306 research outputs found
Atomic-scale analysis of disorder by similarity learning from tunneling spectroscopy
Rapid proliferation of hyperspectral imaging in scanning probe microscopies
creates unique opportunities to systematically capture and categorize higher
dimensional datasets, toward new insights into electronic, mechanical and
chemical properties of materials with nano- and atomic-scale resolution. Here
we demonstrate similarity learning for tunneling spectroscopy acquired on
superconducting material (FeSe) with sparse density of imperfections (Fe
vacancies). Popular methods for unsupervised learning and discrete
representation of the data in terms of clusters of characteristic behaviors
were found to produce inconsistencies with respect to capturing the location
and tunneling characteristics of the vacancy sites. To this end, we applied a
more general, non-linear similarity learning. This approach was found to
outperform several widely used methods for dimensionality reduction and produce
a clear differentiation of the type of tunneling spectra. In particular,
significant spectral weight transfer likely associated with the electronic
reconstruction by the vacancy sites, is systematically captured, as is the
spatial extent of the vacancy region. Given that a great variety of electronic
materials will exhibit similarly smooth variation of the spectral responses due
to random or engineered inhomogeneities in their structure, we believe our
approach will be useful for systematic analysis of hyperspectral imaging with
minimal prior knowledge, as well as prospective comparison of experimental
measurements to theoretical calculations with explicit consideration of
disorder
Many-body effects in nonlinear optical responses of 2D layered semiconductors
We performed ultrafast degenerate pump-probe spectroscopy on monolayer WSe2
near its exciton resonance. The observed differential reflectance signals
exhibit signatures of strong many-body interactions including the
exciton-exciton interaction and free carrier induced band gap renormalization.
The exciton-exciton interaction results in a resonance blue shift which lasts
for the exciton lifetime (several ps), while the band gap renormalization
manifests as a resonance red shift with several tens ps lifetime. Our model
based on the many-body interactions for the nonlinear optical susceptibility
fits well the experimental observations. The power dependence of the spectra
shows that with the increase of pump power, the exciton population increases
linearly and then saturates, while the free carrier density increases
superlinearly, implying that exciton Auger recombination could be the origin of
these free carriers. Our model demonstrates a simple but efficient method for
quantitatively analyzing the spectra, and indicates the important role of
Coulomb interactions in nonlinear optical responses of such 2D materials
Spatial popularity and similarity of watching videos in a large city
With the popularity of watching mobile videos, many works focus on the geographic features of user viewing behaviors, but few study them in the context of an entire metropolitan city. Different regions of a large city have different intensity of economy activities with respect to their different distances to the downtown, and how this will influence video popularity and similarity is still unclear. To quantitatively study the spatial popularity and similarity of watching videos in a large urban environment, we collect a dataset with two-month video view requests from the largest network provider in Shanghai, containing top six content providers, and study the spatial features of video access in regions of different scales. We find that 1) video popularity and similarity exist at different scales of city division; 2) the concentration of video popularity becomes higher as the region is closer to downtown; 3) when comparing the regions of same scale, the similarity of popular videos becomes lower as the region is farther away from the downtown. Finally, we correlate our findings with cache deployment, advertising and video recommendation to illustrate the implication
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