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Jodi Ackerman Frank
Published: 22 October 2021
Scilight, Volume 2021; https://doi.org/10.1063/10.0006721

Abstract:
The iron-oxide nanomagnets could have applications in magnetic hyperthermia to treat cancer.
Jodi Ackerman Frank
Published: 22 October 2021
Scilight, Volume 2021; https://doi.org/10.1063/10.0006879

Abstract:
Efficient parametric down-conversion together with photon-number-resolving detection makes multiphoton quantum metrology robust and scalable.
Anne Cockshott
Published: 22 October 2021
Scilight, Volume 2021; https://doi.org/10.1063/10.0006842

Abstract:
Creating and studying 3D tissue platforms will aid understanding of the fundamental pathology and treatment of diabetes mellitus.
Aili McConnon
Published: 22 October 2021
Scilight, Volume 2021; https://doi.org/10.1063/10.0007040

Abstract:
Heat transfer in transistors at cryogenic temperatures occurs by radiation of atomic vibrations and limits the noise performance of modern microwave amplifiers.
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0062523

Abstract:
In this Perspective, I raise the question whether the terms high entropy ceramics, high entropy nitrides, high entropy oxides, high entropy borides, etc., are meaningful considering the magnitude of the calculated configurational entropy. Here, the origin of Boltzmann's equation is reviewed and the implications for application are discussed. This back to the roots’ excursion may be helpful for approaching an answer to the question raised in the title and for re-evaluating literature reports connecting superior performance with configurational entropy of so-called high entropy ceramics.
Alexander Antonov, ,
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0065190

Abstract:
Single-file transport in pore-like structures constitute an important topic for both theory and experiment. For hardcore interacting particles, a good understanding of the collective dynamics has been achieved recently. Here we study how softness in the particle interaction affects the emergent transport behavior. To this end, we investigate driven Brownian motion of particles in a periodic potential. The particles interact via a repulsive softcore potential with a shape corresponding to a smoothed rectangular barrier. This shape allows us to elucidate effects of mutual particle penetration and particle crossing in a controlled manner. We find that even weak deviations from the hardcore case can have a strong impact on the particle current. Despite of this fact, the knowledge about the transport in a corresponding hardcore system is shown to be useful to describe and interpret our findings for the softcore case. This is achieved by assigning a thermodynamic effective size to the particles based on the equilibrium density functional of hard spheres.
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0070178

Abstract:
It has recently been suggested that a breakdown of electroneutrality occurs in highly confined nanopores that are encompassed by a dielectric material. This work elucidates the conditions for this breakdown. We show that the breakdown within the pore results from the response of the electric field within the dielectric. Namely, we show that this response is highly sensitive to the boundary condition at the dielectric edge. The standard Neumann boundary condition of no-flux predicts that the breakdown doesn't occur. However, a Dirichlet boundary condition for a zero-potential predicts a breakdown. Within this latter scenario, the breakdown exhibits a dependence on the thickness of the dielectric material. Specifically, infinite thickness dielectrics do not exhibit a breakdown, while dielectrics of finite thickness do exhibit a breakdown. Numerical simulations confirm theoretical predictions. The breakdown outcomes are discussed with regard to single pore systems and multiple pore systems.
Elie Belorizky, Pascal H. Fries
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0069362

Abstract:
We consider the longitudinal relaxation rate enhancement (QRE) of a 1H nucleus due to the time fluctuations of the local dipolar magnetic field created by a close quadrupole 14N nucleus, the electric-field gradient (EFG) Hamiltonian of which changes with time because of vibrations/distorsions of its chemical environment. The QRE is expressed analytically as a linear combination of the cosine Fourier transforms of the three quantum time auto-correlation functions GAA(t) of the 14N spin components along the principal axes A = X, Y, Z of the mean (time-averaged) EFG Hamiltonian. Denoting the three transition frequencies between the energy levels of this mean Hamiltonian by νA, the functions GAA(t) oscillate at frequencies νA + sA/(2π) with mono-exponential decays of relaxation time τA, where the frequency dynamic shifts sA and the relaxation times τA are closed expressions of the magnitude of the fluctuations of the instantaneous EFG Hamiltonian about its mean and of the characteristic fluctuation time. Thus, the theoretical QRE is the sum of three Lorentzian peaks centered at νA + sA/(2π) with full widths at half maxima 1/(π τA). The predicted peak widths are nearly equal. The predicted dynamic shifts of the peaks are much smaller than their widths and amazingly keep proportional to the transition frequencies νA for reasonably fast EFG fluctuations. The theory is further improved by correcting the transition frequencies by the Zeeman effects of second order. It is successfully applied to reinterpret the QRE pattern measured by Broche et al. in normal cartilage.
Alexander Y. Choi, Iretomiwa Esho, Bekari Gabritchidze, ,
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0063331

Abstract:
Cryogenic low-noise amplifiers based on high electron mobility transistors (HEMTs) are widely used in applications such as radio astronomy, deep space communications, and quantum computing. Consequently, the physical mechanisms governing the microwave noise figure are of practical interest. In particular, the magnitude of the contribution of thermal noise from the gate at cryogenic temperatures remains unclear owing to a lack of experimental measurements of thermal resistance under these conditions. Here, we report measurements of gate junction temperature and thermal resistance in a HEMT at cryogenic and room temperatures using Schottky thermometry. At temperatures ∼20 K, we observe a nonlinear trend of thermal resistance vs power that is consistent with heat dissipation by phonon radiation. Based on this finding, we consider heat transport by phonon radiation at the low-noise bias and liquid helium temperatures and estimate that the thermal noise from the gate is several times larger than previously assumed owing to self-heating. We conclude that without improvements in thermal management, self-heating results in a practical lower limit for microwave noise figure of HEMTs at cryogenic temperatures.
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0062889

Abstract:
In this work, we developed a calculation method of local stress tensor applicable to non-equilibrium molecular dynamics (NEMD) systems, which evaluates the macroscopic momentum advection and the kinetic term of the stress in the framework of the Method of Plane (MoP), in a consistent way to guarantee the mass and momentum conservation. From the relation between the macroscopic velocity distribution function and the microscopic molecular passage across a fixed control plane, we derived a method to calculate the basic properties of the macroscopic momentum conservation law including the density, the velocity, the momentum flux, the interaction and kinetic terms of the stress tensor defined on a surface with a finite area. Any component of the streaming velocity can be obtained on a control surface, which enables the separation of the kinetic momentum flux into the advection and stress terms in the framework of MoP, and this enebles strict satisfaction of the the mass and momentum conservation for an arbitrary closed control volume (CV) set in NEMD systems. We validated the present method through the extraction of the density, velocity and stress distributions in a quasi-1D steady-state Couette flow system and in a quasi-2D steady-state NEMD system with a moving contact line. We showed that with the present MoP, in contrast to the volume average method (VA), the conservation law was satisfied even for a CV set around the moving contact line, which was located in a strongly inhomogeneous region.
Robin J. Shannon, Helen M. Deeks, Eleanor Burfoot, Edward Clark, Alex J. Jones, ,
Published: 21 October 2021
The Journal of Chemical Physics, Volume 155; https://doi.org/10.1063/5.0062517

Abstract:
The emerging fields of citizen science and gamification reformulate scientific problems as games or puzzles to be solved. Through engaging the wider non-scientific community, significant breakthroughs may be made by analyzing citizen-gathered data. In parallel, recent advances in virtual reality (VR) technology are increasingly being used within a scientific context and the burgeoning field of interactive molecular dynamics in VR (iMD-VR) allows users to interact with dynamical chemistry simulations in real time. Here, we demonstrate the utility of iMD-VR as a medium for gamification of chemistry research tasks. An iMD-VR “game” was designed to encourage users to explore the reactivity of a particular chemical system, and a cohort of 18 participants was recruited to playtest this game as part of a user study. The reaction game encouraged users to experiment with making chemical reactions between a propyne molecule and an OH radical, and “molecular snapshots” from each game session were then compiled and used to map out reaction pathways. The reaction network generated by users was compared to existing literature networks demonstrating that users in VR capture almost all the important reaction pathways. Further comparisons between humans and an algorithmic method for guiding molecular dynamics show that through using citizen science to explore these kinds of chemical problems, new approaches and strategies start to emerge.
Marco Cherubini, , Francesco Mauri
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0062689

Abstract:
Water ice is a unique material presenting intriguing physical properties, like negative thermal expansion and anomalous volume isotope effect (VIE). They arise from the interplay between weak hydrogen bonds and nuclear quantum fluctuations, making theoretical calculations challenging. Here, we employ the stochastic self-consistent harmonic approximation (SSCHA) to investigate how thermal and quantum fluctuations affect the physical properties of ice XI with ab initio accuracy.Regarding the anomalous VIE, our work reveals that quantum effects on hydrogen are so strong to be in a nonlinear regime: when progressively increasing the mass of hydrogen from protium to infinity (classical limit), the volume firstly expands and then contracts, with a maximum slightly above the mass of tritium. We observe an anharmonic renormalization of about 10% in the bending and stretching phonon frequencies probed in IR and Raman experiments.For the first time, we report an accurate comparison of the low energy phonon dispersion with the experimental data, possible only thanks to high-level accuracy in the electronic correlation and nuclear quantum and thermal fluctuations, paving the way for the study of thermal transport in ice from first principles and the simulation of ice under pressure.
Yiheng Qiu,
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0068887

Abstract:
We introduce hybrid gausslet/Gaussian basis sets, where a standard Gaussianbasis is added to a gausslet basis in order to increase accuracy near thenuclei while keeping the spacing of the grid of gausslets relatively large. TheGaussians are orthogonalized to the gausslets, which are already orthonormal,and approximations are introduced to maintain the diagonal property of the twoelectron part of the Hamiltonian, so that it continues to scale as the secondpower of the number of basis functions, rather than the fourth. We introduceseveral corrections to the Hamiltonian designed to enforce certain exactproperties, such as the values of certain two-electron integrals. We alsointroduce a simple universal energy correction which compensates for theincompleteness of the basis stemming from the electron-electron cusps, based onthe measured double occupancy of each basis function. We perform a number ofHartree Fock and full configuration interaction (full-CI) test calculations ontwo electron systems, and Hartree Fock on a ten-atom hydrogen chain, tobenchmark these techniques. The inclusion of the cusp correction allows us toobtain complete basis set full-CI results, for the two electron cases, at thelevel of several microHartrees, and we see similar apparent accuracy forHartree Fock on the ten-atom hydrogen chain.
Mikita Misiura, Alexander M. Berezhkovskii, , Anatoly B. Kolomeisky
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0069917

Abstract:
Trapping by active sites on surfaces plays important roles in various chemical and biological processes, including catalysis, enzymatic reactions, and viral entry into host cells. However, the mechanisms of these processes remain not well understood, mostly because the existing theoretical descriptions are not fully accounting for the role of the surfaces. Here we present a theoretical investigation on the dynamics of surface-assisted trapping by specific active sites. In our model, a diffusing particle can occasionally reversibly bind to the surface and diffuse on it before reaching the final target site. An approximate theoretical framework is developed, and its predictions are tested by Brownian Dynamics computer simulations. It is found that the surface diffusion can be crucial in mediating the association to trapping binding active sites. Our theoretical predictions work reasonably well as long as the size of the active site is much smaller than the overall surface area. Potential applications of our approach are discussed.
Jie Wei, Zhengda He, Wei Chen, , Elizabeth Santos,
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0072069

Abstract:
The activity of Pt(111) electrodes for the hydrogen evolution reaction (HER) in 0.5 M H2SO4 solution is found to increase with continuous potential cycling in the HER potential region. In addition, the basic cyclic voltammograms obtained in 0.5 M H2SO4 saturated with N2 after HER shows several characteristic changes: the current waves for hydrogen adsorption in the region of 0.2 V < E < 0.35 V, and for sulfate adsorption at 0.35 V < E < 0.5 V decrease, and the current spike at 0.44 V for the phase-transition of the sulfate adlayer gradually disappears. We suggest that these changes are caused by the absorption of a small amount of hydrogen in the subsurface layer, and propose a mechanism by which this enhances hydrogen evolution.
Diego Renato Ugarte La Torre,
Published: 21 October 2021
The Journal of Chemical Physics, Volume 155; https://doi.org/10.1063/5.0057278

Abstract:
Biological membranes that play major roles in diverse functions are composed of numerous lipids and proteins, making them an important target for coarse-grained (CG) molecular dynamics (MD) simulations. Recently, we have developed the CG implicit solvent lipid force field, iSoLF, that has a resolution compatible with the widely used Cα protein representation [Ugarte La Torre and Takada, J. Chem. Phys. 153, 2020]. In this study, we extended it and developed a lipid-protein interaction model that allows the combination of the iSoLF and the Cα protein force field, AICG2+. The hydrophobic-hydrophilic interaction is modeled as a modified Lennard-Jones potential, in which parameters were tuned partly to reproduce the experimental transfer free energy and partly based on the free energy profile normal to the membrane surface from previous all-atom MD simulations. Then, the obtained lipid-protein interaction is tested for the configuration and placement of transmembrane proteins, water-soluble proteins, and peripheral proteins, showing good agreement with prior knowledge. The interaction is generally applicable and is implemented in the publicly available software, CafeMol.
Joshua David Finkelstein, Chungho Cheng, , Benjamin Seibold,
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0066008

Abstract:
In light of the recently published complete set of statistically correct Gronbech-Jensen (GJ) methods for discrete-time thermodynamics, we revise a differential operator splitting method for the Langevin equation in order to comply with the basic GJ thermodynamic sampling features, namely the Boltzmann distribution and Einstein diffusion, in linear systems. This revision, which is based on the introduction of time scaling along with flexibility of a discrete-time velocity attenuation parameter, provides a direct link between the ABO splitting formalism and the GJ methods. This link brings about the conclusion that any GJ method has at least weak second order accuracy in the applied time step. It further helps identify a novel half-step velocity, which simultaneously produces both correct kinetic statistics and correct transport measures for any of the statistically sound GJ methods. Explicit algorithmic expressions are given for the integration of the new half-step velocity into the GJ set of methods. Numerical simulations, including quantum-based molecular dynamics (QMD) using the QMD suite LATTE, highlight the discussed properties of the algorithms as well as exhibit the direct application of robust, time step independent stochastic integrators to quantum-based molecular dynamics.
Shashikant Kumar, Babak Sadigh, Siya Zhu, , Sebastien Hamel, Brian Gallagher, Vasily Bulatov, John E Klepeis,
Published: 21 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0065217

Abstract:
Our ability to perform large scale electronic structures calculations is severely limited by the algorithmic difficulties associated with a Kohn-Sham density functional theory (KSDFT) calculation and the absence of accurate density- dependent kinetic energy functionals required for orbital-free density functional theory calculations (OFDFT). There- fore, the development of accurate density-dependent kinetic energy functionals is important for OFDFT calculations of large realistic systems. To this end, we propose a method to train kinetic energy functional models at the exact-exchange level of theory by using a dictionary of physically relevant terms that have been proposed in the literature in conjunction with linear or non-linear regression methods to obtain the fitting coefficients. For our dictionary, we use gradient expansion of the kinetic energy, non-local models proposed in the literature and their nonlinear combinations, such as a model that incorporates spatial correlations between higher order derivatives of electron density at two points. The predictive capabilities of these models are assessed by using a variety of model one-dimensional systems that exhibit diverse bonding characteristics, such as a chain of eight hydrogens, LiF, LiH, C4H2, C4N2 and C3O2. We show that by using data from KSDFT calculations performed using the exact-exchange functional for only a few neutral structures, it is possible to generate models with high accuracy for charged structures as well as structures obtained from a self- consistent field iteration. In addition, we show that it possible to learn both the orbital dependent terms, i.e., the kinetic energy and the exact-exchange energy.
C. Hauenstein, X. de Vries, , P. Imbrasas, , S. Lenk, , , , , et al.
Published: 21 October 2021
Journal of Applied Physics, Volume 130; https://doi.org/10.1063/5.0062926

Abstract:
The efficiency of organic light-emitting diodes that utilize the principle of thermally activated delayed fluorescence (TADF) depends sensitively on the host material in which the TADF emitter molecules (guests) are embedded. Potential loss processes are “deconfinement,” the transfer of excitons from the guest to the host, and “dissociation,” the formation of intermolecular charge-transfer states. We investigate how both processes can be suppressed by studying the photoluminescence efficiency, emission spectrum, and time-resolved emission intensity of eight thin-film systems in which 5 mol. % of the sky-blue TADF emitter 4-carbazolyl-methylphthalimide (abbreviated here as CzPIMe) is embedded in various host materials. Deconfinement is found to be entirely suppressed if the triplet energy of the host is 0.25 eV or more above that of the guest. For systems allowing for deconfinement, the dependence on the energy difference is consistent with a recent theoretical analysis [C. Hauenstein et al., J. Appl. Phys. 128, 075501 (2020)]. Dissociation, due to hole transfer to a host molecule, is found to be suppressed if the host’s highest occupied molecular orbital energy is not more than about 0.2 eV higher than that of the guest. Otherwise, we observe an efficiency loss, a spectral redshift, and the disappearance of distinct prompt and delayed emission regimes. A comprehensive rate-equation model is developed from which we study the sensitivity of these observations to the energy level structure, the intermolecular interaction rates, and the photophysical rates that follow from a fit to the experimental data for the CzPIMe:TCTA[tris(4-carbazoyl-9-ylphenyl)amine] system.
Jianan Zhao, Kenichiro Koshiyama, Huimin Wu
Published: 21 October 2021
Default Digital Object Group, Volume 33; https://doi.org/10.1063/5.0065309.1

Abstract:
Most existing whole lung models neglect the airway deformation kinematics and assume the lung airways are static. However, neglecting the airway deformation effect on pulmonary air-particle flow dynamics significantly limits the modeling capability under disease-specific lung conditions. Therefore, a novel elastic truncated whole-lung (TWL) modeling framework has been developed to simulate the disease-specific airway deformation kinematics simultaneously with pulmonary air-particle flow dynamics using one-way coupled Euler–Lagrange method plus the dynamic mesh method. Specifically, the deformation kinematics of the elastic TWL model was calibrated with clinical data and pulmonary function test results for both healthy lung and lungs with chronic obstructive pulmonary diseases (COPDs). The transport dynamics of spherical sub micrometer and micrometer particles were investigated. Results show that noticeable differences in air-particle flow predictions between static and elastic lung models can be found, which demonstrates the necessity to model airway deformation kinematics in whole-lung models. The elastic TWL model predicted lower deposition fraction in mouth-throat regions and higher deposition fraction in lower airways. The effect of disease-specific airway deformation kinematics on particle transport and deposition in the whole lung was investigated, with a focus on the targeted drug delivery efficiency in small airways from generation (G8) to alveoli as the designated lung sites for COPD treatment using inhalation therapy. Simulation results indicate that with the exacerbation of COPD disease conditions, the highest delivery efficiency of the inhaled drug particles decreases which indicates that delivering aerosolized medications to small airways to treat COPD is more challenging for patients with severe disease conditions.
Samuel Jack Palmer Marlton, Benjamin Ian McKinnon, Phillip Greißel, Oisin John Shiels, Boris Ucur,
Published: 20 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0071847

Abstract:
Certain chemical groups give rise to characteristic excited-state deactivation mechanisms. Here we target the role of a protonated N-N chemical group in the excited-state deactivation of protonated indazole by comparison to its isomer that lacks this group, protonated benzimidazole. Gas-phase protonated indazole and protonated benzimidazole ions are investigated at room temperature using picosecond laser pump-probe photodissociation experiments in a linear ion-trap. Excited state lifetimes are measured across a range of pump energies (4.0 - 5.4 eV). The 1ππ* lifetimes of protonated indazole range from 390 {plus minus} 70 ps using 4.0 eV pump energy to {less than or equal to} 18 ps using 4.6 eV pump energy. The 1ππ* lifetimes of protonated benzimidazole are systematically longer, ranging from 3700 {plus minus} 1100 ps at 4.6 eV pump energy to 400 {plus minus} 200 ps at 5.4 eV. Based on these experimental results, and accompanying quantum chemical calculations and potential energy surfaces (MS-CASPT2), the shorter lifetimes of protonated indazole are attributed to πσ* state mediated elongation of the protonated N-N bond.
Yawar Ali Sheikh, Muhammad Umar Maqbool, , , Ahmed Bilal Awan, Kashif Nisar Paracha, Muhammad Murad Khan
Journal of Renewable and Sustainable Energy; https://doi.org/10.1063/5.0063044

Abstract:
Solar energy is one of the most abundant and widely available renewable energy sources. It can be harnessed using photovoltaic panels on top of buildings to reduce dependence on the electrical grid and to achieve the status of net-zero energy building. However, the rooftop coverage by solar panels can modify the heat interface between the roof surface and its surrounding environment. This can alter the building's energy demand for heating, ventilation, and air conditioning. Such an impact on building's energy demand is highly correlated with its roof structure and climate. In this work, three-dimensional distributed thermal models of the bare and photovoltaic added rooftop ensembles are developed to simulate the heat gain/loss associated with the roof structure for monthly mean diurnal cycles. This work considers the low-rise, high-density building style and hot semi-arid climate of Faisalabad city, Pakistan to quantify the impact of rooftop photovoltaic on the roof-related thermal load of a building. Results depict a 42.58% reduced heat loss from the photovoltaic added roof structure during winter and a 1.98% increase in heat gain during summer. This reduces the electricity demand for indoor heating during winter and slightly increases it for indoor cooling during summer. The obtained results prove the significance of this work and provide guidelines to energy policymakers, the construction industry, and energy consumers. Moreover, this work provides a better understanding of the building's energy demand for heating, ventilation, and air conditioning with rooftop photovoltaic system and its net-zero energy requirements which are pivotal for modern construction.
El Hassane Lahrar, Patrice Simon,
Published: 20 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0065150

Abstract:
Carbon-carbon supercapacitors are high power electrochemical energy storage systems which store energy through reversible ion adsorption at the electrode-electrolyte interface. Due to the complex structure of the porous carbons used as electrodes, extracting structure-property relationships in these systems remains a challenge. In this work, we conduct molecular simulations of two model supercapacitors based on nanoporous electrodes with the same average pore size, a property often used when comparing porous materials, but different morphologies. We show that the carbon with the more ordered structure, with a well defined pore size, has a much higher capacitance than the carbon with the more disordered structure, and broader pore size distribution. We analyze the structure of the confined electrolyte and show that the ions adsorbed in the ordered carbon are present in larger quantities and are also more confined than for the disordered carbon. Both aspects favor a better charge separation and thus a larger capacitance. In addition, the disordered electrodes contain a significant amount of carbon atoms which are never in contact with the electrolyte, carry a close to zero charge and are thus not involved in the charge storage. The total quantities of adsorbed ions and degrees of confinement do not change much with the applied potential and as such, this work opens the door to computationally tractable screening strategies.
Nicolai Ree, Mads Koerstz, Kurt V. Mikkelsen,
Published: 20 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0063694

Abstract:
We present a computational methodology for the screening of a chemical space of 1025 substituted norbornadiene molecules for promising kinetically stable molecular solar thermal (MOST) energy storage systems with high energy densities that absorb in the visible part of the solar spectrum. We use semiempirical tight-binding methods to construct a dataset of nearly 34,000 molecules and train graph convolutional networks to predict energy densities, kinetic stability, and absorption spectra and then use the models together with a genetic algorithm to search the chemical space for promising MOST energy storage systems. We identify 15 kinetically stable molecules, five of which have energy densities greater than 0.45 MJ/kg and the main conclusion of this study is that the largest energy density that can be obtained for a single norbornadiene moiety with the substituents considered here, while maintaining a long half-life and absorption in the visible spectrum, is around 0.55 MJ/kg.
Anastasiia Tukmakova, Dmitry Zykov, Mikhail Novoselov, Ivan Tkhorzhevskiy, Artyom Sedinin, Anna Novotelnova, Anton Zaitsev, Petr Demchenko, Elena Makarova, Natallya Kablukova
Published: 20 October 2021
Default Digital Object Group, Volume 119; https://doi.org/10.1063/5.0062228.1

Abstract:
A room-temperature terahertz (THz) detector based on a thermoelectric frequency selective surface (FSS) has been numerically simulated, designed, fabricated, and tested. The FSS has been fabricated from a 150 nm thin Bi88Sb12 thermoelectric film with the engraved periodic Greek crosses. The detector prototype has been tested under the 0.14 THz radiation exposure and showed a voltage response due to the photo-thermoelectric effect up to 0.13–0.18 mV, and voltage responsivity equal to 14–20 mV/W. The detector based on the FSS has shown voltage responsivity up to three times higher in comparison with the detector based on the continuous film. Thermal imaging has shown a temperature increase in the FSS up to 1.5 K under the THz exposure. The obtained results demonstrate prospects for utilization of the Bi88Sb12 FSS detector as a low cost, compact, high-speed, highly sensitive room-temperature THz detector.
, Alexander Aleksandrowicz Maryewski, , , Zuzana Konopkova, Vitali B. Prakapenka,
Published: 20 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0067828

Abstract:
We have performed a combined experimental and theoretical study of ethane and methane at high pressures up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopy and the USPEX ab-initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (Phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on a good agreement of the experimental results and theoretical predictions. For methane, our x-ray diffraction (XRD) measurements are in agreement with the previously reported high-pressure structures and equation of state (EOS). We have determined the EOSs of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.
Published: 20 October 2021
Physics Today, Volume 2021; https://doi.org/10.1063/pt.6.2.20211020a

Abstract:
New laser technology could be the key to producing inertial fusion energy. But the companies pursuing that goal face a steep climb to engineer a power-generating reactor.
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0065649

Abstract:
Frequency-domain electron spin resonance (FDESR) spectroscopy in the terahertz (THz) region using continuously tunable photomixers was demonstrated. Spectral resolution was greatly improved with the use of a pair of fiber stretchers. In this setup, the amplitude of the THz electric field was determined at each frequency by externally sweeping the optical path difference, resulting in a spectral resolution of about 1 MHz. With this technique, we observed narrow ESR spectra with a 20-MHz linewidth, enabling high-resolution FDESR spectroscopy in a broad frequency range.
Ke Jiang, , Yuxuan Chen, Shanli Zhang, Jianwei Ben, Yang Chen, , Yuping Jia, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0064779

Abstract:
GaN-based ultraviolet (UV) detectors have a considerable application potential in many fields. In this Letter, we report an alternative strategy to realize a high-optical-gain bipolar UV phototransistor based solely on a GaN p-i-n epilayer. The device consists of two tightly adjacent vertical p-i-n structures with a common n-type layer as a floating base. The collector and emitter electrodes are deposited on the two p-type mesas, forming a three-dimensional metal–semiconductor–metal (MSM) like photodetector. As a result, a peak responsivity of 11.7 A/W at a wavelength of 358 nm at 5 V is realized, corresponding to an optical gain of 40 with the assumption of 100% internal quantum efficiency. Different from traditional GaN-based n-p-i-n phototransistors, the optical gain of this detector originates from the accumulated electrons in the n-type floating base upon illumination, which can lower the barrier height between the base and emitter, leading to hole emission from the emitter. Although the structure of this phototransistor is similar to a planar back-to-back Schottky-type MSM photodetector, the response speed is much faster because the gain mainly results from carrier emission rather than MS interface defects.
, Michael S. Shur
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0069072

Abstract:
A non-uniform capacitance profile in the channel of a terahertz (THz) field-effect transistor (TeraFET) could improve the THz detection performance. The analytical solutions and simulations of the hydrodynamic equations for the exponentially varying capacitance vs distance showed ∼10% increase in the responsivity for the 130 nm Si TeraFETs in good agreement with numerical simulations. Using the numerical solutions of the hydrodynamic equations, we compared three different Cg configurations (exponential, linear, and sawtooth). The simulations showed that the sawtooth configuration provides the largest response tunability. We also compared the effects of the non-uniform capacitance profiles for Si, III–V, and p-diamond TeraFETs. The results confirmed a great potential of p-diamond for THz applications. Varying the threshold voltage across the channel could have an effect similar to that of varying the gate-to-channel capacitance. The physics behind the demonstrated improvement in THz detection performance is related to the amplification of boundary asymmetry by the non-uniform device geometry.
, Victor V. Galushka, Ilya O. Kozhevnikov, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0058572

Abstract:
Ongoing active development of modern radio frequency electronic devices operating in the millimeter (V) band, such as fifth-generation wireless communications, demands new materials to control electromagnetic interference, compatibility, and reliability of such systems. This work investigates feasibility absorptive non-reflective thin coatings deposition on dielectric substrates using simultaneous magnetron co-deposition. For this, electromagnetic waves propagation in the millimeter band through in micrometer-thick Al–Si films of varied composition was studied. The co-deposition process was controlled by the ratio of sputtered atoms fluxes. Graded segregation was observed under certain parameters of the co-deposition process, resulting in a depth gradient of an aluminum content, as confirmed by the secondary ion mass spectrometry study. A qualitative model was proposed involving aluminum-induced silicon recrystallization happening in the course of a known aluminum interlayer exchange process. The observed Al–Si segregation effect in micrometer-thick films allows for preparation of the non-reflective and absorptive material for operation in the V-band with reflection losses more than 10 dB and transmission losses around 5 dB in the bandwidth of up to 20 GHz.
Ana C. Feltrin, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0066698

Abstract:
Transition metal borides have a unique combination of high melting point and high chemical stability and are suitable for high temperature applications (>2000 °C). A metastable dual-phase boride (Ti0.25V0.25Zr0.25Hf0.25)B2 with distinct two hexagonal phases and with an intermediate entropy formation ability of 87.9 (eV/atom)−1 as calculated via the density functional theory (DFT) was consolidated by pulsed current sintering. Thermal annealing of the sintered dual-phase boride at 1500 °C promoted the diffusion of metallic elements between the two boride phases leading to chemical homogenization and resulted in the stabilization of a single-phase high-entropy boride. Scanning electron microscopy, in situ high temperature x-ray diffraction, and simultaneous thermal analysis of the as-sintered and annealed high-entropy borides showed the homogenization of a dual-phase to a single-phase. The experimentally obtained single-phase structure was verified by DFT calculations using special quasirandom structures, which were further used for theoretical investigations of lattice distortions and mechanical properties. Experimentally measured mechanical properties of the single-phase boride showed improved mechanical properties with a hardness of 33.2 ± 2.1 GPa, an elastic modulus of 466.0 ± 5.9 GPa, and a fracture toughness of 4.1 ± 0.6 MPa m1/2.
, , , Irfan Saadat
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0063515

Abstract:
Nonvolatile memory technology is a necessary component in many electronic devices. With the scaling down of memory devices to achieve high density and low power consumption, floating gate devices encounter various challenges like high leakage current, which leads to reliability issues and a decrease in charge density. Therefore, the use of metal nanoparticles (NPs) as charge storage centers is becoming a promising candidate due to their excellent scalability and favorable reliability. In this work, we demonstrate the charge storage dependency on the size of a gold-nanoparticle (Au-NP) by using a contact mode atomic force microscope. The individually dispersed Au-NPs are sandwiched between a thin layer (3 nm) of TiO2 blocking layer and SiO2 (2 nm) tunneling layer thin films. The consecutive I–V sweeps on a pristine device of stacking TiO2/Au-NP/SiO2/n-Si show that the threshold voltage (ΔV) increases with the increase in the Au-NP size, whereas the retention shows much more stability time with smaller size NPs, in the range of 10 nm.
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0069970

Abstract:
Cubic boron nitride is an ultrawide-bandgap semiconductor with potential applications in high-power electronics, ultraviolet optoelectronics, and quantum information science. Many of those applications are predicated on the ability to control doping. Using hybrid-functional first-principles calculations, we systematically explore potential donors including group-IV (C, Si, and Ge) and group-VI (O, S, and Se) elements, as well as Li and F. We also address the role of compensation either by substitution on the wrong site or due to native point defects. We identify SiB and ON as promising dopants, as they have the lowest formation energies and do not suffer from self-compensation. However, compensation by boron vacancies, which act as deep acceptors, poses a challenge. We discuss strategies to mitigate these effects.
, Jie Deng, Mengdie Shi, Zeshi Chu, Haowen Li, Rui Dong, Xiaoshuang Chen
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0060033

Abstract:
Plasmonic structures have been widely employed to manipulate the light coupling of infrared detectors to enhance sensitivity and achieve multidimensional light field recognition. Recently, cavity coupled plasmonic resonators as an improved version of plasmonic light coupling structures have received much attention. A variety of ordinary plasmonic structure integrated infrared detectors and cavity coupled plasmonic resonator integrated infrared detectors are reviewed. Compared with ordinary plasmonic structures, cavity coupled plasmonic resonators are more effective in adjusting the light in-coupling efficiency, local field polarization, and light absorption competition, which is beneficial to performance enhancement in infrared detectors. Several features of cavity coupled plasmonic resonator integrated infrared detectors, including deep subwavelength light concentration with high efficiency, Ohmic loss suppression, high-contrast polarization discrimination, are discussed. As a brief outlook, cavity coupled plasmonic resonators for infrared detectors are expected to have optical-electrical joint functions, be compatible with focal plane array technology, and have new features stemming from innovative combinations of different kinds of cavities and plasmonic structures.
Rohit Medwal, Angshuman Deka, Joseph Vimal Vas, Martial Duchamp, Hironori Asada, , Yasuhiro Fukuma,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0064653

Abstract:
Directional specific control on the generation and propagation of magnons is essential for designing future magnon-based logic and memory devices for low power computing. The epitaxy of the ferromagnetic thin film is expected to facilitate anisotropic linewidths, which depend on the crystal cut and the orientation of the thin film. Here, we have shown the growth-induced magneto-crystalline anisotropy in 40 nm epitaxial yttrium iron garnet (YIG) thin films, which facilitate cubic and uniaxial in-plane anisotropy in the resonance field and linewidth using ferromagnetic resonance measurements. The growth-induced cubic and non-cubic anisotropy in epitaxial YIG thin films are explained using the short-range ordering of the Fe3+ cation pairs in octahedral and tetrahedral sublattices with respect to the crystal growth directions. This site-preferred directional anisotropy enables an anisotropic magnon–magnon interaction and opens an avenue to precisely control the propagation of magnonic current for spin-transfer logics using YIG-based magnonic technology.
J. D. Ortiz, J. P. del Risco, , R. Marqués
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0065724

Abstract:
Babinet's principle is widely applied in optics and has been useful for designing metasurfaces with dual behavior. Although this principle can be rigorously demonstrated for infinitely thin perfect conducting screens, it is not exact for any real screen. In fact, metals used in plasmonic metasurfaces are far from good conductors, and the thickness of samples is not negligible in comparison with the typical size of the patterned structure. In this paper, we propose an extension of Babinet's principle valid for plasmonic metasurfaces by redefining the concept of complementary screens and finding impedance relations between such screens that ultimately leads to a simple relation between the transmission matrices of two complementary plasmonic metasurfaces. The theory is valid under the assumptions of the electroquasistatic approximation and plane waves in the far field. It may find applications in the design of optical plasmonic metasurfaces, nanocircuits, and nanoantennas.
Jing Wan, Xiao Gu, Peiyuan Ji, Jien Li, Junlin Lu, Shuang Luo, Bangxing Li, , ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0059392

Abstract:
The ion storage mechanism and ion concentration play crucial roles in determining the electrochemical energy storage performances of multi-ion-based batteries and/or capacitors. Here, we take δ-MnO2-A2SO4 (A = Li, Na, K) as an example system to explore the physical and chemical mechanisms related to electrochemical energy storage using experimental analysis and first-principles calculations. Among the studied systems, superior capacitance performance is found in δ-MnO2-Li2SO4 due to excellent mobility (migration barrier 0.168 eV) of lithium ions. Better cycling stability appears in δ-MnO2-K2SO4, which is attributed to larger adsorption energy (−0.655 eV) between potassium ions and δ-MnO2. Moreover, compared with a pure Li2SO4 electrolyte, our calculations suggest that incorporation of moderate Na2SO4 or K2SO4 into the Li2SO4 electrolyte could considerably elongate the cycling lifetime. Overdose of Na+ or K+ is, however, adverse to the capacitance performance as verified by our experiments. We argue that the dominance role of Na+ or K+ ions played in the hybrid electrolyte originates from the larger formation enthalpy and adsorption energy of Na+ or K+ when reacting with δ-MnO2 compared with those of Li+. Our findings suggest that understanding of the ion storage mechanism can provide useful clues for searching the proper ion concentration ratio, which takes advantages of individual ions in multi-ion-based δ-MnO2 electrochemical energy storage devices.
Zexuan Zhang, Jimy Encomendero, , , Vladimir Protasenko, Kazuki Nomoto, Kevin Lee, Masato Toita, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0066072

Abstract:
A high-conductivity two-dimensional (2D) hole gas is the enabler of wide-bandgap p-channel transistors. Compared to commonly used AlN template substrates with high dislocation densities, the recently available single-crystal AlN substrates are promising to boost the speed and power handling capability of p-channel transistors based on GaN/AlN 2D hole gases (2DHGs) thanks to the much lower dislocation densities and the absence of thermal boundary resistance. Using plasma-assisted molecular beam epitaxy, we report the observation of polarization-induced high-density 2DHGs in undoped pseudomorphic GaN/AlN heterostructures on the single-crystal AlN substrates with high structural quality and atomic steps on the surface. The high-density 2DHG persists down to cryogenic temperatures with a record high mobility exceeding 280 cm2/V s and a density of 2.2 × 1013/cm2 at 10 K. These results shed light on aspects of improving 2D hole mobilities and indicate significant potential of GaN/AlN 2DHG grown on bulk AlN substrates for future high performance wide-bandgap p-channel transistors.
Lea-Sophie Hornberger, David Neusser, Claudia Malacrida, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0056484

Abstract:
Electrochemical doping is an elegant method of controlling the doping level and charge carrier densities of conjugated polymer films and enhancing their thermoelectric figure of merit. Applying this doping technique to films of poly(3-hexylthiophene) (P3HT) results in conductivities with values as high as 200 S/cm. The stability of the doped films in the solid state can be probed by UV-vis-NIR spectroscopy. We found that the choice of the conducting salt in the liquid electrolyte exerts a strong influence over the conductivity. Using TBAPF6 and LiClO4 provides highest conductivities for P3HT films, while LiTFSI and TBABF4 show overall lower performance. This effect is also reflected in cyclic voltammetry measurements coupled with in situ spectroscopy. Overall lower reversibility upon multiplex cycling in LiTFSI and TBABF4 electrolytes suggests strong charge trapping effects, which one might attribute to a considerable fraction of charges (holes/ions) remaining in the films after charge/discharge cycles. The salts with stronger charge irreversibility in the electrochemistry experiments show the poorer solid state conductivities. Our conclusion is that one should carefully choose the electrolyte to ensure good percolation pathways and delocalized charge transport throughout doped films.
Yuan Cao, , Zhenhai Lai, Cheng Wang, Yingfang Yao, Xi Zhu, Zhigang Zou
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0061703

Abstract:
Designing efficient oxygen evolution reaction (OER) electrocatalysts is essential for numerous sustainable energy conversion technologies. An obstacle that impedes the development of OER electrocatalysts is the insufficient emphasis on the spin attribution of electrons. Recently, the different spin configuration of reactants and products in the OER has been recognized as the factor that slows down the reaction kinetics. In this work, Mn substitution was introduced to LaCoO3, which brought about lattice expansion and reduced crystalline field splitting energy. This led to the increase in the effective magnetic moment, which triggers the transfer of Co3+ from low to higher spin states. Thus, the hybridization of Co eg and O 2p states across the Fermi level was strengthened. Specifically, with 25 at. % Mn substitution, LaCoO3 transits from a semiconductor to a half-metal, which benefits the spin-oriented electronic transport and resultantly promotes the OER. This method paves the way for the construction of spin pathways in catalysts.
, , Mingjie Xu, , Rongjie Hong, Chaoyi Zhu, Xueying Dai, , , , et al.
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0066130

Abstract:
In the past decade, the concept of high-entropy alloys (HEAs) or multi-principal element alloys (MPEAs), which are composed of at least four principal elements, significantly expands the compositional space for alloy design. This concept can also be employed in the design of superelastic alloys to promote the development of this functional material field. Here, we report the orientation-dependent superelasticity of a metastable Fe-27.5Ni-16.5Co-10Al-2.2Ta-0.04B (at.%) HEA through in situ micropillar compression tests along ⟨001⟩, ⟨011⟩, and ⟨111⟩ orientations. Our results show that considerable superelastic strains can be achieved along the three orientations in the metastable HEA via a reversible martensitic transformation. Thermoelastic martensite with thin-plate morphology was observed under cryogenic conditions. This work demonstrates that the maximum superelastic strains vary with different orientations, and the ⟨001⟩-oriented specimen shows the largest superelastic strain. The superelastic strains along specific orientations are compared with theoretical values calculated from the lattice deformation method and the energy minimization theory, respectively. The limited number of martensite variants under compression testing may be responsible for the discrepancy that exists in the experimental and the two theoretically predicted transformation strains. This study may provide a feasible strategy for the design of superelastic HEAs with specific orientation for applications in microsystems.
, Rigo A. Carrasco, Ryan Hickey, Nalin S. Fernando, , James Kolodzey
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0064358

Abstract:
The dielectric functions of germanium–tin alloy thin-films, deposited by molecular beam epitaxy on bulk Ge substrates, with relatively high Sn contents from 15 to 27 at. %, were measured by variable angle spectroscopic ellipsometry over the wavelength range from 0.190 to 6 μm, using a combination of ultraviolet-visible and infrared ellipsometers. The band structure critical point energies, specifically the E1 and E1 + Δ1 optical transitions, were extracted from the measurements by a method of parametric oscillator modeling and second derivative analysis. With increasing Sn content, the transitions shifted to lower energies, and for alloys with less than 20% Sn, the numerical values agreed reasonably with predictions based on deformation potential theory that accounted for film strain. For the higher Sn alloys, the critical point energies from measurements agreed less well with deformation potential theory. These results provide information on the band structure of GeSn alloys with high Sn contents, which are increasingly important for long-wave infrared devices and applications.
Zhe Liu, Xiaojuan Sun, Jun Tang, Jing Pan, Ruiheng Pan, ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0066913

Abstract:
The coexistence of polarization and mobile ions is an extraordinary property for hybrid perovskites, as they are proven critical for charge generation and transport. There is a lack of study to elucidate the slow responses of surface polarization and ion accumulation on the electroluminescence (EL) for perovskite light emitting diodes (PeLEDs). Here, we adopt ac-field impedance spectroscopy combined with in situ EL measurements for the study of surface recombination, when a prototypical methylammonium lead bromide (CH3NH3PbBr3) PeLED operates at working conditions. We have found that the surface polarization due to charges and ions has remarkable impact on EL characteristics such as the illumination intensity, full width at half maximum (FWHM), and emissive peak. Such a phenomenon can be explained by the slow surface polarization relaxation and the ion vacancy-polarization interaction. Both of them promote surface band-to-band and trap-assist recombination, while giving rise to the EL intensity. This surface science study is merit for understanding the role of surface polarization and ion accumulation for the EL generation in PeLEDs.
Sriswaroop Dasari, , Todd A. Byers, Gary A. Glass, , ,
Published: 18 October 2021
Applied Physics Letters, Volume 119; https://doi.org/10.1063/5.0065875

Abstract:
The radiation resistance of single-phase high entropy alloys has been reported to be superior to conventional alloys, due to their high lattice distortion and sluggish diffusion kinetics. The current study combines the beneficial effects of a concentrated multi-component solid solution and chemical ordering on the parent lattice of a candidate alloy, Al0.3CoFeNi, to enhance proton radiation resistance. The strong ordering tendency in this alloy results in the formation of Ni-Al-rich short-range ordered (SRO) domains when it is annealed in a single FCC phase field and water quenched. The irradiation of these microstructures with high-fluence MeV-energetic protons aids the transformation of the prior metastable single FCC solid-solution with SRO domains toward a more stable condition with L12 long-range ordered (LRO) domains embedded within the FCC solid solution matrix. Potentially, the creation of radiation-induced vacancy cascades within the FCC solid-solution enhances local diffusivity aiding the transition from SRO domains to LRO L12 domains. Therefore, this can be considered as a recovery mechanism, since the radiation-induced damage is not allowed to accumulate and is minimized via nanometer-scale precipitation of the ordered intermetallic phase. Additionally, preferential Co segregation to defect clusters or dislocation loops was also observed. In comparison, purely thermal activation via annealing at 500 °C for 30 min induces a similar transformation from SRO to LRO in this alloy, driving the system closer to equilibrium.
Guanghan Wang, Tianlin Liu, Adriana Caracciolo, , Nisalak Trongsiriwat, Patrick Walsh, Barbara Marchetti, ,
Published: 17 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0068664

Abstract:
The electronic spectrum of methyl vinyl ketone oxide (MVK-oxide), a four-carbon Criegee intermediate derived from isoprene ozonolysis, is examined on its second π* ← π transition, involving primarily the vinyl group, at UV wavelengths (λ) below 300 nm. A broad and unstructured spectrum is obtained by a UV-induced ground state depletion method with photoionization detection on the parent mass (m/z 86). Electronic excitation of MVK-oxide results in dissociation to O (1D) products that are characterized using velocity map imaging. Electronic excitation of MVK-oxide on the first π* ← π transition associated primarily with the carbonyl oxide group at λ > 300 nm results in a prompt dissociation, and yields broad total kinetic energy release (TKER) and anisotropic angular distributions for the O (1D) + methyl vinyl ketone products. By contrast, electronic excitation at λ {less than or equal to} 300 nm results in bimodal TKER and angular distributions, indicating two distinct dissociation pathways to O (1D) products. One pathway is analogous to that at λ > 300 nm, while the second pathway results in very low TKER and isotropic angular distributions indicative of internal conversion to the ground electronic state and statistical unimolecular dissociation.
Published: 17 October 2021
Abstract:
Ultrafast vectorially polarized pulses have found many applications in information and energy transfer owing mainly to the presence of strong longitudinal components and their space-polarization non-separability. Due to their broad spectrum, such pulses often exhibit space-time couplings, which significantly affect the pulse propagation dynamics. Although such couplings usually result in reduced energy density at the focal spot, they have been utilized to demonstrate pulse shaping as in the case of a rotating or sliding wavefront as the pulse travels through its focal point. Here, we present a new method for the spatio-temporal characterization of ultrashort cylindrical vector pulses based on a combination of spatially resolved Fourier transform spectroscopy and Mach-Zehnder interferometry. The method provides access to spatially resolved spectral amplitudes and phases of all polarization components of the pulse. We demonstrate the capabilities of the method by completely characterizing a 10 fs radially polarized pulse from a Ti:sapphire laser centered at 800 nm.
Hang Xiao, ,
Published: 17 October 2021
The Journal of Chemical Physics; https://doi.org/10.1063/5.0065396

Abstract:
In solid-state NMR, frequency-selective homonuclear dipolar recoupling is key to quantitative distance measurement or selective enhancement of correlations between atoms of interest in multiple-spin systems, which are not amenable to band-selective or broadband recoupling. Previous frequency-selective recoupling is mostly based on the so-called rotational resonance (R2) condition that restricts the application to spin pairs with resonance frequencies differing in integral multiples of the magic-angle spinning (MAS) frequency. Recently, we have proposed a series of frequency-selective homonuclear recoupling sequences called SPR (short for Selective Phase-optimized Recoupling), which have been successfully applied for selective 1H-1H or 13C-13C recoupling under from moderate (~10 kHz) to ultra-fast (150 kHz) MAS frequencies. In this study, we fully analyze the average Hamiltonian theory (AHT) of SPR sequences and reveal the origin of frequency selectivity in recoupling. The theoretical description, as well as numerical simulations and experiments, demonstrates that the frequency selectivity can be easily controlled by the flip angle (p) in the (p)ϕk(p)ϕk+π unit in the pSPR-Nn sequences. Small flip angles lead to frequency-selective recoupling while large flip angles may lead to broadband recoupling in principle. The result shall shed new light on the design of homonuclear recoupling sequences with arbitrary frequency bandwidths.
Yuqing Xie, Limin Song, Wenchao Yan, Shiqi Xia, Liqin Tang, , Jun-Won Rhim, Zhigang Chen
Published: 17 October 2021
Abstract:
We experimentally realize fractal-like photonic lattices by use of the cw-laser-writing technique, thereby observing distinct compact localized states (CLSs) associated with different flatbands in the same lattice setting. Such triangle-shaped lattices, akin to the first generation Sierpinski lattices, possess a band structure where singular non-degenerate and nonsingular degenerate flatbands coexist. By proper phase modulation of an input excitation beam, we demonstrate not only the simplest CLSs but also their superimposition into other complex mode structures. Our experimental and numerical results are corroborated by theoretical analysis. Furthermore, we show by numerical simulation a dynamical oscillation of the flatband states due to beating of the CLSs that have different eigenenergies. These results may provide inspiration for exploring fundamental phenomena arising from fractal structure, flatband singularity, and real-space topology.
Anne Cockshott
Published: 15 October 2021
Scilight, Volume 2021; https://doi.org/10.1063/10.0006747

Abstract:
Implanting minute coils into the brain for focal and directional neuronal stimulation could replace invasive neuromodulation devices and contribute to future neuroscience research.
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