Time Scale Of Several Hours Research Articles

Persistent photocurrent spectroscopy of GaN metal–semiconductor–metal photodetectors on long time scale

Some of the trapping mechanisms responsible for the persistent photocurrent effects in GaN metal–semiconductor–metal photodetectors have been studied on a time scale of several hours by analyzing the photocurrent decay kinetic as a function of temperature. The analysis of the Arrhenius plot of the decay kinetic on a long time scale shows two activation energies of about 140meV and 1eV. Such values are in good agreement with two slope changes observed in the room temperature photocurrent spectrum around 2.40 and 3.25eV, in the GaN energy band gap. According to the Lucovsky theory the 140meV activation energy was interpreted as due to the transition from a deep localized state to the conduction band edge, whereas the 1eV activation energy was interpreted as due to the transition from the valence band edge to a deep localized state. Therefore, the persistent photocurrent on long time scale is primarily due to the presence of donor deep and acceptor deep states generated by gallium vacancies, gallium antisites, and carbon impurities.

X-Ray Flares from a Late B-Type Star, HD 38563S, in NGC 2068 Observed with Chandra X-Ray Observatory

A report is made on the Chandra archival data analysis of NGC 2068, which is one of the nearest ($\sim 400 \,\mathrm{pc}$) and X-ray active star-forming regions, with an estimated age of $10^{5-6} \,\mathrm{yr}$. The observation was made for 100 ks. Using the wavdetect tool, 152 X-ray sources were detected, of which 52 had counterparts in the 2MASS survey. One of the brightest X-ray sources is apparently identified with a $V = 10.42$ late B-type main-sequence star, HD 38563S. During the observation, it exhibited two X-ray flares, with the 0.5–8.0 keV luminosity reaching $\sim 10^{31.6} \,\mathrm{erg} \,\mathrm{s}^{-1}$. The flare X-rays evolved on a time scale of several hours, and showed a thermal spectrum of temperature of $\sim 5 \,\mathrm{keV}$. These results suggests that this star forms an active binary, although contamination by a T Tauri star also remains an alternative possibility.

The VIRGO suspensions: design and recent performance measurements

The success of the VIRGO interferometer to detect gravitational waves, starting from a few Hz, depends on the ability of its suspension systems to isolate the test masses from seismic noise and to maintain them actively at a working position with relative displacement of ∼10 −12 m rms on a time scale of several hours. For this purpose, considerable development effort has been dedicated to realize a multi-stage vibration isolator to suspend the test masses: the SuperAttenuator (SA). It has been designed to provide an attenuation of more than 10 orders of magnitude starting from about 4 Hz. Hierarchical control forces, exerted via coil-magnet actuators placed at three different stages of the SA, damp the fundamental mechanical modes of the system (all below 2 Hz) and maintain the relative orientation and position of the test mass for interferometer alignment and locking. In this Letter a description of the SA is given and recent experimental results are presented.

Thermal stability and partial dewetting of crystalline organic thin films: 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111)

The thermal stability and dewetting effects of crystalline organic thin films on inorganic substrates have been investigated for a model system for organic epitaxy, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111). The thin films deposited under a variety of growth conditions have been annealed stepwise and studied by in situ x-ray diffraction and noncontact atomic force microscopy. It has been found that comparatively smooth films deposited at temperatures Tg≲350 K are metastable, while films deposited at Tg≳350 K are stable against annealing on a time scale of several hours. The thermal expansion of thin PTCDA films and bulk samples has been determined.

An XMM-Newton-based X-ray survey of pre-main sequence stellar emission in the L1551 star-forming complex

We present a study of the X-ray sources present in the nearby L1551 star forming region, based on a deep XMM-Newton observation complemented with Chandra data for the brightest sources. Most known pre-main sequence stars in the region have been detected, most of them with sufficient statistics to allow a detailed study of the temporal and spectral characteristics of their X-ray emission. Significant temporal (and spectral) variability on both short and long time scales is visible for most of the stars. In particular XZ Tau shows large-amplitude variations on time scales of several hours with large changes in the intervening absorption, suggestive of the X-ray emission being eclipsed by the accretion stream (and thus of the X-ray emission being partly or totally accretion-induced). The coronal metal abundance of the WTTS sources is clustered around , while the CTTS sources span almost two orders of magnitudes in coronal Z, even though the photospheric abundance of all stars in the L1551 is likely to be very similar. Some individual elements (notably Ne) appear to be systematically enhanced with respect to Fe in the WTTS stars. The significant differences between the spectral and temporal characteristics of the CTTS and WTTS populations suggest that a different emission mechanism is (at least partly) responsible for the X-ray emission of the two types of stars.

Central Stars of Young Planetary Nebulae - A New Class of Variables

A new class of variable star is proposed. These are variable central stars of young Planetary Nebulae exhibiting roughly sinusoidal (semi)regular photometric and/or radial velocity variations with time scales of several hours. Fourteen of these objects have been identified. Their temperatures are between 25000 and 50000 K and most show hydrogen-rich spectra. The most likely reason for the variability is stellar pulsation. Another possibility would be variable stellar mass loss, but in that case the mechansism causing it must be different from that operating in massive O stars. We speculate that it actually is the stellar pulsations which cause mass loss mdulations.

Unconventional vortex dynamics in superconducting states with broken time-reversal symmetry

We report vortex dynamics in the unconventional superconductors Sr 2 RuO 4 , thoriated UBe 1 3 and compare it with previous data on UPt 3 [A. Amann, A. C. Mota, M. B. Maple, and H.v. Lohneysen, Phys. Rev. B 57, 3640 (1998)]. In all three systems, a pinning mechanism, which is very distinct from the standard pinning by defects, can be associated with the appearance of broken time-reversal symmetry in the superconducting state. The pinning mechanism is so strong that no vortex creep is observed in a time scale of several hours. Our observations could he explained by the presence of domain walls, separating different degenerate superconducting states, as proposed by Sigrist and Agterberg [Prog. Theor. Phys. 102, 965 (1999)]. A conventional vortex approaching such a domain wall can decay into vortices with fractional flux quanta. Domain walls occupied with strongly pinned fractional vortices, represent efficient barriers for vortex motion and thus prevent relaxation towards equilibrium. In the case of UPt 3 and U 0 . 9 7 2 5 Th 0 . 0 2 7 5 Be 1 3 , two consecutive phase transitions are known to occur at H=0, of which the low temperature one leads to a superconducting phase with broken time-reversal symmetry. In both systems, one observes a sharp drop of initial creep rates by more than three orders of magnitude to undetectabely low levels at their second superconducting transition. In Sr 2 RuO 4 time-reversal symmetry is reported to occur right below T c . However, we do not observe unconventional pinning immediately below the superconducting transition, but zero creep sets in only much below T c . While in U 0 . 9 7 2 5 Th 0 . 0 2 7 5 Be 1 3 and UPt 3 , the drop in creep rates at the lower superconducting transition temperature is very sudden and strong, in Sr 2 RuO 4 it looks more like a crossover.

Living tert‐butyllithium initiated anionic polymerization of 2‐vinylnaphthalene in toluene‐tetrahydrofuran mixtures

AbstractThe tert‐butyllithium (t‐BuLi) initiated polymerization of carefully purified 2‐vinylnaphthalene in toluene containing small amounts of tetrahydrofuran with respect to t‐BuLi proceeds on a timescale of several hours without significant deactivation and allows the synthesis of very narrow molecular weight distribution poly‐(2‐vinylnaphthalene) (P2VN) (polydispersities as low as 1.04) and molecular weights between 1000 and 20,000. The absence of P2VN‐Li deactivation at these conditions is also indicated by high degrees of trimethylsilyl end functionalization (>95%) and coupling with dibromoxylene. The respective polymerizations of conventionally purified monomer reveal a complex polymerization profile consistent with deactivation by 2‐acetylnaphthalene during the early stages of the reaction. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3034–3041, 2001

Measurements of dissipation rate in frontal zones

This paper demonstrates that it is possible to use measurements of the Doppler spectrum width from a high-resolution radar to obtain quantitative estimates of dissipation rate during the passage of precipitating frontal zones. Using the high-resolution Chilbolton radar, this method has been applied, for the first time, to infer the rate at which kinetic energy is dissipated by turbulence within mid- and lower-tropospheric frontal zones. Analysis of detailed measurements for one cold-frontal region showed multiple shallow sheets of high dissipation rate, with spatially averaged values around 1 x 10−3 m2s−3. Comparable values were also observed in a warm-frontal zone. The magnitude of the observed dissipation rate supports the hypothesis that mixing played a major role in determining the mean properties of the fronts, in that the turbulence acted to destroy the frontal shear layer on a time-scale of several hours. These issues have particular relevance to the representation of fronts within future very high-resolution numerical weather-prediction models.

Thermal annealing of metamict titanite: A synchrotron radiation and optical birefringence study

The thermal annealing behaviour of metamict titanite from Cardiff, Canada, which shows moderate structural damage from the a-decay of radiogenic elements (U, Th) has been studied using single-crystal synchrotron radiation and optical-birefringence techniques. Progressive and isothermal annealing experiments yield a temperature-dependent response in the diffraction and optical signals. The low-temperature regime (< 673 K) is characterized by a preferred annealing of Frenkel-defects which results in small changes of the unit-cell volume and the Bragg-intensities on a time-scale of several hours. On constant heating at higher temperatures, metamict titanite displays two-stage kinetics with most rapid effects during the first two hours of annealing and less significant changes after the initial annealing period. Fast annealing of Frenkel defects occurs first, followed by recrystallization of more damaged regions. Single-crystal studies reveal lower temperatures for pronounced annealing effects than previously reported in powder-diffraction studies (ca. 100 K).

The dissolution of hectorite: In-situ, real-time observations using atomic force microscopy

The dissolution of individual hectorite (a trioctahedral smectite) particles has been observed at molecular scales in acidic aqueous solution with atomic force microscopy (AFM). A new sample preparation technique was used to attach nanometer-sized hectorite particles to a mica substrate. The reactive surface area of individual hectorite particles was identified and its change during a dissolution experiment was quantified. The dissolution of hectorite under pH 2 conditions takes place exclusively at the edge surfaces. In contrast, the basal surface is completely unreactive within the investigated time scale of several hours. The short edges of the hectorite laths were found to react somewhat more quickly than the long edges. The edge surface area represents 1.5-3.3% of the total surface area. The total surface area has been determined from the actual particle dimensions derived from AFM data to be 730 m 2 /g. The dissolution rate normalized to the reactive edge surface area (ESA) has been determined to be 7.3 × 10 -9 mol hectorite/(m 2 -s), which represents a total surface area (TSA) normalized dissolution rate of 1.9 × 10 -10 mol hectorite/(m 2 -s). The ESA/TSA ratio increases by about 15% within 1 h exposure to a pH 2 aqueous solution at 22 °C.

Faraday Rotation and Models for the Plasma Structure of the Solar Corona

Faraday rotation observations of polarized radiation from natural radio sources are unique among remote diagnostics of the solar corona in that they provide information on the coronal magnetic field. Dual frequency radio polarization measurements yield the rotation measure, a quantity that is proportional to the integral along the line of sight of the product of the electron density and the line-of-sight component of the magnetic field. We made linear polarization observations with the NRAO Very Large Array of 13 polarized radio sources occulted by the solar corona. The observations were made at frequencies of 1465 and 1665 MHz on four days in 1997 May and cover a 20 day period, sampling elongations ranging from about 5 to 14 R☉. The magnitudes of the rotation measures observed range from about 11 to 0 rad m-2. The relatively low values for the rotation measures are due to the solar minimum configuration of the corona at the time of the observations, with the lines of sight to the sources generally not crossing sector boundaries. The largest rotation measure was observed for the extended radio source 3C 79 on 1997 May 11 and corresponds to a case in which the line of sight passed next to the streamer belt at small solar elongations. We have developed a three-dimensional model of the solar corona that is in excellent agreement with the observed rotation measures, as well as being completely consistent with other coronal diagnostics such as coronagraph images. In particular, our observations support the coronal magnetic field model of Patzold et al. (1987); they would be inconsistent with coronal magnetic fields significantly weaker or stronger than this model. The plasma density distribution in the corona is successfully modeled by a dense streamer belt component and a more tenuous coronal hole component. Details of these models are given in § 3 of this paper. The principal disagreement between the model and observations occurs for three lines of sight for which the model predicts nearly zero rotation measure but for which we measure small but significant values of -1 to -2 rad m-2. These lines of sight passed over the solar polar regions. We discuss the possibility that these residual rotation measures are due to static coronal plasma structures, not described by global coronal models, or to very long wavelength coronal Alfven waves. Fluctuations in the rotation measure on timescales of a few hours were observed for some sources and not others. When detected, they were of order 1-2 rad m-2 and occurred on timescales of several hours.

Factors controlling evaporation and energy partitioning beneath a deciduous forest over an annual cycle

The energy balance components were measured above the ground surface of a temperate deciduous forest over an annual cycle using the eddy covariance technique. Over a year, the net radiation at the forest floor was 21.5% of that above the canopy, but this proportion was not constant, primarily because of the distinct phenological stages separated by the emergence and senescence of leaves. The dominant response to seasonal changes in net radiation was through corresponding changes in the sensible heat flux, and both net radiation and sensible heat flux peaked just before leaf emergence. Evaporation at the forest floor was typically less than 0.5 mm per day, and unlike sensible heat flux, was not closely coupled to seasonal changes in net radiation. Instead, evaporation at the forest floor responded primarily to rapid changes in litter water content. Forest floor evaporation was limited by the water-holding capacity of litter, and when the atmospheric demand was large, the litter layer dried on the time scale of several hours. After this rapid period of drying, net radiation and sensible heat flux dominated the energy budget. When leaves were present during the growing season, the sensible and latent energy fluxes at the forest floor were less than 10% of the total canopy fluxes, and the mean Bowen ratio was similar to that above the canopy. However, during the dormant season, the controls of the energy budget at the forest floor largely determine the whole canopy fluxes. On an annual basis, the fluxes from the forest floor are roughly 15–22% of those above the canopy and the evaporation was 86 mm.

Background light in potential sites for the ANTARES undersea neutrino telescope

The ANTARES collaboration has performed a series of in-situ measurements to study the background light for a planned undersea neutrino telescope. Such background can be caused by 40 K decays or by biological activity. We report on measurements at two sites in the Mediterranean Sea at depths of 2400 m and 2700 m, respectively. Three photomultiplier tubes were used to measure single counting rates and coincidence rates for pairs of tubes at various distances. The background rate is seen to consist of three components: a constant rate due to 40 K decays, a continuum rate that varies on a time scale of several hours simultaneously over distances up to at least 40 m, and random bursts a few seconds long that are only correlated in time over distances of the order of a meter. A trigger requiring coincidences between nearby photomultiplier tubes should reduce the trigger rate for a neutrino telescope to a manageable level with only a small loss in efficiency.

Myosin monomer density and exchange in synthetic thick filaments investigated using fluorescence microscopy with single molecule sensitivity

The number of myosin molecules in synthetic thick filaments, prepared by dialysis at 0.12 M NaCl and pH 7.0, was estimated to be between 400 and 800 molecules per micrometre under conditions appropriate for in vitro motility assays. This estimate was based on a number count of Cy3-labelled myosin molecules incorporated into filaments at a nominal ratio of 1:1000. At this dilution, single fluorescent spots were resolved corresponding to individual labelled myosins. The spots usually bleached with a one- or two-step profile but, in around 30% of the cases, fluctuations were observed indicating that additional photophysical or photochemical events had occurred. Myosin molecules were shown to exchange between filaments in suspension on a time-scale of several hours at 4 degrees C, but the reaction was only 75% complete after 48 h, suggesting a non-exchangeable core. However, myosin exchange does not appear to be the predominant source of the fluctuations in fluorescence intensity in the single molecule assays.

水蒸気勾配がGPS可降水量の精度に与える影響

One of the major sources of the error in estimating precipitable water derived from GPS data is horizontal gradient of the moisture. Several characteristics of the error were investigated in comparison with water-vapor radiometer measurements. The GPS-precipitable water (GPS PW) exhibited good agreement with precipitable water derived from water-vapor radiometer measurements. The correlation coefficient and rms error for this comparison were determined as 0.991 and 1.93 mm, respectively. The estimation errors for the GPS PW can be divided into two components based on the time scale of the variation. The first components of the error has low frequency with the time scale of several days, with an amplitude of about 2.5 mm. This estimation error was negatively correlated with the north-south component of the GPS-PW gradient derived from GPS data. The GPS PW was overestimated under the condition in which water vapor increased southward. Another component of the error has high frequency with the time scale of several hours, having an amplitude of at most 5.0 mm. The high frequency component of the error seems to depend on rapid changes of the GPS-PW gradient.

Laser frequency stabilisation for LISA: Experimental progress

In all space-borne laser interferometric gravitational wave detectors, inevitable differences in the lengths of the interferometer arms will potentially allow laser frequency noise to give rise to apparent gravitational wave signals. Active frequency stabilisation to a reference optical cavity and use of a frequency noise correction algorithm are both required to allow the target gravitational wave sensitivity to be achieved. Here we report on an experiment to frequency stabilise a laser over timescales of several hours, and to measure the excess noise using an independent analysing cavity. The residual frequency noise, at the spectral frequencies of interest to LISA, is shown to be significantly above the desired specification. However, in this relatively simple experiment it appears that the residual frequency noise can largely be accounted for by thermal drifts in the frequency discriminator cavity pathlength. Thermal stabilisation of the reference cavity can be expected to yeild a major improvement in absolute laser stability.

ITAL]UBVRI[/ITAL] Observations of the Nucleus of NGC 1275 from 1989 to 1994: Microvariability

Intranight variability of the NGC 1275 nucleus in the optical range has been studied using five series of photoelectric observations obtained over 37 nights from 1989 December 22 to 1994 December 29. The UBVRI monitoring on a single night continued for up to 4.5 hr. In total, 820 measurements were performed simultaneously in each of the UBVRI bands (3600 A ≤ λeff ≤ 8300 A) with the 1.25 m telescope at the Crimean Astrophysical Observatory. Flux densities (F) with their standard deviations (SD) and the measure of intranight variability (SD/F) were calculated for each night of observations. On 40% of the nights, intranight activity in the nucleus was seen at a level of variability SD/F ≥ 3% in all spectral bands. Independent activity in the nucleus in the I band (the spectral region around 8300 A) is suspected on about 10% of nights. The peak amplitude of the variability, Fmax/Fmin ~ 40%, was observed only once during the 37 nights. There was suspected low-level intranight variability in each of the UBVRI spectral bands for nights with SD/F ≤ 3%. The data we obtained lead us to argue that during 1989–1994 a process operated in the nucleus of the galaxy NGC 1275 that caused variations of the optical flux on a timescale of several hours. We obtain preliminary evidence that the causes of intranight and internight variations seem to be different. All of the data obtained are discussed from the point of view of current models of active galactic nuclei.

Rapid variations of T Tauri spectral features: clues to the morphology of the inner regions

We have monitored the optical spectra of six T Tauri stars, AA Tau, BP Tau, CI Tau, DR Tau, RY Tau and SU Aur, approximately hourly over five successive nights. Our spectral range extends from 3800 to 7300 A, including the Balmer series lines from H α to H δ, CaII H+K and various Fe II lines. We construct time series of equivalent widths for each species to study the variations of the strengths of the different lines, and the correlations between them. We can discern a range of physical processes at work, such as the slow rotation of the stars (time-scales of days), magnetic flaring activity (time-scales of about an hour), variable accretion (time-scales of several hours) and obscuration by circumstellar material (again, time-scales of several hours). The sample objects show a range of activity in each of these categories, from DR Tau (dominated entirely by hour-time-scale activity) to those with only slowly varying line activity consistent with rotation (AA Tau). For objects between these extremes, we differentiate the activity according to time-scale, short time-scale events often appearing superimposed on longer time-scale variations. We examine the correlation in time between the rapid activity of the different emission features, which are formed under a wide range of excitation conditions. The lower Balmer lines (H α and H β) and Ca II K are usually strongly correlated with one another, as are the higher Balmer lines (H γ and H δ), but the behaviour of these two groups sometimes becomes decoupled. In one particular case, for the active object DR Tau, we see well-correlated but time-lagged activity, occurring sequentially with the high-energy lines varying first, followed by lines of successively lower excitation temperature. We tentatively conclude that this may be the signature of an accretion shock moving over the stellar limb.

Wheat-germ aspartate transcarbamoylase: revised purification, stability and re-evaluation of regulatory kinetics in terms of the Monod-Wyman-Changeux model.

A revised and simplified purification scheme for aspartate transcarbamoylase (ATCase) from wheat-germ is reported, with an eightfold increase in scale (yielding approximately 10 mg of the pure protein from 4 kg of wheat-germ), and improved characteristics of stability and regulatory kinetics. The ATCase obtained is greater than 96% pure, as judged by polyacrylamide gel electrophoresis. The long-term stability (i.e. on a time-scale of several hours to weeks) of the activity of the purified enzyme, under various storage conditions, was investigated. At 4 degreesC and pH 7.5, stability was found to be strongly dependent on protein concentration (increased stability at high concentration), buffer concentration (decreased stability at high buffer concentration) and the inclusion of glycerol (increased stability with increasing glycerol concentration). The enzyme is routinely stored at 4 degreesC, in 0. 05 m Tris/HCl buffer containing 25% glycerol and at high protein concentration (approximately 1 mg.mL-1, or 10 microm in trimers). Under these conditions, the half-life of the enzyme activity is greater than 300 days. Over the time-scale of kinetic experiments (up to 20 min), the diluted activity (at around 1 nm of ATCase, in the presence of ligands) is completely stable. The specific activity remains constant in the range 0.1-10 nm, in the absence and presence of ligands, showing that dissociation of the trimeric enzyme into its subunits is negligible. Steady-state kinetics were examined using the enzyme at a concentration of 1.3 nm. Initial-rate curves for both allosteric ligands, carbamoylphosphate (CP) and uridine 5'-monophosphate (UMP), showed pronounced sigmoidicity, each in the presence of the other. In the absence of UMP, initial-rate curves for CP are hyperbolic. The initial rate data fit reasonably well to a trimeric Monod-Wyman-Changeux model, suggesting a two-state conformational mechanism, greatly favouring the active (R) state when both ligands are absent, in which the R-state binds CP exclusively (dissociation constant = 23.2 microm), and the T-state binds UMP exclusively (dissociation constant = 0.49 microm). This regulatory behaviour was found to be quite stable, and was indistinguishable from that of the enzyme in a freshly made crude extract, even after storage of the pure sample for 5 months. This enzyme preparation is therefore free of the anomalous allosteric kinetics produced by a previous purification scheme, in which the affinity for UMP was markedly reduced, CP rate curves showed no sigmoidicity, while UMP rate curves had sigmoidicity exaggerated by a low maximum.