175,347 research outputs found
Geometric phases in a scattering process
The study of geometric phase in quantum mechanics has so far be confined to
discrete (or continuous) spectra and trace preserving evolutions. Consider only
the transmission channel, a scattering process with internal degrees of freedom
is neither a discrete spectrum problem nor a trace preserving process. We
explore the geometric phase in a scattering process taking only the
transmission process into account. We find that the geometric phase can be
calculated by the some method as in an unitary evolution. The interference
visibility depends on the transmission amplitude. The dependence of the
geometric phase on the barrier strength and the spin-spin coupling constant is
also presented and discussed.Comment: 4 pages, 5 figure
Effect of spin relaxations on the spin mixing conductances for a bilayer structure
The spin current can result in a spin-transfer torque in the
normal-metal(NM)|ferromagnetic-insulator(FMI) or
normal-metal(NM)|ferromagnetic-metal(FMM) bilayer. In the earlier study on this
issue, the spin relaxations were ignored or introduced phenomenologically. In
this paper, considering the FMM or FMI with spin relaxations described by a
non-Hermitian Hamiltonian, we derive an effective spin-transfer torque and an
effective spin mixing conductance in the non-Hermitian bilayer. The dependence
of the effective spin mixing conductance on the system parameters (such as
insulating gap, \textit{s-d} coupling, and layer thickness) as well as the
relations between the real part and the imaginary part of the effective spin
mixing conductance are given and discussed. We find that the effective spin
mixing conductance can be enhanced in the non-Hermitian system. This provides
us with the possibility to enhance the spin mixing conductance
Photonic band structure of ZnO photonic crystal slab laser
We recently reported on the first realization of ultraviolet photonic crystal
laser based on zinc oxide [Appl. Phys. Lett. {\bf 85}, 3657 (2004)]. Here we
present the details of structural design and its optimization. We develop a
computational super-cell technique, that allows a straightforward calculation
of the photonic band structure of ZnO photonic crystal slab on sapphire
substrate. We find that despite of small index contrast between the substrate
and the photonic layer, the low order eigenmodes have predominantly
transverse-electric (TE) or transverse-magnetic (TM) polarization. Because
emission from ZnO thin film shows strong TE preference, we are able to limit
our consideration to TE bands, spectrum of which can possess a complete
photonic band gap with an appropriate choice of structure parameters. We
demonstrate that the geometry of the system may be optimized so that a sizable
band gap is achieved.Comment: 8 pages, 7 figure
Analytical Bit Error Rate Performance of DS-CDMA Ad Hoc Networks using Large Area Synchronous Spreading Sequences
The performance of large area synchronous (LAS) direct sequence code division multiple access (DS-CDMA) assisted ad hoc networks is investigated in the context of a single-hop infinite mesh of rectilinearly located ad hoc nodes. It is shown that LAS DS-CDMA exhibits a significantly better performance than the family of traditional spreading sequences used in a quasisynchronous DS-CDMA scenario having a low number of resolvable multipath components and a sufficiently high number of RAKE receiver branches. The benefits of LAS codes in ad hoc networks are multifold: (i) Their performance is noise-limited, rather than interference-limited, provided that the multipath and multi-user interference arrives within their interference free window. (ii) Under the same conditions LAS codes are robust against the ânearâfarâ effects imposed by ad hoc networks operating without base-station-aided power control, without accurate synchronisation and without implementationally complex interference cancellers
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