Disordering to Order: de Vries behavior from a Landau theory for smectics

We show that Landau theory for the isotropic, nematic, smectic A, and smectic C phases generically, but not ubiquitously, implies de Vries behavior. I.e., a continuous AC transition can occur with little layer contraction; the birefringence decreases as temperature T is lowered above this transition, and increases again below the transition. This de Vries behavior occurs in models with unusually small orientational order, and is preceded by a first order I − A transition. A first order AC transition with elements of de Vries behavior can also occur. These results correspond well with experimental work to date.Comment: 4 pages, 2 page appendi

Pair Correlations, Short Range Order and Dispersive Excitations in the Quasi-Kagome Quantum Magnet Volborthite

We present spatial and dynamic information on the s=1/2 distorted kagome antiferromagnet volborthite, Cu3V2O7(OD)2.2D2O, obtained by polarized and inelastic neutron scattering. The instantaneous structure factor, S(Q), is dominated by nearest neighbor pair correlations, with short range order at wave vectors Q1=0.65(3) {\AA}^-1 and Q2=1.15(5) {\AA}^-1 emerging below 5 K. The excitation spectrum, S(Q,{\omega}), reveals two steep branches dispersing from Q1 and Q2, and a flat mode at {\omega}=5.0(2) meV. The results allow us to identify the cross-over at T*=1 K in 51V NMR and specific heat measurements as the build-up of correlations at Q_1. We compare our data to theoretical models proposed for volborthite, and demonstrate that the excitation spectrum can be explained by spin-wave-like excitations with anisotropic exchange parameters, as also suggested by recent local density calculations.Comment: Rewritten article resubmitted to Phys. Rev. Lett. 021

Cantor type functions in non-integer bases

Cantor's ternary function is generalized to arbitrary base-change functions in non-integer bases. Some of them share the curious properties of Cantor's function, while others behave quite differently

Scaling of polymers in aligned rods

We study the behavior of self avoiding polymers in a background of vertically aligned rods that are either frozen into random positions or free to move horizontally. We find that in both cases the polymer chains are highly elongated, with vertical and horizontal size exponents that differ by a factor of 3. Though these results are different than previous predictions, our results are confirmed by detailed computer simulations.Comment: 4 pages, 4 figure

17O NMR study of the intrinsic magnetic susceptibility and spin dynamics of the quantum kagome antiferromagnet ZnCu3(OH)6Cl2

We report through 17O NMR, an unambiguous local determination of the intrinsic kagome lattice spin susceptibility as well as that created around non-magnetic defects issued from natural Zn/ Cu exchange in the S=1/2 (Cu2+) herbertsmithite ZnCu3(OH)6Cl2 compound. The issue of a singlet-triplet gap is addressed. The magnetic response around a defect is found to markedly differ from that observed in non-frustrated antiferromagnetic materials. Finally, we discuss our relaxation measurements in the light of Cu and Cl NMR data [cond-mat 070314] and suggest a flat q-dependence of the excitations.Comment: Accepted for publication in Phys. Rev. Lett., 3 jan. 2008 Figure 1 has been modified to include a two-components fit of the 17O NMR spectru

Dust absorption and scattering in the silicon K-edge

The composition and properties of interstellar silicate dust are not well understood. In X-rays, interstellar dust can be studied in detail by making use of the fine structure features in the Si K-edge. The features in the Si K-edge offer a range of possibilities to study silicon-bearing dust, such as investigating the crystallinity, abundance, and the chemical composition along a given line of sight. We present newly acquired laboratory measurements of the silicon K-edge of several silicate-compounds that complement our measurements from our earlier pilot study. The resulting dust extinction profiles serve as templates for the interstellar extinction that we observe. The extinction profiles were used to model the interstellar dust in the dense environments of the Galaxy. The laboratory measurements, taken at the Soleil synchrotron facility in Paris, were adapted for astrophysical data analysis and implemented in the SPEX spectral fitting program. The models were used to fit the spectra of nine low-mass X-ray binaries located in the Galactic center neighborhood in order to determine the dust properties along those lines of sight. Most lines of sight can be fit well by amorphous olivine. We also established upper limits on the amount of crystalline material that the modeling allows. We obtained values of the total silicon abundance, silicon dust abundance, and depletion along each of the sightlines. We find a possible gradient of $0.06\pm0.02$ dex/kpc for the total silicon abundance versus the Galactocentric distance. We do not find a relation between the depletion and the extinction along the line of sight.Comment: 18 pages, 16 figures. Accepted for publication in Astronomy and Astrophysic

The Dynamical Cluster Approximation: Non-Local Dynamics of Correlated Electron Systems

We recently introduced the dynamical cluster approximation(DCA), a new technique that includes short-ranged dynamical correlations in addition to the local dynamics of the dynamical mean field approximation while preserving causality. The technique is based on an iterative self-consistency scheme on a finite size periodic cluster. The dynamical mean field approximation (exact result) is obtained by taking the cluster to a single site (the thermodynamic limit). Here, we provide details of our method, explicitly show that it is causal, systematic, $\Phi$-derivable, and that it becomes conserving as the cluster size increases. We demonstrate the DCA by applying it to a Quantum Monte Carlo and Exact Enumeration study of the two-dimensional Falicov-Kimball model. The resulting spectral functions preserve causality, and the spectra and the CDW transition temperature converge quickly and systematically to the thermodynamic limit as the cluster size increases.Comment: 19 pages, 13 postscript figures, revte