9,610 research outputs found
Linear Vlasov theory of a magnetised, thermally stratified atmosphere
The stability of a collisionless, magnetised plasma to local convective
disturbances is examined, with a focus on kinetic and finite-Larmor-radius
effects. Specific application is made to the outskirts of galaxy clusters,
which contain hot and tenuous plasma whose temperature increases in the
direction of gravity. At long wavelengths (the "drift-kinetic" limit), we
obtain the kinetic version of the magnetothermal instability (MTI) and its
Alfv\'enic counterpart (Alfv\'enic MTI), which were previously discovered and
analysed using a magnetofluid (i.e. Braginskii) description. At sub-ion-Larmor
scales, we discover an overstability driven by the electron temperature
gradient of kinetic-Alfv\'en drift waves -- the electron MTI (eMTI) -- whose
growth rate is even larger than the standard MTI. At intermediate scales, we
find that ion finite-Larmor-radius effects tend to stabilise the plasma. We
discuss the physical interpretation of these instabilities in detail, and
compare them both with previous work on magnetised convection in a collisional
plasma and with temperature-gradient-driven drift-wave instabilities well-known
to the magnetic-confinement-fusion community. The implications of having both
fluid and kinetic scales simultaneously driven unstable by the same temperature
gradient are briefly discussed.Comment: 51 pages, 9 figures; to appear in Journal of Plasma Physic
Simultaneous Measurements of Microwave Photoresistance and Cyclotron Reflection in the Multi-Photon Regime
We simultaneously measure photoresistance with electrical transport and
plasmon-cyclotron resonance (PCR) using microwave reflection spectroscopy in
high mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field.
Multi-photon transitions are revealed as sharp peaks in the resistance and the
cyclotron reflection on samples with various carrier densities. Our main
finding is that plasmon coupling is relevant in the cyclotron reflection
spectrum but has not been observed in the electrical conductivity signal. We
discuss possible mechanisms relevant to reflection or dc conductivity signal to
explain this discrepancy. We further confirm a trend that higher order
multi-photon features can be observed using higher carrier density samples.Comment: 19 pages, 5 figure
Thermopower and Nernst measurements in a half-filled lowest Landau level
Motivated by recent proposal by Potter et al. [Phys. Rev. X 6, 031026 (2016)]
concerning possible thermoelectric signatures of Dirac composite fermions, we
perform a systematic experimental study of thermoelectric transport of an
ultrahigh-mobility GaAs/AlxGa1-xAs two dimensional electron system at filling
factor v = 1/2. We demonstrate that the thermopower Sxx and Nernst Sxy are
symmetric and anti-symmetric with respect to B = 0 T, respectively. The
measured properties of thermopower Sxx at v = 1/2 are consistent with previous
experimental results. The Nernst signals Sxy of v = 1/2, which have not been
reported previously, are non-zero and show a power law relation with
temperature in the phonon-drag dominant region. In the electron-diffusion
dominant region, the Nernst signals Sxy of v = 1/2 are found to be
significantly smaller than the linear temperature dependent values predicted by
Potter et al., and decreasing with temperature faster than linear dependence.Comment: 23 pages, 5 figure
Test for a large amount of entanglement, using few measurements
Bell-inequality violations establish that two systems share some quantum
entanglement. We give a simple test to certify that two systems share an
asymptotically large amount of entanglement, n EPR states. The test is
efficient: unlike earlier tests that play many games, in sequence or in
parallel, our test requires only one or two CHSH games. One system is directed
to play a CHSH game on a random specified qubit i, and the other is told to
play games on qubits {i,j}, without knowing which index is i.
The test is robust: a success probability within delta of optimal guarantees
distance O(n^{5/2} sqrt{delta}) from n EPR states. However, the test does not
tolerate constant delta; it breaks down for delta = Omega~(1/sqrt{n}). We give
an adversarial strategy that succeeds within delta of the optimum probability
using only O~(delta^{-2}) EPR states.Comment: 17 pages, 2 figures. Journal versio
- …