2,236 research outputs found
Flux-free conductance modulation in a helical Aharonov-Bohm interferometer
A novel conductance oscillation in a twisted quantum ring composed of a
helical atomic configuration is theoretically predicted. Internal torsion of
the ring is found to cause a quantum phase shift in the wavefunction that
describes the electron's motion along the ring. The resulting conductance
oscillation is free from magnetic flux penetrating inside the ring, which is in
complete contrast with the ordinary Aharonov-Bohm effect observed in untwisted
quantum rings.Comment: 10 pages, 4 figure
On the laminar solutions and stability of accelerating and decelerating channel flows
We study the effect of acceleration and deceleration on the stability of
channel flows. To do so, we derive an exact solution for laminar profiles of
channel flows with arbitrary, time-varying wall motion and pressure gradient.
This solution then allows us to investigate the stability of any unsteady
channel flow. In particular, we investigate the nonnormal growth of
perturbations in flows with exponentially decaying acceleration and
deceleration, with comparisons to growth in a constant flow (i.e., the
time-invariant simple shear or parabolic profile). We apply this acceleration
and deceleration through the velocity of the walls and through the flow rate.
For accelerating flows, disturbances never grow larger than disturbances in a
constant flow, while decelerating flows show massive amplification of
disturbances -- at a Reynolds number of 800, disturbances in the decelerating
flow grow times larger than disturbances grow in a constant
flow. This amplification increases as we raise the rate of deceleration and the
Reynolds number. We find that this amplification arises due to a transition
from spanwise perturbations leading to the largest amplification, to streamwise
perturbations leading to the largest amplification that only occurs in the
decelerating flow. By evolving the optimal perturbations through the linearized
equations of motion, we reveal that the decelerating case achieves this massive
amplification through a down-gradient Reynolds stress mechanism, which
accelerating and constant flows cannot maintain. Finally, we end the paper by
validating that these perturbations exhibit the same growth behaviors in direct
numerical simulations
Torsion-induced persistent current in a twisted quantum ring
We describe the effects of geometric torsion on the coherent motion of
electrons along a thin twisted quantum ring. The geometric torsion inherent in
the quantum ring triggers a quantum phase shift in the electrons' eigenstates,
thereby resulting in a torsion-induced persistent current that flows along the
twisted quantum ring. The physical conditions required for detecting the
current flow are discussed.Comment: 9 pages, 3 figure
High sensitive X-ray films to detect electron showers in 100 GeV region
Nonscreen type X-ray films were used in emulsion chamber experiments to detect high energy showers in cosmic rays. Ranges of the detection threshold is from about 1 to 2 TeV depending on the exposure conditions. Different types of X-ray films and sheets i.e. high sensitive screen type X-ray films and luminescence sheets were tested. The threshold of the shower detection is found to be about 200 GeV, which is much lower than that of nonscreen type X-ray films. These films are useful to detect showers in the medium energy range, a few hundred GeV, of the cosmic ray electrons
High energy electrons beyond 100 GEV observed by emulsion chamber
Much efforts have been expended to observe the spectrum of electrons in the high energy region with large area emulsion chambers exposed at balloon altitudes, and now 15 electrons beyond 1 TeV have been observed. The observed integral flux at 1 TeV is (3.24 + or - 0.87)x10(-5)/sq m sec sr. The statistics of the data around a few hundred GeV are also improving by using new shower detecting films of high sensitivity. The astrophysical significance of the observed spectrum are discussed for the propagation of electrons based on the leaky box and the nested leaky box model
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