Do strange stars exist in the Universe?

Definitely, an affirmative answer to this question would have implications of fundamental importance for astrophysics (a new class of compact stars), and for the physics of strong interactions (deconfined phase of quark matter, and strange matter hypothesis). In the present work, we use observational data for the newly discovered millisecond X-ray pulsar SAX J1808.4-3658 and for the atoll source 4U 1728-34 to constrain the radius of the underlying compact stars. Comparing the mass-radius relation of these two compact stars with theoretical models for both neutron stars and strange stars, we argue that a strange star model is more consistent with SAX J1808.4-3658 and 4U 1728-34, and suggest that they are likely strange star candidates.Comment: In memory of Bhaskar Datta. -- Invited talk at the Pacific Rim Conference on Stellar Astrophysics (Hong Kong, aug. 1999

Suppression of pulse splitting in two-core optical fibers with Kerr nonlinearity

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The thermal response of A pulsar glitch: The nonspherically symmetric case

We study the thermal evolution of a pulsar after a glitch in which the energy is released from a relatively compact region. A set of relativistic thermal transport and energy balance equations is used to study the thermal evolution, without making the assumption of spherical symmetry. We use an exact cooling model to solve this set of differential equations. Our results could differ significantly from those obtained under the assumption of spherical symmetry. Even for young pulsars with a hot core like the Vela pulsar, a detectable hot spot could be observed after a glitch if a large amount of energy is released in a small region close to the surface of the star. The results suggest that the intensity variation and the relative phases of hard X-ray emissions in different epochs may provide important information on the equation of state. © 1998. The American Astronomical Society. All rights reserved.published_or_final_versio

Optical gain of interdiffused InGaAs-As and AlGaAs-GaAs quantum wells

We have analyzed theoretically the effects of interdiffusion on the gain, differential gain, linewidth enhancement factor, and the injection current density of In0.2Ga0.8As-GaAs and Al0.3Ga0.7As-GaAs quantum-well (QW) lasers. We have calculated the electron and hole subband structures including the effects of valence band mixing and strains. The optical gain is then calculated using the density matrix approach. Our results show that the gain spectrum can be blue-shifted without an enormous increase in the injected current density. Imposing an upper limit (416 A·cm-2) on the injection current density for a typical laser structure, we find that the InGaAs-GaAs and AlGaAs-GaAs QW lasers can be blue-shifted by 24 and 54 mn, respectively. Our theoretical results compare well with the tuning ranges of 53 and 66 meV found for AlGaAs-GaAs QWs in some experiments. This indicates that the interdiffusion technique is useful for the tuning of laser operation wavelength for multiwavelength applications.published_or_final_versio

Modulation instabilities in two-core optical fibers

Modulation instability (MI) of cw states of a two-core fiber, incorporating the effects of coupling-coefficient dispersion (CCD), is studied by solving a pair of generalized, linearly coupled nonlinear Schrödinger equations. CCD refers to the property that the coupling coefficient depends on the optical wavelength, and earlier studies of MI do not account for this physics. CCD does not seriously affect the symmetric/antisymmetric cw, but can drastically modify the MI of the asymmetric state. Generally, new MI frequency bands are produced, and CCD reduces (enhances) the original MI band in the anomalous (normal) dispersion regime. Another remarkable result is the existence of a critical value for the CCD, where the MI gain spectrum undergoes an abrupt change. In the anomalous dispersion regime, a new low-frequency MI band is generated. In the normal dispersion regime, an MI band vanishes, reappears, and then moves up in frequency on crossing this critical value. In both dispersion regimes, the relative magnitude of the low-frequency band and the high-frequency band depends strongly on the total input power.It is possibleto switch the dominantMI frequency between a low frequency and a high frequency by tuning the total input power, providing a promising scheme to manipulate MI-related nonlinear effects in two-core fibers. The MI bands are independent of the third-order dispersion, but can be shifted significantly by self-steepening at a sufficiently high total input power. The evolution of MI from a cw input is also demonstrated with a wave propagation study. © 2011 Optical Society of America.published_or_final_versio

Interdiffusion effect on the gain of InGaAs/InP quantum well laser

Lattice-matched In0.53Ga0.47As/InP quantum well (QW) structures are of considerable interest in photonic application since they enabled device operation in the 1.3micrometers to 1.55micrometers wavelength range which is of importance for optical communication systems. The process of interdiffusion modifies the as-grown square QW to a graded QW which alter the subband structure and optical properties of the QW. Thus it provides a useful tool for bandstructure engineering. The interdiffusion process of InGaAs/InP QW provides more degrees of freedom than AlGaAs/GaAs QW system since interdiffusion can occur for group-III, group-V, and groups III plus V together. These are determined by the temperature and chemical environment used during annealing of the QW structure. The effect of interdiffusion on the laser performances of InGaAs/InP QWs is also studied based on these different types of diffusion processes. It is found that the operating wavelength shows both a red shift and a blue shift depending on the types of diffusion process. It is also found that group-III interdiffusion gives the best performance of InGaAs/InP QW laser when comparing to the other tow types of interdiffusion in terms of a smaller threshold carrier density.published_or_final_versio

Scheduling algorithms for peer-to-peer collaborative file distribution

Peer-to-Peer file sharing applications on the Internet, such as BitTorrent, Gnutella, etc., have been immensely popular. Prior research mainly focuses on peer and content discovery, overlay topology formation, fairness and incentive issues, etc, but seldom investigates the data distribution problem which is also a core component of any file sharing application. In this paper, we present the first effort in addressing this collaborative file distribution problem and formally define the scheduling problem in a simplified context. We suggest several types of algorithms, including a novel Bipartite Matching algorithm, for solving the problem. Simulation results show that our weighted bipartite algorithm finds an optimal solution for all cases tested. Therefore, we believe our algorithm is a promising solution to be employed as the core scheduling module in P2P file sharing applications, shortening the total download time experienced by users. © 2005 IEEE.published_or_final_versio

Switching of ultrashort pulses in nonlinear high-birefringence two-core optical fibers

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Electro-absorption and refraction at 1.5 μm in InGaAs/AlGaAssuperlattice growth on GaAs substrate

High indium concentration In0.65Ga0.35As/Al 0.33Ga0.67As superlattices on GaAs substrates are useful for modulators and optical communication applications. This is due to the lowest loss 1.55 μm optimum wavelength for operation of fiber optic systems. The optical parameters such as absorption coefficient and change in refractive index with applied electric field are investigated.published_or_final_versio

Eigenstates and absorption spectra of interdiffused AlGaAs-GaAs multiple-quantum-well structures

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