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Wintertime Local Wind Dynamics from Scanning Doppler Lidar and Air Quality in the Arve River Valley
摘要： Air quality issues are frequent in urbanized valleys, particularly in wintertime when a temperature inversion forms and the air within the valley is stably stratified over several days. In addition to pollutant sources, local winds can have a significant impact on the spatial distribution and temporal evolution of pollutant concentrations. They can be very complex and difficult to represent in numerical weather prediction models, particularly under stable conditions. Better knowledge of these local winds from observations is also a prerequisite to improving air quality prediction capability. This paper analyses local winds during the Passy-2015 field experiment that took place in a section of the Arve river valley, near Chamonix–Mont-Blanc. This location is one of the worst places in France regarding air quality. The wind analysis, which is mainly based on scanning Doppler lidar data sampling a persistent temperature inversion episode, reveals features consistent with the higher pollutant concentrations observed in this section of the valley as well as their spatial heterogeneities. In particular, an elevated down-valley jet is observed at night in the northern half of the valley, which, combined with a weak daytime up-valley wind, leads to very poor ventilation of the lowest layers. A northeast–southwest gradient in ventilation is observed on a daily-average, and is consistent with the PM10 heterogeneities observed within the valley.
关键词： scanning Doppler wind lidar,Passy-2015 field experiment,cold air pool,local wind dynamics,air quality,alpine valley
更新于2025-09-23 15:23:52
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Topological valley pumping in Weyl semimetals
摘要： We investigate the topological pumping effect in Weyl semimetals, subject to the modulation of two ac electric fields along y and z directions, respectively. We show that the pumping effect originates from the anomalous velocity related to the Berry curvature. The direction of the pumping current is dependent on the chirality of the Weyl fermions. While the total particle current is vanishing because the Weyl points of opposite chirality always come in pairs in Weyl semimetals, the pump gives rise to a net chirality current or valley current. The noiseless valley current generated can be useful in valleytronic applications.
关键词： Valleytronics,Chirality,Topological pumping effect,Valley current,Weyl semimetal
更新于2025-09-23 15:23:52
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Valley engineering by strain in Kekulé-distorted graphene
摘要： A Kekulé bond texture in graphene modi?es the electronic band structure by folding the Brillouin zone and bringing the two inequivalent Dirac points to the center. This can result in the opening of a gap (Kek-O) or the locking of the valley degree of freedom with the direction of motion (Kek-Y). We analyze the effects of uniaxial strain on the band structure of Kekulé-distorted graphene for both textures. Using a tight-binding approach, we introduce strain by considering the hopping renormalization and corresponding geometrical modi?cations of the Brillouin zone. We numerically evaluate the dispersion relation and present analytical expressions for the low-energy limit. Our results indicate the emergence of a Zeeman-like term due to the coupling of the pseudospin with the pseudomagnetic strain potential which separates the valleys by moving them in opposite directions away from the center of the Brillouin zone. For the Kek-O phase, this results in a competition between the Kekulé parameter that opens a gap and the magnitude of strain which closes it, while for the Kek-Y phase, it results in a superposition of two shifted Dirac cones. As the Dirac cones are much closer in the superlattice reciprocal space than in pristine graphene, we propose strain as a control parameter for intervalley scattering.
关键词： Brillouin zone,tight-binding,strain,graphene,valley engineering,Kekulé distortion
更新于2025-09-23 15:22:29
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[IEEE IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Valencia (2018.7.22-2018.7.27)] IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Carbon Dioxide and Water Vapour Fluxes of a Alkaline Fen and Their Dependence on Reflectance
摘要： This study shows results of parallel measurements of greenhouse gases fluxes (carbon dioxide and water vapour) and canopy reflectance of alkaline fen. Fluxes were measured using eddy-covariance technique on micrometeorological station located in the Upper Biebrza Basin (NE Poland) in Rogo?ynek Village. Study site is located in the Biebrza National Park which was established to protect one of the biggest coherent lowland wetland area in the Central Europe. Statistical relations of reflectance and spectral indices with fluxes were calculated based on measurements during two growing season (2015 and 2016). Four types of functions were examined: linear, quadratic, exponential and logarithmic and for two timestamps: half-hour and day. The correlation between carbon dioxide fluxes and reflectance is better than for water vapour fluxes. For both carbon dioxide and water vapour we obtained higher correlations coefficients using selected spectral indices than using reflectance and higher correlations for daily timestamp than for half-hour fluxes.
关键词： greenhouse gases,eddy-covariance technique,spectral indices,Biebrza River valley,wetland
更新于2025-09-23 15:22:29
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Recipe for Dirac Phonon States with a Quantized Valley Berry Phase in Two-Dimensional Hexagonal Lattices
摘要： The topological quantum states in two-dimensional (2D) materials are fascinating subjects of research, which usually highlight electron-related systems. In this work, we present a recipe that leads to Dirac phonon states with quantized valley Berry phase in 2D hexagonal lattices by first-principles calculations. We show that candidates possessing the three-fold rotational symmetry at the corners of the hexagonal Brillouin zone host valley Dirac phonons, which are guaranteed to remain intact with respect to perturbations. We identify that such special topological features populated by Dirac phonons can be realized in various 2D materials. In particular, the monolayer CrI3, an attractive 2D magnetic semiconductor with exotic applications in spintronics, is an ideal platform to investigate nontrivial phonons in experiments. We further confirm that the phonon Berry phase is quantized to ±π at two inequivalent valleys. The phonon edge states terminated at the projection of phonon Dirac cones are clearly visible. This work demonstrates that 2D hexagonal lattices with attractive valley Dirac phonons will extend the knowledge of valley physics, providing wide applications of topological phonons.
关键词： Dirac phonons,first-principles calculations,valley Berry phase,phonon edge states,2D hexagonal lattices
更新于2025-09-23 15:21:21
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Low-frequency spectroscopy for quantum multilevel systems
摘要： A periodically driven quantum system with avoided level crossing experiences both nonadiabatic transitions and wave-function phase changes. These result in coherent interference fringes in the system’s occupation probabilities. For qubits, with repelling energy levels, such interference, named after Landau-Zener-Stückelberg-Majorana, displays arc-shaped resonance lines. In the case of a multilevel system with an avoided level crossing of the two lower levels, we demonstrate that the shape of the resonances can change from convex arcs to concave heart-shaped and harp-shaped resonance lines. Indeed, the whole energy spectrum determines the shape of such resonance fringes and this also provides insight into the slow-frequency system spectroscopy. As a particular example, we consider this for valley-orbit silicon quantum dots, which are important for the emerging field of valleytronics.
关键词： valley-orbit silicon quantum dots,valleytronics,low-frequency spectroscopy,quantum multilevel systems,Landau-Zener-Stückelberg-Majorana interference
更新于2025-09-23 15:21:21
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[IEEE IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Valencia, Spain (2018.7.22-2018.7.27)] IGARSS 2018 - 2018 IEEE International Geoscience and Remote Sensing Symposium - Intercomparison of Field Methods for Acquiring Ground Reflectance at Railroad Valley Playa for Spectral Calibration of Satellite Data
摘要： Ground reflectance was acquired at the Railroad Valley Playa calibration site in Nevada USA using different methods of collection. The data was collected near the time and date of Landsat 8 OLI and Sentinel-2 satellite overpasses so an inter-comparison could be made with the reflectance products to determine which method was more suitable for vicarious calibration. The field spectrometers and reference panels were characterized before the field campaign. A continuous acquisition method was compared to stop and measure collections. Both acquisition methods were collected along an 80 m east-west transect as well as for a series of north-south transects over an 80 x 320 m area, with the stop and measure method being performed at random sampling locations. The measurements were performed using two field spectrometers by three teams of two people to compare the repeatability. The aim of the field campaign was to determine the variability due to the operator and the method of collection.
关键词： Railroad Valley Playa,spectrometer,vicarious calibration,reflectance
更新于2025-09-23 15:21:21
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[NanoScience and Technology] Silicene (Prediction, Synthesis, Application) || Electronic and Topological Properties of Silicene, Germanene and Stanene
摘要： In this chapter, we review the recent progress on electronic and topological properties of monolayer topological insulators including silicene, germanene and stanene. We start with the description of the topological nature of the general Dirac system and then apply it to silicene by introducing the spin and valley degrees of freedom. Based on them, we classify all topological insulators in the general honeycomb system. We discuss topological electronics based on honeycomb systems. We introduce the topological Kirchhoff law, which is a conservation law of topological edge states. Field effect topological transistor is proposed based on the topological edge states. We show that the conductance is quantized even in the presence of random distributed impurities. Monolayer topological insulators will be a key for future topological electronics and spin-valleytronics.
关键词： topological Kirchhoff law,silicene,topological insulators,topological electronics,Dirac system,field effect topological transistor,spin and valley degrees of freedom,stanene,quantized conductance,germanene
更新于2025-09-23 15:21:21
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Moire intralayer excitons in a MoSe <sub/>2</sub> /MoS <sub/>2</sub> heterostructure
摘要： Spatially periodic structures with a long range period, referred to as moir′e pattern, can be obtained in van der Waals bilayers in the presence of a small stacking angle or of lattice mismatch between the monolayers. Theoretical predictions suggest that the resulting spatially periodic variation of the band structure modifies the optical properties of both intra and interlayer excitons of transition metal dichalcogenides heterostructures. Here, we report on the impact of the moir′e pattern formed in a MoSe2/MoS2 heterobilayer encapsulated in hexagonal boron nitride. The periodic in-plane potential results in a splitting of the MoSe2 exciton and trion in both emission and absorption spectra. The observed energy difference between the split peaks is fully consistent with theoretical predictions.
关键词： valley polarization,van der Waals heterostructures,moir′e pattern,moir′e excitons,Transition metal dichalcogenides
更新于2025-09-23 15:21:21
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valley of (In,Al)As/AlAs quantum dots
摘要： The electron-nuclei hyperfine interaction of electrons in indirect band gap (In,Al)As/AlAs quantum dots with type-I band alignment has been experimentally studied by measuring the polarization degree of the photoluminescence in a transverse magnetic field (Hanle effect) and the polarization recovery in a longitudinal magnetic field. The different symmetries of the X valley electron Bloch amplitudes at the As, In, and Al nuclei strongly affect the hyperfine interaction. The hyperfine constants corresponding to these nuclei have been determined.
关键词： X valley,hyperfine constants,Bloch amplitudes,Hanle effect,polarization recovery,photoluminescence,quantum dots,electron-nuclei hyperfine interaction
更新于2025-09-23 15:21:01