1,222 research outputs found
Interfaces within graphene nanoribbons
We study the conductance through two types of graphene nanostructures: nanoribbon junctions in which the width changes from wide to narrow, and curved nanoribbons. In the wide-narrow structures, substantial reflection occurs from the wide-narrow interface, in contrast to the behavior of the much studied electron gas waveguides. In the curved nanoribbons, the conductance is very sensitive to details such as whether regions of a semiconducting armchair nanoribbon are included in the curved structure -- such regions strongly suppress the conductance. Surprisingly, this suppression is not due to the band gap of the semiconducting nanoribbon, but is linked to the valley degree of freedom. Though we study these effects in the simplest contexts, they can be expected to occur for more complicated structures, and we show results for rings as well. We conclude that experience from electron gas waveguides does not carry over to graphene nanostructures. The interior interfaces causing extra scattering result from the extra effective degrees of freedom of the graphene structure, namely the valley and sublattice pseudospins
Interfaces Within Graphene Nanoribbons
We study the conductance through two types of graphene nanostructures:
nanoribbon junctions in which the width changes from wide to narrow, and curved
nanoribbons. In the wide-narrow structures, substantial reflection occurs from
the wide-narrow interface, in contrast to the behavior of the much studied
electron gas waveguides. In the curved nanoribbons, the conductance is very
sensitive to details such as whether regions of a semiconducting armchair
nanoribbon are included in the curved structure -- such regions strongly
suppress the conductance. Surprisingly, this suppression is not due to the band
gap of the semiconducting nanoribbon, but is linked to the valley degree of
freedom. Though we study these effects in the simplest contexts, they can be
expected to occur for more complicated structures, and we show results for
rings as well. We conclude that experience from electron gas waveguides does
not carry over to graphene nanostructures. The interior interfaces causing
extra scattering result from the extra effective degrees of freedom of the
graphene structure, namely the valley and sublattice pseudospins.Comment: 19 pages, published version, several references added, small changes
to conclusion
Edge effects in graphene nanostructures: I. From multiple reflection expansion to density of states
We study the influence of different edge types on the electronic density of
states of graphene nanostructures. To this end we develop an exact expansion
for the single particle Green's function of ballistic graphene structures in
terms of multiple reflections from the system boundary, that allows for a
natural treatment of edge effects. We first apply this formalism to calculate
the average density of states of graphene billiards. While the leading term in
the corresponding Weyl expansion is proportional to the billiard area, we find
that the contribution that usually scales with the total length of the system
boundary differs significantly from what one finds in semiconductor-based,
Schr\"odinger type billiards: The latter term vanishes for armchair and
infinite mass edges and is proportional to the zigzag edge length, highlighting
the prominent role of zigzag edges in graphene. We then compute analytical
expressions for the density of states oscillations and energy levels within a
trajectory based semiclassical approach. We derive a Dirac version of
Gutzwiller's trace formula for classically chaotic graphene billiards and
further obtain semiclassical trace formulae for the density of states
oscillations in regular graphene cavities. We find that edge dependent
interference of pseudospins in graphene crucially affects the quantum spectrum.Comment: to be published in Phys. Rev.
Impurity and edge roughness scattering in armchair graphene nanoribbons: Boltzmann approach
The conductivity of armchair graphene nanoribbons in the presence of
short-range impurities and edge roughness is studied theoretically using the
Boltzmann transport equation for quasi-one-dimensional systems. As the number
of occupied subbands increases, the conductivity due to short-range impurities
converges towards the two-dimensional case. Calculations of the
magnetoconductivity confirm the edge-roughness-induced dips at cyclotron radii
close to the ribbon width suggested by the recent quantum simulations
Drifting inwards in protoplanetary discs I Sticking of chondritic dust at increasing temperatures
Sticking properties rule the early phases of pebble growth in protoplanetary
discs in which grains regularly travel from cold, water-rich regions to the
warm inner part. This drift affects composition, grain size, morphology, and
water content as grains experience ever higher temperatures. In this study we
tempered chondritic dust under vacuum up to 1400 K. Afterwards, we measured the
splitting tensile strength of millimetre-sized dust aggregates. The deduced
effective surface energy starts out as . This value
is dominated by abundant iron-oxides as measured by M\"ossbauer spectroscopy.
Up to 1250 K, continuously decreases by up to a factor five.
Olivines dominate at higher temperature. Beyond 1300 K dust grains
significantly grow in size. The no longer decreases but the large
grain size restricts the capability of growing aggregates. Beyond 1400 K
aggregation is no longer possible. Overall, under the conditions probed, the
stability of dust pebbles would decrease towards the star. In view of a minimum
aggregate size required to trigger drag instabilities it becomes increasingly
harder to seed planetesimal formation closer to a star
Physical Conditoins in Orion's Veil II: A Multi-Component Study of the Line of Sight Toward the Trapezium
Orion's Veil is an absorbing screen that lies along the line of sight to the
Orion H II region. It consists of two or more layers of gas that must lie
within a few parsecs of the Trapezium cluster. Our previous work considered the
Veil as a whole and found that the magnetic field dominates the energetics of
the gas in at least one component. Here we use high-resolution STIS UV spectra
that resolve the two velocity components in absorption and determine the
conditions in each. We derive a volume hydrogen density, 21 cm spin
temperature, turbulent velocity, and kinetic temperature, for each. We combine
these estimates with magnetic field measurements to find that magnetic energy
significantly dominates turbulent and thermal energies in one component, while
the other component is close to equipartition between turbulent and magnetic
energies. We observe molecular hydrogen absorption for highly excited v, J
levels that are photoexcited by the stellar continuum, and detect blueshifted S
III and P III. These ions must arise from ionized gas between the mostly
neutral portions of the Veil and the Trapezium and shields the Veil from
ionizing radiation. We find that this layer of ionized gas is also responsible
for He I absorption in the Veil, which resolves a 40-year-old debate on the
origin of He I absorption towards the Trapezium. Finally, we determine that the
ionized and mostly atomic layers of the Veil will collide in less than 85,000
years.Comment: 43 pages, 15 figures, to be published in Ap
Dual‐Targeted Nanoreactors and Prodrugs: Hydrogen Peroxide Triggers Oxidative Damage and Prodrug Activation for Synergistic Elimination of Cancer Cells
Synergistic strategies by combining nanoreactors and prodrugs hold tremendous potential in anticancer treatment. However, precise death of target cancer cells remains a significant challenge due to the absence of an elaborate cancer targeting strategy. Here, a dual-targeting approach that combines the action of H2O2-producing folate receptor-targeted nanoreactors with a cyclooxygenase-2 (COX-2) targeted prodrug is reported. A folate-modified silica nanoreactor encapsulating glucose oxidase (GOX) is prepared to generate H2O2, which induces oxidative stress and allows the activation of the prodrug by targeted intracellular delivery. A novel prodrug bearing both COX-2 targeting Celecoxib and SN-38 anticancer agent with an H2O2-cleavable thioketal linker to activate the drug is presented. By dual-targeting, the generated H2O2 from GOX triggers the cleavage of a thioketal linker in the prodrug to produce the active form of the SN-38 anticancer drug in cancer cells inducing synergistic cell death. This dual-targeting strategy with a synergistic potency can aid in developing selective and effective anticancer therapeutics
Graphene Rings in Magnetic Fields: Aharonov-Bohm Effect and Valley Splitting
We study the conductance of mesoscopic graphene rings in the presence of a
perpendicular magnetic field by means of numerical calculations based on a
tight-binding model. First, we consider the magnetoconductance of such rings
and observe the Aharonov-Bohm effect. We investigate different regimes of the
magnetic flux up to the quantum Hall regime, where the Aharonov-Bohm
oscillations are suppressed. Results for both clean (ballistic) and disordered
(diffusive) rings are presented. Second, we study rings with smooth mass
boundary that are weakly coupled to leads. We show that the valley degeneracy
of the eigenstates in closed graphene rings can be lifted by a small magnetic
flux, and that this lifting can be observed in the transport properties of the
system.Comment: 12 pages, 9 figure
Semi-Hard Scattering Unraveled from Collective Dynamics by Two-Pion Azimuthal Correlations in 158 A GeV/c Pb + Au Collisions
Elliptic flow and two-particle azimuthal correlations of charged hadrons and
high- pions ( 1 GeV/) have been measured close to mid-rapidity in
158A GeV/ Pb+Au collisions by the CERES experiment. Elliptic flow ()
rises linearly with to a value of about 10% at 2 GeV/. Beyond
1.5 GeV/, the slope decreases considerably, possibly indicating
a saturation of at high . Two-pion azimuthal anisotropies for
1.2 GeV/ exceed the elliptic flow values by about 60% in mid-central
collisions. These non-flow contributions are attributed to near-side and
back-to-back jet-like correlations, the latter exhibiting centrality dependent
broadening.Comment: Submitted to Phys. Rev. Letters, 4 pages, 5 figure
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