5,387 research outputs found
Dissipation of the sectored heliospheric magnetic field near the heliopause: a mechanism for the generation of anomalous cosmic rays
The recent observations of the anomalous cosmic ray (ACR) energy spectrum as
Voyagers 1 and 2 crossed the heliospheric termination shock have called into
question the conventional shock source of these energetic particles. We suggest
that the sectored heliospheric magnetic field, which results from the flapping
of the heliospheric current sheet, piles up as it approaches the heliopause,
narrowing the current sheets that separate the sectors and triggering the onset
of collisionless magnetic reconnection. Particle-in-cell simulations reveal
that most of the magnetic energy is released and most of this energy goes into
energetic ions with significant but smaller amounts of energy going into
electrons. The energy gain of the most energetic ions results from their
reflection from the ends of contracting magnetic islands, a first order Fermi
process. The energy gain of the ions in contracting islands increases their
parallel (to the magnetic field ) pressure until the
marginal firehose condition is reached, causing magnetic reconnection and
associated particle acceleration to shut down. The model calls into question
the strong scattering assumption used to derive the Parker transport equation
and therefore the absence of first order Fermi acceleration in incompressible
flows. A simple 1-D model for particle energy gain and loss is presented in
which the feedback of the energetic particles on the reconnection drive is
included. The ACR differential energy spectrum takes the form of a power law
with a spectral index slightly above 1.5. The model has the potential to
explain several key Voyager observations, including the similarities in the
spectra of different ion species.Comment: Submitted to ApJ; shortened abstract; degraded figure qualit
Surface state reconstruction in ion-damaged SmB_6
We have used ion-irradiation to damage the (001) surfaces of SmB_6 single
crystals to varying depths, and have measured the resistivity as a function of
temperature for each depth of damage. We observe a reduction in the residual
resistivity with increasing depth of damage. Our data are consistent with a
model in which the surface state is not destroyed by the ion-irradiation, but
instead the damaged layer is poorly conducting and the initial surface state is
reconstructed below the damage. This behavior is consistent with a surface
state that is topologically protected.Comment: 5 pages, 3 figure
Evidence for electron-phonon interaction in FeMSb (M=Co, Cr) single crystals
We have measured polarized Raman scattering spectra of the
FeCoSb and FeCrSb (00.5)
single crystals in the temperature range between 15 K and 300 K. The highest
energy symmetry mode shows significant line asymmetry due to phonon
mode coupling width electronic background. The coupling constant achieves the
highest value at about 40 K and after that it remains temperature independent.
Origin of additional mode broadening is pure anharmonic. Below 40 K the
coupling is drastically reduced, in agreement with transport properties
measurements. Alloying of FeSb with Co and Cr produces the B mode
narrowing, i.e. weakening of the electron-phonon interaction. In the case of
A symmetry modes we have found a significant mode mixing
Superconductivity without Fe or Ni in the phosphides BaIr2P2 and BaRh2P2
Heat capacity, resistivity, and magnetic susceptibility measurements confirm
bulk superconductivity in single crystals of BaIrP (T=2.1K) and
BaRhP (T = 1.0 K). These compounds form in the ThCrSi (122)
structure so they are isostructural to both the Ni and Fe pnictides but not
isoelectronic to either of them. This illustrates the importance of structure
for the occurrence of superconductivity in the 122 pnictides. Additionally, a
comparison between these and other ternary phosphide superconductors suggests
that the lack of interlayer bonding favors superconductivity. These
stoichiometric and ambient pressure superconductors offer an ideal playground
to investigate the role of structure for the mechanism of superconductivity in
the absence of magnetism.Comment: Published in Phys Rev B: Rapid Communication
Single crystal growth and physical properties of a new uranium compound URhIn
We have grown the new uranium compound URhIn with the tetragonal
HoCoGa-type by the In self flux method. In contrast to the nonmagnetic
ground state of the isoelectronic analogue URhGa, URhIn is an
antiferromagnet with antiferromagnetic transition temperature = 98
K. The moderately large electronic specific heat coefficient = 50
mJ/Kmol demonstrates the contribution of 5 electrons to the conduction
band. On the other hand, magnetic susceptibility in the paramagnetic state
roughly follows a Curie-Weiss law with a paramagnetic effective moment
corresponding to a localized uranium ion. The crossover from localized to
itinerant character at low temperature may occur around the characteristic
temperature 150 K where the magnetic susceptibility and electrical resistivity
show a marked anomaly.Comment: 7 pages, 7 figure
Centriole assembly and the role of Mps1: defensible or dispensable?
The Mps1 protein kinase is an intriguing and controversial player in centriole assembly. Originally shown to control duplication of the budding yeast spindle pole body, Mps1 is present in eukaryotes from yeast to humans, the nematode C. elegans being a notable exception, and has also been shown to regulate the spindle checkpoint and an increasing number of cellular functions relating to genomic stability. While its function in the spindle checkpoint appears to be both universally conserved and essential in most organisms, conservation of its originally described function in spindle pole duplication has proven controversial, and it is less clear whether Mps1 is essential for centrosome duplication outside of budding yeast. Recent studies of Mps1 have identified at least two distinct functions for Mps1 in centriole assembly, while simultaneously supporting the notion that Mps1 is dispensable for the process. However, the fact that at least one centrosomal substrate of Mps1 is conserved from yeast to humans down to the phosphorylation site, combined with evidence demonstrating the exquisite control exerted over centrosomal Mps1 levels suggest that the notion of being essential may not be the most important of distinctions
Magnetic, thermal and transport properties of Cd doped CeIn
We have investigated the effect of Cd substitution on the archetypal heavy
fermion antiferromagnet CeIn via magnetic susceptibility, specific heat and
resistivity measurements. The suppression of the Neel temperature, T,
with Cd doping is more pronounced than with Sn. Nevertheless, a doping induced
quantum critical point does not appear to be achievable in this system. The
magnetic entropy at and the temperature of the maximum in resistivity are
also systematically suppressed with Cd, while the effective moment and the
Curie-Weiss temperature in the paramagnetic state are not affected. These
results suggest that Cd locally disrupts the AFM order on its neighboring Ce
moments, without affecting the valence of Ce. Moreover, the temperature
dependence of the specific heat below is not consistent with 3D magnons
in pure as well as in Cd-doped CeIn, a point that has been missed in
previous investigations of CeIn and that has bearing on the type of quantum
criticality in this system
A predictive standard model for heavy electron systems
We propose a predictive standard model for heavy electron systems based on a
detailed phenomenological two-fluid description of existing experimental data.
It leads to a new phase diagram that replaces the Doniach picture, describes
the emergent anomalous scaling behavior of the heavy electron (Kondo) liquid
measured below the lattice coherence temperature, T*, seen by many different
experimental probes, that marks the onset of collective hybridization, and
enables one to obtain important information on quantum criticality and the
superconducting/antiferromagnetic states at low temperatures. Because T* is
~J^2\rho/2, the nearest neighbor RKKY interaction, a knowledge of the
single-ion Kondo coupling, J, to the background conduction electron density of
states, \rho, makes it possible to predict Kondo liquid behavior, and to
estimate its maximum superconducting transition temperature in both existing
and newly discovered heavy electron families.Comment: 4 pages, 2 figures, submitted to J. Phys.: Conf. Ser. for SCES 201
An explanation for strongly underwound magnetic field in coârotating rarefaction regions and its relationship to footpoint motion on the the sun
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/94855/1/grl15835.pd
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