244 research outputs found
Ratchet effect for cold atoms in an optical lattice
The realization of a directed current for a quantum particle in a flashing
asymmetric potential is studied. It is found that a positive current, i.e. in
the direction expected for a conventional diffusive ratchet, can be attained at
short times in the limit where the potential is weak and quantum diffusion
dominates, while current reversal is obtained for stronger potentials. A single
parameter, the ratio between the kicking frequency and the optical lattice
potential strength, determines both the degree to which quantum effects
dominate, and the possibility of obtaining a ratchet current. The effect should
be readily observable in experiments.Comment: 4 pages, 3 figure
Quantum Critical Dynamics Simulation of Dirty Boson Systems
Recently the scaling result for the dynamic critical exponent at the
Bose glass to superfluid quantum phase transition has been questioned both on
theoretical and numerical grounds. This motivates a careful evaluation of the
critical exponents in order to determine the actual value of . We study a
model of quantum bosons at T=0 with disorder in 2D using highly effective worm
Monte Carlo simulations. Our data analysis is based on a finite size scaling
approach to determine the scaling of the quantum correlation time from
simulation data for boson world lines. The resulting critical exponents are
and , hence suggesting
that is not satisfied.Comment: 4 pages 4 figure
Critical Scaling Properties at the Superfluid Transition of He in Aerogel
We study the superfluid transition of He in aerogel by Monte Carlo
simulations and finite size scaling analysis. Aerogel is a highly porous silica
glass, which we model by a diffusion limited cluster aggregation model. The
superfluid is modeled by a three dimensional XY model, with excluded bonds to
sites on the aerogel cluster. We obtain the correlation length exponent
, in reasonable agreement with experiments and with previous
simulations. For the heat capacity exponent , both experiments and
previous simulations suggest deviations from the Josephson hyperscaling
relation . In contrast, our Monte Carlo results support
hyperscaling with . We suggest a reinterpretation of
previous experiments, which avoids scaling violations and is consistent with
our simulation results.Comment: 4 pages, 3 figure
Assessment of nitrogen pollution reduction options in the river Nemunas (Lithuania) using FyrisNP model
The paper quantifies and discusses diffuse and point sources total nitrogen (TN) inputs as well as retention and TN reduction options in the catchment of the main Lithuanian River Nemunas. Modelled average TN export between 2000–2006 from the River Nemunas catchment to the Baltic Sea was 37620 tonnes TN yr−1 according to the data oriented FyrisNP model. Loads of TN from diffuse and point sources as well as retention have been estimated for five subcatchments of the River Nemunas including the external load from Belarus. Agriculture contributes 74.6 to 89.5% of the TN load, increasing with the percentage of arable land and load from point sources. The main point source input is poorly treated wastewater at Kaunas city. The contribution from forest land to the TN load increases from 2.2% to 15.8% with an increase in forest land from 28.5 to 56.9% of the total subcatchments area. The highest retention of TN (30.7%) was observed in the Neris river subcatchment with the lowest hydraulic load (5.55 m yr−1). Scenario modelling suggests that the reduction target for Lithuania for nitrogen input to the Baltic Sea by 11700 tonnes can be achieved by installing biological treatment in sewage treatment plants in all district cities and by converting 20% of arable land to pastures or implementation of other equivalent measures in agriculture. Assessment of the FyrisNP model results shows that the model can be successfully applied for river basin management planning in catchments outside the area where the model originally has been developed.
First published online: 11 Oct 201
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