Superconducting transition of a two-dimensional Josephson junction array in weak magnetic fields

The superconducting transition of a two-dimensional (2D) Josephson junction array exposed to weak magnetic fields has been studied experimentally. Resistance measurements reveal a superconducting-resistive phase boundary in serious disagreement with the theoretical and numerical expectations. Critical scaling analyses of the $IV$ characteristics indicate contrary to the expectations that the superconducting-to-resistive transition in weak magnetic fields is associated with a melting transition of magnetic-field-induced vortices directly from a pinned-solid phase to a liquid phase. The expected depinning transition of vortices from a pinned-solid phase to an intermediate floating-solid phase was not observed. We discuss effects of the disorder-induced random pinning potential on phase transitions of vortices in a 2D Josephson junction array.Comment: 9 pages, 7 figures (EPS+JPG format), RevTeX

Luminous Infrared Galaxies With the Submillimeter Array. III. The Dense Kiloparsec Molecular Concentrations of Arp 299

We have used high resolution (~2.3") observations of the local (D = 46 Mpc) luminous infrared galaxy Arp 299 to map out the physical properties of the molecular gas which provides the fuel for its extreme star formation activity. The 12CO J=3-2, 12CO J=2-1 and 13CO J=2-1 lines were observed with the Submillimeter Array and the short spacings of the 12CO J=2-1 and J=3-2 observations have been recovered using James Clerk Maxwell Telescope single dish observations. We use the radiative transfer code RADEX to estimate the physical properties (density, column density and temperature) of the different regions in this system. The RADEX solutions of the two galaxy nuclei, IC 694 and NGC 3690, are consistent with a wide range of gas components, from warm moderately dense gas with T_{kin} > 30 K and n(H_{2}) ~ 0.3 - 3 x 10^{3} cm^{-3} to cold dense gas with T_{kin} ~ 10-30 K and n(H_{2}) > 3 x 10^{3} cm^{-3}. The overlap region is shown to have a better constrained solution with T_{\rm{kin}}$ ~ 10-50 K and n(H_{2}) ~ 1-30 x 10^{3} cm^{-3}. We estimate the gas masses and star formation rates of each region in order to derive molecular gas depletion times. The depletion times of all regions (20-60 Myr) are found to be about 2 orders of magnitude lower than those of normal spiral galaxies. This rapid depletion time can probably be explained by a high fraction of dense gas on kiloparsec scales in Arp 299. We estimate the CO-to-H_{2} factor, \alpha_{co} to be 0.4 \pm 0.3 (3 x 10^{-4}/ x_{CO}) M_{sol} (K km s^{-1} pc^{2})^{-1} for the overlap region. This value agrees well with values determined previously for more advanced merger systems.Comment: 24 pages, 4 figures, ApJ accepte

Spectral dimensions of hierarchical scale-free networks with shortcuts

The spectral dimension has been widely used to understand transport properties on regular and fractal lattices. Nevertheless, it has been little studied for complex networks such as scale-free and small world networks. Here we study the spectral dimension and the return-to-origin probability of random walks on hierarchical scale-free networks, which can be either fractals or non-fractals depending on the weight of shortcuts. Applying the renormalization group (RG) approach to the Gaussian model, we obtain the spectral dimension exactly. While the spectral dimension varies between $1$ and $2$ for the fractal case, it remains at $2$, independent of the variation of network structure for the non-fractal case. The crossover behavior between the two cases is studied through the RG flow analysis. The analytic results are confirmed by simulation results and their implications for the architecture of complex systems are discussed.Comment: 10 pages, 3 figure

Rectifying the Optical-Field-Induced Current in Dielectrics: Petahertz Diode

Investigating a theoretical model of the optical-field-induced current in dielectrics driven by strong few-cycle laser pulses, we propose an asymmetric conducting of the current by forming a heterojunction made of two distinct dielectrics with a low hole mass (m(h)(*) << m(e)(*)) and low electron mass (m(e)(*) << m(h)(*)), respectively. This proposition introduces the novel concept of a petahertz (10(15) Hz) diode to rectify the current in the petahertz domain, which should be a key ingredient for the electric signal manipulation of future light-wave electronics. Further, we suggest the candidate dielectrics for the heterojunction.open

Spin relaxation in mesoscopic superconducting Al wires

We studied the diffusion and the relaxation of the polarized quasiparticle spins in superconductors. To that end, quasiparticles of polarized spins were injected through an interface of a mesoscopic superconducting Al wire in proximity contact with an overlaid ferromagnetic Co wire in the single-domain state. The superconductivity was observed to be suppressed near the spin-injecting interface, as evidenced by the occurrence of a finite voltage for a bias current below the onset of the superconducting transition. The spin diffusion length, estimated from finite voltages over a certain length of Al wire near the interface, was almost temperature independent in the temperature range sufficiently below the superconducting transition but grew as the transition temperature was approached. This temperature dependence suggests that the relaxation of the spin polarization in the superconducting state is governed by the condensation of quasiparticles to the paired state. The spin relaxation in the superconducting state turned out to be more effective than in the normal state.Comment: 9 pages, 8 figure

An annealing-free aqueous-processed anatase TiO2 compact layer for efficient planar heterojunction perovskite solar cells

A facile aqueous-based fabrication scheme is developed for producing annealing-free anatase TiO2 (AF-TiO2) films that exhibit efficient electron transport properties in planar heterojunction perovskite solar cells (PSCs). AF-TiO2 films are fabricated by spin coating on a substrate a colloidal solution of anatase TiO2 nanoparticles (NPs) prepared via a low temperature hydrolytic sol-gel method. The resultant AF-TiO2 films show low electrical resistance, high transmittance in the visible and near-infrared regions and facilitation of high-quality perovskite film formation, which can be attributed to their homogeneous surface morphology and nanocrystallinity. The AF-TiO2 based PSCs achieve a power conversion efficiency (PCE) of 18.29 ± 0.18%, showing significant improvement compared to the control PSCs (PCE = 11.33 ± 0.32%) based on TiO2 films made by high-temperature annealing of amorphous TiO2 (HTA-TiO2)

Tracing the evolutionary stage of Bok globules: CCS and NH3

We pursue the investigation of a previously proposed correlation between chemical properties and physical evolutionary stage of isolated low-mass star-forming regions. In the past, the NH3/CCS abundance ratio was suggested to be a potentially useful indicator for the evolutionary stage of cloud cores. We aim to study its applicability for isolated Bok globules. A sample of 42 Bok globules with and without signs of current star formation was searched for CCS(2-1) emission, the observations were complemented with NH3 measurements available in the literature and own observations. The abundance ratio of both molecules is discussed with respect to the evolutionary stage of the objects and in the context of chemical models. The NH3/CCS ratio could be assessed for 18 Bok globules and is found to be moderately high and roughly similar across all evolutionary stages from starless and prestellar cores towards internally heated cores harbouring protostars of Class 0, Class I or later. Bok globules with extremely high CCS abundance analogous to carbon-chain producing regions in dark cloud cores are not found. The observed range of NH3/CCS hints towards a relatively evolved chemical state of all observed Bok globules.Comment: 12 pages, 6 figures, accepted for publication in Astronomy & Astrophysic