483 research outputs found
Quantum Particles Constrained on Cylindrical Surfaces with Non-constant Diameter
We present a theoretical formulation of the one-electron problem constrained
on the surface of a cylindrical tubule with varying diameter. Because of the
cylindrical symmetry, we may reduce the problem to a one-dimensional equation
for each angular momentum quantum number along the cylindrical axis. The
geometrical properties of the surface determine the electronic structures
through the geometry dependent term in the equation. Magnetic fields parallel
to the axis can readily be incorporated. Our formulation is applied to simple
examples such as the catenoid and the sinusoidal tubules. The existence of
bound states as well as the band structures, which are induced geometrically,
for these surfaces are shown. To show that the electronic structures can be
altered significantly by applying a magnetic field, Aharonov-Bohm effects in
these examples are demonstrated.Comment: 7 pages, 7 figures, submitted to J. Phys. Soc. Jp
Terahertz imaging and spectroscopy of large-area single-layer graphene
We demonstrate terahertz (THz) imaging and spectroscopy of a 15x15-mm^2
single-layer graphene film on Si using broadband THz pulses. The THz images
clearly map out the THz carrier dynamics of the graphene-on-Si sample, allowing
us to measure sheet conductivity with sub-mm resolution without fabricating
electrodes. The THz carrier dynamics are dominated by intraband transitions and
the THz-induced electron motion is characterized by a flat spectral response. A
theoretical analysis based on the Fresnel coefficients for a metallic thin film
shows that the local sheet conductivity varies across the sample from {\sigma}s
= 1.7x10^-3 to 2.4x10^-3 {\Omega}^-1 (sheet resistance, {\rho}s = 420 - 590
{\Omega}/sq).Comment: 6 pages, 5 figure
Reports to the President, 1871 - 1916
For more information about this item, visit https://archivesspace.mit.edu/repositories/2/archival_objects/30888
Orbital Kondo effect in carbon nanotubes
Progress in the fabrication of nanometer-scale electronic devices is opening
new opportunities to uncover the deepest aspects of the Kondo effect, one of
the paradigmatic phenomena in the physics of strongly correlated electrons.
Artificial single-impurity Kondo systems have been realized in various
nanostructures, including semiconductor quantum dots, carbon nanotubes and
individual molecules. The Kondo effect is usually regarded as a spin-related
phenomenon, namely the coherent exchange of the spin between a localized state
and a Fermi sea of electrons. In principle, however, the role of the spin could
be replaced by other degrees of freedom, such as an orbital quantum number.
Here we demonstrate that the unique electronic structure of carbon nanotubes
enables the observation of a purely orbital Kondo effect. We use a magnetic
field to tune spin-polarized states into orbital degeneracy and conclude that
the orbital quantum number is conserved during tunneling. When orbital and spin
degeneracies are simultaneously present, we observe a strongly enhanced Kondo
effect, with a multiple splitting of the Kondo resonance at finite field and
predicted to obey a so-called SU(4) symmetry.Comment: 26 pages, including 4+2 figure
Spontaneous DC Current Generation in a Resistively Shunted Semiconductor Superlattice Driven by a TeraHertz Field
We study a resistively shunted semiconductor superlattice subject to a
high-frequency electric field. Using a balance equation approach that
incorporates the influence of the electric circuit, we determine numerically a
range of amplitude and frequency of the ac field for which a dc bias and
current are generated spontaneously and show that this region is likely
accessible to current experiments. Our simulations reveal that the Bloch
frequency corresponding to the spontaneous dc bias is approximately an integer
multiple of the ac field frequency.Comment: 8 pages, Revtex, 3 Postscript figure
How the other half lives: CRISPR-Cas's influence on bacteriophages
CRISPR-Cas is a genetic adaptive immune system unique to prokaryotic cells
used to combat phage and plasmid threats. The host cell adapts by incorporating
DNA sequences from invading phages or plasmids into its CRISPR locus as
spacers. These spacers are expressed as mobile surveillance RNAs that direct
CRISPR-associated (Cas) proteins to protect against subsequent attack by the
same phages or plasmids. The threat from mobile genetic elements inevitably
shapes the CRISPR loci of archaea and bacteria, and simultaneously the
CRISPR-Cas immune system drives evolution of these invaders. Here we highlight
our recent work, as well as that of others, that seeks to understand phage
mechanisms of CRISPR-Cas evasion and conditions for population coexistence of
phages with CRISPR-protected prokaryotes.Comment: 24 pages, 8 figure
Electron Transport in Very Clean, As-Grown Suspended Carbon Nanotubes
Single walled carbon nanotubes (SWNT) have displayed a wealth of quantum
transport phenomena thus far. Defect free, unperturbed SWNTs with wellbehaved
or tunable metal contacts are important to probing the intrinsic electrical
properties of nanotubes. Meeting these conditions experimentally is nontrivial
due to numerous disorder and randomizing factors. Here we show that ~ 1 um long
fully suspended SWNTs grown-in-place between metal contacts afford SWNT devices
exhibiting well-defined characteristics over much wider energy ranges than
nanotubes pinned on substrates. Various low temperature transport regimes in
true-metallic, small and large bandgap semiconducting nanotubes are observed
including quantum states shell-filling, -splitting and -crossing in magnetic
fields for medium conductance devices. The clean transport data reveals a
correlation between the contact junction resistance and the various transport
regimes in SWNT devices. Further, we show that electrical transport data can be
used to probe the band structures of nanotubes including nonlinear band
dispersion.Comment: Nature Materials, in pres
Population Distribution, Settlement Patterns and Accessibility across Africa in 2010
The spatial distribution of populations and settlements across a country and their interconnectivity and accessibility from urban areas are important for delivering healthcare, distributing resources and economic development. However, existing spatially explicit population data across Africa are generally based on outdated, low resolution input demographic data, and provide insufficient detail to quantify rural settlement patterns and, thus, accurately measure population concentration and accessibility. Here we outline approaches to developing a new high resolution population distribution dataset for Africa and analyse rural accessibility to population centers. Contemporary population count data were combined with detailed satellite-derived settlement extents to map population distributions across Africa at a finer spatial resolution than ever before. Substantial heterogeneity in settlement patterns, population concentration and spatial accessibility to major population centres is exhibited across the continent. In Africa, 90% of the population is concentrated in less than 21% of the land surface and the average per-person travel time to settlements of more than 50,000 inhabitants is around 3.5 hours, with Central and East Africa displaying the longest average travel times. The analyses highlight large inequities in access, the isolation of many rural populations and the challenges that exist between countries and regions in providing access to services. The datasets presented are freely available as part of the AfriPop project, providing an evidence base for guiding strategic decisions
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