5,575 research outputs found
On finite abelian groups realizable as Mislin genera
We study the realizability of finite abelian groups as Mislin genera of finitely generated nilpotent groups with finite commutator subgroup. In particular, we give criteria to decide whether a finite abelian group is realizable as the Mislin genus of a direct product of nilpotent groups of a certain specified type. In the case of a positive answer, we also give an effective way of realizing that abelian group as a genus. Further, we obtain some non-realizability results
Softening of the insulating phase near Tc for the photo-induced insulator-to-metal phase transition in vanadium dioxide
We use optical-pump terahertz-probe spectroscopy to investigate the
near-threshold behavior of the photoinduced insulator-to-metal (IM) transition
in vanadium dioxide thin films. Upon approaching Tc a reduction in the fluence
required to drive the IM transition is observed, consistent with a softening of
the insulating state due to an increasing metallic volume fraction (below the
percolation limit). This phase coexistence facilitates the growth of a
homogeneous metallic conducting phase following superheating via
photoexcitation. A simple dynamic model using Bruggeman effective medium theory
describes the observed initial condition sensitivity.Comment: accepted for publication in Physical Review Letter
Possible explanations for different surface quality in laser cutting with 1 micron and 10 microns beams
In laserăcuttingăofăthickăsteelăsheets,ăqualityădifferenceăisăobservedăbetweenăcutăsurfacesăobtainedăwith 1 micron and 10 micron laser beams. This paper investigates physical mechanisms for this interesting and important problem of the wavelength dependence. First, striation generation process is described, based on a 3D structure of melt flow on a kerf front, which was revealed for the first time by our recent experimental observations. Two fundamental processes are suggested to explain the difference in the cut surface quality: destabilization of the melt flow in the central part of the kerf front and downward displacement of discrete melt accumulations along the side parts of the front. Then each of the processes is analyzed using a simplified analytical model. The results show that in both processes, different angular dependence of the absorptivity of the laser beam can result in the quality difference. Finally we propose use of radial polarization to improve the quality with the 1 micron wavelength
The nuclear scissors mode within two approaches (Wigner function moments versus RPA)
Two complementary methods to describe the collective motion, RPA and Wigner
function moments method, are compared on an example of a simple model -
harmonic oscillator with quadrupole-quadrupole residual interaction. It is
shown that they give identical formulae for eigenfrequencies and transition
probabilities of all collective excitations of the model including the scissors
mode, which here is the subject of our special attention. The exact relation
between the variables of the two methods and the respective dynamical equations
is established. The normalization factor of the "synthetic" scissors state and
its overlap with physical states are calculated analytically. The orthogonality
of the spurious state to all physical states is proved rigorously.Comment: 39 page
Spin injection from perpendicular magnetized ferromagnetic -MnGa into (Al,Ga)As heterostructures
Electrical spin injection from ferromagnetic -MnGa into an (Al,Ga)As
p-i-n light emitting diode (LED) is demonstrated. The -MnGa layers show
strong perpendicular magnetocrystalline anisotropy, enabling detection of spin
injection at remanence without an applied magnetic field. The bias and
temperature dependence of the spin injection are found to be qualitatively
similar to Fe-based spin LED devices. A Hanle effect is observed and
demonstrates complete depolarization of spins in the semiconductor in a
transverse magnetic field.Comment: 4 pages, 3 figure
Corrugated Silicon Platelet Feed Horn Array for CMB Polarimetry at 150 GHz
Next generation cosmic microwave background (CMB) polarization anisotropy
measurements will feature focal plane arrays with more than 600 millimeter-wave
detectors. We make use of high-resolution photolithography and wafer-scale etch
tools to build planar arrays of corrugated platelet feeds in silicon with
highly symmetric beams, low cross-polarization and low side lobes. A compact
Au-plated corrugated Si feed designed for 150 GHz operation exhibited
performance equivalent to that of electroformed feeds: ~-0.2 dB insertion loss,
<-20 dB return loss from 120 GHz to 170 GHz, <-25 dB side lobes and <-23 dB
cross-polarization. We are currently fabricating a 50 mm diameter array with 84
horns consisting of 33 Si platelets as a prototype for the SPTpol and ACTpol
telescopes. Our fabrication facilities permit arrays up to 150 mm in diameter.Comment: 12 pages; SPIE proceedings for Millimeter, Submillimeter, and
Far-Infrared Detectors and Instrumentation for Astronomy V (Conference 7741,
June 2010, San Diego, CA, USA
Comparison of Zn_{1-x}Mn_xTe/ZnTe multiple-quantum wells and quantum dots by below-bandgap photomodulated reflectivity
Large-area high density patterns of quantum dots with a diameter of 200 nm
have been prepared from a series of four Zn_{0.93}Mn_{0.07}Te/ZnTe multiple
quantum well structures of different well width (4 nm, 6 nm, 8 nm and 10 nm) by
electron beam lithography followed by Ar+ ion beam etching. Below-bandgap
photomodulated reflectivity spectra of the quantum dot samples and the parent
heterostructures were then recorded at 10 K and the spectra were fitted to
extract the linewidths and the energy positions of the excitonic transitions in
each sample. The fitted results are compared to calculations of the transition
energies in which the different strain states in the samples are taken into
account. We show that the main effect of the nanofabrication process is a
change in the strain state of the quantum dot samples compared to the parent
heterostructures. The quantum dot pillars turn out to be freestanding, whereas
the heterostructures are in a good approximation strained to the ZnTe lattice
constant. The lateral size of the dots is such that extra confinement effects
are not expected or observed.Comment: 23 pages, LaTeX2e (amsmath, epsfig), 7 EPS figure
The Enriched Element for Finite Element Analysis of Three-Dimensional Elastic Crack Problems
An improved procedure for enriching 'three-dimensional Introduction Finite element procedures have been applied to the analysis of crack problems with varying degrees of success for more than a decade. Experience and expertise was first developed for the simplest case, the set of two-dimensional elastic crack problems. It was found that in order to obtain accurate results with reasonable computer costs the finite element procedure must be specialized to crack problems by explicit incorporation of the asymptotic crack tip solution into the finite element formulation. A number of techniques for handling crack problems (known as specialized elements) have subsequently been developed. These are reviewed in reference More recently, finite element procedures are being applied to analyze two-dimensional elastic-plastic crack problems and three-dimensional elastic crack problems. These calculations involve use of extensions of the two-dimensional, elastic crack tip elements to account for three-dimensional and elasticplastic influences. The enriched element developed by Benzley [2,3] has proven particularly effective for the analysis of twodimensional elastic crack problems. It has been extended to three dimensions by the present author
What young people want from health-related online resources: a focus group study
The growth of the Internet as an information source about health, particularly amongst young people, is well established. The aim of this study was to explore young people's perceptions and experiences of engaging with health-related online content, particularly through social media websites. Between February and July 2011 nine focus groups were facilitated across Scotland with young people aged between 14 and 18 years. Health-related user-generated content seems to be appreciated by young people as a useful, if not always trustworthy, source of accounts of other people's experiences. The reliability and quality of both user-generated content and official factual content about health appear to be concerns for young people, and they employ specialised strategies for negotiating both areas of the online environment. Young people's engagement with health online is a dynamic area for research. Their perceptions and experiences of health-related content seem based on their wider familiarity with the online environment and, as the online environment develops, so too do young people's strategies and conventions for accessing it
- âŚ