13,847 research outputs found
Ordering monomial factors of polynomials in the product representation
The numerical construction of polynomials in the product representation (as
used for instance in variants of the multiboson technique) can become
problematic if rounding errors induce an imprecise or even unstable evaluation
of the polynomial. We give criteria to quantify the effects of these rounding
errors on the computation of polynomials approximating the function . We
consider polynomials both in a real variable and in a Hermitian matrix. By
investigating several ordering schemes for the monomials of these polynomials,
we finally demonstrate that there exist orderings of the monomials that keep
rounding errors at a tolerable level.Comment: Latex2e file, 7 figures, 32 page
Phonon-assisted tunneling in the quantum regime of Mn12-ac
Longitudinal or transverse magnetic fields applied on a crystal of Mn12-ac
allows to observe independent tunnel transitions between m=-S+p and m=S-n-p
(n=6-10, p=0-2 in longitudinal field and n=p=0 in transverse field). We observe
a smooth transition (in longitudinal) from coherent ground-state to thermally
activated tunneling. Furthermore two ground-state relaxation regimes showing a
crossover between quantum spin relaxation far from equilibrium and near
equilibrium, when the environment destroys multimolecule correlations. Finally,
we stress that the complete Hamiltonian of Mn12 should contain odd spin
operators of low order
Magnetic structure of the edge-sharing copper oxide chain compound NaCu2O2
Single-crystal neutron diffraction has been used to determine the
incommensurate magnetic structure of NaCu2O2, a compound built up of chains of
edge-sharing CuO4 plaquettes. Magnetic structures compatible with the lattice
symmetry were identified by a group-theoretical analysis, and their magnetic
structure factors were compared to the experimentally observed Bragg
intensities. In conjunction with other experimental data, this analysis yields
an elliptical helix structure in which both the helicity and the polarization
plane alternate among copper-oxide chains. This magnetic ground state is
discussed in the context of the recently reported multiferroic properties of
other copper-oxide chain compounds
Hysteresis in the de Haas-van Alphen Effect
A hysteresis loop is observed for the first time in the de Haas-van Alphen
(dHvA) effect of beryllium at low temperatures and quantizing magnetic field
applied parallel to the hexagonal axis of the single crystal. The irreversible
behavior of the magnetization occurs at the paramagnetic part of the dHvA
period in conditions of Condon domain formation arising by strong enough dHvA
amplitude. The resulting extremely nonlinear response to a very small
modulation field offers the possibility to find in a simple way the Condon
domain phase diagram. From a harmonic analysis, the shape and size of the
hysteresis loop is constructed.Comment: 4 pages, 5 figures, submitted to PR
{\eta} and {\eta}' mesons from Nf=2+1+1 twisted mass lattice QCD
We determine mass and mixing angles of eta and eta' states using Nf=2+1+1
Wilson twisted mass lattice QCD. We describe how those flavour singlet states
need to be treated in this lattice formulation. Results are presented for three
values of the lattice spacing, a=0.061 fm, a=0.078 fm and a=0.086 fm, with
light quark masses corresponding to values of the charged pion mass in a range
of 230 to 500 MeV and fixed bare strange and charm quark mass values. We obtain
557(15)(45) MeV for the eta mass (first error statistical, second systematic)
and 44(5) degrees for the mixing angle in the quark flavour basis,
corresponding to -10(5) degrees in the octet-singlet basis.Comment: 28 pages, 9 figures, version to appear in JHEP, extended discussion
of autocorrelation times and comparison to results available in the
literature, added a comment for FS-effects and clarified the description of
our blocking procedur
Direct Observation of Condon Domains in Silver by Hall Probes
Using a set of micro Hall probes for the detection of the local induction,
the inhomogeneous Condon domain structure has been directly observed at the
surface of a pure silver single crystal under strong Landau quantization in
magnetic fields up to 10 T. The inhomogeneous induction occurs in the
theoretically predicted part of the H-T Condon domain phase diagram.
Information about size, shape and orientation of the domains is obtained by
analyzing Hall probes placed along and across the long sample axis and by
tilting the sample. On a beryllium surface the induction inhomogeneity is
almost absent although the expected induction splitting here is at least ten
times higher than in silver.Comment: 4 pages, 6 figures, submitted to PR
Assessing Human Error Against a Benchmark of Perfection
An increasing number of domains are providing us with detailed trace data on
human decisions in settings where we can evaluate the quality of these
decisions via an algorithm. Motivated by this development, an emerging line of
work has begun to consider whether we can characterize and predict the kinds of
decisions where people are likely to make errors.
To investigate what a general framework for human error prediction might look
like, we focus on a model system with a rich history in the behavioral
sciences: the decisions made by chess players as they select moves in a game.
We carry out our analysis at a large scale, employing datasets with several
million recorded games, and using chess tablebases to acquire a form of ground
truth for a subset of chess positions that have been completely solved by
computers but remain challenging even for the best players in the world.
We organize our analysis around three categories of features that we argue
are present in most settings where the analysis of human error is applicable:
the skill of the decision-maker, the time available to make the decision, and
the inherent difficulty of the decision. We identify rich structure in all
three of these categories of features, and find strong evidence that in our
domain, features describing the inherent difficulty of an instance are
significantly more powerful than features based on skill or time.Comment: KDD 2016; 10 page
The eta' meson from lattice QCD
We study the flavour singlet pseudoscalar mesons from first principles using
lattice QCD. With N_f=2 flavours of light quark, this is the so-called eta_2
meson and we discuss the phenomenological status of this. Using maximally
twisted-mass lattice QCD, we extract the mass of the eta_2 meson at two values
of the lattice spacing for lighter quarks than previously discussed in the
literature. We are able to estimate the mass value in the limit of light quarks
with their physical masses.Comment: 16 pages: version accepted for publicatio
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