4,996 research outputs found
Probing the interplay between surface and bulk states in the topological Kondo insulator SmB through conductance fluctuation spectroscopy
We present results of resistance fluctuation spectroscopy on single crystals
of the predicted Kondo topological insulator material SmB. Our measurements
show that at low temperatures, transport in this system takes place only
through surface states. The measured noise in this temperature range arises due
to Universal Conductance Fluctuations whose statistics was found to be
consistent with theoretical predictions for that of two-dimensional systems in
the Symplectic symmetry class. At higher temperatures, we find signatures of
glassy dynamics and establish that the measured noise is caused by mobility
fluctuations in the bulk. We find that, unlike the topological insulators of
the dichalcogenide family, the noise in surface and bulk conduction channels in
SmB are completely uncorrelated. Our measurements establish that at
sufficiently low temperatures, the bulk has no discernible contribution to
electrical transport in SmB making it an ideal platform for probing the
physics of topological surface states.Comment: 9 pages, 11 figure
Magnetic properties of geometrically frustrated SrGd2O4
A study of the magnetic properties of the frustrated rare earth oxide SrGd2O4
has been completed using bulk property measurements of magnetization,
susceptibility and specific heat on single crystal samples. Two zero-field
phase transitions have been identified at 2.73 and 0.48 K. For the field, H,
applied along the a and b axes, a single boundary is identified that delineates
the transition from a low field, low temperature magnetically ordered regime to
a high field, high temperature paramagnetic phase. Several field-induced
transitions, however, have been observed with H || c. The measurements have
been used to map out the magnetic phase diagram of SrGd2O4, suggesting that it
is a complex system with several competing magnetic interactions. The
low-temperature magnetic behavior of SrGd2O4 is very different compared to the
other SrLn2O4 (Ln = Lanthanide) compounds, even though all of the SrLn2O4
compounds are isostructural, with the magnetic ions forming a low-dimensional
lattice of zigzag chains that run along the c axis. The differences are likely
to be due to the fact that in the ground state Gd3+ has zero orbital angular
momentum and therefore the spin-orbit interactions, which are crucial for other
SrLn2O4 compounds, can largely be neglected. Instead, given the relatively
short Gd3+-Gd3+ distances in SrGd2O4, dipolar interactions must be taken into
account for this antiferromagnet alongside the Heisenberg exchange terms.Comment: 10 pages, 9 figure
Growth of large single crystals of Rare Earth Hexaborides
Single crystal growth of several rare earth hexaborides has been carried out
by the floating zone technique. A high power Xenon arc lamp image furnace was
used for the crystal growth. Large high quality crystals of LaB,
CeB, PrB and NdB, about 1 cc in volume have been obtained.
Crystals of all these compounds have also been grown using enriched B
isotope for use in neutron scattering experiments.Comment: 3 pages, 3 figures, in press (J. Crystal Growth
System Support for Bandwidth Management and Content Adaptation in Internet Applications
This paper describes the implementation and evaluation of an operating system
module, the Congestion Manager (CM), which provides integrated network flow
management and exports a convenient programming interface that allows
applications to be notified of, and adapt to, changing network conditions. We
describe the API by which applications interface with the CM, and the
architectural considerations that factored into the design. To evaluate the
architecture and API, we describe our implementations of TCP; a streaming
layered audio/video application; and an interactive audio application using the
CM, and show that they achieve adaptive behavior without incurring much
end-system overhead. All flows including TCP benefit from the sharing of
congestion information, and applications are able to incorporate new
functionality such as congestion control and adaptive behavior.Comment: 14 pages, appeared in OSDI 200
Formation of molecular oxygen in ultracold O + OH reaction
We discuss the formation of molecular oxygen in ultracold collisions between
hydroxyl radicals and atomic oxygen. A time-independent quantum formalism based
on hyperspherical coordinates is employed for the calculations. Elastic,
inelastic and reactive cross sections as well as the vibrational and rotational
populations of the product O2 molecules are reported. A J-shifting
approximation is used to compute the rate coefficients. At temperatures T = 10
- 100 mK for which the OH molecules have been cooled and trapped
experimentally, the elastic and reactive rate coefficients are of comparable
magnitude, while at colder temperatures, T < 1 mK, the formation of molecular
oxygen becomes the dominant pathway. The validity of a classical capture model
to describe cold collisions of OH and O is also discussed. While very good
agreement is found between classical and quantum results at T=0.3 K, at higher
temperatures, the quantum calculations predict a larger rate coefficient than
the classical model, in agreement with experimental data for the O + OH
reaction. The zero-temperature limiting value of the rate coefficient is
predicted to be about 6.10^{-12} cm^3 molecule^{-1} s^{-1}, a value comparable
to that of barrierless alkali-metal atom - dimer systems and about a factor of
five larger than that of the tunneling dominated F + H2 reaction.Comment: 9 pages, 8 figure
The Kolar Schist Belt: A possible Archaean suture zone
The Kolar Schist Belt represents a N-S trending discontinuity in the structures, lithologies, and emplacement and metamorphic ages of late Archean gneisses. The suggestion of a much older basement on the west side of the belt is not seen on the east. Within the schist belt amphibolites from each side have distinctly different chemical characteristics, suggesting different sources at similar mantle depths. These amphibolites were probably not part of a single volcanic sequence, but may have formed about the same time in two completely different settings. Could the amphibolites with depleted light REE patterns represent Archean ocean floor volcanics which are derived from a mantle source with a long term depletion of the light REE? Why are the amphibolites giving an age which may be older than the exposed gneisses immediately on either side of the belt? These results suggest that it is necessary to seriously consider whether the Kolar Schist Belt may be a suture between two late Archean continental terranes
Self-assembly of iron nanoclusters on the Fe3O4(111) superstructured surface
We report on the self-organized growth of a regular array of Fe nanoclusters
on a nanopatterned magnetite surface. Under oxidizing preparation conditions
the (111) surface of magnetite exhibits a regular superstructure with
three-fold symmetry and a 42 A periodicity. This superstructure represents an
oxygen terminated (111) surface, which is reconstructed to form a periodically
strained surface. This strain patterned surface has been used as a template for
the growth of an ultrathin metal film. A Fe film of 0.5 A thickness was
deposited on the substrate at room temperature. Fe nanoclusters are formed on
top of the surface superstructure creating a regular array with the period of
the superstructure. We also demonstrate that at least the initial stage of Fe
growth occurs in two-dimensional mode. In the areas of the surface where the
strain pattern is not formed, random nucleation of Fe was observed.Comment: 6 pages, 3 figure
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