2,269 research outputs found
Design of an electron microscope phase plate using a focused continuous-wave laser
We propose a Zernike phase contrast electron microscope that uses an intense
laser focus to convert a phase image into a visible image. We present the
relativistic quantum theory of the phase shift caused by the
laser-electron-interaction, study resonant cavities for enhancing the laser
intensity, and discuss applications in biology, soft materials science, and
atomic and molecular physics.Comment: 5 pages, 3 figure
An assessment of the resolution limitation due to radiation-damage in x-ray diffraction microscopy
X-ray diffraction microscopy (XDM) is a new form of x-ray imaging that is
being practiced at several third-generation synchrotron-radiation x-ray
facilities. Although only five years have elapsed since the technique was first
introduced, it has made rapid progress in demonstrating high-resolution
threedimensional imaging and promises few-nm resolution with much larger
samples than can be imaged in the transmission electron microscope. Both life-
and materials-science applications of XDM are intended, and it is expected that
the principal limitation to resolution will be radiation damage for life
science and the coherent power of available x-ray sources for material science.
In this paper we address the question of the role of radiation damage. We use a
statistical analysis based on the so-called "dose fractionation theorem" of
Hegerl and Hoppe to calculate the dose needed to make an image of a lifescience
sample by XDM with a given resolution. We conclude that the needed dose scales
with the inverse fourth power of the resolution and present experimental
evidence to support this finding. To determine the maximum tolerable dose we
have assembled a number of data taken from the literature plus some
measurements of our own which cover ranges of resolution that are not well
covered by reports in the literature. The tentative conclusion of this study is
that XDM should be able to image frozen-hydrated protein samples at a
resolution of about 10 nm with "Rose-criterion" image quality.Comment: 9 pages, 4 figure
Elementary processes governing the evolution of road networks
Urbanisation is a fundamental phenomenon whose quantitative characterisation
is still inadequate. We report here the empirical analysis of a unique data set
regarding almost 200 years of evolution of the road network in a large area
located north of Milan (Italy). We find that urbanisation is characterised by
the homogenisation of cell shapes, and by the stability throughout time of
high-centrality roads which constitute the backbone of the urban structure,
confirming the importance of historical paths. We show quantitatively that the
growth of the network is governed by two elementary processes: (i)
`densification', corresponding to an increase in the local density of roads
around existing urban centres and (ii) `exploration', whereby new roads trigger
the spatial evolution of the urbanisation front. The empirical identification
of such simple elementary mechanisms suggests the existence of general, simple
properties of urbanisation and opens new directions for its modelling and
quantitative description.Comment: 10 pages, 6 figure
Government institutions and the dynamics of urban growth in China
Economic growth in China in recent decades has largely rested on the dynamism of its cities. High economic growth has coincided with measures aimed at improving the efficiency of local governments and with a mounting political drive to curb corruption. Yet the connection between government institutions and urban growth in China remains poorly understood. This paper is the first to look into the link between government efficiency and corruption, on the one hand, and urban growth in China, on the other hand and to assess what is the role of institutions relative to more traditional factors for economic growth in Chinese cities. Using panel data for 283 cities over the period between 2003 and 2014, the results show that the urban growth in China is a consequence of a combination of favorable human capital, innovation, density, local conditions, foreign direct investment, and city-level government institutions. Both government qualityâespecially for those cities with the best governmentsâand the fight against corruption at the city level have a direct effect on urban growth. Measures to tackle corruption at the provincial level matter in a more indirect way, by raising or lowering the returns of other growth-inducing factors
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