136,430 research outputs found
Dynamics and transport properties of Kondo insulators
A many-body theory of paramagnetic Kondo insulators is described, focusing
specifically on single-particle dynamics, scattering rates, d.c. transport and
optical conductivities. This is achieved by development of a non-perturbative
local moment approach to the symmetric periodic Anderson model within the
framework of dynamical mean-field theory. Our natural focus is the strong
coupling, Kondo lattice regime; in particular the resultant `universal' scaling
behaviour in terms of the single, exponentially small low-energy scale
characteristic of the problem. Dynamics/transport on all relevant ()
scales are considered, from the gapped/activated behaviour characteristic of
the low-temperature insulator through to explicit connection to single-impurity
physics at high and/or ; and for optical conductivities emphasis is
given to the nature of the optical gap, the temperature scale responsible for
its destruction, and the consequent clear distinction between indirect and
direct gap scales. Using scaling, explicit comparison is also made to
experimental results for d.c. transport and optical conductivites of
Ce_3Bi_4Pt_3, SmB_6 and YbB_{12}. Good agreement is found, even quantitatively;
and a mutually consistent picture of transport and optics results.Comment: 49 pages, 23 figure
Automatic classification of eutrophication of inland lakes from spacecraft data
The author has identified the following significant results. Spacecraft data and computer techniques can be used to rapidly map and store onto digital tapes watershed land use information. Software is now available by which this land use information can be rapidly and economically extracted from the tapes and related to coliform counts and other lake contaminants (e.g. phosphorus). These tools are basic elements for determining those land use factors and sources of nutrients that accelerate eutrophication in lakes and reservoirs
Light echoes reveal an unexpectedly cool Eta Carinae during its 19th-century Great Eruption
Eta Carinae (Eta Car) is one of the most massive binary stars in the Milky
Way. It became the second-brightest star in the sky during its mid-19th century
"Great Eruption," but then faded from view (with only naked-eye estimates of
brightness). Its eruption is unique among known astronomical transients in that
it exceeded the Eddington luminosity limit for 10 years. Because it is only 2.3
kpc away, spatially resolved studies of the nebula have constrained the ejected
mass and velocity, indicating that in its 19th century eruption, Eta Car
ejected more than 10 M_solar in an event that had 10% of the energy of a
typical core-collapse supernova without destroying the star. Here we report the
discovery of light echoes of Eta Carinae which appear to be from the 1838-1858
Great Eruption. Spectra of these light echoes show only absorption lines, which
are blueshifted by -210 km/s, in good agreement with predicted expansion
speeds. The light-echo spectra correlate best with those of G2-G5 supergiant
spectra, which have effective temperatures of ~5000 K. In contrast to the class
of extragalactic outbursts assumed to be analogs of Eta Car's Great Eruption,
the effective temperature of its outburst is significantly cooler than allowed
by standard opaque wind models. This indicates that other physical mechanisms
like an energetic blast wave may have triggered and influenced the eruption.Comment: Accepted for publication by Nature; 4 pages, 4 figures, SI: 6 pages,
3 figures, 5 table
Observed Consequences of Presupernova Instability in Very Massive Stars
This chapter concentrates on the deaths of very massive stars, the events
leading up to their deaths, and how mass loss affects the resulting death. The
previous three chapters emphasized the theory of wind mass loss, eruptions, and
core collapse physics, but here we emphasize mainly the observational
properties of the resulting death throes. Mass loss through winds, eruptions,
and interacting binaries largely determines the wide variety of different types
of supernovae that are observed, as well as the circumstellar environments into
which the supernova blast waves expand. Connecting these observed properties of
the explosions to the initial masses of their progenitor stars is, however, an
enduring challenge and is especially difficult for very massive stars.
Superluminous supernovae, pair instability supernovae, gamma ray bursts, and
"failed" supernovae are all end fates that have been proposed for very massive
stars, but the range of initial masses or other conditions leading to each of
these (if they actually occur) are still very certain. Extrapolating to infer
the role of very massive stars in the early universe is essentially
unencumbered by observational constraints and still quite dicey.Comment: 39 pages, 5 figures, to appear as chapter in the book "Very Massive
Stars in the Local Universe", ed. J. Vin
Dissipation signatures of the normal and superfluid phases in torsion pendulum experiments with 3He in aerogel
We present data for energy dissipation factor (Q^{-1}) over a broad
temperature range at various pressures of a torsion pendulum setup used to
study 3He confined in a 98% open silica aerogel. Values for Q^{-1} above T_c
are temperature independent and have a weak pressure dependence. Below T_c, a
deliberate axial compression of the aerogel by 10% widens the range of
metastability for a superfluid Equal Spin Pairing (ESP) state; we observe this
ESP phase on cooling and the B phase on warming over an extended temperature
region. While the dissipation for the B phase tends to zero as T goes to 0,
Q^{-1} exhibits a peak value greater than that at T_c at intermediate
temperatures. Values for Q^{-1} in the ESP phase are consistently higher than
in the B phase and are proportional to \rho_s/\rho until the ESP to B phase
transition is attained. We apply a viscoelastic collision-drag model, which
couples the motion of the helium and the aerogel through a frictional
relaxation time \tau_f. Our dissipation data is not sensitive to the damping
due to the presumed small but non-zero value of \tau_f. The result is that an
additional mechanism to dissipate energy not captured in the collision-drag
model and related to the emergence of the superfluid order must exist. The
extra dissipation below T_c is possibly associated with mutual friction between
the superfluid phases and the clamped normal fluid. The pressure dependence of
the measured dissipation in both superfluid phases is likely related to the
pressure dependence of the gap structure of the "dirty" superfluid. The large
dissipation in the ESP state is consistent with the phase being the A or the
Polar with the order parameter nodes oriented in the plane of the cell and
perpendicular to the aerogel anisotropy axis.Comment: 12 pages, 7 figure
Integer quantum Hall effect on a six valley hydrogen-passivated silicon (111) surface
We report magneto-transport studies of a two-dimensional electron system
formed in an inversion layer at the interface between a hydrogen-passivated
Si(111) surface and vacuum. Measurements in the integer quantum Hall regime
demonstrate the expected sixfold valley degeneracy for these surfaces is
broken, resulting in an unequal occupation of the six valleys and anisotropy in
the resistance. We hypothesize the misorientation of Si surface breaks the
valley states into three unequally spaced pairs, but the observation of odd
filling factors, is difficult to reconcile with non-interacting electron
theory.Comment: 4 pages, 4 figures, to appear in Physical Review Letter
Variable temperature study of the crystal and magnetic structures of the giant magnetoresistant materials LMnAsO (L=La, Nd)
Peer reviewedPublisher PD
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