1,745 research outputs found
Dynamics of drag and force distributions for projectile impact in a granular medium
Our experiments and molecular dynamics simulations on a projectile
penetrating a two-dimensional granular medium reveal that the mean deceleration
of the projectile is constant and proportional to the impact velocity. Thus,
the time taken for a projectile to decelerate to a stop is independent of its
impact velocity. The simulations show that the probability distribution
function of forces on grains is time-independent during a projectile's
penetration of the medium. At all times the force distribution function
decreases exponentially for large forces.Comment: 4 page
Magnetic ordering in EuRh2As2 studied by x-ray resonant magnetic scattering
Element-specific x-ray resonant magnetic scattering investigations were
performed to determine the magnetic structure of Eu in EuRh2As2. In the
temperature range from 46 K down to 6 K, an incommensurate antiferromagnetic
(ICM)structure with a temperature dependent propagation vector (0 0 0.9)
coexists with a commensurate antiferromagnetic (CM) structure.
Angular-dependent measurements of the magnetic intensity indicate that the
magnetic moments lie in the tetragonal basal plane and are ferromagnetically
aligned within the a-b plane for both magnetic structures. The ICM structure is
a spiral-like magnetic structure with a turn angle of 162 deg between adjacent
Eu planes. In the CM structure, this angle is 180 deg. These results are
consistent with band-structure calculations which indicate a strong sensitivity
of the magnetic configuration on the Eu valence.Comment: 5 pages, 5 figures (technical problem with abstract corrected, no
other changes
Channeled blast wave behavior based on longitudinal instabilities
To address the important issue of how kinetic energy of collimated blast
waves is converted into radiation, Pohl & Schlickeiser (2000) have recently
investigated the relativistic two-stream instability of electromagnetic
turbulence. They have shown that swept-up matter is quickly isotropized in the
blast wave, which provides relativistic particles and, as a result, radiation.
Here we present new calculations for the electrostatic instability in such
systems. It is shown that the electrostatic instability is faster than the
electromagnetic instability for highly relativistic beams. However, even after
relaxation of the beam via the faster electrostatic turbulence, the beam is
still unstable with respect to the electromagnetic waves, thus providing the
isotropization required for efficient production of radiation. While the
emission spectra in the model of Pohl and Schlickeiser have to be modified, the
basic characteristics persist.Comment: Astronomy & Astrophysics, in pres
Magnetoresistance of composite fermions at \nu=1/2
We have studied temperature dependence of both diagonal and Hall resistivity
in the vicinity of . Magnetoresistance was found to be positive and
almost independent of temperature: temperature enters resistivity as a
logarithmic correction. At the same time, no measurable corrections to the Hall
resistivity has been found. Neither of these results can be explained within
the mean-field theory of composite fermions by an analogy with conventional
low-field interaction theory. There is an indication that interactions of
composite fermions with fluctuations of the gauge field may reconcile the
theory and experiment.Comment: 9 pages, 4 figure
Influence of Magnetism on Phonons in CaFe2As2 Via Inelastic X-ray Scattering
In the iron pnictides, the strong sensitivity of the iron magnetic moment to
the arsenic position suggests a significant relationship between phonons and
magnetism. We measured the phonon dispersion of several branches in the high
temperature tetragonal phase of CaFe2As2 using inelastic x-ray scattering on
single-crystal samples. These measurements were compared to ab initio
calculations of the phonons. Spin polarized calculations imposing the
antiferromagnetic order present in the low temperature orthorhombic phase
dramatically improve agreement between theory and experiment. This is discussed
in terms of the strong antiferromagnetic correlations that are known to persist
in the tetragonal phase.Comment: 4 pages, 3 figures; added additional information and references about
spin fluctuation
The Aharonov-Bohm Effect in the Fractional Quantum Hall Regime
We have investigated experimentally resonant tunnelling through
single-particle states formed around an antidot by a magnetic field, in the
fractional quantum Hall regime. For 1/3 filling factor around the antidot,
Aharonov-Bohm oscillations are observed with the same magnetic field period as
in the integer quantum Hall regime. All our measurements are consistent with
quasiparticles of fractional charge e*. However, the results are also
consistent with particles of any charge (>= e*) as the system must rearrange
every time the flux enclosed increases by h/e.Comment: Postscript, 4 pages, gzipped (350 kB
Crystallographic snapshots of sulfur insertion by lipoyl synthase
Lipoyl synthase (LipA) catalyzes the insertion of two sulfur atoms at the unactivated C6 and C8 positions of a protein-bound octanoyl chain to produce the lipoyl cofactor. To activate its substrate for sulfur insertion, LipA uses a [4Fe-4S] cluster and S-adenosylmethionine (AdoMet) radical chemistry; the remainder of the reaction mechanism, especially the source of the sulfur, has been less clear. One controversial proposal involves the removal of sulfur from a second (auxiliary) [4Fe-4S] cluster on the enzyme, resulting in destruction of the cluster during each round of catalysis. Here, we present two high-resolution crystal structures of LipA from Mycobacterium tuberculosis: one in its resting state and one at an intermediate state during turnover. In the resting state, an auxiliary [4Fe-4S] cluster has an unusual serine ligation to one of the irons. After reaction with an octanoyllysine-containing 8-mer peptide substrate and 1 eq AdoMet, conditions that allow for the first sulfur insertion but not the second insertion, the serine ligand dissociates from the cluster, the iron ion is lost, and a sulfur atom that is still part of the cluster becomes covalently attached to C6 of the octanoyl substrate. This intermediate structure provides a clear picture of iron-sulfur cluster destruction in action, supporting the role of the auxiliary cluster as the sulfur source in the LipA reaction and describing a radical strategy for sulfur incorporation into completely unactivated substrates. Keywords: iron–sulfur cluster; radical SAM enzyme; lipoic acidNational Science Foundation (U.S.) (Grant MCB-0543833
Colossal Positive Magnetoresistance in a Doped Nearly Magnetic Semiconductor
We report on a positive colossal magnetoresistance (MR) induced by
metallization of FeSb, a nearly magnetic or "Kondo" semiconductor with 3d
ions. We discuss contribution of orbital MR and quantum interference to
enhanced magnetic field response of electrical resistivity.Comment: 5 pages, 5 figure
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