2,477 research outputs found
Symmetry breaking in a mechanical resonator made from a carbon nanotube
Nanotubes behave as semi-flexible polymers in that they can bend by a
sizeable amount. When integrating a nanotube in a mechanical resonator, the
bending is expected to break the symmetry of the restoring potential. Here we
report on a new detection method that allows us to demonstrate such symmetry
breaking. The method probes the motion of the nanotube resonator at nearly
zero-frequency; this motion is the low-frequency counterpart of the second
overtone of resonantly excited vibrations. We find that symmetry breaking leads
to the spectral broadening of mechanical resonances, and to an apparent quality
factor that drops below 100 at room temperature. The low quality factor at room
temperature is a striking feature of nanotube resonators whose origin has
remained elusive for many years. Our results shed light on the role played by
symmetry breaking in the mechanics of nanotube resonators.Comment: manuscript and supplementary material, 7 figure
Exact averages of central values of triple product L-functions
We obtain exact formulas for central values of triple product L-functions averaged over newforms of weight 2 and prime level. We apply these formulas to non-vanishing problems. This paper uses a period formula for the triple product L-function proved by Gross and Kudla
Nuclear-resonant electron scattering
We investigate nuclear-resonant electron scattering as occurring in the
two-step process of nuclear excitation by electron capture (NEEC) followed by
internal conversion. The nuclear excitation and decay are treated by a
phenomenological collective model in which nuclear states and transition
probabilities are described by experimental parameters. We present capture
rates and resonant strengths for a number of heavy ion collision systems
considering various scenarios for the resonant electron scattering process. The
results show that for certain cases resonant electron scattering can have
significantly larger resonance strengths than NEEC followed by the radiative
decay of the nucleus. We discuss the impact of our findings on the possible
experimental observation of NEEC.Comment: 24 pages, 2 plots, 5 table
Relativistic theory of the double photoionization of helium-like atoms
A fully relativistic calculation of the double photoionization of helium-like
atoms is presented. The approach is based on the partial-wave representation of
the Dirac continuum states and accounts for the retardation in the
electron-electron interaction as well as the higher-order multipoles of the
absorbed photon. The electron-electron interaction is taken into account to the
leading order of perturbation theory. The relativistic effects are shown to
become prominent already for the medium-Z ions, changing the shape and the
asymptotic behaviour of the photon energy dependence of the ratio of the
double-to-single photoionization cross section
Asymptotic channels and gauge transformations of the time-dependent Dirac equation for extremely relativistic heavy-ion collisions
We discuss the two-center, time-dependent Dirac equation describing the
dynamics of an electron during a peripheral, relativistic heavy-ion collision
at extreme energies. We derive a factored form, which is exact in the
high-energy limit, for the asymptotic channel solutions of the Dirac equation,
and elucidate their close connection with gauge transformations which transform
the dynamics into a representation in which the interaction between the
electron and a distant ion is of short range. We describe the implications of
this relationship for solving the time-dependent Dirac equation for extremely
relativistic collisions.Comment: 12 pages, RevTeX, 2 figures, submitted to PR
Nanoladder cantilevers made from diamond and silicon
We present a "nanoladder" geometry that minimizes the mechanical dissipation
of ultrasensitive cantilevers. A nanoladder cantilever consists of a
lithographically patterned scaffold of rails and rungs with feature size
100 nm. Compared to a rectangular beam of the same dimensions, the mass and
spring constant of a nanoladder are each reduced by roughly two orders of
magnitude. We demonstrate a low force noise of zN and zN in a one-Hz bandwidth for devices made from silicon and
diamond, respectively, measured at temperatures between 100--150 mK. As opposed
to bottom-up mechanical resonators like nanowires or nanotubes, nanoladder
cantilevers can be batch-fabricated using standard lithography, which is a
critical factor for applications in scanning force microscopy
Tuning the Josephson current in carbon nanotubes with the Kondo effect
We investigate the Josephson current in a single wall carbon nanotube
connected to superconducting electrodes. We focus on the parameter regime in
which transport is dominated by Kondo physics. A sizeable supercurrent is
observed for odd number of electrons on the nanotube when the Kondo temperature
Tk is sufficiently large compared to the superconducting gap. On the other hand
when, in the center of the Kondo ridge, Tk is slightly smaller than the
superconducting gap, the supercurrent is found to be extremely sensitive to the
gate voltage Vbg. Whereas it is largely suppressed at the center of the ridge,
it shows a sharp increase at a finite value of Vbg. This increase can be
attributed to a doublet-singlet transition of the spin state of the nanotube
island leading to a pi shift in the current phase relation. This transition is
very sensitive to the asymmetry of the contacts and is in good agreement with
theoretical predictions.Comment: 5 pages, 4 figure
Relativistic calculations of the K-K charge transfer and K-vacancy production probabilities in low-energy ion-atom collisions
The previously developed technique for evaluation of charge-transfer and
electron-excitation processes in low-energy heavy-ion collisions [I.I. Tupitsyn
et al., Phys. Rev. A 82, 042701(2010)] is extended to collisions of ions with
neutral atoms. The method employs the active electron approximation, in which
only the active electron participates in the charge transfer and excitation
processes while the passive electrons provide the screening DFT potential. The
time-dependent Dirac wave function of the active electron is represented as a
linear combination of atomic-like Dirac-Fock-Sturm orbitals, localized at the
ions (atoms). The screening DFT potential is calculated using the overlapping
densities of each ions (atoms), derived from the atomic orbitals of the passive
electrons. The atomic orbitals are generated by solving numerically the
one-center Dirac-Fock and Dirac-Fock-Sturm equations by means of a
finite-difference approach with the potential taken as the sum of the exact
reference ion (atom) Dirac-Fock potential and of the Coulomb potential from the
other ion within the monopole approximation. The method developed is used to
calculate the K-K charge transfer and K-vacancy production probabilties for the
Ne -- F collisions at the F projectile
energies 130 keV/u and 230 keV/u. The obtained results are compared with
experimental data and other theoretical calculations. The K-K charge transfer
and K-vacancy production probabilities are also calculated for the Xe --
Xe collision.Comment: 16 pages, 4 figure
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