bluuebunny/crossref_metadata_2025 huggingface dataset:
An extended Vlasov–Fokker–Planck approach for kinetic simulations of laser plasmas
Authors: Shaffer N. R. , Sherlock M. , Maximov A. V. , Goncharov V. N.
Published: April 2023

Vlasov–Fokker–Planck simulation codes occupy an important niche in modeling laser-produced plasmas, since they are well suited to studying the effect of collisions on electron kinetic phenomena, especially energy transport. One of the most important elements of energy transport is the absorption of laser light by the plasma; however, simulating this in detail requires resolving oscillations of the laser light, whose characteristic timescale is orders of magnitude shorter than the simulation time needed to study transport physics. For this reason, most Vlasov–Fokker–Planck codes used to study electron transport in laser plasmas rely on simplified models of the laser–plasma coupling. Their underlying assumptions nominally preclude their use for modeling laser light having short-scale structure in space or time, such as broadband lasers. In this work, we derive a more general computational framework suitable for arbitrarily structured laser fields. Our approach is based on an extended set of Vlasov–Fokker–Planck equations that separately solve for the low- and high-frequency plasma response. We implement these extended Vlasov–Fokker–Planck equations in the spherical harmonic code K2 and demonstrate the performance of the method on several laser absorption test problems, with particular attention to the judicious selection of time steps, time integrators, and spherical harmonic truncation, according to the intensity and spectrum of the laser light under consideration. Comparison with the widely used Langdon absorption operator shows the Langdon operator performs remarkably well for predicting laser heating in the simple cases considered here, even in situations that would seem to violate its underlying assumptions.
Impedance switching for neural recording with ferroelectric microelectrodes
Authors: Becker Maximilian T. , Oldroyd Poppy , Strkalj Nives , Müller Moritz L. , Malliaras George G. , MacManus-Driscoll Judith L.
Published: April 2023

We demonstrate ferroelectric and bipolar impedance switching behavior in 18-nm-thick epitaxial BaTiO3 (BTO) films in an electrolyte–ferroelectric–semiconductor (EFS) configuration. The system is explored for its potential as a ferroelectric microelectrode in bioelectronics. Cyclic voltammetry measurements in EFS configuration, with a phosphate-buffered saline solution acting as the liquid electrolyte top contact, indicate characteristic ferroelectric switching peaks in the bipolar current–voltage loop. Moreover, small-signal electrochemical impedance spectroscopy measurements (applied root mean square voltage VRMS = 10 mV) on pre-poled EFS devices indicate bipolar impedance switching behavior. Also, a maximum ratio of the two different impedance magnitudes of ∼1.5 was observed at frequency f = 100 Hz. The observed impedance switching corresponds to a resistive switching effect, which could be explained by the modulation of the space charge region at the BTO/electrolyte interface via fixed ferroelectric polarization charges. Our approach represents a key step toward neural recordings with ferroelectric microelectrodes.
Perspective on active submillimeter electromagnetic wave imaging using CMOS integrated circuits technologies
Authors: O Kenneth K. , Choi Wooyeol , Han Ruonan
Published: April 2023

The performance of CMOS transmitters and receivers operating at the submillimeter electromagnetic wave frequencies have sufficiently improved for use in active transmission and reflection-mode imaging applications that have the potential for broad deployment and utilization. Imaging integrated circuits have the potential to be large in area to support a high number of pixels along with digital backend processing circuits. For high volume imaging applications that may eventually be included in automobiles, smartphones, laptops, tablets, and others, a large manufacturing capacity to support the volume of large area ICs is necessary. For this, the use of CMOS technologies with a much larger manufacturing capacity is favored. It should be possible to improve the performance of CMOS circuits to increase the range, and operation margin and frequency. The electronically steerable submillimeter-wave reflector technology holds the promise for improving the performance and energy efficiency of submillimeter-wave imaging systems by multiple orders of magnitude, and it is a critical research area. Increasing the operating frequency from 430 to 850 GHz using CMOS integrated circuits to improve the angular resolution by 2X at a given form factor (∼0.15° for a reflector diameter of 15 cm) can make the submillimeter-wave imaging competitive to the LIDAR angular-resolution performance, while providing superior capabilities in visually impaired conditions and making the imaging devices more affordable.
Platinum contacts for 9-atom-wide armchair graphene nanoribbons
Authors: Hsu Chunwei , Rohde Michael , Borin Barin Gabriela , Gandus Guido , Passerone Daniele , Luisier Mathieu , Ruffieux Pascal , Fasel Roman , van der Zant Herre S. J. , Abbassi Maria El
Published: April 2023

Creating a good contact between electrodes and graphene nanoribbons (GNRs) has been a long-standing challenge in searching for the next GNR-based nanoelectronics. This quest requires the controlled fabrication of sub-20 nm metallic gaps, a clean GNR transfer minimizing damage and organic contamination during the device fabrication, as well as work function matching to minimize the contact resistance. Here, we transfer 9-atom-wide armchair-edged GNRs (9-AGNRs) grown on Au(111)/mica substrates to pre-patterned platinum electrodes, yielding polymer-free 9-AGNR field-effect transistor devices. Our devices have a resistance in the range of 106–108 Ω in the low-bias regime, which is 2–4 orders of magnitude lower than previous reports. Density functional theory calculations combined with the non-equilibrium Green's function method explain the observed p-type electrical characteristics and further demonstrate that platinum gives strong coupling and higher transmission in comparison to other materials, such as graphene.
Local N-electron valence state perturbation theory using pair-natural orbitals based on localized virtual molecular orbitals
Authors: Uemura Kazuma , Saitow Masaaki , Ishimaru Takaki , Yanai Takeshi
Published: April 2023

Second-order N-electron valence state perturbation theory (NEVPT2) is an exactly size-consistent and intruder-state-free multi-reference theory. To accelerate the NEVPT2 computation, Guo and Neese combined it with the local pair-natural orbital (PNO) method using the projected atomic orbitals (PAOs) as the underlying local basis [Guo et al., J. Chem. Phys. 144, 094111 (2016)]. In this paper, we report the further development of the PNO-NEVPT2 method using the orthonormal and non-redundant localized virtual molecular orbitals (LVMOs) instead of PAOs. The LVMOs were previously considered to perform relatively poor compared to PAOs because the resulting orbital domains were unacceptably large. Our prior work, however, showed that this drawback can be remedied by re-forming the domain construction scheme using differential overlap integrals [Saitow et al., J. Chem. Phys. 157, 084101 (2022)]. In this work, we develop further refinements to enhance the feasibility of using LVMOs. We first developed a two-level semi-local approach for screening out so-called weak-pairs to select or truncate the pairs for PNO constructions more flexibly. As a refinement specific to the Pipek–Mezey localization for LVMOs, we introduced an iterative scheme to truncate the Givens rotations using varying thresholds. We assessed the LVMO-based PNO-NEVPT2 method through benchmark calculations for linear phenyl alkanes, which demonstrate that it performs comparably well relative to the PAO-based approach. In addition, we evaluated the Co–C bond dissociation energies for the cobalamin derivatives composed of 200 or more atoms, which confirms that the LVMO-based method can recover more than 99.85% of the canonical NEVPT2 correlation energy.
Near-infrared absorption of fused core-modified expanded porphyrins for dye-sensitized solar cells
Authors: Menéndez María Isabel , Montenegro-Pohlhammer Nicolas , Pino-Rios Ricardo , Urzúa-Leiva Rodrigo , Morales-Lovera Simone , Borges-Martínez Merlys , Granados-Tavera Kevin , López Ramón , Cárdenas-Jirón Gloria
Published: April 2023

Photophysical, photovoltaic, and charge transport properties of fused core-modified expanded porphyrins containing two pyrroles, one dithienothiophene (DTT) unit, and 1–4 thiophenes (1–4) were inspected by using density functional theory (DFT) and time-dependent DFT. Compounds 1–3 have been investigated experimentally before, but 4 is a theoretical proposal whose photophysical features match those extrapolated from 1 to 3. They exhibit absorption in the range of 700–970 nm for their Q bands and 500–645 nm for their Soret bands. The rise of thiophene rings placed in front of the DTT unit in the expanded porphyrin ring causes a bathochromic shift of the longest absorption wavelength, leading to near-infrared absorptions, which represent 49% of the solar energy. All the systems show a thermodynamically favorable process for the electron injection from the dye to TiO2 and adsorption on a finite TiO2 model. The electron regeneration of the dye is only thermodynamically feasible for the smallest expanded porphyrins 1 and 2 when I−/I3− electrolyte is used. The charge transport study shows that for voltages lower than 0.4 V, junctions featuring pentaphyrin 1 and octaphyrin 4 are more conductive than those containing hexaphyrin 2 or heptaphyrin 3. The results showed that the four fused core-modified expanded porphyrins investigated are potential dyes for applications in dye-sensitized solar cells, mainly pentaphyrin 1 and hexaphyrin 2. Moreover, increasing the number of thiophene rings in the macrocycle proved fruitful in favoring absorption in the near-infrared region, which is highly desired for dye-sensitized solar cells.
Large eddy simulations of fully developed turbulent flows over additively manufactured rough surfaces
Authors: Garg Himani , Wang Lei , Sahut Guillaume , Fureby Christer
Published: April 2023

In the last decade, with the growing demand for efficient and more sustainable products that reduce our CO2 footprint, progresses in Additive Manufacturing (AM) have paved the way for optimized heat exchangers, whose disruptive design will heavily depend on predictive numerical simulations. Typical AM rough surfaces show limited resemblance to the artificially constructed rough surfaces that have been the basis of most prior fundamental research on turbulent flow over rough walls. Hence, current wall models used in steady and unsteady three-dimensional (3D) Navier–Stokes simulations do not consider such characteristics. Therefore, a high-fidelity Large Eddy Simulation (LES) database is built to develop and assess novel wall models for AM. This article investigates the flow in rough pipes built from the surfaces created using AM techniques at Siemens based on Nickel Alloy IN939 material. We developed a code to generate the desired rough pipes from scanned planar surfaces. We performed high-fidelity LES of turbulent rough pipe flows at Reynolds number, Re = 11 700, to reveal the influence of roughness parameters on turbulence, mainly the average roughness height and the effective slope. The equivalent sand-grain roughnesses, ks, of the present AM rough surfaces are predicted using the Colebrook correlation. The main contributors to the skin friction coefficient are found to be turbulence and drag forces. In the present study, the existence of a logarithmic layer is marked even for high values of ks. The mean flow, the velocity fluctuations, and the Reynolds shear stresses show turbulence's strong dependence on the roughness topography. Profiles of turbulence statistics are compared by introducing an effective wall-normal distance defined as zero-plane displacement. The effective distance collapses the shear stresses and the velocity fluctuations outside the roughness sublayer; thus, Townsend's similarity of the streamwise mean velocity is marked for the present roughnesses. Furthermore, a mixed scaling is introduced to improve the collapse of turbulence statistics in the roughness sublayer. In addition, an attempt to investigate the impact of surface roughness on flow physics using the acquired LES results based on quadrant analysis of the Reynolds shear stresses and anisotropy of turbulence is made.
Numerical investigation of electrohydrodynamically enhanced flow boiling inside minichannels: The seeding model
Authors: Soleimani Mehran , Ahmadpour Ali , Amani Ehsan
Published: April 2023

In the present study, three-dimensional numerical simulations are performed to examine the effect of applied direct current electric fields on the subcooled flow boiling heat transfer in a vertical minichannel. The volume of fluid model is used to capture the liquid–vapor interfaces, and the conservation laws, together with the electrohydrodynamic (EHD) equations, are solved using the finite volume method. Two mass transfer models of Lee and Fourier are evaluated, and a new seeding algorithm is developed based on the physics of the bubble formation and departure on the heated walls, integrated with the Fourier model. The early transition from slug to churn/annular flow regimes, due to EHD forces, is observed in the numerical solutions with both models, in agreement with the available experiment. Nevertheless, the Lee model tuned for the electric-free condition fails to predict the EHD-induced heat transfer enhancement, while the Fourier model with the new seeding algorithm captures this phenomenon with reasonable accuracy. Based on the present numerical results, this effect can be attributed to the migration of small bubbles toward the walls under the effect of electric fields, forming recirculating flow regions near the walls, augmenting the flow mixing, and mitigating hot spots.
Effect of layer structure of AlN interlayer on the strain in GaN layers during metal-organic vapor phase epitaxy on Si substrates
Authors: Deura Momoko , Nakahara Takuya , Lee Wan Chi , Momose Takeshi , Nakano Yoshiaki , Sugiyama Masakazu , Shimogaki Yukihiro
Published: April 2023

It is highly challenging to grow high-quality gallium nitride (GaN) layers on silicon (Si) substrates due to the intrinsic mismatching of their structural and thermal properties. Aluminum nitride (AlN) interlayers have been used to induce a compressive strain to GaN layers during growth, which compensates for the tensile strain in these layers on Si substrates during cooling. In this study, we investigated the effect of the growth temperature and layer structure of the AlN interlayer to understand the relationship between surface flatness and relaxation ratio of the AlN interlayer and the compressive strain in the overlying GaN layer. Low-temperature (LT) growth enhanced lattice relaxation of the AlN interlayer, whereas the AlN surface was atomically flat at high temperature (HT). We also examined a two-step growth to combine the advantages of LT- and HT-AlN. This approach resulted in a surface with multiple flat regions separated by grooves, which had the largest compressive strain in the overlying GaN layer at the early stages of growth. At later stages, the strain was the largest on the HT-AlN interlayer. In both cases, the experimentally measured compressive strain exceeded simulated predictions. Finally, possible solutions for inducing a larger compressive strain in the GaN layer using interlayers were discussed.
Ionization induced by the ponderomotive force in intense and high-frequency laser fields
Authors: Zhu Mingyu , Liu Yuxiang , Wei Chunli , Ni Hongcheng , Wei Qi
Published: April 2023

Atomic stabilization is a universal phenomenon that occurs when atoms interact with intense and high-frequency laser fields. In this work, we systematically study the influence of the ponderomotive (PM) force, present around the laser focus, on atomic stabilization. We show that the PM force could induce tunneling and even over-barrier ionization to the otherwise stabilized atoms. Such effect may overweigh the typical multiphoton ionization under moderate laser intensities. Our work highlights the importance of an improved treatment of atomic stabilization that includes the influence of the PM force.
High energy storage performance in Sr0.64Bi0.18Na0.18TiO3-ZnO hybrid solid solution ceramics
Authors: Guo Jian , Yu Hao-Ran , Zhang Shan-Tao , Yang Bin
Published: April 2023

Ceramic-based capacitors with excellent recoverable energy storage density (Wrec) and efficiency (η) are greatly desired for pulsed capacitors but challenge as well. In this work, the high energy storage performance with an Wrec of 5.1 J/cm3, an η of 93.4%, and an ultrafast discharge–charge speed t0.9 of 72 ns is achieved in unusual interstitial and substitutional hybrid solid solution of Sr0.64Bi0.18Na0.18TiO3-ZnO (SBNT-Zn), in which two Zn2+ ions occupy one A-site, while one Zn2+ ion occupies one B-site of ABO3 perovskite lattice simultaneously. The addition of Zn2+ increases lattice distortion and breaks long-range ferroelectric order, resulting in more highly dynamic polar nanoregions, higher relaxation behaviors as well as larger polarization (ΔP). Moreover, the extraordinary hybrid solid solution configuration enhances the bandgap of SBNT-Zn and ZnO as a sintering aid refines grain size, both help to enhance electric breakdown strength. This work affirms the feasibility and reliability of forming a hybrid solid solution to develop high-performance ferroelectric materials.
Numerical validation of an effective slender fault source solution for past tsunami scenarios
Authors: Gomez Bernabe , Kadri Usama
Published: April 2023

To estimate tsunami hazards, it is first necessary to have reliable data relating to the rupture characteristics, such as epicenter, fault geometry, uplift speed, and duration. We made use of a mathematical model that combines analytical and machine learning technique capable of retrieving rupture characteristics from acoustic data. The model was applied with short computational times to data recorded by the comprehensive nuclear-test-Ban Treaty organization hydrophones during four tectonic events that were reported to trigger tsunami waves. The presented inverse problem model for acoustic waves with adequate tsunami propagation tools can be used as a complementary technique alongside tsunami warning systems due to the high propagating speeds of the sound in the ocean. In this paper, the validity of the solutions provided by the inverse problem model is tested by using the calculated earthquake parameters as input to the Cornell multi-grid coupled tsunami numerical model, which, in turn, output surface wave elevations (tsunami) to be compared against deep-ocean assessment and reporting of tsunamis buoy data.
Phonons in the 1/f noise of topological insulators
Authors: Mihaila M. , Dinulescu S. , Varasteanu P.
Published: April 2023

In topological insulators, such as (Bi,Sb)2Te3 and BiSbTeSe1.6, the 1/f noise intensity features intriguing peaks, which develop at some specific temperatures. In search for their microscopic origin, we compared this noise structure with either phonon density of states or Raman spectrum of each topological insulator (TI), respectively. In (Bi,Sb)2Te3, the comparison revealed that the noise peaks track the van Hove singularities in the phonon density of states. The most intense noise peak observed in (Bi,Sb)2Te3 at 50 K is attributed to the thermal motion of the Bi atoms. Other less intense noise peaks are assigned to either a single phonon mode or multi-phonon combinations. We found that thermal vibrations of Bi and Te2 atoms in different symmetry directions are involved in most of the phonon combinations, which stand for the signature of the lattice anharmonicity in noise. The noise increase observed in (Bi,Sb)2Te3 and BiSbTeSe1.6 above a specific temperature threshold is attributed to the strengthening of the carrier–phonon coupling induced by anharmonicity. In the case of BiSbTeSe1.6, we show that all noise singularities are mirrored in the Raman spectrum of a structurally close TI (BiSbTeSe2) in the whole temperature range. This indicates that although transport can be at the surface or in the bulk or both of them, the carrier–phonon interaction is the only source of 1/f fluctuations in TIs. Inherently, these results imply that the microscopic origin of 1/f noise in solid is in the perpetual thermal motion of the atoms.
Superconductor digital circuits with π junctions alone
Authors: Li Feng , Takeshita Yuto , Tanaka Masamitsu , Fujimaki Akira
Published: April 2023

We adopt superconductor/ferromagnet/insulator/superconductor (SFIS) Josephson junctions (JJs) as both switching JJs and intrinsic π phase shifters in superconductor digital circuits. The critical current density (Jc) and characteristic voltage (Vc) of the SFIS junctions are about 22 A/cm2 and 22 μV, respectively. The intrinsic π phase shift is confirmed by measuring the suppressed nominal critical current Icn and half-period-shifted modulation pattern of a π–π–π superconducting quantum interference device (SQUID) that contains three π-JJs in a superconducting loop. A single-flux-quantum (SFQ) circuit composed of a DC/SFQ, Josephson transmission line (JTL), and SFQ/DC converter based on SFIS JJs alone is demonstrated at 4.2 K. The energy dissipation of the SFQ/DC converter decreases by 80% because some JJs are self-biased by the π phase shifter. The intrinsic circulating currents induced by the π phase shifters lead to a narrow bias margin (±5%) and even error function, which can be solved by parameters optimization or circuit initialization in the future. The half-modulation period (Φ0/2) of a half-flux-quantum (HFQ) SQUID (a partial HFQ JTL) exhibits propagation of HFQ between π–π–π SQUIDs, indicating that more complex HFQ circuits can be developed with π-JJs alone in the future.
Scalable fabrication of hemispherical solid immersion lenses in silicon carbide through grayscale hard-mask lithography
Authors: Bekker Christiaan , Arshad Muhammad Junaid , Cilibrizzi Pasquale , Nikolatos Charalampos , Lomax Peter , Wood Graham S. , Cheung Rebecca , Knolle Wolfgang , Ross Neil , Gerardot Brian , Bonato Cristian
Published: April 2023

Grayscale lithography allows the creation of micrometer-scale features with spatially controlled height in a process that is fully compatible with standard lithography. Here, solid immersion lenses are demonstrated in silicon carbide using a fabrication protocol combining grayscale lithography and hard-mask techniques to allow nearly hemispherical lenses of 5μm radius to be etched into the substrate. Lens performance was benchmarked by studying the enhancement obtained in the optical collection efficiency for single quantum emitters hosted in silicon carbide. Enhancement by a factor of 4.4 ± 1.0 was measured for emitters not registered to the center of the lens, consistent with devices fabricated through other methods. The grayscale hard-mask technique is highly reproducible, scalable, and compatible with CMOS technology, and device aspect ratios can be tuned after resist patterning by controlling the chemistry of the subsequent dry etch. These results provide a reproducible, low-cost, high-throughput and industrially relevant alternative to focused ion beam milling for the creation of high-aspect-ratio, rounded microstructures for quantum technology, and microphotonic applications.
Flow imaging as an alternative to non-intrusive measurements and surrogate models through vision transformers and convolutional neural networks
Authors: Miotto Renato F. , Wolf William R.
Published: April 2023

A numerical framework is proposed whereby flow imaging data are leveraged to extract relevant information from flowfield visualizations. To this end, a vision transformer (ViT) model is developed to predict quantities of interest from images of unsteady flows. Here, the unsteady pressure distribution, the aerodynamic coefficients, and the skin friction coefficient are computed for an airfoil under dynamic stall as an example. The network is capable of identifying relevant flow features present in the images and associate them to the airfoil response. Results demonstrate that the model is effective in interpolating and extrapolating between flow regimes and for different airfoil motions, meaning that ViT-based models may offer a promising alternative for sensors in experimental campaigns and for building robust surrogate models of complex unsteady flows. In addition, we uniquely treat the image semantic segmentation as an image-to-image translation task that infers semantic labels of structures from the input images in a supervised way. Given an input image of the velocity field, a resulting convolutional neural network generates synthetic images of any corresponding fluid property of interest. In particular, we convert the velocity field data into pressure in order to subsequently estimate the pressure distribution over the airfoil in a robust manner. This approach proves to be effective in mapping between flowfield properties.
Modeling the electrical degradation of AlGaN-based UV-C LEDs by combined deep-level optical spectroscopy and TCAD simulations
Authors: Roccato Nicola , Piva Francesco , De Santi Carlo , Buffolo Matteo , Fregolent Manuel , Pilati Marco , Susilo Norman , Vidal Daniel Hauer , Muhin Anton , Sulmoni Luca , Wernicke Tim , Kneissl Michael , Meneghesso Gaudenzio , Zanoni Enrico , Meneghini Matteo
Published: April 2023

The long-term stability of ultraviolet (UV)-C light-emitting diodes (LEDs) is of major importance for many applications. To improve the understanding in this field, we analyzed the degradation of AlGaN-based UVC LEDs and modeled the variation of electrical characteristics by 2D simulations based on the results of deep-level optical spectroscopy (DLOS). The increase in the forward leakage current observed during ageing was ascribed an increase in trap-assisted tunneling. The analysis of the degradation kinetics suggests the role of a defect diffusion process, possibly involving impurities coming from the p-type layers.
Plasmon mediated extra-ordinary optical transmission through an apertureless plasmonic metagrating
Authors: Verma Sagar Kumar , Srivastava Sachin Kumar
Published: April 2023

Extra-ordinary optical transmission (EOT) through subwavelength plasmonic nanoapertures is possible due to the funneling of light via surface plasmons (SPs) at the resonant wavelengths through the apertures. In this Letter, we experimentally demonstrate EOT through a plasmonic metagrating which does not have any open apertures. The plasmonic metagrating was fabricated by deposition of silver (Ag) on a one-dimensionally patterned flexible and transparent polydimethylsiloxane grating obtained via pattern imprinting and subsequent peeling off a commercially available blue ray disk. For normal incidence of transverse magnetic-polarized light on the top surface of plasmonic metagrating, transmission of light through it was obtained in the visible wavelength range of electromagnetic spectrum. Control experiments on variation of Ag film thickness were performed to attain optimal parameters for maximum transmission, followed by polarization and refractive index (RI) dependent performance of the plasmonic metagrating. Electric fields and Poynting vector profiles were simulated using a finite element method to explain the interaction of light with the plasmonic metagrating and the mechanism of plasmon mediated optical transmission. Such a large optical transmission is possible because the SP modes generated at metal–air interface penetrate through metagrating and couple with those supported by the metal–substrate interface. As a model application, RI sensing using the plasmonic metagrating was demonstrated. The present study shows that optical transmission is possible from apertureless structures and enriches literature with better understanding of EOT. Moreover, it opens avenues for development of flexible, cost-effective plasmonic metagratings for sensors, spectral filters, polarizers, etc.
Unraveling the decarboxylation dynamics of the fluorescein dianion with fragment action spectroscopy
Authors: Gibbard Jemma A. , Verlet Jan R. R.
Published: April 2023

The decarboxylation dynamics of the doubly deprotonated fluorescein dianion, Fl2−, are investigated by recording fragment action spectra for the anion, Fl−, and its decarboxylated analog, Fl–CO2−, using a new reflectron secondary mass spectrometer. The formation of the anion, Fl−, is directly investigated by photoelectron imaging. The Fl− and Fl–CO2− action spectra indicate that, for λ < 400 nm, one-photon dissociative photodetachment, i.e., simultaneous decarboxylation and electron loss, competes with photodetachment, whereas for λ > 400 nm, decarboxylation only proceeds following electron loss via a sequential two-photon process. The primary decarboxylation pathway is the ready loss of CO2 from the relatively short-lived intermediate excited state, Fl−[D1], which is formed by electron loss from the dianion via resonant tunneling through the repulsive Coloumb barrier associated with a high-lying excited dianion state, Fl2−[S2].
Stochastic modeling of blob-like plasma filaments in the scrape-off layer: Theoretical foundation
Authors: Losada J. M. , Theodorsen A. , Garcia O. E.
Published: April 2023

A stochastic model for a superposition of uncorrelated pulses with a random distribution of amplitudes, sizes, and velocities is presented. The pulses are assumed to move radially with fixed shape and amplitudes decaying exponentially in time due to linear damping. The pulse velocities are taken to be time-independent but randomly distributed. The implications of a distribution of pulse amplitudes, sizes, and velocities are investigated. Closed-form expressions for the cumulants and probability density functions for the process are derived in the case of exponential pulses and a discrete uniform distribution of pulse velocities. The results describe many features of the boundary region of magnetically confined plasmas, such as high average particle densities, broad and flat radial profiles, and intermittent large-amplitude fluctuations. The stochastic model elucidates how these phenomena are related to the statistical properties of blob-like structures. In particular, the presence of fast pulses generally leads to flattened far scrape-off layer profiles and enhanced intermittency, which amplifies plasma–wall interactions.
Theoretical and experimental studies of W-band relativistic surface-wave oscillator of planar geometry
Authors: Zaslavsky V. Yu. , Palitsin A. V. , Rodin Yu. V. , Malkin A. M. , Sergeev A. S. , Fedotov A. E. , Goykhman M. B. , Gromov A. V. , Guznov Yu. M. , Panin A. N. , Gulovskii D. R. , Peskov N. Yu. , Ginzburg N. S.
Published: April 2023

This paper presents the results of theoretical and experimental studies of a surface-wave oscillator (SWO) of planar geometry excited by a ribbon high-current relativistic electron beam. Within the framework of the quasi-optical approach and direct three-dimensional particle-in-cell modeling, we demonstrate the advantages of open transverse edged configuration against the closed one for effective mode selection at a fairly large oversize factor. In the experiments carried out on the basis of the SINUKI accelerator (Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, 1 kA/650 keV/17 ns), we form a magnetically guided ribbon electron beam with a cross-section of 0.3 × 20 mm2, which moves parallel to a slow-wave structure with a period of 1.75 mm. A planar W-band SWO both of open and closed edged configuration has been experimentally tested. In full agreement with modeling, the open configuration exhibits much more stable operation, where at a frequency of 75 GHz, we observe pulse generation with duration of about 7 ns. The output power measured by the calorimetric method reaches 25 MW.
Microbial stir bars: Light-activated rotation of tethered bacterial cells to enhance mixing in stagnant fluids
Authors: Gurung Jyoti P. , Navvab Kashani Moein , de Silva Charitha M. , Baker Matthew A. B.
Published: April 2023

Microfluidics devices are gaining significant interest in biomedical applications. However, in a micron-scale device, reaction speed is often limited by the slow rate of diffusion of the reagents. Several active and passive micro-mixers have been fabricated to enhance mixing in microfluidic devices. Here, we demonstrate external control of mixing by rotating a rod-shaped bacterial cell. This rotation is driven by ion transit across the bacterial flagellar stator complex. We first measured the flow fields generated by rotating a single bacterial cell rotationally locked to rotate either clockwise (CW) or counterclockwise (CCW). Micro-particle image velocimetry (μPIV) and particle tracking velocimetry results showed that a bacterial cell of ∼ 2.75 μm long, rotating at 5.75 ± 0.39 Hz in a counterclockwise direction could generate distinct micro-vortices with circular flow fields with a mean velocity of 4.72 ± 1.67 μm/s and maximum velocity of 7.90 μm/s in aqueous solution. We verified our experimental data with a numerical simulation at matched flow conditions, which revealed vortices of similar dimensions and speed. We observed that the flow-field diminished with increasing z-height above the plane of the rotating cell. Lastly, we showed that we could activate and tune rotational mixing remotely using strains engineered with proteorhodopsin, where rotation could be activated by controlled external illumination using green laser light (561 nm).
Biomimetic micropump: Leveraging a novel propagative rhythmic membrane function
Authors: Kumar Mahesh , Mansukhani Jaikishan , Tripathy Arijeet , Mondal Pranab Kumar
Published: April 2023

We discuss bioinspired pumping mechanism in a microfluidic configuration, consistent with the newly formulated propagative rhythmic contraction–expansion of a membrane attached to the channel wall. We aptly demonstrate the kinematics of the proposed membrane function and describe the wall profile developed due to the propagative expansion–contraction phases of the actuation cycle. The transport equations governing the flow dynamics are solved analytically in the purview of the lubrication theory. An analysis of the proposed model establishes that the fluidic functionality of a channel equipped with a single elastic membrane that operates following the propagation expansion-contraction modes can produce a unidirectional flow, and acts as a micropump in the process. By depicting the flow velocity, velocity contours, and streamlines patterns in the flow pathway, we discuss and demonstrate the eventual consequence of these flow parameters to the net throughput during both the compression and expansion phases of the process. We establish the efficacy of the novel membrane function by demonstrating augmented net throughput obtained from the proposed configuration. The membrane function developed in this endeavor will provide greater flexibility to the researchers to carry out further research in the field and will make the underlying analysis even easier in systems that utilize electromagnetic fields in the flow system.
Faithful guiding-center orbits in an axisymmetric magnetic field
Authors: Brizard Alain J. , Hodgeman Brook C.
Published: April 2023

The problem of the charged-particle motion in an axisymmetric magnetic geometry is used to assess the validity of higher-order Hamiltonian guiding-center theory, which includes higher-order corrections associated with gyrogauge invariance as well as guiding-center polarization induced by magnetic-field non-uniformity. Two axisymmetric magnetic geometries are considered: a magnetic mirror geometry and a simple tokamak geometry. When a magnetically confined charged-particle orbit is regular (i.e., its guiding-center magnetic moment is adiabatically invariant), the guiding-center approximation, which conserves both energy and azimuthal canonical angular momentum, is shown to be faithful to the particle orbit when higher-order corrections are taken into account.
Moiré fringes in conductive atomic force microscopy
Authors: Richarz L. , He J. , Ludacka U. , Bourret E. , Yan Z. , van Helvoort A. T. J. , Meier D.
Published: April 2023

Moiré physics plays an important role in characterization of functional materials and engineering of physical properties in general, ranging from strain-driven transport phenomena to superconductivity. Here, we report on the observation of moiré fringes in conductive atomic force microscopy (cAFM) scans gained on the model ferroelectric Er(Mn,Ti)O3. By performing a systematic study of the impact of key experimental parameters on the emergent moiré fringes, such as scan angle and pixel density, we demonstrate that the observed fringes arise due to a superposition of the applied raster scanning and sample-intrinsic properties, classifying the measured modulation in conductance as a scanning moiré effect. Our findings are important for the investigation of local transport phenomena in moiré engineered materials by cAFM, providing a general guideline for distinguishing extrinsic from intrinsic moiré effects. Furthermore, the experiments provide a possible pathway for enhancing the sensitivity, pushing the resolution limit of local transport measurements by probing conductance variations at the spatial resolution limit via more long-ranged moiré patterns.
Glass softening in the limit of high heating rates: Heterogeneous devitrification kinetics on nano, meso, and micrometer scale
Authors: Kaur Rinipal , Bhattacharya Deepanjan , Cubeta Ulyana S. , Sadtchenko Vlad
Published: April 2023

When heated rapidly, glasses often devitrify heterogeneously, i.e., by a softening front that originates at the surface of an amorphous film. Yet the fundamentals of this devitrification regime are not completely understood; depending on experimental conditions, the reported front propagation distances differ by an order of magnitude. Using a high-resolution fast scanning calorimetry technique, we have investigated the softening of glassy methylbenzene films with thicknesses between 30 and 1400 nm. We confirm first that, in all films, the devitrification process begins with the formation of a softening front that propagates through the films over distances of ∼50 nm and that the front propagation kinetics at this stage follow an Arrhenius law. However, we also show that, in films with thicknesses above 165 nm, the front propagation does not terminate with the onset of bulk softening. Specifically, increasing the films’ thicknesses above 165 nm yields sharp, clearly discernible endotherms that precede the bulk softening endotherms and that are consistent with a two-fold increase in the enthalpic barrier to front propagation at a well-defined critical temperature. We term this phenomenon “Arrhenius discontinuity” and use reaction rate and continuum front dynamics theories to explain its origins and the physical nature of the resulting distinct heterogeneous devitrification processes. Finally, we discuss the findings in the context of recent theoretical, computational, and experimental studies of heterogeneous devitrification by other research groups.
Dynamics of non-spherical particles resting on a flat surface in a viscous fluid
Authors: Simmons Tiffany , Daghooghi Mohsen , Borazjani Iman
Published: April 2023

The resting dynamics of non-spherical particles on a flat surface can be considered the last phase in settling a particle, which has yet to be fully investigated. This last phase for the non-spherical particle is numerically investigated, for the first time, using a sharp-interface immersed boundary method coupled with a kinematic-based collision model. The collision model guarantees a realistic, stable/settled position of non-spherical-shaped particles, contrary to alternative models that implement a repulsive penalty force. In the simulations, a single particle is released with a constant velocity downwards close to the wall until the collision occurs. Hydrodynamic moments alter the settling dynamics depending on the Reynolds number (Re) by opposing the gravity-driven motion of particles. It was observed that the settling trajectories/angles were generally not affected for each particle, but their rate of change, i.e., angular velocities, reduced as the Reynolds number decreased. A simplified model for the hydrodynamic moment was explored based on a modified Stokes drag moment for spherical particles, which includes a shape factor Kn for relating non-spherical particles to spherical ones. It was found that using the projected area of non-spherical particles provided the best overall scaling to find their equivalent spheres because it provided the lowest Kn values. In addition, Kn was found to deviate from the constant theoretical value because of the build-up pressure between the particle and the wall which changed with Re. A linear relation between the mean Kn and Re was found to be a good approximation. This work demonstrates how particle-resolved simulations can provide the data required for developing simplified models for non-spherical particles.
Comprehensive analysis of the contribution from the heat capacity in thermal measurement on nanoscale thin films
Authors: Zhou Jing , Fan Xuanhui , Zhang Zhongyin , Li Donghao , Zhu Jie , Tang Dawei
Published: April 2023

Nanofilms (NFs) have been widely used in many emerging applications, such as microelectronic devices, spintronics, and optical instruments. Characterizing the thermal conductivity (k) of NFs, kNFs, is nontrivial for both fundamental science and industrial applications. Time-domain thermoreflectance (TDTR) is a powerful technique for thermal characterizations under nano-to-micro-scales. However, both the kNFs and the interface thermal resistance between layers are generally unknown parameters when analyzing TDTR signals. So that an effective thermal resistance model (ETRM) is often utilized, where the impact of heat capacity of NFs, cNFs, has been always ignored. Previous studies have proposed viewpoints on how to validate this assumption, however, which still needs to be verified further by considering the parameters' sensitivities for TDTR signals. In this work, we have highlighted the significance of sensitivity analysis for the investigation of the impact of cNFs with the example experiments on Ni NFs. The error of ETRM has been quantitatively and systematically studied. We found that the application requirements of ETRM in TDTR data analysis rely on the sensitivity relations between kNFs and interface thermal resistance. Finally, suggestions for future characterization of the kNFs have been discussed for reference. Our results and conclusions deepen the understanding of both the transient thermal transport process of multilayer NFs and the ability of TDTR on characterizing kNFs.
Electro-optic modulation using lithium niobate metasurfaces with topological corner state
Authors: Gu Zhidong , Tan Haiyun , Peng Jiaxin , Chen Jiaxin , Zhang Di , Xie Fei , Wu Wei , Cai Wei , Ren Mengxin , Xu Jingjun
Published: April 2023

Electro-optically (EO) tunable metasurfaces have received considerable attention owing to their capability for dynamic light field control. Here, we report an EO lithium niobate metasurface mediated by topological corner states. Such a supercell of metasurface is constructed by two kinds of finite-sized arrays possessing different topological properties via the generalized two-dimensional (2D) Su–Schrieffer–Heeger model. The generated narrow-linewidth resonance with high-quality factor and strong field localization are very suitable for EO modulation. The results reveal that the required driving external voltage to fully shift the resonance can be well below 15 V. Furthermore, the 0.46 rad of phase modulation is also demonstrated along with transmission intensity modulation. The results offer a fundamental path of potential for tunable displays, light detection, spatial light modulators, and Floquet topological metasurfaces.
Tunable spin texture transformation with field-free biskyrmion over a broad temperature range in rare-earth ferrimagnets
Authors: Xu Jiawang , Wang Zhan , Li Zhuolin , Gao Yang , Hu Chaoqun , Wang Dingsong , Zhang Jingyan , Zheng Xinqi , Cai Jianwang , Zhao Tongyun , Wang Shouguo , Zhang Ying , Shen Baogen
Published: April 2023

Exploring and discovering various types of skyrmions has enriched the fundamental study and the active skyrmionics aiming at using skyrmions in spintronics due to the advantages of global stability as high-density magnetic information bit. The unique capability to introduce abundant spin structures, physical phenomena, and dynamics due to the interactions of 4f and 3d electrons push the rare-earth–transition metal (RE–TM) magnets into the research frontier again. Herein, the exotic magnetic domain configurations are discovered in traditional ThMn12-type RE–TM magnets. It is clearly demonstrated that the magnetic anisotropy alteration and magnetic coupling between the respective RE and TM sublattices are responsible for the abundant magnetic domain evolution. In particular, the field-free biskyrmions at room temperature and tunable spin texture transformation are successfully obtained via manipulating the in-plane magnetic anisotropy, which has broadened the physical mechanism and application exploration of manipulating ferrimagnetic order to generate topological spin structures in strategic rare-earth magnets.
Field-free spin–orbit devices via heavy-metal alloy with opposite spin Hall angles for in-memory computing
Authors: Lan Xiukai , Liu Xiangyu , Bekele Zelalem Abebe , Lei Kun , Wang Kaiyou
Published: April 2023

With the advantages of high speed, low energy consumption, and non-volatility, spin–orbit devices are promising to be used in the field of in-memory computing. However, for large-scale integration, a simpler field-free switching scheme needs to be further explored. Here, we prepared field-free spin–orbit devices based on the PtW alloy layer with competing spin currents. The preparation of such devices is friendly to integration, because there is no requirement of introducing additional processing technology. Only the traditional heavy-metal layer is needed to be replaced by an alloy layer with opposite spin Hall angles. A series of positive and negative pulsed current tests have shown a stable field-free magnetization switching in the Ta/PtW/Co/AlOx/Pt device. The programmable Boolean logic of NAND and NOR were performed in a single device by changing the initial magnetization state. In addition, a pair of devices were connected with always opposite magnetizations to implement the XNOR logic gate, which can be applied to perform the dot product operation in the binary neural network. Based on the spin XNOR gates, a three-layer binary neural network achieves 89% recognition accuracy of handwritten digits. Our findings pave the way to efficient in-memory computing applications.
Research on the β-Ga2O3 Schottky barrier diodes with oxygen-containing plasma treatment
Authors: He Yun-Long , Sheng Bai-Song , Hong Yue-Hua , Liu Peng , Lu Xiao-Li , Zhang Fang , Wang Xi-Chen , Li Yuan , Zheng Xue-Feng , Ma Xiao-Hua , Hao Yue
Published: April 2023

This Letter reports two kinds of oxygen-containing plasma treated β-Ga2O3 Schottky barrier diodes (SBDs), including N2O plasma treatment and O2 plasma treatment, and the SBD without plasma is prepared for comparison. I–V characteristics, breakdown characteristics, and trap state characteristics of three devices have been studied. It is found that the turn-on voltage of SBDs with N2O plasma can reduce to 0.6 V, and the better current density of 750 A/cm2 and an on-resistance of 3.5 mΩ cm2 are obtained after the N2O plasma treatment. Moreover, the breakdown voltage of SBDs with N2O plasma is 50.2% higher than the conventional one, whose value reaches 323 V. In addition, the trap states' characteristics of the devices are studied, which show that the oxygen-containing plasma can reduce the deep level trap states density partly in the anode region, which can improve the surface quality effectively.