16,783 research outputs found
Specific heat and energy for the three-dimensional O(2) model
We investigate the three-dimensional O(2) model on lattices of size 8^3 to
160^3 close to the critical point at zero magnetic field. We confirm explicitly
the value of the critical coupling J_c found by Ballesteros et al. and estimate
there the universal values of g_r and xi/L. At the critical point we study the
finite size dependencies of the energy density epsilon and the specific heat C.
We find that the nonsingular part of the specific heat C_{ns} is linearly
dependent on 1/alpha. From the critical behaviour of the specific heat for T
not T_c on the largest lattices we determine the universal amplitude ratio
A+/A-. The alpha- dependence of this ratio is close to the phenomenological
relation A+/A- = 1-4alpha.Comment: Lattice2001(spin), 3 pages, 4 figure
The chiral transition of N_f=2 QCD with fundamental and adjoint fermions
We study QCD with two staggered Dirac fermions both in the fundamental (QCD)
and the adjoint representation (aQCD) near the chiral transition. The aim is to
find the universality class of the chiral transition and to verify Goldstone
effects below the transition. We investigate aQCD, because in that theory the
deconfinement and the chiral transitions occur at different temperatures
T_d<T_c. Here, we show that the scaling behaviour of the chiral condensate in
the vicinity of \beta_c is in full agreeement with that of the 3d O(2)
universality class. In the region T_d<T<T_c we confirm the quark mass
dependence of the chiral condensate which is expected due to the existence of
Goldstone modes like in 3d O(N) spin models. For fundamental QCD we use the
p4-action. Here, we find Goldstone effects below T_c like in aQCD and the 3d
O(N) spin models, however no O(2)/O(4) scaling near the chiral transition
point. The result for QCD may be a consequence of the coincidence of the
deconfinement transition with the chiral transition.Comment: 6 pages, 5 figures, poster contribution to Lattice 2005 (Nonzero
temperature and density), one reference added, figure 2 change
Optomechanics with molecules in a strongly pumped ring cavity
Cavity cooling of an atom works best on a cyclic optical transition in the
strong coupling regime near resonance, where small cavity photon numbers
suffice for trapping and cooling. Due to the absence of closed transitions a
straightforward application to molecules fails: optical pumping can lead the
particle into uncoupled states. An alternative operation in the far
off-resonant regime generates only very slow cooling due to the reduced
field-molecule coupling. We predict to overcome this by using a strongly driven
ring-cavity operated in the sideband cooling regime. As in the optomechanical
setups one takes advantage of a collectively enhanced field-molecule coupling
strength using a large photon number. A linearized analytical treatment
confirmed by full numerical quantum simulations predicts fast cooling despite
the off-resonant small single molecule - single photon coupling. Even ground
state cooling can be obtained by tuning the cavity field close to the
Anti-stokes sideband for sufficiently high trapping frequency. Numerical
simulations show quantum jumps of the molecules between the lowest two trapping
levels, which can be be directly and continuously monitored via scattered light
intensity detection
Ionization by bulk heating of electrons in capacitive radio frequency atmospheric pressure microplasmas
Electron heating and ionization dynamics in capacitively coupled radio
frequency (RF) atmospheric pressure microplasmas operated in helium are
investigated by Particle in Cell simulations and semi-analytical modeling. A
strong heating of electrons and ionization in the plasma bulk due to high bulk
electric fields are observed at distinct times within the RF period. Based on
the model the electric field is identified to be a drift field caused by a low
electrical conductivity due to the high electron-neutral collision frequency at
atmospheric pressure. Thus, the ionization is mainly caused by ohmic heating in
this "Omega-mode". The phase of strongest bulk electric field and ionization is
affected by the driving voltage amplitude. At high amplitudes, the plasma
density is high, so that the sheath impedance is comparable to the bulk
resistance. Thus, voltage and current are about 45{\deg} out of phase and
maximum ionization is observed during sheath expansion with local maxima at the
sheath edges. At low driving voltages, the plasma density is low and the
discharge becomes more resistive resulting in a smaller phase shift of about
4{\deg}. Thus, maximum ionization occurs later within the RF period with a
maximum in the discharge center. Significant analogies to electronegative low
pressure macroscopic discharges operated in the Drift-Ambipolar mode are found,
where similar mechanisms induced by a high electronegativity instead of a high
collision frequency have been identified
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