201 research outputs found
Interaction-driven topological insulator states in strained graphene
The electronic properties of graphene can be manipulated via mechanical
deformations, which opens prospects for studying the Dirac fermions in new
regimes and for new device applications. Certain natural configurations of
strain generate large nearly uniform pseudo-magnetic fields, which have
opposite signs in the two valleys, and give rise to flat spin- and
valley-degenerate pseudo Landau levels (PLLs). Here we consider the effect of
the Coulomb interactions in strained graphene with uniform pseudo-magnetic
field. We show that the spin/valley degeneracies of the PLLs get lifted by the
interactions, giving rise to topological insulator-like states. In particular,
when a nonzero PLL is quarter- or three-quarter filled, an anomalous quantum
Hall state spontaneously breaking time-reversal symmetry emerges. At
half-filled PLL, weak spin-orbital interaction stabilizes
time-reversal-symmetric quantum spin-Hall state. These many-body states are
characterized by the quantized conductance and persist to a high temperature
scale set by the Coulomb interactions, which we estimate to be a few hundreds
Kelvin at moderate strain values. At fractional fillings, fractional quantum
Hall states breaking valley symmetry emerge. These results suggest a new route
to realizing robust topological insulator states in mesoscopic graphene.Comment: 5 page
Fermi-Edge Resonance and Tunneling in Nonequilibrium Electron Gas
Fermi-edge singularity changes in a dramatic way in a nonequilibrium system,
acquiring features which reflect the structure of energy distribution. In
particular, it splits into several components if the energy distribution
exhibits multiple steps. While conventional approaches, such as bosonization,
fail to describe the nonequilibrium problem, an exact solution for a generic
energy distribution can be obtained with the help of the method of functional
determinants. In the case of a split Fermi distribution, while the `open loop'
contribution to Green's function has power law singularities, the tunneling
density of states profile exhibits broadened peaks centered at Fermi
sub-levels.Comment: 5 pages, 1 figur
Nature of the spin liquid state of the Hubbard model on honeycomb lattice
Recent numerical work (Nature 464, 847 (2010)) indicates the existence of a
spin liquid phase (SL) that intervenes between the antiferromagnetic and
semimetallic phases of the half filled Hubbard model on a honeycomb lattice. To
better understand the nature of this exotic phase, we study the quantum
spin model on the honeycomb lattice, which provides an effective
description of the Mott insulating region of the Hubbard model. Employing the
variational Monte Carlo approach, we analyze the phase diagram of the model,
finding a phase transition between antiferromagnet and an unusual SL
state at , which we identify as the SL phase of the
Hubbard model. At higher we find a transition to a
dimerized state with spontaneously broken rotational symmetry.Comment: 5 pages, 6 figure
Peierls-type Instability and Tunable Band Gap in Functionalized Graphene
Functionalizing graphene was recently shown to have a dramatic effect on the
electronic properties of this material. Here we investigate spatial ordering of
adatoms driven by the RKKY-type interactions. In the ordered state, which
arises via a Peierls-instability-type mechanism, the adatoms reside mainly on
one of the two graphene sublattices. Bragg scattering of electron waves induced
by sublattice symmetry breaking results in a band gap opening, whereby Dirac
fermions acquire a finite mass. The band gap is found to be immune to the
adatoms' positional disorder, with only an exponentially small number of
localized states residing in the gap. The gapped state is stabilized in a wide
range of electron doping. Our findings show that controlled adsorption of
adatoms or molecules provides a route to engineering a tunable band gap in
graphene.Comment: 6 pgs, 3 fg
Tunable Electron Interactions and Fractional Quantum Hall States in Graphene
The recent discovery of fractional quantum Hall states in graphene raises the
question of whether the physics of graphene and its bilayer offers any
advantages over GaAs-based materials in exploring strongly-correlated states of
two-dimensional electrons. Here we propose a method to continuously tune the
effective electron interactions in graphene and its bilayer by the dielectric
environment of the sample. Using this method, the charge gaps of prominent FQH
states, including \nu=1/3 or \nu=5/2 states, can be increased several times, or
reduced all the way to zero. The tunability of the interactions can be used to
realize and stabilize various strongly correlated phases in the FQH regime, and
to explore the transitions between them.Comment: 4.2 pages, 5 figure
Quantized Transport in Graphene p-n Junctions in Magnetic Field
Recent experimental work on locally gated graphene layers resulting in p-n
junctions have revealed quantum Hall effect in their transport behavior. We
explain the observed conductance quantization which is fractional in the
bipolar regime and integer in the unipolar regime in terms of quantum Hall edge
modes propagating along and across the p-n interface. In the bipolar regime the
electron and hole modes can mix at the p-n boundary, leading to current
partition and quantized shot noise plateaus similar to those of conductance,
while in the unipolar regime transport is noiseless. These quantum Hall
phenomena reflect the massless Dirac character of charge carriers in graphene,
with particle-hole interplay manifest in mode mixing and noise in the bipolar
regime.Comment: 4 pages, 3 figures, available online at:
http://www.sciencemag.org/cgi/content/abstract/114467
Ordering of magnetic impurities and tunable electronic properties of topological insulators
We study collective behavior of magnetic adatoms randomly distributed on the
surface of a topological insulator. As a consequence of the spin-momentum
locking on the surface, the RKKY-type interactions of two adatom spins depend
on the direction of the vector connecting them, thus interactions of an
ensemble of adatoms are frustrated. We show that at low temperatures the
frustrated RKKY interactions give rise to two phases: an ordered ferromagnetic
phase with spins pointing perpendicular to the surface, and a disordered
spin-glass-like phase. The two phases are separated by a quantum phase
transition driven by the magnetic exchange anisotropy. Ferromagnetic ordering
occurs via a finite-temperature phase transition. The ordered phase breaks
time-reversal symmetry spontaneously, driving the surface states into a gapped
state, which exhibits an anomalous quantum Hall effect and provides a
realization of the parity anomaly. We find that the magnetic ordering is
suppressed by potential scattering. Our work indicates that controlled
deposition of magnetic impurities provides a way to modify the electronic
properties of topological insulators.Comment: 4+ pages, 2 figure
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