21 research outputs found
Magnetic field dependence of the atomic collapse state in graphene
Quantum electrodynamics predicts that heavy atoms ()
will undergo the process of atomic collapse where electrons sink into the
positron continuum and a new family of so-called collapsing states emerges. The
relativistic electrons in graphene exhibit the same physics but at a much lower
critical charge () which has made it possible to confirm this
phenomenon experimentally. However, there exist conflicting predictions on the
effect of a magnetic field on atomic collapse. These theoretical predictions
are based on the continuum Dirac-Weyl equation, which does not have an exact
analytical solution for the interplay of a supercritical Coulomb potential and
the magnetic field. Approximative solutions have been proposed, but because the
two effects compete on similar energy scales, the theoretical treatment varies
depending on the regime which is being considered. These limitations are
overcome here by starting from a tight-binding approach and computing exact
numerical results. By avoiding special limit cases, we found a smooth evolution
between the different regimes. We predict that the atomic collapse effect
persists even after the magnetic field is activated and that the critical
charge remains unchanged. We show that the atomic collapse regime is
characterized: 1) by a series of Landau level anticrossings and 2) by the
absence of scaling of the Landau levels with regard to magnetic
field strength
Realization of a Tunable Artificial Atom at a Supercritically Charged Vacancy in Graphene
The remarkable electronic properties of graphene have fueled the vision of a
graphene-based platform for lighter, faster and smarter electronics and
computing applications. One of the challenges is to devise ways to tailor its
electronic properties and to control its charge carriers. Here we show that a
single atom vacancy in graphene can stably host a local charge and that this
charge can be gradually built up by applying voltage pulses with the tip of a
scanning tunneling microscope (STM). The response of the conduction electrons
in graphene to the local charge is monitored with scanning tunneling and Landau
level spectroscopy, and compared to numerical simulations. As the charge is
increased, its interaction with the conduction electrons undergoes a transition
into a supercritical regime 6-11 where itinerant electrons are trapped in a
sequence of quasi-bound states which resemble an artificial atom. The
quasi-bound electron states are detected by a strong enhancement of the density
of states (DOS) within a disc centered on the vacancy site which is surrounded
by halo of hole states. We further show that the quasi-bound states at the
vacancy site are gate tunable and that the trapping mechanism can be turned on
and off, providing a new mechanism to control and guide electrons in grapheneComment: 18 pages and 5 figures plus 14 pages and 15 figures of supplementary
information. Nature Physics advance online publication, Feb 22 (2016
Tuning a Circular p-n Junction in Graphene from Quantum Confinement to Optical Guiding
The motion of massless Dirac-electrons in graphene mimics the propagation of
photons. This makes it possible to control the charge-carriers with components
based on geometrical-optics and has led to proposals for an all-graphene
electron-optics platform. An open question arising from the possibility of
reducing the component-size to the nanometer-scale is how to access and
understand the transition from optical-transport to quantum-confinement. Here
we report on the realization of a circular p-n junction that can be
continuously tuned from the nanometer-scale, where quantum effects are
dominant, to the micrometer scale where optical-guiding takes over. We find
that in the nanometer-scale junction electrons are trapped in states that
resemble atomic-collapse at a supercritical charge. As the junction-size
increases, the transition to optical-guiding is signaled by the emergence of
whispering-gallery modes and Fabry-Perot interference. The creation of tunable
junctions that straddle the crossover between quantum-confinement and
optical-guiding, paves the way to novel design-architectures for controlling
electronic transport.Comment: 16 pages, 4 figure