11 research outputs found
Moiré Superlattice Effects and Band Structure Evolution in Near-30-Degree Twisted Bilayer Graphene
Pnictogens Allotropy and Phase Transformation during van der Waals Growth
Pnictogens have multiple allotropic forms resulting from their ns2 np3
valence electronic configuration, making them the only elemental materials to
crystallize in layered van der Waals (vdW) and quasi-vdW structures throughout
the group. Light group VA elements are found in the layered orthorhombic A17
phase such as black phosphorus, and can transition to the layered rhombohedral
A7 phase at high pressure. On the other hand, bulk heavier elements are only
stable in the A7 phase. Herein, we demonstrate that these two phases not only
co-exist during the vdW growth of antimony on weakly interacting surfaces, but
also undertake a spontaneous transformation from the A17 phase to the
thermodynamically stable A7 phase. This metastability of the A17 phase is
revealed by real-time studies unraveling its thickness-driven transition to the
A7 phase and the concomitant evolution of its electronic properties. At a
critical thickness of ~4 nm, A17 antimony undergoes a diffusionless shuffle
transition from AB to AA stacked alpha-antimonene followed by a gradual
relaxation to the A7 bulk-like phase. Furthermore, the electronic structure of
this intermediate phase is found to be determined by surface self-passivation
and the associated competition between A7- and A17-like bonding in the bulk.
These results highlight the critical role of the atomic structure and
interfacial interactions in shaping the stability and electronic
characteristics of vdW layered materials, thus enabling a new degree of freedom
to engineer their properties using scalable processes
Revisiting the Origin of Low Work Function Areas in Pattern Forming Reaction Systems: Electropositive Contaminants or Subsurface Oxygen?
Moiré Superlattice Effects and Band Structure Evolution in Near-30-Degree Twisted Bilayer Graphene
In stacks of two-dimensional crystals, mismatch of their lattice constants and misalignment of crystallographic axes lead to formation of moiré patterns. We show that moiré superlattice effects persist in twisted bilayer graphene (tBLG) with large twists and short moiré periods. Using angle-resolved photoemission, we observe dramatic changes in valence band topology across large regions of the Brillouin zone, including the vicinity of the saddle point at M and across 3 eV from the Dirac points. In this energy range, we resolve several moiré minibands and detect signatures of secondary Dirac points in the reconstructed dispersions. For twists θ > 21.8°, the low-energy minigaps are not due to cone anticrossing as is the case at smaller twist angles but rather due to moiré scattering of electrons in one graphene layer on the potential of the other which generates intervalley coupling. Our work demonstrates the robustness of the mechanisms which enable engineering of electronic dispersions of stacks of two-dimensional crystals by tuning the interface twist angles. It also shows that large-angle tBLG hosts electronic minigaps and van Hove singularities of different origin which, given recent progress in extreme doping of graphene, could be explored experimentally
Moiré Superlattice Effects and Band Structure Evolution in Near-30-Degree Twisted Bilayer Graphene
In stacks of two-dimensional crystals, mismatch of their lattice constants
and misalignment of crystallographic axes lead to formation of moir\'{e}
patterns. We show that moir\'{e} superlattice effects persist in twisted
bilayer graphene (tBLG) with large twists and short moir\'{e} periods. Using
angle-resolved photoemission, we observe dramatic changes in valence band
topology across large regions of the Brillouin zone, including the vicinity of
the saddle point at and across 3 eV from the Dirac points. In this energy
range, we resolve several moir\'{e} minibands and detect signatures of
secondary Dirac points in the reconstructed dispersions. For twists
, the low-energy minigaps are not due to cone
anti-crossing as is the case at smaller twist angles but rather due to
moir\'{e} scattering of electrons in one graphene layer on the potential of the
other which generates intervalley coupling. Our work demonstrates robustness of
mechanisms which enable engineering of electronic dispersions of stacks of
two-dimensional crystals by tuning the interface twist angles. It also shows
that large-angle tBLG hosts electronic minigaps and van Hove singularities of
different origin which, given recent progress in extreme doping of graphene,
could be explored experimentally.Comment: main text: 25 pages, 5 figures; supplement: 22 pages, 7 figure
Magnetic-field-induced domain-wall motion in permalloy nanowires with modified Gilbert damping
Domain wall (DW) depinning and motion in the viscous regime induced by magnetic fields, are investigated in planar permalloy nanowires in which the Gilbert damping α is tuned in the range 0.008–0.26 by doping with Ho. Real time, spatially resolved magneto-optic Kerr effect measurements yield depinning field distributions and DW mobilities. Depinning occurs at discrete values of the field which are correlated with different metastable DW states and changed by the doping. For α<0.033, the DW mobilities are smaller than expected while for α≥0.033, there is agreement between the measured DW mobilities and those predicted by the standard one-dimensional model of field-induced DW motion. Micromagnetic simulations indicate that this is because as α increases, the DW spin structure becomes increasingly rigid. Only when the damping is large can the DW be approximated as a pointlike quasiparticle that exhibits the simple translational motion predicted in the viscous regime. When the damping is small, the DW spin structure undergoes periodic distortions that lead to a velocity reduction. We therefore show that Ho doping of permalloy nanowires enables engineering of the DW depinning and mobility, as well as the extent of the viscous regime