1,563 research outputs found
Freeze–thaw cycles have minimal effect on the mineralisation of low molecular weight, dissolved organic carbon in Arctic soils
Warmer winters in Arctic regions may melt
insulating snow cover and subject soils to more freeze–
thaw cycles. The effect of freeze–thaw cycles on the
microbial use of low molecular weight, dissolved organic
carbon (LMW-DOC) is poorly understood. In this study,
soils from the Arctic heath tundra, Arctic meadow tundra
and a temperate grassland were frozen to -7.5 C and
thawed once and three times. Subsequently, the mineralisation of 3 LMW-DOC substrates types (sugars, amino
acids and peptides) was measured over an 8-day period and
compared to controls which had not been frozen. This
allowed the comparison of freeze–thaw effects between
Arctic and temperate soil and between different substrates.
The results showed that freeze–thaw cycles had no significant effect on C mineralisation in the Arctic tundra
soils. In contrast, for the same intensity freeze–thaw cycles,
a significant effect on C mineralisation was observed for all
substrate types in the temperate soil although the response
was substrate specific. Peptide and amino acid mineralisation were similarly affected by FT, whilst glucose had a
different response. Further work is required to fully
understand microbial use of LMW-DOC after freeze–thaw,
yet these results suggest that relatively short freeze–thaw
cycles have little effect on microbial use of LMW-DOC in
Arctic tundra soils after thaw
A gamma- and X-ray detector for cryogenic, high magnetic field applications
As part of an experiment to measure the spectrum of photons emitted in
beta-decay of the free neutron, we developed and operated a detector consisting
of 12 bismuth germanate (BGO) crystals coupled to avalanche photodiodes (APDs).
The detector was operated near liquid nitrogen temperature in the bore of a
superconducting magnet and registered photons with energies from 5 keV to 1000
keV. To enlarge the detection range, we also directly detected soft X-rays with
energies between 0.2 keV and 20 keV with three large area APDs. The
construction and operation of the detector is presented, as well as information
on operation of APDs at cryogenic temperatures
Out-of-equilibrium evolution of quantum fields in the hybrid model with quantum back reaction
The hybrid model with a scalar "inflaton" field coupled to a "Higgs" field
with a broken symmetry potential is one of the promising models for inflation
and (p)reheating after inflation. We consider the nonequilibrium evolution of
the quantum fields of this model with quantum back reaction in the Hartree
approximation, in particular the transition of the Higgs field from the
metastable "false vacuum" to the broken symmetry phase. We have performed the
renormalization of the equations of motion, of the gap equations and of the
energy density, using dimensional regularization. We study the influence of the
back reaction on the evolution of the classical fields and of the quantum
fluctuations. We observe that back reaction plays an important role over a wide
range of parameters. Some implications of our investigation for the preheating
stage after cosmic inflation are presented.Comment: 35 pages, 16 eps figures, revtex4; v2: typos corrected and references
added, accepted for publication in Physical Review
Vortex lattice of a Bose-Einstein Condensate in a rotating anisotropic trap
We study the vortex lattices in a Bose-Einstein Condensate in a rotating
anisotropic harmonic trap. We first investigate the single particle
wavefunctions obtained by the exact solution of the problem and give simple
expressions for these wavefunctions in the small anisotropy limit. Depending on
the strength of the interactions, a few or a large number of vortices can be
formed. In the limit of many vortices, we calculate the density profile of the
cloud and show that the vortex lattice stays triangular. We also find that the
vortex lattice planes align themselves with the weak axis of the external
potential. For a small number of vortices, we numerically solve the
Gross-Pitaevskii equation and find vortex configurations that are very
different from the vortex configurations in an axisymmetric rotating trap.Comment: 15 pages,4 figure
A Grand Canonical Ensemble Approach to the Thermodynamic Properties of the Nucleon in the Quark-Gluon Coupling Model
In this paper, we put forward a way to study the nucleon's thermodynamic
properties such as its temperature, entropy and so on, without inputting any
free parameters by human hand, even the nucleon's mass and radius. First we use
the Lagrangian density of the quark gluon coupling fields to deduce the Dirac
Equation of the quarks confined in the gluon fields. By boundary conditions we
solve the wave functions and energy eigenvalues of the quarks, and thus get
energy-momentum tensor, nucleon mass, and density of states. Then we utilize a
hybrid grand canonical ensemble, to generate the temperature and chemical
potentials of quarks, antiquarks of three flovars by the four conservation laws
of the energy and the valence quark numbers, after which, all other
thermodynamic properties are known. The only seemed free paremeter, the nucleon
radius is finally determined by the grand potential minimal principle.Comment: 5 pages, LaTe
Dynamics of coupled bosonic systems with applications to preheating
Coupled, multi-field models of inflation can provide several attractive
features unavailable in the case of a single inflaton field. These models have
a rich dynamical structure resulting from the interaction of the fields and
their associated fluctuations. We present a formalism to study the
nonequilibrium dynamics of coupled scalar fields. This formalism solves the
problem of renormalizing interacting models in a transparent way using
dimensional regularization. The evolution is generated by a renormalized
effective Lagrangian which incorporates the dynamics of the mean fields and
their associated fluctuations at one-loop order. We apply our method to two
problems of physical interest: (i) a simple two-field model which exemplifies
applications to reheating in inflation, and (ii) a supersymmetric hybrid
inflation model. This second case is interesting because inflation terminates
via a smooth phase transition which gives rise to a spinodal instability in one
of the fields. We study the evolution of the zero mode of the fields and the
energy density transfer to the fluctuations from the mean fields. We conclude
that back reaction effects can be significant over a wide parameter range. In
particular for the supersymmetric hybrid model we find that particle production
can be suppressed due to these effects.Comment: 23 pages, 16 eps-figures, minor changes in the text, references
added, accepted for publication in PR
Quantum Extremism: Effective Potential and Extremal Paths
The reality and convexity of the effective potential in quantum field
theories has been studied extensively in the context of Euclidean space-time.
It has been shown that canonical and path-integral approaches may yield
different results, thus resolving the `convexity problem'. We discuss the
transferral of these treatments to Minkowskian space-time, which also
necessitates a careful discussion of precisely which field configurations give
the dominant contributions to the path integral. In particular, we study the
effective potential for the N=1 linear sigma model.Comment: 11 pages, 4 figure
Vortices and dynamics in trapped Bose-Einstein condensates
I review the basic physics of ultracold dilute trapped atomic gases, with
emphasis on Bose-Einstein condensation and quantized vortices. The hydrodynamic
form of the Gross-Pitaevskii equation (a nonlinear Schr{\"o}dinger equation)
illuminates the role of the density and the quantum-mechanical phase. One
unique feature of these experimental systems is the opportunity to study the
dynamics of vortices in real time, in contrast to typical experiments on
superfluid He. I discuss three specific examples (precession of single
vortices, motion of vortex dipoles, and Tkachenko oscillations of a vortex
array). Other unusual features include the study of quantum turbulence and the
behavior for rapid rotation, when the vortices form dense regular arrays.
Ultimately, the system is predicted to make a quantum phase transition to
various highly correlated many-body states (analogous to bosonic quantum Hall
states) that are not superfluid and do not have condensate wave functions. At
present, this transition remains elusive. Conceivably, laser-induced synthetic
vector potentials can serve to reach this intriguing phase transition.Comment: Accepted for publication in Journal of Low Temperature Physics,
conference proceedings: Symposia on Superfluids under Rotation (Lammi,
Finland, April 2010
W Boson Inclusive Decays to Quarkonium at the LHC
In this paper, the production rates of quarkonia eta_c, J/psi, eta_b,
Upsilon, B_c and B_c^* through W boson decay at the LHC are calculated, at the
leading order in both the QCD coupling constant and in v, the typical velocity
of the heavy quark inside of mesons. It shows that a sizable number of
quarkonia from W boson decay will be produced at the LHC. Comparison with the
predictions by using quark fragmentation mechanism is also discussed. Results
show that, for the charmonium production through W decay, the difference
between predictions by the fragmentation mechanism and complete leading order
calculation is around 3%, and it is insensitive to the uncertainties of
theoretical parameters; however, for the bottomonium and B_c^(*) productions,
the difference cannot be ignored as the fragmentation mechanism is less
applicable here due to the relatively large ratio mb/mw.Comment: Updated to match the published version in EPJ
Self-adapting method for the localization of quantum critical points using Quantum Monte Carlo techniques
A generalization to the quantum case of a recently introduced algorithm (Y.
Tomita and Y. Okabe, Phys. Rev. Lett. {\bf 86}, 572 (2001)) for the
determination of the critical temperature of classical spin models is proposed.
We describe a simple method to automatically locate critical points in
(Quantum) Monte Carlo simulations. The algorithm assumes the existence of a
finite correlation length in at least one of the two phases surrounding the
quantum critical point. We illustrate these ideas on the example of the
critical inter-chain coupling for which coupled antiferromagnetic S=1 spin
chains order at T=0. Finite-size scaling relations are used to determine the
exponents, and in agreement with previous
estimates.Comment: 5 pages, 3 figures, published versio
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