35 research outputs found
Lattice Expansion in Seamless Bi layer Graphene Constrictions at High Bias
Our understanding of sp2 carbon nanostructures is still emerging and is
important for the development of high performance all carbon devices. For
example, in terms of the structural behavior of graphene or bi-layer graphene
at high bias, little to nothing is known. To this end we investigated bi-layer
graphene constrictions with closed edges (seamless) at high bias using in situ
atomic resolution transmission electron microscopy. We directly observe a
highly localized anomalously large lattice expansion inside the constriction.
Both the current density and lattice expansion increase as the bi-layer
graphene constriction narrows. As the constriction width decreases below 10 nm,
shortly before failure, the current density rises to 4 \cdot 109 A cm-2 and the
constriction exhibits a lattice expansion with a uniaxial component showing an
expansion approaching 5 % and an isotropic component showing an expansion
exceeding 1 %. The origin of the lattice expansion is hard to fully ascribe to
thermal expansion. Impact ionization is a process in which charge carriers
transfer from bonding states to antibonding states thus weakening bonds. The
altered character of C-C bonds by impact ionization could explain the
anomalously large lattice expansion we observe in seamless bi-layer graphene
constrictions. Moreover, impact ionization might also contribute to the
observed anisotropy in the lattice expansion, although strain is probably the
predominant factor.Comment: to appear in NanoLetter
Experimental Realization of a Three-Dimensional Dirac Semimetal
The three dimensional (3D) Dirac semimetal, which has been predicted
theoretically, is a new electronic state of matter. It can be viewed as 3D
generalization of graphene, with a unique electronic structure in which
conduction and valence band energies touch each other only at isolated points
in momentum space (i.e. the 3D Dirac points), and thus it cannot be classified
either as a metal or a semiconductor. In contrast to graphene, the Dirac points
of such a semimetal are not gapped by the spin-orbit interaction and the
crossing of the linear dispersions is protected by crystal symmetry. In
combination with broken time-reversal or inversion symmetries, 3D Dirac points
may result in a variety of topologically non-trivial phases with unique
physical properties. They have, however, escaped detection in real solids so
far. Here we report the direct observation of such an exotic electronic
structure in cadmium arsenide (Cd3As2) by means of angle-resolved photoemission
spectroscopy (ARPES). We identify two momentum regions where electronic states
that strongly disperse in all directions form narrow cone-like structures, and
thus prove the existence of the long sought 3D Dirac points. This electronic
structure naturally explains why Cd3As2 has one of the highest known bulk
electron mobilities. This realization of a 3D Dirac semimetal in Cd3As2 not
only opens a direct path to a wide spectrum of applications, but also offers a
robust platform for engineering topologically-nontrivial phases including Weyl
semimetals and Quantum Spin Hall systems.Comment: Submitted on the 27th of September 201