122 research outputs found
Spectrum of pi electrons in bilayer graphene nanoribbons and nanotubes: an analytical approach
We present an analytical description of pi electrons of a finite size bilayer
graphene within a framework of the tight-binding model. The bilayered
structures considered here are characterized by a rectangular geometry and have
a finite size in one or both directions with armchair- and zigzag-shaped edges.
We provide an exact analytical description of the spectrum of pi electrons in
the zigzag and armchair bilayer graphene nanoribbons and nanotubes. We analyze
the dispersion relations, the density of states, and the conductance
quantization.Comment: 8 figure
Topological lattice using multi-frequency radiation
We describe a novel technique for creating an artificial magnetic field for
ultra-cold atoms using a periodically pulsed pair of counter propagating Raman
lasers that drive transitions between a pair of internal atomic spin states: a
multi-frequency coupling term. In conjunction with a magnetic field gradient,
this dynamically generates a rectangular lattice with a non-staggered magnetic
flux. For a wide range of parameters, the resulting Bloch bands have
non-trivial topology, reminiscent of Landau levels, as quantified by their
Chern numbers.Comment: Replaced with a revised version, 15 pages, 6 figure
Formation of solitons in atomic Bose-Einstein condensates by dark-state adiabatic passage
We propose a new method of creating solitons in elongated Bose-Einstein
Condensates (BECs) by sweeping three laser beams through the BEC. If one of the
beams is in the first order (TEM10) Hermite-Gaussian mode, its amplitude has a
transversal phase slip which can be transferred to the atoms creating a
soliton. Using this method it is possible to circumvent the restriction set by
the diffraction limit inherent to conventional methods such as phase
imprinting. The method allows one to create multicomponent (vector) solitons of
the dark-bright form as well as the dark-dark combination. In addition it is
possible to create in a controllable way two or more dark solitons with very
small velocity and close to each other for studying their collisional
properties.Comment: 10 figure
Non-Abelian gauge potentials for ultra-cold atoms with degenerate dark states
We show that the adiabatic motion of ultracold, multilevel atoms in spatially varying laser fields can give rise to effective non-Abelian gauge fields if degenerate adiabatic eigenstates of the atom-laser interaction exist. A pair of such degenerate dark states emerges, e.g., if laser fields couple three internal states of an atom to a fourth common one under pairwise two-photon-resonance conditions. For this so-called tripod scheme we derive general conditions for truly non-Abelian gauge potentials and discuss special examples. In particular we show that using orthogonal laser beams with orbital angular momentum an effective magnetic field can be generated that has a monopole component
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