The electron spin transport in an asymmetrical magnetic superlattice ͑MSL͒, which is due to the periodic barriers with abnormal one in height, has been theoretically studied. The effects of the abnormal barrier in the MSL have been introduced by a defined asymmetry factor, which corresponds to the changeable height of the abnormal barrier. The results show that in the ballistic transport region, the spin-up electron transmission and conductance can be dramatically depressed by the enhancement of the asymmetry factor. However, the spindown transmission and conductance are only slightly affected. It means that the spin transport in the asymmetric MSL is obviously different from that in the symmetric MSL. In addition, the value of spin polarization of the conductance can be well controlled by the asymmetry factor in a wide range of energy, and highly polarized transport is possible. Furthermore, the number of the abnormal barriers can also strongly affect the oscillation of the spin polarization.
Using classical constant-pressure molecular dynamics simulations and the force constants model, radial breathing mode ͑RBM͒ transition of single-wall carbon nanotubes under hydrostatic pressure is reported. With the pressure increased, the RBM shifts linearly toward higher frequency, and the RBM transition occurs at the same critical pressure as the structural transition. The group theory indicates that the RBMs are all Ramanactive; however, due to the effect of the frequency transition and the electronic structure change for tube radial deformation, the Raman intensity of the modes becomes so weak as not to be experimentally detected, which is in agreement with a recent experiment by S. Lebedkin et al. ͓Phys. Rev. B 73, 094109 ͑2006͔͒. Furthermore, the calculated RBM transition pressure is well fitted to the cube of diameter ͑ϳ1/d 3 ͒.
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