365 research outputs found
Spin Polarized and Valley Helical Edge Modes in Graphene Nanoribbons
Inspired by recent progress in fabricating precisely zigzag-edged graphene
nanoribbons and the observation of edge magnetism, we find that spin polarized
edge modes with well-defined valley index can exist in a bulk energy gap opened
by a staggered sublattice potential such as that provided by a hexagonal
Boron-Nitride substrate. Our result is obtained by both tight-binding model and
first principles calculations. These edge modes are helical with respect to the
valley degree of freedom, and are robust against scattering, as long as the
disorder potential is smooth over atomic scale, resulting from the protection
of the large momentum separation of the valleys.Comment: 4 pages, 4 figure
Edge states in Graphene: from gapped flat band to gapless chiral modes
We study edge-states in graphene systems where a bulk energy gap is opened by
inversion symmetry breaking. We find that the edge-bands dispersion can be
controlled by potentials applied on the boundary with unit cell length scale.
Under certain boundary potentials, gapless edge-states with valley-dependent
velocity are found, exactly analogous to the spin-dependent gapless chiral
edge-states in quantum spin Hall systems. The connection of the edge-states to
bulk topological properties is revealed
Quantum Anomalous Hall Effect in Graphene from Rashba and Exchange Effects
We investigate the possibility of realizing quantum anomalous Hall effect in
graphene. We show that a bulk energy gap can be opened in the presence of both
Rashba spin-orbit coupling and an exchange field. We calculate the Berry
curvature distribution and find a non-zero Chern number for the valence bands
and demonstrate the existence of gapless edge states. Inspired by this finding,
we also study, by first principles method, a concrete example of graphene with
Fe atoms adsorbed on top, obtaining the same result.Comment: 4 papges, 5 figure
Magnetic control of the valley degree of freedom of massive Dirac fermions with application to transition metal dichalcogenides
We study the valley-dependent magnetic and transport properties of massive
Dirac fermions in multivalley systems such as the transition metal
dichalcogenides. The asymmetry of the zeroth Landau level between valleys and
the enhanced magnetic susceptibility can be attributed to the different orbital
magnetic moment tied with each valley. This allows the valley polarization to
be controlled by tuning the external magnetic field and the doping level. As a
result of this magnetic field induced valley polarization, there exists an
extra contribution to the ordinary Hall effect. All these effects can be
captured by a low energy effective theory with a valley-orbit coupling term.Comment: 9 pages, 6 figure
Blue Phosphorene Oxide: Strain-tunable Quantum Phase Transitions and Novel 2D Emergent Fermions
Tunable quantum phase transitions and novel emergent fermions in solid state
materials are fascinating subjects of research. Here, we propose a new stable
two-dimensional (2D) material, the blue phosphorene oxide (BPO), which exhibits
both. Based on first-principles calculations, we show that its equilibrium
state is a narrow-bandgap semiconductor with three bands at low energy.
Remarkably, a moderate strain can drive a semiconductor-to-semimetal quantum
phase transition in BPO. At the critical transition point, the three bands
cross at a single point at Fermi level, around which the quasiparticles are a
novel type of 2D pseudospin-1 fermions. Going beyond the transition, the system
becomes a symmetry-protected semimetal, for which the conduction and valence
bands touch quadratically at a single Fermi point that is protected by
symmetry, and the low-energy quasiparticles become another novel type of 2D
double Weyl fermions. We construct effective models characterizing the phase
transition and these novel emergent fermions, and we point out several exotic
effects, including super Klein tunneling, supercollimation, and universal
optical absorbance. Our result reveals BPO as an intriguing platform for the
exploration of fundamental properties of quantum phase transitions and novel
emergent fermions, and also suggests its great potential in nanoscale device
applications.Comment: 23 pages, 5 figure
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