15 research outputs found
Onset voltage shift due to non-zero Landau ground state level in coherent magnetotransport
Coherent electron transport in double-barrier heterostructures with parallel
electric and magnetic fields is analyzed theoretically and with the aid of a
quantum simulator accounting for 3-dimensional transport effects. The
onset-voltage shift induced by the magnetic field in resonant tunneling diodes,
which was previously attributed to the cyclotron frequency inside the
well is found to arise from an upward shift of the non-zero ground (lowest)
Landau state energy in the entire quantum region where coherent transport takes
place. The spatial dependence of the cyclotron frequency is accounted for and
verified to have a negligible impact on resonant tunneling for the device and
magnetic field strength considered. A correction term for the onset-voltage
shift arising from the magnetic field dependence of the chemical potential is
also derived. The Landau ground state with its nonvanishing finite harmonic
oscillator energy is verified however to be the principal
contributor to the onset voltage shift at low temperatures.Comment: 13 pages, and 3 figures. Accepted for publication in Phys. Rev.
Phase diagrams of period-4 spin chains consisting of three kinds of spins
We study a period-4 antiferromagnetic mixed quantum spin chain consisting of
three kinds of spins. When the ground state is singlet, the spin magnitudes in
a unit cell are arrayed as (s-t, s, s+t, s) with integer or half-odd integer s
and t (0 <= t < s). The spin Hamiltonian is mapped onto a nonlinear sigma model
(NLSM) in a previously developed method. The resultant NLSM includes only two
independent parameters originating from four exchange constants for fixed s and
t. The topological angle in the NLSM determines the gapless phase boundaries
between disordered phases in the parameter space. The phase diagrams for
various s and t shows rich structures. We systematically explain the phases in
the singlet-cluster-solid picture.Comment: 8 pages (16 figures included
Digital particle image velocimetry measurements of the downwash distribution of a desert locust Schistocerca gregaria.
Actuator disc models of insect flight are concerned solely with the rate of momentum transfer to the air that passes through the disc. These simple models assume that an even pressure is applied across the disc, resulting in a uniform downwash distribution. However, a correction factor, k, is often included to correct for the difference in efficiency between the assumed even downwash distribution, and the real downwash distribution. In the absence of any empirical measurements of the downwash distribution behind a real insect, the values of k used in the literature have been necessarily speculative. Direct measurement of this efficiency factor is now possible, and could be used to compare the relative efficiencies of insect flight across the Class. Here, we use Digital Particle Image Velocimetry to measure the instantaneous downwash distribution, mid-downstroke, of a tethered desert locust (Schistocerca gregaria). By integrating the downwash distribution, we are thereby able to provide the first direct empirical measurement of k for an insect. The measured value of k=1.12 corresponds reasonably well with that predicted by previous theoretical studies