604 research outputs found
Non-equilibrium dynamics of simple spherical spin models
We investigate the non-equilibrium dynamics of spherical spin models with
two-spin interactions. For the exactly solvable models of the d-dimensional
spherical ferromagnet and the spherical Sherrington-Kirkpatrick model the
asymptotic dynamics has for large times and for large waiting times the same
formal structure. In the limit of large waiting times we find in both models an
intermediate time scale, scaling as a power of the waiting time with an
exponent smaller than one, and thus separating the time-translation invariant
short-time dynamics from the aging regime. It is this time scale on which the
fluctuation-dissipation regime is violated. Aging in these models is similar to
that observed in spin glasses at the level of correlation functions, but
different at the level of response functions, and thus different at the level
of experimentally accessible quantities like the thermoremanent magnetization.Comment: 8 pages, 1 eps figur
Composite Differential Evolution for Constrained Evolutionary Optimization
When solving constrained optimization problems (COPs) by evolutionary algorithms, the search algorithm plays a crucial role. In general, we expect that the search algorithm has the capability to balance not only diversity and convergence but also constraints and objective function during the evolution. For this purpose, this paper proposes a composite differential evolution (DE) for constrained optimization, which includes three different trial vector generation strategies with distinct advantages. In order to strike a balance between diversity and convergence, one of these three trial vector generation strategies is able to increase diversity, and the other two exhibit the property of convergence. In addition, to accomplish the tradeoff between constraints and objective function, one of the two trial vector generation strategies for convergence is guided by the individual with the least degree of constraint violation in the population, and the other is guided by the individual with the best objective function value in the population. After producing offspring by the proposed composite DE, the feasibility rule and the ϵ constrained method are combined elaborately for selection in this paper. Moreover, a restart scheme is proposed to help the population jump out of a local optimum in the infeasible region for some extremely complicated COPs. By assembling the above techniques together, a constrained composite DE is proposed. The experiments on two sets of benchmark test functions with various features, i.e., 24 test functions from IEEE CEC2006 and 18 test functions with 10 dimensions and 30 dimensions from IEEE CEC2010, have demonstrated that the proposed method shows better or at least competitive performance against other state-of-the-art methods
Superconductivity at 22.3 K in SrFe2-xIrxAs2
By substituting the Fe with the 5d-transition metal Ir in SrFe2As2, we have
successfully synthesized the superconductor SrFe2-xIrxAs2 with Tc = 22.3 K at x
= 0.5. X-ray diffraction indicates that the material has formed the
ThCr2Si2-type structure with a space group I4/mmm. The temperature dependence
of resistivity and dc magnetization both reveal sharp superconducting
transitions at around 22 K. An estimate on the diamagnetization signal reveals
a high Meissner shielding volume. Interestingly, the normal state resistivity
exhibits a roughly linear behavior up to 300 K. The superconducting transitions
at different magnetic fields were also measured yielding a slope of -dHc2/dT =
3.8 T/K near Tc. Using the Werthamer-Helfand-Hohenberg (WHH) formula, the upper
critical field at zero K is found to be about 58 T. Counting the possible
number of electrons doped into the system in SrFe2-xIrxAs2, we argue that the
superconductivity in the Ir-doped system is different from the Co-doped case,
which should add more ingredients to the underlying physics of the iron
pnictide superconductors.Comment: 4 pages, 4 figure
A controllable superconducting electromechanical oscillator with a suspended membrane
We fabricate a microscale electromechanical system, in which a suspended
superconducting membrane, treated as a mechanical oscillator, capacitively
couples to a superconducting microwave resonator. As the microwave driving
power increases, nonmonotonic dependence of the resonance frequency of the
mechanical oscillator on the driving power has been observed. We also
demonstrate the optical switching of the resonance frequency of the mechanical
oscillator. Theoretical models for qualitative understanding of our
experimental observations are presented. Our experiment may pave the way for
the application of a mechanical oscillator with its resonance frequency
controlled by the electromagnetic and/or optical fields, such as a
microwave-optical interface and a controllable element in a
superqubit-mechanical oscillator hybrid system.Comment: 8 pages,4 figure
SDSS quasars in the WISE preliminary data release and quasar candidate selection with optical/infrared colors
We present a catalog of 37,842 quasars in the SDSS Data Release 7, which have
counterparts within 6" in the WISE Preliminary Data Release. The overall WISE
detection rate of the SDSS quasars is 86.7%, and it decreases to less than
50.0% when the quasar magnitude is fainter than . We derive the median
color-redshift relations based on this SDSS-WISE quasar sample and apply them
to estimate the photometric redshifts of the SDSS-WISE quasars. We find that by
adding the WISE W1- and W2-band data to the SDSS photometry we can increase the
photometric redshift reliability, defined as the percentage of sources with the
photometric and spectroscopic redshift difference less than 0.2, from 70.3% to
77.2%. We also obtain the samples of WISE-detected normal and late-type stars
with SDSS spectroscopy, and present a criterion in the versus
color-color diagram, , to separate quasars from stars.
With this criterion we can recover 98.6% of 3089 radio-detected SDSS-WISE
quasars with redshifts less than four and overcome the difficulty in selecting
quasars with redshifts between 2.2 and 3 from SDSS photometric data alone. We
also suggest another criterion involving the WISE color only, , to
efficiently separate quasars with redshifts less than 3.2 from stars. In
addition, we compile a catalog of 5614 SDSS quasars detected by both WISE and
UKIDSS surveys and present their color-redshift relations in the optical and
infrared bands. By using the SDSS , UKIDSS YJHK and WISE W1- and W2-band
photometric data, we can efficiently select quasar candidates and increase the
photometric redshift reliability up to 87.0%. We discuss the implications of
our results on the future quasar surveys. An updated SDSS-WISE quasar catalog
consisting of 101,853 quasars with the recently released WISE all-sky data is
also provided.Comment: 27 pages, 9 figures and 5 tables. Revised to match the published
version in the Astronomical Journal. 5 tables are available electronically at
(http://vega.bac.pku.edu.cn/~wuxb/sdsswiseqso.htm). A new SDSS-WISE quasar
catalog consisting of 101,853 quasars with the WISE all-sky data is available
as Table
Effects of siRNA targeting BMPR-II on the biological activities of human liver cancer cells and its mechanism
Activated dissociation of O2 on Pb(111) surfaces by Pb adatoms
We investigate the dissociation of O2 on Pb(111) surface using
first-principles calculations. It is found that in a practical high-vacuum
environment, the adsorption of molecular O2 takes place on clean Pb surfaces
only at low temperatures such as 100 K, but the O2 easily desorbs at (elevated)
room temperatures. It is further found that the Pb adatoms enhance the
molecular adsorption and activate the adsorbed O2 to dissociate during
subsequent room-temperature annealing. Our theory explains the observation of a
two-step oxidation process on the Pb surfaces by the unique role of Pb adatoms
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