with long range interactions
We simulate the collective dynamics in spin lattices with long range
interactions and collective decay in one, two and three dimensions. Starting
from a dynamical mean-field approach derived by local factorization of the
density operator we improve the numerical approximation of the full master
equation by including pair correlations at any distance. This truncations
enable us to drastically increase the number of spins in our numerical
simulations from about ten spins in case of the full quantum model to several
ten-thousands in the mean-field approximation and a few hundreds if pair
correlations are included. Extensive numerical tests help us identify
interaction strengths and geometric configurations where these approximations
perform well and allow us to state fairly simple error estimates. By simulating
systems of increasing size we show that in one and two dimensions we can
include as many spins as needed to capture the properties of infinite size
systems with high accuracy, while in 3D the method does not converge to desired
accuracy within the system sizes we can currently implement. Our approach is
well suited to give error estimates of magic wavelength optical lattices for
atomic clock applications and corresponding super radiant lasers
Hemodynamic and inotropic effects of endothelin-1 in vivo
Endothelin-1 (ET-1) is known to have strong vasoactive properties. Contradictory results have been reported with regard to its inotropic effects. This study examined the dose-dependent (500, 1000, 2500, 5000 and 10,000 ng ET-1/kg vs. NaCl controls) hemodynamic and inotropic effects of ET-1 in 53 open-chest rats during and after a 7-min infusion. Besides measurements in the intact circulation the myocardial function was examined by isovolumic registrations independent of peripheral vascular effects. A transient ET-1 induced (500, 1000, 2500, 5000 ng ET-1/kg) decrease of the left ventricular systolic pressure (LVSP) and the mean aortic pressure (AoPmean) was followed by a dose-related rise of these pressures (LVSP: -1%, -1%, +8%, +16% vs. preinfusion values; AoPmean: -11%, +9%, +39%, +52%). Heart rate (HR) was not influenced by ET-1. Due to the dose-dependent decrease of the stroke volume (SV) the cardiac output (CO) was reduced (CO: -8%, -23%, -40%, -50%). After an initial vasodilatation ET-1 elevates the total peripheral resistance (TPR: -1%, +49%, +139%, +215%) dose-dependently. 10,000 ng ET-1/kg was a lethal dose resulting in cardiac failure within minutes (low output). Since the maximum of the isovolumic LVSP (peak LVSP) and the corresponding dP/dtmax (peak dP/dtmax) were unchanged under ET-1, the isovolumic measurements do not indicate a positive inotropic effect of ET-1 in vivo in contrast to published results of in vitro experiments. It may be possible that a direct positive inotropic effect of ET-1 observed in in vitro studies is counterbalanced in vivo by an indirect negative inotropic effect due to the coronary-constrictive effect of ET-1
Higgs radiation off quarks in supersymmetric theories at e^+e^- colliders
Yukawa couplings between Higgs bosons and quarks in supersymmetric theories
can be measured in the processes e^+e^- -> Q Qbar + Higgs. We have determined
the cross sections of these processes in the minimal supersymmetric model
including the complete set of next-to-leading order QCD corrections for all
channels.Comment: 12 pages, latex, 3 figure
Precision Calculations for Associated WH and ZH Production at Hadron Colliders
Recently the next-to-next-to-leading order QCD corrections and the
electroweak O(alpha) corrections to the Higgs-strahlung processes ppbar/pp ->
WH/ZH + X have been calculated. Both types of corrections are of the order of
5-10%. In this article the various corrections are briefly discussed and
combined into state-of-the-art predictions for the cross sections. The
theoretical uncertainties from renormalization/factorization scales and from
the parton distribution functions are discussed.Comment: 8 pages, Contributed to 3rd Les Houches Workshop: Physics at TeV
Collider
Spin Configuration in the 1/3 Magnetization Plateau of Azurite Determined by NMR
High magnetic field Cu NMR spectra were used to determine the local
spin polarization in the 1/3 magnetization plateau of azurite,
Cu(CO)(OH), which is a model system for the distorted diamond
antiferromagnetic spin-1/2 chain. The spin part of the hyperfine field of the
Cu2 (dimer) sites is found to be field independent, negative and strongly
anisotropic, corresponding to 10 % of fully polarized spin in a
-orbital. This is close to the expected configuration of the "quantum"
plateau, where a singlet state is stabilized on the dimer. However, the
observed non-zero spin polarization points to some triplet admixture, induced
by strong asymmetry of the diamond bonds and .Comment: Phys. Rev. Lett. 102, in press (2009
U(1)-Symmetry breaking and violation of axial symmetry in TlCuCl3 and other insulating spin systems
We describe the Bose-Einstein condensate of magnetic bosonic quasiparticles
in insulating spin systems using a phenomenological standard functional method
for T = 0. We show that results that are already known from advanced
computational techniques immediately follow. The inclusion of a perturbative
anisotropy term that violates the axial symmetry allows us to remarkably well
explain a number of experimental features of the dimerized spin-1/2 system
TlCuCl3. Based on an energetic argument we predict a general intrinsic
instability of an axially symmetric magnetic condensate towards a violation of
this symmetry, which leads to the spontaneous formation of an anisotropy gap in
the energy spectrum above the critical field. We, therefore, expect that a true
Goldstone mode in insulating spin systems, i.e., a strictly linear
energy-dispersion relation down to arbitrarily small excitations energies,
cannot be observed in any real material.Comment: 6 pages, 3 figure
Dimensional reduction by pressure in the magnetic framework material CuF(DO)pyz: from spin-wave to spinon excitations
Metal organic magnets have enormous potential to host a variety of electronic
and magnetic phases that originate from a strong interplay between the spin,
orbital and lattice degrees of freedom. We control this interplay in the
quantum magnet CuF(DO)pyz by using high pressure to drive the
system through a structural and magnetic phase transition. Using neutron
scattering, we show that the low pressure state, which hosts a two-dimensional
square lattice with spin-wave excitations and a dominant exchange coupling of
0.89 meV, transforms at high pressure into a one-dimensional spin-chain
hallmarked by a spinon continuum and a reduced exchange interaction of 0.43
meV. This direct microscopic observation of a magnetic dimensional crossover as
a function of pressure opens up new possibilities for studying the evolution of
fractionalised excitations in low dimensional quantum magnets and eventually
pressure-controlled metal--insulator transitions
Spatially Resolved Magnetization in the Bose-Einstein Condensed State of BaCuSi2O6: Evidence for Imperfect Frustration
In order to understand the nature of the two-dimensional Bose-Einstein
condensed (BEC) phase in BaCuSi2O6, we performed detailed 63Cu and 29Si NMR
above the critical magnetic field, Hc1= 23.4 T. The two different alternating
layers present in the system have very different local magnetizations close to
Hc1; one is very weak, and its size and field dependence are highly sensitive
to the nature of inter-layer coupling. Its precise value could only be
determined by "on-site" 63Cu NMR, and the data are fully reproduced by a model
of interacting hard-core bosons in which the perfect frustration associated to
tetragonal symmetry is slightly lifted, leading to the conclusion that the
population of the less populated layers is not fully incoherent but must be
partially condensed
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