14,323 research outputs found

    On the Superradiance of Spin-1 Waves in an Equatorial Wedge around a Kerr Hole

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    Recently Van Putten has suggested that superradiance of magnetosonic waves in a toroidal magnetosphere around a Kerr black hole may play a role in the central engine of gamma-ray bursts. In this context, he computed (in the WKB approximation) the superradiant amplification of scalar waves confined to a thin equatorial wedge around a Kerr hole and found that the superradiance is higher than for radiation incident over all angles. This paper presents calculations of both spin-0 (scalar) superradiance (integrating the radial equation rather than using the WKB method) and and spin-1 (electromagnetic/magnetosonic) superradiance, in Van Putten's wedge geometry. In contrast to the scalar case, spin-1 superradiance decreases in the wedge geometry, decreasing the likelihood of its astrophysical importance.Comment: Submitted to The Astrophysical Journal Letter

    Hermaphroditic Skipjack

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    Volume: 15Start Page: 294End Page: 29

    Comment on ''Phase Diagram of La2x_{2-x}Srx_xCuO4_4 Probed in the Infrared: Imprints of Charge Stripe Excitations''

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    Recently Lucarelli {\it et al.} have reported\cite{lucarelli} temperature-dependence of the in-plane optical reflectivity of La2x_{2-x}Srx_xCuO4_4 over a wide doping range, focusing on the infrared peaks at 30 cm1^{-1} (for xx=0.12), 250 cm1^{-1} and 510 cm1^{-1}. They interpreted the first peak (30 cm1^{-1}) as a signature of charge stripe ordering, while the latter two (250 cm1^{-1} and 510 cm1^{-1}) are attributed to the polaronic charge excitations. However, careful readers would notice that the reported spectra are largely different from those so far measured on the same system. As we illustrate below, all these peaks are caused by an uncontrolled leakage of the c-axis reflectivity into the measured spectra.Comment: 1 page, 1 figure, accepted for publication in Phys. Rev. Lett 91 (2003

    Sliding Density-Wave in Sr_{14}Cu_{24}O_{41} Ladder Compounds

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    We used transport and Raman scattering measurements to identify the insulating state of self-doped spin 1/2 two-leg ladders of Sr_{14}Cu_{24}O_{41} as a weakly pinned, sliding density wave with non-linear conductivity and a giant dielectric response that persists to remarkably high temperatures

    NMR and NQR study of pressure-induced superconductivity and the origin of critical-temperature enhancement in the spin-ladder cuprate Sr2_2Ca12_{12}Cu24_{24}O41_{41}

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    Pressure-induced superconductivity was studied for a spin-ladder cuprate Sr2_2Ca12_{12}Cu24_{24}O41_{41} using nuclear magnetic resonance (NMR) under pressures up to the optimal pressure 3.8 GPa. Pressure application leads to a transitional change from a spin-gapped state to a Fermi-liquid state at temperatures higher than TcT_c. The relaxation rate 1/T11/T_1 shows activated-type behavior at an onset pressure, whereas Korringa-like behavior becomes predominant at the optimal pressure, suggesting that an increase in the density of states (DOS) at the Fermi energy leads to enhancement of TcT_c. Nuclear quadrupole resonance (NQR) spectra suggest that pressure application causes transfer of holes from the chain to the ladder sites. The transfer of holes increases DOS below the optimal pressure. A dome-shaped TcT_c versus pressure curve arises from naive balance between the transfer of holes and broadening of the band width
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