1,456 research outputs found
The enigma of the pseudogap phase of the cuprate superconductors
The last few years have seen significant experimental progress in
characterizing the copper-based hole-doped high temperature superconductors in
the regime of low hole density, p. Quantum oscillations, NMR, X-ray, and STM
experiments have shed much light on the nature of the ordering at low
temperatures. We review evidence that the order parameter in the
non-Lanthanum-based cuprates is a d-form factor density-wave. This novel order
acts as an unexpected window into the electronic structure of the pseudogap
phase at higher temperatures in zero field: we argue in favor of a
`fractionalized Fermi liquid' (FL*) with 4 pockets of spin S=1/2, charge +e
fermions enclosing an area specified by p.Comment: 37 pages, 19 figures, 100+ references. Proceedings of the 50th
Karpacz Winter School of Theoretical Physics, 2-9 March 2014, Karpacz, Polan
Higgs criticality in a two-dimensional metal
We analyze a candidate theory for the strange metal near optimal hole-doping
in the cuprate superconductors. The theory contains a quantum phase transition
between metals with large and small Fermi surfaces of spinless fermions
carrying the electromagnetic charge of the electron, but the transition does
not directly involve any broken global symmetries. The two metals have emergent
SU(2) and U(1) gauge fields respectively, and the transition is driven by the
condensation of a real Higgs field, carrying a finite lattice momentum and an
adjoint SU(2) gauge charge. This Higgs field measures the local
antiferromagnetic correlations in a "rotating reference frame". We propose a
global phase diagram around this Higgs transition, and describe its
relationship to a variety of recent experiments on the cuprate superconductors.Comment: 30 pages, 7 figures; (v2) added new figur
Quantum butterfly effect in weakly interacting diffusive metals
We study scrambling, an avatar of chaos, in a weakly interacting metal in the
presence of random potential disorder. It is well known that charge and heat
spread via diffusion in such an interacting disordered metal. In contrast, we
show within perturbation theory that chaos spreads in a ballistic fashion. The
squared anticommutator of the electron field operators inherits a light-cone
like growth, arising from an interplay of a growth (Lyapunov) exponent that
scales as the inelastic electron scattering rate and a diffusive piece due to
the presence of disorder. In two spatial dimensions, the Lyapunov exponent is
universally related at weak coupling to the sheet resistivity. We are able to
define an effective temperature-dependent butterfly velocity, a speed limit for
the propagation of quantum information, that is much slower than microscopic
velocities such as the Fermi velocity and that is qualitatively similar to that
of a quantum critical system with a dynamical critical exponent .Comment: 15 pages in two-column format, 7 figure
Small-Object Detection in Remote Sensing Images with End-to-End Edge-Enhanced GAN and Object Detector Network
The detection performance of small objects in remote sensing images is not
satisfactory compared to large objects, especially in low-resolution and noisy
images. A generative adversarial network (GAN)-based model called enhanced
super-resolution GAN (ESRGAN) shows remarkable image enhancement performance,
but reconstructed images miss high-frequency edge information. Therefore,
object detection performance degrades for small objects on recovered noisy and
low-resolution remote sensing images. Inspired by the success of edge enhanced
GAN (EEGAN) and ESRGAN, we apply a new edge-enhanced super-resolution GAN
(EESRGAN) to improve the image quality of remote sensing images and use
different detector networks in an end-to-end manner where detector loss is
backpropagated into the EESRGAN to improve the detection performance. We
propose an architecture with three components: ESRGAN, Edge Enhancement Network
(EEN), and Detection network. We use residual-in-residual dense blocks (RRDB)
for both the ESRGAN and EEN, and for the detector network, we use the faster
region-based convolutional network (FRCNN) (two-stage detector) and single-shot
multi-box detector (SSD) (one stage detector). Extensive experiments on a
public (car overhead with context) and a self-assembled (oil and gas storage
tank) satellite dataset show superior performance of our method compared to the
standalone state-of-the-art object detectors.Comment: This paper contains 27 pages and accepted for publication in MDPI
remote sensing journal. GitHub Repository:
https://github.com/Jakaria08/EESRGAN (Implementation
Topological excitations and the dynamic structure factor of spin liquids on the kagome lattice
Recent neutron scattering experiments on the spin-1/2 kagome lattice antiferromagnet ZnCu(OH)C (Herbertsmithite) provide the first evidence of fractionalized excitations in a quantum spin liquid state in two spatial dimensions. In contrast to existing theoretical models of spin liquids, the measured dynamic structure factor reveals an excitation continuum which is remarkably at as a function of frequency and has almost no momentum dependence along several high-symmetry directions. Here we show that many experimentally observed features can be explained by the presence of topological vison excitations in a Z spin liquid. These visons form at bands on the kagome lattice, and thus act as a momentum sink for spin-carrying excitations which are probed by neutron scattering. We compute the dynamic structure factor for two di fferent Z spin liquids and find that one of them describes Herbertsmithite well above a very low energy cutoff.Physic
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Nematic Order in the Vicinity of a Vortex in Superconducting FeSe
We present a phenomenological theory of the interplay between nematic order and superconductivity in the vicinity of a vortex induced by an applied magnetic field. Nematic order can be strongly enhanced in the vortex core. As a result, the vortex cores become elliptical in shape. For the case where there is weak bulk nematic order at zero magnetic field, the field-induced eccentricity of the vortex core has a slow power-law decay away from the core. Conversely, if the nematic order is field induced, then the eccentricity is confined to the vortex core. We discuss the relevance of our results to recent scanning tunneling microscopy experiments on FeSe [Song et al. Science 332 1410 (2011)].Engineering and Applied SciencesPhysic
Development of a spectral unmixing procedure using a genetic algorithm and spectral shape
xvi, 85 leaves : ill. (chiefly col.) ; 29 cmSpectral unmixing produces spatial abundance maps of endmembers or ‘pure’ materials using sub-pixel scale decomposition. It is particularly well suited to extracting a greater portion of the rich information content in hyperspectral data in support of real-world issues such as mineral exploration, resource management, agriculture and food security, pollution detection, and climate change. However, illumination or shading effects, signature variability, and the noise are problematic. The Least Square (LS) based spectral unmixing technique such as Non-Negative Sum Less or Equal to One (NNSLO) depends on “shade” endmembers to deal with the amplitude errors. Furthermore, the LS-based method does not consider amplitude errors in abundance constraint calculations, thus, often leads to abundance errors. The Spectral Angle Constraint (SAC) reduces the amplitude errors, but the abundance errors remain because of using fully constrained condition. In this study, a Genetic Algorithm (GA) was adapted to resolve these issues using a series of iterative computations based on the Darwinian strategy of ‘survival of the fittest’ to improve the accuracy of abundance estimates. The developed GA uses a Spectral Angle Mapper (SAM) based fitness function to calculate abundances by satisfying a SAC-based weakly constrained condition. This was validated using two hyperspectral data sets: (i) a simulated hyperspectral dataset with embedded noise and illumination effects and (ii) AVIRIS data acquired over Cuprite, Nevada, USA. Results showed that the new GA-based unmixing method improved the abundance estimation accuracies and was less sensitive to illumination effects and noise compared to existing spectral unmixing methods, such as the SAC and NNSLO. In case of synthetic data, the GA increased the average index of agreement between true and estimated abundances by 19.83% and 30.10% compared to the SAC and the NNSLO, respectively. Furthermore, in case of real data, GA improved the overall accuracy by 43.1% and 9.4% compared to the SAC and NNSLO, respectively
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