Magnetic flux locking in two weakly coupled superconducting rings

We have analyzed the quantum interference effects in the macroscopic ''superconducting molecule''. The composite system consists of two massive superconducting rings, each interrupted by a Josephson junction, which are at the same time weakly coupled with one another. The special case of coupling via the Josephson four-terminal junction is considered. The structure of the macroscopic quantum states in an applied magnetic field is calculated. It is shown, that depending on the values of the magnetic fluxes through each ring, the system displays two groups of states, the ''orthostates'' with both induced currents going in the same direction, and the ''parastates'' with the opposite currents and with the total induced flux locked to zero value. The transition to the flux locked state with changing of the total applied flux is sudden and is preserved in a certain interval which is determined by the difference of the fluxes applied through each ring. It makes the system sensitive to small gradients of the external magnetic field.Comment: 8 pages in Latex, 3 figures (eps

HI and OH absorption at z=0.89

We report on WSRT observations of HI and OH absorption at z=0.885 towards the radio lens PKS 1830-21, mm wave transitions of several molecular species have already been observed at this redshift. At mm wavelengths the source structure is dominated by two extremely compact components, the northeast (NE) and southwest (SW) components. At lower frequencies the continuum emission is much more extended and there is also a broad Einstein ring connecting the NE and SW components. This means that the HI and OH spectra sample a much larger region of the absorber than the mm wave spectra. The HI spectrum that we obtain is asymmetric, with a peak at -147 km/s with respect to the main molecular line redshift of z=0.88582. Weak mm wave molecular absorption has also been detected towards the NE component at this same velocity. The HI absorption, however, covers a total velocity width of 300 km/sec, i.e. including velocities well to the red of molecular features suggesting that it is spatially widespread. In OH we detect both the 1667 and the 1665 MHz transitions. The OH spectrum has a velocity width comparable to that of the HI spectrum, suggesting that it too is widespread in the absorber. The lack of a prominent HI peak in the spectrum at the velocity corresponding to the SW component, suggests that the galaxy responsible for the absorption at z=0.885 has a central molecular disk many kpc in size, and that HI is deficient in this central region. Our observations are sensitive to the large scale kinematics of the absorber, and to first order the implied dynamical mass is consistent with the lens models of Nair et. al. (1993).Comment: 4 pages, 3 figure

On hyperovals of polar spaces

We derive lower and upper bounds for the size of a hyperoval of a finite polar space of rank 3. We give a computer-free proof for the uniqueness, up to isomorphism, of the hyperoval of size 126 of H(5, 4) and prove that the near hexagon E-3 has up to isomorphism a unique full embedding into the dual polar space DH(5, 4)

WSRT Faraday tomography of the Galactic ISM at \lambda \sim 0.86 m

We investigate the distribution and properties of Faraday rotating and synchrotron emitting regions in the Galactic ISM in the direction of the Galactic anti-centre. We apply Faraday tomography to a radio polarization dataset that we obtained with the WSRT. We developed a new method to calculate a linear fit to periodic data, which we use to determine rotation measures from our polarization angle data. From simulations of a Faraday screen + noise we could determine how compatible the data are with Faraday screens. An unexpectedly large fraction of 14% of the lines-of-sight in our dataset show an unresolved main component in the Faraday depth spectrum. For lines-of-sight with a single unresolved component we demonstrate that a Faraday screen in front of a synchrotron emitting region that contains a turbulent magnetic field component can explain the data.Comment: 5 pages, 5 figures. Accepted for publication as a Letter to the Editor in A&

A time-delay determination from VLA light curves of the CLASS gravitational lens B1600+434

We present Very Large Array (VLA) 8.5-GHz light curves of the two lens images of the Cosmic Lens All Sky Survey (CLASS) gravitational lens B1600+434. We find a nearly linear decrease of 18-19% in the flux densities of both lens images over a period of eight months (February-October) in 1998. Additionally, the brightest image A shows modulations up to 11% peak-to-peak on scales of days to weeks over a large part of the observing period. Image B varies significantly less on this time scale. We conclude that most of the short-term variability in image A is not intrinsic source variability, but is most likely caused by microlensing in the lens galaxy. The alternative, scintillation by the ionized Galactic ISM, is shown to be implausible based on its strong opposite frequency dependent behavior compared with results from multi-frequency WSRT monitoring observations (Koopmans & de Bruyn 1999). From these VLA light curves we determine a median time delay between the lens images of 47^{+5}_{-6} d (68%) or 47^{+12}_{-9} d (95%). We use two different methods to derive the time delay; both give the same result within the errors. We estimate an additional systematic error between -8 and +7 d. If the mass distribution of lens galaxy can be described by an isothermal model (Koopmans, de Bruyn & Jackson 1998), this time delay would give a value for the Hubble parameter, H_0=57^{+14}_{-11} (95% statistical) ^{+26}_{-15} (systematic) km/s/Mpc (Omega_m=1 and Omega_Lambda=0). Similarly, the Modified-Hubble-Profile mass model would give H_0=74^{+18}_{-15} (95% statistical) ^{+22}_{-22} (systematic) km/s/Mpc. For Omega_m=0.3 and Omega_Lambda=0.7, these values increase by 5.4%. ... (ABRIDGED)Comment: 14 pages, 6 figures, accepted for publication in Astronomy & Astrophysics (Figs 1 and 3 with degraded resolution

Occupy this: a dialogic derive

This dialogue is the text-based component of an evolving performative multi-media lecture. By re-reading Guy Debord\u27s The Society of the Spectacle, in relation to the global Occupy movement and the rise of social media, we ask: in what ways does the proliferation of digital imagery enable and limit this recent form of political activism? By subjectively responding to selective quotations from Debord\u27s writing, we link the triumvirate of global capitalism, public space and digital technology, producing commentary on the displacement imposed by contemporary \u27spectacular\u27 technologies, the networked \u27technical image\u27 and the politics of public space

Geometric Hyperplanes of the Near Hexagon L_3 times GQ(2, 2)

Having in mind their potential quantum physical applications, we classify all geometric hyperplanes of the near hexagon that is a direct product of a line of size three and the generalized quadrangle of order two. There are eight different kinds of them, totalling to 1023 = 2^{10} - 1 = |PG(9, 2)|, and they form two distinct families intricately related with the points and lines of the Veldkamp space of the quadrangle in question.Comment: 10 pages, 5 figures and 2 tables; Version 2 - more detailed discussion of the properties of hyperplane

Calibration artefacts in radio interferometry. I. Ghost sources in WSRT data

This work investigates a particular class of artefacts, or ghost sources, in radio interferometric images. Earlier observations with (and simulations of) the Westerbork Synthesis Radio Telescope (WSRT) suggested that these were due to calibration with incomplete sky models. A theoretical framework is derived that validates this suggestion, and provides predictions of ghost formation in a two-source scenario. The predictions are found to accurately match the result of simulations, and qualitatively reproduce the ghosts previously seen in observational data. The theory also provides explanations for many previously puzzling features of these artefacts (regular geometry, PSF-like sidelobes, seeming independence on model flux), and shows that the observed phenomenon of flux suppression affecting unmodelled sources is due to the same mechanism. We demonstrate that this ghost formation mechanism is a fundamental feature of calibration, and exhibits a particularly strong and localized signature due to array redundancy. To some extent this mechanism will affect all observations (including those with non-redundant arrays), though in most cases the ghosts remain hidden below the noise or masked by other instrumental artefacts. The implications of such errors on future deep observations are discussed.Comment: 19 pages, 15 figures, submitted to MNRA

Response of thin-film SQUIDs to applied fields and vortex fields: Linear SQUIDs

In this paper we analyze the properties of a dc SQUID when the London penetration depth \lambda is larger than the superconducting film thickness d. We present equations that govern the static behavior for arbitrary values of \Lambda = \lambda^2/d relative to the linear dimensions of the SQUID. The SQUID's critical current I_c depends upon the effective flux \Phi, the magnetic flux through a contour surrounding the central hole plus a term proportional to the line integral of the current density around this contour. While it is well known that the SQUID inductance depends upon \Lambda, we show here that the focusing of magnetic flux from applied fields and vortex-generated fields into the central hole of the SQUID also depends upon \Lambda. We apply this formalism to the simplest case of a linear SQUID of width 2w, consisting of a coplanar pair of long superconducting strips of separation 2a, connected by two small Josephson junctions to a superconducting current-input lead at one end and by a superconducting lead at the other end. The central region of this SQUID shares many properties with a superconducting coplanar stripline. We calculate magnetic-field and current-density profiles, the inductance (including both geometric and kinetic inductances), magnetic moments, and the effective area as a function of \Lambda/w and a/w.Comment: 18 pages, 20 figures, revised for Phys. Rev. B, the main revisions being to denote the effective flux by \Phi rather than