6,712 research outputs found
Uniform approximation of barrier penetration in phase space
A method to approximate transmission probabilities for a nonseparable
multidimensional barrier is applied to a waveguide model. The method uses
complex barrier-crossing orbits to represent reaction probabilities in phase
space and is uniform in the sense that it applies at and above a threshold
energy at which classical reaction switches on. Above this threshold the
geometry of the classically reacting region of phase space is clearly reflected
in the quantum representation. Two versions of the approximation are applied. A
harmonic version which uses dynamics linearised around an instanton orbit is
valid only near threshold but is easy to use. A more accurate and more widely
applicable version using nonlinear dynamics is also described
How should we interpret the two transport relaxation times in the cuprates ?
We observe that the appearance of two transport relaxation times in the
various transport coefficients of cuprate metals may be understood in terms of
scattering processes that discriminate between currents that are even, or odd
under the charge conjugation operator. We develop a transport equation that
illustrates these ideas and discuss its experimental and theoretical
consequences.Comment: 19 pages, RevTeX with 8 postscript figures included. To appear in
``Non Fermi Liquid Physics'', J. Phys:Cond. Matt. (1997
Empirical Study of Ground Proximity Effects for Small-scale Electroaerodynamic Thrusters
Electroaerodynamic (EAD) propulsion, where thrust is produced by collisions
between electrostatically-accelerated ions and neutral air, is a potentially
transformative method for indoor flight owing to its silent and solid-state
nature. Like rotors, EAD thrusters exhibit changes in performance based on
proximity to surfaces. Unlike rotors, they have no fragile and quickly spinning
parts that have to avoid those surfaces; taking advantage of the efficiency
benefits from proximity effects may be a route towards longer-duration indoor
operation of ion-propelled fliers. This work presents the first empirical study
of ground proximity effects for EAD propulsors, both individually and as
quad-thruster arrays. It focuses on multi-stage ducted centimeter-scale
actuators suitable for use on small robots envisioned for deployment in
human-proximal and indoor environments. Three specific effects (ground,
suckdown, and fountain lift), each occurring with a different magnitude at a
different spacing from the ground plane, are investigated and shown to have
strong dependencies on geometric parameters including thruster-to-thruster
spacing, thruster protrusion from the fuselage, and inclusion of flanges or
strakes. Peak thrust enhancement ranging from 300 to 600% is found for certain
configurations operated in close proximity (0.2 mm) to the ground plane and as
much as a 20% increase is measured even when operated centimeters away
Terahertz Hall Measurements On Optimally Doped Single Crystal Bi-2212
The infrared Hall angle in optimally doped single crystal was measured from 3.05 to 21.75 meV as a continuous function of
temperature from 25 to 300\,K. In the normal state, the temperature dependence
of the real part of the cotangent of the infrared Hall angle obeys the same
power law as dc measurements. The measured Hall frequency is
significantly larger than the expected value based upon ARPES data analyzed in
terms of the relaxation time approximation. This discrepancy as well as the
temperature dependence of and is well
described by a Fermi liquid theory in which current vertex corrections produced
by electron-magnon scattering are included.Comment: 10 pages, 2 figure
Terahertz Kerr and Reflectivity Measurements on the Topological Insulator Bi2Se3
We report the first terahertz Kerr measurements on bulk crystals of the
topological insulator Bi2Se3. At T=10K and fields up to 8T, the real and
imaginary Kerr angle and reflectance measurements utilizing both linearly and
circularly polarized incident radiation were measured at a frequency of
5.24meV. A single fluid free carrier bulk response can not describe the
line-shape. Surface states with a small mass and surprisingly large associated
spectral weight quantitatively fit all data. However, carrier concentration
inhomogeneity has not been ruled out. A method employing a gate is shown to be
promising for separating surface from bulk effects.Comment: 10 pages, 5 figure
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