9,446 research outputs found
Determination of the Fermi Velocity by Angle-dependent Periodic Orbit Resonance Measurements in the Organic Conductor alpha-(BEDT-TTF)2KHg(SCN)4
We report detailed angle-dependent studies of the microwave (f=50 to 90 GHz)
interlayer magneto-electrodynamics of a single crystal sample of the organic
charge-density-wave (CDW) conductor alpha-(BEDT-TTF)2KHg(SCN)4. Recently
developed instrumentation enables both magnetic field (B) sweeps for a fixed
sample orientation and, for the first time, angle sweeps at fixed f/B. We
observe series' of resonant absorptions which we attribute to periodic orbit
resonances (POR) - a phenomenon closely related to cyclotron resonance. The
angle dependence of the POR indicate that they are associated with the low
temperature quasi-one-dimensional (Q1D) Fermi surface (FS) of the title
compound; indeed, all of the resonance peaks collapse beautifully onto a single
set of f/B versus angle curves, generated using a semiclassical
magneto-transport theory for a single Q1D FS. We show that Q1D POR measurements
provide one of the most direct methods for determining the Fermi velocity,
without any detailed assumptions concerning the bandstructure; our analysis
yields an average value of v_F=6.5x10^4 m/s. Quantitative analysis of the POR
harmonic content indicates that the Q1D FS is strongly corrugated. This is
consistent with the assumption that the low-temperature FS derives from a
reconstruction of the high temperature quasi-two-dimensional FS, caused by the
CDW instability. Detailed analysis of the angle dependence of the POR yields
parameters associated with the CDW superstructure which are consistent with
published results. Finally, we address the issue as to whether or not the
interlayer electrodynamics are coherent in the title compound.Comment: 28 pages, including 6 figures. Submitted to PR
Anomalous behaviour of the in-plane electrical conductivity of the layered superconductor -(BEDT-TTF)Cu(NCS)
The quasiparticle scattering rates in high-quality crystals of the
quasi-two-dimensional superconductor -(BEDT-TTF)Cu(NCS) ~are
studied using the Shubnikov-de Haas effect and MHz penetration-depth
experiments. There is strong evidence that the broadening of the Landau-levels
is primarily caused by spatial inhomogeneities, indicating a quasiparticle
lifetime for the Landau states ps. In contrast to the predictions of
Fermi-liquid theory, the scattering time derived from the intralayer
conductivity is found to be much shorter ( ps)
Critical State Behaviour in a Low Dimensional Metal Induced by Strong Magnetic Fields
We present the results of magnetotransport and magnetic torque measurements
on the alpha-(BEDT-TTF)2KHg(SCN)4 charge-transfer salt within the high magnetic
field phase, in magnetic fields extending to 33 T and temperatures as low as 27
mK. While the high magnetic field phase (at fields greater than ~ 23 T) is
expected, on theoretical grounds, to be either a modulated charge-density wave
phase or a charge/spin-density wave hybrid, the resistivity undergoes a
dramatic drop below ~ 3 K within the high magnetic field phase, falling in an
approximately exponential fashion at low temperatures, while the magnetic
torque exhibits pronounced hysteresis effects. This hysteresis, which occurs
over a broad range of fields, is both strongly temperature-dependent and has
several of the behavioural characteristics predicted by critical-state models
used to describe the pinning of vortices in type II superconductors in strong
magnetic fields. Thus, rather than exhibiting the usual behaviour expected for
a density wave ground state, both the transport and the magnetic properties of
alpha-(BEDT-TTF)2KHg(SCN)4, at high magnetic fields, closely resembles those of
a type II superconductor
Comparison of the sidereal angular velocity of subphotospheric layers and small bright coronal structures during the declining phase of solar cycle 23
Context. We compare solar differential rotation of subphotospheric layers
derived from local helioseismology analysis of GONG++ dopplergrams and the one
derived from tracing small bright coronal structures (SBCS) using EIT/SOHO
images for the period August 2001 - December 2006, which correspond to the
declining phase of solar cycle 23. Aims. The study aims to find a relationship
between the rotation of the SBCS and the subphotospheric angular velocity. The
northsouth asymmetries of both rotation velocity measurements are also
investigated. Methods. Subphotospheric differential rotation was derived using
ring-diagram analysis of GONG++ full-disk dopplergrams of 1 min cadence. The
coronal rotation was derived by using an automatic method to identify and track
the small bright coronal structures in EIT full-disk images of 6 hours cadence.
Results. We find that the SBCS rotate faster than the considered upper
subphotospheric layer (3Mm) by about 0.5 deg/day at the equator. This result
joins the results of several other magnetic features (sunspots, plages,
faculae, etc.) with a higher rotation than the solar plasma. The rotation rate
latitudinal gradients of the SBCS and the subphotospheric layers are very
similar. The SBCS motion shows an acceleration of about 0.005 deg/day/month
during the declining phase of solar cycle 23, whereas the angular velocity of
subsurface layers does not display any evident variation with time, except for
the well known torsional oscillation pattern. Finally, both subphotospheric and
coronal rotations of the southern hemisphere are predominantly larger than
those of the northern hemisphere. At latitudes where the north-south asymmetry
of the angular velocity increases (decreases) with activity for the SBCS, it
decreases (increases) for subphotospheric layers.Comment: 6pages, 8 figures, Accepted for publication in Astronomy and
Astrophysic
A SIMULATION BASED SCHEDULING MODEL FOR CALL CENTERS WITH UNCERTAIN ARRIVAL RATES
In this paper we develop a two stage algorithm for scheduling call centers with strict SLAs and arrival rate uncertainty. The first cut schedule can be developed in less than a minute using a constructive heuristic. The schedule is then refined via a simulation based optimization approach. We find that when allowed to run for five minutes or less this two stage process can create a schedule with a total expected cost within a few percentage points of schedules generated using much more computationally intensive methods. This rapid scheduling process is designed to support front line managers who wish to evaluate multiple scheduling options in a what if analysis mode
Fermi Surface Properties of Low Concentration CeLaB: dHvA
The de Haas-van Alphen effect is used to study angular dependent extremal
areas of the Fermi Surfaces (FS) and effective masses of CeLaB alloys for between 0 and 0.05. The FS of these alloys was previously
observed to be spin polarized at low Ce concentration ( = 0.05). This work
gives the details of the initial development of the topology and spin
polarization of the FS from that of unpolarized metallic LaB to that of
spin polarized heavy Fermion CeB .Comment: 7 pages, 9 figures, submitted to PR
Top Quark Physics at the Tevatron
The discovery of the top quark in 1995, by the CDF and D0 collaborations at
the Fermilab Tevatron, marked the dawn of a new era in particle physics. Since
then, enormous efforts have been made to study the properties of this
remarkable particle, especially its mass and production cross section. In this
article, we review the status of top quark physics as studied by the two
collaborations using the p-pbar collider data at sqrt(s) = 1.8 TeV. The
combined measurement of the top quark mass, m_t = 173.8 +- 5.0 GeV/c^2, makes
it known to a fractional precision better than any other quark mass. The
production cross sections are measured as sigma (t-tbar) = 7.6 -1.5 +1.8 pb by
CDF and sigma (t-tbar) = 5.5 +- 1.8 pb by D0. Further investigations of t-tbar
decays and future prospects are briefly discussed.Comment: 119 pages, 59 figures, 17 tables Submitted to Int. J. Mod. Phys. A
Fixed some minor error
Methane Emissions from Process Equipment at Natural Gas Production Sites in the United States: Liquid Unloadings
Methane emissions from liquid unloadings were measured at 107 wells in natural gas production regions throughout the United States. Liquid unloadings clear wells of accumulated liquids to increase production, employing a variety of liquid lifting mechanisms. In this work, wells with and without plunger lifts were sampled. Most wells without plunger lifts unload less than 10 times per year with emissions averaging 21 000–35 000 scf methane (0.4–0.7 Mg) per event (95% confidence limits of 10 000–50 000 scf/event). For wells with plunger lifts, emissions averaged 1000–10 000 scf methane (0.02–0.2 Mg) per event (95% confidence limits of 500–12 000 scf/event). Some wells with plunger lifts are automatically triggered and unload thousands of times per year and these wells account for the majority of the emissions from all wells with liquid unloadings. If the data collected in this work are assumed to be representative of national populations, the data suggest that the central estimate of emissions from unloadings (270 Gg/yr, 95% confidence range of 190–400 Gg) are within a few percent of the emissions estimated in the EPA 2012 Greenhouse Gas National Emission Inventory (released in 2014), with emissions dominated by wells with high frequencies of unloadings
Analysis of Resonant Inelastic X-Ray Scattering in Stripe-Ordered Nickelate
We analyze theoretically the resonant inelastic x-ray scattering (RIXS) at
the Ni K edge in the stripe-ordered state of La_{2-x}Sr_xNiO_4 at x=1/3. In the
calculation of RIXS spectra, the stripe-ordered ground state is described
within the Hartree-Fock approximation by using a realistic tight-binding model
for Ni3d\gamma and O2p_{x, y} orbitals, and the electron correlations in the
electronic excitation processes are taken into account within the random-phase
approximation. The calculated RIXS spectrum shows a tail toward the low-energy
region when the momentum transfer of photons equals the stripe vector Q, being
consistent with a recent experimental result. The origin of this anomalous
momentum dependence of RIXS spectra is discussed microscopically.Comment: 23 pages, 9 figures. Published version in J. Phys. Soc. Jp
Quantum Tunneling Effect in Oscillating Friedmann Cosmology
It is shown that the tunneling effect in quantum cosmology is possible not
only at the very beginning or the very end of the evolution, but also at the
moment of maximum expansion of the universe. A positive curvature expanding
Friedmann universe changes its state of evolution spontaneously and completely,
{\it without} any changes in the matter content, avoiding recollapse, and
falling into oscillations between the nonzero values of the scale factor. On
the other hand, an oscillating nonsingular universe can tunnel spontaneously to
a recollapsing regime. The probability of such kind of tunneling is given
explicitly. It is inversely related to the amount of nonrelativistic matter
(dust), and grows from a certain fixed value to unity if the negative
cosmological constant approaches zero.Comment: 18 pages Latex + 2 figures available by fax upon reques
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