An access alternative for mobile satellite networks

Conceptually, this paper discusses strategies of digital satellite communication networks for a very large number of low density traffic stations. These stations can be either aeronautical, land mobile, or maritime. The techniques can be applied to international, domestic, regional, and special purpose satellite networks. The applications can be commercial, scientific, military, emergency, navigational or educational. The key strategy is the use of a non-orthogonal access method, which tolerates overlapping signals. With n being either time or frequency partitions, and with a single overlapping signal allowed, a low cost mobile satellite system can be designed with n squared (n squared + n + 1) number of terminals

Novel valley depolarization dynamics and valley Hall effect of exciton in mono- and bilayer MoS2_2

We investigate the valley depolarization dynamics and valley Hall effect of exciton due to the electron-hole exchange interaction in mono- and bilayer MoS$_2$ by solving the kinetic spin Bloch equations. The effect of the exciton energy spectra by the electron-hole exchange interaction is explicitly considered. For the valley depolarization dynamics, in the monolayer MoS$_2$, it is found that in the strong scattering regime, the conventional motional narrowing picture is no longer valid, and a novel valley depolarization channel is opened. For the valley Hall effect of exciton, in both the mono- and bilayer MoS$_2$, with the exciton equally pumped in the K and K' valleys, the system can evolve into the equilibrium state where the valley polarization is parallel to the effective magnetic field due to the exchange interaction. With the drift of this equilibrium state by applied uniaxial strain, the exchange interaction can induce the {\it momentum-dependent} valley/photoluminesence polarization, which leads to the valley/photoluminesence Hall current. Specifically, the disorder strength dependence of the valley Hall conductivity is revealed. In the strong scattering regime, the valley Hall conductivity decreases with the increase of the disorder strength; whereas in the weak scattering regime, it saturates to a constant, which can be much larger than the one in Fermi system due to the absence of the Pauli blocking.Comment: 14 pages, 7 figure

Modulation of the dephasing time for a magnetoplasma in a quantum well

We investigate the femtosecond kinetics of optically excited 2D magneto-plasma. We calculate the femtosecond dephasing and relaxation kinetics of the laser pulse excited magneto-plasma due to bare Coulomb potential scattering, because screening is under these conditions of minor importance. By taking into account four Landau subbands in both the conduction band and the valence band, we are now able to extend our earlier study [Phys. Rev. B {\bf 58}, 1998,in print (see also cond-mat/9808073] to lower magnetic fields. We can also fix the magnetic field and change the detuning to further investigate the carrier density-dependence of the dephasing time. For both cases, we predict strong modulation in the dephasing time.Comment: RevTex, 3 figures, to be published in Solid. Stat. Commu

Electron spin relaxation in bilayer graphene

Electron spin relaxation due to the D'yakonov-Perel' mechanism is investigated in bilayer graphene with only the lowest conduction band being relevant. The spin-orbit coupling is constructed from the symmetry group analysis with the parameters obtained by fitting to the numerical calculation according to the latest report by Konschuh {\it et al.} [Phys. Rev. B {\bf 85}, 115423 (2012)] from first principles. In contrast to single-layer graphene, the leading term of the out-of-plane component of the spin-orbit coupling in bilayer graphene shows a Zeeman-like term with opposite effective magnetic fields in the two valleys. This Zeeman-like term opens a spin relaxation channel in the presence of intervalley scattering. It is shown that the intervalley electron-phonon scattering, which has not been reported in the previous literature, strongly suppresses the in-plane spin relaxation time at high temperature whereas the intervalley short-range scattering plays an important role in the in-plane spin relaxation especially at low temperature. A marked nonmonotonic dependence of the in-plane spin relaxation time on temperature with a minimum of several hundred picoseconds is predicted in the absence of the short-range scatterers. This minimum is comparable to the experimental data. Moreover, a peak in the electron density dependence of the in-plane spin relaxation time at low temperature, which is very different from the one in semiconductors, is predicted. We also find a rapid decrease in the in-plane spin relaxation time with increasing initial spin polarization at low temperature, which is opposite to the situation in both semiconductors and single-layer graphene. ......(The remaining is cut due to the limit of space)Comment: 15 pages, 9 figures, PRB in pres