1,070 research outputs found
Diffraction catastrophes and semiclassical quantum mechanics for Veselago lensing in graphene
We study the effect of trigonal warping on the focussing of electrons by n-p
junctions in graphene. We find that perfect focussing, which was predicted for
massless Dirac fermions, is only preserved for one specific sample orientation.
In the general case, trigonal warping leads to the formation of cusp caustics,
with a different position of the focus for graphene's two valleys. We develop a
semiclassical theory to compute these positions and find very good agreement
with tight-binding simulations. Considering the transmission as a function of
potential strength, we find that trigonal warping splits the single Dirac peak
into two distinct peaks, leading to valley polarization. We obtain the
transmission curves from tight-binding simulations and find that they are in
very good agreement with the results of a billiard model that incorporates
trigonal warping. Furthermore, the positions of the transmission maxima and the
scaling of the peak width are accurately predicted by our semiclassical theory.
Our semiclassical analysis can easily be carried over to other Dirac materials,
which generally have different Fermi surface distortions.Comment: 6 pages, 4 figures, plus supplemental material. Important reference
added and text update
Atomistic simulations of structural and thermodynamic properties of bilayer graphene
We study the structural and thermodynamic properties of bilayer graphene, a
prototype two-layer membrane, by means of Monte Carlo simulations based on the
empirical bond order potential LCBOPII. We present the temperature dependence
of lattice parameter, bending rigidity and high temperature heat capacity as
well as the correlation function of out-of-plane atomic displacements. The
thermal expansion coefficient changes sign from negative to positive above
K, which is lower than previously found for single layer graphene
and close to the experimental value of bulk graphite. The bending rigidity is
twice as large than for single layer graphene, making the out-of-plane
fluctuations smaller. The crossover from correlated to uncorrelated
out-of-plane fluctuations of the two carbon planes occurs for wavevectors
shorter than nmComment: 6 pages, 7 figures
Motion of domain walls and the dynamics of kinks in the magnetic Peierls potential
We study the dynamics of magnetic domain walls in the Peierls potential due
to the discreteness of the crystal lattice. The propagation of a narrow domain
wall (comparable to the lattice parameter) under the effect of a magnetic field
proceeds through the formation of kinks in its profile. We predict that,
despite the discreteness of the system, such kinks can behave like sine-Gordon
solitons in thin films of materials such as yttrium iron garnets, and we derive
general conditions for other materials. In our simulations we also observe
long-lived breathers. We provide analytical expressions for the effective mass
and limiting velocity of the kink in excellent agreement with our numerical
results.Comment: 12 pages, 9 figures (incl. supp. mat.
Mechanics of thermally fluctuating membranes
Besides having unique electronic properties, graphene is claimed to be the
strongest material in nature. In the press release of the Nobel committee it is
claimed that a hammock made of a squared meter of one-atom thick graphene could
sustain the wight of a 4 kg cat. More practically important are applications of
graphene like scaffolds and sensors which are crucially dependent on the
mechanical strength. Meter-sized graphene is even being considered for the
lightsails in the starshot project to reach the star alpha centaury. The
predicted strength of graphene is based on its very large Young modulus which
is, per atomic layer, much larger than that of steel. This reasoning however
would apply to conventional thin plates but does not take into account the
peculiar properties of graphene as a thermally fluctuating crystalline
membrane. It was shown recently both experimentally and theoretically that
thermal fluctuations lead to a dramatic reduction of the Young modulus and
increase of the bending rigidity for micron-sized graphene samples in
comparison with atomic scale values. This makes the use of the standard
F\"oppl-von Karman elasticity (FvK) theory for thin plates not directly
applicable to graphene and other single atomic layer membranes. This fact is
important because the current interpretation of experimental results is based
on the FvK theory. In particular, we show that the FvK-derived Schwerin
equation, routinely used to derive the Young modulus from indentation
experiments has to be essentially modified for graphene at room temperature and
for micron sized samples. Based on scaling analysis and atomistic simulation we
investigate the mechanics of graphene under transverse load up to breaking. We
determine the limits of applicability of the FvK theory and provide
quantitative estimates for the different regimes.Comment: to appear in npj 2D Materials and Application
Dirac electrons and domain walls: a realization in junctions of ferromagnets and topological insulators
We study a system of Dirac electrons with finite density of charge carriers
coupled to an external electromagnetic field in two spatial dimensions, with a
domain wall (DW) mass term. The interface between a thin-film ferromagnet and a
three-dimensional topological insulator provides a condensed-matter realization
of this model, when an out-of-plane domain wall magnetization is coupled to the
TI surface states. We show how, for films with very weak intrinsic in-plane
anisotropies, the torque generated by the edge electronic current flowing along
the DW competes with an effective in-plane anisotropy energy, induced by
quantum fluctuations of the chiral electrons bound to the wall, in a mission to
drive the internal angle of the DW from a Bloch configuration towards a N\'eel
configuration. Both the edge current and the induced anisotropy contribute to
stabilize the internal angle, so that for weak intrinsic in-plane anisotropies
DW motion is still possible without suffering from an extremely early Walker
breakdown.Comment: 18 pages, 3 figure
Electronic Structure of gated graphene and graphene ribbons
We study the electronic structure of gated graphene sheets. We consider both
infinite graphene and finite width ribbons. The effect of Coulomb interactions
between the electrically injected carriers and the coupling to the external
gate are computed self-consistently in the Hartree approximation. We compute
the average density of extra carriers, , the number of occupied
subbands and the density profiles as a function of the gate potential . We
discuss quantum corrections to the classical capacitance and we calculate the
threshold above which semiconducting armchair ribbons conduct. We find
that the ideal conductance of perfectly transmitting wide ribbons is
proportional to the square root of the gate voltage.Comment: 8 pages, 6 figure
Scaling behavior and strain dependence of in-plane elastic properties of graphene
We show by atomistic simulations that, in the thermodynamic limit, the
in-plane elastic moduli of graphene at finite temperature vanish with system
size as a power law with , in
agreement with the membrane theory. Our simulations clearly reveal the size and
strain dependence of graphene's elastic moduli, allowing comparison to
experimental data. Although the recently measured difference of a factor 2
between the asymptotic value of the Young modulus for tensilely strained
systems and the value from {\it ab initio} calculations remains unsolved, our
results do explain the experimentally observed increase of more than a factor 2
for a tensile strain of only a few permille. We also discuss the scaling of the
Poisson ratio, for which our simulations disagree with the predictions of the
self-consistent screening approximation.Comment: 5 figure
Zero modes in magnetic systems: general theory and an efficient computational scheme
The presence of topological defects in magnetic media often leads to normal
modes with zero frequency (zero modes). Such modes are crucial for long-time
behavior, describing, for example, the motion of a domain wall as a whole.
Conventional numerical methods to calculate the spin-wave spectrum in magnetic
media are either inefficient or they fail for systems with zero modes. We
present a new efficient computational scheme that reduces the magnetic
normal-mode problem to a generalized Hermitian eigenvalue problem also in the
presence of zero modes. We apply our scheme to several examples, including
two-dimensional domain walls and Skyrmions, and show how the effective masses
that determine the dynamics can be calculated directly. These systems highlight
the fundamental distinction between the two types of zero modes that can occur
in spin systems, which we call special and inertial zero modes. Our method is
suitable for both conservative and dissipative systems. For the latter case, we
present a perturbative scheme to take into account damping, which can also be
used to calculate dynamical susceptibilities.Comment: 64 pages, 15 figure
First-principles Theory of Nonlocal Screening in Graphene
Using the quasiparticle self-consistent GW (QSGW) and local-density (LD)
approximations, we calculate the q-dependent static dielectric function, and
derive an effective 2D dielectric function corresponding to screening of point
charges. In the q-to-0 limit, the 2D function is found to scale approximately
as the square root of the macroscopic dielectric function. Its value is ~4, a
factor approximately 1.5 larger than predictions of Dirac model. Both kinds of
dielectric functions depend strongly on q, in contrast with the Dirac model.
The QSGW approximation is shown to describe QP levels very well, with small
systematic errors analogous to bulk sp semiconductors. Local-field effects are
rather more important in graphene than in bulk semiconductors.Comment: 9 pages, 2 figure
Chirality-dependent transmission of spin waves through domain walls
Spin-wave technology (magnonics) has the potential to further reduce the size
and energy consumption of information processing devices. In the submicrometer
regime (exchange spin waves), topological defects such as domain walls may
constitute active elements to manipulate spin waves and perform logic
operations. We predict that spin waves that pass through a domain wall in an
ultrathin perpendicular-anisotropy film experience a phase shift that depends
on the orientation of the domain wall (chirality). The effect, which is absent
in bulk materials, originates from the interfacial Dzyaloshinskii-Moriya
interaction and can be interpreted as a geometric phase. We demonstrate
analytically and by means of micromagnetic simulations that the phase shift is
strong enough to switch between constructive and destructive interference. The
two chirality states of the domain wall may serve as a memory bit or spin-wave
switch in magnonic devices.Comment: 11 pages, 10 figures (incl. supp. mat.); Phys. Rev. Lett. (accepted
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