arXiv subjects

M. Siegel

Publications and source records attributed to M. Siegel.

65 records · Page 4Linked to original sources

arXiv2007

The ACS Survey of Galactic Globular Clusters. III. The Double Subgiant Branch of NGC 1851

Photometry with HST's ACS reveals that the subgiant branch (SGB) of the globular cluster NGC 1851 splits into two well-defined branches. If the split is due only to an age effect, the two SGBs would imply two star formation episodes separated by $\sim$ 1 Gyr. We discuss other anomalies in NGC 1851 which could be interpreted in terms of a double stellar population. Finally, we compare the case of NGC 1851 with the other two globulars known to host multiple stellar populations, and show that all three clusters differ in several important respects.

astro-ph↗

arXiv2006

The ACS Survey of Galactic Globular Clusters. I. Overview and Clusters Without Previous HST Photometry

We present the first results of a large ACS Survey of Galactic globular clusters. This Hubble Space Telescope (HST) Treasury project is designed to obtain photometry with S/N > ~10 for main sequence stars with masses > ~0.2Msun in a sample of globulars using the Advanced Camera for Surveys (ACS) Wide Field Channel. Here we focus on clusters without previous HST imaging data. These include NGC 5466, 6779, 5053, 6144, Palomar 2, E 3, Lynga 7, Palomar 1, and NGC 6366. Our CMDs extend reliably from the horizontal branch to as much as seven magnitudes fainter than the main sequence turnoff and represent the deepest CMDs published to-date for these clusters. Using fiducial sequences for three standard clusters (M92, NGC 6752, and 47 Tuc) with well-known metallicities and distances, we perform main sequence fitting on the target clusters in order to obtain estimates of their distances and reddenings. These comparisons along with fitting the cluster main sequences to theoretical isochrones yield ages for the target clusters. We find that the majority of the clusters have ages that are consistent with the standard clusters at their metallicities. The exceptions are E 3 which appears ~2 Gyr younger than 47 Tuc, and Pal 1, which could be as much as 8 Gyr younger than 47 Tuc.

astro-ph↗

arXiv2006

Commensurability effects in superconducting Nb films with quasiperiodic pinning arrays

We study experimentally the critical depinning current Ic versus applied magnetic field B in Nb thin films which contain 2D arrays of circular antidots placed on the nodes of quasiperiodic (QP) fivefold Penrose lattices. Close to the transition temperature Tc we observe matching of the vortex lattice with the QP pinning array, confirming essential features in the Ic(B) patterns as predicted by Misko et al. [Phys. Rev. Lett, vol.95, 177007 (2005)]. We find a significant enhancement in Ic(B) for QP pinning arrays in comparison to Ic in samples with randomly distributed antidots or no antidots.

cond-mat.supr-con↗

arXiv2006

Spectroscopy of the fractional vortex eigenfrequency in a long Josephson 0-kappa junction

Fractional Josephson vortices carry a magnetic flux Phi, which is a fraction of the magnetic flux quantum Phi_0 ~ 2.07x10^{-15} Wb. Their properties are very different from the properties of the usual integer fluxons. In particular, fractional vortices are pinned and have an oscillation eigenfrequency which is expected to be within the Josephson plasma gap. Using microwave spectroscopy, we investigate the dependence of the eigenfrequency of a fractional Josephson vortex on its magnetic flux $Φ$ and on the bias current. The experimental results are in good agreement with the theoretical predictions.

cond-mat.supr-con↗

arXiv2006

Multiple 0 - pi transitions in SIFS Josephson tunnel junctions

We report on experimental studies of superconducting coupling through a thin Ni76Al24 film. A new patterning process has been developed, which allows in combination with the wedge shaped deposition technique the in situ deposition of 20 single Nb/Al/Al2O3/Ni3Al/Nb multilayers, each with its own well defined Ni3Al thickness. Every single multilayer consists of 10 different sized Josephson junctions, showing a high reproducibility and scaling with its junction area. Up to six damped oscillations of the critical current density against F-layer thickness were observed, revealing three single 0-pi transitions in the ground state of Josephson junctions. Contrary to former experimental studies, the exponential decay length is one magnitude larger than the oscillation period defining decay length. The theoretical predictions based on linearized Eilenberger equations result in excellent agreement of theory and experimental results.

cond-mat.supr-con↗

arXiv2005

Fluctuation effects in superconducting nanostrips

Superconducting fluctuations in long and narrow strips made from ultrathin NbN films, have been investigated. For large bias currents close to the critical current fluctuations led to localized, temporary transitions into the normal conducting state, which were detected as voltage transients developing between the strip ends. We present models based on fluctuations in the Cooper pair density and current-assisted thermal-unbinding of vortex-antivortex pairs, which explain the current and temperature dependence of the experimental fluctuation rates.

cond-mat.supr-con↗

arXiv2005

High efficiency deterministic Josephson Vortex Ratchet

We investigate experimentally a Josephson vortex ratchet -- a fluxon in an asymmetric periodic potential driven by a deterministic force with zero time average. The highly asymmetric periodic potential is created in an underdamped annular long Josephson junction by means of a current injector providing efficiency of the device up to 91%. We measured the ratchet effect for driving forces with different spectral content. For monochromatic high-frequency drive the rectified voltage becomes quantized. At high driving frequencies we also observe chaos, sub-harmonic dynamics and voltage reversal due to the inertial mass of a fluxon.

cond-mat.supr-con↗

arXiv2005

Non-ideal artificial phase discontinuity in long Josephson 0-kappa-junctions

We investigate the creation of an arbitrary $κ$-discontinuity of the Josephson phase in a long Nb-AlO_x-Nb Josephson junction (LJJ) using a pair of tiny current injectors, and study the formation of fractional vortices formed at this discontinuity. The current I_inj, flowing from one injector to the other, creates a phase discontinuity kappa ~ I_inj. The calibration of injectors is discussed in detail. The small but finite size of injectors leads to some deviations of the properties of such a 0-kappa-LJJ from the properties of a LJJ with an ideal kappa-discontinuity. These experimentally observed deviations in the dependence of the critical current on I_inj$ and magnetic field can be well reproduced by numerical simulation assuming a finite injector size. The physical origin of these deviations is discussed.

cond-mat.supr-con↗

arXiv2005

Density of states anomalies in hybrid superconductor-ferromagnet-normal metal structures

The results of calculations of the spatially-resolved density of states (DoS) in an S(F/N) bilayer are presented (S is a superconductor, F is a metallic ferromagnet, N is a normal metal) within quasiclassical theory in the dirty limit. Analytical solutions are obtained in the case of thin F, N layers which demonstrate the peculiar features of DoS in this system. The dependencies of the minigap and the DoS peak positions on the exchange energy and parameters of the layers are studied numerically

cond-mat.supr-con↗

arXiv2002

Exploring Halo Substructure with Giant Stars IV: The Extended Structure of the Ursa Minor Dwarf Spheroidal

We present a large area photometric survey of the Ursa Minor dSph. We identify UMi giant star candidates extending to ~3 deg from the center of the dSph. Comparison to previous catalogues of stars within the tidal radius of UMi suggests that our photometric luminosity classification is 100% accurate. Over a large fraction of the survey area, blue horizontal branch stars associated with UMi can also be identified. The spatial distribution of both the UMi giant stars and the BHB stars are remarkably similar, and a large fraction of both samples of stars are found outside the tidal radius of UMi. An isodensity contour map of the stars within the tidal radius of UMi reveals two morphological peculiarities: (1) The highest density of dSph stars is offset from the center of symmetry of the outer isodensity contours. (2) The overall shape of the outer contours appear S-shaped. We find that previously determined King profiles with ~50' tidal radii do not fit well the distribution of our UMi stars. A King profile with a larger tidal radius produces a reasonable fit, however a power law with index -3 provides a better fit for radii > 20'. The existence of UMi stars at large distances from the core of the galaxy, the peculiar morphology of the dSph within its tidal radius, and the shape of its surface density profile all suggest that UMi is evolving significantly due to the tidal influence of the Milky Way. However, the photometric data on UMi stars alone does not allow us to determine if the candidate extratidal stars are now unbound or if they remain bound to the dSph within an extended dark matter halo. (Abridged)

astro-ph↗

arXiv2002

Effects of small surface tension in Hele-Shaw multifinger dynamics: an analytical and numerical study

We study the singular effects of vanishingly small surface tension on the dynamics of finger competition in the Saffman-Taylor problem, using the asymptotic techniques described in [S. Tanveer, Phil. Trans. R. Soc. Lond. A 343, 155 (1993)]and [M. Siegel, and S. Tanveer, Phys. Rev. Lett. 76, 419 (1996)] as well as direct numerical computation, following the numerical scheme of [T. Hou, J. Lowengrub, and M. Shelley,J. Comp. Phys. 114, 312 (1994)]. We demonstrate the dramatic effects of small surface tension on the late time evolution of two-finger configurations with respect to exact (non-singular) zero surface tension solutions. The effect is present even when the relevant zero surface tension solution has asymptotic behavior consistent with selection theory.Such singular effects therefore cannot be traced back to steady state selection theory, and imply a drastic global change in the structure of phase-space flow. They can be interpreted in the framework of a recently introduced dynamical solvability scenario according to which surface tension unfolds the structually unstable flow, restoring the hyperbolicity of multifinger fixed points.

nlin.PS↗