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For rooting the development of GaN-based optoelectronic devices, understanding the roles of midgap recombination centers (MGRCs), namely, nonradiative recombination centers and deep-state radiative recombination centers, on the carrier recombination dynamics is an essential task. By using the combination of time-resolved photoluminescence and positron annihilation spectroscopy (PAS) measurements, the origins of major MGRCs in the state-of-the-art GaN epilayers, bulk crystals, and Mg-implanted layers were identified, and their concentrations were quantified for deriving the capture coefficients of minority carriers. In this article, potential standardization of the room-temperature photoluminescence lifetime for the near-band-edge emission (τ PL RT ) as the concentration of major MGRCs well below the detection limit of PAS is proposed. For n-GaN substrates and epilayers grown from the vapor phase, τ PL RT was limited by the concentration of carbon on N sites or divacancies comprising a Ga vacancy (VGa) and a N vacancy (VN), [VGaVN], when carbon concentration was higher or lower, respectively, than approximately 1016 cm−3. Here, carbon and VGaVN act as major deep-state radiative and nonradiative recombination centers, respectively, while major MGRCs in bulk GaN crystals were identified as VGa(VN)3 vacancy clusters in Na-flux GaN and VGa or VGaVN buried by a hydrogen and/or VGa decorated with oxygen on N sites, VGa(ON)3–4, in ammonothermal GaN. The values of τ PL RT in n-GaN samples are compared with those of p-GaN, in which τ PL RT was limited by the concentration of VGa(VN)2 in Mg-doped epilayers and by the concentrations of VGaVN and (VGaVN)3 in Mg-implanted GaN right after the implantation and after appropriate activation annealing, respectively. [ABSTRACT FROM AUTHOR]

Although the global market for wind energy is growing rapidly, leading-edge erosion is a critical issue hindering the development of wind power. The impact force of a droplet colliding with flat surfaces has been investigated in previous studies. However, the impact force exerted on curved surfaces, such as that experienced by eroded wind turbine blades, is not well understood. This study discusses the relationship between the impact force generated on a solid surface by a water droplet and the radius of curvature of the impacting surface. The impact force by a droplet was measured using a force sensor mounted on semi-cylindrical caps with different radii of curvature. The measurement results showed that the impact force decreased as the radius of curvature decreased. A computational fluid dynamics model solving incompressible flows showed that, unlike the case of a curved surface, the initial momentum of the droplet was mostly transferred to the flat surface. This resulted in a high impulse for an impact with a flat surface. The falling droplet was blocked by the surface, and the lateral jet was accelerated sideward. This acceleration was moderate for curved surfaces. When colliding with a flat surface, a higher impact force was generated owing to the wider area of the excited surface pressure compared with that of the curved surface. Finally, the relationship between the peak of the impact force and the surface curvature was derived, suggesting that the force peak is inversely proportional to the curvature. [ABSTRACT FROM AUTHOR]

We explore the phase behaviors of simple molecular crystals in order to investigate the molecular basis of the stability mechanism relative to their liquid counterparts. The free energies of the face centered cubic crystals of Xe and CO2 are calculated as a collection of oscillators, and those of the liquids are from an equation of state via molecular dynamics simulations. The vibrational free energy in the solid is separated into the harmonic and anharmonic terms. The harmonic free energies decrease harshly with the expansion of the volume manifested as the large positive Grüneisen parameters, but the anharmonic free energies are positive and increase with volume, both of which originate from the deviation of the potential surface from the parabolic curve. The anharmonic free energies, though less significant in magnitude and destabilize the solids thermodynamically, serve to enhance their mechanical stability. The solid–liquid phase boundaries cannot be settled correctly without the exquisite balance between the two opposing contributions. A sharp contrast regarding the solid free energy is found in low-pressure ice, where the harmonic free energy does not decrease monotonically with volume and its anharmonic free energy is negative. [ABSTRACT FROM AUTHOR]

Poly(N,N-diethylacrylamide) (PdEA), one of the thermoresponsive polymers, in aqueous solutions has attracted much attention because of its characteristic properties, such as coil-to-globule (CG) transition. We performed two-dimensional infrared spectroscopy and molecular dynamics (MD) simulations to understand the hydration dynamics in the vicinity of the CG transition at the molecular level via vibrational frequency fluctuations of the carbonyl stretching modes in the side chains of PdEA. Furthermore, N,N-diethylpropionamide, a repeating monomer unit of PdEA, is also investigated for comparison. From decays of the frequency–frequency time correlation functions (FFTCFs) of the carbonyl stretching modes, we consider that inhomogeneity of the hydration environments originates from various backbone configurations of PdEA. The degree of the inhomogeneity depends on temperature. Hydration water molecules near the carbonyl groups are influenced by the confinements of the polymers. The restricted reorientation of the embedded water, the local torsions of the backbone, and the rearrangement of the whole structure contribute to the slow spectral diffusion. By performing MD simulations, we calculated the FFTCFs and dynamical quantities, such as fluctuations of the dihedral angles of the backbone and the orientation of the hydration water molecules. The simulated FFTCFs match well with the experimental results, indicating that the retarded water reorientations via the excluded volume effect play an important role in the vibrational frequency fluctuations of the carbonyl stretching mode. It is also found the embedded water molecules are influenced by the local torsions of the backbone structure within the time scales of the spectral diffusion. [ABSTRACT FROM AUTHOR]

An area-selectable Mg doping via ion implantation (I/I) is essential to realize gallium nitride (GaN) based power switching devices. Conventional post-implantation annealing forms considerable defects in the GaN, resulting in extremely low activation efficiency. The recent invention of ultrahigh-pressure annealing (UHPA) has substantially improved the p-type activation efficiency; however, the UHPA causes an unexpected Mg diffusion. Thus, both annealing processes resulted in a much lower Mg concentration in the GaN matrix than the Mg dose. In this study, the effect of a sequential N I/I for p-type Mg-implanted GaN was investigated by the correlative cathodoluminescence, transfer length method, scanning transmission electron microscopy, and atom probe tomography (APT) analyses. APT results have revealed that the sequential N I/I can successfully maintain the Mg concentration in the GaN matrix in the higher range of 1018 cm−3 or more. Our investigation suggests that sequential N I/I is a promising technique to maintain the Mg concentration higher and improve the p-type activation efficiency. [ABSTRACT FROM AUTHOR]

Illumination stress (IS) and negative bias under illumination stress (NBIS) cause considerable device instability in thin-film transistors based on amorphous In–Ga–Zn–O (a-IGZO). Models using in-gap states are suggested to explain device instability. Therefore, to provide reliably their density of states (DOS), this study investigated the valence band, conduction band, and in-gap states of an a-IGZO thin film. The DOS of in-gap states was directly determined in a dynamic range of six orders of magnitude through constant final state yield spectroscopy (CFS-YS) using low-energy and low-flux photons. Furthermore, light irradiation irreversibly induced extra in-gap states near the Fermi level and shifted the Fermi level to the vacuum level side, which should be related to the device instability due to IS and NBIS. Hard x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy using synchrotron radiation observed the large DOS of in-gap states near the Fermi level as in previous works. Here, we reveal that they are not intrinsic electronic states of undamaged a-IGZO, but induced by the intense measurement light of synchrotron radiation. This study demonstrates that CFS-YS is useful for determining the reliable DOS of the in-gap states for samples that are sensitive to light irradiation. The absorption spectrum measured through photothermal deflection spectroscopy is interpreted based on DOS directly determined via photoemission spectroscopies. This indicates that the line shape in the energy region below the region assigned to the Urbach tail in previous works actually roughly reflects the DOS of occupied in-gap states. [ABSTRACT FROM AUTHOR]

To efficiently generate H − ions from positive atomic or molecular hydrogen beams injected onto a solid surface, it has been suggested to use a material with a low-work function as the target material. However, it is not clear under what conditions the most efficient H − production is realized for incident beam parameters or reflection angles. Therefore, we studied the interaction between low-energy atomic and molecular hydrogen beams (less than 1 keV/nucleon) with a low-work function electride 12CaO ⋅ 7 A l 2 O 3 (C12A7). The production ratio of H − to H + ions from the C12A7 electride was much higher than Mo targets with higher work functions, especially at smaller incident and smaller reflection angles. The H − to H + production ratio slightly increased as the incident energies were decreased, but there was no significant difference between the electride and Mo targets. These results indicate that smaller incident angles and lower beam energies of the incident hydrogen beam are favorable for the enhancement of the production ratio of H − to H + ions in C12A7. The higher production ratio appeared at the vertical beam energies less than on the order of 100 eV, where quantum mechanical processes may become important. [ABSTRACT FROM AUTHOR]

The molecular dynamics of regioregulated poly(3-hexylthiophene) P3HT is investigated using muon spin relaxation (μ SR). The response of the μ SR spectra to a longitudinal magnetic field (B LF , parallel to the initial muon spin direction) indicates that the implanted muons form both muonated radicals localized on the thiophene ring and diamagnetic states with comparable yields. Moreover, the unpaired electron in the radical undergoes hyperfine interactions with muon bound to thiophene and with neighboring protons, whose fluctuations can serve as a measure of the molecular dynamics. The B LF dependence of the longitudinal muon spin relaxation rate (1 / T 1 μ ) measured in detail at several temperatures is found to be well reproduced by the spectral density function J (ω) derived from the local susceptibility that incorporates the Havriliak–Negami (H–N) function used in the analysis of dielectric relaxation, χ (ω) ∝ 1 / [ 1 − i (ω / ν ~) δ ] γ (where ν ~ is the mean fluctuation rate and 0 < γ , δ ≤ 1). The magnitude of ν ~ and its temperature dependence deduced from the analysis of 1 / T 1 μ are found to be consistent with the motion of hexyl chains and thiophene rings suggested by 13 C-NMR. The present result marks a methodological milestone in the application of μ SR to the dynamics of complex systems with coexisting fluctuations over a wide range of time scales, such as polymers. [ABSTRACT FROM AUTHOR]

We discuss the structure of threading dislocations in α-Ga2O3 thin films grown on c- and m-plane sapphire substrates. The thickness-dependent threading dislocation density in both films directly affects the electrical properties of the films including carrier concentration and mobility. Two distinct types of threading dislocations are identified for each of the c- and m-plane α-Ga2O3 thin films. The c-plane α-Ga2O3 thin film shows Burgers vectors of 1 / 3 [ 1 1 ¯ 01 ] and 1 / 3 [ 11 2 ¯ 0 ] , while the m-plane α-Ga2O3 thin film displays Burgers vectors of 1 / 3 [ 2 1 ¯ 1 ¯ 0 ] and 1 / 3 [ 1 1 ¯ 01 ]. This paper presents a detailed structure of the threading dislocations in α-Ga2O3, which has been little disclosed thus far mainly due to the difficulty in synthesizing the metastable α-Ga2O3. [ABSTRACT FROM AUTHOR]

Fourier-transform microwave spectroscopy has been applied for the 13C/18O-substituted tropolone to observe tunneling-rotation transitions as well as pure rotational transitions. The tunneling-rotation transitions were observed between the 13C-4 and -6 forms as well as between 13C-3 and -7, between 13C-1 and -2, and between 18O-8 and -9 (we denote these tunneling pairs as 13C-46, etc., below) although they have an asymmetric tunneling potential due to the difference in the zero point energy (ZPE). From the observed tunneling splittings ΔEij (0.9800–1.6824 cm−1), the differences in ZPE Δij for the 13C-46, -37, -12, and 18O-89 species are derived to be −0.1104, 0.5652, −1.3682, and 1.3897 cm−1 to agree well with the DFT calculation. The state mixing ratio of the tunneling states decreases drastically from (44%:56%) to (8.7%:91.3%) for 13C-46 and 18O-89 with an increase in the asymmetry Δij of the tunneling potential function. The observed tunneling-rotation interaction constants Fij decrease from 16.001 to 9.224 cm−1 as the differences in ZPE Δij increase, while the diagonal tunneling-rotation interaction constants Fu increase from 1.767 to 13.70 cm−1, explained well by the mixing ratio of the tunneling states. [ABSTRACT FROM AUTHOR]

We observed a modification of transition pathways in polarized resonance Raman spectroscopy during tip-enhanced Raman spectroscopy (TERS) analysis of metallic carbon nanotubes (CNTs). At a spatial resolution reaching up to the sub-nanometer regime, the signal intensity of the typical D-band is observed to be even higher than the intensity of the G-band all over the probed CNTs in TERS imaging. The measured D-band is attributed to the non-vertical transitions of electrons in k-space that are facilitated by highly confined near-field light at the tip–sample junction of our scanning tunneling microscope based TERS system. The D-band signal was observed even when the CNTs were excited by light polarized perpendicular to the tube axis that corresponds to electronic excitations between different cutting line numbers of a CNT. By combining the electron pathways brought about by both the near-field light and its polarization, we found a unique optical transition of electrons of CNTs in near-field Raman spectroscopy. [ABSTRACT FROM AUTHOR]

Thermodynamic and dynamic anomalies of water play a crucial role in supporting life on our planet. The two-state theory attributes these anomalies to a dynamic equilibrium between locally favored tetrahedral structures (LFTSs) and disordered normal liquid structures. This theory provides a straightforward, phenomenological explanation for water's unique thermodynamic and dynamic characteristics. To validate this two-state feature, it is critical to unequivocally identify these structural motifs in a dynamically fluctuating disordered liquid. In this study, we employ a recently introduced structural parameter (θavg) that characterizes the local angular order within the first coordination shell to identify these LFTSs through molecular dynamics simulations. We employ both realistic water models with a liquid–liquid critical point (LLCP) and a coarse-grained water model without an LLCP to study water's anomalies in low-pressure regions below 2 kbar. The two-state theory consistently describes water's thermodynamic anomalies in these models, both with and without an LLCP. This suggests that the anomalies predominantly result from the two-state features rather than criticality, particularly within experimentally accessible temperature–pressure regions. [ABSTRACT FROM AUTHOR]

We performed Ge doping of α-Ga2O3 thin films grown on m-plane sapphire substrates using mist chemical vapor deposition. Although the typical growth rate was high at 4 μm/h, the resultant α-Ga2O3 thin films exhibited high crystallinity. We controlled the carrier density in the range of 8.2 × 1016–1.6 × 1019 cm−3 using bis[2-carboxyethylgermanium(IV)]sesquioxide as the Ge source. The highest mobility achieved was 66 cm2 V−1 s−1 at a carrier concentration of 6.3 × 1017 cm−3. Through secondary ion mass spectrometry analysis, a linear relationship between the Ge concentration in the α-Ga2O3 thin films and the molar ratio of Ge to Ga in the source solution was established. The quasi-vertical Schottky barrier diode fabricated using the Ge-doped α-Ga2O3 thin films exhibited an on-resistance of 7.6 mΩ cm2 and a rectification ratio of 1010. These results highlight the good performance of the fabricated device and the significant potential of Ge-doped α-Ga2O3 for power-device applications. [ABSTRACT FROM AUTHOR]

A frequency-dependent dielectric constant characterizes the dielectric response of a medium and also represents the time scale of system's collective dynamics. Although it is valuable not only academically but also practically for developing advanced devices, getting the value of a solution at the interface with a solid or electrode surface is challenging both experimentally and computationally. Here, we propose a computational method that imitates the dielectric spectroscopy and AC impedance measurement. It combines a time-dependent voltage applied molecular dynamics simulation with an equivalent circuit representation of a system composed of a solution confined between two identical electrodes. It gives the frequency-dependent dielectric constants of the bulk solution and the interface simultaneously. Unlike the conventional method, it does not require computation of a dipole autocorrelation function and its Fourier transformation. Application of the method on a system of water confined between polarizable Pt electrodes gives the static dielectric constant and the relaxation time of the bulk water in good agreement with previous simulation results and experimental values. In addition, it gives a much smaller static dielectric constant at the interface, consistent with previous observations. The outline of the dielectric dispersion curve of the interface seems similar to that of the bulk, but the relaxation time is several times faster. [ABSTRACT FROM AUTHOR]

We conducted spin Hall magnetoresistance (SMR) measurements to investigate the Ru-doping effect on the Morin transition of α-Fe2O3(0001) films, which is the transition from the low-temperature antiferromagnetic state with c-axis magnetic moments to the high-temperature weak ferromagnetic state with c-plane magnetic moments, under a thickness of 6–100 nm. We clarified that the Morin temperature of the 2.6 at. % Ru-doped α-Fe2O3 film was greater than 400 K when the thickness was 100 nm and decreased with a decrease in thickness. Our results demonstrated that SMR measurements are a valid verification method for the Morin transition of very thin α-Fe2O3 films down to several nm. [ABSTRACT FROM AUTHOR]

This article explores the concept of nucleation, which is the formation of small nuclei that eventually lead to the creation of a new phase. Nucleation is involved in various processes such as ice formation, drug crystallization, and the setting of concrete. The classical theory of nucleation, developed by Gibbs, is widely used but has limitations. The article emphasizes the need for new theories and computational methods to accurately describe nucleation mechanisms, considering factors like size, non-classical pathways, and stochastic effects. The collection of articles in this issue provides an overview of nucleation in different scientific fields, including ice formation, biomedical applications, and functional materials. Recent research in this area has utilized experimental techniques, simulations, and theoretical models to investigate factors influencing nucleation, such as surface properties and composition. Understanding nucleation mechanisms is crucial for applications in materials science, nanotechnology, and environmental science. The article highlights the progress made in recent years and the potential for further advancements in our understanding of nucleation processes. [Extracted from the article]

Ice is different from ordinary crystals because it contains randomness, which means that statistical treatment based on ensemble averaging is essential. Ice structures are constrained by topological rules known as the ice rules, which give them unique anomalous properties. These properties become more apparent when the system size is large. For this reason, there is a need to produce a large number of sufficiently large crystals that are homogeneously random and satisfy the ice rules. We have developed an algorithm to quickly generate ice structures containing ions and defects. This algorithm is provided as an independent software module that can be incorporated into crystal structure generation software. By doing so, it becomes possible to simulate ice crystals on a previously impossible scale. [ABSTRACT FROM AUTHOR]

Alkali metal dosing has nowadays been extensively used in angle-resolved photoemission spectroscopy (ARPES) for the in situ surface electron doping of materials to provide access to the unoccupied states. This technique also gives rise to nontrivial physical phenomena, such as the appearance of quantum well states and effects due to alkali metal intercalation. Here, we uncovered a previously unobserved type of electronic behavior induced by alkali metal dosing. By employing ARPES to study the evolution of the electronic structure of the Ti2O3 thin film upon rubidium (Rb) dosing, we found that the electron chemical potential of the system remains unchanged throughout the process. Interestingly, a series of electron-like band dispersions first appear with Rb dosing. A further increase in the Rb dosage leads to the eventual disappearance of the electron-like bands and the emergence of a set of hole-like bands. Our finding enriches the phenomenology brought about by alkali metal surface dosing, suggesting a novel functionality of this popular surface doping technique. [ABSTRACT FROM AUTHOR]

Hole mobility anisotropy in 4H-SiC was investigated based on both experimental and theoretical approaches. First, the authors established a complete database of the anisotropic hole mobility along both directions parallel and perpendicular to the c -axis in 4H-SiC over the wide acceptor density and temperature ranges by preparing Hall bar structures on p-type SiC(11 2 ¯ 0) epitaxial layers. Empirical equations for the mobility along each direction vs the acceptor density and temperature were determined, which should be useful for the simulation and designing of any SiC devices. In addition to that, the anisotropy in the hole mobility was extracted from the experimental results, and its origin was discussed focusing on that in the effective mass (m ∗) of holes. The obtained mobility ratio was far from the m ∗ ratio at the valence band maximum, and an averaged m ∗ along each direction was determined by theoretical calculation taking into account the energy distribution of holes. Consequently, the authors revealed that the anisotropic hole mobility is explained quantitatively by the anisotropic m ∗ considering the E – k dispersion over the entire first Brillouin zone. [ABSTRACT FROM AUTHOR]

Poly(ethylene oxide) (PEO) is a well-known biocompatible polymer and has widely been used for medical applications. Recently, we have investigated the dynamic behavior of hydration water in the vicinity of PEO chains at physiological temperature and shown the presence of slow water with diffusion coefficient one order of magnitude less than that of bulk water. This could be evidence for the intermediate water that is critical for biocompatibility; however, its detailed dynamical features were not established. In this article, we analyze the quasi-elastic neutron scattering from hydration water through mode distribution analysis and present a microscopic picture of hydration water as well as its relation to cold crystallization. [ABSTRACT FROM AUTHOR]

We propose a statistical mechanical theory for the thermodynamic stability of clathrate hydrates, considering the influence of the guest–guest interaction on the occupancies of the cages. A mean field approximation is developed to examine the magnitude of the influence. Our new method works remarkably well, which is manifested by two sorts of grandcanonical Monte Carlo (GCMC) simulations. One is full GCMC, and the other is designed in the present study for clathrate hydrates, called lattice-GCMC, in which each guest can be adsorbed at one of the centers of the cage. In the latter simulation, only the guest–guest interaction is explicitly treated, incorporating the host–guest interaction into the free energy of the cage occupation without other guests. Critical phenomena for guest species, such as large density fluctuations, are observed when the temperature is low or the guest–guest interaction is strong. [ABSTRACT FROM AUTHOR]

Uncovering the mystery of efficient and directional energy transfer in photosynthetic organisms remains a critical challenge in quantum biology. Recent experimental evidence and quantum theory developments indicate the significance of quantum features of molecular vibrations in assisting photosynthetic energy transfer, which provides the possibility of manipulating the process by controlling molecular vibrations. Here, we propose and theoretically demonstrate efficient manipulation of photosynthetic energy transfer by using vibrational strong coupling between the vibrational state of a Fenna–Matthews–Olson (FMO) complex and the vacuum state of an optical cavity. Specifically, based on a full-quantum analytical model to describe the strong coupling effect between the optical cavity and molecular vibration, we realize efficient manipulation of energy transfer efficiency (from 58% to 92%) and energy transfer time (from 20 to 500 ps) in one branch of FMO complex by actively controlling the coupling strength and the quality factor of the optical cavity under both near-resonant and off-resonant conditions, respectively. Our work provides a practical scenario to manipulate photosynthetic energy transfer by externally interfering molecular vibrations via an optical cavity and a comprehensible conceptual framework for researching other similar systems. [ABSTRACT FROM AUTHOR]

To spectroscopically qualify strength in the π-electron conjugation, the electronic spectra of jet-cooled 1,4-bis(phenylethynyl)benzene (BPEB) in the region of the transition to the lowest excited singlet (S1) 1B1u state are measured by the fluorescence excitation and the single-vibronic-level dispersed fluorescence methods. Strength is defined as the difference in potential energies between the planar and perpendicular conformations. BPEB possesses two large-amplitude torsional motions, out-of-phase 24 and in-phase 29 modes. The most stable is the planar conformation, and barrier heights at the perpendicular conformation are coincident in torsional potentials for the two modes. Torsional levels are successively observed up to 19± and 16− quantum levels in the ground state, respectively. Strength is determined to be 293 cm−1 (3.51 kJmol−1) with an accuracy of an error range smaller than 1 cm−1. In the excited state, strength is estimated to be 1549 ± 73 cm−1. Combination levels of two torsional modes are also measured up to high quantum levels. A systematic decrease in frequencies is observed with increasing the quantum number. Quantum-chemistry calculations of B3LYP, CAM-B3PLYP, WB97XD, and M062X with basis sets of aug-cc-pVDZ are performed, where B3LYP theories are carried out with the dispersion correlation. The calculated strength is 1.1–2.1 times larger than observed. [ABSTRACT FROM AUTHOR]

Polymer chains at a buried interface with an inorganic solid play a critical role in the performance of polymer nanocomposites and adhesives. Sum frequency generation (SFG) vibrational spectroscopy with a sub-nanometer depth resolution provides valuable information regarding the orientation angle of functional groups at interfaces. However, in the case of conventional SFG, since the signal intensity is proportional to the square of the second-order nonlinear optical susceptibility and thereby loses phase information, it cannot be unambiguously determined whether the functional groups face upward or downward. This problem can be solved by phase-sensitive SFG (ps-SFG). We here applied ps-SFG to poly(methyl methacrylate) (PMMA) chains in direct contact with a quartz surface, shedding light on the local conformation of chains adsorbed onto the solid surface. The measurements made it possible to determine the absolute orientation of the ester methyl groups of PMMA, which were oriented toward the quartz interface. Combining ps-SFG with all-atomistic molecular dynamics simulation, the distribution of the local conformation and the driving force are also discussed. [ABSTRACT FROM AUTHOR]

We study epitaxial growth, crystal structures, and magnetic properties of Fe–As compound thin films grown on GaAs (111)B substrates at various values of the As4:Fe flux ratio γ, using molecular beam epitaxy. The samples grown at low As4 flux (γ = 0.3, sample A) show mainly a body-centered-cubic (bcc) crystal structure, exhibiting ferromagnetic properties similar to bcc Fe. Meanwhile, the Fe–As samples grown at medium γ (2.7–4.5, sample group B) comprise regions of Ni2In-type FeAs (a hexagonal crystal with lattice constants of a = 0.399 nm and c = 0.536 nm), which are grown at the bottom and interface with the GaAs buffer layer, and a layer of non-stoichiometric FeAs with a DO3 structure (a = 0.522 nm) formed on the top. The DO3-structure FeAs phase contains partially transformed regions, which are characterized by thin stripes in a scanning transmission electron microscopy image. Furthermore, in the sample grown with high γ = 8.5 (sample C), a hexagonal Fe–As crystal with a large in-plane lattice constant (a = 0.691 nm and c = 0.542 nm) and threefold screw axes are observed. None of these crystal structures of Fe–As compounds has ever been reported. While sample C shows no ferromagnetism, the samples in group B exhibit strong ferromagnetism with high Curie temperature TC above 400 K. These new ferromagnetic Fe–As compounds are promising for spintronic device applications. [ABSTRACT FROM AUTHOR]

The intrinsic carrier density of 4H-SiC at temperatures ranging from 294 to 595 K was derived by analyzing a collector current in an npn-type SiC bipolar junction transistor, the structure of which was designed based on a device simulation. The obtained intrinsic carrier density was in good agreement with the value calculated from the bandgap and effective densities of states taking multiple and non-parabolic SiC bands into account. The coincidence of the intrinsic carrier density obtained by these two different approaches indicates the usefulness of the proposed method and the validity of the evaluated value of intrinsic carrier density. The temperature dependence of the bandgap was also estimated from the deduced intrinsic carrier density and compared with an empirical formula. The derived bandgap agreed well with the empirical formula showing bandgap shrinkage at high temperatures. The errors in evaluating the intrinsic carrier density and the bandgap caused by the estimation of the hole density and electron mobility in the base layer are also discussed for the proposed method. [ABSTRACT FROM AUTHOR]

Many experimental and theoretical studies on CH4–CO2 hydrates have been performed aiming at the extraction of CH4 as a relatively clean energy resource and concurrent sequestration of CO2. However, vague or insufficient characterization of the environmental conditions prevents us from a comprehensive understanding of even equilibrium properties of CH4–CO2 hydrates for this substitution. We propose possible reaction schemes for the substitution, paying special attention to the coexisting phases, the aqueous and/or the fluid, where CO2 is supplied from and CH4 is transferred to. We address the two schemes for the substitution operating in three-phase and two-phase coexistence. Advantages and efficiencies of extracting CH4 in the individual scheme are estimated from the chemical potentials of all the components in all the phases involved in the substitution on the basis of a statistical mechanical theory developed recently. It is found that although substitution is feasible in the three-phase coexistence, its working window in temperature–pressure space is much narrower compared to the two-phase coexistence condition. Despite that the substitution normally generates only a small amount of heat, a large endothermic substitution is suggested in the medium pressure range, caused by the vaporization of liquid CO2 due to mixing with a small amount of the released CH4. This study provides the first theoretical framework toward the practical use of hydrates replacing CH4 with CO2 and serves as a basis for quantitative planning. [ABSTRACT FROM AUTHOR]

Phosphorus (P) doping in ZnTe grown by molecular beam epitaxy (MBE) under alternating source supply method was investigated to achieve p-type P-doped ZnTe (ZnTe:P) thin films using InP as a P dopant source, and the result was compared with those grown under a simultaneous MBE growth where the source beams were supplied simultaneously. As a result, P concentration in ZnTe thin films was found to increase with increasing the InP flux, and high P concentration up to 6.6 × 1019 cm−3 was confirmed by secondary ion mass spectroscopy (SIMS) analyses. However, In incorporation was also observed in the ZnTe:P thin films, despite that the detected In concentration by SIMS was more than one order of magnitude lower than the P concentration and almost two order of magnitude lower than those grown by the simultaneous MBE. Photoluminescence measurement of ZnTe:P thin film grown under alternating source supply showed a P-related acceptor bound exciton (Ia) peak at 2.37 eV, and the intensity of Ia emission increased after the annealing treatment, indicating the activation of P acceptor. The annealing also decreases the resistivity of the film. The results clearly indicate that the alternating source supply growth is effective to obtain ZnTe:P thin films with better P doping properties. [ABSTRACT FROM AUTHOR]

We present a strategic approach for enhancing the ionic conductivity of block copolymer electrolytes. This was achieved by introducing mixed ionic liquids (ILs) with varying molar ratios, wherein the imidazolium cation was paired with either tetrafluoroborate (BF4) anion or bis(trifluoromethylsulfonyl)imide (TFSI) anion. Two polymer matrices, poly(4-styrenesulfonate)-b-polymethylbutylene (SSMB) and poly(4-styrenesulfonyl (trifluoromethanesulfonyl)imide)-b-polymethylbutylene (STMB), were synthesized for this purpose. All the SSMB and STMB containing mixed ILs showed hexagonal cylindrical structures, but the type of tethered acid group significantly influenced the interfacial properties. STMB electrolytes demonstrated enhanced segregation strength, which was attributed to strengthened Coulomb and hydrogen bonding interactions in the ionic domains, where the ILs were uniformly distributed. In contrast, the SSMB electrolytes exhibited increased concentration fluctuations because the BF4 anions were selectively sequestered at the block interfaces. This resulted in the effective confinement of imidazolium TFSI along the ionic domains, thereby preventing ion trapping in dead zones and facilitating rapid ion diffusion. Consequently, the SSMB electrolytes with mixed ILs demonstrated significantly improved ionic conductivities, surpassing the expected values based on the arithmetic average of the conductivities of each IL, whereas the ionic conductivity of the STMB was aligned with the expected average. The methodology explored in this study holds great promise for the development of solid-state polymer electrolytes. [ABSTRACT FROM AUTHOR]

Rutile-type wide and ultrawide band-gap oxide semiconductors are emerging materials for high-power electronics and deep ultraviolet optoelectronics applications. A rutile-type GeO2-SnO2 alloy (r-GexSn1–xO2) recently found is one of such materials. Herein, we report low-temperature electron transport properties of r-GexSn1−xO2 thin films with x = 0.28 and 0.41. Based on resistivity and magnetoresistance measurements, along with the theory of quantum interference, it is suggested that Efros–Shklovskii variable-range hopping, i.e., hopping over the states within the Coulomb gap, is dominant at lower temperatures (T ≤ 10 and 15 K) in both r-Ge0.41Sn0.59O2 and r-Ge0.28Sn0.72O2. The negative and positive magnetoresistances observed at low temperatures are attributable to the quantum interference and field-induced spin alignment, respectively. The magnetoresistance measurements at higher temperatures suggest that both Mott variable–range hopping and thermally activated band conduction occur at T < 100 K and that almost pure thermally activated band conduction takes place at T ≥ 150 K. [ABSTRACT FROM AUTHOR]

During devitrification, pre-existing crystallites grow by adding particles to their surface via a process that is either thermally activated (diffusive mode) or happens without kinetic barriers (fast crystal growth mode). It is yet unclear what factors determine the crystal growth mode and how to predict it. With simulations of repulsive hard-sphere-like (Weeks–Chandler–Andersen) glasses, we show for the first time that the same system at the same volume fraction and temperature can devitrify via both modes depending on the preparation protocol of the glass. We prepare two types of glass: conventional glass (CG) via fast quenching and uniform glass (UG) via density homogenization. First, we bring either glass into contact with a crystal (X) and find the inherent structure (CGX/UGX). During energy minimization, the crystal front grows deep into the CG interface, while the growth is minimal for UG. When thermal noise is added, this behavior is reflected in different crystallization dynamics. CGX exhibits a density drop at the crystal growth front, which correlates with enhanced dynamics at the interface and a fast growth mode. This mechanism may explain the faster crystal growth observed below the glass transition experimentally. In contrast, UGX grows via intermittent avalanche-like dynamics localized at the interface, a combination of localized mechanical defects and the exceptional mechanical stability imposed by the UG glass phase. [ABSTRACT FROM AUTHOR]

Large bulk single crystals of InGaZn3O6 (InGaO3(ZnO)3 or IGZO-13) have been successfully grown using an optical floating zone method under a flow of dry-air at high pressure. These crystals exhibit a distinct blue hue in their as-grown state, with notable enhancement in transparency attainable through O2 annealing. Comparative analysis with IGZO-11 reveals a substantial reduction in conductivity following O2 annealing. Moreover, a pronounced in-plane/out-of-plane conductive anisotropy ranging from 100 to 450 is observed, surpassing that of IGZO-11 under similar in-plane conductivity conditions. This increase in electrical conducting anisotropy is ascribed to the incorporation of ZnO blocks amid the two InO layers, aligned along the c axis. Based on these findings, it is concluded that the contribution of In-5s orbitals likely assumes a predominant role, surpassing that of other cations from an experimental standpoint. [ABSTRACT FROM AUTHOR]

We develop a statistical mechanical theory on clathrate hydrates in order to explore the phase behaviors of clathrate hydrates containing two kinds of guest species and apply it to CH4–CO2 binary hydrates. The two boundaries separating water and hydrate and hydrate and guest fluid mixtures are estimated, which are extended to the lower temperature and the higher pressure region far distant from the three-phase coexisting conditions. The chemical potentials of individual guest components can be calculated from free energies of cage occupations, which are available from intermolecular interactions between host water and guest molecules. This allows us to derive all thermodynamic properties pertinent to the phase behaviors in the whole space of thermodynamic variables of temperature, pressure, and guest compositions. It is found that the phase boundaries of CH4–CO2 binary hydrates with water and with fluid mixtures locate between simple CH4 and CO2 hydrates, but the composition ratios of CH4 guests in hydrates are disproportional to those in fluid mixtures. Such differences arise from the affinities of each guest species to the large and small cages of CS-I hydrates and significantly affect occupation of each cage type, which results in a deviation of the guest composition in hydrates from that in fluid on the two-phase equilibrium conditions. The present method provides a basis for the evaluation of the efficiency of the guest CH4 replacement to CO2 at the thermodynamic limit. [ABSTRACT FROM AUTHOR]

Nanostructure control is of fundamental and technological importance because various properties such as vortex pinning in superconductors are determined by the nanostructure. While the nanostructure in superconducting oxides is well controlled, the nanostructure control of iron-based superconductors is required. The nanostructure has not yet been well controlled even in Fe(Te,Se) with the simplest structure among Fe-based superconductors because the anion may be non-stoichiometric and anionic substitution is possible. In this study, we observed the compositional inhomogeneity originating from phase separation in Fe(Te,Se) films, which were prepared by pulsed laser deposition. The films deposited at lower temperature do not show nanoscale compositional inhomogeneity. On the other hand, the high-temperature deposited films contain an inhomogeneous anion distribution with the size of ∼8 nm due to phase separation. The spacing of the compositional inhomogeneity is ∼16 nm. This structure obtained by phase separation is expected to be an effective pinning center in high magnetic fields after further optimization. Thus, the method of nanostructure control other than nanocomposite formation in oxides is demonstrated for Fe(Te,Se). [ABSTRACT FROM AUTHOR]

We investigate quasi-two-dimensional buckled colloidal monolayers on a triangular lattice with tunable depletion interactions. Without depletion attraction, the experimental system provides a colloidal analog of the well-known geometrically frustrated Ising antiferromagnet [Y. Han et al., Nature 456, 898–903 (2008)]. In this contribution, we show that the added depletion attraction can influence both the magnitude and sign of an Ising spin coupling constant. As a result, the nearest-neighbor Ising "spin" interactions can be made to vary from antiferromagnetic to para- and ferromagnetic. Using a simple theory, we compute an effective Ising nearest-neighbor coupling constant, and we show how competition between entropic effects permits for the modification of the coupling constant. We then experimentally demonstrate depletion-induced modification of the coupling constant, including its sign, and other behaviors. Depletion interactions are induced by rod-like surfactant micelles that change length with temperature and thus offer means for tuning the depletion attraction in situ. Buckled colloidal suspensions exhibit a crossover from an Ising antiferromagnetic to paramagnetic phase as a function of increasing depletion attraction. Additional dynamical experiments reveal structural arrest in various regimes of the coupling-constant, driven by different mechanisms. In total, this work introduces novel colloidal matter with "magnetic" features and complex dynamics rarely observed in traditional spin systems. [ABSTRACT FROM AUTHOR]

We have investigated the impact of high-temperature Mg-implantation in GaN layers on distribution of Mg-enriched defects using scanning transmission electron microscopy and atom probe tomography. For this, 1 × 1019 cm−3 Mg ions have been implanted in GaN layers at room temperature (RT) and 1000 °C, followed by annealing at 1300 °C. A smaller number of Mg-enriched defects were observed in the sample implanted at 1000 °C in comparison to the sample implanted at RT. The implantation of Mg ions at 1000 °C resulted in a higher amount of randomly distributed Mg in the GaN matrix, which, in turn, leads to more uniform and enhanced donor–acceptor pair emission, leading to higher Mg activation. [ABSTRACT FROM AUTHOR]

In this study, the transmission frequency dependence of the random waveguide, which is the waveguide having a randomly corrugated boundary, on waveguide length is investigated using an accurate numerical method. While resonances in the low-frequency region are caused by the interference among the dominant modes, those in the high-frequency region are caused by the mode conversion from the dominant mode to the second mode. Furthermore, their characteristics have not been investigated in detail. Numerical analysis shows that the resonant frequencies in the high-frequency region are nearly independent of the length of the random waveguide within the numerical error. It indicates that random waveguides can be regarded as a stack of independent resonators in high-frequency regions. [ABSTRACT FROM AUTHOR]

Because the work function (WF) of the thermionic cathode of an electron gun is the primary factor for determining emission current density, the precise monitoring of the WF in actual operational conditions under high temperatures is essential for tracking the status of the cathode surface. We adjusted a theoretical model of quantum efficiency under high temperatures as proposed by R. H. Fowler to include the final energy state in the photoexcitation process. This modification is necessary for estimating the WF precisely with the measurement results obtained by photoelectron yield spectroscopy using a tunable pulsed laser in the wavelength range from 410 to 709 nm. As the first step of the investigation, we conducted WF measurements of a single-crystal CeB6 cathode, which is employed for the SACLA x-ray free-electron laser. The WF of the unused fresh CeB6 was 2.44 ± 0.02 eV at a high temperature of 836 °C. Our future plans for WF measurements are also discussed below. [ABSTRACT FROM AUTHOR]

The electrical properties of semi-insulating GaN substrates doped with iron (Fe), carbon (C), or manganese (Mn) grown by hydride vapor phase epitaxy are presented. Hall effect measurements were performed at temperatures ranging from 300 to 800 K. At all of the investigated temperatures, the Mn-doped samples exhibited the highest resistivity. The Fe-doped samples showed n-type conduction, whereas the C-doped samples and the Mn-doped sample with a Mn concentration of 1 × 1019 cm−3 showed p-type conduction. A detailed analysis of the temperature dependence of the carrier concentration showed that all of the impurities formed acceptor levels at EC −(0.59–0.61) eV for Fe, at EV +(0.90–1.07) eV for C, and at EV +1.55 eV for Mn. The Mn-doped sample with a Mn concentration of 8 × 1017 cm−3 showed a negative Hall coefficient (suggesting n-type conduction) at high temperatures, contradicting the formation of acceptor levels by Mn. We successfully explained the negative value by considering the conduction of both holes and electrons with different mobilities. On the basis of the results, we calculated the relationship between the resistivity and doping concentration for each dopant. The calculations indicated that the highest resistivity can be realized in Mn-doped GaN with an optimized doping concentration (depending on the residual donor concentration). All of the dopants can effectively realize high resistivity at room temperature. Mn is an effective dopant for attaining high resistivity, especially at high temperatures (e.g., 800 K). [ABSTRACT FROM AUTHOR]

Despite the rapid progresses in the field of organic semiconductors, aided by the development of high-mobility organic materials, their high carrier mobilities are often unipolar, being sufficiently high only for either electrons or holes. Yet, the basic mechanisms underlying such significant mobility asymmetry largely remains elusive. We perform angle-resolved photoelectron spectroscopy to reveal the occupied band structures and the many-body interactions for low-mobility hole carriers in a typical n-type semiconductor perylene diimide derivative. The band dispersion exhibits strong renormalization to the calculated non-interacting electronic structure. The analysis including many-body interactions elucidate that the significant mass enhancement can be understood in terms of strong charge–phonon coupling, leading to an important mechanism of polaron band transport of low intrinsic carrier mobility in organic semiconductors. [ABSTRACT FROM AUTHOR]

Perfectly diffuse sound fields play an important role in architectural acoustics and there are established theoretical characterizations of perfect diffuseness. Although sound fields in real rooms are diffuse to some extent, they are not perfectly diffuse, and therefore theories are required to describe pseudo-perfectly diffuse sound fields. Here, we aim to spatially characterize pseudo-perfect diffuseness via directional characterization of that, finite-degree spherical harmonic diffuseness. Our results show that finite-degree diffuse sound fields yield local spatial diffuseness, suggesting that spatial pseudo-perfect diffuseness is characterized using the effective radius of diffuseness. [ABSTRACT FROM AUTHOR]

We present characterization and storage methods for a high-finesse nanofiber Fabry–Pérot resonator. Reflection spectroscopy from both ends of the resonator allows for the evaluation of the mirror transmittances and optical loss inside the resonator. To maintain the quality of the nanofiber resonator after the fabrication, we have developed a portable storage container. By filling the container with dry, clean nitrogen gas, we can prevent contamination of the nanofiber during storage. This approach allows us to minimize the additional optical loss to less than 0.08% over a week. The portable container facilitates both the fabrication and subsequent experimentation with the resonator in different locations. This flexibility expands the range of applications, including quantum optics, communication, and sensing. [ABSTRACT FROM AUTHOR]

A Compton camera is a gamma-ray imaging device, especially in the sub-mega-electron volt to higher than mega-electron volt range. Compton cameras have recently attracted attention as an environmental survey tool. However, owing to their limited sensitivity, Compton camera images often suffer from various artifacts, especially when the event statistics are low. To address this challenge, several deep learning models have been proposed to enhance the quality of reconstructed images with limited statistics. However, during the event selection phase of a typical Compton camera image reconstruction, a significant number of events that potentially reflect the source distribution are generally discarded. Effective utilization of these discarded events has the potential to estimate an accurate source distribution from limited statistical data. Thus, we initially developed ComptonNet-v1, a framework designed to directly estimate source distribution by integrating all measured events into a single model. To explicitly implement the difference in contribution between events that interact solely with scatterers, solely with absorbers, or with both, we developed ComptonNet-v2, which integrates these events individually. Consequently, our proposed models exhibited superior performance in both quantitative and qualitative assessments compared with existing models, even under low event statistics. In the future, we plan to implement a more memory-efficient model to estimate the distribution of complex source shapes. [ABSTRACT FROM AUTHOR]

We evaluated topological protection for edge and higher-order corner states in topological diffusion systems based on the breathing kagome lattice. In the kagome lattice, the corner states appear at the corner boundary where all three Wannier centers in nontrivial unit cells are located. The three Wannier centers in a unit cell can be placed on the obtuse- and acute-corner boundaries utilizing the armchair boundary, generating topological acute- and obtuse-corner states. For another representative zigzag boundary, only the acute-corner unit cell has three Wannier centers located at the boundary; hence, only the acute-corner state appears. Our band analysis and numerical studies show that the topologically protected decay behavior for armchair boundaries is as robust as that for zigzag boundaries, unlike wave phenomena with space and time periodicities. Our findings can guide the flexible design of topological diffusion applications such as heat localization and recovery systems. [ABSTRACT FROM AUTHOR]

Leveraging the quantization properties inherent in superconductors, we present the development of an intrinsic superconducting phase battery. This advancement is achieved by integrating a ferromagnetic π-phase Josephson junction (π-JJ) within a superconducting ring. The core innovation lies in harnessing the potential energy of the π-JJ to generate an intrinsic circulating current, thereby inducing a phase difference as the current goes through the geometric inductance. This mechanism allows for tuning the phase bias φ (0 < φ < π) through an arrangement of the geometric inductance in the battery. We integrate the intrinsic phase batteries into superconducting quantum interference devices, where we verified the effectiveness of the induced phase bias. The polarity of the phase battery is determined by the direction of the intrinsic circulating current, which can be initialized by an external magnetic field. The design methodology for precise intrinsic phase bias has been established. Our findings not only show the feasibility of generating an intrinsic and adjustable phase bias using established fabrication techniques but also open new avenues for enhancing the design, efficiency, and functionality of superconducting electronics, promising to accelerate advancements in digital and quantum computing technologies. [ABSTRACT FROM AUTHOR]

In recent years, the application of positron emission tomography (PET) for the dose range verification of proton therapy has been proposed. However, the positron distribution is determined by the nuclear reaction cross section; hence, PET may not accurately reflect the dose range primarily influenced by ionization. Consequently, a proton dose range verification system based on scattered proton measurements has been suggested owing to the similarity in the reaction cross section between Rutherford scattering and ionization. While previous investigations have only verified the feasibility of dose range estimation through simple simulations, the objective of this study is to demonstrate this feasibility through experimental investigation. In this paper, we established an experimental framework for capturing scattered protons and introduced an algorithm that compares measured signal patterns with a reference database to estimate the dose range. A therapeutic beam was irradiated onto the abdominal region of a human phantom, and scattered protons were measured using scintillation detectors placed on the phantom surface. Consequently, the dose range was estimated with error margins of 4.22 ± 3.68 and 0.60 ± 1.03 mm along the beam axis and perpendicular directions to the Bragg peak, respectively. While providing the same level of Bragg peak positioning accuracy as conventional methods, our system features small size, cost-effectiveness, and system simplicity. One notable limitation of our method is the challenge in achieving precise detector positioning, which is crucial for accurate dose range estimation. Future research will focus on improving detector-position accuracy and exploring advanced algorithms for signal analysis to further refine dose range estimations. [ABSTRACT FROM AUTHOR]

The perovskite oxides ReNiO3 (Re = rare-earth elements) are promising functional materials due to their strongly correlated electrons. Except for the well-known intrinsic metal-insulating transition in these materials, recent progresses have proved that protonation of ReNiO3 can bring about interesting Mott transition in this series. To date, in these protonated species (H-ReNiO3), huge resistance switching, fast ionic diffusion, and their applications as an iontronic transistor, memristor, and fuel cell are reported. In this work, the thermal conductivities of H-ReNiO3 (Re = La, Nd, Sm, and Eu) epitaxial thin films are investigated. The protonation-induced Mott transition can effectively modulate the electronic thermal conductivity while the lattice thermal conductance is less affected. Hence, at room temperature, the metallic LaNiO3 and NdNiO3 exhibit reversible wide thermal conductivity modulation, in ranges of 2.6–12.0 and 1.6–8.0 W m−1 K−1, respectively. These values are much larger than other thermal regulation materials based on transition metal oxides. Thus, our work reveals the great potential of ReNiO3 being applied as a thermal-regulating material. The fast ionic diffusion in H-ReNiO3 also guarantees that a fast response and wide-range thermal transistor can be realized by H-LaNiO3 and H-NdNiO3 in the future. [ABSTRACT FROM AUTHOR]

Helical dichroism (HD), which is defined as the difference in optical absorption between chiral pairs of lights involving left-handed (LH) and right-handed (RH) optical vortices (OVs) carrying orbital angular momentum (OAM), is a promising way to characterize chiral materials. In the current major methods of OV generation using spatial light modulators (SLMs), the speed of OAM switching is typically as slow as 100 Hz, which is comparable to low-frequency noise, making precise chiral detection difficult. Here, we theoretically propose and experimentally demonstrate a rapid modulation of the LH and RH OVs at around 50 kHz. This modulation is achieved through a rapid modulation of circularly polarized lights carrying spin angular momentum (SAM), combined with a SAM–OAM conversion technique. We establish a theory not only for rapid OV modulation but also for HD measurements using the modulated OVs. We experimentally verify the theory using helical phase holograms drawn on a SLM as a pseudo-HD active sample. Our work addresses the limitations of current methods and offers a new avenue for precise HD measurements, paving the way for the development of sensitive chiral-optical spectroscopy techniques. [ABSTRACT FROM AUTHOR]