923 research outputs found
Orbital order in bilayer graphene at filling factor
In a graphene bilayer with Bernal stacking both and orbital
Landau levels have zero kinetic energy. An electronic state in the N=0 Landau
level consequently has three quantum numbers in addition to its guiding center
label: its spin, its valley index or , and an orbital quantum
number The two-dimensional electron gas (2DEG) in the bilayer supports
a wide variety of broken-symmetry states in which the pseudospins associated
these three quantum numbers order in a manner that is dependent on both filling
factor and the electric potential difference between the layers. In this
paper, we study the case of in an external field strong enough to
freeze electronic spins. We show that an electric potential difference between
layers drives a series of transitions, starting from interlayer-coherent states
(ICS) at small potentials and leading to orbitally coherent states (OCS) that
are polarized in a single layer. Orbital pseudospins carry electric dipoles
with orientations that are ordered in the OCS and have Dzyaloshinskii-Moriya
interactions that can lead to spiral instabilities. We show that the microwave
absorption spectra of ICSs, OCSs, and the mixed states that occur at
intermediate potentials are sharply distinct.Comment: 21 pages, 14 figure
Renormalized Landau Levels and Particle-Hole Symmetry in Graphene
In this proceedings paper we report on a calculation of graphene's Landau
levels in a magnetic field. Our calculations are based on a self-consistent
Hartree-Fock approximation for graphene's massless-Dirac continuum model. We
find that because of graphene's chiral band structure interactions not only
shift Landau-level energies, as in a non-relativistic electron gas, but also
alter Landau level wavefunctions. We comment on the subtle continuum model
regularization procedure necessary to correctly maintain the lattice-model's
particle hole symmetry properties
Orbital and interlayer Skyrmions crystals in bilayer graphene
A graphene bilayer in a transverse magnetic field has a set of Landau levels
with energies where
is the effective cyclotron frequency and
All Landau levels but N=0 are four times degenerate counting spin and valley
degrees of freedom. The Landau level N=0 has an extra degeneracy due to the
fact that orbitals and both have zero kinetic energies. At integer
filling factors, Coulomb interactions produce a set of broken-symmetry states
with partial or full alignement in space of the valley and orbital pseudospins.
These quantum Hall pseudo-ferromagnetic states support topological charged
excitations in the form of orbital and valley Skyrmions. Away from integer
fillings, these topological excitations can condense to form a rich variety of
Skyrme crystals with interesting properties. We study in this paper different
crystal phases that occur when an electric field is applied between the layers.
We show that orbital Skyrmions, in analogy with spin Skyrmions, have a texture
of electrical dipoles that can be controlled by an in-plane electric field.
Moreover, the modulation of electronic density in the crystalline phases are
experimentally accessible through a measurement of their local density of
statesComment: 18 pages with 13 figure
The Role of Electron-electron Interactions in Graphene ARPES Spectra
We report on a theoretical study of the influence of electron-electron
interactions on ARPES spectra in graphene that is based on the
random-phase-approximation and on graphene's massless Dirac equation continuum
model. We find that level repulsion between quasiparticle and plasmaron
resonances gives rise to a gap-like feature at small k. ARPES spectra are
sensitive to the electron-electron interaction coupling strength and might enable an experimental determination of this material parameter.Comment: 5 Pages, 4 Figures, Submitte
Intra-Landau level Cyclotron Resonance in Bilayer Graphene
Interaction driven integer quantum Hall effects are anticipated in graphene
bilayers because of the near-degeneracy of the eight Landau levels which appear
near the neutral system Fermi level. We predict that an intra-Landau-level
cyclotron resonance signal will appear at some odd-integer filling factors,
accompanied by collective modes which are nearly gapless and have approximate
dispersion. We speculate on the possibility of unususal localization
physics associated with these modes.Comment: 5 pages, 2 figure
Experimental and numerical validation of active flaps for wind turbine blades
An industrial active flap concept for wind turbine rotor blades is validated numerically by means of CFD, as well as experimentally in a wind tunnel environment. This paper presents the numerical and experimental results, as well as a discussion regarding the testing of airfoils equipped with active flaps with a highly loaded aft portion. A conceptual implementation for an offshore wind turbine and the potential for load reduction is shown by means of aeroelastic calculations. The work presented herein is conducted within the frame of the Induflap2 project and is partially funded by the Danish funding board EUDP
Numerical studies of the fractional quantum Hall effect in systems with tunable interactions
The discovery of the fractional quantum Hall effect in GaAs-based
semiconductor devices has lead to new advances in condensed matter physics, in
particular the possibility for exotic, topological phases of matter that
possess fractional, and even non-Abelian, statistics of quasiparticles. One of
the main limitations of the experimental systems based on GaAs has been the
lack of tunability of the effective interactions between two-dimensional
electrons, which made it difficult to stabilize some of the more fragile
states, or induce phase transitions in a controlled manner. Here we review the
recent studies that have explored the effects of tunability of the interactions
offered by alternative two-dimensional systems, characterized by non-trivial
Berry phases and including graphene, bilayer graphene and topological
insulators. The tunability in these systems is achieved via external fields
that change the mass gap, or by screening via dielectric plate in the vicinity
of the device. Our study points to a number of different ways to manipulate the
effective interactions, and engineer phase transitions between quantum Hall
liquids and compressible states in a controlled manner.Comment: 9 pages, 4 figures, updated references; review for the CCP2011
conference, to appear in "Journal of Physics: Conference Series
Vaccine-like and Prophylactic Treatments of EAE with Novel IDomain Antigen Conjugates (IDAC): Targeting Multiple Antigenic Peptides to APC
The objective of this work is to utilize novel I-domain antigenic-peptide conjugates (IDAC) for
targeting antigenic peptides to antigen-presenting cells (APC) to simulate tolerance in
experimental autoimmune encephalomyelitis (EAE). IDAC-1 and IDAC-3 molecules are
conjugates between the I-domain protein and PLP-Cys and Ac-PLP-Cys-NH2 peptides,
respectively, tethered to N-terminus and Lys residues on the I-domain. The hypothesis is that the
I-domain protein binds to ICAM-1 and PLP peptide binds to MHC-II on the surface of APC; this
binding event inhibits the formation of the immunological synapse at the APC-T-cell interface to
alter T-cell differentiation from inflammatory to regulatory phenotypes. Conjugation of peptides
to the I-domain did not change the secondary structure of IDAC molecules as determined by
circular dichroism spectroscopy. The efficacies of IDAC-1 and -3 were evaluated in EAE mice by
administering i.v or s.c. injections of IDAC in a prophylactic or a vaccine-like dosing schedule.
IDAC-3 was better than IDAC-1 in suppressing and delaying the onset of EAE when delivered in
prophylactic and vaccine-like manners. IDAC-3 also suppressed subsequent relapse of the disease.
The production of IL-17 was lowered in the IDAC-33 treated mice compared to those treated with
PBS. In contrast, the production of IL-10 was increased, suggesting that there is a shift from
inflammatory to regulatory T-cell populations in IDAC-33treated mice. In conclusion, the Idomain
can effectively deliver antigenic peptides in a vaccine-like or prophylactic manner for
inducing immunotolerance in the EAE mouse model
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