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First-principles calculations of defects and electron–phonon interactions play a critical role in the design and optimization of materials for electronic and optoelectronic devices. The late Audrius Alkauskas made seminal contributions to developing rigorous first-principles methodologies for the computation of defects and electron–phonon interactions, especially in the context of understanding the fundamental mechanisms of carrier recombination in semiconductors. Alkauskas was also a pioneer in the field of quantum defects, helping to build a first-principles understanding of the prototype nitrogen-vacancy center in diamond, as well as identifying novel defects. Here, we describe the important contributions made by Alkauskas and his collaborators and outline fruitful research directions that Alkauskas would have been keen to pursue. Audrius Alkauskas’ scientific achievements and insights highlighted in this article will inspire and guide future developments and advances in the field.

A multi-scale simulation framework for ion-solid interactions in plasma-exposed materials provides crucial insight into advancing fusion energy and space electric propulsion. Leveraging binary-collision approximation (BCA) simulations, the framework uniquely predicts sputter yields and analyzes material transport within volumetrically complex materials. This approach, grounded in the validated BCA code TRI3DYN, addresses key limitations in existing models by accurately capturing ion-solid interaction physics. A case study is presented, highlighting the framework’s ability to replicate experimental sputter yield results, underscoring its reliability and potential for designing durable materials in harsh plasma environments. Insights into sputtering transport phenomenology mark a significant advancement in material optimization for improved resilience in plasma-facing applications.

There is an important difference between piezoelectric polymer films and solid crystals for the application of piezoelectric generators. In the case of polymers, the optimal piezoelectric response imposes a large bending regime. Starting from the linear Curie's constitutive equations, we develop an analytical model under the assumption of the large bending regime resulting from bulge testing configuration. This model shows a specific non-linear piezoelectric response, which follows a power of 2 / 3 of the mechanical excitation. The piezoelectric voltage and the corresponding power are then studied experimentally as a function of the angular frequency ω of the mechanical excitation, the load resistance R , and the thickness of the film ℓ. The experimental results carried out on piezoelectric polyvinylidene fluoride (PVDF) films validate the model. [ABSTRACT FROM AUTHOR]

We study the effects of uniaxial pressure on the thermal conductivity between two nanoparticles using atomistic simulation. While the system is compressed, we analyze the evolution of contact area, the relative density, and the dislocation density. Lattice thermal conductivity is calculated by non-equilibrium molecular dynamics simulations at several stages of the compression. Despite the increment of dislocation defects, thermal conductivity increases with pressure due to the increase in relative density and contact radius. The behavior of the contact radius is compared with the Johnson–Kendall–Roberts (JKR) model. While there is good agreement at low strain, after significant plasticity, signaled by the emission of dislocations from the contact region, the discrepancy with JKR grows larger with the dislocation density. The results for thermal conductivity show good agreement with previous studies at zero strain, and a theoretical model is used to accurately explain its behavior vs strain-dependent contact radius. Both the Kapitza resistance and thermal resistance decrease with strain but with very different evolution. Simulations of a bulk sample under uniaxial strain were also carried out, allowing for a clear distinction between the role of compressive stress, which increases the conductivity, vs the role of dislocations, which decrease the conductivity. For the NP system, there is the additional role of contact area, which increases with stress and also modifies conductivity. An analytical model with a single free parameter allows for a description of all these effects and matches both our bulk and NP simulation results. [ABSTRACT FROM AUTHOR]

2'-Deoxy-ATP (dATP), a naturally occurring near analog of ATP, is a well-documented myosin activator that has been shown to increase contractile force, improve pump function, and enhance lusitropy in the heart. Calcium transients in cardiomyocytes with elevated levels of dATP show faster calcium decay compared with cardiomyocytes with basal levels of dATP, but the mechanisms behind this are unknown. Here, we design and utilize a multiscale computational modeling framework to test the hypothesis that dATP acts on the sarcoendoplasmic reticulum calcium-ATPase (SERCA) pump to accelerate calcium re-uptake into the sarcoplasmic reticulum during cardiac relaxation. Gaussian accelerated molecular dynamics simulations of human cardiac SERCA2A in the E1 apo, ATP-bound and dATP-bound states showed that dATP forms more stable contacts in the nucleotide binding pocket of SERCA and leads to increased closure of cytosolic domains. These structural changes ultimately lead to changes in calcium binding, which we assessed using Brownian dynamics simulations. We found that dATP increases calcium association rate constants to SERCA and that dATP binds to apo SERCA more rapidly than ATP. Using a compartmental ordinary differential equation model of human cardiomyocyte excitation-contraction coupling, we found that these increased association rate constants contributed to the accelerated rates of calcium transient decay observed experimentally. This study provides clear mechanistic evidence of enhancements in cardiac SERCA2A pump function due to interactions with dATP.

The Hf/Ti ratio was precisely controlled at monolayer thickness using atomic partial layer deposition (APLD). HfxTi1−xO2 films with varying Hf concentrations were deposited by adjusting the pulse time of Hf precursors within a single atomic layer. Characterization using x-ray reflectivity, x-ray photoelectron spectroscopy, and spectroscopic ellipsometry confirmed the presence of Hf, Ti, and O in the films. Increasing the Hf content caused the binding energies of the O 1s peak to shift to higher values, indicating a chemical environment change from TiO2-like to HfO2-like. A higher Hf content also increased the relative atomic percentages of Hf, Ti, and O, altering the film properties. The mass density and optical properties were notably sensitive to changes in the Hf/Ti ratio at monolayer thickness. The potential of APLD to reduce dimensionality through precise control of both thickness and composition renders it especially appropriate for applications requiring highly specific material properties. [ABSTRACT FROM AUTHOR]

The Microwave SQUID Multiplexer (μ MUX) is the device of choice for the readout of a large number of low-temperature detectors in a wide variety of experiments within the fields of astronomy and particle physics. While it offers large multiplexing factors, the system noise performance is highly dependent on the cold- and warm-readout electronic systems used to read it out, as well as the demodulation domain and parameters chosen. In order to understand the impact of the readout systems in the overall detection system noise performance, first, we extended the available μ MUX simulation frameworks, including additive and multiplicative noise sources in the probing tones (i.e., phase and amplitude noise), along with the capability of demodulating the scientific data, either in the resonator's phase or the scattering amplitude. Then, considering the additive noise as a dominant noise source, the optimum readout parameters to achieve minimum system noise were found for both open-loop and flux-ramp demodulation schemes in the aforementioned domains. Later, we evaluated the system noise sensitivity to multiplicative noise sources under the optimum readout parameters. Finally, as a case study, we evaluated the optimal demodulation domain and the expected system noise level for a typical software-defined radio readout system. This work leads to an improved system performance prediction and noise engineering based on the available readout electronics and the selected demodulation domain. [ABSTRACT FROM AUTHOR]

This work presents a theoretical analysis of the oscillations of a system of degenerate electrons in metallic submicron clusters, taking into account the presence of a large-scale spatial inhomogeneity caused by quantum shell effects. The analysis was carried out in a model formulation in which the large-scale effect was taken into account by the presence of an effective external field acting on the electrons. Two approaches to the description of electron dynamics were considered: kinetic and hydrodynamic. The results obtained within these approaches are in good qualitative and quantitative agreement with each other and demonstrate that for certain cluster sizes, a spectrum of oscillations of the electronic system appears with frequencies differing by the value Δω ∼ ω0/R20 and localized in the vicinity of plasmon frequency ω0 (R0, radius of cluster). The impact of the large-scale effect on the processes of light and electron scattering by metal clusters is estimated. It is shown that maxima appear in the absorption spectrum of the electromagnetic waves, and they turn out to be shifted from the plasmon frequency by value Δω ∼ ω0/R20. Within the framework of the Born approximation, it is shown that a maximum at kR0 ∼ 4π appears in the cross section of elastic scattering of electrons on a metal cluster. The results obtained are important from the point of view of interpreting experiments on plasmonic structures, in particular, on metal films. [ABSTRACT FROM AUTHOR]

This study investigates the use of atomic layer deposition (ALD) to mitigate multipacting phenomena inside superconducting radio frequency cavities used in particle accelerators while preserving high quality factors in the 1010 range. The unique ALD capability to control the film thickness down to the atomic level on arbitrary complex shape objects enables the fine-tuning of TiN film resistivity and total electron emission yield (TEEY) from coupons to devices. This level of control allows us to adequately choose a TiN film thickness that provides both high resistivity to prevent Ohmic losses and a low TEEY to mitigate multipacting for the application of interest. The methodology presented in this work can be scaled to other domains and devices subject to RF fields in vacuum and sensitive to multipacting or electron discharge processes with their own requirements in resistivities and TEEY values. [ABSTRACT FROM AUTHOR]

We report an updated isothermal equation of state (EoS) of molybdenum (Mo) obtained by compression in beveled and toroidal diamond-anvil cells (DACs). For an improved compression environment, we developed a copper (Cu) pressure-transmitting medium (PTM) for the toroidal diamond-anvil cell samples, as it is a soft metal compared to Mo with a well calibrated EoS. A Ne PTM was used for the conventional beveled DAC samples. The unit-cell volumes of Mo were measured to 336(1) GPa in the Cu PTM and 231.2(6) GPa in the Ne PTM at room temperature. We additionally calculated elastic stiffness and compliance constants and evaluated the uniaxial stress of Mo and Cu with pressure. A new EoS for Mo is presented from data collected in all sample environments and compared to our theoretical predictions as well as previous compression studies of Mo. The (200) lattice plane of Mo produced the lowest volumes across the pressure range of this study for all compression environments, suggesting that it is less affected by nonhydrostatic stresses in the DAC compared to the other observed diffraction planes. The presented Mo EoS is compatible with extrapolations of EoS fits of Mo in helium (He) within ∼1% at 330 GPa. Results from this work demonstrate that compressing a sample in a softer metal in the toroidal DAC can improve the compression environment and result in measured sample volumes comparable to those collected in noble-gas media at multi-megabar conditions. [ABSTRACT FROM AUTHOR]

Using the density-functional theory approach, including Hubbard U correction, we investigate the defect structures consisting of vanadium (V) atoms embedded in a monolayer silicene. Specifically, we consider V–V atom pairs in antiferromagnetic (AFM), ferromagnetic (FM), and non-magnetic states, which are embedded in substitutional and interstitial sites. We determine the ground-state structures, formation and binding energies, electronic structures, induced magnetization, as well as the spin-exchange coupling between the V–V pair. For the substitutional vanadium atom pair, the stability of the AFM and FM spin configurations depends on the sublattice sites in which the V atoms are sited. When the V pair is located on a similar sublattice site type, the AFM spin alignment is more energetically favored, whereas when the pair is located in a different sublattice site, the FM interactions are more stable. However, the relative stability of the AFM or FM configurations changes rapidly as the separation between the V pair increases. Regarding the interstitial-hole V–V pair configurations, the most stable structure is when the pair is at the nearest-neighbor hole sites and is in an FM alignment. Also, at larger separations, the AFM or FM hole configurations are approximately degenerate in energy. Furthermore, we elucidate on the Ruderman–Kittel–Kasuya–Yosida, direct-exchange, and the superexchange interaction mechanisms in the vanadium-embedded silicene. In addition, we estimate a Curie temperature (Tc) of up to ∼500 K for a silicene structure containing a V pair in the FM spin alignment. Such a high Tc, in addition to the stability of the material, suggests that vanadium-embedded silicene is a potential candidate material for spintronic device applications. [ABSTRACT FROM AUTHOR]

The mixed zinc-ferrite spinel magnetic nanoparticles (MNPs) with the general formula ZnxFe3−xO4 are among the most extensively studied families of Fe oxides due to their interesting and diverse chemical, electronic, and magnetic properties. These systems offer the possibility of surface functionalization and possess high biocompatibility, making them highly attractive for applications in biomedicine, such as magnetic fluid hyperthermia (MFH). The efficiency of the MFH process relies on the magnetic, structural and morphological properties of the MNPs. The substitution with the Zn ion and the cationic distribution, as well as the synthesis process employed, have a direct impact on the final properties of these oxides. Therefore, it is essential to have tools that enable a comprehensive characterization of the system to assess its performance in MFH. In this study, we have synthesized four ZnxFe3−xO4 MNP systems using three different methods: two by thermal decomposition at high temperatures, one by co-precipitation, and another by co-precipitation followed by ball milling. We analyze the effect of these various synthesis processes on the magnetic and crystallographic properties, aiming to correlate them with the response of each system in MFH. Neutron diffraction data are employed to determine the cation site occupation and to investigate the correlation with the synthesis method. MFH measurements were conducted in media of diverse viscosities, revealing different values of specific loss power, thus demonstrating a clear dependence on the synthesis process and Zn content. [ABSTRACT FROM AUTHOR]

The article discusses a theoretical and experimental investigation of the reflection of slow electrons from the surfaces of single-crystal and polycrystalline tungsten thermionic cathodes. The findings challenge traditional ideas as they confirm that the effective reflection coefficient, reff, can reach values close to unity contrary to prior belief. The reason for this occurrence has been established, which is the additional reflection of slow electrons from a potential barrier near polycrystalline surfaces. A method has been developed to separately measure electron reflection coefficients at the surfaces of thermionic cathodes and at the potential barrier of electrode spot fields with different work functions. The study reveals that the maximum values of reff are achieved on polycrystalline surfaces. Additionally, the work functions and reflection coefficients rhkl have been determined for the faces of single crystals of (110), (112), (100), (111), and (116) oriented tungsten. The proposed method enables control over cathode emission inhomogeneity and makes it possible to mitigate the negative effects of secondary electron emission by suppressing electric fields near the cathode surface. [ABSTRACT FROM AUTHOR]

The response of polycrystalline nitinol with solely austenite structure was studied in three series of planar impact tests characterized by loading of the nitinol samples of 0.5–10 mm thickness by 1 mm thick aluminum impactor accelerated up to velocities of about 387, 429, and 567 m/s. In all the tests, the velocities of the free surfaces of the samples were monitored by a laser velocity interferometer. It was found that in all three test series, the amplitude of elastic precursor wave, being initially greater than 4 GPa, rapidly decays with the propagation distance down to ∼2.5 GPa, below which the decay is hindered by atomic clusters of the nanometer size. Based on the part of the velocity histories indicating the shock-induced austenite–martensite transformation, the initial, of about 2.5 × 103 s−1, and the maximum, up to 1 × 105 s−1, rates of the transformation were determined. As well, the impact stress slightly greater than 4 GPa was determined as that required for the onset of the B2 → B19′ transformation under shock loading. The unloading parts of the same velocity histories allowed a rough estimate of the fraction of the shock-transformed martensite and the elucidation of the virtually complete reversibility of the transformation. [ABSTRACT FROM AUTHOR]

The editorial in the Journal of Applied Physics discusses the importance of defects in semiconductor materials for designing semiconductor-based devices. It covers various types of defects, their impact on device performance, and advancements in defect modeling and experimentation. The document also highlights research areas such as defects in oxides, III-V semiconductors, silicon carbide, 2D materials, and detector/absorber materials. The study emphasizes the ongoing research in understanding defects in semiconductors and their implications for fields like quantum information science, power electronics, and energy storage and conversion. [Extracted from the article]

Based on experimental data, we propose a model to evaluate the energy dissipated during quantum tunneling processes in solid-state junctions. This model incorporates a nonlinear friction force expressed in the general form f (x) = γ v (x) α , where γ is the frictional coefficient, which is fitted to data. We study this by applying voltages just below the barrier height up to near breakdown voltages. Furthermore, by lowering the temperature and adjusting the applied voltage to the junction, the effect on dissipation caused by the variation in barrier height is examined. We underline that the crucial dependency of dissipation on the fraction of particle energy lost is modulated by two primary mechanisms: the application of voltage and the variation of temperature. The fraction of energy dissipated decreases, in general, for increasing energies of the tunneling particles at a given temperature. However, for a given energy of the tunneling particle, the present work demonstrates a turning point at a temperature of 137 K, after which the dissipated energy starts increasing for higher temperatures. The latter can possibly be due to the increase of electron–phonon interactions, which become predominant over barrier height reduction at higher temperatures, and hence, we identify T = 137 K as a critical temperature for a change in the dissipative characteristics of the solid-state junction under consideration. Notably, the study also identifies significant changes in dissipation parameters, γ and α, above 137 K, exhibiting a linear decline and underscoring the importance of further research at higher temperatures. [ABSTRACT FROM AUTHOR]

High gain and large bandwidth of traveling-wave parametric amplifier exploiting the nonlinearity of Josephson Junctions can be achieved by fulfilling the so-called phase-matching condition. This condition is usually addressed by placing resonant structures along the waveguide or by periodic modulations of its parameters, creating gaps in the waveguide's dispersion. Here, we propose to employ the Josephson junctions, which constitute the centerline of the amplifier, as resonant elements for phase matching. By numerical simulations in JoSIM (and WRspice) software, we show that Josephson plasma oscillations can be utilized to create wave vector mismatch sufficient for phase matching as well as to prevent the conversion of the pump energy to higher harmonics. The proposed traveling wave parametric amplifier design has a gain of 15 dB and a 3 GHz bandwidth, which is comparable to the state-of-the-art TWPAs. [ABSTRACT FROM AUTHOR]

We exhaustively investigate the temporal correlations between the ultrahigh-frequency (of the order of 1–6 GHz), very high-frequency (of the order of 10–100 MHz), and x-ray (photons with energies from 5 keV to 1 MeV) emissions together with the optical emissions in the near-infrared (within 700–1100 nm) and near-ultraviolet (within 300–400 nm) wavelength regions. The emissions are produced by a laboratory atmospheric discharge developing in an 55 cm air gap at voltages up to 1 MV. When registering various electromagnetic emissions, the discharge current and voltage are measured, as well as nanosecond imaging of the discharge evolution in its own glow is performed. The spatiotemporal localization of the discharge regions associated with the x-ray generation is carried out by employing the group of fast scintillation detectors, and the evolution of plasma structures during the x-ray generation is traced. A chronological map is constructed providing an in-depth understanding of the temporal character and correlations of various electromagnetic emissions. The map allows one to analyze fast ionization processes occurring in the gas-discharge medium and triggering the generation of the corresponding emissions. The generation mechanisms of the considered emissions in an extended high-voltage discharge are discussed. The findings can be helpful in revealing the sources of various electromagnetic emissions accompanying the formation of laboratory and atmospheric discharges. [ABSTRACT FROM AUTHOR]

In a recent work, assuming a Beer–Lambert optical absorption and a Gaussian laser time profile, it was shown that the exact solutions for a 1D photoacoustic (PA) boundary-value-problem predict a null pressure for optically strong absorbent materials. In order to overcome this inconsistency, a heuristic correction was introduced by assuming that heat flux travels a characteristic length during the duration of the laser pulse [M. Ruiz-Veloz et al., J. Appl. Phys. 130, 025104 (2021)] τ p. In this work, we obtained exact analytical solutions in the frequency domain for a 1D boundary-value-problem for the Dual-Phase-Lag (DPL) heat equation coupled with a 1D PA-boundary-value-problem via the acoustic wave equation. Temperature and pressure solutions were studied by assuming that the sample and its surroundings have a similar characteristic thermal lag response time τ T ; therefore, the whole system is assumed to have a similar thermal relaxation. A second assumption for τ T is that it is considered as a free parameter that can be adjusted to reproduce experimental results. Solutions for temperature and pressure were obtained for a one-layer 1D system. It was found that for τ T < τ p , the DPL temperature has a similar thermal profile of the Fourier heat equation; however, when τ T ≥ τ p , this profile is very different from the Fourier case. Additionally, via a numerical Fourier transform, the wave-like behavior of DPL temperature is explored, and it was found that as τ T increases, thermal wave amplitude is increasingly attenuated. Exact solutions for pressure were compared with experimental PA signals, showing a close resemblance between both data sets, particularly in time domain, for an appropriated value of τ T ; the transference function was also calculated, which allowed us to find the maximum response in frequency for the considered experimental setup. [ABSTRACT FROM AUTHOR]

The temporal, spectral, and angular characteristics of the x-ray emissions (photons with energies within 5–1000 keV) are exhaustively investigated during the discharge formation at voltages up to 1 MV in approximately 55 cm air gaps. The temporal correlations between the x-ray emissions and discharge voltage and current waveforms are established. The evolution of the discharge plasma structures developing in the time periods of the x-ray generation is traced with a nanosecond resolution. Based on statistical data, theoretical analysis, and estimates the regularities in the x-ray emission characteristics are revealed together with their relationship with the ionization processes occurring in the gas-discharge medium. On the basis of the obtained experimental data, the probable x-ray generation mechanisms are discussed. The findings can provide a deeper understanding of the physics behind the sources of hard x rays arising during the development of laboratory and atmospheric discharges. [ABSTRACT FROM AUTHOR]

The transformation of 2 ¯ 01 β - Ga 2 O 3 to h- GaN under exposure to RF nitrogen plasma was monitored in situ by reflection high-energy electron diffraction. Analysis of the reaction kinetics reveals that the nitridation process is initiated by the formation of an oxynitride phase and proceeds via two-dimensional nucleation and growth of wurtzite GaN grains. X-ray photoelectron spectra suggest a Ga − (N x O 1 − x) type configuration dominates the surface early in the nitridation process. The surface restructuring is followed by a diffusion-fed phase transformation, which propagates the wurzite GaN structure into the substrate upon reaching 70 % nitrogen anion site occupation, corresponding to the oxygen solubility in h - GaN. A direct correlation is observed between the nitridated film morphology and the epitaxial film crystallinity, demonstrating control of the residual strain, lateral coherence, and mosaicity in subsequent GaN epitaxy by the nitridation conditions. This study provides mechanistic details of the nitridation reaction of 2 ¯ 01 β - Ga 2 O 3 facilitating the optimization of the nitridation process toward improving GaN - 2 ¯ 01 β - Ga 2 O 3 heterojunctions. [ABSTRACT FROM AUTHOR]

The development of Ge-rich GeSbTe (GGST) alloys significantly enhanced the high-temperature stability required for Phase-Change Memory technology. Previous studies on Ge enrichment in GeSbTe (GST) materials with Sb-over-Te ratio lower than one (Sb / Te < 1) highlighted the segregation into cubic Ge and cubic GST phases. Such a segregated cubic GST phase is metastable and presents a polycrystalline structure with disordered grain boundaries that could lead to structural relaxation and then to drift phenomena. In this work, using resistivity measurements, Raman spectroscopy, and in situ x-ray diffraction analyses, we demonstrate for the first time to our knowledge that GGST with Sb/Te higher than one (Sb / Te > --> 1) upon annealing leads to the direct formation of a GST hexagonal phase featuring a high growth speed, bypassing the cubic metastable phase. Combined with Ge enrichment, the increased value of the activation energy of the nucleation of Sb / Te > --> 1 GGST alloys ensures a high stability of the amorphous phase. Finally, nitrogen introduction further stabilizes the system against the crystallization, without compromising the high crystalline growth speed and the formation of the stable GST hexagonal phase in alloys with Sb / Te > --> 1. These results demonstrate the possibility to tune the crystalline structure of the segregated phases in Ge-rich GeSbTe alloys, combining the stability at high temperature of the amorphous phase with the high crystallization speed and uniformity (with larger grains) of a targeted GST phase. [ABSTRACT FROM AUTHOR]

Noncontact transportation and separation techniques using airborne ultrasound are attractive for use in industrial fields in which micrometer- to millimeter-sized objects can be levitated, rotated, and transported in the air using acoustic standing-wave and traveling-wave fields. This paper discusses a method to rotate a small object in the air without physical contact using ultrasound. The experimental system comprises a vibrating disk with four bolt-clamped Langevin-type ultrasound transducers and two semicircular reflectors. The flexural vibration of the disk generates an acoustic standing wave between them, and a small object can be levitated at the nodal position. An acoustic traveling wave was generated in the horizontal direction in an asymmetric acoustic field by inclining one of the two reflectors, which induced rotation of the object in the clockwise or counterclockwise direction. The acoustic intensity in the circumferential direction acting on the object was then calculated, and the directions of rotation predicted by the calculations corresponded with the experimental results. Higher input currents produced higher rotation speeds; the rotation speed reached a maximum value of 6.8 ± 0.9 rps at an input current of 1.1 App. [ABSTRACT FROM AUTHOR]

The lack of suitable p-type dopant for β-Ga2O3 remains a hurdle for vertical power device applications. Epitaxy of GaN on Ga2O3 substrates was demonstrated as an alternative. (–201)-oriented β-Ga2O3 was converted into (0001)-oriented hexagonal GaN via nitrogen plasma in a plasma-assisted molecular beam epitaxy chamber, as verified by XRD and RHEED. The resulting nitridated GaN layers were characterized by TEM, x-ray reflectivity, and AFM to relate the nitridation conditions to crystallinity, layer thickness, and surface roughness. The crystallinity of subsequently grown epitaxial GaN films was quantified via XRD rocking curves and related to the nitridation layer properties across varying nitridation conditions. Specifically, the effect of the grain size and nitridation layer thickness was investigated to determine their role in threading screw dislocation management. [ABSTRACT FROM AUTHOR]

A scattering matrix technique is used to calculate the longitudinal and transverse energy dependence of the transmission probability through various heterostructures using both the BenDaniel–Duke (BD) and the lesser known Zhu–Kroemer (ZK) boundary conditions to take into account the spatial dependence of the effective mass. We first illustrate the large difference in the transmission probabilities calculated using both boundary conditions for the simple problems of tunneling through a potential step, a single rectangular barrier, and a resonant tunneling device. Then, we present numerical calculations of the external electric field dependence of the field emission (FE) current from a n-doped GaAs semiconductor/vacuum interface using both boundary conditions, showing that the BD boundary conditions underestimate the FE current for large values of the applied external electrostatic field. A comparison of calculated FE characteristics with FE data may be a way to determine the appropriate boundary conditions to solve tunneling problems through heterostructures with spatially varying effective mass. [ABSTRACT FROM AUTHOR]

The emission properties of a single Si microdisk resonator with a deterministically embedded GeSi quantum dot (QD) stack have been investigated. The results demonstrate selective excitation of different modes of the resonator depending on the position of QDs. The photoluminescence (PL) spectrum changes dramatically depending on the location of the QDs in the resonator. For the central QD position, the excitation of low Q-factor Mie modes with high field concentration in the center of resonator results in the appearance of a broad PL band. When the stack of QDs is shifted from the center to the edge of the Si resonator, the quenching of this PL band is observed and narrow PL peaks corresponding to whispering gallery modes (WGMs) appear in the PL spectrum. It is found that resonator modes can be excited not only by QDs but also by the radiation of the wetting layer. It is shown that a GeSi island on the top of the QD stack, not covered by silicon, can play the role of a nanoantenna, redirecting radiation to the upper half-space, which is especially important for WGMs that usually radiate sideways. [ABSTRACT FROM AUTHOR]

In this paper, we report on the electrical impedance measurement of human endothelial cellular networks and show the existence of emergent power law behavior in its admittance. In particular, we find that the admittance scales with the frequency ω as ωα, with the exponent that varies with the degree of the disruption caused by the inflammation in the endothelial cellular network. We demonstrate that the power law of the measured electrical admittance can be understood by a simple percolation model of a large R–C (resistor–capacitor) network, which allows us to relate quantitatively and the intensity of inflammation. Our results suggest that the electrical properties of heterogeneous biomaterials, like living tissues, behave as a complex microstructural network. [ABSTRACT FROM AUTHOR]

The Arrhenius equation was used to describe the dynamics of two-state switching in mesoscale, ferromagnetic particles. Using square permalloy dots as an idealized two-state switching system, measurements of the prefactor of the Arrhenius law changed by 26 decades over barrier heights from 30 to 700 meV. Measurements of the prefactor ratios for a two well system revealed significant deviations from the common interpretation of the Arrhenius law. The anomalous Arrhenius prefactors and the prefactor ratios can be fitted to a modified model that includes entropic contributions to two-state transitions. Similar considerations are likely for the application of the Arrhenius law to other mesoscale systems. [ABSTRACT FROM AUTHOR]

Low-frequency (LF) noise measurements are compared for Schottky-gate AlGaN/GaN heterostructure planar and fin field-effect transistors (FinFETs) as functions of gate voltage and measuring temperature. The noise of each device type is consistent with a carrier number fluctuation model. Similar effective defect-energy Eo distributions are derived for each of the two device architectures from measurements of excess drain-voltage noise-power spectral density vs temperature from 80 to 380 K. Defect- and/or impurity-related peaks are observed in the inferred energy distributions for Eo < 0.2 eV, Eo ≈ 0.45 eV, and Eo > 0.6 eV. Significant contributions to the LF noise are inferred for nitrogen vacancies and ON and FeGa impurity complexes. Ga dangling bonds at fin interfaces with gate metal are likely candidates for enhanced noise observed in FinFETs, relative to planar devices. Reducing the concentrations of these defects and impurity complexes should reduce the LF noise and enhance the performance, reliability, and radiation tolerance of GaN-based high electron mobility transistors. [ABSTRACT FROM AUTHOR]

The quantum thermal transport of elastic excitations through a two-dimensional elastic waveguide between two thermal reservoirs is studied. We solve the classical Kirchhoff–Love equation for rectangular plates and explore the dispersion relation for both the symmetric and antisymmetric solutions. Then, we study the phonon transport of these modes within the second quantization framework by analyzing the mean quadratic displacement, from which the energy density current, the temperature field, and conductance are determined. We identify the relevant modes contributing to thermal transport and explore the average temperature difference to reach the high-temperature limit. We expect our results to pave the way for understanding phonon-mediated thermal transport in two-dimensional mesoscopic quantum devices. [ABSTRACT FROM AUTHOR]

In the search for materials alternate to bulk HgCdTe for high performance infrared imaging applications, colloidal quantum dots (CQDs), particularly HgTe CQDs, have gained traction owing to acceptable detector performance with easy preparation and low cost. In this article, we evaluate α -Sn CQDs, an environmentally less reactive and less toxic alternative to HgTe, for infrared sensing applications. Ab initio density functional theory calculations are used to study the shape-dependent stability, electronic bandgap, and absorption coefficient of α -Sn CQD nanoparticles (NPs). We consider three possible CQD shape constructions—Wulff, shell-by-shell, and spherical. The CQD of Wulff construction is predicted to be the most stable. However, we find that the size, not the shape, of the NP has a strong effect on the bandgap and absorption coefficient. Consequently, a sharp absorption edge is expected even in an ensemble of CQDs with different shapes. Importantly, the shape determines the position of the band edges with respect to vacuum, and thus offers a possibility of choosing the shape to improve alignment with the energy levels of ligands to enable efficient drift transport, instead of a slower and less efficient hopping transport. [ABSTRACT FROM AUTHOR]

Results of experimental research and two-dimensional hydrodynamical simulations of close to critically damped microsecond timescale underwater electrical explosions of butterfly-shaped foils for six different materials are presented. Using current and voltage waveforms along with multi-frame shadow images of the shocks generated in water, the values of the specific action integral, h, were determined. It is shown that values of h can be calculated based on the average current density and that its value (within error bars) does not change in the range of current densities (0.5–1) × 108 A/cm2. The values of h were found to be consistent with those obtained for sub-microsecond underwater electrical explosions of wires made of the same material but differ from those obtained in earlier research with explosion of wires in vacuum. [ABSTRACT FROM AUTHOR]

Shock experiments give a unique insight into the behavior of matter subjected to extremely high pressures and temperatures. Understanding the behavior of materials under such extreme conditions is key to modeling material failure and deformation dynamics under impact. While studies on pure silica are extensive, the shock behavior of other commercial silicates that contain additional oxides has not been systematically investigated. To better understand the role of composition in the dynamic behavior of silicates, we performed laser-driven dynamic compression experiments on soda-lime glass (SLG) up to 315 GPa. Using the accurate pulse shaping offered by the long pulse laser system at the Matter in Extreme Conditions end-station at the Linac Coherent Light Source, SLG was shock compressed along the Hugoniot to multiple pressure-temperature points. Velocity Interferometer System for Any Reflector was used to measure the velocity and determine the pressure inside the SLG. The U s -u p relationship obtained agrees well with the previous parallel plate impact studies. Within the error bars, no transformation to the crystalline phase was observed up to 70 GPa, which is in contrast to the behavior of pure silica under shock compression. Our studies show that the glass composition strongly influences the shock compression behavior of the silicate glasses.

We demonstrate the possibility to tune the saturation magnetization, coercivity, and uniaxial in-plane anisotropy constant in amorphous bilayers and multilayers of Co85 (Al70Zr30)15 and Sm11Co82Ti7 through the interface density. From magnetometry and x-ray circular dichroism (XMCD) measurements, we conclude that the easy-axis coercivity μ0 Hc increases four times when the number of bilayer repetitions, N, increases from 1 to 10 within a constant total sample thickness of 20 nm. At the same time, the anisotropy constant Ku also increases by a factor four, whereas the saturation magnetization Ms decreases slightly. The Co spin and orbital moments, ms and ml, are found to be approximately constant within the sample series. The average total Co moment is only 0.8-0.9 μB/atom, but the ml/ms ratio is strongly enhanced compared to pure Co. Magnetization curves extracted from XMCD measurements show that the Co and Sm moments are ferromagnetically coupled for all samples.

A magnetocaloric effect (MCE) due to adiabatic change of temperature was directly measured in an Fe47.1Mn26.1Ga26.8 alloy undergoing martensitic transformation. Its values in the high-temperature region were positive, while in the temperature range below temperatures of martensitic transformation, the adiabatic change of temperature in the magnetic field was negative. The x-ray diffraction analysis revealed the presence of a Heusler L21 (B2) phase and a γ-phase in the Fe47.1Mn26.1Ga26.8 alloy above temperature of martensitic transformation. The features of field-dependent magnetization and temperature variation in MCE indicate the occurrence of ferromagnetic-to-antiferromagnetic transition in the γ-phase, which is responsible for the observed inverse magnetocaloric effect. [ABSTRACT FROM AUTHOR]

In this work, we studied the nature and dilemma of the inverse magnetocaloric effect using Ni50Mn36In14 magnetic shape memory alloy. In this context, the inverse magnetocaloric effects of Ni50Mn36In14 magnetic shape memory alloy polycrystalline samples were investigated as a function of annealing heat treatments by the thermo-magnetometry method. Two forms of Ni49TiMn36In14 magnetic shape memory alloy were studied: one that predominantly undergoes a magnetic phase transition and the other that exhibits both a magnetic and martensitic phase transition. The magnetic behaviors and magnetocaloric properties of these alloys were analyzed to investigate the competition between magnetic and lattice contributions to the total entropy change. Finally, the mutually contradictory role of magnetic and lattice contributions was demonstrated. [ABSTRACT FROM AUTHOR]

Five high entropy carbide ceramics, (Hf0.2,Nb0.2,Ta0.2,Ti0.2,Zr0.2)C, (Cr0.2,Hf0.2,Ta0.2,Ti0.2,Zr0.2)C, (Hf0.2,Mo0.2,Ta0.2,Ti0.2,Zr0.2)C, (Hf0.2,Ta0.2,Ti0.2,W0.2,Zr0.2)C, and (Hf0.2,Mo0.2,Ti0.2,W0.2,Zr0.2)C, were synthesized by carbothermal reduction of oxides and direct current sintering. The five high entropy carbide ceramics were determined to be nominally phase-pure with relative densities of more than 98.9% and mean grain sizes of less than 5 μm. Average Vickers hardness values ranged from 19.2 ± 0.4 GPa for (Hf0.2,Nb0.2,Ta0.2,Ti0.2,Zr0.2)C at a load of 2 kgf to 43.5 ± 0.4 GPa for (Hf0.2,Mo0.2,Ti0.2,W0.2,Zr0.2)C at a load of 0.05 kgf. Hardness generally increased with increasing the valence electron concentration and strain as measured by the Williamson–Hall analysis. However, neither correlation was conclusive enough to be a clear indicator of hardness. Instead, it was determined that a combination of effects that includes the valence electron concentration, lattice strain, and grain size all contribute to the hardness of high entropy carbide ceramics. [ABSTRACT FROM AUTHOR]

Zinc germanium diphosphide (ZnGeP2) is a ternary semiconductor best known for its nonlinear optical properties. A primary application is optical parametric oscillators operating in the mid-infrared region. Controlled donor doping provides a method to minimize the acceptor-related absorption bands that limit the output power of these devices. In the present study, a ZnGeP2 crystal is doped with selenium during growth. Selenium substitutes for phosphorus and serves as a deep donor. Significant concentrations of native defects (zinc vacancies, germanium-on-zinc antisites, and phosphorous vacancies) are also present in the crystal. Electron paramagnetic resonance (EPR) is used to establish the atomic-level model for the neutral charge state of the selenium donor. The S = 1/2 signal from the neutral donors is produced at 6 K by illuminating with 633 nm light (electrons excited from doubly ionized Zn vacancies convert Se P + donors to Se P 0 donors). A g matrix, with principal values of 2.088, 2.203, and 1.904, is extracted from the angular dependence of the EPR spectrum. The principal-axis direction associated with the 1.904 principal value is close to a Se–Ge bond. This indicates an asymmetric distribution of unpaired spin density around the selenium ion and thus predicts the deep donor behavior. [ABSTRACT FROM AUTHOR]

We generalize a native Nb 2 O 5 dissolution model [G. Ciovati, Appl. Phys. Lett. 89, 022507 (2006)] to sequential overlayer dissolutions, multilayer dissolution, and realistic temperature profiles, which may be applicable to other materials. The model is applied to secondary ion mass spectrometry depth profile measurements for varying temperature profiles and two-step oxide dissolution in Nb and found to agree well. In the context of the Meissner screening response due to impurity profiles on the length scale of the London penetration depth, the shallow diffusion of O impurities results in a substantial decrease in the peak supercurrent density near the surface. In this framework, oxide dissolution and oxygen diffusion can account for a rise in peak supportable magnetic field in SRF cavities with baking time and a suppression after the optimal baking time is reached, in good agreement with peak-field baking temperatures and times as well as recent quench field measurements. [ABSTRACT FROM AUTHOR]

The effect of the YBCO superconducting (SC) state on the magnetic properties of as-grown YBCO/Co/IrMn heterostructures has been systematically studied using magnetometry and ferromagnetic resonance. The obtained data showed that the superconductivity of the YBCO substrate strongly affects the ferromagnetic properties of the deposited Co layer deeper (up to 50 nm) than the coherence length of the YBCO (≃ 4 nm) by an exchange interaction between the Co magnetic moments and the superconducting pairs at the YBCO/Co interface. The interfacial exchange interaction, switched on while the YBCO enters the SC state, pins Co spins and yields an enhancement of the Co magnetic properties. [ABSTRACT FROM AUTHOR]

The recent decade brought many advances to plasmonics, but high power density plasmonic antennas designed to behave as heaters or operate in high temperature environments are still facing material stability challenges preventing their ultimate use. Gold has been the optimal choice among plasmonic materials but experiences morphology changes at temperature that result in device efficiency reduction and failure. Bulk titanium nitride has been explored as a solution but has deal-breaking tradeoffs in device quality factor. In this paper, we explore via proof-of-concept the use of a metal-ceramic composite structure to determine whether a bulk Au nanorod can provide strong plasmonic resonances while coated with an ultrathin conformal layer of titanium nitride or silica to provide morphological stability and sufficient plasmonic activity without excessive resonance quality degradation. We show SEM-level morphological stability for temperatures up to 500 °C with coatings below 4 nm. Computer modeling suggests the ultrathin titanium nitride has detrimental effects on the strong plasmonic resonances of a Au nanorod. We then looked into other possible coatings for solutions to stabilize high power density plasmonic antennas including plasmonic oxides, metal adhesion layers, and silica, the latter appearing to be the best option while lowering the overall peak electric field intensity, the silica increases the electric field intensity at its boundary.

Pre-orientation stretching of high-density polyethylene improves the charge stability of electrets produced in corona discharge. Electrets made of oriented polyethylene charged in the corona under uniaxial tensile mechanical stress have even greater stability. The increased stability of electrets is due to the capture of electrons in deep structural traps formed after the rupture of macromolecules during orientation and mechanical stress. By the method of thermally stimulated electron emission, it is shown that the charge of electrets is formed at the expense of electrons. New convenient formulas for calculation of electrical voltages in multilayer insulation with electrets were obtained. [ABSTRACT FROM AUTHOR]

Ridge microstructures were prepared by etching through samples consisting of a series of stacked InAs x P 1 − x quantum wells (QWs) with step graded composition grown on InP by molecular beam epitaxy. Different etching techniques were used: wet etching with HCl/H 2 O and reactive ion etching with CH 4 /H 2. These microstructures were characterized by low-temperature micro-photoluminescence. The photoluminescence (PL) emission associated with each QW was clearly identified. The PL was measured in detail across etched ridge stripes of various widths. Variations of the integrated PL intensities across the etched stripes were observed. The PL intensities for all QWs increase gradually from the edge to the center of the ridge microstructures. The PL intensity measured at the ridge center is systematically reduced for ridges which are 10 or 20 μ m wide as compared to ridges which are 30 μ m wide or larger. On the other hand, the spectral peak position of the PL lines remained constant with high accuracy (0.2–0.4 meV) across the microstructures. These observations are discussed in terms of the different possible mechanisms which determine the PL intensity variations, namely, non-radiative recombination at the etched walls and effects of stray electric fields which result from the etching process. Based on this discussion, we compare quantitatively the different etching processes which we have used. Altogether, this study illustrates the contribution that specially designed test structures, coupled with advanced spectroscopic characterization, can provide to the development of semiconductor photonic devices (e.g., lasers or waveguides) involving etching processes. [ABSTRACT FROM AUTHOR]

Rare-earth ion doped oxide thin films integrated on silicon substrates provide a route toward scalable, chip-scale platforms for quantum coherent devices. Erbium-doped TiO 2 is an attractive candidate: the Er 3 + optical transition is compatible with C-band optical fiber communications, while TiO 2 is an insulating dielectric compatible with silicon process technology. Through structural and optical studies of Er-doped TiO 2 thin films grown via molecular beam deposition on silicon, SrTiO 3 , and sapphire substrates, we have explored the impact of polycrystallinity and microstructure on the optical properties of the Er emission. Comparing polycrystalline TiO 2 (rutile)/Si with single-crystalline TiO 2 (rutile)/r-sapphire and polycrystalline TiO 2 (anatase)/Si with single-crystalline TiO 2 (anatase)/ SrTiO 3 , we observe that the inhomogeneous linewidth (Γ inh) of the most prominent peak in the Er spectrum (the Y 1 – Z 1 transition, 1520 and 1533 nm in rutile and anatase TiO 2) is significantly narrower in the polycrystalline case. This implies a relative insensitivity to extended structural defects and grain boundaries in such films (as opposed to, e.g., point defects). We show that the growth of an undoped, underlying TiO 2 buffer on Si can reduce Γ inh by a factor of 4–5. Expectedly, Γ inh also reduces with decreasing Er concentrations: we observe a ∼ 2 order of magnitude reduction from ∼ 1000 ppm Er to ∼ 10 ppm Er. Γ inh then gets limited to a residual value of ∼ 5 GHz that is insensitive to further reduction in the Er concentration. Based upon the above results, we argue that the optical properties in these thin films are limited by the presence of high "grown-in" point defect concentrations. [ABSTRACT FROM AUTHOR]

The continuous effort in making artificial neural networks more alike to human brain calls for the hardware elements to implement biological synapse-like functionalities. The recent experimental demonstration of ferroelectric-like FETs promises low-power operation as compared to the conventional ferroelectric switching devices. This work presents an in-house numerical tool, which self-consistently solves the electrostatics and time-dependent electronic and ionic transport. The tool is exploited to analyze the effect that various physical parameters such as mobility and ion concentration could have on the design of the ferroelectric-like FETs. Their suitability in emulating different functions of the biological synapses is also demonstrated. [ABSTRACT FROM AUTHOR]

Laser pulses were applied to a target mounted on a ballistic pendulum to study the momentum imparted by a laser shock impact. Photonic Doppler Velocimetry was used to assess the momentum imparted by each laser pulse. To increase the momentum produced, a layer of polymer transparent to the laser wavelength was applied to the surface of the targets to confine the plasma generated as a result of the laser–matter interaction. This yielded momentum coupling coefficients one hundred times higher than those obtained for equivalent laser parameters in the classical direct regime configuration. The study was completed by simulating the experiments with the one-dimensional Lagrangian hydrodynamics code ESTHER, which showed good agreement with the experimental results. [ABSTRACT FROM AUTHOR]

X-ray self-heating is a common by-product of X-ray Free Electron Laser (XFEL) techniques that can affect targets, optics, and other irradiated materials. Diagnosis of heating and induced changes in samples may be performed using the x-ray beam itself as a probe. However, the relationship between conditions created by and inferred from x-ray irradiation is unclear and may be highly dependent on the material system under consideration. Here, we report on a simple case study of a titanium foil irradiated, heated, and probed by a MHz XFEL pulse train at 18.1 keV delivered by the European XFEL using measured x-ray diffraction to determine temperature and finite element analysis to interpret the experimental data. We find a complex relationship between apparent temperatures and sample temperature distributions that must be accounted for to adequately interpret the data, including beam averaging effects, multivalued temperatures due to sample phase transitions, and jumps and gaps in the observable temperature near phase transformations. The results have implications for studies employing x-ray probing of systems with large temperature gradients, particularly where these gradients are produced by the beam itself. Finally, this study shows the potential complexity of studying nonlinear sample behavior, such as phase transformations, where biasing effects of temperature gradients can become paramount, precluding clear observation of true transformation conditions. [ABSTRACT FROM AUTHOR]

Partial shading can significantly impair the efficiency of thin-film solar cells. When exposed to partial shading, cells within the array tend to become reverse biased, leading to thermal runaway events and the emergence of hotspots. In Cu (In , Ga) Se 2 (CIGS) solar cells such hotspots are also associated with so-called worm-like defects. Both theoretical and experimental studies have shown that in CIGS, a positive-feedback loop leads to instability and thermal runaway events. However, we observe an inconsistency between published simulation results and recently published experimental work. In a recent experimental study, it was shown that under certain conditions, a hotspot develops within 1 m s , showing signs of melting of the CIGS in an area with a 5 μ m radius. However, in published simulation results, the time for such high temperatures to develop is in the order of seconds, a discrepancy of three orders of magnitude. In this work, we argue that this discrepancy is explained by the size of the seed defect, demonstrating that the origin of these experimentally observed, fast-developing hotspots is likely microscopic defects. To this end, we developed an electro-thermal finite element model, with very high temporal and spatial resolution. We demonstrate that, assuming a seed defect with a 10 n m radius, we can reproduce the experimental results with respect to the size of the defect and the time it took to develop. [ABSTRACT FROM AUTHOR]

Radio-frequency (RF) cavities, previously employed in particle physics, quantum computing, and gravitational wave research, offer unique advantages in terms of sensitivity and non-invasiveness as a method of sensing motion in both macroscopic and microscopic systems. This research aims to address how an RF cavity can effectively detect and characterize the low-frequency vibratory motion of a room-temperature mm-scale levitated particle. In this case, the particle in question is a diamagnetically levitated slab of highly oriented pyrolytic graphite. Cavity-based identification of the slab's rigid-body modes is substantiated by calculations of the force acting on the particle and validated through slow-motion video object tracking. We find that this system can accurately measure oscillations in all six center-of-mass degrees of freedom. Calculations indicate that this system could potentially detect forces on the scale of tens of femto-Newtons and center of mass displacements of less than 10 nm. This work provides a non-invasive method of conducting position and vibration measurements in the field of levitodynamics without the ultra-cold temperatures or bulky precision laser setups that superconducting quantum interference devices and conventional interferometric methods utilize. [ABSTRACT FROM AUTHOR]

A study on the neutron production from single and double gas puff Z-pinches on the CESZAR linear transformer driver with ∼0.45 MA current and 170 ns rise time is presented. Total neutron yield measurements made with a LaBr activation detector are compared for three configurations, using a double nozzle setup. When a single, hollow, deuterium gas shell was used, reliable implosions could only be attained at higher load mass than the optimal value to match implosion time with the driver rise time, with neutron yields of ∼106 per pulse. The use of a double gas puff configuration with a deuterium center jet allowed a reduction in the shell density and operation closer to machine-matched conditions, recording up to (4.1 ± 0.3) × 107 neutrons/pulse when either Kr or D2 was used in the shell. For a comparable mass and implosion time, using a higher atomic-number gas in the outer shell results in more unstable plasma surface and smaller plasma radius at the location of instability bubbles, which, however, do not seem to consistently correlate with a higher neutron yield. Comparing implosion dynamics with models and neutron yields with literature scaling suggests that the machine current is not well coupled to the plasma during the final stages of compression. Optimizing current and energy coupling to the pinched plasma is critical to improving performance, particularly in low-impedance drivers. [ABSTRACT FROM AUTHOR]