2,451 research outputs found
Conductance of electrolytes in 1-propanol solutions from −40 to 25°C
Conductance data for solutions of LiCl, NaBr, NaI, KI, KSCN, RbI, Et4NI, Pr4NI, Bu4NI, Bu4NClO4, n-Am4NI, i-Am4NI, n-Hept4NI, Me2Bu2NI, MeBu3NI, EtBu3NI, i-Am3BuNI, and i-Am3BuNBPh4 in 1-propanol at –40, –30, –20, –10, 0, 10, and 25°C are communicated and discussed. Evaluation of the data is performed on the basis of a conductance equation that includes a term in c3/2. Single ion conductances at 25 and 10°C are determined with the help of transference numbers t o + (KSCN/PrOH); the data are compared to data estimated by other methods. Ion-pair association constants and their temperature dependence are discussed in terms of contact and solvent separated ion pairs, and the role of non-coulombic forces is shown with the help of an appropriate splitting of the Gibbs energy of ion-pair formation
Entanglement scaling in critical two-dimensional fermionic and bosonic systems
We relate the reduced density matrices of quadratic bosonic and fermionic
models to their Green's function matrices in a unified way and calculate the
scaling of bipartite entanglement of finite systems in an infinite universe
exactly. For critical fermionic 2D systems at T=0, two regimes of scaling are
identified: generically, we find a logarithmic correction to the area law with
a prefactor dependence on the chemical potential that confirms earlier
predictions based on the Widom conjecture. If, however, the Fermi surface of
the critical system is zero-dimensional, we find an area law with a
sublogarithmic correction. For a critical bosonic 2D array of coupled
oscillators at T=0, our results show that entanglement follows the area law
without corrections.Comment: 4 pages, 4 figure
Crystal lattice properties fully determine short-range interaction parameters for alkali and halide ions
Accurate models of alkali and halide ions in aqueous solution are necessary
for computer simulations of a broad variety of systems. Previous efforts to
develop ion force fields have generally focused on reproducing experimental
measurements of aqueous solution properties such as hydration free energies and
ion-water distribution functions. This dependency limits transferability of the
resulting parameters because of the variety and known limitations of water
models. We present a solvent-independent approach to calibrating ion parameters
based exclusively on crystal lattice properties. Our procedure relies on
minimization of lattice sums to calculate lattice energies and interionic
distances instead of equilibrium ensemble simulations of dense fluids. The gain
in computational efficiency enables simultaneous optimization of all parameters
for Li+, Na+, K+, Rb+, Cs+, F-, Cl-, Br-, and I- subject to constraints that
enforce consistency with periodic table trends. We demonstrate the method by
presenting lattice-derived parameters for the primitive model and the
Lennard-Jones model with Lorentz-Berthelot mixing rules. The resulting
parameters successfully reproduce the lattice properties used to derive them
and are free from the influence of any water model. To assess the
transferability of the Lennard-Jones parameters to aqueous systems, we used
them to estimate hydration free energies and found that the results were in
quantitative agreement with experimentally measured values. These
lattice-derived parameters are applicable in simulations where coupling of ion
parameters to a particular solvent model is undesirable. The simplicity and low
computational demands of the calibration procedure make it suitable for
parametrization of crystallizable ions in a variety of force fields.Comment: 9 pages, 5 table
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Enhancing Terminal Deoxynucleotidyl Transferase Activity on Substrates with 3' Terminal Structures for Enzymatic De Novo DNA Synthesis.
Enzymatic oligonucleotide synthesis methods based on the template-independent polymerase terminal deoxynucleotidyl transferase (TdT) promise to enable the de novo synthesis of long oligonucleotides under mild, aqueous conditions. Intermediates with a 3' terminal structure (hairpins) will inevitably arise during synthesis, but TdT has poor activity on these structured substrates, limiting its usefulness for oligonucleotide synthesis. Here, we described two parallel efforts to improve the activity of TdT on hairpins: (1) optimization of the concentrations of the divalent cation cofactors and (2) engineering TdT for enhanced thermostability, enabling reactions at elevated temperatures. By combining both of these improvements, we obtained a ~10-fold increase in the elongation rate of a guanine-cytosine hairpin
Magnetism, coherent many-particle dynamics, and relaxation with ultracold bosons in optical superlattices
We study how well magnetic models can be implemented with ultracold bosonic
atoms of two different hyperfine states in an optical superlattice. The system
is captured by a two-species Bose-Hubbard model, but realizes in a certain
parameter regime actually the physics of a spin-1/2 Heisenberg magnet,
describing the second order hopping processes. Tuning of the superlattice
allows for controlling the effect of fast first order processes versus the
slower second order ones.
Using the density-matrix renormalization-group method, we provide the
evolution of typical experimentally available observables. The validity of the
description via the Heisenberg model, depending on the parameters of the
Hubbard model, is studied numerically and analytically. The analysis is also
motivated by recent experiments [S. Foelling et al., Nature 448, 1029 (2007);
S. Trotzky et al., Sience 319, 295 (2008)] where coherent two-particle dynamics
with ultracold bosonic atoms in isolated double wells were realized. We provide
theoretical background for the next step, the observation of coherent
many-particle dynamics after coupling the double wells. Contrary to the case of
isolated double wells, relaxation of local observables can be observed. The
tunability between the Bose-Hubbard model and the Heisenberg model in this
setup could be used to study experimentally the differences in equilibration
processes for nonintegrable and Bethe ansatz integrable models. We show that
the relaxation in the Heisenberg model is connected to a phase averaging
effect, which is in contrast to the typical scattering driven thermalization in
nonintegrable models. We discuss the preparation of magnetic groundstates by
adiabatic tuning of the superlattice parameters.Comment: 20 pages, 24 figures; minor changes, published versio
Early Growth and Efficient Accretion of Massive Black Holes at High Redshift
Black-hole masses of the highest redshift quasars (4 <~ z <~ 6) are estimated
using a previously presented scaling relationship, derived from reverberation
mapping of nearby quasars, and compared to quasars at lower redshift. It is
shown that the central black holes in luminous z >~ 4 quasars are very massive
(>~ 10^9 solar masses). It is argued that the mass estimates of the high-z
quasars are not subject to larger uncertainties than those for nearby quasars.
Specifically, the large masses are not overestimates and the lack of similarly
large black-hole masses in the nearby Universe does not rule out their
existence at high-z. However, AGN host galaxies do not typically appear fully
formed or evolved at these early epochs. This supports scenarios in which black
holes build up mass very fast in a radiatively inefficient (or obscured) phase
relative to the stars in their galaxies. Additionally, upper envelopes of
black-hole mass of approximately 10^{10} solar masses and bolometric luminosity
of ~ 10^{48} erg/s are observed at all redshifts.Comment: 17 pages including 7 figures (5 in color) and 1 table. To appear in
ApJ, v600, January 1, 200
Dephasing and the steady state in quantum many-particle systems
We discuss relaxation in bosonic and fermionic many-particle systems. For
integrable systems, the time evolution can cause a dephasing effect, leading
for finite subsystems to certain steady states. We give an explicit derivation
of those steady subsystem states and devise sufficient prerequisites for the
dephasing to take place. We also find simple scenarios, in which dephasing is
ineffective and discuss the dependence on dimensionality and criticality. It
follows further that, after a quench of system parameters, bipartite
entanglement entropy will become extensive. This provides a way of creating
strong entanglement in a controlled fashion.Comment: 4 pages, 3 figures, minor change
Probing the BLR in AGNs using time variability of associated absorption line
It is know that most of the clouds producing associated absorption in the
spectra of AGNs and quasars do not completely cover the background source
(continuum + broad emission line region, BLR). We note that the covering factor
derived for the absorption is the fraction of photons occulted by the absorbing
clouds, and is not necessarily the same as the fractional area covered. We show
that the variability in absorption lines can be produced by the changes in the
covering factor caused by the variation in the continuum and the finite light
travel time across the BLR. We discuss how such a variability can be
distinguished from the variability caused by other effects and how one can use
the variability in the covering factor to probe the BLR.Comment: 12 pages, latex(aaspp4.sty), 2 figures, (To appear in ApJ
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