3,547 research outputs found
Signatures of electronic polarons in LaSrMnO observed by electron energy-loss spectroscopy
The dielectric properties of LaSrMnO single crystals with
x = 0, 0.125, 0.25, and 0.5 were studied by means of electron energy-loss
spectroscopy as a function of temperature and momentum transfer. A clear
signature of the doped holes is observed around 1.65 eV energy loss, where
spectral weight emerges with increasing x. For all , this
doping-induced excitation can propagate within the ab-plane, as revealed by a
clear upward dispersion of the corresponding loss peak with increasing momentum
transfer. The hole-induced excitation also shifts to higher energies with the
onset of magnetic correlations for x = 0.5, implying a strong coupling of
charge and spin dynamics. We conclude that (i) the loss feature at 1.65 eV is a
signature of electronic polarons, which are created around doped holes, and
that (ii) this low-energy excitation involves the charge transfer between
manganese and oxygen. The finite dispersion of these excitations further
indicates significant polaron-polaron interactions.Comment: 7 pages, 4 figure
Softly broken supersymmetric Yang-Mills theories: Renormalization and non-renormalization theorems
We present a minimal version for the renormalization of softly broken
Super-Yang-Mills theories using the extended model with a local gauge coupling.
It is shown that the non-renormalization theorems of the case with unbroken
supersymmetry are valid without modifications and that the renormalization of
soft-breaking parameters is completely governed by the renormalization of the
supersymmetric parameters. The symmetry identities in the present context are
peculiar, since the extended model contains two anomalies: the Adler-Bardeen
anomaly of the axial current and an anomaly of supersymmetry in the presence of
the local gauge coupling. From the anomalous symmetries we derive the exact
all-order expressions for the beta functions of the gauge coupling and of the
soft-breaking parameters. They generalize earlier results to arbitrary
normalization conditions and imply the NSVZ expressions for a specific
normalization condition on the coupling.Comment: 24 pages, LaTeX, v2: one reference adde
First results of a cryogenic optical photon counting imaging spectrometer using a DROID array
Context. In this paper we present the first system test in which we
demonstrate the concept of using an array of Distributed Read Out Imaging
Devices (DROIDs) for optical photon detection. Aims. After the successful S-Cam
3 detector the next step in the development of a cryogenic optical photon
counting imaging spectrometer under the S-Cam project is to increase the field
of view using DROIDs. With this modification the field of view of the camera
has been increased by a factor of 5 in area, while keeping the number of
readout channels the same. Methods. The test has been performed using the
flexible S-Cam 3 system and exchanging the 10x12 Superconducting Tunnel
Junction array for a 3x20 DROID array. The extra data reduction needed with
DROIDs is performed offline. Results. We show that, although the responsivity
(number of tunnelled quasiparticles per unit of absorbed photon energy, e- /eV)
of the current array is too low for direct astronomical applications, the
imaging quality is already good enough for pattern detection, and will improve
further with increasing responsivity. Conclusions. The obtained knowledge can
be used to optimise the system for the use of DROIDs.Comment: 7 pages, 9 figures, accepted for publicaiton in A&
Generic Modal Cut Elimination Applied to Conditional Logics
We develop a general criterion for cut elimination in sequent calculi for
propositional modal logics, which rests on absorption of cut, contraction,
weakening and inversion by the purely modal part of the rule system. Our
criterion applies also to a wide variety of logics outside the realm of normal
modal logic. We give extensive example instantiations of our framework to
various conditional logics. For these, we obtain fully internalised calculi
which are substantially simpler than those known in the literature, along with
leaner proofs of cut elimination and complexity. In one case, conditional logic
with modus ponens and conditional excluded middle, cut elimination and
complexity were explicitly stated as open in the literature
Black holes in three dimensional higher spin gravity: A review
We review recent progress in the construction of black holes in three
dimensional higher spin gravity theories. Starting from spin-3 gravity and
working our way toward the theory of an infinite tower of higher spins coupled
to matter, we show how to harness higher spin gauge invariance to consistently
generalize familiar notions of black holes. We review the construction of black
holes with conserved higher spin charges and the computation of their partition
functions to leading asymptotic order. In view of the AdS/CFT correspondence as
applied to certain vector-like conformal field theories with extended conformal
symmetry, we successfully compare to CFT calculations in a generalized Cardy
regime. A brief recollection of pertinent aspects of ordinary gravity is also
given.Comment: 49 pages, harvmac, invited contribution to J. Phys. A special volume
on "Higher Spin Theories and AdS/CFT" edited by M. R. Gaberdiel and M.
Vasilie
Microtesla MRI of the human brain combined with MEG
One of the challenges in functional brain imaging is integration of
complementary imaging modalities, such as magnetoencephalography (MEG) and
functional magnetic resonance imaging (fMRI). MEG, which uses highly sensitive
superconducting quantum interference devices (SQUIDs) to directly measure
magnetic fields of neuronal currents, cannot be combined with conventional
high-field MRI in a single instrument. Indirect matching of MEG and MRI data
leads to significant co-registration errors. A recently proposed imaging method
- SQUID-based microtesla MRI - can be naturally combined with MEG in the same
system to directly provide structural maps for MEG-localized sources. It
enables easy and accurate integration of MEG and MRI/fMRI, because microtesla
MR images can be precisely matched to structural images provided by high-field
MRI and other techniques. Here we report the first images of the human brain by
microtesla MRI, together with auditory MEG (functional) data, recorded using
the same seven-channel SQUID system during the same imaging session. The images
were acquired at 46 microtesla measurement field with pre-polarization at 30
mT. We also estimated transverse relaxation times for different tissues at
microtesla fields. Our results demonstrate feasibility and potential of human
brain imaging by microtesla MRI. They also show that two new types of imaging
equipment - low-cost systems for anatomical MRI of the human brain at
microtesla fields, and more advanced instruments for combined functional (MEG)
and structural (microtesla MRI) brain imaging - are practical.Comment: 8 pages, 5 figures - accepted by JM
Complex Visibilities of Cosmic Microwave Background Anisotropies
We study the complex visibilities of the cosmic microwave background
anisotropies that are observables in interferometric observations of the cosmic
microwave background, using the multipole expansion methods commonly adopted in
analyzing single-dish experiments. This allows us to recover the properties of
the visibilities that is obscured in the flat-sky approximation. Discussions of
the window function, multipole resolution, instrumental noise, pixelization,
and polarization are given.Comment: 22 pages, 1 figure include
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