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

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