1,956 research outputs found
Fermi Surface of CrV across the Quantum Critical Point
We have measured de Haas-van Alphen oscillations of CrV, , at high fields for samples on both sides of the quantum critical
point at . For all samples we observe only those oscillations
associated with a single small hole band with magnetic breakdown orbits of the
reconstructed Fermi surface evident for . The absence of oscillations
from Fermi surface sheets most responsible for the spin density wave (SDW) in
Cr for is further evidence for strong fluctuation scattering of these
charge carriers well into the paramagnetic regime. We find no significant mass
enhancement of the carriers in the single observed band at any . An
anomalous field dependence of the dHvA signal for our crystal at
particular orientations of the magnetic field is identified as due to magnetic
breakdown that we speculate results from a field induced SDW transition at high
fields.Comment: 8 pages with 7 figure
High Field de Haas - van Alphen Studies of the Fermi Surfaces of LaMIn (M = Co, Rh, Ir)
We report measurements of the de Haas - van Alphen effect on a series of
compounds, LaMIn (M = Co, Rh, Ir). The results show that each of the Co
and Ir Fermi surfaces (FSs) exhibit some portions that are two dimensional and
some portions that are three dimensional. The most two dimensional character is
exhibited in LaCoIn, less two dimensional behavior is seen in
LaIrIn, no part of Fermi surface of LaRhIn is found to have a two
dimensional character. Thus the two dimensionality of portions of the FSs is
largely determined by the d character of the energy bands while all of the
effective masses remain 1.2. This fact has implications for the causes
of the heavy fermion nature of superconductivity and magnetism in the Ce-based
compounds having the similar composition and structure. All of the measurements
were performed at the National High Magnetic Field Laboratory using either
cantilever magnetometry or field modulation methods.Comment: 10 pages, 4 figure
Observations of the magnetic field and plasma flow in Jupiter's magnetosheath
Large scale (many minutes to 10 hours) magnetic field structures consisting predominantly of nearly north-south field direction were discovered in Jupiter's magnetosheath from the data of Voyagers 1 and 2 and Pioneer 10 during their outbound encounter trajectories. The Voyager 2 data, and that of Voyager 1 to a lesser extent, show evidence of a quasi-period of 10 hours (and occasionally 5 hours) for these structures. The north-south components of the field and plasma velocity were strongly correlated in the outbound magnetosheath as observed by Voyagers 1 and 2, and the components orthogonal to the north-south direction showed weak correlations. For both Voyager encounters the sense (positive and negative) of the north-south correlations were directly related to the direction of the ecliptic plane component of the interplanetary magnetic field using the field and plasma measurements of the non-encountering spacecraft
The Fermi surface of CeCoIn5: dHvA
Measurements of the de Haas - van Alphen effect in the normal state of the
heavy Fermion superconductor CeCoIn5 have been carried out using a torque
cantilever at temperatures ranging from 20 to 500 mK and in fields up to 18
tesla. Angular dependent measurements of the extremal Fermi surface areas
reveal a more extreme two dimensional sheet than is found in either CeRhIn5 or
CeIrIn5. The effective masses of the measured frequencies range from 9 to 20
m*/m0.Comment: 4 pages, 2 figures, submitted to PRB Rapid
Predicting magnetopause crossings at geosynchronous orbit during the Halloween storms
[1] In late October and early November of 2003, the Sun unleashed a powerful series of events known as the Halloween storms. The coronal mass ejections launched by the Sun produced several severe compressions of the magnetosphere that moved the magnetopause inside of geosynchronous orbit. Such events are of interest to satellite operators, and the ability to predict magnetopause crossings along a given orbit is an important space weather capability. In this paper we compare geosynchronous observations of magnetopause crossings during the Halloween storms to crossings determined from the Lyon-Fedder-Mobarry global magnetohydrodynamic simulation of the magnetosphere as well to predictions of several empirical models of the magnetopause position. We calculate basic statistical information about the predictions as well as several standard skill scores. We find that the current Lyon-Fedder-Mobarry simulation of the storm provides a slightly better prediction of the magnetopause position than the empirical models we examined for the extreme conditions present in this study. While this is not surprising, given that conditions during the Halloween storms were well outside the parameter space of the empirical models, it does point out the need for physics-based models that can predict the effects of the most extreme events that are of significant interest to users of space weather forecasts
The Radio Afterglow and the Host Galaxy of the X-Ray Rich GRB 981226
We report the discovery of a radio transient VLA 232937.2-235553, coincident
with the proposed X-ray afterglow for the gamma-ray burst GRB 981226. This GRB
has the highest ratio of X-ray to gamma-ray fluence of all the GRBs detected by
BeppoSAX so far and yet no corresponding optical transient was detected. The
radio light curve of VLA 232937.2-235553 is qualitatively similar to that of
several other radio afterglows. At the sub-arcsecond position provided by the
radio detection, optical imaging reveals an extended R=24.9 mag object, which
we identify as the host galaxy of GRB 981226. Afterglow models which invoke a
jet-like geometry for the outflow or require an ambient medium with a radial
density dependence, such as that produced by a wind from a massive star, are
both consistent with the radio data. Furthermore, we show that the observed
properties of the radio afterglow can explain the absence of an optical
transient without the need for large extinction local to the GRB.Comment: Accepted for publication in the Astrophysical Journal Letters.
Thirteen pages. Three Postscript figure
The afterglows of gamma-ray bursts
Gamma-ray burst astronomy has undergone a revolution in the last three years, spurred by the discovery of fading long-wavelength counterparts. We now know that at least the long duration GRBs lie at cosmological distances with estimated electromagnetic energy release of 10^51–10^53 erg, making these the brightest explosions in the Universe. In this article we review the current observational state, beginning with the statistics of X-ray, optical, and radio afterglow detections. We then discuss the insights these observations have given to the progenitor population, the energetics of the GRB events, and the physics of the afterglow emission. We focus particular attention on the evidence linking GRBs to the explosion of massive stars. Throughout, we identify remaining puzzles and uncertainties, and emphasize promising observational tools for addressing them. The imminent launch of HETE-2 and the increasingly sophisticated and coordinated ground-based and space-based observations have primed this field for fantastic growth
The afterglows of gamma-ray bursts
Gamma-ray burst astronomy has undergone a revolution in the last three years, spurred by the discovery of fading long-wavelength counterparts. We now know that at least the long-duration GRBs lie at cosmological distances with estimated electromagnetic energy release of 10^51–10^53 erg, making these the brightest explosions in the Universe. In this article we review the current observational state, beginning with the statistics of X-ray, optical, and radio afterglow detections. We then discuss the insights these observations have given to the progenitor population, the energetics of the GRB events, and the physics of the afterglow emission. We focus particular attention on the evidence linking GRBs to the explosion of massive stars. Throughout, we identify remaining puzzles and uncertainties, and emphasize promising observational tools for addressing them. The imminent launch of HETE-2 and the increasingly sophisticated and coordinated ground-based and space-based observations have primed this field for fantastic growth
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