903 research outputs found
Transient interference of transmission and incidence
Due to a transient quantum interference during a wavepacket collision with a
potential barrier, a particular momentum, that depends on the potential
parameters but is close to the initial average momentum, becomes suppressed.
The hole left pushes the momentum distribution outwards leading to a
significant constructive enhancement of lower and higher momenta. This is
explained in the momentum complex-plane language in terms of a saddle point and
two contiguous ``structural'' poles, which are not associated with resonances
but with incident and transmitted components of the wavefunction.Comment: 4 pages of text, 6 postscript figures, revte
Decoherence in a quantum harmonic oscillator monitored by a Bose-Einstein condensate
We investigate the dynamics of a quantum oscillator, whose evolution is
monitored by a Bose-Einstein condensate (BEC) trapped in a symmetric double
well potential. It is demonstrated that the oscillator may experience various
degrees of decoherence depending on the variable being measured and the state
in which the BEC is prepared. These range from a `coherent' regime in which
only the variances of the oscillator position and momentum are affected by
measurement, to a slow (power law) or rapid (Gaussian) decoherence of the mean
values themselves.Comment: 4 pages, 3 figures, lette
Explicit solution for a Gaussian wave packet impinging on a square barrier
The collision of a quantum Gaussian wave packet with a square barrier is
solved explicitly in terms of known functions. The obtained formula is suitable
for performing fast calculations or asymptotic analysis. It also provides
physical insight since the description of different regimes and collision
phenomena typically requires only some of the terms.Comment: To be published in J. Phys.
A method to determine the acoustic reflection and absorption coefficients of porous media by using modal dispersion in a waveguide
The measurement of acoustic material characteristics using a standard impedance tube method is generally limited to the plane wave regime below the tube cut-on frequency. This implies that the size of the tube and, consequently, the size of the material specimen must remain smaller than a half of the wavelength. This paper presents a method that enables the extension of the frequency range beyond the plane wave regime by at least a factor of 3, so that the size of the material specimen can be much larger than the wavelength. The proposed method is based on measuring of the sound pressure at different axial locations and applying the spatial Fourier transform. A normal mode decomposition approach is used together with an optimization algorithm to minimize the discrepancy between the measured and predicted sound pressure spectra. This allows the frequency and angle dependent reflection and absorption coefficients of the material specimen to be calculated in an extended frequency range. The method has been tested successfully on samples of melamine foam and wood fiber. The measured data are in close agreement with the predictions by the equivalent fluid model for the acoustical properties of porous media
G-CSF increases mesenchymal precursor cell numbers in the bone marrow via an indirect mechanism involving osteoclast-mediated bone resorption
AbstractDuring the course of studies to investigate whether MPC circulate in response to G-CSF, the agent most frequently used to induce mobilization of hematopoietic progenitors, we observed that while G-CSF failed to increase the number of MPC in circulation (assayed in vitro as fibroblast colony-forming cells, CFU-F), G-CSF administration nevertheless resulted in a time-dependent increase in the absolute number of CFU-F within the BM, peaking at Day 7. Treatment of BM cells from G-CSF-treated mice with hydroxyurea did not alter CFU-F numbers, suggesting that the increase in their numbers in response to G-CSF administration is not due to proliferation of existing CFU-F. Given previous studies demonstrating that G-CSF potently induces bone turnover in mice, we hypothesized that the increase in CFU-F may be triggered by the bone resorption that occurs following G-CSF administration. In accord with this hypothesis, administration of an inhibitor of osteoclast differentiation, osteoprotegerin (OPG), prevented the increase of CFU-F numbers induced by G-CSF. In conclusion, these data indicate that the cytokine treatment routinely used to mobilize hematopoietic stem cells could provide a readily applicable method to induce in vivo expansion of MPC for clinical applications
The k-j-j′ vector correlation in inelastic and reactive scattering
Quasi-classical trajectory (QCT) methods are presented which allow characterization of the angular momentum depolarization of the products of inelastic and reactive scattering. The particular emphasis of the theory is on three-vector correlations, and on the connection with the two-vector correlation between the initial and final angular momenta, j and j′, which is amenable to experimental measurement. The formal classical theory is presented, and computational results for NO(A) + He are used to illustrate the type of mechanistic information provided by analysis of the two- and three-vector correlations. The classical j-j′ two-vector correlation results are compared with quantum mechanical calculations, and are shown to be in good agreement. The data for NO(A) + He support previous conclusions [M. Brouard, H. Chadwick, Y.-P. Chang, R. Cireasa, C. J. Eyles, A. O. L. Via, N. Screen, F. J. Aoiz, and J. Kos, J. Chem. Phys. 131, 104307 (2009)]10.1063/1.3212608 that this system is only weakly depolarizing. Furthermore, it is shown that the projection of j along the kinematic apse is nearly conserved for this system under thermal collision energy conditions. © 2011 American Institute of Physics
Time scale of forerunners in quantum tunneling
The forerunners preceding the main tunneling signal of the wave created by a
source with a sharp onset or by a quantum shutter, have been generally
associated with over-the-barrier (non-tunneling) components. We demonstrate
that, while this association is true for distances which are larger than the
penetration lenght, for smaller distances the forerunner is dominated by
under-the-barrier components. We find that its characteristic arrival time is
inversely proportional to the difference between the barrier energy and the
incidence energy, a tunneling time scale different from both the phase time and
the B\"uttiker-Landauer (BL) time.Comment: Revtex4, 14 eps figure
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