854 research outputs found
Distinct quasiparticle interference patterns for surface impurity scattering on various Weyl semimetals
We examine the response of the Fermi arc in the context of quasi-particle
interference (QPI) with regard to a localized surface impurity on various
three-dimensional Weyl semimetals (WSMs). Our study also reveals the variation
of the local density of states (LDOS), obtained by Fourier transforming the QPI
profile, on the two-dimensional surface. We use the -matrix formalism to
numerically (analytically and numerically) capture the details of the momentum
space scattering in QPI (real space decay in LDOS), considering relevant
tight-binding lattice and/or low-energy continuum models modeling a range of
different WSMs. In particular, we consider multi-WSM (mWSM), hosting multiple
Fermi arcs between two opposite chirality Weyl nodes (WNs), where we find a
universal -decay ( measuring the radial distance from the impurity
core) of the impurity-induced LDOS, irrespective of the topological charge.
Interestingly, the inter-Fermi arc scattering is only present for triple WSMs,
where we find an additional -decay as compared to double and single
WSMs. The untilted single (double) [triple] WSM shows a straight-line
(leaf-like) [oval-shaped] QPI profile. The above QPI profiles are canted for
hybrid WSMs where type-I and type-II Weyl nodes coexist, however, hybrid single
WSM demonstrates strong non-uniformity, unlike the hybrid double and triple
WSMs. We also show that the chirality and the positions of the Weyl nodes
imprint marked signatures in the QPI profile. This allows us to distinguish
between different WSMs, including the time-reversal-broken WSMs from the
time-reversal-invariant WSM, even though both of the WSMs can host two pairs of
Weyl nodes. Our study can thus shed light on experimentally obtainable complex
QPI profiles and help differentiate different WSMs and their surface band
structures.Comment: 18 pages, 6 figure
Experimental Research on Corrosion-Induced Cracking Monitoring Based on Optical Fiber Sensor
Corrosion-induced cracking is a widely existent issue for coastal infrastructure, which leads to its advanced failure. The monitoring of corrosion-induced cracking is an important means of evaluating its influence on structure normal use and safe operation. However, traditional sensors such as strain gage are unsuitable to embed into concrete and to record long-term strain of concrete caused by steel bar corrosion. The optical fiber sensor, Brillouin Optical Time Domain Analysis (BOTDA) can effectively avoid the undetected phenomenon existing in point-wise test method, and it has the characteristic of automated monitoring. The optical fiber sensor is also electrical insulation and anti-electromagnetic interference; so it is suitable for detecting the corrosion-induced cracks. In this article, experimental research on corrosion-induced crack monitoring based on BOTDA is introduced. The distributed optical fiber is embedded into concrete around the steel bar to record expansion force of concrete. An accelerated corrosion test is performed to investigate the relationship between tiny geometrical changes of steel bar and concrete expansion force. Different rates of corrosion current are applied to the specimen. The accelerated corrosion test approved that the optical fiber sensor can effectively monitor the whole process of corrosion-induced cracking
Tip induced unconventional superconductivity on Weyl semimetal TaAs
Weyl fermion is a massless Dirac fermion with definite chirality, which has
been long pursued since 1929. Though it has not been observed as a fundamental
particle in nature, Weyl fermion can be realized as low-energy excitation
around Weyl point in Weyl semimetal, which possesses Weyl fermion cones in the
bulk and nontrivial Fermi arc states on the surface. As a firstly discovered
Weyl semimetal, TaAs crystal possesses 12 pairs of Weyl points in the momentum
space, which are topologically protected against small perturbations. Here, we
report for the first time the tip induced superconductivity on TaAs crystal by
point contact spectroscopy. A conductance plateau and sharp double dips are
observed in the point contact spectra, indicating p-wave like unconventional
superconductivity. Furthermore, the zero bias conductance peak in low
temperature regime is detected, suggesting potentially the existence of
Majorana zero modes. The experimentally observed tunneling spectra can be
interpreted with a novel mirror-symmetry protected topological superconductor
induced in TaAs, which can exhibit zero bias and double finite bias peaks, and
double conductance dips in the measurements. Our work can open a broad avenue
in search for new topological superconducting phases from topological Weyl
materials and trigger intensive investigations for pursuing Majorana fermions
Effect of low-Raman window position on correlated photon-pair generation in a chalcogenide Ge11.5As24Se64.5 nanowire
We investigated correlated photon-pair generation via spontaneous four-wave mixing in an integrated chalcogenideGe11.5As24Se64.5photonicnanowire. The coincidence to accidental ratio, a key measurement for the quality of correlated photon-pair sources, was measured to be only 0.4 when the photon pairs were generated at 1.9 THz detuning from the pump frequency due to high spontaneous Raman noise in this regime. However, the existence of a characteristic low-Raman window at around 5.1 THz in this material's Raman spectrum and dispersion engineering of the nanowire allowed us to generate photon pairs with a coincidence to accidental ratio of 4.5, more than 10 times higher than the 1.9 THz case. Through comparing the results with those achieved in chalcogenide As2S3waveguides which also exhibit a low Raman-window but at a larger detuning of 7.4 THz, we find that the position of the characteristic low-Raman window plays an important role on reducing spontaneous Raman noise because the phonon population is higher at smaller detuning. Therefore the ultimate solution for Raman noise reduction in Ge11.5As24Se64.5 is to generate photon pairs outside the Raman gain band at more than 10 THz detuning
Two-Phase Iteration for Value Function Approximation and Hyperparameter Optimization in Gaussian-Kernel-Based Adaptive Critic Design
Adaptive Dynamic Programming (ADP) with critic-actor architecture is an effective way to perform online learning control. To avoid the subjectivity in the design of a neural network that serves as a critic network, kernel-based adaptive critic design (ACD) was developed recently. There are two essential issues for a static kernel-based model: how to determine proper hyperparameters in advance and how to select right samples to describe the value function. They all rely on the assessment of sample values. Based on the theoretical analysis, this paper presents a two-phase simultaneous learning method for a Gaussian-kernel-based critic network. It is able to estimate the values of samples without infinitively revisiting them. And the hyperparameters of the kernel model are optimized simultaneously. Based on the estimated sample values, the sample set can be refined by adding alternatives or deleting redundances. Combining this critic design with actor network, we present a Gaussian-kernel-based Adaptive Dynamic Programming (GK-ADP) approach. Simulations are used to verify its feasibility, particularly the necessity of two-phase learning, the convergence characteristics, and the improvement of the system performance by using a varying sample set
Arrival time in quantum field theory
Via the proper-time eigenstates (event states) instead of the proper-mass
eigenstates (particle states), free-motion time-of-arrival theory for massive
spin-1/2 particles is developed at the level of quantum field theory. The
approach is based on a position-momentum dual formalism. Within the framework
of field quantization, the total time-of-arrival is the sum of the single
event-of-arrival contributions, and contains zero-point quantum fluctuations
because the clocks under consideration follow the laws of quantum mechanics.Comment: 15 pages, A detailed version with a appendix, to be published in
Physics Letters
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