By using multiple-scale method,we analytically study the nonlinear dynamical properties of an electromagnetically induced transparency medium with three-level atoms and a strong coupling light field. It is shown that a weak probe light beam can form spatial weak-light dark solitons. When it propagates along axial direction,the dark soliton will transforms into a steady spatial weak-light ring dark soliton,which results from the balance of the interplay between beam diffraction and refractive nonlinearity.
According to a tight-binding model and the Green's function formalism, we investigate the electronic transport in hybrid contact of doubly stacked zigzag graphene nanoribbons. Our study shows that the next nearest neighbor interlayer coupling, the hybrid contact length and gate voltage each have a significant modulation effect on the electron transmission spectrum. Due to the next nearest neighbor interlayer coupling, the transmission spectrum of the hybrid contact exhibits an electron-hole asymmetry, which is consistent with the experimental result. There exist some transmission gap (T=0) and quantum step (T=1) within the first subband below the Fermi energy, meaning that electrons can reflect and/or transmit completely. It is also observed that the transmission coefficient oscillates within 1 as the contact length increases, showing a quantum interference effect. Under a gate voltage in the bilayer regime, the transmission coefficient can be changed from 1 to 0, showing that a switching effect exists here. The results is useful for the design and the application of the graphene-based device.
We investigate the co-doping of potassium and bromine in single-walled carbon nanotubes (SWCNTs) and doublewalled carbon nanotubes (DWCNTs) based on density functional theory. In the co-doped (6,0) SWCNTs, the 4s electron of potassium is transferred to nanotube and Br, leading to the n-type feature of SWCNTs. When potassium is intercalated into inner tube and bromine is put on outer tube, the positive and negative charges reside on the outer and inner tubes of the (7,0)@(16,0) DWCNT, respectively. It is expected that DWCNTs would be an ideal candidate for p-n junction and diode applications.
Based on lattice dynamics theory and force constant model, we calculated phonon dispersion relation, density of states, and specific heat of graphene ribbons.It is shown that the characteristics of phonon spectra behaves as those of a system between one_dimensional carbon nanotube and two-dimensional graphene. The high and intermediate phonon branches of armchair and zigzag nanoribbons resemble those of zigzag and armchair carbon nanotubes, respectively. With the ribbon width increasing, interestingly, it is found that the low frequency peaks in density of states shifted to lower frequency, indicating a phonon confinement effect. At both low and high temperature,specific heat decreased with the increase of the band width,showing up as a singular quantum size effect.The specific heat at 300K can be fitted by CV=CVg+A/n, where CVg is specific heat of graphene and the A/n term represents the effect of edge on specific heat of ribbons.
Considering both the gradient decay of the real disorder and the contact scattering, we investigate the electronic transport in quasi-one-dimensional nanowires by developing a decomposition elimination method for Green's function matrix. In the presence the contact scattering, the conductance oscillates with energy. For some energies of incident electrons, an abnormal enhancement is obtained in the average conductance due to the destroyed coherence by the introduction of much low disorder, showing that there appears a new conductance peak. In the absence of disorder gradient, the average conductance firstly decreases then increases with disorder strength, indicating that there exists a localization-delocalization transition. In the presence of linearly decaying disorder, the average conductance increases slightly in a strong disorder region. In the case of the Gaussian-type decaying disorder, the average conductance decreases exponentially and the localization-delocalization transition disappears, which is different from previous thereotical result. The results are helpful for the design and the application of quasi-one-dimensional nanowires device.
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