662 research outputs found
Long wavelength local density of states oscillations near graphene step edges
Using scanning tunneling microscopy and spectroscopy, we have studied the
local density of states (LDOS) of graphene over step edges in boron nitride.
Long wavelength oscillations in the LDOS are observed with maxima parallel to
the step edge. Their wavelength and amplitude are controlled by the energy of
the quasiparticles allowing a direct probe of the graphene dispersion relation.
We also observe a faster decay of the LDOS oscillations away from the step edge
than in conventional metals. This is due to the chiral nature of the Dirac
fermions in graphene.Comment: 5 pages, 4 figures, to appear in Phys. Rev. Let
Electronic excitation spectrum of metallic carbon nanotubes
We have studied the discrete electronic spectrum of closed metallic nanotube
quantum dots. At low temperatures, the stability diagrams show a very regular
four-fold pattern that allows for the determination of the electron addition
and excitation energies. The measured nanotube spectra are in excellent
agreement with theoretical predictions based on the nanotube band structure.
Our results permit the complete identification of the electron quantum states
in nanotube quantum dots.Comment: 4 pages, 3 figure
Real Time Electron Tunneling and Pulse Spectroscopy in Carbon Nanotube Quantum Dots
We investigate a Quantum Dot (QD) in a Carbon Nanotube (CNT) in the regime
where the QD is nearly isolated from the leads. An aluminum single electron
transistor (SET) serves as a charge detector for the QD. We precisely measure
and tune the tunnel rates into the QD in the range between 1 kHz and 1 Hz,
using both pulse spectroscopy and real - time charge detection and measure the
excitation spectrum of the isolated QD.Comment: 12 pages, 5 figure
Dispersal-induced resilience to stochastic environmental fluctuations in populations with Allee effect
Many species are unsustainable at small population densities (Allee Effect).
This implies that for population densities below a threshold, named Allee
threshold, the population decreases instead of growing. In a closed local
population, this makes that environmental fluctuations always leads to
extinction. Here, we show how, in spatially extended habitats, dispersal can
lead to a sustainable population in a region, provided the amplitude of
environmental fluctuations is below an extinction threshold. We have identified
two types of sustainable populations: high-density and low-density populations
(through a mean-field approximation, valid in the limit of large dispersal
length). Our results show that close to global extinction patches where
population density is high, low or extinct coexist (even for homogeneous
habitats). The extinction threshold increases proportionally to the squared
root of the dispersal rate, decreases with the Allee threshold, and it is
maximum for characteristic dispersal distances much larger than the spatial
scale of synchrony of environmental fluctuations. The low-density population
solution can be particularly interesting for future applications, as to
understand non-recovery events after harvesting. This theoretical framework
allows novel approaches to address the impact of other factors, as habitat
fragmentation, on the population resilience to environmental fluctuations.Comment: 24 pages, 3 figures, 1 tabl
Spatial and Ecological Scaling of Stability in Spatial Community Networks
There are many scales at which to quantify stability in spatial and
ecological networks. Local-scale analyses focus on specific nodes of the
spatial network, while regional-scale analyses consider the whole network.
Similarly, species- and community-level analyses either account for single
species or for the whole community. Furthermore, stability itself can be
defined in multiple ways, including resistance (the inverse of the relative
displacement caused by a perturbation), initial resilience (the rate of return
after a perturbation), and invariability (the inverse of the relative amplitude
of the population fluctuations). Here, we analyze the scale-dependence of these
stability properties. More specifically, we ask how spatial scale (local vs
regional) and ecological scale (species vs community) influence these stability
properties. We find that regional initial resilience is the weighted arithmetic
mean of the local initial resiliences. The regional resistance is the harmonic
mean of local resistances, which makes regional resistance particularly
vulnerable to nodes with low stability, unlike regional initial resilience.
Analogous results hold for the relationship between community- and
species-level initial resilience and resistance. Both resistance and initial
resilience are ``scale-free'' properties: regional and community values are
simply the biomass-weighted means of the local and species values,
respectively. Thus, one can easily estimate both stability metrics of whole
networks from partial sampling. In contrast, invariability generally is greater
at the regional and community-level than at the local and species-level,
respectively. Hence, estimating the invariability of spatial or ecological
networks from measurements at the local or species level is more complicated,
requiring an unbiased estimate of the network (i.e. region or community) size
Electronic Transport Spectroscopy of Carbon Nanotubes in a Magnetic Field
We report magnetic field spectroscopy measurements in carbon nanotube quantum
dots exhibiting four-fold shell structure in the energy level spectrum. The
magnetic field induces a large splitting between the two orbital states of each
shell, demonstrating their opposite magnetic moment and determining transitions
in the spin and orbital configuration of the quantum dot ground state. We use
inelastic cotunneling spectroscopy to accurately resolve the spin and orbital
contributions to the magnetic moment. A small coupling is found between
orbitals with opposite magnetic moment leading to anticrossing behavior at zero
field.Comment: 7 pages, 4 figure
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