33,357 research outputs found
Parametric Nanomechanical Amplification at Very High Frequency
Parametric resonance and amplification are important in both fundamental physics and technological applications. Here we report very high frequency (VHF) parametric resonators and mechanical-domain amplifiers based on nanoelectromechanical systems (NEMS). Compound mechanical nanostructures patterned by multilayer, top-down nanofabrication are read out by a novel scheme that parametrically modulates longitudinal stress in doubly clamped beam NEMS resonators. Parametric pumping and signal amplification are demonstrated for VHF resonators up to ~ 130 MHz and provide useful enhancement of both resonance signal amplitude and quality factor. We find that Joule heating and reduced thermal conductance in these nanostructures ultimately impose an upper limit to device performance. We develop a theoretical model to account for both the parametric response and nonequilibrium thermal transport in these composite nanostructures. The results closely conform to our experimental observations, elucidate the frequency and threshold-voltage scaling in parametric VHF NEMS resonators and sensors, and establish the ultimate sensitivity limits of this approach
Systemic risk in dynamical networks with stochastic failure criterion
Complex non-linear interactions between banks and assets we model by two
time-dependent Erd\H{o}s Renyi network models where each node, representing
bank, can invest either to a single asset (model I) or multiple assets (model
II). We use dynamical network approach to evaluate the collective financial
failure---systemic risk---quantified by the fraction of active nodes. The
systemic risk can be calculated over any future time period, divided on
sub-periods, where within each sub-period banks may contiguously fail due to
links to either (i) assets or (ii) other banks, controlled by two parameters,
probability of internal failure and threshold ("solvency" parameter).
The systemic risk non-linearly increases with and decreases with average
network degree faster when all assets are equally distributed across banks than
if assets are randomly distributed. The more inactive banks each bank can
sustain (smaller ), the smaller the systemic risk---for some values
in I we report a discontinuity in systemic risk. When contiguous spreading
becomes stochastic (ii) controlled by probability ---a condition for the
bank to be solvent (active) is stochastic---the systemic risk decreases with
decreasing . We analyse asset allocation for the U.S. banks.Comment: 7 pages, 7 figure
Bias-controllable intrinsic spin polarization in a quantum dot
We propose a novel scheme to efficiently polarize and manipulate the electron
spin in a quantum dot. This scheme is based on the spin-orbit interaction and
it possesses following advantages: (1) The direction and the strength of the
spin polarization is well controllable and manipulatable by simply varying the
bias or the gate voltage. (2) The spin polarization is quite large even with a
weak spin-orbit interaction. (3) Both electron-electron interaction and
multi-energy levels do not weaken but strengthen the spin polarization. (4) It
has the short spin flip time. (5) The device is free of a magnetic field or a
ferromagnetic material. (6) It can be easily realized with present technology.Comment: 9 pages, 5 figure
Global behavior of cosmological dynamics with interacting Veneziano ghost
In this paper, we shall study the dynamical behavior of the universe
accelerated by the so called Veneziano ghost dark energy component locally and
globally by using the linearization and nullcline method developed in this
paper. The energy density is generalized to be proportional to the Hawking
temperature defined on the trapping horizon instead of Hubble horizon of the
Friedmann-Robertson-Walker (FRW) universe. We also give a prediction of the
fate of the universe and present the bifurcation phenomenon of the dynamical
system of the universe. It seems that the universe could be dominated by dark
energy at present in some region of the parameter space.Comment: 8 pages, 7 figures, accepted for publication in JHE
Cancer-associated epithelial cell adhesion molecule (EpCAM; CD326) enables epidermal Langerhans cell motility and migration in vivo
After activation, Langerhans cells (LC), a distinct subpopulation of epidermis-resident dendritic cells, migrate from skin to lymph nodes where they regulate the magnitude and quality of immune responses initiated by epicutaneously applied antigens. Modulation of LC-keratinocyte adhesion is likely to be central to regulation of LC migration. LC express high levels of epithelial cell adhesion molecule (EpCAM; CD326), a cell-surface protein that is characteristic of some epithelia and many carcinomas and that has been implicated in intercellular adhesion and metastasis. To gain insight into EpCAM function in a physiologic context in vivo, we generated conditional knockout mice with EpCAM-deficient LC and characterized them. Epidermis from these mice contained increased numbers of LC with normal levels of MHC and costimulatory molecules and T-cell-stimulatory activity in vitro. Migration of EpCAM-deficient LC from skin explants was inhibited, but chemotaxis of dissociated LC was not. Correspondingly, the ability of contact allergen-stimulated, EpCAM-deficient LC to exit epidermis in vivo was delayed, and strikingly fewer hapten-bearing LC subsequently accumulated in lymph nodes. Attenuated migration of EpCAM-deficient LC resulted in enhanced contact hypersensitivity responses as previously described in LC-deficient mice. Intravital microscopy revealed reduced translocation and dendrite motility in EpCAM-deficient LC in vivo in contact allergen-treated mice. These results conclusively link EpCAM expression to LC motility/migration and LC migration to immune regulation. EpCAM appears to promote LC migration from epidermis by decreasing LC-keratinocyte adhesion and may modulate intercellular adhesion and cell movement within in epithelia during development and carcinogenesis in an analogous fashion
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