14,216 research outputs found
Efficient AUC Optimization for Information Ranking Applications
Adequate evaluation of an information retrieval system to estimate future
performance is a crucial task. Area under the ROC curve (AUC) is widely used to
evaluate the generalization of a retrieval system. However, the objective
function optimized in many retrieval systems is the error rate and not the AUC
value. This paper provides an efficient and effective non-linear approach to
optimize AUC using additive regression trees, with a special emphasis on the
use of multi-class AUC (MAUC) because multiple relevance levels are widely used
in many ranking applications. Compared to a conventional linear approach, the
performance of the non-linear approach is comparable on binary-relevance
benchmark datasets and is better on multi-relevance benchmark datasets.Comment: 12 page
Kondo effect in complex mesoscopic structures
We study the Kondo effect of a quantum dot placed in a complex mesoscopic
structure. Assuming that electronic interactions are taking place solely on the
dot, and focusing on the infinite Hubbard interaction limit, we use a
decoupling scheme to obtain an explicit analytic approximate expression for the
dot Green function, which fulfills certain Fermi-liquid relations at zero
temperature. The details of the complex structure enter into this expression
only via the self-energy for the non-interacting case. The effectiveness of the
expression is demonstrated for the single impurity Anderson model and for the
T-shaped network.Comment: 12 pages 6 figure
Coherent laser control of the current through molecular junctions
The electron tunneling through a molecular junction modeled by a single site
weakly coupled to two leads is studied in the presence of a time-dependent
external field using a master equation approach. In the case of small bias
voltages and high carrier frequencies of the external field, we observe the
phenomenon of coherent destruction of tunneling, i.e. the current through the
molecular junction vanishes completely for certain parameters of the external
field. In previous studies the tunneling within isolated and open multi-site
systems was suppressed; it is shown here that the tunneling between a single
site and electronic reservoirs, i.e. the leads, can be suppressed as well. For
larger bias voltages the current does not vanish any more since further
tunneling channels participate in the electron conduction and we also observe
photon-assisted tunneling which leads to steps in the current-voltage
characteristics. The described phenomena are demonstrated not only for
monochromatic fields but also for laser pulses and therefore could be used for
ultrafast optical switching of the current through molecular junctions.Comment: 6 pages and 4 figure
Combined fit to BaBar and Belle data on e+e- to pi+pi- psi(2S)
A combined fit is performed to the BaBar and Belle measurements of the e+e-
to pi+pi-psi(2S) cross sections for center-of-mass energy between threshold and
5.5 GeV. The resonant parameters of the Y(4360) and Y(4660) are determined. The
mass is 4355^{+9}_{-10}\pm 9 MeV/c^2 and the width is 103^{+17}_{-15}\pm 11
MeV/c^2 for the Y(4360), and the mass is 4661^{+9}_{-8}\pm 6 MeV/c^2 and the
width is 42^{+17}_{-12}\pm 6 MeV/c^2 for the Y(4660). The production of the
Y(4260) in pi+pi-psi(2S) mode is found to be at 2\sigma level, and B(Y(4260) to
pi+pi-psi(2S))\Gamma_{e+e-} is found to be less than 4.3 eV/c^2 at the 90%
confidence level, or equal to 7.4^{+2.1}_{-1.7} eV/c^2 depending on it
interferes with the Y(4360) constructively or destructively. These information
will shed light on the understanding of the nature of the Y states observed in
initial state radiation processes.Comment: 8 pages, 4 figure
Plaquette order and deconfined quantum critical point in the spin-1 bilinear-biquadratic Heisenberg model on the honeycomb lattice
We have precisely determined the ground state phase diagram of the quantum
spin-1 bilinear-biquadratic Heisenberg model on the honeycomb lattice using the
tensor renormalization group method. We find that the ferromagnetic,
ferroquadrupolar, and a large part of the antiferromagnetic phases are stable
against quantum fluctuations. However, around the phase where the ground state
is antiferroquadrupolar ordered in the classical limit, quantum fluctuations
suppress completely all magnetic orders, leading to a plaquette order phase
which breaks the lattice symmetry but preserves the spin SU(2) symmetry. On the
evidence of our numerical results, the quantum phase transition between the
antiferromagnetic phase and the plaquette phase is found to be either a direct
second order or a very weak first order transition.Comment: 6 pages, 9 figures, published versio
Submillimeter continuum observations of Sagittarius B2 at subarcsecond spatial resolution
We report the first high spatial resolution submillimeter continuum
observations of the Sagittarius B2 cloud complex using the Submillimeter Array
(SMA). With the subarcsecond resolution provided by the SMA, the two massive
star-forming clumps Sgr B2(N) and Sgr B2(M) are resolved into multiple compact
sources. In total, twelve submillimeter cores are identified in the Sgr B2(M)
region, while only two components are observed in the Sgr B2(N) clump. The gas
mass and column density are estimated from the dust continuum emission. We find
that most of the cores have gas masses in excess of 100 M and column
densities above 10 cm. The very fragmented appearance of Sgr
B2(M), in contrast to the monolithic structure of Sgr B2 (N), suggests that the
former is more evolved. The density profile of the Sgr B2(N)-SMA1 core is well
fitted by a Plummer density distribution. This would lead one to believe that
in the evolutionary sequence of the Sgr B2 cloud complex, a massive star forms
first in an homogeneous core, and the rest of the cluster forms subsequently in
the then fragmenting structure.Comment: 4 pages, 2 figures, accepted by A&A letter
Two-dimensional structures of ferroelectric domain inversion in LiNbO3 by direct electron beam lithography
We report on the fabrication of domain-reversed structures in LiNbO3 by means
of direct electron beam lithography at room temperature without any static
bias. The LiNbO3 crystals were chemically etched after the exposure of electron
beam and then, the patterns of domain inversion were characterized by atomic
force microscopy (AFM). In our experiment, an interesting phenomenon occurred
when the electron beam wrote a one-dimensional (1-D) grating on the negative
c-face: a two-dimensional (2-D) dotted array was observed on the positive c-
face, which is significant for its potential to produce 2-D and
three-dimensional photonic crystals. Furthermore, we also obtained 2-D
ferroelectric domain inversion in the whole LiNbO3 crystal by writing the 2-D
square pattern on the negative c-face. Such a structure may be utilized to
fabricate 2-D nonlinear photonic crystal. AFM demonstrates that a 2-D
domain-reversed structure has been achieved not only on the negative c-face of
the crystal, but also across the whole thickness of the crystal.Comment: 17 pages, 4 figure
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