15,691 research outputs found
Resonant Phonon Scattering in Quantum Hall Systems Driven by dc Electric Fields
Using dc excitation to spatially tilt Landau levels, we study resonant
acoustic phonon scattering in two-dimensional electron systems. We observe that
dc electric field strongly modifies phonon resonances, transforming resistance
maxima into minima and back into maxima. Further, phonon resonances are
enhanced dramatically in the non-linear dc response and can be detected even at
low temperatures. Most of our observations can be explained in terms of
dc-induced (de)tuning of the resonant acoustic phonon scattering and its
interplay with intra-Landau level impurity scattering. Finally, we observe a
dc-induced zero-differential resistance state and a resistance maximum which
occurs when the electron drift velocity approaches the speed of sound.Comment: 4 pages, 4 figures, to appear in Phys. Rev. Let
Magnetoresistance Oscillations in Two-dimensional Electron Systems Induced by AC and DC Fields
We report on magnetotransport measurements in a high-mobility two-dimentional
electron system subject simultaneously to AC (microwave) and DC (Hall) fields.
We find that DC excitation affects microwave photoresistance in a nontrivial
way. Photoresistance maxima (minima) evolve into minima (maxima) and back,
reflecting strong coupling and interplay of AC- and DC-induced effects. Most of
our observations can be explained in terms of indirect electron transitions
using a new, ``combined'' resonant condition. Observed quenching of
microwave-induced zero resistance by a DC field cannot be unambiguously linked
to a domain model, at least until a systematic theory treating both excitation
types within a single framework is developed
THI APPLICATION TO INSURING AGAINST HEAT STRESS IN DAIRY COWS
Heat stress is associated with reduced milk production in dairy cows. Insurance instruments based on an index of ambient temperature and relative humidity measured at Macon, Georgia and Tallahassee, Florida are shown to reduce net revenue risk for a representative farm in south-central Georgia.Risk and Uncertainty,
Giant microwave photoresistivity in a high-mobility quantum Hall system
We report the observation of a remarkably strong microwave photoresistivity
effect in a high-mobility two-dimensional electron system subject to a weak
magnetic field and low temperature. The effect manifests itself as a giant
microwave-induced resistivity peak which, in contrast to microwave-induced
resistance oscillations, appears only near the second harmonic of the cyclotron
resonance and only at sufficiently high microwave frequencies. Appearing in the
regime linear in microwave intensity, the peak can be more than an order of
magnitude stronger than the microwave-induced resistance oscillations and
cannot be explained by existing theories.Comment: 4 pages, 4 figure
Revolutionaries and spies: Spy-good and spy-bad graphs
We study a game on a graph played by {\it revolutionaries} and
{\it spies}. Initially, revolutionaries and then spies occupy vertices. In each
subsequent round, each revolutionary may move to a neighboring vertex or not
move, and then each spy has the same option. The revolutionaries win if of
them meet at some vertex having no spy (at the end of a round); the spies win
if they can avoid this forever.
Let denote the minimum number of spies needed to win. To
avoid degenerate cases, assume |V(G)|\ge r-m+1\ge\floor{r/m}\ge 1. The easy
bounds are then \floor{r/m}\le \sigma(G,m,r)\le r-m+1. We prove that the
lower bound is sharp when has a rooted spanning tree such that every
edge of not in joins two vertices having the same parent in . As a
consequence, \sigma(G,m,r)\le\gamma(G)\floor{r/m}, where is the
domination number; this bound is nearly sharp when .
For the random graph with constant edge-probability , we obtain constants
and (depending on and ) such that is near the
trivial upper bound when and at most times the trivial lower
bound when . For the hypercube with , we have
when , and for at least spies are
needed.
For complete -partite graphs with partite sets of size at least , the
leading term in is approximately
when . For , we have
\sigma(G,2,r)=\bigl\lceil{\frac{\floor{7r/2}-3}5}\bigr\rceil and
\sigma(G,3,r)=\floor{r/2}, and in general .Comment: 34 pages, 2 figures. The most important changes in this revision are
improvements of the results on hypercubes and random graphs. The proof of the
previous hypercube result has been deleted, but the statement remains because
it is stronger for m<52. In the random graph section we added a spy-strategy
resul
Temperature Dependence of Microwave Photoresistance in 2D Electron Systems
We report on the temperature dependence of microwave-induced resistance
oscillations in high-mobility two-dimensional electron systems. We find that
the oscillation amplitude decays exponentially with increasing temperature, as
, where scales with the inverse magnetic field.
This observation indicates that the temperature dependence originates primarily
from the modification of the single particle lifetime, which we attribute to
electron-electron interaction effects.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
Generalised geometry, eleven dimensions and E11
We construct the non-linear realisation of E11 and its first fundamental
representation in eleven dimensions at low levels. The fields depend on the
usual coordinates of space-time as well as two form and five form coordinates.
We derive the terms in the dynamics that contain the three form and six form
fields and show that when we restricted their field dependence to be only on
the usual space-time we recover the correct self-duality relation. Should this
result generalise to the gravity fields then the non-linear realisation is an
extension of the maximal supergravity theory, as previously conjectured. We
also comment on the connections between the different approaches to generalised
geometry.Comment: 17 pages, Trivial typos corrected in version one and a substantial
note added which gives the equation of motion relating the gravity field to
its dua
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