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General Volume-Preserving Mechanical Systems
In this letter, we present the general form of equations that generate a
volume-preserving flow on a symplectic manifold (M, \omega). It is shown that
every volume-preserving flow has some 2-forms acting the role of the
Hamiltonian functions in the Hamiltonian mechanics and the ordinary Hamilton
equations are included as a special case with a 2-form \frac{1}{n-1} H \omega
where H is the corresponding Hamiltonian.Comment: Plain LaTeX, 13 pages, no figure
Optical transitions between Landau levels: AA-stacked bilayer graphene
The low-frequency optical excitations of AA-stacked bilayer graphene are
investigated by the tight-binding model. Two groups of asymmetric LLs lead to
two kinds of absorption peaks resulting from only intragroup excitations. Each
absorption peak obeys a single selection rule similar to that of monolayer
graphene. The excitation channel of each peak is changed as the field strength
approaches a critical strength. This alteration of the excitation channel is
strongly related to the setting of the Fermi level. The peculiar optical
properties can be attributed to the characteristics of the LL wave functions of
the two LL groups. A detailed comparison of optical properties between
AA-stacked and AB-stacked bilayer graphenes is also offered. The compared
results demonstrate that the optical properties are strongly dominated by the
stacking symmetry. Furthermore, the presented results may be used to
discriminate AABG from MG, which can be hardly done by STM
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