3,260 research outputs found
The gravitational Vavilov-Cherenkov effect
In this essay we show that an uncharged black-hole moving superluminally in a
transparent dielectric medium violates Hawking's area theorem. The violation is
overcome through the emission of radiation. Since modes cannot emerge from the
black hole itself, this radiation must originate from a collective effect in
the medium, in complete analogy with the Vavilov-Cherenkov effect. However,
because the black-hole is uncharged, the emission mechanism must be different.
We discuss the physical origin of the effect and obtain a Newtonian estimative.
Then we obtain the appropriate equations in the relativistic case and show that
the field which is radiated away is a combination of gravitational and
electromagnetic degrees of freedom. Possible astrophysical relevance for the
detection of primordial black-holes and binary systems is discussed.Comment: 9 pages, Honorable Mention from the Gravity Research Foundation, 199
IUE spectra of a flare in HR 5110: A flaring RS CVn or Algol system?
Ultraviolet spectra of the RS CVn type binary system HR 5110 were obtained with IUE on May 31, 1979 during a period of intense radio flaring of this star. High temperature transition region lines are present, but are not enhanced above observed quiescent strengths. The similarities of HR 5110 to the Algol system, As Eri, suggest that the 1979 May to June flare may involve mass exchange rather than annihilation of coronal magnetic fields
Geometrical Magnetic Frustration in Rare Earth Chalcogenide Spinels
We have characterized the magnetic and structural properties of the CdLn2Se4
(Ln = Dy, Ho), and CdLn2S4 (Ln = Ho, Er, Tm, Yb) spinels. We observe all
compounds to be normal spinels, possessing a geometrically frustrated
sublattice of lanthanide atoms with no observable structural disorder. Fits to
the high temperature magnetic susceptibilities indicate these materials to have
effective antiferromagnetic interactions, with Curie-Weiss temperatures theta ~
-10 K, except CdYb2S4 for which theta ~ -40 K. The absence of magnetic long
range order or glassiness above T = 1.8 K strongly suggests that these
materials are a new venue in which to study the effects of strong geometrical
frustration, potentially as rich in new physical phenomena as that of the
pyrochlore oxides.Comment: 17 pages, 5 figures, submitted to Phys Rev B; added acknowledgement
Nanoengineered Curie Temperature in Laterally-Patterned Ferromagnetic Semiconductor Heterostructures
We demonstrate the manipulation of the Curie temperature of buried layers of
the ferromagnetic semiconductor (Ga,Mn)As using nanolithography to enhance the
effect of annealing. Patterning the GaAs-capped ferromagnetic layers into
nanowires exposes free surfaces at the sidewalls of the patterned (Ga,Mn)As
layers and thus allows the removal of Mn interstitials using annealing. This
leads to an enhanced Curie temperature and reduced resistivity compared to
unpatterned samples. For a fixed annealing time, the enhancement of the Curie
temperature is larger for narrower nanowires.Comment: Submitted to Applied Physics Letters (minor corrections
Valence-bond crystal in a {111} slice of the pyrochlore antiferromagnet
We investigate theoretically the ordering effect of quantum spin fluctuations
in a Heisenberg antiferromagnet on a two-dimensional network of corner sharing
tetrahedra. This network is obtained as a {111} slice of the highly frustrated
pyrochlore lattice, from which it inherits the equivalence of all three pairs
of opposite bonds of each tetrahedron. The lowest-order (in 1/S) quantum
corrections partially lift the huge degeneracy of the classical ground state
and select an ensemble of states with long-range valence-bond order.Comment: 4 pages, 2 EPS figures. Minor revision: clarifications in response to
referee comments, additional reference
Ga-NMR local susceptibility of the kagome-based magnet SrCr_9pGa_(12-9p)O_19. A high temperature study
We report a high- Ga-NMR study in the kagome-based antiferromagnetic
compound SrCrGaO (), and present a
refined mean-field analysis of the high T local NMR susceptibility of Cr
frustrated moments. We find that the intralayer kagome coupling is K,
and the interlayer coupling through non-kagome Cr moments is K. The ratio confirms the common belief that
the frustrated entity is a pyrochlore slab.Comment: 8 pages, 4 figures Conference paper: Highly Frustrated Magnetism
2000, Waterloo (Canada) Submitted to Canadian Journal of Physic
Measurements of Nanoscale Domain Wall Flexing in a Ferromagnetic Thin Film
We use the high spatial sensitivity of the anomalous Hall effect in the
ferromagnetic semiconductor Ga1-xMnxAs, combined with the magneto-optical Kerr
effect, to probe the nanoscale elastic flexing behavior of a single magnetic
domain wall in a ferromagnetic thin film. Our technique allows position
sensitive characterization of the pinning site density, which we estimate to be
around 10^14 cm^{-3}. Analysis of single site depinning events and their
temperature dependence yields estimates of pinning site forces (10 pN range) as
well as the thermal deactivation energy. Finally, our data hints at a much
higher intrinsic domain wall mobility for flexing than previously observed in
optically-probed micron scale measurements
Quantum spin liquids: a large-S route
This paper explores the large-S route to quantum disorder in the Heisenberg
antiferromagnet on the pyrochlore lattice and its homologues in lower
dimensions. It is shown that zero-point fluctuations of spins shape up a
valence-bond solid at low temperatures for one two-dimensional lattice and a
liquid with very short-range valence-bond correlations for another. A
one-dimensional model demonstrates potential significance of quantum
interference effects (as in Haldane's gap): the quantum melting of a
valence-bond order yields different valence-bond liquids for integer and
half-integer values of S.Comment: Proceedings of Highly Frustrated Magnetism 2003 (Grenoble), 6 LaTeX
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