X-ray Variability Characteristics of the Seyfert 1 Galaxy NGC 3783

We have characterized the energy-dependent X-ray variability properties of the Seyfert~1 galaxy NGC 3783 using archival XMM-Newton and Rossi X-ray Timing Explorer data. The high-frequency fluctuation power spectral density function (PSD) slope is consistent with flattening towards higher energies. Light curve cross correlation functions yield no significant lags, but peak coefficients generally decrease as energy separation of the bands increases on both short and long timescales. We have measured the coherence between various X-ray bands over the temporal frequency range of 6e-8 to 1e-4 Hz; this range includes the temporal frequency of the low-frequency power spectral density function (PSD) break tentatively detected by Markowitz et al. and includes the lowest temporal frequency over which coherence has been measured in any AGN to date. Coherence is generally near unity at these temporal frequencies, though it decreases slightly as energy separation of the bands increases. Temporal frequency-dependent phase lags are detected on short time scales; phase lags are consistent with increasing as energy separation increases or as temporal frequency decreases. All of these results are similar to those obtained previously for several Seyfert galaxies and stellar-mass black hole systems. Qualitatively, these results are consistent with the variability models of Kotov et al. and Lyubarskii, wherein the X-ray variability is due to inwardly propagating variations in the local mass accretion rate.Comment: Accepted for publication in The Astrophysical Journal, 2005, vol. 635, p. 180; version 2 has minor grammatical changes; 23 pages; uses emulateapj

In-plane magnetic field-induced spin polarization and transition to insulating behavior in two-dimensional hole systems

Using a novel technique, we make quantitative measurements of the spin polarization of dilute (3.4 to 6.8*10^{10} cm^{-2}) GaAs (311)A two-dimensional holes as a function of an in-plane magnetic field. As the field is increased the system gradually becomes spin polarized, with the degree of spin polarization depending on the orientation of the field relative to the crystal axes. Moreover, the behavior of the system turns from metallic to insulating \textit{before} it is fully spin polarized. The minority-spin population at the transition is ~8*10^{9} cm^{-2}, close to the density below which the system makes a transition to an insulating state in the absence of a magnetic field.Comment: 4 pages with figure

Low-field magnetoresistance in GaAs 2D holes

We report low-field magnetotransport data in two-dimensional hole systems in GaAs/AlGaAs heterostructures and quantum wells, in a large density range, $2.5 \times 10^{10} \leq p \leq 4.0 \times 10^{11}$ cm$^{-2}$, with primary focus on samples grown on (311)A GaAs substrates. At high densities, $p \gtrsim 1 \times 10^{11}$ cm$^{-2}$, we observe a remarkably strong positive magnetoresistance. It appears in samples with an anisotropic in-plane mobility and predominantly along the low-mobility direction, and is strongly dependent on the perpendicular electric field and the resulting spin-orbit interaction induced spin-subband population difference. A careful examination of the data reveals that the magnetoresistance must result from a combination of factors including the presence of two spin-subbands, a corrugated quantum well interface which leads to the mobility anisotropy, and possibly weak anti-localization. None of these factors can alone account for the observed positive magnetoresistance. We also present the evolution of the data with density: the magnitude of the positive magnetoresistance decreases with decreasing density until, at the lowest density studied ($p = 2.5 \times 10^{10}$ cm$^{-2}$), it vanishes and is replaced by a weak negative magnetoresistance.Comment: 8 pages, 8 figure

Trivial compiler equivalence: A large scale empirical study of a simple, fast and effective equivalent mutant detection technique

Identifying equivalent mutants remains the largest impediment to the widespread uptake of mutation testing. Despite being researched for more than three decades, the problem remains. We propose Trivial Compiler Equivalence (TCE) a technique that exploits the use of readily available compiler technology to address this long-standing challenge. TCE is directly applicable to real-world programs and can imbue existing tools with the ability to detect equivalent mutants and a special form of useless mutants called duplicated mutants. We present a thorough empirical study using 6 large open source programs, several orders of magnitude larger than those used in previous work, and 18 benchmark programs with hand-analysis equivalent mutants. Our results reveal that, on large real-world programs, TCE can discard more than 7% and 21% of all the mutants as being equivalent and duplicated mutants respectively. A human- based equivalence verification reveals that TCE has the ability to detect approximately 30% of all the existing equivalent mutants

Valley splitting of AlAs two-dimensional electrons in a perpendicular magnetic field

By measuring the angles at which the Landau levels overlap in tilted magnetic fields (the coincidence method), we determine the splitting of the conduction-band valleys in high-mobility two-dimensional (2D) electrons confined to AlAs quantum wells. The data reveal that, while the valleys are nearly degenerate in the absence of magnetic field, they split as a function of perpendicular magnetic field. The splitting appears to depend primarily on the magnitude of the perpendicular component of the magnetic field, suggesting electron-electron interaction as its origin.Comment: Revtex4: 4 pages, 4 figures. Submitted for publicatio

Detecting Trivial Mutant Equivalences via Compiler Optimisations

Mutation testing realises the idea of fault-based testing, i.e., using artificial defects to guide the testing process. It is used to evaluate the adequacy of test suites and to guide test case generation. It is a potentially powerful form of testing, but it is well-known that its effectiveness is inhibited by the presence of equivalent mutants. We recently studied Trivial Compiler Equivalence (TCE) as a simple, fast and readily applicable technique for identifying equivalent mutants for C programs. In the present work, we augment our findings with further results for the Java programming language. TCE can remove a large portion of all mutants because they are determined to be either equivalent or duplicates of other mutants. In particular, TCE equivalent mutants account for 7.4% and 5.7% of all C and Java mutants, while duplicated mutants account for a further 21% of all C mutants and 5.4% Java mutants, on average. With respect to a benchmark ground truth suite (of known equivalent mutants), approximately 30% (for C) and 54% (for Java) are TCE equivalent. It is unsurprising that results differ between languages, since mutation characteristics are language-dependent. In the case of Java, our new results suggest that TCE may be particularly effective, finding almost half of all equivalent mutants

Comment on "Theory of metal-insulator transitions in gated semiconductors" (B. L. Altshuler and D. L. Maslov, Phys. Rev. Lett. 82, 145 (1999))

In a recent Letter, Altshuler and Maslov propose a model which attributes the anomalous temperature and field dependence of the resistivity of two-dimensional electron (or hole) systems to the charging and discharging of traps in the oxide (spacer), rather than to intrinsic behavior of interacting particles associated with a conductor-insulator transition in two dimensions. We argue against this model based on existing experimental evidence.Comment: 1 page; submitted to PR

Jahn-Teller effect and stability of the charge-ordered state in La1-xCaxMnO3 (0.5<x<0.9) manganites

The longitudinal ultrasonic sound velocity and attenuation, the resistivity, and lattice parameters were studied as a function of temperature from 30 K to 300 K in La1-xCaxMnO3 (0.5<x<0.9). For all the samples, a dramatic stiffening of the sound velocity below the charge ordering transition temperature TCO was directly driven by distinct changes of the lattice parameters due to the formation of long range ordering of Jahn-Teller distorted MnO6 octahedra. The relative change of the sound velocity (DeltaV/V) below TCO depends on the Ca concentration x and reaches the maximum at x=0.75, implying that the effective strength of electron-lattice interaction with the Jahn-Teller distortion is the strongest at x=0.75 and hence the charge ordered state is mostly stabilized near x=0.75 and insensitive to the application of a magnetic field, which is supported by the charge transport properties under high magnetic fields up to 14T.Comment: 16 pages, 5 figures, PD

Defining ship structural and machinery onboard measurement campaign for energy efficient operations

In the day-to-day ship operations, structural and machinery failures may lead to major accidents, endangering crew and passengers lives onboard, posing a threat to the environment and having a great impact in terms of ship performance and business losses. The ship onboard measurement methodology aims in addressing which systems should be monitored, when the scheduling of monitoring should occur, who will carry out the measurements and how. Information is acquired from data for machinery and structures collected from sources such as Classification Societies, ship structural drawings, planned maintenance systems, dry-dock reports and real time monitoring data. Through the examination of the above, the specifications of the various ship system parameters to be measured are conducted alongside the number of sensors to be installed. Different scenarios for monitoring the condition and performance of systems either on a day-to-day basis or periodically using either permanent sensors installed onboard the ship or portable equipment or also a combination of them is evaluated. Moreover, parameter measurements are concentrated on various systems temperatures, pressures and monitoring techniques such as vibration monitoring. The onboard measurement outcome will be further utilised for input in tools capable of calculating and assessing the performance and reliability of the ship, thus ensuring safe and efficient vessel operation. The deployment of the developed onboard system will be studied under realistic operational conditions for three different ship types, i.e. tanker, bulk carrier and container ship