48 research outputs found
Interactions of atoms with highâfield singleâcycle infrared radiation
We have studied ionization and excitation of Rydberg atoms by intense subâpicosecond electromagnetic field pulses of less than one cycle duration. The ionization threshold electric field scales as the binding energy (i.e., nââ2, rather than the nââ4 threshold scaling characteristic of static field ionization and high order multiphoton ionization. This altered behavior is due to the short duration and the subcycle nature of the field. We also find a wide distribution of final states produced by these pulses, which are effectively ââin resonanceââ for dozens of transitions simultaneously.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87507/2/227_1.pd
Self-similar Approximants of the Permeability in Heterogeneous Porous Media from Moment Equation Expansions
We use a mathematical technique, the self-similar functional renormalization,
to construct formulas for the average conductivity that apply for large
heterogeneity, based on perturbative expansions in powers of a small parameter,
usually the log-variance of the local conductivity. Using
perturbation expansions up to third order and fourth order in
obtained from the moment equation approach, we construct the general functional
dependence of the transport variables in the regime where is of
order 1 and larger than 1. Comparison with available numerical simulations give
encouraging results and show that the proposed method provides significant
improvements over available expansions.Comment: Latex, 14 pages + 5 ps figure
Ultrafast-ultrafine probing of high-speed electrical waveforms using a scanning force microscope with photoconductive gating
Picosecond photoconductivity in low-temperature-grown GaAs (LT GaAs) has been used to provide temporal resolution both in rigid probes and in scanning force microscope probes. This article reviews the fabrication and use of such probes. 2.5 ps temporal resolution and few microvolts sensitivity are obtained at arbitrary points on circuits with a spatial definition of 100 nm. Rigid probes are tested in application to analogue and digital circuits. As an alternative to electron beam testing, scanning force probes are applied to in situ imaging and waveform measurement. Finally, the use of time-resolved waveform analysis with scanning-force microscopy probes with semiconductor laser sources is demonstrated.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43337/1/11082_2004_Article_BF00820152.pd
NuSTAR and Chandra observations of new X-ray transients in the central parsec of the Galaxy
We report NuSTAR and Chandra observations of two X-ray transients, SWIFT
J174540.7290015 (T15) and SWIFT J174540.2290037 (T37), which were
discovered by the Neil Gehrels Swift Observatory in 2016 within pc of
Sgr A*. NuSTAR detected bright X-ray outbursts from T15 and T37, likely in the
soft and hard states, with 3-79~keV luminosities of and
erg/s, respectively. No X-ray outbursts have previously been
detected from the two transients and our Chandra ACIS analysis puts an upper
limit of erg/s on their quiescent 2-8 keV
luminosities. No pulsations, significant QPOs, or type I X-ray bursts were
detected in the NuSTAR data. While T15 exhibited no significant red noise, the
T37 power density spectra are well characterized by three Lorentzian
components. The declining variability of T37 above Hz is typical
of black hole (BH) transients in the hard state. NuSTAR spectra of both
transients exhibit a thermal disk blackbody, X-ray reflection with broadened Fe
atomic features, and a continuum component well described by Comptonization
models. Their X-ray reflection spectra are most consistent with high BH spin
() and large disk density ( cm).
Based on the best-fit ionization parameters and disk densities, we found that
X-ray reflection occurred near the inner disk radius, which was derived from
the relativistic broadening and thermal disk component. These X-ray
characteristics suggest the outbursting BH-LMXB scenario for both transients
and yield the first BH spin measurements from X-ray transients in the central
100 pc region.Comment: 15 pages, 7 figures, accepted for publication in Ap
Relationships between woody vegetation and geomorphological patterns in three gravel-bed rivers with different intensities of anthropogenic disturbance
We compared three gravel-bed rivers in north-eastern Italy (Brenta, Piave, Tagliamento) having similar bioclimate, geology and fluvial morphology, but affected by different intensities of anthropogenic disturbance related particularly to hydropower dams, training works and instream gravel mining. Our aim was to test whether a corresponding difference in the interactions between vegetation and geomorphological patterns existed among the three rivers. In equally spaced and sized plots (n = 710) we collected descriptors of geomorphic conditions, and presence-absence of woody species. In the less disturbed river (Tagliamento), spatial succession of woody communities from the floodplain to the channel followed a profile where higher elevation floodplains featured more developed tree communities, and lower elevation islands and bars were covered by pioneer communities. In the intermediate-disturbed river (Piave), islands and floodplains lay at similar elevation and both showed species indicators of mature developed communities. In the most disturbed river (Brenta), all these patterns were simplified, all geomorphic units lay at similar elevations, were not well characterized by species composition, and presented similar persistence age. This indicates that in human-disturbed rivers, channel and vegetation adjustments are closely linked in the long term, and suggests that intermediate levels of anthropogenic disturbance, such as those encountered in the Piave River, could counteract the natural, more dynamic conditions that may periodically fragment vegetated landscapes in natural rivers. © 2015 Elsevier Ltd