On the Integrability, B\"Acklund Transformation and Symmetry Aspects of a Generalized Fisher Type Nonlinear Reaction-Diffusion Equation

The dynamics of nonlinear reaction-diffusion systems is dominated by the onset of patterns and Fisher equation is considered to be a prototype of such diffusive equations. Here we investigate the integrability properties of a generalized Fisher equation in both (1+1) and (2+1) dimensions. A Painlev\'e singularity structure analysis singles out a special case ($m=2$) as integrable. More interestingly, a B\"acklund transformation is shown to give rise to a linearizing transformation for the integrable case. A Lie symmetry analysis again separates out the same $m=2$ case as the integrable one and hence we report several physically interesting solutions via similarity reductions. Thus we give a group theoretical interpretation for the system under study. Explicit and numerical solutions for specific cases of nonintegrable systems are also given. In particular, the system is found to exhibit different types of travelling wave solutions and patterns, static structures and localized structures. Besides the Lie symmetry analysis, nonclassical and generalized conditional symmetry analysis are also carried out.Comment: 30 pages, 10 figures, to appear in Int. J. Bifur. Chaos (2004

Supra-oscillatory critical temperature dependence of Nb-Ho bilayers

We investigate the critical temperature Tc of a thin s-wave superconductor (Nb) proximity coupled to a helical rare earth ferromagnet (Ho). As a function of the Ho layer thickness, we observe multiple oscillations of Tc superimposed on a slow decay, that we attribute to the influence of the Ho on the Nb proximity effect. Because of Ho inhomogeneous magnetization, singlet and triplet pair correlations are present in the bilayers. We take both into consideration when solving the self consistent Bogoliubov-de Gennes equations, and we observe a reasonable agreement. We also observe non-trivial transitions into the superconducting state, the zero resistance state being attained after two successive transitions which appear to be associated with the magnetic structure of Ho.Comment: Main article: 5 pages, 4 figures; Supplementary materials: 4 pages, 5 figure

X-ray Spectral and timing properties of the black hole x-ray transient Swift J1753.5-0127

We have carried out detailed analysis on the black hole candidate (BHC) X- ray transient Swift J1753.5-0127 observed by the Rossi X-Ray Timing Explorer (RXTE) during its outburst in 2005 {2006. The spectral analysis shows that the emissions are dominated by the hard X-rays, thus revealing the low/hard state of the source during the outburst. The peak luminosity is found lower than the typical value of balancing the mass flow and evaporation of the inner edge of disk (Meyer-Hofmeister 2004). As a result, the disk is prevented from extending inward to produce strong soft X-rays, corresponding to the so-called high/soft state. These are the typical characteristics for a small subset of BHCs, i.e. those soft X-ray transients stay at the low/hard state during the outburst. In most observational time, the QPO frequencies are found to vary roughly linearly with the fluxes and the spectral indices, while the deviation from this relationship at the peak luminosity might provide the first observational evidence of a partially evaporated inner edge of the accretion disk. The anti-correlation between the QPO frequency and spectral color suggests that the global disk oscillation model proposed by Titarchuk & Osherovich (2000) is not likely at work.Comment: Accepted by ApJ, 24 pages, 13 figures, 3 table

Efficient unknown tag identification protocols in large-scale RFID systems

PublishedJournal ArticleOwing to its attractive features such as fast identification and relatively long interrogating range over the classical barcode systems, radio-frequency identification (RFID) technology possesses a promising prospect in many practical applications such as inventory control and supply chain management. However, unknown tags appear in RFID systems when the tagged objects are misplaced or unregistered tagged objects are moved in, which often causes huge economic losses. This paper addresses an important and challenging problem of unknown tag identification in large-scale RFID systems. The existing protocols leverage the Aloha-like schemes to distinguish the unknown tags from known tags at the slot level, which are of low time-efficiency, and thus can hardly satisfy the delay-sensitive applications. To fill in this gap, two filtering-based protocols (at the bit level) are proposed in this paper to address the problem of unknown tag identification efficiently. Theoretical analysis of the protocol parameters is performed to minimize the execution time of the proposed protocols. Extensive simulation experiments are conducted to evaluate the performance of the protocols. The results demonstrate that the proposed protocols significantly outperform the currently most promising protocols.This work was supported by NSFC (Grant Nos. 60973117, 61173160, 61173162, and 60903154), New Century Excellent Talents in University (NCET) of Ministry of Education of China, The Research Fund for the Doctoral Program of Higher Education (Program No. 20130041110019) and the National Science Foundation for Distinguished Young Scholars of China (Grant No. 61225010)

Experimental investigation of non-uniform heating effect on flow boiling instabilities in a microchannel-based heat sink

Copyright @ 2011 ElsevierTwo-phase flow boiling in microchannels is one of the most promising cooling technologies for coping with high heat fluxes produced by the next generation of central processor units (CPUs). If flow boiling is to be used as a thermal management method for high heat flux electronics it is necessary to understand the behaviour of a non-uniform heat distribution, which is typically the case observed in a real operating CPU. The work presented is an experimental study of two-phase boiling in a multi-channel silicon heat sink with non-uniform heating, using water as the cooling liquid. Thin nickel film sensors, integrated on the back side of the heat sinks were used in order to gain insight related to temperature fluctuations caused by two-phase flow instabilities under non-uniform heating. The effect of various hotspot locations on the temperature profile and pressure drop has been investigated. It was observed that boiling inside microchannels with axially non-uniform heating leads to high temperature non-uniformity in the transverse direction.This research was supported by the UK Engineering and Physical Sciences Research Council through grant EP/D500109/1

Rotor-position detection in permanent-magnet wheel motor to ensure smooth startup from standstill

In this paper, an innovative rotor-position-detection method for a permanent-magnet wheel motor (PMWM) that operates from standstill to low speed is presented. The neutral voltage, which is sensed through phaseshifted pulse width modulation, overcomes the limitations of the conventional back electromotive force (EMF)-based position-detection method, which is more suitable for high-speed operation. In addition, a technique that ensures a transition between the two position-detection methods is presented to cover the full speed range. Computer simulations are employed to design and assess the neutral-voltage-based and EMF-based position-detection methods. The results of the position detection and angle error are presented starting from standstill to low speed. A step current (iq) corresponding to motor torque demand is applied for the starting process in the two position-detection methods. The experimental studies of the new position-detection method are conducted. The method is successfully applied to drive a 60-kW PMWM that operates from standstill to high speed. This demonstrates the effectiveness and performance of the presented method

Evolution of Hard X-Ray Spectra Along the Branches in Cir X-1

Using the data from the PCA and HEXTE on board the RXTE satellite, we investigate the evolution of the 3-200 keV spectra of the peculiar low mass X-ray binary (LMXB) Cir X-1 along the branches on its hardness-intensity diagram (HID) from the vertical horizontal branch (VHB), through the horizontal horizontal branch (HHB) and normal branch (NB), to the flaring branch (FB). We detect a power-law hard component in the spectra. It is found that the derived photon indices ($\Gamma$) of the power-law hard component are correlated with the position on the HID. The power-law component dominates the X-ray emission of Cir X-1 in the energy band higher than $\sim 20$ keV. The fluxes of the power-law component are compared with those of the bremsstrahlung component in the spectra. A possible origin of the power-law hard component is discussed.Comment: 14 pages, 5 figures, ApJ Letter accepte

The Additional Line Component within the Iron K\alpha Profile in MCG-6-30-15: Evidence for Blob Ejection?

The EPIC data of MCG -6-30-15 observed by XMM-Newton were analyzed for the complexities of the iron K-alpha line. Here we report that the additional line component (ALC) at 6.9 keV undoubtedly appears within the broad iron Kalpha; line profile at the high state, whereas it disappears at the low state. These state-dependent behaviors exclude several possible origins and suggest an origin of the ALC in matter being ejected from the vicinity of the black hole. At the low state, the newborn blob ejected from the accretion disk is so Thomson-thick that hard X-rays are blocked from ionizing the old blobs, leading to the disappearance of the ALC. When the blob becomes Thomson-thin as a result of expansion, the hard X-ray will penetrate it and ionize the old ones, emitting the ALC at the high state. The blob ejection is the key to switching the ALC on or off.Comment: 6 pages, 4 Figure

Gate-Voltage Control of Chemical Potential and Weak Anti-localization in Bismuth Selenide

We report that Bi$_2$Se$_3$ thin films can be epitaxially grown on SrTiO$_{3}$ substrates, which allow for very large tunablity in carrier density with a back-gate. The observed low field magnetoconductivity due to weak anti-localization (WAL) has a very weak gate-voltage dependence unless the electron density is reduced to very low values. Such a transition in WAL is correlated with unusual changes in longitudinal and Hall resistivities. Our results suggest much suppressed bulk conductivity at large negative gate-voltages and a possible role of surface states in the WAL phenomena. This work may pave a way for realizing three-dimensional topological insulators at ambient conditions.Comment: 5 pages, 4 figures

The Cross-Spectra of Cir X-1: Evolution of Time Lags

Earlier work showed that the track in the X-ray hardness-intensity diagram of Cir X-1 corresponds to a Z track in its color-color diagram. In this paper, we study the cross spectrum of Cir X-1 in different regions of the hardness-intensity diagram with $RXTE$/PCA data. Comparing the light curves of Cir X-1 for the energy band 1.8-5.1 keV to those for 5.1-13.1 keV, we find that Cir X-1 exhibits a hard time lag on the horizontal branch, and a soft time lag on both the normal and the flaring branch. This indicates that Cir X-1 is similar to GX 5-1 and Cyg X-2 on the horizontal branch, but is different from them on the normal branch. We briefly discuss the mechanism of the time lags in the context of Comptonization models.Comment: 12 pages, 4 figures, accepted for publication in Ap