5,843 research outputs found
Persistence exponents for fluctuating interfaces
Numerical and analytic results for the exponent \theta describing the decay
of the first return probability of an interface to its initial height are
obtained for a large class of linear Langevin equations. The models are
parametrized by the dynamic roughness exponent \beta, with 0 < \beta < 1; for
\beta = 1/2 the time evolution is Markovian. Using simulations of
solid-on-solid models, of the discretized continuum equations as well as of the
associated zero-dimensional stationary Gaussian process, we address two
problems: The return of an initially flat interface, and the return to an
initial state with fully developed steady state roughness. The two problems are
shown to be governed by different exponents. For the steady state case we point
out the equivalence to fractional Brownian motion, which has a return exponent
\theta_S = 1 - \beta. The exponent \theta_0 for the flat initial condition
appears to be nontrivial. We prove that \theta_0 \to \infty for \beta \to 0,
\theta_0 \geq \theta_S for \beta
1/2, and calculate \theta_{0,S} perturbatively to first order in an expansion
around the Markovian case \beta = 1/2. Using the exact result \theta_S = 1 -
\beta, accurate upper and lower bounds on \theta_0 can be derived which show,
in particular, that \theta_0 \geq (1 - \beta)^2/\beta for small \beta.Comment: 12 pages, REVTEX, 6 Postscript figures, needs multicol.sty and
epsf.st
Normal-superfluid interaction dynamics in a spinor Bose gas
Coherent behavior of spinor Bose-Einstein condensates is studied in the
presence of a significant uncondensed (normal) component. Normal-superfluid
exchange scattering leads to a near-perfect local alignment between the spin
fields of the two components. Through this spin locking, spin-domain formation
in the condensate is vastly accelerated as the spin populations in the
condensate are entrained by large-amplitude spin waves in the normal component.
We present data evincing the normal-superfluid spin dynamics in this regime of
complicated interdependent behavior.Comment: 5 pages, 4 fig
Observation of anomalous spin-state segregation in a trapped ultra-cold vapor
We observe counter-intuitive spin segregation in an inhomogeneous sample of
ultra-cold, non-condensed Rubidium atoms in a magnetic trap. We use spatially
selective microwave spectroscopy to verify a model that accounts for the
differential forces on two internal spin states. In any simple understanding of
the cloud dynamics, the forces are far too small to account for the dramatic
transient spin polarizations observed. The underlying mechanism remains to be
elucidated.Comment: 5 pages, 3 figure
Output coupling of a Bose-Einstein condensate formed in a TOP trap
Two distinct mechanisms are investigated for transferring a pure 87Rb
Bose-Einstein condensate in the F = 2, mF = 2 state into a mixture of
condensates in all the mF states within the F = 2 manifold. Some of these
condensates remain trapped whilst others are output coupled in the form of an
elementary pulsed atom laser. Here we present details of the condensate
preparation and results of the two condensate output coupling schemes. The
first scheme is a radio frequency technique which allows controllable transfer
into available mF states, and the second makes use of Majorana spin flips to
equally populate all the manifold sub-states.Comment: 12 Pages, 5 Figures, submitted to J. Phys.
Quadratic Forms in Singular Normal Variables
16 pages, 1 article*Quadratic Forms in Singular Normal Variables* (Rodhe, C. A.; Urquhart, N. S.; Searle, S. R.) 16 page
Effects of climate-induced changes in isoprene emissions after the eruption of Mount Pinatubo
In the 1990s the rates of increase of greenhouse gas concentrations, most notably of methane, were observed to change, for reasons that have yet to be fully determined. This period included the eruption of Mt. Pinatubo and an El Nino warm event, both of which affect biogeochemical processes, by changes in temperature, precipitation and radiation. We examine the impact of these changes in climate on global isoprene emissions and the effect these climate dependent emissions have on the hydroxy radical, OH, the dominant sink for methane. We model a reduction of isoprene emissions in the early 1990s, with a maximum decrease of 40 Tg(C)/yr in late 1992 and early 1993, a change of 9%. This reduction is caused by the cooler, drier conditions following the eruption of Mt. Pinatubo. Isoprene emissions are reduced both directly, by changes in temperature and a soil moisture dependent suppression factor, and indirectly, through reductions in the total biomass. The reduction in isoprene emissions causes increases of tropospheric OH which lead to an increased sink for methane of up to 5 Tg(CH4)/year, comparable to estimated source changes over the time period studied. There remain many uncertainties in the emission and oxidation of isoprene which may affect the exact size of this effect, but its magnitude is large enough that it should remain important
Renormalization Group Study of the A+B->0 Diffusion-Limited Reaction
The diffusion-limited reaction, with equal initial densities
, is studied by means of a field-theoretic renormalization
group formulation of the problem. For dimension an effective theory is
derived, from which the density and correlation functions can be calculated. We
find the density decays in time as a,b \sim C\sqrt{\D}(Dt)^{-d/4} for , with \D = n_0-C^\prime n_0^{d/2} + \dots, where is a universal
constant, and is non-universal. The calculation is extended to the
case of unequal diffusion constants , resulting in a new
amplitude but the same exponent. For a controlled calculation is not
possible, but a heuristic argument is presented that the results above give at
least the leading term in an expansion. Finally, we address
reaction zones formed in the steady-state by opposing currents of and
particles, and derive scaling properties.Comment: 17 pages, REVTeX, 13 compressed figures, included with epsf. Eq.
(6.12) corrected, and a moderate rewriting of the introduction. Accepted for
publication in J. Stat. Phy
TeV-scale electron Compton scattering in the Randall-Sundrum scenario
The spin-2 graviton excitations in the Randall-Sundrum gravity model provides
a t-channel contribution to electron Compton scattering which competes
favourably with the standard QED contributions. The phenomenological
implications of these contributions to the unpolarized and polarized
cross-sections are evaluated.Comment: 11 pages, 5 figure
Global Persistence Exponent for Critical Dynamics
A `persistence exponent' is defined for nonequilibrium critical
phenomena. It describes the probability, , that the
global order parameter has not changed sign in the time interval following
a quench to the critical point from a disordered state. This exponent is
calculated in mean-field theory, in the limit of the model,
to first order in , and for the 1-d Ising model. Numerical
results are obtained for the 2-d Ising model. We argue that is a new
independent exponent.Comment: 4 pages, revtex, one figur
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