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Search Results: 1 - 10 of 353002 matches for " G. D. Gu "
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Hot Jupiters` Tidal Interaction with their Host Stars
D. N. C. Lin,P. G. Gu
Revista mexicana de astronomía y astrofísica , 2004,
Abstract:
Optical excitations in Sr$_{2}$CuO$_{3}$
K. W. Kim,G. D. Gu
Physics , 2008, DOI: 10.1103/PhysRevB.79.085121
Abstract: We investigated excitation spectra of the one-dimensional chain compound Sr$_{2}$CuO$_{3}$. The small peak at 2.3 eV in the loss function was turned to correspond to the strong charge transfer transition at 1.8 eV in conductivity. It has the excitonic character expected in one dimensional extended Hubbard model of the transition from the lower Hubberd band to the Zhang-Rice singlet state. The strongest peak at 2.7 eV in the loss function is attributed to the continuum excitation of the excitonic charge transfer transition. The spectral weight sum rule is satisfied within these transitions.
High-transition-temperature superconductivity in the absence of the magnetic-resonance mode
J. Hwang,T. Timusk,G. D. Gu
Physics , 2004, DOI: 10.1038/nature02347
Abstract: The fundamental mechanism that gives rise to high-transition-temperature (high-Tc) superconductivity in the copper oxide materials has been debated since the discovery of the phenomenon. Recent work has focussed on a sharp 'kink' in the kinetic energy spectra of the electrons as a possible signature of the force that creates the superconducting state. The kink has been related to a magnetic resonance and also to phonons. Here we report that infrared spectra of Bi2Sr2CaCu2O(8+d), (Bi-2212) show that this sharp feature can be separated from a broad background and, interestingly, weakens with doping before disappearing completely at a critical doping level of 0.23 holes per copper atom. Superconductivity is still strong in terms of the transition temperature (Tc approx 55 K), so our results rule out both the magnetic resonance peak and phonons as the principal cause of high-Tc superconductivity. The broad background, on the other hand, is a universal property of the copper oxygen plane and a good candidate for the 'glue' that binds the electrons.
Transparency of an instantaneously created electron-positron-photon plasma
D. Bégué,G. V. Vereshchagin
Physics , 2014, DOI: 10.1093/mnras/stu011
Abstract: The problem of the expansion of a relativistic plasma generated when a large amount of energy is released in a small volume has been considered by many authors. We use the analytical solution of Bisnovatyi-Kogan and Murzina (1995) for the spherically symmetric relativistic expansion. The light-curves and the spectra from transparency of an electron-positron-photon plasma are obtained. We compare our results with the work of Goodman (1986).
Doping dependent optical properties of Bi2Sr2CaCu2O8+d
J. Hwang,T. Timusk,G. D. Gu
Physics , 2006, DOI: 10.1088/0953-8984/19/12/125208
Abstract: We report on the ab-plane reflectance of the high temperature superconductor Bi2Sr2CaCu2O8+d (Bi-2212). Samples spanning the doping range from under doped with Tc = 67 K(UD), to optimally doped with Tc = 96 K (OPT), to over doped with Tc = 60 K (OD) were measured from room temperature down to the superconducting state. The measured reflectance data were analyzed to extract the optical conductivity and the real and imaginary parts of the free carrier optical self-energy. We get an estimate of the dc resistivity from the low frequency extrapolation of the optical conductivity and the superfluid density from the imaginary part of the optical conductivity. The conductivity sum rule can be related to the changes of the kinetic energy of the system. When this system becomes a superconductor, the kinetic energy decreases in the underdoped samples and increases in overdoped ones. The optical self-energy, obtained from the extended Drude model, is dominated by two channels of interaction, a sharp mode and a broad background. The amplitude of the mode is strongly doping and temperature dependent whereas the background decreases weakly with doping and is nearly temperature independent.
Extremely Correlated Fermi Liquid Description of Normal State ARPES in Cuprates
G. -H. Gweon,B. S. Shastry,G. D. Gu
Physics , 2011, DOI: 10.1103/PhysRevLett.107.056404
Abstract: The normal state single particle spectral function of the high temperature superconducting cuprates, measured by the angle resolved photoelectron spectroscopy (ARPES), has been considered both anomalous and crucial to understand. Here we show that an unprecedentedly detailed description of the data is provided by a spectral function arising from the Extremely Correlated Fermi Liquid state of the t-J model proposed recently by Shastry. The description encompasses both laser and conventional synchrotron ARPES data on optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$, and also conventional synchrotron ARPES data on the La$_{1.85}$Sr$_{0.15}$CuO$_4$ materials. {\em It fits all data sets with the same physical parameter values}, satisfies the particle sum rule and successfully addresses two widely discussed "kink" anomalies in the dispersion.
Lutte contre les ravageurs des stocks de céréales et de légumineuses au Sénégal et en Afrique occidentale : synthèse bibliographique
Guèye, MT.,Seck, D.,Wathelet, JP.,Lognay, G.
Biotechnologie, Agronomie, Société et Environnement , 2011,
Abstract: Controlling pests of cereals and legumes in Senegal and West Africa: a review. Post-harvest losses of cereals and legumes are a major problem in Senegal and West Africa. The solutions to eliminate insects, major pests of stored products were mainly chemical. However, due to pollution associated with pesticides use, selection of resistant strains, environmental pollution, poisoning, the search for alternatives is needed. It is reported on different methods of protecting stocks performed alternatively or in combination with pesticides. The major pest species encountered, particularly Prostephanus truncatus (Horn), insect emerging in Senegal, could be controlled by alternative methods including specially the use of insecticide plants. Different aspects related to this alternative way to chemical pesticides are reviewed herein.
Monte Carlo simulations of the photospheric emission in GRBs
D. Bégué,I. A. Siutsou,G. V. Vereshchagin
Physics , 2013, DOI: 10.1088/0004-637X/767/2/139
Abstract: We studied the decoupling of photons from ultra-relativistic spherically symmetric outflows expanding with constant velocity by means of Monte-Carlo (MC) simulation. For outflows with finite width we confirm the existence of two regimes: photon thick and photon thin introduced recently by Ruffini, Siutsou, Vereshchagin (2011), hereafter RSV. The probability density function of photon last scattering is shown to be very different in these two cases. We also obtained spectra as well as light curves. In photon thick case, the time integrated spectrum is much broader than the Planck function and its shape is well described by the fuzzy photosphere approximation introduced by RSV. In the photon thin case we confirm the crucial role of photon diffusion, hence the probability density of decoupling has a maximum near the diffusion radius, well below the photosphere. Its spectrum has Band shape. It is produced when the outflow is optically thick and its peak is formed at diffusion radius.
Transport in a harmonic trap: shortcuts to adiabaticity and robust protocols
D. Guéry-Odelin,J. G. Muga
Physics , 2014, DOI: 10.1103/PhysRevA.90.063425
Abstract: We study the fast transport of a particle or a Bose-Einstein condensate in a harmonic potential. An exact expression for the final excitation energy in terms of the Fourier transform of the trap acceleration is used to engineer optimal transport protocols (with no final excitation) that are robust with respect to spring-constant errors. The same technique provides a way to design the simultaneous and robust transport of a few non-interacting species that experience different harmonic trap frequencies in the same trap.
Robust charge stripe order under high electric fields in Nd1.67Sr0.33NiO4
M. Huecker,M. v. Zimmermann,G. D. Gu
Physics , 2006, DOI: 10.1103/PhysRevB.75.041103
Abstract: The influence of high electric fields on the charge stripe order in Nd1.67Sr0.33NiO4 was studied by means of simultaneous hard x-ray diffraction and electrical transport experiments. Direct measurements of the charge stripe satellite peaks in zero and high electric fields provide no evidence for a deformation or a sliding of the stripe lattice, which contradicts previous indications from non-linear conductance effects. By using the order parameter of a structural phase transition for instant sample temperature measurements, non-linear transport effects can be attributed to resistive heating. Implications for the pinning of stripes in the nickelates are discussed.
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