Thermal behavior of Quantum Cellular Automaton wires

We investigate the effect of a finite temperature on the behavior of logic circuits based on the principle of Quantum Cellular Automata (QCA) and of ground state computation. In particular, we focus on the error probability for a wire of QCA cells that propagates a logic state. A numerical model and an analytical, more approximate, model are presented for the evaluation of the partition function of such a system and, consequently, of the desired probabilities. We compare the results of the two models, assessing the limits of validity of the analytical approach, and provide estimates for the maximum operating temperature.Comment: 15 pages, 7 figures, uses revte

Precise Measurements of the Kilohertz Quasi-Periodic Oscillations in 4U 1728-34

We have analyzed seventeen observations of the low-mass X-ray binary and atoll source 4U 1728-34, carried out by the Rossi X-ray Timing Explorer in 1996 and 1997. We obtain precise measurements of the frequencies of the two simultaneous kilohertz quasi-periodic oscillations (kHz QPOs) in this source. We show that the frequency separation between the two QPO, $\Delta \nu$, is always significantly smaller than the frequency of the nearly-coherent oscillations seen in this source during X-ray bursts, even at the lowest inferred mass accretion rate, when $\Delta \nu$ seems to reach its maximum value. We also find that $\Delta \nu$ decreases significantly, from $349.3 \pm 1.7$ Hz to $278.7 \pm 11.6$ Hz, as the frequency of the lower frequency kHz QPO increases from 615 to 895 Hz. This is the first time that variations of the kHz QPO peak separation are measured in a source which shows nearly-coherent oscillations during bursts.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses AAS LaTex v4.0 (5 pages plus 4 postscript figures

Realist Evaluation : an overview

This report summarises the discussions and presentations of the Expert Seminar ‘Realist Evaluation’ with Gill Westhorp, which took place in Wageningen on March 29, 2011. The Expert Seminar was organised by the Wageningen UR Centre for Development Innovation in collaboration with Learning by Design and Context, international cooperation

Will the Three Gorges Dam affect the underwater light climate of Vallisneria spiralis L. and food habitat of Siberian crane in Poyang Lake?

Almost 95% of the entire population of the Siberian crane (Grus leucogeranus) winter in Poyang Lake, China, where they forage on the tubers of the submerged aquatic macrophyte Vallisneria spiralis. The Three Gorges Dam on the Yangtze River may possibly affect this food source of the Siberian crane by affecting the light intensity reaching the top of the V. spiralis canopy. In this study, the photosynthetically active radiation at the top of the V. spiralis canopy (PARtc) in Lake Dahuchi was modeled from 1998 to 2006, and the potential impacts of changes in water level and turbidity on the underwater light climate of V. spiralis were analyzed. PARtc was calculated from incident irradiance while the losses due to reflection at the water surface, absorption, and scattering within the water column were taken into consideration. The results indicated significant differences in PARtc between years. Six years of water level and Secchi disk depth records revealed a seasonal switching of the lake from a turbid state at low water levels in autumn, winter, and spring to a clear state at high water levels during the monsoon in summer. The highest PARtc occurred at intermediate water levels, which were reached when the Yangtze River forces Lake Dahuchi out of its turbid state in early summer and the water becomes clear. The intended operation of the Three Gorges Dam, which will increase water levels in May and June, may advance the moment when Lake Dahuchi switches from turbid to clear. We suggest that this might increase production of V. spiralis and possibly improve the food habitat conditions for wintering Siberian crane in Poyang Lake

Колгоспні ліси у відомчому конфлікті Ніжинського головного лісництва та земельних органів окупаційної влади

During sediment transport downstream, river systems mix sediments from different parts of their catchments. During deposition, sediments are often unmixed again in different depositional environments (facies). During fluvial transport, between erosion and deposition of sediment, the sediment is sorted too. A commonly observed phenomenon is downstream fining (Frings, 2007), which is the tendency for bed sediments of many rivers to become finer downstream. All processes mentioned above make that fluvial sediment, and thus the fluvial record, is spatially variable

Spin-dependent transport in metal/semiconductor tunnel junctions

This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. Measurements are presented of the circular-polarization-dependent photocurrent (the so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/τ-MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of τ-MnAl along the tunnelling direction; the latter is corroborated by bandstructure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52

We present new results based on observations carried out with the Rossi X-ray Timing Explorer during the decay of an outburst of the low-mass X-ray binary (LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in 4U 1608-52, one of the long-standing issues about the phenomenology of the kilohertz quasi-periodic oscillations (kHz QPOs), namely, the lack of a unique relation between the frequency of the kHz QPOs and the X-ray flux. We show that despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs is monotonically related to the position of the source in the color-color diagram. Our findings strengthen the idea that, as in the case of Z sources, in the atoll sources the X-ray flux is not a good indicator of $\dot M$, and that the observed changes in the frequency of the kHz QPOs in LMXBs are driven by changes in $\dot M$. These results raise some concern about the recently reported detection of the orbital frequency at the innermost stable orbit in 4U 1820-30.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses AAS LaTex v4.0 (5 pages plus 4 postscript figures

Discovery of a ~7 Hz Quasi-Periodic Oscillation in the low-luminosity low-mass X-ray binary 4U 1820-30

We have discovered a 7.06+-0.08 Hz quasi-periodic oscillation (QPO) in the X-ray flux of the low-luminosity low-mass X-ray binary (LMXB) and atoll source 4U 1820-30. This QPO was only observable at the highest observed mass accretion rate, when the source was in the uppermost part of the banana branch, at a 2-25 keV luminosity of 5.4x10^37 erg/s (for a distance of 6.4 kpc). The QPO had a FWHM of only 0.5+-0.2 Hz during small time intervals (32-s of data), and showed erratic shifts in the centroid frequency between 5.5 and 8 Hz. The rms amplitude over the energy range 2-60 keV was 5.6%+-0.2%. The amplitude increased with photon energy from 3.7%+-0.5% between 2.8 and 5.3 keV to 7.3%+-0.6% between 6.8 and 9.3 keV, above which it remained approximately constant at ~7%. The time lag of the QPO between 2.8-6.8 and 6.8-18.2 keV was consistent with being zero (-1.2+-3.4 ms). The properties of the QPO (i.e., its frequency and its presence only at the highest observed mass accretion rate) are similar to those of the 5-20 Hz QPO observed in the highest luminosity LMXBs (the Z sources) when they are accreting near the Eddington mass accretion limit. If this is indeed the same phenomenon, then models explaining the 5-20 Hz QPO in the Z sources, which require the near-Eddington accretion rates, will not hold. Assuming isotropic emission, the 2-25 keV luminosity of 4U 1820-30 at the time of the 7 Hz QPOs is at maximum only 40% (for a companion star with cosmic abundances), but most likely ~20% (for a helium companion star) of the Eddington accretion limit.Comment: Accepted for publication in ApJ Letters (6 pages, including 3 figures