1,459 research outputs found
Magnetic resonance imaging (MRI) of heavy-metal transport and fate in an artificial biofilm
Unlike planktonic systems, reaction rates in biofilms are often limited by mass transport, which controls the rate of supply of contaminants into the biofilm matrix. To help understand this phenomenon, we investigated the potential of magnetic resonance imaging (MRI) to spatially quantify copper transport and fate in biofilms. For this initial study we utilized an artificial biofilm composed of a 50:50 mix of bacteria and agar. MRI successfully mapped Cu2+ uptake into the artificial biofilm by mapping T2 relaxation rates. A calibration protocol was used to convert T2 values into actual copper concentrations. Immobilization rates in the artificial biofilm were slow compared to the rapid equilibration of planktonic systems. Even after 36 h, the copper front had migrated only 3 mm into the artificial biofilm and at this distance from the copper source, concentrations were very low. This slow equilibration is a result of (1) the time it takes copper to diffuse over such distances and (2) the adsorption of copper onto cell surfaces, which further impedes copper diffusion. The success of this trial run indicates MRI could be used to quantitatively map heavy metal transport and immobilization in natural biofilms
Bound and free waves in non-collinear second harmonic generation
We analyze the relationship between the bound and the free waves in the
noncollinear SHG scheme, along with the vectorial conservation law for the
different components arising when there are two pump beams impinging on the
sample with two different incidence angles. The generated power is
systematically investigated, by varying the polarization state of both
fundamental beams, while absorption is included via the Herman and Hayden
correction terms. The theoretical simulations, obtained for samples which are
some coherence length thick show that the resulting polarization mapping is an
useful tool to put in evidence the interference between bound and free waves,
as well as the effect of absorption on the interference patternComment: 10 pages, 7 figure. to be published on Optics Expres
Quantum theory of light and noise polarization in nonlinear optics
We present a consistent quantum theory of the electromagnetic field in
nonlinearly responding causal media, with special emphasis on
media. Starting from QED in linearly responding causal media, we develop a
method to construct the nonlinear Hamiltonian expressed in terms of the complex
nonlinear susceptibility in a quantum mechanically consistent way. In
particular we show that the method yields the nonlinear noise polarization,
which together with the linear one is responsible for intrinsic quantum
decoherence.Comment: 4 pages, no figure
A time-dependent density functional theory scheme for efficient calculations of dynamic (hyper)polarizabilities
We present an efficient perturbative method to obtain both static and dynamic
polarizabilities and hyperpolarizabilities of complex electronic systems. This
approach is based on the solution of a frequency dependent Sternheimer
equation, within the formalism of time-dependent density functional theory, and
allows the calculation of the response both in resonance and out of resonance.
Furthermore, the excellent scaling with the number of atoms opens the way to
the investigation of response properties of very large molecular systems. To
demonstrate the capabilities of this method, we implemented it in a real-space
(basis-set free) code, and applied it to benchmark molecules, namely CO, H2O,
and paranitroaniline (PNA). Our results are in agreement with experimental and
previous theoretical studies, and fully validate our approach.Comment: 9 pages, 4 figure
On rational delegations in liquid democracy
Liquid democracy is a proxy voting method where proxies are delegable. We propose and study a game-theoretic model of liquid democracy to address the following question: when is it rational for a voter to delegate her vote? We study the existence of pure-strategy Nash equilibria in this model, and how group accuracy is affected by them. We complement these theoretical results by means of agent-based simulations to study the effects of delegations on group's accuracy on variously structured social networks
On rational delegations in liquid democracy
Liquid democracy is a proxy voting method where proxies are delegable. We propose and study a game-theoretic model of liquid democracy to address the following question: when is it rational for a voter to delegate her vote? We study the existence of pure-strategy Nash equilibria in this model, and how group accuracy is affected by them. We complement these theoretical results by means of agent-based simulations to study the effects of delegations on group's accuracy on variously structured social networks
Hole Doping Effects on Spin-gapped Na2Cu2TeO6 via Topochemical Na Deficiency
We report the magnetic susceptibility and NMR studies of a spin-gapped
layered compound
Na2Cu2TeO6 (the spin gap 250 K), the hole doping effect on the
Cu2TeO6 plane via a topochemical Na deficiency by soft chemical treatment, and
the static spin vacancy effect by nonmagnetic impurity Zn substitution for Cu.
A finite Knight shift at the Te site was observed for pure
Na2Cu2TeO6.
The negative hyperfine coupling constant is an evidence for
the existence of a superexchange pathway of the Cu-O-Te-O-Cu bond. It turned
out that both the Na deficiency and Zn impurities induce a Curie-type magnetism
in the uniform spin susceptibility in an external magnetic field of 1 T, but
only the Zn impurities enhance the low-temperature Na nuclear
spin-lattice relaxation rate whereas the Na deficiency suppresses it. A spin
glass behavior was observed for the Na-deficient samples but not for the
Zn-substituted samples. The dynamics of the unpaired moments of the doped holes
are different from that of the spin vacancy in the spin-gapped Cu2TeO6 planes.Comment: 4 pages, 7 figures, to be published in J. Phys. Soc. Jpn. Vol. 75,
No. 8 (2006
Comparison of Quantum and Classical Local-field Effects on Two-Level Atoms in a Dielectric
The macroscopic quantum theory of the electromagnetic field in a dielectric
medium interacting with a dense collection of embedded two-level atoms fails to
reproduce a result that is obtained from an application of the classical
Lorentz local-field condition. Specifically, macroscopic quantum
electrodynamics predicts that the Lorentz redshift of the resonance frequency
of the atoms will be enhanced by a factor of the refractive index n of the host
medium. However, an enhancement factor of (n*n+2)/3 is derived using the
Bloembergen procedure in which the classical Lorentz local-field condition is
applied to the optical Bloch equations. Both derivations are short and
uncomplicated and are based on well-established physical theories, yet lead to
contradictory results. Microscopic quantum electrodynamics confirms the
classical local-field-based results. Then the application of macroscopic
quantum electrodynamic theory to embedded atoms is proved false by a specific
example in which both the correspondence principle and microscopic theory of
quantum electrodynamics are violated.Comment: Published version with rewritten abstract and introductio
Influence of damping on the vanishing of the electro-optic effect in chiral isotropic media
Using first principles, it is demonstrated that radiative damping alone
cannot lead to a nonvanishing electro-optic effect in a chiral isotropic
medium. This conclusion is in contrast with that obtained by a calculation in
which damping effects are included using the standard phenomenological model.
We show that these predictions differ because the phenomenological damping
equations are valid only in regions where the frequencies of the applied
electromagnetic fields are nearly resonant with the atomic transitions. We also
show that collisional damping can lead to a nonvanishing electrooptic effect,
but with a strength sufficiently weak that it is unlikely to be observable
under realistic laboratory conditions
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