1,031 research outputs found
Sums of hermitian squares and the BMV conjecture
Recently Lieb and Seiringer showed that the Bessis-Moussa-Villani conjecture
from quantum physics can be restated in the following purely algebraic way: The
sum of all words in two positive semidefinite matrices where the number of each
of the two letters is fixed is always a matrix with nonnegative trace. We show
that this statement holds if the words are of length at most 13. This has
previously been known only up to length 7. In our proof, we establish a
connection to sums of hermitian squares of polynomials in noncommuting
variables and to semidefinite programming. As a by-product we obtain an example
of a real polynomial in two noncommuting variables having nonnegative trace on
all symmetric matrices of the same size, yet not being a sum of hermitian
squares and commutators.Comment: 21 pages; minor changes; a companion Mathematica notebook is now
available in the source fil
Modeling of Ultra Low Capacitance Transient Voltage Suppression Diode for High ESD Protection
To improve key properties such as ultra-low capacitance (ULC) and high-voltage (HV) breakdown, we have performed a simulation work about transient voltage suppression (TVS) diodes. ULC-TVS diode was designed to employ a double deep trench to cut off the various parasitic effects that may degrade the device performance. The electrostatic discharge (ESD) protection is the targeting for the best applications in high-frequency and high-speed ICs. In this work, the device could present excellent performance in terms of very responsive ESD properties, high breakdown voltage, low leakage current, and very low capacitance level. The double trenches are aligned to the top electrode contact to restrict field crowding effects by the strong electric field intensity. The performance would be sufficient for the robust ESD nature up to IEC61000-4-2 (30 kV) and compatible with strong surge protection IEC61000-4-5 (10A). Their electrical properties have been evaluated for structure from simulation and the results are obtained at the device parameters. Several process of device design related effects on the electrical capability and can be optimized. Keywords: ULC-TVS diode, simulation (TCAD), characteristics, capacitance, ESD protection
Topological quantization and degeneracy in Josephson-junction arrays
We consider the conductivity quantization in two-dimensional arrays of
mesoscopic Josephson junctions, and examine the associated degeneracy in
various regimes of the system. The filling factor of the system may be
controlled by the gate voltage as well as the magnetic field, and its
appropriate values for quantization is obtained by employing the Jain hierarchy
scheme both in the charge description and in the vortex description. The
duality between the two descriptions then suggests the possibility that the
system undergoes a change in degeneracy while the quantized conductivity
remains fixed.Comment: To appear in Phys. Rev.
Lattice effects on the current-voltage characteristics of superconducting arrays
The lattice effects on the current-voltage characteristics of two-dimensional
arrays of resistively shunted Josephson junctions are investigated. The lattice
potential energies due to the discrete lattice structure are calculated for
several geometries and directions of current injection. We compare the energy
barrier for vortex-pair unbinding with the lattice pinning potential, which
shows that lattice effects are negligible in the low-current limit as well as
in the high-current limit. At intermediate currents, on the other hand, the
lattice potential becomes comparable to the barrier height and the lattice
effects may be observed in the current-voltage characteristics.Comment: 5 pages including 5 figures in two columns, to appear in Phys. Rev.
A T-odd asymmetry in neutralino production and decay
We study CP-violating effects in neutralino production and subsequent decay
within the Minimal Supersymmetric Standard Model with complex parameters M_1
and mu. The observable we propose is a T-odd asymmetry based on a triple
product in neutralino production e^+ e^- -> tilde{chi}^0_i tilde{chi}^0_2, i =
1,...,4, with subsequent leptonic three-body decay tilde{chi}^0_2 ->
tilde{chi}^0_1 l^+ l^-, l = e, mu, at an e^+ e^- linear collider with sqrt{s} =
500 GeV and polarised beams. We provide compact analytical formulae for the
cross section and the T-odd asymmetry taking into account the complete spin
correlations between production and decay. We give numerical predictions for
the cross section and the T-odd asymmetry. The asymmetry can go up to 10 %.Comment: 28 pages, LaTeX, 10 figures; v2: typos corrected, published versio
The , , and electromagnetic form factors
The rainbow truncation of the quark Dyson-Schwinger equation is combined with
the ladder Bethe-Salpeter equation for the meson amplitudes and the dressed
quark-photon vertex in a self-consistent Poincar\'e-invariant study of the pion
and kaon electromagnetic form factors in impulse approximation. We demonstrate
explicitly that the current is conserved in this approach and that the obtained
results are independent of the momentum partitioning in the Bethe-Salpeter
amplitudes. With model gluon parameters previously fixed by the condensate, the
pion mass and decay constant, and the kaon mass, the charge radii and spacelike
form factors are found to be in good agreement with the experimental data.Comment: 8 pages, 6 figures, Revte
Could Large CP Violation Be Detected at Colliders?
We argue that CP--violation effects below a few tenths of a percent are
probably undetectable at hadron and electron colliders. Thus only operators
whose contributions interfere with tree--level Standard Model amplitudes are
detectable. We list these operators for Standard Model external particles and
some two and three body final state reactions that could show detectable
effects. These could test electroweak baryogenesis scenarios.Comment: 11pp, LaTeX, UM--TH--92--27(massaged to make TeX output cleaner), no
picture
Implications of Compressed Supersymmetry for Collider and Dark Matter Searches
Martin has proposed a scenario dubbed ``compressed supersymmetry'' (SUSY)
where the MSSM is the effective field theory between energy scales M_{\rm weak}
and M_{\rm GUT}, but with the GUT scale SU(3) gaugino mass M_3<< M_1 or M_2. As
a result, squark and gluino masses are suppressed relative to slepton, chargino
and neutralino masses, leading to a compressed sparticle mass spectrum, and
where the dark matter relic density in the early universe may be dominantly
governed by neutralino annihilation into ttbar pairs via exchange of a light
top squark. We explore the dark matter and collider signals expected from
compressed SUSY for two distinct model lines with differing assumptions about
GUT scale gaugino mass parameters. For dark matter signals, the compressed
squark spectrum leads to an enhancement in direct detection rates compared to
models with unified gaugino masses. Meanwhile, neutralino halo annihilation
rates to gamma rays and anti-matter are also enhanced relative to related
scenarios with unified gaugino masses but, depending on the halo dark matter
distribution, may yet be below the sensitivity of indirect searches underway.
In the case of collider signals, we compare the rates for the potentially
dominant decay modes of the stop_1 which may be expected to be produced in
cascade decay chains at the LHC: \tst_1\to c\tz_1 and \tst_1\to bW\tz_1. We
examine the extent to which multilepton signal rates are reduced when the
two-body decay mode dominates. For the model lines that we examine here, the
multi-lepton signals, though reduced, still remain observable at the LHC.Comment: 22 pages including 24 eps figure
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