119,252 research outputs found
The Eect of Mechanical Loading on the Frequency of an Oscillator Circuit
We investigate the effect of mechanical strain on the frequency of an electronic oscillator embedded in an integrated circuit. This analysis is aimed at explaining a 1% inaccuracy in the oscillator frequency under test conditions prescribed by a leading supplier of semi-conductor devices. During the test the package containing the oscillator was clamped to a circuit board by mechanical pressure. By considering the nature of the oscillator we show that tensile strains of the order of 10^-4 could explain the observations via the piezoresistance effect. Both a simple one-dimensional analysis based on the beam equation and an elastic finite element simulation show that strains of this magnitude can be generated during the test
Gauge Theory of Gravity Requires Massive Torsion Field
One of the greatest unsolved issues of the physics of this century is to find
a quantum field theory of gravity. According to a vast amount of literature
unification of quantum field theory and gravitation requires a gauge theory of
gravity which includes torsion and an associated spin field. Various models
including either massive or massless torsion fields have been suggested. We
present arguments for a massive torsion field, where the probable rest mass of
the corresponding spin three gauge boson is the Planck mass.Comment: 3 pages, Revte
Hardness of Graph Pricing through Generalized Max-Dicut
The Graph Pricing problem is among the fundamental problems whose
approximability is not well-understood. While there is a simple combinatorial
1/4-approximation algorithm, the best hardness result remains at 1/2 assuming
the Unique Games Conjecture (UGC). We show that it is NP-hard to approximate
within a factor better than 1/4 under the UGC, so that the simple combinatorial
algorithm might be the best possible. We also prove that for any , there exists such that the integrality gap of
-rounds of the Sherali-Adams hierarchy of linear programming for
Graph Pricing is at most 1/2 + .
This work is based on the effort to view the Graph Pricing problem as a
Constraint Satisfaction Problem (CSP) simpler than the standard and complicated
formulation. We propose the problem called Generalized Max-Dicut(), which
has a domain size for every . Generalized Max-Dicut(1) is
well-known Max-Dicut. There is an approximation-preserving reduction from
Generalized Max-Dicut on directed acyclic graphs (DAGs) to Graph Pricing, and
both our results are achieved through this reduction. Besides its connection to
Graph Pricing, the hardness of Generalized Max-Dicut is interesting in its own
right since in most arity two CSPs studied in the literature, SDP-based
algorithms perform better than LP-based or combinatorial algorithms --- for
this arity two CSP, a simple combinatorial algorithm does the best.Comment: 28 page
Black hole as an Information Eraser
We discuss the identity of black hole entropy and show that the first law of
black hole thermodynamics, in the case of a Schwarzschild black hole, can be
derived from Landauer's principle by assuming that the black hole is one of the
most efficient information erasers in systems of a given temperature. The term
"most efficient" implies that minimal energy is required to erase a given
amount of information. We calculate the discrete mass spectra and the entropy
of a Schwarzschild black hole assuming that the black hole processes
information in unit of bits. The black hole entropy acquires a sub-leading
contribution proportional to the logarithm of its mass-squared in addition to
the usual mass-squared term without an artificial cutoff. We also argue that
the minimum of the black hole mass is .Comment: 12 pages, 4 figures, minor change
Non-linear resonance in nearly geodesic motion in low-mass X-ray binaries
We have explored the ideas that parametric resonance affects nearly geodesic
motion around a black hole or a neutron star, and that it may be relevant to
the high frequency (twin) quasi-periodic oscillations occurring in some
low-mass X-ray binaries. We have assumed the particles or fluid elements of an
accretion disc to be subject to an isotropic perturbation of a hypothetical but
rather general form. We find that the parametric resonance is indeed excited
close to the radius where epicyclic frequencies of radial and meridional
oscillations are in a 2:3 ratio. The location and frequencies of the highest
amplitude excitation vary with the strength of the perturbation. These results
agree with actual frequency ratios of twin kHz QPOs that have been reported in
some black hole candidates, and they may be consistent also with correlation of
the twin peaks in Sco X-1.Comment: 5 pages; accepted for publication in PAS
Temperature-dependent properties of the magnetic order in single-crystal BiFeO3
We report neutron diffraction and magnetization studies of the magnetic order
in multiferroic BiFeO3. In ferroelectric monodomain single crystals, there are
three magnetic cycloidal domains with propagation vectors equivalent by
crystallographic symmetry. The cycloid period slowly grows with increasing
temperature. The magnetic domain populations do not change with temperature
except in the close vicinity of the N{\P}eel temperature, at which, in
addition, a small jump in magneti- zation is observed. No evidence for the
spin-reorientation transitions proposed in previous Raman and dielectric
studies is found. The magnetic cycloid is slightly anharmonic for T=5 K. The
an- harmonicity is much smaller than previously reported in NMR studies. At
room temperature, a circular cycloid is observed, within errors. We argue that
the observed anharmonicity provides important clues for understanding
electromagnons in BiFeO3.Comment: In Press at PR
A low-noise ferrite magnetic shield
Ferrite materials provide magnetic shielding performance similar to commonly
used high permeability metals but have lower intrinsic magnetic noise generated
by thermal Johnson currents due to their high electrical resistivity.
Measurements inside a ferrite shield with a spin-exchange relaxation-free
atomic magnetometer reveal a noise level of 0.75 fT Hz^(-1/2), 25 times lower
than what would be expected in a comparable mu-metal shield. The authors
identify a 1/f component of the magnetic noise due to magnetization
fluctuations and derive general relationships for the Johnson current noise and
magnetization noise in cylindrical ferromagnetic shields in terms of their
conductivity and complex magnetic permeability.Comment: 4 pages, 3 figures. Published in Appl. Phys. Lett.; replacement
reflects published wor
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