2,431 research outputs found
Non-equilibrium transitions in fully frustrated Josephson junction arrays
We study the effect of thermal fluctuations in a fully frustrated Josephson
junction array driven by a current I larger than the apparent critical current
I_c(T). We calculate numerically the behavior of the chiral order parameter of
Z_2 symmetry and the transverse helicity modulus (related to the U(1) symmetry)
as a function of temperature. We find that the Z_2 transition occurs at a
temperature T_{Z_2}(I) which is lower than the temperature T_{U(1)}(I) for the
U(1) transition. Both transitions could be observed experimentally from
measurements of the longitudinal and transverse voltages.Comment: 4 pages, 4 figure
Mixed-valent regime of the two-channel Anderson impurity as a model for UBe_13
We investigate the mixed-valent regime of a two-configuration Anderson
impurity model for uranium ions, with separate quadrupolar and magnetic
doublets. With a new Monte Carlo approach and the non-crossing approximation we
find: (i) A non-Fermi-liquid fixed point with two-channel Kondo model critical
behavior; (ii) Distinct energy scales for screening the low-lying and excited
doublets; (iii) A semi-quantitative explanation of magnetic-susceptibility data
for UThBe assuming 60-70% quadrupolar doublet ground-state
weight, supporting the quadrupolar-Kondo interpretation.Comment: 4 Pages, 3 eps figures; submitted to Phys. Rev. Let
Planetary Candidates Observed by Kepler. VII. The First Fully Uniform Catalog Based on The Entire 48 Month Dataset (Q1-Q17 DR24)
We present the seventh Kepler planet candidate catalog, which is the first to
be based on the entire, uniformly processed, 48 month Kepler dataset. This is
the first fully automated catalog, employing robotic vetting procedures to
uniformly evaluate every periodic signal detected by the Q1-Q17 Data Release 24
(DR24) Kepler pipeline. While we prioritize uniform vetting over the absolute
correctness of individual objects, we find that our robotic vetting is overall
comparable to, and in most cases is superior to, the human vetting procedures
employed by past catalogs. This catalog is the first to utilize artificial
transit injection to evaluate the performance of our vetting procedures and
quantify potential biases, which are essential for accurate computation of
planetary occurrence rates. With respect to the cumulative Kepler Object of
Interest (KOI) catalog, we designate 1,478 new KOIs, of which 402 are
dispositioned as planet candidates (PCs). Also, 237 KOIs dispositioned as false
positives (FPs) in previous Kepler catalogs have their disposition changed to
PC and 118 PCs have their disposition changed to FP. This brings the total
number of known KOIs to 8,826 and PCs to 4,696. We compare the Q1-Q17 DR24 KOI
catalog to previous KOI catalogs, as well as ancillary Kepler catalogs, finding
good agreement between them. We highlight new PCs that are both potentially
rocky and potentially in the habitable zone of their host stars, many of which
orbit solar-type stars. This work represents significant progress in accurately
determining the fraction of Earth-size planets in the habitable zone of
Sun-like stars. The full catalog is publicly available at the NASA Exoplanet
Archive.Comment: Accepted to the Astrophysical Journal Supplement Series. 30 pages, 9
figures, 7 tables. We make the DR24 robovetter decision code publicly
available at http://github.com/JeffLCoughlin/robovetter, with input and
output examples provided using the same data as contained in the full paper's
table
Multi-Messenger Astronomy with Extremely Large Telescopes
The field of time-domain astrophysics has entered the era of Multi-messenger
Astronomy (MMA). One key science goal for the next decade (and beyond) will be
to characterize gravitational wave (GW) and neutrino sources using the next
generation of Extremely Large Telescopes (ELTs). These studies will have a
broad impact across astrophysics, informing our knowledge of the production and
enrichment history of the heaviest chemical elements, constrain the dense
matter equation of state, provide independent constraints on cosmology,
increase our understanding of particle acceleration in shocks and jets, and
study the lives of black holes in the universe. Future GW detectors will
greatly improve their sensitivity during the coming decade, as will
near-infrared telescopes capable of independently finding kilonovae from
neutron star mergers. However, the electromagnetic counterparts to
high-frequency (LIGO/Virgo band) GW sources will be distant and faint and thus
demand ELT capabilities for characterization. ELTs will be important and
necessary contributors to an advanced and complete multi-messenger network.Comment: White paper submitted to the Astro2020 Decadal Surve
Planetary Candidates Observed by Kepler VI: Planet Sample from Q1-Q16 (47 Months)
\We present the sixth catalog of Kepler candidate planets based on nearly 4
years of high precision photometry. This catalog builds on the legacy of
previous catalogs released by the Kepler project and includes 1493 new Kepler
Objects of Interest (KOIs) of which 554 are planet candidates, and 131 of these
candidates have best fit radii <1.5 R_earth. This brings the total number of
KOIs and planet candidates to 7305 and 4173 respectively. We suspect that many
of these new candidates at the low signal-to-noise limit may be false alarms
created by instrumental noise, and discuss our efforts to identify such
objects. We re-evaluate all previously published KOIs with orbital periods of
>50 days to provide a consistently vetted sample that can be used to improve
planet occurrence rate calculations. We discuss the performance of our planet
detection algorithms, and the consistency of our vetting products. The full
catalog is publicly available at the NASA Exoplanet Archive.Comment: 18 pages, to be published in the Astrophysical Journal Supplement
Serie
Ge-rich graded-index Si_1-xGex waveguides with broadband tight mode confinement and flat anomalous dispersion for nonlinear mid-infrared photonics
This work explores the use of Ge-rich graded-index Si1-xGex rib waveguides as building blocks to develop integrated nonlinear optical devices for broadband operation in the mid-IR. The vertical Ge gradient concentration in the waveguide core renders unique properties to the guided optical mode, providing tight mode confinement over a broadband mid-IR wavelength range from λ = 3 µm to 8 µm. Additionally, the gradual vertical confinement pulls the optical mode upwards in the waveguide core, overlapping with the Ge-rich area where the nonlinear refractive index is larger. Moreover, the Ge-rich graded-index Si1-xGex waveguides allow efficient tailoring of the chromatic dispersion curves, achieving flat anomalous dispersion for the quasi-TM optical mode with D ≤ 14 ps/nm/km over a ~1.4 octave span while retaining an optimum third-order nonlinear parameter, γeff. These results confirm the potential of Ge-rich graded-index Si1-xGex waveguides as an attractive platform to develop mid-IR nonlinear approaches requiring broadband dispersion engineering
Bacterial growth inhibitory activity for nanostructured copper minerals obtained from the Marañon region: comparison with commercial copper
En el presente trabajo se reporta la actividad inhibitoria del crecimiento bacteriano por nanopartÃculas de cobre cementado y de cobre comercial. Se utilizaron las cepas de Staphylococcus aureus ATCC 6538 (Gram positiva) y Escherichia coli ATCC 35218 (Gram negativa) para determinar el efecto inhibitorio mediante la concentración mÃnima inhibitoria de las nanopartÃculas diluidas en caldo de cultivo nutritivo y distribuidas en placas de ELISA. Las muestras de cobre cementado (obtenidas por procesos hidrometalúrgicos) y de cobre comercial fueron nanoestructuradas empleando un equipo de molienda mecánica. Los resultados indican que las nanopartÃculas de cobre comercial (a 2.5 horas de molienda) muestran acción inhibitoria del crecimiento de la cepa S. aureus y no asà en la cepa E. coli. Asimismo, se determinó que la concentración mÃnima inhibitoria de la muestra de cobre comercial fue de 20 μg/mL frente a S. aureus. El cobre cementado (en su forma sólida y nanoestructurada) no mostró efecto inhibitorio del crecimiento en ninguna de las dos cepas estudiadas.In this paper, we report on the bacterial growth inhibitory activity of nanoparticles of cemented and commercial copper. Strains of Staphylococcus aureus ATCC 6538 (Gram positive) and Escherichia coli ATCC 35218 (Gram negative) were used to determine the inhibitory effect by the minimal inhibitory concentration of the nanoparticles diluted in nutrient culture broth and distributed in ELISA plates. The copper cements (obtained from hydrometallurgical processes) and the commercial one were nanostructured employing a mechanical milling equipment. The results indicate that commercial copper nanoparticles (after 2.5 hours of milling) show growth inhibitory action of S. aureus strain. However, in the case of E. coli strains no inhibitory action has been observed. It was also determined that the minimal inhibitory concentration of the commercial copper is 20 μg/mL against S. aureus. On the other hand, copper cements (in solid and nanostructured form) do not show inhibitory effects
Adaptive fractional PID control of biped robots with time-delayed feedback
This paper presents the application of Fractional Order Time- Delay adaptive neural networks to the trajectory tracking for chaos synchronization between Fractional Order delayed plant, reference and Fractional Order Time-Delay adaptive neural networks. The proposed new control scheme is applied via simulations to control of a 4-DOF Biped Robot [1]. The main methodologies, on which the approach is based, are Fractional Order PID the Fractional Order Lyapunov-Krasovskii functions methodology. The structure of the biped robot is designed with two degrees of freedom per leg, corresponding to the knee and hip joints. Since torso and ankle are not considered, it is obtained a 4-DOF system, and each leg, we try to force this biped robot to track a reference signal given by undamped Duffing equation. The tracking error is globally asymptotically stabilized by two control laws derived based on a Lyapunov-Krasovski functional
The Need for Laboratory Measurements and Ab Initio Studies to Aid Understanding of Exoplanetary Atmospheres
We are now on a clear trajectory for improvements in exoplanet observations
that will revolutionize our ability to characterize their atmospheric
structure, composition, and circulation, from gas giants to rocky planets.
However, exoplanet atmospheric models capable of interpreting the upcoming
observations are often limited by insufficiencies in the laboratory and
theoretical data that serve as critical inputs to atmospheric physical and
chemical tools. Here we provide an up-to-date and condensed description of
areas where laboratory and/or ab initio investigations could fill critical gaps
in our ability to model exoplanet atmospheric opacities, clouds, and chemistry,
building off a larger 2016 white paper, and endorsed by the NAS Exoplanet
Science Strategy report. Now is the ideal time for progress in these areas, but
this progress requires better access to, understanding of, and training in the
production of spectroscopic data as well as a better insight into chemical
reaction kinetics both thermal and radiation-induced at a broad range of
temperatures. Given that most published efforts have emphasized relatively
Earth-like conditions, we can expect significant and enlightening discoveries
as emphasis moves to the exotic atmospheres of exoplanets.Comment: Submitted as an Astro2020 Science White Pape
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