65,059 research outputs found
Fermionic entanglement ambiguity in non-inertial frames
We analyse an ambiguity in previous works on entanglement of fermionic fields
in non-inertial frames. This ambiguity, related to the anticommutation
properties of field operators, leads to non-unique results when computing
entanglement measures for the same state. We show that the ambiguity disappears
when we introduce detectors, which are in any case necessary as a means to
probe the field entanglement.Comment: 9 pages, 4 figures. Revtex 4.1, added Journal referenc
Non-Kramers Freezing and Unfreezing of Tunneling in the Biaxial Spin Model
The ground state tunnel splitting for the biaxial spin model in the magnetic
field, H = -D S_{x}^2 + E S_{z}^2 - g \mu_B S_z H_z, has been investigated
using an instanton approach. We find a new type of spin instanton and a new
quantum interference phenomenon associated with it: at a certain field, H_2 =
2SE^{1/2}(D+E)^{1/2}/(g \mu_B), the dependence of the tunneling splitting on
the field switches from oscillations to a monotonic growth. The predictions of
the theory can be tested in Fe_8 molecular nanomagnets.Comment: 7 pages, minor changes, published in EP
Turbine blade-tip clearance excitation forces
The results of an effort to assess the existing knowledge and plan the required experimentation in the area of turbine blade tip excitation forces is summarized. The work was carried out in three phases. The first was a literature search and evaluation, which served to highlight the state of the art and to expose the need for an articulated theoretical experimental effort to provide not only design data, but also a rational framework for their extrapolation to new configurations and regimes. The second phase was a start in this direction, in which several of the explicit or implicit assumptions contained in the usual formulations of the Alford force effect were removed and a rigorous linearized flow analysis of the behavior of a nonsymmetric actuator disc was carried out. In the third phase a preliminary design of a turbine test facility that would be used to measure both the excitation forces themselves and the flow patterns responsible for them were conducted and do so over a realistic range of dimensionless parameters
Cross-correlation of the CMB and radio galaxies in real, harmonic and wavelet spaces: detection of the integrated Sachs-Wolfe effect and dark energy constraints
We report the first detection of the ISW effect in wavelet space, at scales
in the sky around 7 degrees with a significance of around 3.3 sigma, by
cross-correlating the WMAP first-year data and the NRAO VLA Sky Survey (NVSS).
In addition, we present a detailed comparison among the capabilities of three
different techniques for two different objectives: to detect the ISW and to put
constraints in the nature of the dark energy. The three studied techniques are:
the cross-angular power spectrum (CAPS, harmonic space), the correlation
function (CCF, real space) and the covariance of the Spherical Mexican Hat
Wavelet (SMHW) coefficients (CSMHW, wavelet space). We prove that the CSMHW is
expected to provide a higher detection of the ISW effect for a certain scale.
This prediction has been corroborated by the analysis of the data. The SMHW
analysis shows that the cross-correlation signal is caused neither by
systematic effects nor foreground contamination. However, by taking into
account the information encoded in all the multipoles/scales/angles, the CAPS
provides slightly better constraints than the SMHW in the cosmological
parameters that define the nature of the dark energy. The limits provided by
the CCF are wider than for the other two methods. Two different cases have been
studied: 1) a flat Lambda-CDM universe and 2) a flat universe with an equation
of state parameter different from -1. In the first case, the CAPS provides (for
a bias value of b = 1.6) 0.59 < Lambda density < 0.84 (at 1 sigma CL).
Moreover, the CAPS rejects the range Lambda density < 0.1 at 3.5 sigma, which
is the highest detection of the dark energy reported up to date. In the second
case, the CAPS gives 0.50 < dark energy density < 0.82 and -1.16 < w < 0.43 (at
1 sigma CL).Comment: 12 pages, 7 figures, accepted for publication in MNRAS. Analysis
redone. Changes in the estimation of the cosmological parametres. Additional
comparison between wavelets and more standard technique
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