10,583 research outputs found
Anomalous Strength Characteristics of Tilt Grain Boundaries in Graphene
Using molecular dynamics simulations and first principles calculations, we
have studied the structure and mechanical strength of tilt grain boundaries in
graphene sheets that arise during CVD growth of graphene on metal substrates.
Surprisingly, we find that for tilt boundaries in the vicinity of both the
zig-zag and arm-chair orientations, large angle boundaries with a higher
density of 5-7 defect pairs are stronger than the low-angle boundaries which
are comprised of fewer defects per unit length. Interestingly, the trends in
our results cannot be explained by a continuum Griffith-type fracture mechanics
criterion, which predicts the opposite trend due to that fact that it does not
account for the critical bonds that are responsible for the failure mechanism.
We have identified the highly-strained bonds in the 7-member rings that lead to
the failure of the sheets, and we have found that large angle boundaries are
able to better accommodate the strained 7-rings. Our results provide guidelines
for designing growth methods to obtain grain boundary structures that can have
strengths close to that of pristine graphene
Variational method to study vortex matter in mesoscopic superconductors
A simple variational model is proposed to analyze the superconducting state
in long cylindrical type-II superconductor placed in the external magnetic
field. In the framework of this model, it is possible to solve the
Ginzburg-Landau equations for the states with axially symmetric distributions
of the order parameter. Phase transitions between different superconducting
states are studied in the presence of external magnetic field and an
equilibrium phase diagram of thin cylinder is obtained. The lower critical
field of the cylindrical type-II superconductor with arbitrary values of radius
and Ginzburg-Landau parameter is found. The field dependence of the
magnetization of thin cylinder, which can carry several magnetic flux quanta,
is calculated.Comment: 10 pages, 5 figures, submitted to Physica
Non-local transport and the hydrodynamic shear viscosity in graphene
Motivated by recent experimental progress in preparing encapsulated graphene
sheets with ultra-high mobilities up to room temperature, we present a
theoretical study of dc transport in doped graphene in the hydrodynamic regime.
By using the continuity and Navier-Stokes equations, we demonstrate
analytically that measurements of non-local resistances in multi-terminal Hall
bar devices can be used to extract the hydrodynamic shear viscosity of the
two-dimensional (2D) electron liquid in graphene. We also discuss how to probe
the viscosity-dominated hydrodynamic transport regime by scanning probe
potentiometry and magnetometry. Our approach enables measurements of the
viscosity of any 2D electron liquid in the hydrodynamic transport regime.Comment: 12 pages, 4 multi-panel figure
Failure of conductance quantization in two-dimensional topological insulators due to non-magnetic impurities
Despite topological protection and the absence of magnetic impurities,
two-dimensional topological insulators display quantized conductance only in
surprisingly short channels, which can be as short as 100 nm for atomically
thin materials. We show that the combined action of short-range nonmagnetic
impurities located near the edges and on site electron-electron interactions
effectively creates noncollinear magnetic scatterers, and, hence, results in
strong backscattering. The mechanism causes deviations from quantization even
at zero temperature and for a modest strength of electron-electron
interactions. Our theory provides a straightforward conceptual framework to
explain experimental results, especially those in atomically thin crystals,
plagued with short-range edge disorder.Comment: 8 pages, 9 figures, 5 appendice
Magnetoresistance of a 2-dimensional electron gas in a random magnetic field
We report magnetoresistance measurements on a two-dimensional electron gas
(2DEG) made from a high mobility GaAs/AlGaAs heterostructure, where the
externally applied magnetic field was expelled from regions of the
semiconductor by means of superconducting lead grains randomly distributed on
the surface of the sample. A theoretical explanation in excellent agreement
with the experiment is given within the framework of the semiclassical
Boltzmann equation.Comment: REVTEX 3.0, 11 pages, 3 Postscript figures appended. The manuscript
can also be obtained from our World Wide Web server:
http://roemer.fys.ku.dk/randmag.ht
Novel A-B type oscillations in a 2-D electron gas in inhomogenous magnetic fields
We present results from a quantum and semiclassical theoretical study of the
and resistivities of a high mobility 2-D electron gas
in the presence of a dilute random distribution of tubes with magnetic flux
and radius , for arbitrary values of and . We
report on novel Aharonov-Bohm type oscillations in and ,
related to degenerate quantum flux tube resonances, that satisfy the selection
rule , with an integer. We discuss possible
experimental conditions where these oscillations may be observed.Comment: 11 pages REVTE
Enabling single-mode behavior over large areas with photonic Dirac cones
Many of graphene's unique electronic properties emerge from its Dirac-like
electronic energy spectrum. Similarly, it is expected that a nanophotonic
system featuring Dirac dispersion will open a path to a number of important
research avenues. To date, however, all proposed realizations of a photonic
analog of graphene lack fully omnidirectional out-of-plane light confinement,
which has prevented creating truly realistic implementations of this class of
systems. Here we report on a novel route to achieve all-dielectric
three-dimensional photonic materials featuring Dirac-like dispersion in a
quasi-two-dimensional system. We further discuss how this finding could enable
a dramatic enhancement of the spontaneous emission coupling efficiency (the
\beta-factor) over large areas, defying the common wisdom that the \beta-factor
degrades rapidly as the size of the system increases. These results might
enable general new classes of large-area ultralow-threshold lasers,
single-photon sources, quantum information processing devices and energy
harvesting systems
Nonlinear behavior of vibrating molecules on suspended graphene waveguides
Suspended graphene waveguides were deposited on micron-scale periodic metal
(plasmonic) structures. Raman scattering of test molecules (B. Megaterium),
deposited on the waveguides' surface, exhibited azimuthal cycles upon rotation:
at these micron scales, spontaneous Raman ought to be independent of phase
matching conditions. In addition, we observed angular-selective quadratic
intensity dependence contrary to the typical linear behavior of spontaneous
Raman. The effects were observed at very modest pump laser intensities (<10
MW/cm2 at the sample surface, oftenly used in Raman experiments). We attributed
these observations to nonlinear coupling between the vibrating molecules and
surface plasmon polariton (SPP) modes at the molecular vibration frequency. It
was assessed that the polariton mode propagates through fairly long distances
(over 100 microns).Comment: 18 pages; 3 figures; a journal pape
Colossal infrared and terahertz magneto-optical activity in a two-dimensional Dirac material
When two-dimensional electron gases (2DEGs) are exposed to magnetic field,
they resonantly absorb electromagnetic radiation via electronic transitions
between Landau levels (LLs). In 2DEGs with a Dirac spectrum, such as graphene,
theory predicts an exceptionally high infrared magneto-absorption, even at zero
doping. However, the measured LL magneto-optical effects in graphene have been
much weaker than expected because of imperfections in the samples available so
far for such experiments. Here we measure magneto-transmission and Faraday
rotation in high-mobility encapsulated monolayer graphene using a custom
designed setup for magneto-infrared microspectroscopy. Our results show a
strongly enhanced magneto-optical activity in the infrared and terahertz ranges
characterized by a maximum allowed (50%) absorption of light, a 100% magnetic
circular dichroism as well as a record high Faraday rotation. Considering that
sizeable effects have been already observed at routinely achievable magnetic
fields, our findings demonstrate a new potential of magnetic tuning in 2D Dirac
materials for long-wavelength optoelectronics and plasmonics.Comment: 14 pages, 4 figure
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