26,788 research outputs found
Photonic realization of topologically protected bound states in domain-wall waveguide arrays
We present an analytical theory of topologically protected photonic states
for the two-dimensional Maxwell equations for a class of continuous periodic
dielectric structures, modulated by a domain wall. We further numerically
confirm the applicability of this theory for three-dimensional structures.Comment: 6 pages, 5 figures. To appear in the Phys. Rev.
Effects of rapid thermal annealing on device characteristics of InGaAs/GaAs quantum dot infrared photodetectors
In this work, rapid thermal annealing was performed on InGaAs/GaAs quantum dot infrared photodetectors (QDIPs) at different temperatures. The photoluminescence showed a blueshifted spectrum in comparison with the as-grown sample when the annealing temperature was higher than 700 °C, as a result of thermal interdiffusion of the quantum dots (QDs). Correspondingly, the spectral response from the annealed QDIP exhibited a redshift. At the higher annealing temperature of 800 °C, in addition to the largely redshifted photoresponse peak of 7.4 µm (compared with the 6.1 µm of the as-grown QDIP), a high energy peak at 5.6 µm (220 meV) was also observed, leading to a broad spectrum linewidth of 40%. This is due to the large interdiffusion effect which could greatly vary the composition of the QDs and thus increase the relative optical absorption intensity at higher energy. The other important detector characteristics such as dark current, peak responsivity, and detectivity were also measured. It was found that the overall device performance was not affected by low annealing temperature, however, for high annealing temperature, some degradation in device detectivity (but not responsivity) was observed. This is a consequence of increased dark current due to defect formation and increased ground state energy. © 2006 American Institute of Physic
Radio Galaxy Zoo: Knowledge Transfer Using Rotationally Invariant Self-Organising Maps
With the advent of large scale surveys the manual analysis and classification
of individual radio source morphologies is rendered impossible as existing
approaches do not scale. The analysis of complex morphological features in the
spatial domain is a particularly important task. Here we discuss the challenges
of transferring crowdsourced labels obtained from the Radio Galaxy Zoo project
and introduce a proper transfer mechanism via quantile random forest
regression. By using parallelized rotation and flipping invariant Kohonen-maps,
image cubes of Radio Galaxy Zoo selected galaxies formed from the FIRST radio
continuum and WISE infrared all sky surveys are first projected down to a
two-dimensional embedding in an unsupervised way. This embedding can be seen as
a discretised space of shapes with the coordinates reflecting morphological
features as expressed by the automatically derived prototypes. We find that
these prototypes have reconstructed physically meaningful processes across two
channel images at radio and infrared wavelengths in an unsupervised manner. In
the second step, images are compared with those prototypes to create a
heat-map, which is the morphological fingerprint of each object and the basis
for transferring the user generated labels. These heat-maps have reduced the
feature space by a factor of 248 and are able to be used as the basis for
subsequent ML methods. Using an ensemble of decision trees we achieve upwards
of 85.7% and 80.7% accuracy when predicting the number of components and peaks
in an image, respectively, using these heat-maps. We also question the
currently used discrete classification schema and introduce a continuous scale
that better reflects the uncertainty in transition between two classes, caused
by sensitivity and resolution limits
Coulomb Drag at the Onset of Anderson Insulators
It is shown that the Coulomb drag between two identical layers in the
Anderson insulting state indicates a striking difference between the Mott and
Efros-Shklovskii (ES) insulators. In the former, the trans-resistance
is monotonically increasing with the localization length ; in the latter,
the presence of a Coulomb gap leads to an opposite result: is enhanced
with a decreasing , with the same exponential factor as the single layer
resistivity. This distinction reflects the relatively pronounced role of
excited density fluctuations in the ES state, implied by the enhancement in the
rate of hopping processes at low frequencies. The magnitude of drag is
estimated for typical experimental parameters in the different cases. It is
concluded that a measurement of drag can be used to distinguish between
interacting and non-interacting insulating state.Comment: 15 pages, revte
A different view of the quantum Hall plateau-to-plateau transitions
We demonstrate experimentally that the transitions between adjacent integer
quantum Hall (QH) states are equivalent to a QH-to-insulator transition
occurring in the top Landau level, in the presence of an inert background of
the other completely filled Landau levels, each contributing a single unit of
quantum conductance, , to the total Hall conductance of the system.Comment: 10 pages, 4 figures, Revtex 3.
Spin-Electromagnetic Hydrodynamics and Magnetization Induced by Spin-Magnetic Interaction
The hydrodynamic model including the spin degree of freedom and the
electromagnetic field was discussed. In this derivation, we applied
electromagnetism for macroscopic medium proposed by Minkowski. For the equation
of motion of spin, we assumed that the hydrodynamic representation of the Pauli
equation is reproduced when the many-body effect is neglected. Then the
spin-magnetic interaction in the Pauli equation was converted to a part of the
magnetization. The fluid and spin stress tensors induced by the many-body
effect were obtained by employing the algebraic positivity of the entropy
production in the framework of the linear irreversible thermodynamics,
including the mixing effect of the irreversible currents. We further
constructed the constitutive equation of the polarization and the
magnetization. Our polarization equation is more reasonable compared to another
result obtained using electromagnetism for macroscopic medium proposed by de
Groot-Mazur.Comment: 24 pages, no figure, the discussion for the modifed thermodynamic
relation is added, several errors are corrected, accepted for publication in
PR
Theory for a Hanbury Brown Twiss experiment with a ballistically expanding cloud of cold atoms
We have studied one-body and two-body correlation functions in a
ballistically expanding, non-interacting atomic cloud in the presence of
gravity. We find that the correlation functions are equivalent to those at
thermal equilibrium in the trap with an appropriate rescaling of the
coordinates. We derive simple expressions for the correlation lengths and give
some physical interpretations. Finally a simple model to take into account
finite detector resolution is discussed
Transmission Through Carbon Nanotubes With Polyhedral Caps
We study electron transport between capped carbon nanotubes and a substrate,
and relate the transmission probability to the local density of states in the
cap. Our results show that the transmission probability mimics the behavior of
the density of states at all energies except those that correspond to localized
states in the cap. Close proximity of a substrate causes hybridization of the
localized state. As a result, new transmission paths open from the substrate to
nanotube continuum states via the localized states in the cap. Interference
between various transmission paths gives rise to antiresonances in the
transmission probability, with the minimum transmission equal to zero at
energies of the localized states. Defects in the nanotube that are placed close
to the cap cause resonances in the transmission probability, instead of
antiresonances, near the localized energy levels. Depending on the spatial
position of defects, these resonant states are capable of carrying a large
current. These results are relevant to carbon nanotube based studies of
molecular electronics and probe tip applications
Disentangling the Imaginary-Time Formalism at Finite Temperature
We rewrite the imaginary-time formalism of finite temperature field theory in
a form that all graphs used in calculating physical processes do not have any
loops. Any production of a particle from a heat bath which is itself not
thermalized or the decay and absorption of a similar particle in the bath is
expressed entirely in terms of the sum of particle interaction processes. These
are themselves very general in meaning. They can be straight forward
interactions or the more subtle and less well-known purely interference
processes that do not have a counter part in the vacuum.Comment: 14 pages revtex style, 20 embedded EPS figures, added discussion of
the connection with the real-time formalism + reference
Exact Conductance through Point Contacts in the Fractional Quantum Hall Effect
The conductance for tunneling through a point contact between two
quantum Hall edges is described by a universal scaling function, which has
recently been measured experimentally. We compute this universal function
exactly, by using the thermodynamic Bethe ansatz and a Boltzmann equation.Comment: 10 pages, 1 figur
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