138,734 research outputs found
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
Spin Hall effect in infinitely large and finite-size diffusive Rashba two-dimensional electron systems: A helicity-basis nonequilibrium Green's function approach
A nonequilibrium Green's function approach is employed to investigate the
spin-Hall effect in diffusive two-dimensional electron systems with Rashba
spin-orbit interaction. Considering a long-range electron-impurity scattering
potential in the self-consistent Born approximation, we find that the spin-Hall
effect arises from two distinct interband polarizations in helicity basis: a
disorder-unrelated polarization directly induced by the electric field and a
polarization mediated by electron-impurity scattering. The disorder-unrelated
polarization is associated with all electron states below the Fermi surface and
produces the original intrinsic spin-Hall current, while the disorder-mediated
polarization emerges with contribution from the electron states near the Fermi
surface and gives rise to an additional contribution to the spin-Hall current.
Within the diffusive regime, the total spin-Hall conductivity vanishes in {\it
infinitely large} samples, independently of temperature, of the spin-orbit
coupling constant, of the impurity density, and of the specific form of the
electron-impurity scattering potential. However, in a {\it finite-size} Rashba
two-dimensional semiconductor, the spin-Hall conductivity no longer always
vanishes. Depending on the sample size in the micrometer range, it can be
positive, zero or negative with a maximum absolute value reaching as large as
order of magnitude at low temperatures. As the sample size increases,
the total spin-Hall conductivity oscillates with a decreasing amplitude. We
also discuss the temperature dependence of the spin-Hall conductivity for
different sample sizes.Comment: 9 pages, 3 figures, extended version of cond-mat/041162
Intrinsic electron-doping in nominal "non-doped" superconducting (La,Y)CuO thin films grown by dc magnetron sputtering
The superconducting nominal "non-doped" (LYCO) thin
films are successfully prepared by dc magnetron-sputtering and in situ
post-annealing in vacuum. The best more than 13K is achieved in the
optimal LYCO films with highly pure c-axis oriented T'-type structure. In the
normal state, the quasi-quadratic temperature dependence of resistivity, the
negative Hall coefficient and effect of oxygen content in the films are quite
similar to the typical Ce-doped T'-214 cuprates, suggesting that T'-LYCO shows
the electron-doping nature like known n-type cuprates, and is not a band
superconductor as proposed previously. The charge carriers are considered to be
induced by oxygen deficiency.Comment: 5 pages, 7 figure
Ion-Beam Induced Current in High-Resistance Materials
The peculiarities of electric current in high-resistance materials, such as
semiconductors or semimetals, irradiated by ion beams are considered. It is
shown that after ion--beam irradiation an unusual electric current may arise
directed against the applied voltage. Such a negative current is a transient
effect appearing at the initial stage of the process. The possibility of using
this effect for studying the characteristics of irradiated materials is
discussed. A new method for defining the mean projected range of ions is
suggested.Comment: 1 file, 7 pages, RevTex, no figure
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