Strongly nonexponential time-resolved fluorescence of quantum-dot ensembles in three-dimensional photonic crystals

We observe experimentally that ensembles of quantum dots in three-dimensional (3D) photonic crystals reveal strongly nonexponential time-resolved emission. These complex emission decay curves are analyzed with a continuous distribution of decay rates. The log-normal distribution describes the decays well for all studied lattice parameters. The distribution width is identified with variations of the radiative emission rates of quantum dots with various positions and dipole orientations in the unit cell. We find a striking sixfold change of the width of the distribution by varying the lattice parameter. This interpretation qualitatively agrees with the calculations of the 3D projected local density of states. We therefore conclude that fluorescence decay of ensembles of quantum dots is highly nonexponential to an extent that is controlled by photonic crystals

The Kazhdan-Lusztig conjecture for W-algebras

The main result in this paper is the character formula for arbitrary irreducible highest weight modules of W algebras. The key ingredient is the functor provided by quantum Hamiltonian reduction, that constructs the W algebras from affine Kac-Moody algebras and in a similar fashion W modules from KM modules. Assuming certain properties of this functor, the W characters are subsequently derived from the Kazhdan-Lusztig conjecture for KM algebras. The result can be formulated in terms of a double coset of the Weyl group of the KM algebra: the Hasse diagrams give the embedding diagrams of the Verma modules and the Kazhdan-Lusztig polynomials give the multiplicities in the characters.Comment: uuencoded file, 29 pages latex, 5 figure

21-cm synthesis observations of VIRGOHI 21 - a possible dark galaxy in the Virgo Cluster

Many observations indicate that dark matter dominates the extra-galactic Universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously we have suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line-width (~200 km/s) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral-hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~10^8 years ago with a perturber of ~10^11 solar masses. This we take as additional evidence for the massive nature of VIRGOHI 21 as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the HI and find none down to an I band surface brightness limit of 31.1 +/- 0.2 mag/sq. arcsec.Comment: 8 pages, accepted to ApJ, uses emulateapj.cls. Mpeg animation (Fig. 2) available at ftp://ftp.naic.edu/pub/publications/minchin/video2.mp

Gate-tunable Superconductivity in Hybrid InSb-Pb Nanowires

We present a report on hybrid InSb-Pb nanowires that combine high spin-orbit coupling with a high critical field and a large superconducting gap. Material characterization indicates the Pb layer of high crystal quality on the nanowire side facets. Hard induced superconducting gaps and gate-tunable supercurrent are observed in the hybrid nanowires. These results showcase the promising potential of this material combination for a diverse range of applications in hybrid quantum transport devices.Comment: 11 pages, 3 figure

Anyons in discrete gauge theories with Chern-Simons terms

We study the effect of a Chern-Simons term in a theory with discrete gauge group H, which in (2+1)-dimensional space time describes (non-abelian) anyons. As in a previous paper, we emphasize the underlying algebraic structure, namely the Hopf algebra D(H). We argue on physical grounds that the addition of a Chern-Simons term in the action leads to a non-trivial 3-cocycle on D(H). Accordingly, the physically inequivalent models are labelled by the elements of the cohomology group H^3(H,U(1)). It depends periodically on the coefficient of the Chern-Simons term which model is realized. This establishes a relation with the discrete topological field theories of Dijkgraaf and Witten. Some representative examples are worked out explicitly.Comment: 18 page

A Dark Hydrogen Cloud in the Virgo Cluster

VIRGOHI21 is an HI source detected in the Virgo Cluster survey of Davies et al. (2004) which has a neutral hydrogen mass of 10^8 M_solar and a velocity width of Delta V_20 = 220 km/s. From the Tully-Fisher relation, a galaxy with this velocity width would be expected to be 12th magnitude or brighter; however deep CCD imaging has failed to turn up a counterpart down to a surface-brightness level of 27.5 B mag/sq. arcsec. The HI observations show that it is extended over at least 16 kpc which, if the system is bound, gives it a minimum dynamical mass of ~10^11 M_solar and a mass to light ratio of M_dyn/L_B > 500 M_solar/L_solar. If it is tidal debris then the putative parents have vanished; the remaining viable explanation is that VIRGOHI21 is a dark halo that does not contain the expected bright galaxy. This object was found because of the low column density limit of our survey, a limit much lower than that achieved by all-sky surveys such as HIPASS. Further such sensitive surveys might turn up a significant number of the dark matter halos predicted by Dark Matter models.Comment: Accepted by ApJ

De dubbele waterput uit het laat-Romeinse castellum van Oudenburg (prov. West-Vlaanderen): tafonomie, chronologie en interpretatie

This article focuses on a remarkable well structure that was brought to light by the Flemish Heritage Institute during recent archaeological research at the south-western corner of the Saxon Shore fort at Oudenburg (2001-2005). The site of Oudenburg is situated 8 km from the Flemish coastline, in the polder area between Bruges and Ostend. During Roman times however, positioned strategically on an elevated sandy ridge, the site overlooked the coastal plain consisting of mudflats and marshes intersected by natural gullies. The remains of the fort at Oudenburg were discovered in 1956-1957 by J. Mertenslater excavation campaigns in 1960 and 1970 on the western defence area revealed a sequence of three successive forts. The 1960s excavations on two late Roman military cemeteries more than 400 m to the west of the castellum revealed burials of 4th-century fort inhabitants with rich grave goods. During archaeological research within the fort walls in 1976-1977 the first information was collected about the inner organisation of the fort and the remains of a stone building of late 3rd-century date were excavated. It was only in 2001 that new excavations could take place on the fort area. This systematic research resulted in a finer chronology for the occupation of the castellum. A succession of five main fort periods was revealed, dating between ca. 200 and the beginning of the 5th century AD. These excavations yielded insight into the spatial organisation of the south west area of the fort, which had different functions in each successive fort period. The first three phases belonged to wood and earthen fortstemporary installations in times of trouble and Germanic threat. Probably in the later 3rd century AD, a more permanent fort measuring 153 by 176 m was built in stonethis was renovated and reoccupied during the second quarter of the 4th century AD. The characteristics of the ground plan, its topographical position and several finds pointing to a close link with the Saxon Shore forts on the coast of south Britannia, suggest that Oudenburg was probably part of the Litus Saxonicum. In this paper the so-called double well, a context of the fifth fort period (4th centurybeginning 5th century), is analysed. During this period the south-western area of the castellum was dominated by a stone bath building with hypocaust system. Later in the 4th century, long fences were constructed to divide the area into yards, a timber-framed construction with simple plan may be identified as a stable structure, and a large oak basin was probably a reservoir for drinking water. The double well, which received feature number OS 2562, seems to be a key context for this fort period