501 research outputs found
New Dirac points and multiple Landau level crossings in biased trilayer graphene
Recently a new high-mobility Dirac material, trilayer graphene, was realized
experimentally. The band structure of ABA-stacked trilayer graphene consists of
a monolayer-like and a bilayer-like pairs of bands. Here we study electronic
properties of ABA-stacked trilayer graphene biased by a perpendicular electric
field. We find that the combination of the bias and trigonal warping gives rise
to a set of new Dirac points: in each valley, seven species of Dirac fermions
with small masses of order of a few meV emerge. The positions and masses of the
emergent Dirac fermions are tunable by bias, and one group of Dirac fermions
becomes massless at a certain bias value. Therefore, in contrast to bilayer
graphene, the conductivity at the neutrality point is expected to show
non-monotonic behavior, becoming of the order of a few e^2/h when some Dirac
masses vanish. Further, we analyze the evolution of Landau level spectrum as a
function of bias. Emergence of new Dirac points in the band structure
translates into new three-fold-degenerate groups of Landau levels. This leads
to an anomalous quantum Hall effect, in which some quantum Hall steps have a
height of 3e^2/h. At an intermediate bias, the degeneracies of all Landau
levels get lifted, and in this regime all quantum Hall plateaus are spaced by
e^2/h. Finally, we show that the pattern of Landau level crossings is very
sensitive to certain band structure parameters, and can therefore provide a
useful tool for determining their precise values.Comment: 11 pages, 6 figures; v2: expanded introduction, new references added,
a few typos correcte
Magnetic order and paramagnetic phases in the quantum J1-J2-J3 honeycomb model
Recent work shows that a quantum spin liquid can arise in realistic fermionic
models on a honeycomb lattice. We study the quantum spin-1/2 Heisenberg
honeycomb model, considering couplings J1, J2, and J3 up to third nearest
neighbors. We use an unbiased pseudofermion functional renormalization group
method to compute the magnetic susceptibility and determine the ordered and
disordered states of the model. Aside from antiferromagnetic, collinear, and
spiral order domains, we find a large paramagnetic region at intermediate J2
coupling. For larger J2 within this domain, we find a strong tendency to
staggered dimer ordering, while the remaining paramagnetic regime for low J2
shows only weak plaquet and staggered dimer response. We suggest this regime to
be a promising region to look for quantum spin liquid states when charge
fluctuations would be included.Comment: 4 pages, 3 figure
Charge and Spin Transport at the Quantum Hall Edge of Graphene
Landau level bending near the edge of graphene, described using 2d Dirac
equation, provides a microscopic framework for understanding the quantum Hall
Effect (QHE) in this material. We review properties of the QHE edge states in
graphene, with emphasis on the novel phenomena that arise due to Dirac
character of electronic states. A method of mapping out the dispersion of the
edge states using scanning tunneling probes is proposed. The Zeeman splitting
of Landau levels is shown to create a particularly interesting situation around
the Dirac point, where it gives rise to counter-circulating modes with opposite
spin. These chiral spin modes lead to a rich variety of spin transport
phenomena, including spin Hall effect, spin filtering and injection, and
electric detection of spin current. The estimated Zeeman spin gap, enhanced by
exchange, of a few hundred Kelvin, makes graphene an attractive system for
spintronics. Comparison to recent transport measurements near nu=0 is
presented.Comment: 10 pages, 6 figures, invited pape
Exponentially slow heating in periodically driven many-body systems
We derive general bounds on the linear response energy absorption rates of
periodically driven many-body systems of spins or fermions on a lattice. We
show that for systems with local interactions, energy absorption rate decays
exponentially as a function of driving frequency in any number of spatial
dimensions. These results imply that topological many-body states in
periodically driven systems, although generally metastable, can have very long
lifetimes. We discuss applications to other problems, including decay of highly
energetic excitations in cold atomic and solid-state systems.Comment: v1 to v2: several typos corrected. 5 page
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