2,037 research outputs found
Geometric approach to Fletcher's ideal penalty function
Original article can be found at: www.springerlink.com Copyright Springer. [Originally produced as UH Technical Report 280, 1993]In this note, we derive a geometric formulation of an ideal penalty function for equality constrained problems. This differentiable penalty function requires no parameter estimation or adjustment, has numerical conditioning similar to that of the target function from which it is constructed, and also has the desirable property that the strict second-order constrained minima of the target function are precisely those strict second-order unconstrained minima of the penalty function which satisfy the constraints. Such a penalty function can be used to establish termination properties for algorithms which avoid ill-conditioned steps. Numerical values for the penalty function and its derivatives can be calculated efficiently using automatic differentiation techniques.Peer reviewe
Ytterbium divalency and lattice disorder in near-zero thermal expansion YbGaGe
While near-zero thermal expansion (NZTE) in YbGaGe is sensitive to
stoichiometry and defect concentration, the NZTE mechanism remains elusive. We
present x-ray absorption spectra that show unequivocally that Yb is nearly
divalent in YbGaGe and the valence does not change with temperature or with
nominally 1% B or 5% C impurities, ruling out a valence-fluctuation mechanism.
Moreover, substantial changes occur in the local structure around Yb with B and
C inclusion. Together with inelastic neutron scattering measurements, these
data indicate a strong tendency for the lattice to disorder, providing a
possible explanation for NZTE in YbGaGe.Comment: 4 pages, 4 figure, supplementary inf
Quantum ergodicity for restrictions to hypersurfaces
Quantum ergodicity theorem states that for quantum systems with ergodic
classical flows, eigenstates are, in average, uniformly distributed on energy
surfaces. We show that if N is a hypersurface in the position space satisfying
a simple dynamical condition, the restrictions of eigenstates to N are also
quantum ergodic.Comment: 22 pages, 1 figure; revised according to referee's comments. To
appear in Nonlinearit
Phonon self-energy and origin of anomalous neutron scattering spectra in SnTe and PbTe thermoelectrics
The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe
and PbTe are investigated with inelastic neutron scattering (INS) and
first-principles calculations. The experiments show that, surprisingly,
although SnTe is closer to the ferroelectric instability, phonon spectra in
PbTe exhibit a more anharmonic character. This behavior is reproduced in
first-principles calculations of the temperature-dependent phonon self-energy.
Our simulations reveal how the nesting of phonon dispersions induces prominent
features in the self-energy, which account for the measured INS spectra and
their temperature dependence. We establish that the phase-space for
three-phonon scattering processes, rather than just the proximity to the
lattice instability, is the mechanism determining the complex spectrum of the
transverse-optical ferroelectric mode
Quantum critical behavior in the heavy Fermion single crystal Ce(NiPd)Ge
We have performed magnetic susceptibility, specific heat, resistivity, and
inelastic neutron scattering measurements on a single crystal of the heavy
Fermion compound Ce(NiPd)Ge, which is believed to
be close to a quantum critical point (QCP) at T = 0. At lowest
temperature(1.8-3.5 K), the magnetic susceptibility behaves as with m/mole
(0.0025 emu/mole). For 1 K, the specific heat can be fit to the formula
with of order 700 mJ/mole-K.
The resistivity behaves as for temperatures below 2
K. This low temperature behavior for and is in accord
with the SCR theory of Moriya and Takimoto\cite{Moriya}. The inelastic neutron
scattering spectra show a broad peak near 1.5 meV that appears to be
independent of ; we interpret this as Kondo scattering with 17 K. In
addition, the scattering is enhanced near =(1/2, 1/2, 0) with maximum
scattering at = 0.45 meV; we interpret this as scattering from
antiferromagnetic fluctuations near the antiferromagnetic QCP.Comment: to be published in J. Phys: Conference Serie
The Strauss conjecture on asymptotically flat space-times
By assuming a certain localized energy estimate, we prove the existence
portion of the Strauss conjecture on asymptotically flat manifolds, possibly
exterior to a compact domain, when the spatial dimension is 3 or 4. In
particular, this result applies to the 3 and 4-dimensional Schwarzschild and
Kerr (with small angular momentum) black hole backgrounds, long range
asymptotically Euclidean spaces, and small time-dependent asymptotically flat
perturbations of Minkowski space-time. We also permit lower order perturbations
of the wave operator. The key estimates are a class of weighted Strichartz
estimates, which are used near infinity where the metrics can be viewed as
small perturbations of the Minkowski metric, and the assumed localized energy
estimate, which is used in the remaining compact set.Comment: Final version, to appear in SIAM Journal on Mathematical Analysis. 17
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