33,649 research outputs found
Degenerate states of narrow semiconductor rings in the presence of spin orbit coupling: Role of time-reversal and large gauge transformations
The electron Hamiltonian of narrow semiconductor rings with the Rashba and
Dresselhaus spin orbit terms is invariant under time-reversal operation
followed by a large gauge transformation. We find that all the eigenstates are
doubly degenerate when integer or half-integer quantum fluxes thread the
quantum ring. The wavefunctions of a degenerate pair are related to each other
by the symmetry operation. These results are valid even in the presence of a
disorder potential. When the Zeeman term is present only some of these
degenerate levels anticross
Machine Learning Classification of SDSS Transient Survey Images
We show that multiple machine learning algorithms can match human performance
in classifying transient imaging data from the Sloan Digital Sky Survey (SDSS)
supernova survey into real objects and artefacts. This is a first step in any
transient science pipeline and is currently still done by humans, but future
surveys such as the Large Synoptic Survey Telescope (LSST) will necessitate
fully machine-enabled solutions. Using features trained from eigenimage
analysis (principal component analysis, PCA) of single-epoch g, r and
i-difference images, we can reach a completeness (recall) of 96 per cent, while
only incorrectly classifying at most 18 per cent of artefacts as real objects,
corresponding to a precision (purity) of 84 per cent. In general, random
forests performed best, followed by the k-nearest neighbour and the SkyNet
artificial neural net algorithms, compared to other methods such as na\"ive
Bayes and kernel support vector machine. Our results show that PCA-based
machine learning can match human success levels and can naturally be extended
by including multiple epochs of data, transient colours and host galaxy
information which should allow for significant further improvements, especially
at low signal-to-noise.Comment: 14 pages, 8 figures. In this version extremely minor adjustments to
the paper were made - e.g. Figure 5 is now easier to view in greyscal
Simultaneous Measurements of Microwave Photoresistance and Cyclotron Reflection in the Multi-Photon Regime
We simultaneously measure photoresistance with electrical transport and
plasmon-cyclotron resonance (PCR) using microwave reflection spectroscopy in
high mobility GaAs/AlGaAs quantum wells under a perpendicular magnetic field.
Multi-photon transitions are revealed as sharp peaks in the resistance and the
cyclotron reflection on samples with various carrier densities. Our main
finding is that plasmon coupling is relevant in the cyclotron reflection
spectrum but has not been observed in the electrical conductivity signal. We
discuss possible mechanisms relevant to reflection or dc conductivity signal to
explain this discrepancy. We further confirm a trend that higher order
multi-photon features can be observed using higher carrier density samples.Comment: 19 pages, 5 figure
Microwave photoresistance of a high-mobility two-dimensional electron gas in a triangular antidot lattice
The microwave (MW) photoresistance has been measured on a high-mobility
two-dimensional electron gas patterned with a shallow triangular antidot
lattice, where both the MW-induced resistance oscillations (MIRO) and
magnetoplasmon (MP) resonance are observed superposing on sharp commensurate
geometrical resonance (GR). Analysis shows that the MIRO, MP, and GR are
decoupled from each other in these experiments.Comment: 5 pages, 4 figures, paper accepted by PR
Photovoltaic Oscillations Due to Edge-Magnetoplasmon Modes in a Very-High Mobility 2D Electron Gas
Using very-high mobility GaAs/AlGaAs 2D electron Hall bar samples, we have
experimentally studied the photoresistance/photovoltaic oscillations induced by
microwave irradiation in the regime where both 1/B and B-periodic oscillations
can be observed. In the frequency range between 27 and 130 GHz we found that
these two types of oscillations are decoupled from each other, consistent with
the respective models that 1/B oscillations occur in bulk while the
B-oscillations occur along the edges of the Hall bars. In contrast to the
original report of this phenomenon (Ref. 1) the periodicity of the
B-oscillations in our samples are found to be independent of L, the length of
the Hall bar section between voltage measuring leads.Comment: 4 pages, 4 figure
Observation of a cyclotron harmonic spike in microwave-induced resistances in ultraclean GaAs/AlGaAs quantum wells
We report the observation of a colossal, narrow resistance peak that arises
in ultraclean (mobility 3X10^7cm^2/Vs) GaAs/AlGaAs quantum wells (QWs) under
millimeterwave irradiation and a weak magnetic field. Such a spike is
superposed on the 2nd harmonic microwave-induced resistance oscillations (MIRO)
but having an amplitude > 300% of the MIRO, and a typical FWHM ~50 mK,
comparable with the Landau level width. Systematic studies show a correlation
between the spike and a pronounced negative magnetoresistance in these QWs,
suggesting a mechanism based on the interplay of strong scatterers and smooth
disorder. Alternatively, the spike may be interpreted as a manifestation of
quantum interference between the quadrupole resonance and the higher-order
cyclotron transition in well-separated Landau levels.Comment: 4pages, 4figure
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