20 research outputs found
Switchable valley functionalities of an junction in 2D semiconductors
We show that an junction in 2D semiconductors can flexibly
realize two basic valleytronic functions, i.e. valley filter and valley source,
with gate controlled switchability between the two. Upon carrier flux passing
through the junction, the valley filter and valley source functions are enabled
respectively by intra- and inter-valley scatterings, and the two functions
dominate respectively at small and large band-offset between the and
regions. It can be generally shown that, the valley filter effect has
an angular dependent polarity and vanishes under angular integration, by the
same constraint from time-reversal symmetry that leads to its absence in
one-dimension. These findings are demonstrated for monolayer transition metal
dichalcogenides and graphene using tight-binding calculations. We further show
that junction along chiral directions can concentrate the valley pump in an
angular interval largely separated from the bias direction, allowing efficient
havest of valley polarization in a cross-bar device
Transient probing of the symmetry and the asymmetry of electron interference
The transient processes of electron transport in nano-scale devices exhibit
special phenomena that exist only in the transient regime. Besides how fast the
steady states are approached, one interesting aspect of transient transport
arises from its strong dependence on the initial state of the system. Here we
address the issue of how the symmetries embedded in the initial state interplay
with those of the system structure in the course of transient transports. We
explicitly explore the transient currents arising from various initial
occupations in a double-quantum-dot Aharonov-Bohm interferometer. We find
symmetry relations between the transient in-tunneling and out-tunneling
dynamics for initially empty or full quantum dots when the energy levels in the
electrodes are symmetrically distributed with respect to the energy levels in
the QDs. This is true for whatever applied fluxes. We also find the flux-even
components of the currents and the flux-odd components of the currents exhibit
distinct cross-lead symmetric relations.Comment: 10 figure