Multidimensional optical fractionation with holographic verification

The trajectories of colloidal particles driven through a periodic potential energy landscape can become kinetically locked in to directions dictated by the landscape's symmetries. When the landscape is realized with forces exerted by a structured light field, the path a given particle follows has been predicted to depend exquisitely sensitively on such properties as the particle's size and refractive index These predictions, however, have not been tested experimentally. Here, we describe measurements of colloidal silica spheres' transport through arrays of holographic optical traps that use holographic video microscopy to track individual spheres' motions in three dimensions and simultaneously to measure each sphere's radius and refractive index with part-per-thousand resolution. These measurements confirm previously untested predictions for the threshold of kinetically locked-in transport, and demonstrate the ability of optical fractionation to sort colloidal spheres with part-per-thousand resolution on multiple characteristics simultaneously.Comment: 4 pages, 2 figures. Accepted for publication in Physical Review Letter

Large magnetothermal conductivity of HoMnO_3 single crystals and its relation to the magnetic-field induced transitions of magnetic structure

We study the low-temperature heat transport of HoMnO_3 single crystals to probe the magnetic structures and their transitions induced by magnetic field. It is found that the low-T thermal conductivity (\kappa) shows very strong magnetic-field dependence, with the strongest suppression of nearly 90% and the biggest increase of 20 times of \kappa compared to its zero-field value. In particular, some ``dip"-like features show up in \kappa(H) isotherms for field along both the ab plane and the c axis. These behaviors are found to shed new light on the complex H-T phase diagram and the field-induced re-orientations of Mn^{3+} and Ho^{3+} spin structures. The results also demonstrate a significant spin-phonon coupling in this multiferroic compound.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev.

On graded polynomial identities with an antiautomorphism

AbstractLet G be a commutative monoid with cancellation and let R be a strongly G-graded associative algebra with finite G-grading and with antiautomorphism. Suppose that R satisfies a graded polynomial identity with antiautomorphism. We show that R is a PI algebra

A Cosmology-Independent Calibration of Gamma-Ray Burst Luminosity Relations and the Hubble Diagram

An important concern in the application of gamma-ray bursts (GRBs) to cosmology is that the calibration of GRB luminosity/energy relations depends on the cosmological model, due to the lack of a sufficient low-redshift GRB sample. In this paper, we present a new method to calibrate GRB relations in a cosmology-independent way. Since objects at the same redshift should have the same luminosity distance and since the distance moduli of Type Ia supernovae (SNe Ia) obtained directly from observations are completely cosmology independent, we obtain the distance modulus of a GRB at a given redshift by interpolating from the Hubble diagram of SNe Ia. Then we calibrate seven GRB relations without assuming a particular cosmological model and construct a GRB Hubble diagram to constrain cosmological parameters. From the 42 GRBs at $1.4<z\le6.6$, we obtain $\Omega_{\rm M}=0.25_{-0.05}^{+0.04}$, $\Omega_{\Lambda}=0.75_{-0.04}^{+0.05}$ for the flat $\Lambda$CDM model, and for the dark energy model with a constant equation of state $w_0=-1.05_{-0.40}^{+0.27}$, which is consistent with the concordance model in a 1-$\sigma$ confidence region.Comment: 7 pages, 3 figures, 1 table, now matches the editorially revised version; accepted for publication in ApJ (vol 685)