587 research outputs found

### Evidence of 1D behaviour of He$^4$ confined within carbon-nanotube bundles

We present the first low-temperature thermodynamic investigation of the
controlled physisorption of He$^{4}$ gas in carbon single-wall nanotube (SWNT)
samples. The vibrational specific heat measured between 100 mK and 6 K
demonstrates an extreme sensitivity to outgassing conditions. For bundles with
a few number of NTs the extra contribution to the specific heat, C$_{ads}$,
originating from adsorbed He$^{4}$ at very low density displays 1D behavior,
typical for He atoms localized within linear channels as grooves and
interstitials, for the first time evidenced. For larger bundles, C$_{ads}$
recovers the 2D behaviour akin to the case of He$^{4}$ films on planar
substrates (grafoil).Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

### Ferromagnetic semiconductor single wall carbon nanotube

Possibility of a ferromagnetic semiconductor single wall carbon nanotube
(SWCNT), where ferromagnetism is due to coupling between doped magnetic
impurity on a zigzag SWCNT and electrons spin, is investigate. We found, in the
weak impurity-spin couplings, at low impurity concentrations the spin up
electrons density of states remain semiconductor while the spin down electrons
density of states shows a metallic behavior. By increasing impurity
concentrations the semiconducting gap of spin up electrons in the density of
states is closed, hence a semiconductor to metallic phase transition is take
place. In contrast, for the case of strong coupling, spin up electrons density
of states remain semiconductor and spin down electron has metallic behavior.
Also by increasing impurity spin magnitude, the semiconducting gap of spin up
electrons is increased.Comment: 10 pages and 9 figure

### Interplay between structure and magnetism in $Mo_{12} S_9 I_9$ nanowires

We investigate the equilibrium geometry and electronic structure of
Mo$_{12}$S$_{9}$I$_{9}$ nanowires using ab initio Density Functional
calculations. The skeleton of these unusually stable nanowires consists of
rigid, functionalized Mo octahedra, connected by flexible, bi-stable sulphur
bridges. This structural flexibility translates into a capability to stretch up
to approximate 20% at almost no energy cost. The nanowires change from
conductors to narrow-gap magnetic semiconductors in one of their structural
isomers.Comment: 4 pages with PRL standards and 3 figure

### CO2-Kompensation von Flugreisen, Klimarettung oder Ablasshandel?

Die Verringerung der CO2-Emissionen der Luftfahrt ist ein wichtiger Bau-stein auf dem Weg zum Wunschzustand eines klimaneutralen Verkehrs. In der Ă–ffentlichkeit werden hierzu zahlreiche MĂ¶glichkeiten diskutiert, darunter auch die CO2-Kompensation von Flugreisen. Handelt es sich dabei
um ein effizientes Klimaschutzinstrument? Wie seriĂ¶s sind die Angebote von Kompensationsanbietern aus Verbraucherschutzsicht? Der Autor analysiert das System und weist Wege zu einer Reform des Kompensationswesens

### Four-point resistance of individual single-wall carbon nanotubes

We have studied the resistance of single-wall carbon nanotubes measured in a
four-point configuration with noninvasive voltage electrodes. The voltage drop
is detected using multiwalled carbon nanotubes while the current is injected
through nanofabricated Au electrodes. The resistance at room temperature is
shown to be linear with the length as expected for a classical resistor. This
changes at cryogenic temperature; the four-point resistance then depends on the
resistance at the Au-tube interfaces and can even become negative due to
quantum-interference effects.Comment: 4 pages, 4 figure

### Coherent Bremsstrahlung in Imperfect Periodic Atomic Structures

Coherent bremsstrahlung of high energy electrons moving in a
three-dimensional imperfect periodic lattice consisting of a complicated system
of atoms is considered. On the basis of the normalized probability density
function of the distribution of atomic centers in the fundamental cell the
relations describing coherent and incoherent contributions into cross sections
are obtained. In particular, the cross section of coherent bremsstrahlung in
complex polyatomic single crystals is found.
The peculiarities of formation and possibilities of utilization of coherent
processes are discussed.Comment: 23 pages, 8 figures, to be published in Phys. Rev.

### Low-Prandtl-number B\'enard-Marangoni convection in a vertical magnetic field

The effect of a homogeneous magnetic field on surface-tension-driven
B\'{e}nard convection is studied by means of direct numerical simulations. The
flow is computed in a rectangular domain with periodic horizontal boundary
conditions and the free-slip condition on the bottom wall using a
pseudospectral Fourier-Chebyshev discretization. Deformations of the free
surface are neglected. Two- and three-dimensional flows are computed for either
vanishing or small Prandtl number, which are typical of liquid metals. The main
focus of the paper is on a qualitative comparison of the flow states with the
non-magnetic case, and on the effects associated with the possible
near-cancellation of the nonlinear and pressure terms in the momentum equations
for two-dimensional rolls. In the three-dimensional case, the transition from a
stationary hexagonal pattern at the onset of convection to three-dimensional
time-dependent convection is explored by a series of simulations at zero
Prandtl number.Comment: 26 pages, 9 figure

### Structure of the Wake of a Magnetic Obstacle

We use a combination of numerical simulations and experiments to elucidate
the structure of the flow of an electrically conducting fluid past a localized
magnetic field, called magnetic obstacle. We demonstrate that the stationary
flow pattern is considerably more complex than in the wake behind an ordinary
body. The steady flow is shown to undergo two bifurcations (rather than one)
and to involve up to six (rather than just two) vortices. We find that the
first bifurcation leads to the formation of a pair of vortices within the
region of magnetic field that we call inner magnetic vortices, whereas a second
bifurcation gives rise to a pair of attached vortices that are linked to the
inner vortices by connecting vortices.Comment: 4 pages, 5 figures, corrected two typos, accepted for PR

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