SEARCH · Searcharxiv

Results for “physics.app-ph”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 433 records · Page 24Linked to original sources

arXiv2019

Quad-rotor Flight Simulation in Realistic Atmospheric Conditions

In trajectory planning and control design for unmanned air vehicles, highly simplified models are typically used to represent the vehicle dynamics and the operating environment. The goal of this work is to perform real-time, but realistic flight simulations and trajectory planning for quad-copters in low altitude (<500m) atmospheric conditions. The aerodynamic model for rotor performance is adapted from blade element momentum theory and validated against experimental data. Large-eddy simulations of the atmospheric boundary layer are used to accurately represent the operating environment of unmanned air vehicles. A reduced-order version of the atmospheric boundary layer data as well as the popular Dryden model are used to assess the impact of accuracy of the wind field model on the predicted vehicle performance and trajectory. The wind model, aerodynamics and control modules are integrated into a six-degree-of-freedom flight simulation environment with a fully nonlinear flight controller. Simulations are performed for two representative flight paths, namely, straight and circular paths. Results for different wind models are compared and the impact of simplifying assumptions in representing rotor aerodynamics is discussed. The simulation framework and codes are open-sourced for use by the community.

physics.flu-dyn↗

arXiv2019

On the collective behavior of interacting polydisperse microbubbles

Exploiting the full potential of MBs for applications requires a good understanding of their complex dynamics. Improved understanding of MB oscillations can lead to further enhancement in optimizing their efficacy in many applications and also invent new ones. Previous studies have predominantly addressed the behavior of a single isolated MB in an infinite liquid domain, whereas most applications employ MBs in clusters. Oscillating MBs have been shown to generate secondary pressure waves that modify the dynamics of the MBs in their proximity. A modified Keller-Miksis equation is used to account for inter-bubble interactions. The oscillatory dynamics of each MB within clusters was computed by numerically solving the resulting system of coupled nonlinear second order differential equations. Frequency response analysis and bifurcation diagrams were employed to track the dynamics of interacting MBs. Here we investigate the dynamics of polydisperse MB clusters over a wide range of acoustic and geometric parameters. An emergent collective behavior within bubble clusters was observed whereby individual dynamics of smaller bubbles were suppressed resulting in a collective behavior dominated by the dynamics of the largest MB within the cluster. The emergent dynamics of smaller MBs within bubble clusters can be characterized by constructive and destructive inter-bubble interactions. In constructive interactions, the radial oscillations of smaller bubbles matched those of the largest MB and their oscillations are amplified. In destructive interactions, the oscillations of smaller bubbles are suppressed so that their oscillations match those of the largest MB. Furthermore, a special case of constructive interactions is presented where dominant MB (largest) can force smaller MBs into period doubling and subharmonic oscillations.

physics.app-ph↗

arXiv2019

Coupling of Wideband Impulses Generated by Granular Chains into Liquids

An ultrasonic transducer technology to generate wideband impulses using a one-dimensional chain of spheres was previously presented. The Hertzian contact between the spheres causes the nonlinearity of the system to increase, which transforms high amplitude narrowband sinusoidal input into a train of wideband impulses. Generation of short duration ultrasonic pulses is desirable both in diagnostic and therapeutic ultrasound. Nevertheless, the biggest challenge in terms of adaptation to biomedical ultrasound is the coupling of the ultrasonic energy into biological tissue. An analytical model was created to address the coupling issue. Effect of the matching layer was modelled as a flexible thin plate clamped from the edges. Model was verified against hydrophone measurements. Different coupling materials, such as glass, aluminium, acrylic, silicon rubber, and vitreous carbon, was analysed with this model. Results showed that soft matching layers such as acrylic and rubber inhibit the generation of higher order harmonics. Between the hard matching materials, vitreous carbon achieved the best results due to its acoustic impedance.

physics.med-ph↗

arXiv2019

Magnetic design of twin aperture cos-theta superconducting dipoles with a semi-analytic approach

The magnetic design is a basic aspect of the superconducting magnets for particle accelerators. When dealing with single aperture cos-theta type dipoles, at the first order, the design can be performed with an analytic approach based on a sector dipole approximation followed by a numerical optimization. For double aperture dipoles the magnetic cross-talk between apertures makes this approach unfeasible. We have developed a semi-analytic model, which starting from a sector dipole approximation, allows to consider the cross-talk between the two apertures. We also demonstrate that the iron yoke contribution to harmonics, although dominant, does not change the optimal configuration found in its absence. As examples, we show two possible electromagnetic designs for the D2 dipole of the High Luminosity upgrade of LHC. The semi-analytic model can be generalized to a larger class of magnets.

physics.acc-ph↗

arXiv2019

Exploring co-sputtering of ZnO:Al and SiO2 for efficient electron-selective contacts on silicon solar cells

In recent years, considerable efforts have been devoted to developing novel electron-selective materials for crystalline Si (c-Si) solar cells with the attempts to simplify the fabrication process and improve efficiency. In this study, ZnO:Al (AZO) is co-sputtered with SiO2 to form AZO:SiO2 films with different SiO2 content. These nanometer-scale films, deposited on top of thin intrinsic hydrogenated amorphous silicon films and capped with low-work-function metal (such as Al and Mg), are demonstrated to function effectively as electron-selective contacts in c-Si solar cells. On the one hand, AZO:SiO2 plays an important role in such electron-selective contact and its thickness is a critical parameter, thickness of 2 nm showing the best. On the other hand, at the optimal thickness of AZO:SiO2, the open circuit voltage (VOC) of the solar cells is found to be relatively insensitive to either the work function or the band gap of AZO:SiO2. Whereas, regarding the fill factor (FF), AZO without SiO2 content exhibits to be the optimal choice. By using AZO/Al as electron-selective contact, we successfully realize a 19.5%-efficient solar cell with VOC over 700 mV and FF around 75%, which is the best result among c-Si solar cells using ZnO as electron-selective contact. Also, this work implies that efficient carrier-selective film can be made by magnetron sputtering method.

physics.app-ph↗

arXiv2019

Nanoscale optical and structural characterisation of silk

Background: Nanoscale composition of silk defining its unique properties via a hierarchical structural anisotropy has to be analysed at the highest spatial resolution of tens-of-nanometers corresponding to the size of fibrils made of b-sheets, which are the crystalline building blocks of silk. Results: Nanoscale optical and structural properties of silk have been measured from 100-nm thick longitudinal slices of silk fibers with ~10 nm resolution, the highest so far. Optical sub-wavelength resolution in hyperspectral mapping of absorbance and molecular orientation were carried out for comparison at IR wavelengths 2-10 micrometers using synchrotron radiation. Conclusion: Reliable distinction of transmission changes by only 1-2% due to anisotropy of amide bands was obtained from nano-thin slices of silk.

physics.app-ph↗

arXiv2019

Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics

Dipolar excitons offer a rich playground for both design of novel optoelectronic devices and fundamental many-body physics. Wide GaN/(AlGa)N quantum wells host a new and promising realization of dipolar excitons. We demonstrate the in-plane confinement and cooling of these excitons, when trapped in the electrostatic potential created by semitransparent electrodes of various shapes deposited on the sample surface. This result is a prerequisite for the electrical control of the exciton densities and fluxes, as well for studies of the complex phase diagram of these dipolar bosons at low temperature.

physics.app-ph↗

arXiv2019

Sputtered Spontaneously Nano-porous VO2-based Films via PTFE Self-Template: Localized Surface Plasmon Resonance Induced Robust Optical Performance for Solar Glazing Application

The PTFE (Teflon) has been selected as the self-template structural material in preparation of VO2 films using reactive magnetron sputtering systems and post annealing progress. Spontaneous random nano-porous structures of VO2 films growing on quartz glasses have been deliberately established via bottom-up processing through this novel and facile approach. The nano-porous VO2 films exhibit an excellent optical performance based on localized surface plasmon resonance (LSPR), with ultrahigh luminous transmittance (Tlum) up to 78.0% and the promoted solar modulation ability (ΔTsol) of 14.1%. Meanwhile, the ingenious microstructure of film provides an antireflection function from multiple perspectives in visible light, with the potential of the windshield on vehicles for smart solar modulation. The nano-porous films expand the practical application of thermochromic VO2 to a fire-new field, breaking the optical performance envelope of single-layer dense VO2 film away, and offering a universal method to prepare homogeneous nano-porous structures for thin films.

physics.app-ph↗

arXiv2019

Effect of shielding gas composition and welding speed on autogenous welds of unalloyed tungsten plates

Tungsten usually exhibits poor weldability and marked brittleness at room temperature. This cause tungsten welds to be affected by the evolution of cracks along the weld bead, which can be eliminated by using a pre-heating step to reduce thermal straining. In this study, based on the tungsten inert gas welding process, a working envelope, focussed on varying welding speed and five different shielding gas mixtures of argon and helium, has been defined with the view of producing crack-free autogenous welds. The bead appearance and the microstructure of the different welds were correlated to the welding parameters, whose main effects have been analysed. Welding defects such as humping occurred when using gas mixtures with relatively low content of helium, and when using relatively high welding speeds. Crack-free autogenous welds have been produced without pre-heating when using a high content of helium and relatively low welding speeds. Thus, this study has demonstrated that a helium-rich shielding gas is required for welding thick tungsten plates. Moreover, the low thermal shock induced by the process, coupled with the purity of the tungsten plates used, strongly contributed to avoid the occurrence of any crack.

physics.app-ph↗

arXiv2019

Water vapour pressure as determining control parameter to fabricate high efficiency perovskite solar cells at ambient conditions

Although perovskite solar cells have demonstrated impressive efficiencies in research labs (above 23%), there is a need of experimental procedures that allow their fabrication at ambient conditions, which would decrease substantially manufacturing costs. However, under ambient conditions, a delicate control of the moisture level in the atmosphere has to be enforced to achieve efficient and highly stable devices. In this work, we show that it is the absolute content of water measured in the form of partial water vapour pressure (WVP) the only determining control parameter that needs to be considered during preparation. Following this perspective, MAPbI3 perovskite films were deposited under different WVP by changing the relative humidity (RH) and the lab temperature. We found that efficient and reproducible devices can be obtained at given values of WVP. Furthermore, it is demonstrated that small temperature changes, at the same value of the RH, result in huge changes in performance, due to the non-linear dependence of the WVP on temperature. We have extended the procedure to accomplish high-efficient FA0.83MA0.17PbI3 devices at ambient conditions by adjusting DMSO proportion in precursor solution as a function of WVP only. As an example of the relevance of this paramater, a WVP value of around of 1.6 kPa appears to be an upper limit for safe fabrication of high efficiency devices at ambient conditions, regardless the RH and lab temperature.

physics.app-ph↗

arXiv2019

Designing multi-level resistance states for multi-bit storage using half doped manganites

Designing nonvolatile multi-level resistive devices is the necessity of time to go beyond traditional one-bit storage systems, thus enhancing the storage density. Here, we explore the electronic phase competition scenario to design multi-level resistance states using a half doped CE-type charge ordered insulating bulk manganite, $Sm_{0.5}Ca_{0.25}Sr_{0.25}MnO_3$ (SCSMO). By introducing electronic phase coexistence in a controllable manner in SCSMO, we show that the system can be stabilized into several metastable states, against thermal cycling, up to 62 K. As a result the magnetization (and the resistivity) remains unaltered during the thermal cycling. Monte Carlo calculations using two-band double exchange model, including super-exchange, electron-phonon coupling, and quenched disorder, show that the system freezes into a phase coexistence metastable state during the thermal cycling due to the chemical disorder in SCSMO. Using the obtained insights we outline a pathway by utilizing four reversible metastable resistance states to design a prototype multi-bit memory device.

physics.app-ph↗

arXiv2019

Development of Wire + Arc Additive Manufacturing for the production of large-scale unalloyed tungsten components

The manufacturing of refractory-metals components presents some limitations induced by the materials characteristic low-temperature brittleness and high susceptibility to oxidation. Powder metallurgy is typically the manufacturing process of choice. Recently, Wire + Arc Additive Manufacturing has proven capable to produce fully-dense large-scale metal parts at relatively low cost, by using high-quality wire as feedstock. In this study, this technique has been used for the production of large-scale tungsten linear structures. The orientation of the wire feeding has been studied and optimised to obtain defect-free tungsten deposits. In particular, front wire feeding eliminated the occurrence of pores and micro-cracks, when compared to side wire feeding. The microstructure, the occurrence of defects and their relationship with the deposition process have also been discussed. Despite the repetitive thermal cycles and the inherent brittleness of the material, the as-deposited structures were free from internal cracks and the layer dimensions were stable during the entire deposition process. This enabled the production of a relatively large-scale component, with the dimension of 210 x 75 x 12 mm. This study has demonstrated that Wire + Arc Additive Manufacture can be used to produce large-scale parts in unalloyed tungsten by complete fusion, presenting a potential alternative to the powder metallurgy manufacturing route.

physics.app-ph↗

arXiv2018

Electro-opto-mechanical radio-frequency oscillator driven by guided acoustic waves in standard single-mode fiber

An opto-electronic radio-frequency oscillator that is based on forward scattering by the guided acoustic modes of a standard single-mode optical fiber is proposed and demonstrated. An optical pump wave is used to stimulate narrowband, resonant guided acoustic modes, which introduce phase modulation to a co-propagating optical probe wave. The phase modulation is converted to an intensity signal at the output of a Sagnac interferometer loop. The intensity waveform is detected, amplified and driven back to modulate the optical pump. Oscillations are achieved at a frequency of 319 MHz, which matches the resonance of the acoustic mode that provides the largest phase modulation of the probe wave. Oscillations at the frequencies of competing acoustic modes are suppressed by at least 40 dB. The linewidth of the acoustic resonance is sufficiently narrow to provide oscillations at a single longitudinal mode of the hybrid cavity. Competing longitudinal modes are suppressed by at least 38 dB as well. Unlike other opto-electronic oscillators, no radio-frequency filtering is required within the hybrid cavity. The frequency of oscillations is entirely determined by the fiber opto-mechanics.

physics.app-ph↗

arXiv2018

$π$/2 Mode Converters and Vortex Generators for Electrons

In optics, mode conversion is an elegant way to switch between Hermite Gaussian and Laguerre Gaussian beam profiles and thereby impart orbital angular momentum onto the beam and to create vortices. In optics such vortex beams can be produced in a setup consisting of two identical cylinder lenses. In electron optics, quadrupole lenses can be used for the same purpose. Here we investigate generalized asymmetric designs of a quadrupole mode converter that may be realized within the constraints of existing electron microscopes and can steer the development of dedicated vortex generators for high brilliance electron vortex probes of atomic scale.

physics.app-ph↗

arXiv2019

An Adaptive Soft Plasmonic Nanosheet Resonator

Current micro nanomechanical system are usually based on rigid crystalline semiconductors that normally have high quality factors but lack adaptive responses to variable frequencies, a capability ubiquitous for communications in the biological world, such as bat and whale calls. Here, we demonstrate a soft mechanical resonator based on a freestanding organic-inorganic hybrid plasmonic superlattice nanosheet, which can respond adaptively to either incident light intensity or wavelength. This is achieved because of strong plasmonic coupling in closely-packed nanocrystals which can efficiently concentrate and convert photons into heat. The heat causes the polymer matrix to expand, leading to a change in the nanomechanical properties of the plasmonic nanosheet. Notably, the adaptive frequency responses are also reversible and the responsive ranges are fine-tunable by adjusting the constituent nanocrystal building blocks. We believe that our plasmonic nanosheets may open a new route to design next-generation intelligent bio-mimicking opto-mechanical resonance systems.

physics.app-ph↗

arXiv2019

Record-high photocatalytic rate at single atomic metal oxide

Metal oxides have been extensively investigated and applied in environmental remediation and protection, energy conversion and storage. Most of these diverse applications are results of a large diversity of the electronic states of metal oxides. Noticeably, however, numerous metal oxides have obstacles for applications in catalysis because of low density of active sites in these materials. Size reduction of oxide catalyst is a strategy to improve the active site density. Here, we demonstrate the fabrication of single atomic metal oxide in which the oxide size reaches its minimum. However, the catalytic mechanism in this SATO is determined by a quasi atom physics which is fundamentally distinct from the traditional size effect, and also is in contrast to the standard condensed matter physics. SATO results in a record high and stable sunlight photocatalytic degradation rate of 0.24, which exceeds those of available photocatalysis by approximately two orders of magnitude. The photocatalytic process is enabled by a quasi atom physical mechanism, in which, an electron in the spin up channel is excited from HOMO to LUMO+1 state, which can only occur in SATO with W5+.

physics.app-ph↗

arXiv2019

An Antenna Array Initial Condition Calibration Method for Integrated Optical Phased Array

This paper presents a modified rotating element electric field vector (modified REV) method to calibrate the antenna array initial condition of an optical phased array (OPA) device. The new method follows a similar sequential individual antenna phase calibration process while it modifies the algorithm to avoid possible π phase error in traditional REV when large initial phase distribution and finite optical power measurement accuracy present. We show that the method produces statistically more accurate and predictable calibration result which is highly desired in practice.

physics.app-ph↗

arXiv2019

Magnetically Tunable Organic Semiconductors with Superparamagnetic Nanoparticles

Magnetic nanoparticles (MNPs) exhibiting superparamagnetic properties might generate large magnetic dipole-dipole interaction with electron spins in organic semiconductors (OSECs). This concept could be considered analogous to the effect of hyperfine interaction (HFI). In order to investigate this model, Fe3O4 MNPs are used as a dopant for generating random hyperfine-like magnetic fields in a HFI-dominant π-conjugated polymer host, poly(2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MeH-PPV). The magnetoconductance (MC) response in organic light emitting diodes made by MeH-PPV/MNP blends is used to estimate the effective hyperfine field in the blends. Firstly, we find that the shape of the MC response essentially remains the same regardless of the MNP concentration, which is attributed to the similar functionality between the nuclear spins and the MNPs. Secondly, the width of MC increases with increasing MNP concentration. Magneto-optical Kerr effect experiments and micromagnetic simulation indicate that the additional increase of the MC width is associated with the strength of the magnetization of the blend. Finally, the MC broadening has the same temperature dependent trend as the magnetization of the MNPs where the unique effect of the MNPs in their superparamagnetic and ferromagnetic regimes on the MC response is observed. Magneto-photoinduced absorption (MPA) spectroscopy confirms that the MC broadening is not due to defects introduced by the MNPs, but is a result of unique superparamagnetic behavior. Our study yields a new pathway for tuning OSECs' magnetic functionality, which is essential to organic optoelectronic devices and magnetic sensor applications.

physics.app-ph↗