Giant optical activity in dielectric planar metamaterials with 2D chirality

For the first time, all-dielectric planar chiral metamaterials consisting of arrays of silicon nitride gammadions on fused silica substrates have been fabricated, and shown to be capable of inducing large changes to the polarization states of transmitted light in a manner that is dependent on the two-dimensional chirality of the microstructured silicon nitride film. The polarization response is found to reverse for opposite enantiomers, and also for the same enantiomer when it is illuminated from opposite sides of the structure. In addition, the polarization states of the various diffracted beams are found to be non-reversible. These structures therefore appear to display elements of non-reciprocal behaviour. The polarization responses of complementary designs, different chiral geometries and various silicon nitride film thicknesses have also been studied. As a result we conclude that multiple reflections within the patterned silicon nitride layer play an important role in defining the mechanism by which these structures are able to modify the polarization states of diffracted light

Solid weak BCC-algebras

We characterize weak BCC-algebras in which the identity $(xy)z=(xz)y$ is satisfied only in the case when elements $x,y$ belong to the same branch

Auxiliary-field quantum Monte Carlo study of first- and second-row post-d elements

A series of calculations for the first- and second-row post-d elements (Ga-Br and In-I) are presented using the phaseless auxiliary-field quantum Monte Carlo (AF QMC) method. This method is formulated in a Hilbert space defined by any chosen one-particle basis, and maps the many-body problem into a linear combination of independent-particle solutions with external auxiliary fields. The phase/sign problem is handled approximately by the phaseless formalism using a trial wave function, which in our calculations was chosen to be the Hartree-Fock solution. We used the consistent correlated basis sets of Peterson and coworkers, which employ a small core relativistic pseudopotential. The AF QMC results are compared with experiment and with those from density-functional (GGA and B3LYP) and coupled-cluster CCSD(T) calculations. The AF QMC total energies agree with CCSD(T) to within a few milli-hartrees across the systems and over several basis sets. The calculated atomic electron affinities, ionization energies, and spectroscopic properties of dimers are, at large basis sets, in excellent agreement with experiment.Comment: 10 pages, 2 figures. To be published in Journal of Chemical Physic

Radio-mode feedback in local AGNs: dependence on the central black hole parameters

Radio mode feedback, in which most of the energy of an active galactic nucleus (AGN) is released in a kinetic form via radio-emitting jets, is thought to play an important role in the maintenance of massive galaxies in the present-day Universe. We study the link between radio emission and the properties of the central black hole in a large sample of local radio galaxies drawn from the Sloan Digital Sky Survey (SDSS), based on the catalogue of Best and Heckman (2012). Our sample is mainly dominated by massive black holes (mostly in the range $10^8-10^9 M_{\odot}$) accreting at very low Eddington ratios (typically $\lambda < 0.01$). In broad agreement with previously reported trends, we find that radio galaxies are preferentially associated with the more massive black holes, and that the radio loudness parameter seems to increase with decreasing Eddington ratio. We compare our results with previous studies in the literature, noting potential biases. The majority of the local radio galaxies in our sample are currently in a radiatively inefficient accretion regime, where kinetic feedback dominates over radiative feedback. We discuss possible physical interpretations of the observed trends in the context of a two-stage feedback process involving a transition in the underlying accretion modes.Comment: accepted for publication in Monthly Notices of the Royal Astronomical Societ

Auxiliary-field quantum Monte Carlo calculations of molecular systems with a Gaussian basis

We extend the recently introduced phaseless auxiliary-field quantum Monte Carlo (QMC) approach to any single-particle basis, and apply it to molecular systems with Gaussian basis sets. QMC methods in general scale favorably with system size, as a low power. A QMC approach with auxiliary fields in principle allows an exact solution of the Schrodinger equation in the chosen basis. However, the well-known sign/phase problem causes the statistical noise to increase exponentially. The phaseless method controls this problem by constraining the paths in the auxiliary-field path integrals with an approximate phase condition that depends on a trial wave function. In the present calculations, the trial wave function is a single Slater determinant from a Hartree-Fock calculation. The calculated all-electron total energies show typical systematic errors of no more than a few milli-Hartrees compared to exact results. At equilibrium geometries in the molecules we studied, this accuracy is roughly comparable to that of coupled-cluster with single and double excitations and with non-iterative triples, CCSD(T). For stretched bonds in H$_2$O, our method exhibits better overall accuracy and a more uniform behavior than CCSD(T).Comment: 11 pages, 5 figures. submitted to JC

Finite temperature damping of collective modes of a BCS-BEC crossover superfluid

A new mechanism is proposed to explain the puzzling damping of collective excitations, which was recently observed in the experiments of strongly interacting Fermi gases below the superfluid critical temperature on the fermionic (BCS) side of Feshbach resonance. Sound velocity, superfluid density and damping rate are calculated with effective field theory. We find that a dominant damping process is due to the interaction between superfluid phonons and thermally excited fermionic quasiparticles, in contrast to the previously proposed pair-breaking mechanism. Results from our effective model are compared quantitatively with recent experimental findings, showing a good agreement.Comment: final version, 9 pages, 4 figure

A Multiperiod OPF Model Under Renewable Generation Uncertainty and Demand Side Flexibility

Renewable energy sources such as wind and solar have received much attention in recent years and large amount of renewable generation is being integrated to the electricity networks. A fundamental challenge in power system operation is to handle the intermittent nature of the renewable generation. In this paper we present a stochastic programming approach to solve a multiperiod optimal power flow problem under renewable generation uncertainty. The proposed approach consists of two stages. In the first stage operating points for conventional power plants are determined. Second stage realizes the generation from renewable resources and optimally accommodates it by relying on demand-side flexibility. The benefits from its application are demonstrated and discussed on a 4-bus and a 39-bus systems. Numerical results show that with limited flexibility on the demand-side substantial benefits in terms of potential additional re-dispatch costs can be achieved. The scaling properties of the approach are finally analysed based on standard IEEE test cases upto 300 buses, allowing to underlined its computational efficiency.Comment: 8 pages, 10 figure

The Central Engines of Gamma-Ray Bursts

Leading models for the "central engine" of long, soft gamma-ray bursts (GRBs) are briefly reviewed with emphasis on the collapsar model. Growing evidence supports the hypothesis that GRBs are a supernova-like phenomenon occurring in star forming regions, differing from ordinary supernovae in that a large fraction of their energy is concentrated in highly relativistic jets. The possible progenitors and physics of such explosions are discussed and the important role of the interaction of the emerging relativistic jet with the collapsing star is emphasized. This interaction may be responsible for most of the time structure seen in long, soft GRBs. What we have called "GRBs" may actually be a diverse set of phenomena with a key parameter being the angle at which the burst is observed. GRB 980425/SN 1988bw and the recently discovered hard x-ray flashes may be examples of this diversity.Comment: 8 pages, Proc. Woods Hole GRB meeting, Nov 5 - 9 WoodsHole Massachusetts, Ed. Roland Vanderspe

Bond breaking with auxiliary-field quantum Monte Carlo

Bond stretching mimics different levels of electron correlation and provides a challenging testbed for approximate many-body computational methods. Using the recently developed phaseless auxiliary-field quantum Monte Carlo (AF QMC) method, we examine bond stretching in the well-studied molecules BH and N$_2$, and in the H$_{50}$ chain. To control the sign/phase problem, the phaseless AF QMC method constrains the paths in the auxiliary-field path integrals with an approximate phase condition that depends on a trial wave function. With single Slater determinants from unrestricted Hartree-Fock (UHF) as trial wave function, the phaseless AF QMC method generally gives better overall accuracy and a more uniform behavior than the coupled cluster CCSD(T) method in mapping the potential-energy curve. In both BH and N$_2$, we also study the use of multiple-determinant trial wave functions from multi-configuration self-consistent-field (MCSCF) calculations. The increase in computational cost versus the gain in statistical and systematic accuracy are examined. With such trial wave functions, excellent results are obtained across the entire region between equilibrium and the dissociation limit.Comment: 8 pages, 3 figures and 3 tables. Submitted to JC

Yang-Mills condensate dark energy coupled with matter and radiation

The coincidence problem is studied for the dark energy model of effective Yang-Mills condensate in a flat expanding universe during the matter-dominated stage. The YMC energy $\rho_y(t)$ is taken to represent the dark energy, which is coupled either with the matter, or with both the matter and the radiation components. The effective YM Lagrangian is completely determined by quantum field theory up to 1-loop order. It is found that under very generic initial conditions and for a variety of forms of coupling, the existence of the scaling solution during the early stages and the subsequent exit from the scaling regime are inevitable. The transition to the accelerating stage always occurs around a redshift $z\simeq (0.3\sim 0.5)$. Moreover, when the Yang-Mills condensate transfers energy into matter or into both matter and radiation, the equation of state $w_y$ of the Yang-Mills condensate can cross over -1 around $z\sim 2$, and takes on a current value $\simeq -1.1$. This is consistent with the recent preliminary observations on supernovae Ia. Therefore, the coincidence problem can be naturally solved in the effective YMC dark energy models.Comment: 24 pages, 18 figure