Dynamical precursor of nematic order in a dense fluid of hard ellipsoids of revolution

We investigate hard ellipsoids of revolution in a parameter regime where no long range nematic order is present but already finite size domains are formed which show orientational order. Domain formation leads to a substantial slowing down of a collective rotational mode which separates well from the usual microscopic frequency regime. A dynamic coupling of this particular mode into all other modes provides a general mechanism which explains an excess peak in spectra of molecular fluids. Using molecular dynamics simulation on up to 4096 particles and on solving the molecular mode coupling equation we investigate dynamic properties of the peak and prove its orientational origin.Comment: RevTeX4 style, 7 figure

Theoretical bounds for the exponent in the empirical power-law advance-time curve for surface flow

A fundamental and widely applied concept used to study surface flow processes is the advance-time curve characterized by an empirical power law with an exponent r and a numerical prefactor p (i.e., x = p*t^r). In the literature, different values of r have been reported for various situations and types of surface irrigation. Invoking concepts from percolation theory, we related the exponent r to the backbone fractal dimension Db, whose value depends on two factors: dimensionality of the system (e.g., two or three dimensions) and percolation class (e.g., random or invasion percolation with/without trapping). We showed that the theoretical bounds of Db are in well agreement with experimental ranges of r reported in the literature for two furrow and border irrigation systems. We also used the value of Db from the optimal path class of percolation theory to estimate the advance-time curves of four furrows and seven irrigation cycles. Excellent agreement was obtained between the estimated and observed curves

Electrophoretic Properties of Highly Charged Colloids: A Hybrid MD/LB Simulation Study

Using computer simulations, the electrophoretic motion of a positively charged colloid (macroion) in an electrolyte solution is studied in the framework of the primitive model. Hydrodynamic interactions are fully taken into account by applying a hybrid simulation scheme, where the charged ions (i.e. macroion and electrolyte), propagated via molecular dynamics (MD), are coupled to a Lattice Boltzmann (LB) fluid. In a recent experiment it was shown that, for multivalent salt ions, the mobility $\mu$ initially increases with charge density $\sigma$, reaches a maximum and then decreases with further increase of $\sigma$. The aim of the present work is to elucidate the behaviour of $\mu$ at high values of $\sigma$. Even for the case of monovalent microions, we find a decrease of $\mu$ with $\sigma$. A dynamic Stern layer is defined that includes all the counterions that move with the macroion while subject to an external electrical field. The number of counterions in the Stern layer, $q_0$, is a crucial parameter for the behavior of $\mu$ at high values of $\sigma$. In this case, the mobility $\mu$ depends primarily on the ratio $q_0/Q$ (with $Q$ the valency of the macroion). The previous contention that the increase in the distortion of the electric double layer (EDL) with increasing $\sigma$ leads to the lowering of $\mu$ does not hold for high $\sigma$. In fact, we show that the deformation of the EDL decreases with increase of $\sigma$. The role of hydrodynamic interactions is inferred from direct comparisons to Langevin simulations where the coupling to the LB fluid is switched off. Moreover, systems with divalent counterions are considered. In this case, at high values of $\sigma$ the phenomenon of charge inversion is found.Comment: accepted in J. Chem Phys., 10 pages, 9 figure

RR Lyrae Stars in NGC 6388 and NGC 6441: A New Oosterhoff Group?

NGC 6388 and NGC 6441 are anomalies among Galactic globular clusters in that they cannot be readily place into either Oosterhoff group I or Oosterhoff group II despite their significant numbers of RR Lyrae variables. The mean pulsation periods, , of their RRab variables, at 0.71 d and 0.76 d, respectively, are even larger than for Oosterhoff II clusters. Moreover, Oosterhoff II clusters are very metal-poor, whereas NGC 6388 and NGC 6441 are the most metal-rich globular clusters known to contain RR Lyrae stars. The location of the NGC 6388 and NGC 6441 RRab variables in the period-amplitude diagram implies that the RR Lyrae stars in those two clusters are brighter than expected for their metallicities. Our results therefore indicate that a universal relationship may not exist between the luminosity and the metallicity of RR Lyrae variables.Comment: Four pages, three figures. ApJ (Letters), in pres

RR Lyrae Stars in the Andromeda Halo from Deep Imaging with the Advanced Camera for Surveys

We present a complete census of RR Lyrae stars in a halo field of the Andromeda galaxy. These deep observations, taken as part of a program to measure the star formation history in the halo, spanned a period of 41 days with sampling on a variety of time scales, enabling the identification of short and long period variables. Although the long period variables cannot be fully characterized within the time span of this program, the enormous advance in sensitivity provided by the Advanced Camera for Surveys on the Hubble Space Telescope allows accurate characterization of the RR Lyrae population in this field. We find 29 RRab stars with a mean period of 0.594 days, 25 RRc stars with a mean period of 0.316 days, and 1 RRd star with a fundamental period of 0.473 days and a first overtone period of 0.353 days. These 55 RR Lyrae stars imply a specific frequency S_RR=5.6, which is large given the high mean metallicity of the halo, but not surprising given that these stars arise from the old, metal-poor tail of the distribution. This old population in the Andromeda halo cannot be clearly placed into one of the Oosterhoff types: the ratio of RRc/RRabc stars is within the range seen in Oosterhoff II globular clusters, the mean RRab period is in the gap between Oosterhoff types, and the mean RRc period is in the range seen in Oosterhoff I globular clusters. The periods of these RR Lyraes suggest a mean metallicity of [Fe/H]=-1.6, while their brightness implies a distance modulus to Andromeda of 24.5+/-0.1, in good agreement with the Cepheid distance.Comment: 15 pages, latex. Accepted for publication in The Astronomical Journa

Functional renormalization group approach to zero-dimensional interacting systems

We apply the functional renormalization group method to the calculation of dynamical properties of zero-dimensional interacting quantum systems. As case studies we discuss the anharmonic oscillator and the single impurity Anderson model. We truncate the hierarchy of flow equations such that the results are at least correct up to second order perturbation theory in the coupling. For the anharmonic oscillator energies and spectra obtained within two different functional renormalization group schemes are compared to numerically exact results, perturbation theory, and the mean field approximation. Even at large coupling the results obtained using the functional renormalization group agree quite well with the numerical exact solution. The better of the two schemes is used to calculate spectra of the single impurity Anderson model, which then are compared to the results of perturbation theory and the numerical renormalization group. For small to intermediate couplings the functional renormalization group gives results which are close to the ones obtained using the very accurate numerical renormalization group method. In particulare the low-energy scale (Kondo temperature) extracted from the functional renormalization group results shows the expected behavior.Comment: 22 pages, 8 figures include

Weak-coupling expansions for the attractive Holstein and Hubbard models

Weak-coupling expansions (conserving approximations) are carried out for the attractive Holstein and Hubbard models (on an infinite-dimensional hypercubic lattice) that include all bandstructure and vertex correction effects. Quantum fluctuations are found to renormalize transition temperatures by factors of order unity, but may be incorporated into the superconducting channel of Migdal-Eliashberg theory by renormalizing the phonon frequency and the interaction strength.Comment: 10 pages, (five figures available from the author by request) typeset with ReVTeX, preprint NSF-ITP-93-10

Inelastic neutron and x-ray scattering as probes of the sign structure of the Fe-pnictide superconducting gap

Neutron spin-flip scattering observations of a resonance in the superconducting state is often taken as evidence of an unconventional superconducting state in which the gap changes sign $\Delta(k+Q)=-\Delta(k)$ for momentum transfers $Q$ which play an important role in the pairing. Recently questions regarding this identification for the Fe-pnictide superconductors have been raised and it has been suggested that $\Delta(k+Q)=\Delta(k)$. Here we propose that inelastic neutron or x-ray scattering measurements of the spectral weight of a phonon of momentum $Q$ can distinguish between these two pairing scenarios.Comment: 4 pages, 4 figure

Theory of exciton-exciton correlation in nonlinear optical response

We present a systematic theory of Coulomb interaction effects in the nonlinear optical processes in semiconductors using a perturbation series in the exciting laser field. The third-order dynamical response consists of phase-space filling correction, mean-field exciton-exciton interaction, and two-exciton correlation effects expressed as a force-force correlation function. The theory provides a unified description of effects of bound and unbound biexcitons, including memory-effects beyond the Markovian approximation. Approximations for the correlation function are presented.Comment: RevTex, 35 pages, 10 PostScript figs, shorter version submitted to Physical Review

Collective oscillations driven by correlation in the nonlinear optical regime

We present an analytical and numerical study of the coherent exciton polarization including exciton-exciton correlation. The time evolution after excitation with ultrashort optical pulses can be divided into a slowly varying polarization component and novel ultrafast collective modes. The frequency and damping of the collective modes are determined by the high-frequency properties of the retarded two-exciton correlation function, which includes Coulomb effects beyond the mean-field approximation. The overall time evolution depends on the low-frequency spectral behavior. The collective mode, well separated from the slower coherent density evolution, manifests itself in the coherent emission of a resonantly excited excitonic system, as demonstrated numerically.Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letter