3,929 research outputs found
Preferential antiferromagnetic coupling of vacancies in graphene on SiO_2: Electron spin resonance and scanning tunneling spectroscopy
Monolayer graphene grown by chemical vapor deposition and transferred to
SiO_2 is used to introduce vacancies by Ar^+ ion bombardment at a kinetic
energy of 50 eV. The density of defects visible in scanning tunneling
microscopy (STM) is considerably lower than the ion fluence implying that most
of the defects are single vacancies. The vacancies are characterized by
scanning tunneling spectroscopy (STS) on graphene and HOPG exhibiting a peak
close to the Fermi level. The peak persists after air exposure up to 180 min,
albeit getting broader. After air exposure for less than 60 min, electron spin
resonance (ESR) at 9.6 GHz is performed. For an ion flux of 10/nm^2, we find a
signal corresponding to a g-factor of 2.001-2.003 and a spin density of 1-2
spins/nm^2. The ESR signal consists of a mixture of a Gaussian and a Lorentzian
of equal weight exhibiting a width down to 0.17 mT, which, however, depends on
details of the sample preparation. The g-factor anisotropy is about 0.02%.
Temperature dependent measurements reveal antiferromagnetic correlations with a
Curie-Weiss temperature of -10 K. Albeit the electrical conductivity of
graphene is significantly reduced by ion bombardment, the spin resonance
induced change in conductivity is below 10^{-5}.Comment: 10 pages, 5 figures, discussion on STM images in the literature of
defects in graphene adde
Towards a fully consistent Milky Way disc model - II. The local disc model and SDSS data of the NGP region
We have used the self-consistent vertical disc models of the solar
neighbourhood presented in Just & Jahreiss (2010), which are based on different
star formation histories (SFR) and fit the local kinematics of main sequence
stars equally well, to predict star counts towards the North Galactic Pole
(NGP). We combined these four different models with the local main sequence in
the filter system of the SDSS and predicted the star counts in the NGP field
with b>80deg. All models fit the Hess diagrams in the F-K dwarf regime better
than 20 percent and the star number densities in the solar neighbourhood are
consistent with the observed values. The chi^2 analysis shows that model A is
clearly preferred with systematic deviations of a few percent only. The SFR of
model A is characterised by a maximum at an age of 10Gyr and a decline by a
factor of four to the present day value of 1.4Msun/pc^2/Gyr. The thick disc can
be modelled very well by an old isothermal simple stellar population. The
density profile can be approximated by a sech^(alpha_t) function. We found a
power law index alpha_t=1.16 and a scale height of 800pc corresponding to a
vertical velocity dispersion of 45.3km/s. About 6 percent of the stars in the
solar neighbourhood are thick disc stars.Comment: 11 pages, 12 figures, accepted by MNRAS. The qualtity of figures 10
and 12 are much better in the .ps version than the .pdf versio
Stability domains for time-delay feedback control with latency
We generalize a known analytical method for determining the stability of
periodic orbits controlled by time-delay feedback methods when latencies
associated with the generation and injection of the feedback signal cannot be
ignored. We discuss the case of extended time-delay autosynchronization (ETDAS)
and show that nontrivial qualitative features of the domain of control observed
in experiments can be explained by taking into account the effects of both the
unstable eigenmode and a single stable eigenmode in the Floquet theory.Comment: 9 pages, 6 figures; Submitted to Physical Review
Characterizing the radial oxygen abundance distribution in disk galaxies
We examine the possible dependence of the radial oxygen abundance
distribution on non-axisymmetrical structures (bar/spirals) and other
macroscopic parameters such as the mass, the optical radius R25, the color g-r,
and the surface brightness of the galaxy. A sample of disk galaxies from the
CALIFA DR3 is considered. We adopted the Fourier amplitude A2 of the surface
brightness as a quantitative characteristic of the strength of non-axisymmetric
structures in a galactic disk, in addition to the commonly used morphologic
division for A, AB, and B types based on the Hubble classification. To
distinguish changes in local oxygen abundance caused by the non-axisymmetrical
structures, the multiparametric mass--metallicity relation was constructed as a
function of parameters such as the bar/spiral pattern strength, the disk size,
color index g-r in the SDSS bands, and central surface brightness of the disk.
The gas-phase oxygen abundance gradient is determined by using the R
calibration. We find that there is no significant impact of the
non-axisymmetric structures such as a bar and/or spiral patterns on the local
oxygen abundance and radial oxygen abundance gradient of disk galaxies.
Galaxies with higher mass, however, exhibit flatter oxygen abundance gradients
in units of dex/kpc, but this effect is significantly less prominent for the
oxygen abundance gradients in units of dex/R25 and almost disappears when the
inner parts are avoided. We show that the oxygen abundance in the central part
of the galaxy depends neither on the optical radius R25 nor on the color g-r or
the surface brightness of the galaxy. Instead, outside the central part of the
galaxy, the oxygen abundance increases with g-r value and central surface
brightness of the disk.Comment: 11 pages, 6 figures; accepted for publication in A&
The Isomorphism Relation Between Tree-Automatic Structures
An -tree-automatic structure is a relational structure whose domain
and relations are accepted by Muller or Rabin tree automata. We investigate in
this paper the isomorphism problem for -tree-automatic structures. We
prove first that the isomorphism relation for -tree-automatic boolean
algebras (respectively, partial orders, rings, commutative rings, non
commutative rings, non commutative groups, nilpotent groups of class n >1) is
not determined by the axiomatic system ZFC. Then we prove that the isomorphism
problem for -tree-automatic boolean algebras (respectively, partial
orders, rings, commutative rings, non commutative rings, non commutative
groups, nilpotent groups of class n >1) is neither a -set nor a
-set
Use of the LUS in sequence allele designations to facilitate probabilistic genotyping of NGS-based STR typing results
Some of the expected advantages of next generation sequencing (NGS) for short tandem repeat (STR) typing include enhanced mixture detection and genotype resolution via sequence variation among non-homologous alleles of the same length. However, at the same time that NGS methods for forensic DNA typing have advanced in recent years, many caseworking laboratories have implemented or are transitioning to probabilistic genotyping to assist the interpretation of complex autosomal STR typing results. Current probabilistic software programs are designed for length-based data, and were not intended to accommodate sequence strings as the product input. Yet to leverage the benefits of NGS for enhanced genotyping and mixture deconvolution, the sequence variation among same-length products must be utilized in some form. Here, we propose use of the longest uninterrupted stretch (LUS) in allele designations as a simple method to represent sequence variation within the STR repeat regions and facilitate – in the nearterm – probabilistic interpretation of NGS-based typing results. An examination of published population data indicated that a reference LUS region is straightforward to define for most autosomal STR loci, and that using repeat unit plus LUS length as the allele designator can represent greater than 80% of the alleles detected by sequencing. A proof of concept study performed using a freely available probabilistic software demonstrated that the LUS length can be used in allele designations when a program does not require alleles to be integers, and that utilizing sequence information improves interpretation of both single-source and mixed contributor STR typing results as compared to using repeat unit information alone. The LUS concept for allele designation maintains the repeat-based allele nomenclature that will permit backward compatibility to extant STR databases, and the LUS lengths themselves will be concordant regardless of the NGS assay or analysis tools employed. Further, these biologically based, easy-to-derive designations uphold clear relationships between parent alleles and their stutter products, enabling analysis in fully continuous probabilistic programs that model stutter while avoiding the algorithmic complexities that come with string based searches. Though using repeat unit plus LUS length as the allele designator does not capture variation that occurs outside of the core repeat regions, this straightforward approach would permit the large majority of known STR sequence variation to be used for mixture deconvolution and, in turn, result in more informative mixture statistics in the near term. Ultimately, the method could bridge the gap from current length-based probabilistic systems to facilitate broader adoption of NGS by forensic DNA testing laboratories
Time--delay autosynchronization of the spatio-temporal dynamics in resonant tunneling diodes
The double barrier resonant tunneling diode exhibits complex spatio-temporal
patterns including low-dimensional chaos when operated in an active external
circuit. We demonstrate how autosynchronization by time--delayed feedback
control can be used to select and stabilize specific current density patterns
in a noninvasive way. We compare the efficiency of different control schemes
involving feedback in either local spatial or global degrees of freedom. The
numerically obtained Floquet exponents are explained by analytical results from
linear stability analysis.Comment: 10 pages, 16 figure
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