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Annealing Mg-implanted homoepitaxial GaN at temperatures above 1400 °C eliminates the formation of inversion domains and leads to improved dopant activation efficiency. Extended defects, in the form of inversion domains, contain electrically inactive Mg after post-implantation annealing at temperatures as high as 1300 °C (one GPa N2 overpressure), which results in a low dopant activation efficiency. Triple-axis x-ray data reveal that implant-induced strain is fully relieved after annealing at 1300 °C for 10 min, indicating that strain-inducing point defects formed during implantation have reconfigured and inversion domains are formed. However, annealing at temperatures of 1400–1500 °C (one GPa N2 overpressure) eliminates the presence of the inversion domains. While residual defects, such as dislocation loops, still exist after annealing at and above 1400 °C, chemical analysis at multiple dislocation loops shows no sign of Mg segregation. Meanwhile, an overall decreasing trend in the dislocation loop density is observed after annealing at the higher temperatures and longer times. Additionally, once inversion domains are formed and the samples are cooled to room temperature, they are shown to dissolve with subsequent annealing above 1400 °C. While such defects have been observed before, the important finding that such defects can be dissolved with a short, higher temperature step is key. Earlier work [Breckenridge et al., J. Appl. Phys. Lett. 118, 022101 (2021)] addressing electrical measurements of these types of samples showed that annealing at 1400 °C leads to a dopant activation efficiency that is an order of magnitude higher than that observed at 1300 °C. This work complements earlier work by identifying the inversion domains, which incorporate Mg, and points to the benefits, in terms of defect density and p-type dopant activation, of using higher temperature (>1400 °C) annealing cycles to activate Mg in GaN, even if the Mg-containing inversion domains had been formed during lower temperature annealing. [ABSTRACT FROM AUTHOR]

X-ray polarization-splitting crystals separate incident x rays into two components with perpendicular polarization by Bragg reflections at 45° from paired sets of internal planes. Here, the polarization-splitting properties of a germanium crystal are verified using incompletely polarized synchrotron radiation. Cleaner data would have come from a beam with a higher degree of polarization, which is achievable with small changes in the experimental geometry. [ABSTRACT FROM AUTHOR]

La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray diffraction and high-resolution electron microscopy reveal that the (110) LCMO films are epitaxial and anisotropically in-plane strained, with higher relaxation along the [1–10] direction than along the [001] direction; x-ray absorption spectroscopy data signaled the existence of a single intermediate Mn3+/4+ 3d-state at the film surface. Their magnetic properties are compared to those of (001) LCMO films grown simultaneously on (001) STO substrates It is found that (110) LCMO films present a higher Curie temperature (TC) and a weaker decay of magnetization when approaching TC than their (001) LCMO counterparts. These improved films have been subsequently covered by nanometric STO layers. Conducting atomic-force experiments have shown that STO layers, as thin as 0.8 nm, grown on top of the (110) LCMO electrode, display good insulating properties. We will show that the electric conductance across (110) STO layers, exponentially depending on the barrier thickness, is tunnel-like. The barrier height in STO (110) is found to be similar to that of STO (001). These results show that the (110) LCMO electrodes can be better electrodes than (001) LCMO for magnetic tunnel junctions, and that (110) STO are suitable insulating barriers. [ABSTRACT FROM AUTHOR]

Studies of vibrational spectra of ice I and amorphous ice in the stretching mode frequency range were extended to include (a) the observed effect of a full range of isotopic dilution on the infrared spectra and (b) computational modeling of the observed influence of each dilution step on the properties of vibrationally excited states and on infrared and Raman spectra. The quantum-mechanical computational scheme included effects of frequency lowering due to hydrogen bonding, and of intra- and intermolecular coupling between bonds. The H2O/D2O mixtures can be viewed as a collection of clusters of one X2O isotopomer embedded in a matrix of the other isotopomer. The properties of the vibrationally excited states and of the spectra are determined by complex interplay between the size distribution of the embedded clusters, and the inter- and intramolecular coupling. Vibrational excitations are delocalized over large portions of the embedded clusters. In the limit of a pure crystalline isotopomer, the excitations are delocalized over the entire system and thus proton disorder alone is insufficient to induce localization. The excitations in pure amorphous ice show more pronounced localization effects at the band edges. Throughout the entire composition range, the vibrations of molecules in the low frequency regime retain symmetric stretch character, and the vibrations in the high frequency regime retain antisymmetric stretch character. The perpendicularly polarized Raman spectrum peaks in the region of the latter states. The parallel-polarized Raman spectrum peaks in the low frequency end of the band where the states are globally symmetric, i.e., the contributions of excitations of all bonds to a state are of the same sign. The infrared spectrum extends over the entire band and follows roughly the density of states. [ABSTRACT FROM AUTHOR]

The use of a new method of preparing micron-thick deposits of nanocrystals of ice for Fourier transform infrared sampling, with the nanocrystals supported on a vertical infrared window, has greatly improved the signal-to-noise levels of the spectra available for large ice clusters. High quality spectra of modes of the surface molecules are reported, even for regions that underlie the intense bands of the bulk ice modes. These experimental features are most clearly displayed through the use of difference spectra. For example, the difference between spectra obtained for nanocrystals, before and after an annealing cycle that significantly increases the average cluster size, reflects the decrease in number of surface groups and the corresponding increase in number of interior molecules. Similarly, differences between spectra of bare and adsorbate-covered nanocrystals, obtained at the same temperature for the same ice sample, show the significant shifts of ‘‘surface-localized’’ ice modes caused by the adsorbate molecules. These difference spectra, and similar spectra for amorphous ice, are rich with information about the (three) distinct types of ice surface water molecules and their interactions with small adsorbate molecules. The extraction of that information has been initiated by comparison of the experimental difference spectra from two sizes of D2O cubic ice nanocrystals with simulated difference spectra for a relaxed cubic ice surface compared to bulk cubic ice.From these comparisons specific experimental features have been assigned to modes of the three categories of surface D2O(HDO) molecules: (a) three-coordinated molecules with dangling-D—2725 (2713) cm-1; (b) three-coordinated D2O molecules with dangling-O—2645 (∼2600) cm-1; (c) relaxed four-coordinated molecules—∼2580 (∼2550) cm-1. Also, information has been obtained on the approximate positions (cm-1) of other modes of surface... [ABSTRACT FROM AUTHOR]

We report on the study of the structural, magnetic, and electronic properties of SrTiO3 capped La2/3Ca1/3MnO3 electrodes grown on (001) and (110) SrTiO3 substrates. Magnetic properties of the (001) and (110) capped electrodes evolve differently when the capping layer thickness increases, revealing a reduction of the saturation magnetization for the (001) ones. Electronic properties are studied combining 55Mn nuclear magnetic resonance (NMR) and x-ray photoemission spectroscopy (XPS). NMR experiments highlight that electronic phase separation in the (001) electrodes is enhanced by the presence of the SrTiO3 capping layer and XPS measurements show that the electronic state of interfacial Mn ions from (001) electrode is more sensitive to the capping layer. [ABSTRACT FROM AUTHOR]

We report here on the structural and magnetic characterization of La2/3Ca1/3MnO3 epitaxial films on (001) and (110) SrTiO3 substrates. The (001) films display depressed magnetic properties (lower Curie temperature and magnetization) with respect to the bulk counterpart values. In contrast, the (110) films invariably display optimal magnetic properties. 55Mn nuclear magnetic resonance experiments show that, in contrast to (001) films, the (110) films do not show traces of electronic phase separation and thus provide evidence that no weakening of double exchange coupling at interfaces takes place in (110) films. [ABSTRACT FROM AUTHOR]

By using magnetotransport measurements as well as nuclear magnetic resonance as local magnetic probe, we conclusively show that, irrespective of the film/substrate structural mismatch and film thickness, there are nonconductive regions at the film/substrate interface. We thus discard dimensional effects and structural mismatch as the main reasons for the suppression of conductivity and the weakening of the ferromagnetic coupling in nanometric thin films of manganites. We shall argue that this behavior is related to a charge localization which also leads to a weakening of the magnetic interactions and a decrease of the Curie temperature. [ABSTRACT FROM AUTHOR]

In this work, we report on the impact of distinct growth parameters that affect the roughness and surface morphology of La[sub 2/3]Ca[sub 1/3]MnO[sub 3] epitaxial thin films grown by rf sputtering, namely, the film thickness and the deposition temperature. Data for films with thicknesses ranging from 2.4 to 108 nm will be presented. A correlation with transport measurements is also reported: whereas films thicker than 6 nm show the typical metal-to-insulator transition, the thinner film is insulating. The resistivity is strongly enhanced when decreasing film thickness. Nuclear magnetic resonance measurements have been used to monitor the relative concentration of the localized Mn[sup 4+] and delocalized Mn[sup 3+/4+] states. It is found that the relative intensity of the delocalized Mn[sup 3+/4+] configuration (I[sup 3+/4+]) progressively lowers when reducing film thickness. Of significance could be the observation that I[sup 3+/4+] remains finite for the thicknesses co! rresponding to insulating films, thus suggesting that an electrically inhomogeneous state is formed in a region close to the interface with the substrate. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A [sup 59]Co NMR experiment has been used to investigate the structure of two series of Co films with thicknesses varying from 5 to 400 nm which were electrodeposited on Cu at the electrolyte pH value of 2.1 and 3.7, respectively. It was shown that the overall structure of studied Co films consists of a very good quality fcc phase and a heavily faulted hcp phase in about equal proportions. The exceptions are very thin Co layers (below 20 nm) where the hcp structure was stabilized at pH 3.7 and overpotential of 0.9 V. This effect is attributed to the formation of hexagonal cobalt hydroxide in the early stage of deposition, which acts as a buffer layer stabilizing Co hcp structure. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports that a structural study using 59Co nuclear magnetic resonance has been performed on a series of Co/Cu multilayers with Cu thickness of 9 angstrom and Co thickness ranging from 10 to 50 angstrom. Structure of the Co layer; Effect of increase in Co thickness on hcp Co content.

Presents a study that examined [sup59]cobalt nuclear magnetic resonance. Anisotropy of the neodymium sublattice; Factor that determines the canted magnetic structure of the system; Switch of the system to a planar structure.

Focuses on a study which investigated the molecular dynamics simulation of liquid water modeled by ab initio two- and three-body potentials. Discussion on the Monte Carlo studies of liquid water; Details of cubical sample of water molecules with periodic boundary conditions and Ewald summation; Information on the partial radial distribution functions.

Through the analysis of the magnetic properties and of the nuclear magnetic resonance response of La[sub 2/3]Ca[sub 1/3]MnO[sub 3] ceramics with different grain sizes, we have found that poorly conducting regions, some ferromagnetic and some weakly magnetic, are located at the surface of the grains. We state that these regions constitute the tunnel barrier responsible for the low-field magnetoresistance usually observed in powders of half-metallic oxides. In addition, the spin disorder accompanying the coexistence of phases with different magnetoelectronic character could contribute to the large high-field magnetoresistance also typical of such ceramic samples. From a more general perspective, these findings can be of relevance to understand the microscopic origin of phase separation in manganites. [ABSTRACT FROM AUTHOR]

Details the application of reciprocal space mapping to investigate superlattices (SL) grown on misoriented substrate. Calculation of the parameters describing superlattice geometry; Comparison of the accuracy of the results with that of other techniques; Use in the absence of the nodal SL reflection.

Presents an abstract of the study 'NMR Study of Nd[sub2](Fe[sub1-x]Co[subx])[sub14]B for Low Co Content,' by M. Wojcik, J.L. Budnick, P. Panissod, A.T. Pedziwiatr and W.E. Wallace.

To provide optimal depth resolution with a coded-aperture Laue diffraction microscope, an accurate position of the coded-aperture and its scanning geometry need to be known. However, finding the geometry by trial and error is a time-consuming and often challenging process because of the large number of parameters involved. In this paper, we propose an optimization approach to automate the focusing process after data is collected. We demonstrate the robustness and efficiency of the proposed approach with experimental data taken at a synchrotron facility.

Motivated by the advanced photon source upgrade, a new hard X-ray microscope called "Velociprobe" has been recently designed and built for fast ptychographic imaging with high spatial resolution. We are addressing the challenges of high-resolution and fast scanning with novel hardware designs, advanced motion controls, and new data acquisition strategies, including the use of high-bandwidth interferometric measurements. The use of granite, air-bearing-supported stages provides the necessary long travel ranges for coarse motion to accommodate real samples and variable energy operation while remaining highly stable during fine scanning. Scanning the low-mass zone plate enables high-speed and high-precision motion of the probe over the sample. With an advanced control algorithm implemented in a closed-loop feedback system, the setup achieves a position resolution (3σ) of 2 nm. The instrument performance is evaluated by 2D fly-scan ptychography with our developed data acquisition strategies. A spatial resolution of 8.8 nm has been demonstrated on a Au test sample with a detector continuous frame rate of 200 Hz. Using a higher flux X-ray source provided by double-multilayer monochromator, we achieve 10 nm resolution for an integrated circuit sample in an ultrafast scan with a detector's full continuous frame rate of 3000 Hz (0.33 ms per exposure), resulting in an outstanding imaging rate of 9 × 10 4 resolution elements per second.

Vibrationally state selective overtone spectroscopy and dynamics of weakly bound Ne-H 2 O complexes (D 0 (para) = 31.67 cm -1 , D 0 (ortho) = 34.66 cm -1 ) are reported for the first time, based on near infrared excitation of van der Waals cluster bands correlating with v OH = 2 ← 0 overtone transitions (|02 - ⟩←|00 + ⟩ and |02 + ⟩←|00 + ⟩) out of the ortho (1 01 ) and para (0 00 ) internal rotor states of the H 2 O moiety. Quantum theoretical calculations for nuclear motion on a high level ab initio potential energy surface (CCSD(T)/VnZ-f12 (n = 3,4), corrected for basis set superposition error and extrapolated to the complete basis set limit) are employed for assignment of Σ←Σ,Π←Σ, and Σ←Π infrared bands in the overtone spectra, where Σ(K = 0) and Π (K = 1) represent approximate projections (K) of the body angular momentum along the Ne-H 2 O internuclear axis. End-over-end tumbling of the ortho Ne-H 2 O cluster is evident via rotational band contours observed, with band origins and rotational progressions in excellent agreement with ab initio frequency and intensity predictions. A clear Q branch in the corresponding |02 + ⟩ f Π(1 11 )← e Σ(0 00 ) para Ne-H 2 O spectrum provides evidence for a novel e/f parity-dependent metastability in these weakly bound clusters, in agreement with ab initio bound state calculations and attributable to the symmetry blocking of an energetically allowed channel for internal rotor predissociation. Finally, Boltzmann analysis of the rotational spectra reveals anomalously low jet temperatures (T rot ≈ 4(1) K), which are attributed to "evaporative cooling" of weakly bound Ne-H 2 O clusters and provide support for similar cooling dynamics in rare gas-tagging studies.

We propose a semi-empirical theory which describes the geminate electron-hole separation probability in both homogeneous systems and donor-acceptor heterojunction systems applicable in organic photovoltaics. The theory is based on the results of extensive simulation calculations, which were carried out using various lattice models of the medium and different charge-carrier hopping mechanisms, over the parameter ranges typical for organic solar cells. It is found that the electron-hole separation probability can be conveniently described in terms of measurable parameters by a formula whose functional form is derived from the existing recombination theories, and which contains only one empirical parameter. For homogeneous systems, this parameter is determined by the structure of the medium and only weakly depends on the charge-carrier hopping mechanism. In the case of donor-acceptor heterojunction systems, this empirical parameter shows a simple power-law dependence on the product of the dielectric constant and inter-molecular contact distance. We also study the effect of heterojunction structure on the electron-hole separation probability and show that this probability decreases with increasing roughness of the heterojunction. By analyzing the simulation results obtained for systems under the influence of an external electric field, we find that the field effect on the electron-hole separation probability in donor-acceptor heterojunction systems is weaker than in homogeneous systems. We also describe this field effect by a convenient empirical formula.

We study effective mobility in 2 dimensional (2D) and 3 dimensional (3D) systems, where hopping transitions of carriers are described by the Marcus equation under a Gaussian density of states in the dilute limit. Using an effective medium approximation (EMA), we determined the coefficient Cd for the effective mobility expressed by μeff∝exp-λ/4kBT-Cdσ(2)/kBT(2)/λ(kBT)(3/2), where λ is the reorganization energy, σ is the standard deviation of the Gaussian density of states, and kBT takes its usual meaning. We found Cd = 1/2 for both 2D and 3D. While various estimates of the coefficient Cd for 3D systems are available in the literature, we provide for the first time the expected Cd value for a 2D system. By means of kinetic Monte-Carlo simulations, we show that the effective mobility is well described by the equation shown above under certain conditions on λ. We also give examples of analysis of experimental data for 2D and 3D systems based on our theoretical results.

The kinetic problem of a diffusion-mediated reaction, in which minority reactants are immobile and majority reactants are mobile, is known as the target problem. The standard theory of the target problem ignores the excluded volume interaction between the mobile reactants. Recently, a new theory of the target problem was proposed where the effect of excluded volume interaction was analytically investigated using a lattice model with prohibited double occupancy of the lattice sites. The results of that theory are approximate and need verification. In this work, we perform Monte Carlo simulations on lattices and use their results to assess the accuracy of the analytical theory. We also generalize our theory to the case of different dimensionality and perform calculations for lattices in one- and two-dimensional systems. The analytical results accurately reproduce the simulation results except in the dilute limit in one dimension. For any dimensions, the decay of the target survival probability is accelerated by the presence of excluded volume interaction., (© 2011 American Institute of Physics.)

This paper deals with the exact extension of the original Onsager theory of the escape probability to the case of finite recombination rate at nonzero reaction radius. The empirical theories based on the Eigen model and the Braun model, which are applicable in the absence and presence of an external electric field, respectively, are based on a wrong assumption that both recombination and separation processes in geminate recombination follow exponential kinetics. The accuracies of the empirical theories are examined against the exact extension of the Onsager theory. The Eigen model gives the escape probability in the absence of an electric field, which is different by a factor of 3 from the exact one. We have shown that this difference can be removed by operationally redefining the volume occupied by the dissociating partner before dissociation, which appears in the Eigen model as a parameter. The Braun model gives the escape probability in the presence of an electric field, which is significantly different from the exact one over the whole range of electric fields. Appropriate modification of the original Braun model removes the discrepancy at zero or low electric fields, but it does not affect the discrepancy at high electric fields. In all the above theories it is assumed that recombination takes place only at the reaction radius. The escape probability in the case when recombination takes place over a range of distances is also calculated and compared with that in the case of recombination only at the reaction radius.

The photoluminescence in amorphous semiconductors decays according to the power law t(-delta) at long times. The photoluminescence is controlled by dispersive transport of electrons. The latter is usually characterized by the power alpha of the transient current observed in the time-of-flight experiments. Geminate recombination occurs by radiative tunneling which has a distance dependence. In this paper, we formulate ways to calculate reaction rates and survival probabilities in the case carriers execute dispersive diffusion with long-range reactivity. The method is applied to obtain tunneling recombination rates under dispersive diffusion. The theoretical condition of observing the relation delta=alpha/2+1 is obtained and theoretical recombination rates are compared to the kinetics of observed photoluminescence decay in the whole time range measured.

Quantum-state-resolved reactive-scattering dynamics of F+D(2)O-->DF+OD have been studied at E(c.m.)=5(1) kcal/mol in low-density crossed supersonic jets, exploiting pulsed discharge sources of F atom and laser-induced fluorescence to detect the nascent OD product under single-collision conditions. The product OD is formed exclusively in the v(OD)=0 state with only modest rotational excitation ( =0.50(1) kcal/mol), consistent with the relatively weak coupling of the 18.1(1) kcal/mol reaction exothermicity into "spectator" bond degrees of freedom. The majority of OD products [68(1)%] are found in the ground ((2)Pi(32) (+/-)) spin-orbit state, which adiabatically correlates with reaction over the lowest and only energetically accessible barrier (DeltaE( not equal) approximately 4 kcal/mol). However, 32(1)% of molecules are produced in the excited spin-orbit state ((2)Pi(12) (+/-)), although from a purely adiabatic perspective, this requires passage over a DeltaE( not equal) approximately 25 kcal/mol barrier energetically inaccessible at these collision energies. This provides unambiguous evidence for nonadiabatic surface hopping in F+D(2)O atom abstraction reactions, indicating that reactive-scattering dynamics even in simple atom+polyatom systems is not always isolated on the ground electronic surface. Additionally, the nascent OD rotational states are well fitted by a two-temperature Boltzmann distribution, suggesting correlated branching of the reaction products into the DF(v=2,3) vibrational manifold.