12,576 research outputs found
Scanning Tunneling Spectroscopy of Suspended Single-Wall Carbon Nanotubes
We have performed low-temperature STM measurements on single-wall carbon
nanotubes that are freely suspended over a trench. The nanotubes were grown by
CVD on a Pt substrate with predefined trenches etched into it. Atomic
resolution was obtained on the freestanding portions of the nanotubes.
Spatially resolved spectroscopy on the suspended portion of both metallic and
semiconducting nanotubes was also achieved, showing a Coulomb-staircase
behavior superimposed on the local density of states. The spacing of the
Coulomb blockade peaks changed with tip position reflecting a changing tip-tube
capacitance
A Characterization of Infinite LSP Words
G. Fici proved that a finite word has a minimal suffix automaton if and only
if all its left special factors occur as prefixes. He called LSP all finite and
infinite words having this latter property. We characterize here infinite LSP
words in terms of -adicity. More precisely we provide a finite set of
morphisms and an automaton such that an infinite word is LSP if
and only if it is -adic and all its directive words are recognizable by
Search for magnetic fields in particle-accelerating colliding-wind binaries
Some colliding-wind massive binaries, called particle-accelerating
colliding-wind binaries (PACWB), exhibit synchrotron radio emission, which is
assumed to be generated by a stellar magnetic field. However, no measurement of
magnetic fields in these stars has ever been performed. We aim at quantifying
the possible stellar magnetic fields present in PACWB to provide constraints
for models. We gathered 21 high-resolution spectropolarimetric observations of
9 PACWB available in the ESPaDOnS, Narval and HarpsPol archives. We analysed
these observations with the Least Squares Deconvolution method. We separated
the binary spectral components when possible. No magnetic signature is detected
in any of the 9 PACWB stars and all longitudinal field measurements are
compatible with 0 G. We derived the upper field strength of a possible field
that could have remained hidden in the noise of the data. While the data are
not very constraining for some stars, for several stars we could derive an
upper limit of the polar field strength of the order of 200 G. We can therefore
exclude the presence of strong or moderate stellar magnetic fields in PACWB,
typical of the ones present in magnetic massive stars. Weak magnetic fields
could however be present in these objects. These observational results provide
the first quantitative constraints for future models of PACWB.Comment: Accepted in A&
Recommended from our members
Natural and anthropogenic forest fires recorded in the Holocene pollen record from a Jinchuan peat bog, northeastern China
Pollen and charcoal particles from a Jinchuan peat (northeastern China) were examined to investigate the fire origin and interaction between
climate, vegetation, fire and human activity during the Holocene. Pollen results show that: (i) a broadleaved deciduous forest was dominant during
the early Holocene; (ii) from ~5500 cal. yr B.P. there was a gradual increase in coniferous trees (mainly Pinus), and a decrease in broadleaved
deciduous trees (e.g. Quercus, Juglans, and UlmusâZelkova); (iii) after ~4200 cal. yr B.P., the deciduous forest was replaced by a mixed forest of
coniferous and deciduous trees; (iv) coniferous trees including Pinus, Abies and Picea further increased after ~2000 cal. yr B.P., reflecting a
cooler and drier climate after ~5500â4200 cal. yr B.P. Two layers of abundant microfossil charcoal particles (250â10 ÎŒm) and the coexistence of
macrofossil particles (N2 mm) suggest two local fires: fire event 1 (5120±66 cal. yr B.P.) and fire event 2 (1288±8 cal. yr B.P., AD 662±8).
Charcoal layer 1, with a large amount of Monolete psilate spores, is superimposed on the long-term trend of vegetation changes, indicating a
natural origin for fire event 1 that was probably facilitated by drying environmental conditions since the mid-Holocene. Cerealia-type pollen and a
low percentage of Monolete psilate spores were observed in charcoal layer 2, indicating that fire event 2 was caused by clearing. We suggest that
fire event 2 may be related to the spread of the Han farming culture accompanied by the territorial expansion of the Tang Dynasty to the studied
area in AD 668
Magnetohydrostatic solar prominences in near-potential coronal magnetic fields
We present numerical magnetohydrostatic solutions describing the
gravitationally stratified, bulk equilibrium of cool, dense prominence plasma
embedded in a near-potential coronal field. These solutions are calculated
using the FINESSE magnetohydrodynamics equilibrium solver and describe the
morphologies of magnetic field distributions in and around prominences and the
cool prominence plasma that these fields support. The equilibrium condition for
this class of problem is usually different in distinct subdomains, separated by
free boundaries, across which solutions are matched by suitable continuity or
jump conditions describing force balance. We employ our precise finite element
elliptic solver to calculate solutions not accessible by previous analytical
techniques with temperature or entropy prescribed as free functions of the
magnetic flux function, including a range of values of the polytropic index,
temperature variations mainly across magnetic field lines and photospheric
field profiles sheared close to the polarity inversion line. Out of the many
examples computed here, perhaps the most noteworthy is one which reproduces
precisely the three-part structure often encountered in observations: a cool
dense prominence within a cavity/flux rope embedded in a hot corona. The
stability properties of these new equilibria, which may be relevant to solar
eruptions, can be determined in the form of a full resistive MHD spectrum using
a companion hyperbolic stability solver.Comment: To appear in ApJ August 200
Phonon runaway in nanotube quantum dots
We explore electronic transport in a nanotube quantum dot strongly coupled
with vibrations and weakly with leads and the thermal environment. We show that
the recent observation of anomalous conductance signatures in single-walled
carbon nanotube (SWCNT) quantum dots can be understood quantitatively in terms
of current driven `hot phonons' that are strongly correlated with electrons.
Using rate equations in the many-body configuration space for the joint
electron-phonon distribution, we argue that the variations are indicative of
strong electron-phonon coupling requiring an analysis beyond the traditional
uncorrelated phonon-assisted transport (Tien-Gordon) approach.Comment: 8 pages, 6 figure
Influence of positional correlations on the propagation of waves in a complex medium with polydisperse resonant scatterers
We present experimental results on a model system for studying wave
propagation in a complex medium exhibiting low frequency resonances. These
experiments enable us to investigate a fundamental question that is relevant
for many materials, such as metamaterials, where low-frequency scattering
resonances strongly influence the effective medium properties. This question
concerns the effect of correlations in the positions of the scatterers on the
coupling between their resonances, and hence on wave transport through the
medium. To examine this question experimentally, we measure the effective
medium wave number of acoustic waves in a sample made of bubbles embedded in an
elastic matrix over a frequency range that includes the resonance frequency of
the bubbles. The effective medium is highly dispersive, showing peaks in the
attenuation and the phase velocity as functions of the frequency, which cannot
be accurately described using the Independent Scattering Approximation (ISA).
This discrepancy may be explained by the effects of the positional correlations
of the scatterers, which we show to be dependent on the size of the scatterers.
We propose a self-consistent approach for taking this "polydisperse
correlation" into account and show that our model better describes the
experimental results than the ISA
The Pion Light-Cone Wave Function Phi_pi on the lattice: a partonic signal?
We determine the conditions required to study the pion light-cone wave
function Phi_pi with a new method: a direct display of the partons constituting
the pion. We present the preliminary results of a lattice computation of Phi_pi
following this direction. An auxiliary scalar-quark is introduced. The
spectroscopy of its bound states is studied. We observe some indications of a
partonic behavior of the system of this scalar-quark and the anti-quark.Comment: 3 pages, 4 figures, Lattice2001(matrixelement
Light hadron spectroscopy on the lattice with the non-perturbatively improved Wilson action
We present results for the light meson masses and decay constants as obtained
from calculations with the non-perturbatively improved (`Alpha') action and
operators on a 24^3 \times 64 lattice at beta = 6.2, in the quenched
approximation. The analysis was performed in a way consistent with O(a)
improvement. We obtained: reasonable agreement with experiment for the
hyperfine splitting; f_K=156(17) MeV, f_pi =139(22) MeV, f_K/f_pi = 1.13(4) ;
f_{K*}=219(7) MeV, f_rho =199(15) MeV, f_phi =235(4) MeV; f_{K*}^{T}(2 GeV) =
178(10) MeV, f_rho^{T}(2 GeV) =165(11) MeV, where f_V^{T} is the coupling of
the tensor current to the vector mesons; the chiral condensate
^\bar{MS} (2 GeV)= - (253 +/- 25 MeV)^3. Our results are compared to
those obtained with the unimproved Wilson action. We also verified that the
free-boson lattice dispersion relation describes our results very accurately
for a large range of momenta.Comment: 29 pages (LaTeX), 14 Postscript figure
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