371 research outputs found
Strain dependent conductivity in biased bilayer graphene
Intrinsic bilayer graphene is a gapless semimetal. Under the application of a
bias field it becomes a semiconductor with a direct band gap that is
proportional to the applied field. Under a layer-asymmetric strain (where the
upper layer undergoes compression and lower layer tension or visa-versa) we
find that the band gap of a biased bilayer graphene ribbon becomes indirect
and, for higher strains, becomes negative returning the material its original
semimetal state. As a result, the conductivity of the ribbon increases and can
be almost an order of magnitude larger that of the intrinsic unbiased material
- a change that can be induced with a strain of only ~2-3%. The conductivity is
proportional to the applied strain and the magnitude of the effect is tunable
with the bias field. Such layer-asymmetric strains can be achieved by bending,
with forces on the order of ~1nN resulting in a layer-asymmetric strain of ~1%.
This new electromechanical effect has a wide potential for application in the
areas of nano-force microscopy and pressure sensing on the atomic scale.Comment: Updated to journal version with new title and revised figures. 8
pages, 6 figure
Large displacement strain theory and its application to graphene
Under the application of a force, a material will deform and, hence, the
crystal lattice will experience strain. This induced strain will alter the
electronic properties of the material. In particular, strain in graphene
generates an artificial vector potential which, if spatially varying, admits a
pseudo-magnetic field. Current theories for spatially varying strain use linear
or finite strain theory whose derivation is based on small displacements of
infinitesimal length vectors. Here we apply a differential geometry method to
derive a strain theory for large displacements of finite length vectors. This
method gives a finite displacement term whose contribution is comparable to
that of the linear strain term. Further to this, we show that a 'domain
wall'-like pseudo-magnetic field profile can be generated when a wide graphene
ribbon is subjected to a pair of opposing point forces (point stretch). The
resulting field is a function of the new finite displacement term only and
displays a maximum strength of over three times that which is predicted by the
linear strain theory. These results extend the current theories of strain,
which are based on the transformation of infinitesimal length vectors, to
finite length vectors, thus providing an accurate description of
pseudo-magnetic field structures in strained materials.Comment: Updated to journal version with revised figure 3 and new title. 7
pages, 5 figure
Optical properties of topological insulator Bragg gratings: Faraday rotation enhancement for TM polarized light at large incidence angles
Using the transfer matrix formalism, we study the transmission properties of
a Bragg grating constructed from a layered axionic material. Such a material
can be realized by a topological insulator subject to a time-symmetry breaking
perturbation, such as an external magnetic field or surface magnetic
impurities. Whilst the reflective properties of the structure are only
negligibly changed by the presence of the axionic material, the grating induces
a Faraday rotation and ellipticity in the transmitted light. We find that for
TM polarized light incident on a 16 layer structure at 76 degrees to the normal
the Faraday rotation can approach ~232 mrad (~13 degrees), whilst interference
from the multi-layered structure ensures high transmission. This is
significantly higher than Faraday rotations for the TM polarization at normal
incidences or the TE polarization at any incident angle. Thus, Bragg gratings
in this geometry show a strong optical signal of the magneto-electric and,
hence, provide an ideal system in which to observe this effect by optical
means.Comment: Updated to journal version with new title and new results. 8 pages, 6
figure
Quantum coherence induced second plateau in high-sideband generation
Optically excited electron-hole pairs, driven by a strong terahertz (THz)
field, create high-sidebands in the optical spectrum. The sideband spectrum
exhibits a 'plateau' up to a cutoff of 3.17Up, where Up is the ponderomotive
energy. This cutoff is determined, semi-classically, from the maximum kinetic
energy an electron-hole pair can gain from the THz field along a closed
trajectory. A full quantum treatment reveals a second, classically forbidden,
plateau with a cutoff of 8Up, the maximum kinetic energy an electron-hole pair
can gain from the THz field along an open trajectory. The second plateau
appears because a spatially separated electron and hole can still recombine if
the classical excursion is within the coherence length of the electron-hole
wavefunction or, equivalently, the coherence time is longer than the excursion
time (half the THz field period). This effect broadens the range of materials
and excitation conditions where high-sideband generations can occur, thereby
providing a wealth of novel systems for ultrafast electro-optical applications.Comment: Updated to journal version with revised figure 1. 5 pages, 3 figure
The Electromagnetic Green's Function for Layered Topological Insulators
The dyadic Green's function of the inhomogeneous vector Helmholtz equation
describes the field pattern of a single frequency point source. It appears in
the mathematical description of many areas of electromagnetism and optics
including both classical and quantum, linear and nonlinear optics, dispersion
forces (such as the Casimir and Casimir-Polder forces) and in the dynamics of
trapped atoms and molecules. Here, we compute the Green's function for a
layered topological insulator. Via the magnetoelectric effect, topological
insulators are able to mix the electric, E, and magnetic induction, B, fields
and, hence, one finds that the TE and TM polarizations mix on reflection
from/transmission through an interface. This leads to novel field patterns
close to the surface of a topological insulator.Comment: 16 pages, 9 figure
Casimir-Polder Shift and Decay Rate in the Presence of Nonreciprocal Media
We calculate the Casimir-Polder frequency shift and decay rate for an atom in
front of a nonreciprocal medium by using macroscopic quantum electrodynamics.
The results are a generalization of the respective quantities for matter with
broken time-reversal symmetry which does not fulfill the Lorentz reciprocity
principle. As examples, we contrast the decay rates, the resonant and
nonresonant frequency shifts of a perfectly conducting (reciprocal) mirror to
those of a perfectly reflecting nonreciprocal mirror. We find different power
laws for the distance dependence of all quantities in the retarded and
nonretarded limits. As an example of a more realistic nonreciprocal medium, we
investigate a topological insulator subject to a time-symmetry breaking
perturbation.Comment: 11 pages, 6 figure
Faraday rotations, ellipticity and circular dichroism in the magneto-optical spectrum of moir\'e superlattices
We study the magneto-optical conductivity of a number of Van der Waals
heterostructures, namely, twisted bilayer graphene, AB-AB and AB-BA stacked
twisted double bilayer graphene and monolayer graphene and AB-stacked bilayer
graphene on hexagonal boron nitride. As magnetic field increases, the
absorption spectrum exhibits a self-similar recursive pattern reflecting the
fractal nature of the energy spectrum. Whilst twisted bilayer graphene displays
only weak circular dichroism, monolayer graphene and AB-stacked bilayer
graphene on hexagonal boron nitride show specifically strong circular
dichroism, owing to strong inversion symmetry breaking properties of the
hexagonal boron nitride layer. As, the left and right circularly polarized
light interact with these structures differently, plane polarized incident
light undergoes a Faraday rotation and gains an ellipticity when transmitted.
The size of the respective angles is on the order of a degree.Comment: 24 pages, 9 figure
Quasicrystalline electronic states in twisted bilayers and the effects of interlayer and sublattice symmetries
We study the electronic structure of quasicrystals composed of incommensurate
stacks of atomic layers. We consider two systems: a pair of square lattices
with a relative twist angle of and a pair of hexagonal
lattices with a relative twist angle of , with various
interlayer interaction strengths. This constitutes every two-dimensional
bilayer quasicrystal system. We investigate the resonant coupling governing the
quasicrystalline order in each quasicrystal symmetry, and calculate the
quasi-band dispersion. The resonant interaction emerges in bilayer
quasicrystals if all the dominant interlayer interactions occur between the
atomic orbitals that have the same magnetic quantum number. Thus, not only the
quasicrystal composed of the widely studied graphene, but also those composed
of transition metal dichalcogenides will exhibit the quasicrystalline states.
We find that some quasicrystalline states, which are usually obscured by
decoupled monolayer states, are more prominent, i.e., "exposed", in the systems
with strong interlayer interaction. We also show that we can switch the states
between quasicrystalline configuration and its layer components, by turning on
and off the interlayer symmetry.Comment: 18 pages, 10 figure
Trigonal Quasicrystalline States in Rotated Double Moir\'{e} Superlattices
We study the lattice configuration and electronic structure of a double
moir\'{e} superlattice, which is composed of a graphene layer encapsulated by
two other layers in a way such that the two hexagonal moir\'{e} patterns are
arranged in a dodecagonal quasicrystalline configuration. We show that there
are between 0 and 4 such configurations depending on the lattice mismatch
between graphene and the encapsulating layer. We then reveal the resonant
interaction, which is distinct from the conventional 2-, 3-, 4-wave mixing of
moir\'{e} superlattices, that brings together and hybridizes twelve degenerate
Bloch states of monolayer graphene. These states do not fully satisfy the
dodecagonal quasicrystalline rotational symmetry due to the symmetry of the
wave vectors involved. Instead, their wave functions exhibit trigonal
quasicrystalline order, which lacks inversion symmetry, at the energies much
closer to the charge neutrality point of graphene.Comment: 12 pages, 6 figure
Thermal Casimir-Polder shifts in Rydberg atoms near metallic surfaces
The Casimir-Polder (CP) potential and transition rates of a Rydberg atom
above a plane metal surface at finite temperature are discussed. As an example,
the CP potential and transition rates of a rubidium atom above a copper surface
at room temperature is computed. Close to the surface we show that the
quadrupole correction to the force is significant and increases with increasing
principal quantum number n. For both the CP potential and decay rates one finds
that the dominant contribution comes from the longest wavelength transition and
the potential is independent of temperature. We provide explicit scaling laws
for potential and decay rates as functions of atom-surface distance and
principal quantum number of the initial Rydberg state.Comment: Updated to journal version with corrected figures. 4 Pages, 2 figure
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