331 research outputs found
Frequency tuning, nonlinearities and mode coupling in circular graphene resonators
We study circular nanomechanical graphene resonators by means of continuum
elasticity theory, treating them as membranes. We derive dynamic equations for
the flexural mode amplitudes. Due to geometrical nonlinearity these can be
modeled by coupled Duffing equations. By solving the Airy stress problem we
obtain analytic expressions for eigenfrequencies and nonlinear coefficients as
functions of radius, suspension height, initial tension, back-gate voltage and
elastic constants, which we compare with finite element simulations. Using
perturbation theory, we show that it is necessary to include the effects of the
non-uniform stress distribution for finite deflections. This correctly
reproduces the spectrum and frequency tuning of the resonator, including
frequency crossings.Comment: 21 pages, 7 figures, 3 table
Multi-flavor bosonic Hubbard models in the first excited Bloch band of an optical lattice
We propose that by exciting ultra cold atoms from the zeroth to the first
Bloch band in an optical lattice, novel multi-flavor bosonic Hubbard
Hamiltonians can be realized in a new way. In these systems, each flavor hops
in a separate direction and on-site exchange terms allow pairwise conversion
between different flavors. Using band structure calculations, we determine the
parameters entering these Hamiltonians and derive the mean field ground state
phase diagram for two effective Hamiltonians (2D, two-flavors and 3D, three
flavors). Further, we estimate the stability of atoms in the first band using
second order perturbation theory and find lifetimes that can be considerable
(10-100 times) longer than the relevant time scale associated with inter-site
hopping dynamics, suggesting that quasi-equilibrium can be achieved in these
meta-stable states.Comment: 26 pages, 18 figure
Mechanical Cooper pair transportation as a source of long distance superconducting phase coherence
Transportation of Cooper-pairs by a movable single Cooper-pair-box placed
between two remote superconductors is shown to establish coherent coupling
between them. This coupling is due to entanglement of the movable box with the
leads and is manifested in the supression of quantum fluctuations of the
relative phase of the order parameters of the leads. It can be probed by
attaching a high resistance Josephson junction between the leads and measuring
the current through this junction. The current is suppressed with increasing
temperature.Comment: 4 pages, 4 figures, RevTeX; Updated version, typos correcte
Impact of van der Waals forces on the classical shuttle instability
The effects of including the van der Waals interaction in the modelling of
the single electron shuttle have been investigated numerically. It is
demonstrated that the relative strength of the vdW-forces and the elastic
restoring forces determine the characteristics of the shuttle instability. In
the case of weak elastic forces and low voltages the grain is trapped close to
one lead, and this trapping can be overcome by Coulomb forces by applying a
bias voltage larger than a threshold voltage . This allows for
grain motion leading to an increase in current by several orders of magnitude
above the transition voltage . Associated with the process is also
hysteresis in the I-V characteristics.Comment: minor revisions, updated references, Article published in Phys. Rev.
B 69, 035309 (2004
Diffusion-induced bistability of driven nanomechanical resonators
We study nanomechanical resonators with frequency fluctuations due to
diffusion of absorbed particles. The diffusion depends on the vibration
amplitude through inertial effect. We find that, if the diffusion coefficient
is sufficiently large, the resonator response to periodic driving displays
bistability. The lifetime of the coexisting vibrational states scales
exponentially with the diffusion coefficient. It also displays a characteristic
scaling dependence on the distance to bifurcation points.Comment: 4 pages, 3 figure
Superfluid-insulator transitions of two-species Bosons in an optical lattice
We consider a realization of the two-species bosonic Hubbard model with
variable interspecies interaction and hopping strength. We analyze the
superfluid-insulator (SI) transition for the relevant parameter regimes and
compute the ground state phase diagram for odd filling at commensurate
densities. We find that in contrast to the even commensurate filling case, the
superfluid-insulator transition occurs with (a) simultaneous onset of
superfluidity of both species or (b) coexistence of Mott insulating state of
one species and superfluidity of the other or, in the case of unit filling, (c)
complete depopulation of one species. The superfluid-insulator transition can
be first order in a large region of the phase diagram. We develop a variational
mean-field method which takes into account the effect of second order quantum
fluctuations on the superfluid-insulator transition and corroborate the
mean-field phase diagram using a quantum Monte Carlo study.Comment: 12 pages, 11 figure
Mechanical cat states in graphene resonators
We study the quantum dynamics of a symmetric nanomechanical graphene
resonator with degenerate flexural modes. Applying voltage pulses to two back
gates, flexural vibrations of the membrane can be selectively actuated and
manipulated. For graphene, nonlinear response becomes important already for
amplitudes comparable to the magnitude of zero point fluctuations. We show,
using analytical and numerical methods, that this allows for creation of
cat-like superpositions of coherent states as well as superpositions of
coherent cat-like non-product states.Comment: 4 pages, 3 figure
Parametric resonances in electrostatically interacting carbon nanotube arrays
We study, numerically and analytically, a model of a one-dimensional array of
carbon nanotube resonators in a two-terminal configuration. The system is
brought into resonance upon application of an AC-signal superimposed on a
DC-bias voltage. When the tubes in the array are close to each other,
electrostatic interactions between tubes become important for the array
dynamics. We show that both transverse and longitudinal parametric resonances
can be excited in addition to primary resonances. The intertube electrostatic
interactions couple modes in orthogonal directions and affect the mode
stability.Comment: 11 pages, 12 figures, RevTeX
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