324 research outputs found
Неразрушающий контроль качества конструкции управляемых по скорости двигателей-маховиков для систем ориентации космических аппаратов на этапах экспериментальной отработки
Достоверность результатов определения динамических качеств космического аппарата существенным образом зависит от достоверности определения силовых и моментных возмущений, действующих на космический аппарат со стороны его функциональных систем. Интегральная оценка динамического состояния управляемого по скорости двигателя-маховика выполняется посредством определения его силомоментной и вибрационной характеристик
Multi-wall carbon nanotubes as quantum dots
We have measured the differential conductance dI/dV of individual multi-wall
carbon nanotubes (MWNT) of different lengths. A cross-over from wire-like (long
tubes) to dot-like (short tubes) behavior is observed. dI/dV is dominated by
random conductance fluctuations (UCF) in long MWNT devices (L=2...7 ),
while Coulomb blockade and energy level quantization are observed in short ones
(L=300 nm). The electron levels of short MWNT dots are nearly four-fold
degenerate (including spin) and their evolution in magnetic field (Zeeman
splitting) agrees with a g-factor of 2. In zero magnetic field the sequential
filling of states evolves with spin S according to S=0 -> 1/2 -> 0... In
addition, a Kondo enhancement of the conductance is observed when the number of
electrons on the tube is odd.Comment: 10 pages, 4 figure
Ultrasensitive Displacement Noise Measurement of Carbon Nanotube Mechanical Resonators
Mechanical resonators based on a single carbon nanotube are exceptional
sensors of mass and force. The force sensitivity in these ultra-light
resonators is often limited by the noise in the detection of the vibrations.
Here, we report on an ultra-sensitive scheme based on a RLC resonator and a
low-temperature amplifier to detect nanotube vibrations. We also show a new
fabrication process of electromechanical nanotube resonators to reduce the
separation between the suspended nanotube and the gate electrode down to ~nm. These advances in detection and fabrication allow us to reach
displacement sensitivity. Thermal
vibrations cooled cryogenically at 300~mK are detected with a signal-to-noise
ratio as high as 17~dB. We demonstrate
force sensitivity, which is the best force sensitivity achieved thus far with a
mechanical resonator. Our work is an important step towards imaging individual
nuclear spins and studying the coupling between mechanical vibrations and
electrons in different quantum electron transport regimes.Comment: 9 pages, 5 figure
Coherent Control of Photocurrents in Graphene and Carbon Nanotubes
Coherent one photon () and two photon () electronic
excitations are studied for graphene sheets and for carbon nanotubes using a
long wavelength theory for the low energy electronic states. For graphene
sheets we find that coherent superposition of these excitations produces a
polar asymmetry in the momentum space distribution of the excited carriers with
an angular dependence which depends on the relative polarization and phases of
the incident fields. For semiconducting nanotubes we find a similar effect
which depends on the square of the semiconducting gap, and we calculate its
frequency dependence.
We find that the third order nonlinearity which controls the direction of the
photocurrent is robust for semiconducting t ubes and vanishes in the continuum
theory for conducting tubes. We calculate corrections to these results arising
from higher order crystal field effects on the band structure and briefly
discuss some applications of the theory.Comment: 12 pages in RevTex, 6 epsf figure
Electron fractionalization induced dephasing in Luttinger liquids
Using the appropriate fractionalization mechanism, we correctly derive the
temperature (T) and interaction dependence of the electron lifetime in
Luttinger liquids. For strong enough interactions, we report that
, with being the standard Luttinger exponent; This
reinforces that electrons are {\it not} good quasiparticles. We immediately
emphasize that this is of importance for the detection of electronic
interferences in ballistic 1D rings and carbon nanotubes, inducing
``dephasing'' (strong reduction of Aharonov-Bohm oscillations).Comment: 5 pages, 1 figure (Final version for PRB Brief Report
Quantum Interference Effects in Electronic Transport through Nanotube Contacts
Quantum interference has dramatic effects on electronic transport through
nanotube contacts. In optimal configuration the intertube conductance can
approach that of a perfect nanotube (). The maximum conductance
increases rapidly with the contact length up to 10 nm, beyond which it exhibits
long wavelength oscillations. This is attributed to the resonant cavity-like
interference phenomena in the contact region. For two concentric nanotubes
symmetry breaking reduces the maximum intertube conductance from to
. The phenomena discussed here can serve as a foundation for building
nanotube electronic circuits and high speed nanoscale electromechanical
devices
Paraconductivity in Carbon Nanotubes
We report the calculation of paraconductivity in carbon nanotubes above the
superconducting transition temperature. The complex behavior of
paraconductivity depending upon the tube radius, temperature and magnetic field
strength is analyzed. The results are qualitatively compared with recent
experimental observations in carbon nanotubes of an inherent transition to the
superconducting state and pronounced thermodynamic fluctuations above .
The application of our results to single-wall and multi-wall carbon nanotubes
as well as ropes of nanotubes is discussed.Comment: 7 pages, 1 figur
Electronic Structure of Carbon Nanotube Ropes
We present a tight binding theory to analyze the motion of electrons between
carbon nanotubes bundled into a carbon nanotube rope. The theory is developed
starting from a description of the propagating Bloch waves on ideal tubes, and
the effects of intertube motion are treated perturbatively in this basis.
Expressions for the interwall tunneling amplitudes between states on
neighboring tubes are derived which show the dependence on chiral angles and
intratube crystal momenta. We find that conservation of crystal momentum along
the tube direction suppresses interwall coherence in a carbon nanorope
containing tubes with random chiralities. Numerical calculations are presented
which indicate that electronic states in a rope are localized in the transverse
direction with a coherence length corresponding to a tube diameter.Comment: 15 pages, 10 eps figure
A Mechanical Mass Sensor with Yoctogram Resolution
Nanoelectromechanical systems (NEMS) have generated considerable interest as
inertial mass sensors. NEMS resonators have been used to weigh cells,
biomolecules, and gas molecules, creating many new possibilities for biological
and chemical analysis [1-4]. Recently, NEMS-based mass sensors have been
employed as a new tool in surface science in order to study e.g. the phase
transitions or the diffusion of adsorbed atoms on nanoscale objects [5-7]. A
key point in all these experiments is the ability to resolve small masses. Here
we report on mass sensing experiments with a resolution of 1.7 yg (1 yg =
10^-24 g), which corresponds to the mass of one proton, or one hydrogen atom.
The resonator is made of a ~150 nm long carbon nanotube resonator vibrating at
nearly 2 GHz. The unprecedented level of sensitivity allows us to detect
adsorption events of naphthalene molecules (C10H8) and to measure the binding
energy of a Xe atom on the nanotube surface (131 meV). These ultrasensitive
nanotube resonators offer new opportunities for mass spectrometry,
magnetometry, and adsorption experiments.Comment: submitted version of the manuscrip
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