By irradiating a flat Al target with femtosecond laser pulses at moderate intensities of ∼1017 W/cm2, we obtained stable collimated quasimonoenergetic electrons in the specular direction but deviated somewhat toward the target normal. An associated local minimum located on the other side of the specular direction seems to indicate that the peak actually results from the deflection of the collimated electrons from their initial ejection direction. We have proposed a two-step model in which some laser-accelerated electrons are able to leave the plasma in a narrow phase-locked window of the moving wave interference pattern, and are then steered toward the target normal by the ponderomotive force of the interference field. The periodic repetition of the electron emission leads to a pulse train of collimated quasimonoenergetic electrons with subcycle duration.
Electron emission from individual graphene nanoribbons (GNRs) driven by an internal electric field was studied for the first time Inside a high resolution transmission electron microscope equipped with a state-of-art scanning tunneling microscope sample holder with independent twin probes. Electrons were driven out from Individual GNRs under an internal driving voltage of less than 3 V with an emission current increasing exponentially with the driving voltage. The emission characteristics were analyzed by taking Into account monatomic thickness of GNRs. While deviating from the two-dimensional Richardson equation for thermionic emission, they were well described by the recently proposed by us phonon-assisted electron emission model. Different from widely studied field electron emission from graphene edges, electrons were found to be emitted perpendicularly to the atomic graphene surfaces with an emission density as high as 12.7 A/cm(2). The internally driven electron emission is expected to be less sensitive to the microstructures of an emitter as compared to field emission. The low driving voltage, high emission density, and internal field driving character make the regarded electron emission highly promising for electron source applications.
We explore the electronic and transport properties out of a biased multilayer hexagonal boron nitride (h-BN) by first-principles calculations. The band gaps of multilayer h-BN decrease almost linearly with increasing perpendicular electric field, irrespective of the layer number N and stacking manner. The critical electric filed (E0) required to close the band gap decreases with the increasing N and can be approximated by E0 = 3.2 / (N − 1) (eV). We provide a quantum transport simulation of a dual-gated 4-layer h-BN with graphene electrodes. The transmission gap in this device can be effectively reduced by double gates, and a high on-off ratio of 3000 is obtained with relatively low voltage. This renders biased MLh-BN a promising channel in field effect transistor fabrication.
The analysis is based a multiple-case study.The analytical framework is developed from sociology theories and organizational theories.This study argues that vocational colleges in China have been experiencing substantial transformations.At the macro-level,influenced by the neo-liberal arguments for globalization,they have been transforming from social institutions to industries,with cultivating employability as their new mission.At the micro-level,under the influence of the new mission,there have been considerable changes in program goal,program development,curriculum development,dominant pedagogy,faculty development,and internal management.These changes fit the rhetoric of the employability as core competency,as well as the impact-reaction model for organizational change.The implication of these changes is that Chinese vocational higher education institutions may be degenerated to employment training organizations.