5,938 research outputs found
Detection of electronic nematicity using scanning tunneling microscopy
Electronic nematic phases have been proposed to occur in various correlated
electron systems and were recently claimed to have been detected in scanning
tunneling microscopy (STM) conductance maps of the pseudogap states of the
cuprate high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212). We
investigate the influence of anisotropic STM tip structures on such
measurements and establish, with a model calculation, the presence of a
tunneling interference effect within an STM junction that induces
energy-dependent symmetry-breaking features in the conductance maps. We
experimentally confirm this phenomenon on different correlated electron
systems, including measurements in the pseudogap state of Bi-2212, showing that
the apparent nematic behavior of the imaged crystal lattice is likely not due
to nematic order but is related to how a realistic STM tip probes the band
structure of a material. We further establish that this interference effect can
be used as a sensitive probe of changes in the momentum structure of the
sample's quasiparticles as a function of energy.Comment: Accepted for publication (PRB - Rapid Communications). Main text (5
pages, 4 figures) + Supplemental Material (4 pages, 4 figures
Flight test of the X-29A at high angle of attack: Flight dynamics and controls
The NASA Dryden Flight Research Center has flight tested two X-29A aircraft at low and high angles of attack. The high-angle-of-attack tests evaluate the feasibility of integrated X-29A technologies. More specific objectives focus on evaluating the high-angle-of-attack flying qualities, defining multiaxis controllability limits, and determining the maximum pitch-pointing capability. A pilot-selectable gain system allows examination of tradeoffs in airplane stability and maneuverability. Basic fighter maneuvers provide qualitative evaluation. Bank angle captures permit qualitative data analysis. This paper discusses the design goals and approach for high-angle-of-attack control laws and provides results from the envelope expansion and handling qualities testing at intermediate angles of attack. Comparisons of the flight test results to the predictions are made where appropriate. The pitch rate command structure of the longitudinal control system is shown to be a valid design for high-angle-of-attack control laws. Flight test results show that wing rock amplitude was overpredicted and aileron and rudder effectiveness were underpredicted. Flight tests show the X-29A airplane to be a good aircraft up to 40 deg angle of attack
The AGN Population in X-ray Selected Galaxy Groups at
We use Chandra data to study the incidence and properties of Active Galactic
Nuclei (AGN) in 16 intermediate redshift () X-ray-selected
galaxy groups in the Chandra Deep Field-South. We measure an AGN fraction of
at ,
approximately a factor of two higher than the AGN fraction found for rich
clusters at comparable redshift. This extends the trend found at low redshift
for groups to have higher AGN fractions than clusters. Our estimate of the AGN
fraction is also more than a factor of 3 higher than that of low redshift
X-ray-selected groups. Using optical spectra from various surveys, we also
constrain the properties of emission-line selected AGN in these groups.
Contrary to the large population of X-ray AGN ( erg/s) =
25), we find only 4 emission-line AGN, 3 of which are also X-ray bright.
Furthermore, most of the X-ray AGN in our groups are optically-dull (i.e. lack
strong emission-lines) similar to those found in low redshift X-ray groups and
clusters of galaxies. This contrasts with the AGN population found in low
redshift optically-selected groups which are dominated by emission-line AGN.
The differences between the optically and X-ray-selected AGN populations in
groups are consistent with a scenario where most AGN in the densest
environments are currently in a low accretion state.Comment: 8 pages, 4 figures, accepted for publication in Ap
Meta-Learning Probabilistic Inference For Prediction
This paper introduces a new framework for data efficient and versatile
learning. Specifically: 1) We develop ML-PIP, a general framework for
Meta-Learning approximate Probabilistic Inference for Prediction. ML-PIP
extends existing probabilistic interpretations of meta-learning to cover a
broad class of methods. 2) We introduce VERSA, an instance of the framework
employing a flexible and versatile amortization network that takes few-shot
learning datasets as inputs, with arbitrary numbers of shots, and outputs a
distribution over task-specific parameters in a single forward pass. VERSA
substitutes optimization at test time with forward passes through inference
networks, amortizing the cost of inference and relieving the need for second
derivatives during training. 3) We evaluate VERSA on benchmark datasets where
the method sets new state-of-the-art results, handles arbitrary numbers of
shots, and for classification, arbitrary numbers of classes at train and test
time. The power of the approach is then demonstrated through a challenging
few-shot ShapeNet view reconstruction task
Discrete Holomorphicity at Two-Dimensional Critical Points
After a brief review of the historical role of analyticity in the study of
critical phenomena, an account is given of recent discoveries of discretely
holomorphic observables in critical two-dimensional lattice models. These are
objects whose correlation functions satisfy a discrete version of the
Cauchy-Riemann relations. Their existence appears to have a deep relation with
the integrability of the model, and they are presumably the lattice versions of
the truly holomorphic observables appearing in the conformal field theory (CFT)
describing the continuum limit. This hypothesis sheds light on the connection
between CFT and integrability, and, if verified, can also be used to prove that
the scaling limit of certain discrete curves in these models is described by
Schramm-Loewner evolution (SLE).Comment: Invited talk at the 100th Statistical Mechanics Meeting, Rutgers,
December 200
- …