18,189 research outputs found
Coupled equations for Kähler metrics and Yang-Mills connections
We study equations on a principal bundle over a compact complex manifold
coupling a connection on the bundle with a Kahler structure on the base. These
equations generalize the conditions of constant scalar curvature for a Kahler
metric and Hermite-Yang-Mills for a connection. We provide a moment map
interpretation of the equations and study obstructions for the existence of
solutions, generalizing the Futaki invariant, the Mabuchi K-energy and geodesic
stability. We finish by giving some examples of solutions.Comment: 61 pages; v2: introduction partially rewritten; minor corrections and
improvements in presentation, especially in Section 4; added references; v3:
To appear in Geom. Topol. Minor corrections and improvements, following
comments by referee
Quantum spin Hall phase in multilayer graphene
The so called quantum spin Hall phase is a topologically non trivial
insulating phase that is predicted to appear in graphene and graphene-like
systems. In this work we address the question of whether this topological
property persists in multilayered systems. We consider two situations: purely
multilayer graphene and heterostructures where graphene is encapsulated by
trivial insulators with a strong spin-orbit coupling. We use a four orbital
tight-binding model that includes the full atomic spin-orbit coupling and we
calculate the topological invariant of the bulk states as well as the
edge states of semi-infinite crystals with armchair termination. For
homogeneous multilayers we find that even when the spin-orbit interaction opens
a gap for all the possible stackings, only those with odd number of layers host
gapless edge states while those with even number of layers are trivial
insulators. For the heterostructures where graphene is encapsulated by trivial
insulators, it turns out that the interlayer coupling is able to induce a
topological gap whose size is controlled by the spin-orbit coupling of the
encapsulating materials, indicating that the quantum spin Hall phase can be
induced by proximity to trivial insulators.Comment: 7 pages, 6 figure
Real space mapping of topological invariants using artificial neural networks
Topological invariants allow to characterize Hamiltonians, predicting the
existence of topologically protected in-gap modes. Those invariants can be
computed by tracing the evolution of the occupied wavefunctions under twisted
boundary conditions. However, those procedures do not allow to calculate a
topological invariant by evaluating the system locally, and thus require
information about the wavefunctions in the whole system. Here we show that
artificial neural networks can be trained to identify the topological order by
evaluating a local projection of the density matrix. We demonstrate this for
two different models, a 1-D topological superconductor and a 2-D quantum
anomalous Hall state, both with spatially modulated parameters. Our neural
network correctly identifies the different topological domains in real space,
predicting the location of in-gap states. By combining a neural network with a
calculation of the electronic states that uses the Kernel Polynomial Method, we
show that the local evaluation of the invariant can be carried out by
evaluating a local quantity, in particular for systems without translational
symmetry consisting of tens of thousands of atoms. Our results show that
supervised learning is an efficient methodology to characterize the local
topology of a system.Comment: 9 pages, 6 figure
On the exposure to mobile phone radiation in trains
This report presents theoretical estimates of the Power Density levels which
may be reached inside trains. Two possible sources of high levels of radiation
are discussed. The first one arises since the walls of the wagons are metallic
and therefore bounce back almost all radiation impinging on them. The second is
due to the simultaneous emission of a seemingly large number of nearby
telephones. The theoretical study presented here shows that Power Densities
stay at values below reference levels always.Comment: 9 pages, 1 figur
Theory of extraordinary transmission of light through quasiperiodic arrays of subwavelength holes
By using a theoretical formalism able to work in both real and k-spaces, the
physical origin of the phenomenon of extraordinary transmission of light
through quasi-periodic arrays of holes is revealed. Long-range order present in
a quasiperiodic array selects the wavevector(s) of the surface electromagnetic
mode(s) that allows an efficient transmission of light through subwavelength
holes.Comment: 4 pages, 4 figure
A yeast three-hybrid system that reconstitutes mammalian hypoxia inducible factor regulatory machinery
Background: Several human pathologies, including neoplasia and ischemic cardiovascular diseases, course with an unbalance between oxygen supply and demand ( hypoxia). Cells within hypoxic regions respond with the induction of a specific genetic program, under the control of the Hypoxia Inducible Factor (HIF), that mediates their adaptation to the lack of oxygen. The activity of HIF is mainly regulated by the EGL-nine homolog (EGLN) enzymes that hydroxylate the alpha subunit of this transcription factor in an oxygen-dependent reaction. Hydroxylated HIF is then recognized and ubiquitinilated by the product of the tumor suppressor gene, pVHL, leading to its proteosomal degradation. Under hypoxia, the hydroxylation of HIF by the EGLNs is compromised due to the lack of oxygen, which is a reaction cosubstrate. Thus, HIF escapes degradation and drives the transcription of its target genes. Since the progression of the aforementioned pathologies might be influenced by activation of HIF-target genes, development of small molecules with the ability to interfere with the HIF-regulatory machinery is of great interest.Results: Herein we describe a yeast three-hybrid system that reconstitutes mammalian HIF regulation by the EGLNs and VHL. In this system, yeast growth, under specific nutrient restrictions, is driven by the interaction between the beta domain of VHL and a hydroxyproline-containing HIF alpha peptide. In turn, this interaction is strictly dependent on EGLN activity that hydroxylates the HIFa peptide. Importantly, this system accurately preserves the specificity of the hydroxylation reaction toward specific substrates. We propose that this system, in combination with a matched control, can be used as a simple and inexpensive assay to identify molecules that specifically modulate EGLN activity. As a proof of principle we show that two known EGLN inhibitors, dimethyloxaloylglycine (DMOG) and 6-chlor-3-hydroxychinolin-2-carbonic acid-N-carboxymethylamide (S956711), have a profound and specific effect on the yeast HIF/EGLN/VHL system.Conclusion: The system described in this work accurately reconstitutes HIF regulation while preserving EGLN substrate specificity. Thus, it is a valuable tool to study HIF regulation, and particularly EGLN biochemistry, in a cellular context. In addition, we demonstrate that this system can be used to identify specific inhibitors of the EGLN enzymes
Bayesian Road Estimation Using Onboard Sensors
This paper describes an algorithm for estimating the road ahead of a host vehicle based on the measurements from several onboard sensors: a camera, a radar, wheel speed sensors,and an inertial measurement unit.We propose a novel road model that is able to describe the road ahead with higher accuracy than the usual polynomial model. We also develop a Bayesian fusionsystem that uses the following information from the surroundings: lane marking measurements obtained by the camera and leading vehicle and stationary object measurements obtained bya radar–camera fusion system. The performance of our fusion algorithm is evaluated in several drive tests. As expected, the more information we use, the better the performance is.Index Terms—Camera, information fusion, radar, road geometry,unscented Kalman filter (UKF)
FeNi-based magnetoimpedance multilayers: Tailoring of the softness by magnetic spacers
The microstructure and magnetic properties of sputtered permalloy films and FeNi(170 nm)/X/FeNi(170 nm) (X=Co, Fe, Gd, Gd-Co) sandwiches were studied. Laminating of the thick FeNi film with various spacers was done in order to control the magnetic softness of FeNi-based multilayers. In contrast to the Co and Fe spacers, Gd and Gd-Co magnetic spacers improved the softness of the FeNi/X/FeNi sandwiches. The magnetoimpedance responses were measured for [FeNi/Ti(6 nm)] 2/FeNi and [FeNi/Gd(2 nm)] 2/FeNi multilayers in a frequency range of 1-500 MHz: for all frequencies under consideration the highest magnetoimpedance variation was observed for [FeNi/Gd(2 nm)] 2/FeNi multilayers. © 2012 American Institute of Physics
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