204 research outputs found
Currentâvoltage curves of bipolar membranes
Bipolar membranes consist of a layered ionâexchange structure composed of a cation selective membrane joined to an anion selective membrane. They are analogous to semiconductor pân devices as both of them present currentâvoltage curves exhibiting similar rectification properties. In this article, we present some currentâvoltage curves obtained for different bipolar membranes at several temperatures. The results can be interpreted in terms of a simple model for ion transport and fieldâenhanced water dissociation previously developed. The mechanism responsible for water splitting is assumed to be a catalytic proton transfer reaction between the charged groups and the water at the membrane interface. The effects of temperature are taken into account by introducing an Arrheniusâtype relationship for the dependence of the forward rate constant of the reaction on temperature. Finally, comparison between theory and experiments provides reasonable values for the parameters introduced in the theoretical model. The analysis aims at developing a better physical understanding of a process in which chemical reactions and transport phenomena are coupled in such a way that the potential technological applications depend strongly on this [email protected]
Charakterisierung des elektrochemischen Mikroanalysesystems ELMAS
Am Forschungszentrum Karlsruhe wurde im Rahmen des Projekts Mikrosystemtechnik das modular aufgebaute elektrochemische Mikroanalysesystem ELMAS unter Verwendung von ISFET-Mikrosensoren und gasfördernden Mikromembranpumpen entwickelt. Der Lösungstransport erfolgt indirekt ĂŒber die DruckĂ€nderungen im Luft- volumen, das sich ĂŒber der Lösung im VorratsbehĂ€lter befindet. Die pH-ISFETs zeigen ein schnelles Ansprechverhalten und mit FluĂraten von 200-300 ”l/min sind Zykluszeiten fĂŒr die Messung und Sensorkalibrierung von 30-60 s realisierbar. Bei Erhöhung der FluĂraten sinkt die Zykluszeit, der Lösungsverbrauch hingegen nimmt zu. FĂŒr eine Messung sind ca. 50 ”l Analyt und maximal 70 ”l Kalibrierlösung erforderlich. Aufgrund des Gaspuffers im VorratsgefÀà ist der Fluidstrom gegenĂŒber den Schaltsignalen der Pumpen um 5 s verzögert
Theoretical study of incoherent phi photoproduction on a deuteron target
We study the photoproduction of phi mesons in deuteron, paying attention to
the modification of the cross section from bound protons to the free ones with
the aim of comparing with recent results at LEPS. For this purpose we take into
account Fermi motion in single scattering and rescattering of the phi to
account for phi absorption on a second nucleon as well as the rescattering of
the proton. We find that the contribution of the double scattering is much
smaller than the typical cross section of gamma p to phi p in free space, which
implies a very small screening of the phi production in deuteron. The
contribution from the proton rescattering, on the other hand, is found to be
not negligible compared to the cross section of gamma p to phi p in free space,
and leads to a moderate reduction of the phi photoproduction cross section on a
deuteron at forward angles if LEPS set up is taken into account. The Fermi
motion allows contribution of the single scattering in regions forbidden by
phase space in the free case. In particular, we find that for momentum
transferred squared close to the maximum value, the Fermi motion changes
drastically the shape of d sigma / dt, to the point that the ratio of this
cross section to the free one becomes very sensitive to the precise value of t
chosen, or the size of the bin used in an experimental analysis. Hence, this
particular region of t does not seem the most indicated to find effects of a
possible phi absorption in the deuteron. This reaction is studied theoretically
as a function of t and the effect of the experimental angular cuts at LEPS is
also discussed, providing guidelines for future experimental analyses of the
reaction.Comment: 17 pages, 16 figure
Diquarks: condensation without bound states
We employ a bispinor gap equation to study superfluidity at nonzero chemical
potential: mu .neq. 0, in two- and three-colour QCD. The two-colour theory,
QC2D, is an excellent exemplar: the order of truncation of the quark-quark
scattering kernel: K, has no qualitative impact, which allows a straightforward
elucidation of the effects of mu when the coupling is strong. In rainbow-ladder
truncation, diquark bound states appear in the spectrum of the three-colour
theory, a defect that is eliminated by an improvement of K. The corrected gap
equation describes a superfluid phase that is semi-quantitatively similar to
that obtained using the rainbow truncation. A model study suggests that the
width of the superfluid gap and the transition point in QC2D provide reliable
quantitative estimates of those quantities in QCD.Comment: 7 pages, 3 figures, REVTEX, epsfi
Renormalization Group Flow Equation at Finite Density
For the linear sigma model with quarks we derive renormalization group flow
equations for finite temperature and finite baryon density using the heat
kernel cutoff. At zero temperature we evolve the effective potential to the
Fermi momentum and compare the solutions of the full evolution equation with
those in the mean field approximation. We find a first order phase transition
either from a massive constituent quark phase to a mixed phase, where both
massive and massless quarks are present, or from a metastable constituent quark
phase at low density to a stable massless quark phase at high density. In the
latter solution, the formation of droplets of massless quarks is realized even
at low density.Comment: 30 pages, 9 figures; typos corrected, section 3 revised, one
reference added, two references updated, submitted to Phys. Rev.
NA60 results on thermal dimuons
The NA60 experiment at the CERN SPS has measured muon pairs with
unprecedented precision in 158A GeV In-In collisions. A strong excess of pairs
above the known sources is observed in the whole mass region 0.2<M<2.6 GeV. The
mass spectrum for M<1 GeV is consistent with a dominant contribution from
pi+pi- -> rho -> mu+mu- annihilation. The associated rho spectral function
shows a strong broadening, but essentially no shift in mass. For M>1 GeV, the
excess is found to be prompt, not due to enhanced charm production, with
pronounced differences to Drell-Yan pairs. The slope parameter Teff associated
with the transverse momentum spectra rises with mass up to the rho, followed by
a sudden decline above. The rise for M<1 GeV is consistent with radial flow of
a hadronic emission source. The seeming absence of significant flow for M>1 GeV
and its relation to parton-hadron duality is discussed in detail, suggesting a
dominantly partonic emission source in this region. A comparison of the data to
the present status of theoretical modeling is also contained. The accumulated
empirical evidence, including also a Planck-like shape of the mass spectra at
low pT and the lack of polarization, is consistent with a global interpretation
of the excess dimuons as thermal radiation. We conclude with first results on
omega in-medium effects.Comment: 10 pages, 12 figures, submitted to Eur. Phys. J.
Quantum Computing and Quantum Simulation with Group-II Atoms
Recent experimental progress in controlling neutral group-II atoms for
optical clocks, and in the production of degenerate gases with group-II atoms
has given rise to novel opportunities to address challenges in quantum
computing and quantum simulation. In these systems, it is possible to encode
qubits in nuclear spin states, which are decoupled from the electronic state in
the S ground state and the long-lived P metastable state on the
clock transition. This leads to quantum computing scenarios where qubits are
stored in long lived nuclear spin states, while electronic states can be
accessed independently, for cooling of the atoms, as well as manipulation and
readout of the qubits. The high nuclear spin in some fermionic isotopes also
offers opportunities for the encoding of multiple qubits on a single atom, as
well as providing an opportunity for studying many-body physics in systems with
a high spin symmetry. Here we review recent experimental and theoretical
progress in these areas, and summarise the advantages and challenges for
quantum computing and quantum simulation with group-II atoms.Comment: 11 pages, 7 figures, review for special issue of "Quantum Information
Processing" on "Quantum Information with Neutral Particles
Last Call for RHIC Predictions
This paper contains the individual contributions of all speakers of the
session on 'Last Call for RHIC Predictions' at Quark Matter 99, and a summary
by the convenor.Comment: 56 pages, psfig, epsf, epsfig, graphicx style files required,
Proceedings of the XIV Int. Conf. on Nucleus-Nucleus Collisions, Quark Matter
99, Torino, Italy, May 10 - 15, 1999. Typographical mistakes corrected and
figure numbers change
Study of the electromagnetic transition form-factors in \eta -> \mu^+\mu^-\gamma and \omega -> \mu^+\mu^-\pi^0 decays with NA60
The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A
GeV In-In collisions. The mass and pT spectra associated with peripheral
collisions can quantitatively be described by the known neutral meson decays.
The high data quality has allowed to remeasure the electromagnetic transition
form factors of the Dalitz decays \eta -> \mu^+\mu^-\gamma and \omega ->
\mu^+\mu^-\pi^0. Using the usual pole approximation F =
(1-M_{\mu\mu}^{2}/\Lambda^{2})^{-1} for the form factors, we find \Lambda^{-2}
(in GeV^{-2}) to be 1.95+-0.17(stat.)+-0.05(syst.) for the \eta and
2.24+-0.06(stat.)+-0.02(syst.) for the \omega. While the values agree with
previous results from the Lepton-G experiment, the errors are greatly improved,
confirming now on the level of 10\sigma the strong enhancement of the \omega
form factor beyond the expectation from vector meson dominance. An improved
value of the branching ratio BR(\omega -> \mu^+\mu^-\pi^0) =
[1.73+-0.25(stat.)+-0.14(syst.)]*10^{-4} has been obtained as a byproduct.Comment: Submitted to Phys. Lett.
Three-quark clusters at finite temperatures and densities
We present a relativistic three-body equation to study correlations in a
medium of finite temperatures and densities. This equation is derived within a
systematic Dyson equation approach and includes the dominant medium effects due
to Pauli blocking and self energy corrections. Relativity is implemented
utilizing the light front form. The equation is solved for a zero-range force
for parameters close to the confinement-deconfinement transition of QCD. We
present correlations between two- and three-particle binding energies and
calculate the three-body Mott transition.Comment: 7 pages, 7 figure
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