Hawking Radiation, Covariant Boundary Conditions and Vacuum States

The basic characteristics of the covariant chiral current $$and the covariant chiral energy-momentum tensor$$ are obtained from a chiral effective action. These results are used to justify the covariant boundary condition used in recent approaches \cite{Isowilczek,Isoumtwilczek,shailesh,shailesh2,Banerjee} of computing the Hawking flux from chiral gauge and gravitational anomalies. We also discuss a connection of our results with the conventional calculation of nonchiral currents and stress tensors in different (Unruh, Hartle-Hawking and Boulware) states.Comment: 18 pages, no figures, minor changes, to appear in Phys. Rev.

Hawking Radiation and Covariant Anomalies

Generalising the method of Wilczek and collaborators we provide a derivation of Hawking radiation from charged black holes using only covariant gauge and gravitational anomalies. The reliability and universality of the anomaly cancellation approach to Hawking radiation is also discussed.Comment: Minor changes, conforms to published versio

Inhomogeneous vortex-state-driven enhancement of superconductivity in nanoengineered ferromagnet-superconductor heterostructures

Thin film heterostructures provide a powerful means to study the antagonism between superconductivity (SC) and ferromagnetism (FM). One interesting issue in FM-SC hybrids which defies the notion of antagonistic orders is the observation of magnetic field induced superconductivity (FIS). Here we show that in systems where the FM domains/islands produce spatial inhomogeneities of the SC order parameter, the FIS can derive significant contribution from different mobilities of the magnetic flux identified by two distinct critical states in the inhomogeneous superconductor. Our experiments on nanoengineered bilayers of ferromagnetic CoPt and superconducting NbN where CoPt/NbN islands are separated by a granular NbN, lend support to this alternative explanation of FIS in certain class of FM-SC hybrids.Comment: 5 figure

Fractional vortices in the XY model with π\pi bonds

We define a new set of excitations in the XY model which we call ``fractional vortices''. In the frustrated XY model containing $\pi$ bonds, we make the ansatz that the ground state configurations can be characterized by pairs of oppositely charged fractional vortices. For a chain of $\pi$ bonds, the ground state energy and the phase configurations calculated on the basis of this ansatz agree well with the results from direct numerical simulations. Finally, we discuss the possible connection of these results to some recent experiments by Kirtley {\it et al} [Phys. Rev. B {\bf 51}, R12057 (1995)] on high-T$_c$ superconductors where fractional flux trapping was observed along certain grain boundaries.Comment: 13 pages, 14 figures included (.eps). No essential differences to previous version, however more compact forma

A Luminous Companion to SGR 1806-20

We have obtained infrared spectra of the star suggested to be the counterpart of the soft gamma-ray repeater (SGR) 1806-20. We found strong emission lines similar to those seen in the spectra of the rare Luminous Blue Variables and B[e] stars. A He I absorption line is also seen, from which we infer a spectral type O9--B2. This classification, in combination with the minimum distance of \simgt6 kpc inferred from its extinction, makes the star one of the most luminous in the Galaxy. We infer that it is a companion to SGR 1806-20, and suggest that the presence of a companion is somehow related to the SGR phenomenon.Comment: 5 pages, AASTEX text+table and 2 PostScript figures (needs LaTeX style files aaspptwo.sty, epsf.sty and rotate.sty). In case of problems, contact [email protected]. Postscript file of complete article available on request. (Replaced because first version had one wrong reference in it

Gamma-Ray Burst Spectral Features: Interpretation as X-ray Emission From A Photoionized Plasma

Numerous reports have been made of features, either in emission or absorption, in the 10 - 1000 keV spectra of some gamma-ray bursts. Originally interpreted in the context of Galactic neutron star models as cyclotron line emission and $e^+ - e^-$ annihilation features, the recent demonstration that the majority of GRBs lie at cosmological distances make these explanations unlikely. In this letter, we adopt a relativistic fireball model for cosmological GRBs in which dense, metal rich blobs or filaments of plasma are entrained in the relativistic outflow. In the context of this model, we investigate the conditions under which broadband features, similar to those detected, can be observed. We find a limited region of parameter space capable of reproducing the observed GRB spectra. Finally, we discuss possible constraints further high-energy spectral observations could place on fireball model parameters.Comment: Accepted for publication in Astrophysical Journal Letters Four pages, 2 figure

Magnetic properties and complex magnetic phase diagram in non centrosymmetric EuRhGe3_3 and EuIrGe3_3 single crystals

We report the magnetic properties of two Eu based compounds, single crystalline EuIrGe$_3$ and EuRhGe$_3$, inferred from magnetisation, electrical transport, heat capacity and $^{151}$Eu M\"{o}ssbauer spectroscopy. These previously known compounds crystallise in the non-centrosymmetric, tetragonal, $I4mm$, BaNiSn$_3$-type structure. Single crystals of EuIrGe$_3$ and EuRhGe$_3$ were grown using high temperature solution growth method using In as flux. EuIrGe$_3$ exhibits two magnetic orderings at $T_{\rm N1}$ = 12.4 K, and $T_{\rm N2}$ = 7.3 K. On the other hand EuRhGe$_3$ presents a single magnetic transition with a $T_{\rm N}$ = 12 K. $^{151}$Eu M\"{o}ssbauer spectra present evidence for a cascade of transitions from paramagnetic to incommensurate amplitude modulated followed by an equal moment antiferromagnetic phase at lower temperatures in EuIrGe$_3$, the transitions having a substantial first order character. On the other hand the $^{151}$Eu M\"{o}ssbauer spectra at 4.2 and 9 K in EuRhGe$_3$ present evidence of a single magnetic transition. In both compounds a superzone gap is observed for the current density $J\parallel$ [001], which enhances with transverse magnetic field. The magnetisation measured up to 14 T shows the occurrence of field induced transitions, which are well documented in the magnetotransport data as well. The magnetic phase diagram constructed from these data is complex, revealing the presence of many phases in the $H-T$ phase space