Logit and probit (L/P) models are a mainstay of binary time-series cross-sectional (BTSCS) analyses. Researchers include cubic splines or time polynomials to acknowledge the temporal element inherent in these data. However, L/P models cannot easily accommodate three other aspects of the data’s temporality: whether covariate effects are conditional on time, whether the process of interest is causally complex, and whether our functional form assumption regarding time’s effect is correct. Failing to account for any of these issues amounts to misspecification bias, threatening our inferences’ validity. We argue scholars should consider using Cox duration models when analyzing BTSCS data, as they create fewer opportunities for such misspecification bias, while also having the ability to assess the same hypotheses as L/P. We use Monte Carlo simulations to bring new evidence to light showing Cox models perform just as well—and sometimes better—than logit models in a basic BTSCS setting, and perform considerably better in more complex BTSCS situations. In addition, we highlight a new interpretation technique for Cox models—transition probabilities—to make Cox model results more readily interpretable. We use an application from interstate conflict to demonstrate our points.

Using index matching and particle tracking, we measure the three-dimensional velocity field in an isotropic porous medium composed of randomly packed solid spheres. This high-resolution experimental dataset provides new insights into the dynamics of dispersion and stretching in porous media. Dynamic-range velocity measurements indicate that the distribution of the velocity magnitude, $U$, is flat at low velocity (probability density function $(U)\propto U^{0}$). While such a distribution should lead to a persistent anomalous dispersion process for advected non-diffusive point particles, we show that the dispersion of non-diffusive tracers nonetheless becomes Fickian beyond a time set by the smallest effective velocity of the tracers. We derive expressions for the onset time of the Fickian regime and the longitudinal and transverse dispersion coefficients as a function of the velocity field properties. The experimental velocity field is also used to study, by numerical advection, the stretching histories of fluid material lines. The mean and the variance of the line elongations are found to grow exponentially in time and the distribution of elongation is log-normal. These results confirm the chaotic nature of advection within three-dimensional porous media. By providing the laws of dispersion and stretching, the present study opens the way to a complete description of mixing in porous media.

The use of duration models in political science continues to grow, more than a decade after Box-Steffensmeier and Jones (2004). However, several common misconceptions about the models still persist. To improve scholars’ use and interpretation of duration models, we point out that they are a type of regression model and therefore follow the same rules as other more commonly used regression models. In this article, we present four maxims as guidelines. We survey the various duration model interpretation strategies and group them into four categories, which is an important organizational exercise that does not appear elsewhere. We then discuss the strengths and weaknesses of these strategies, noting that all are correct from a technical perspective. However, some strategies make more sense than others for nontechnical reasons, which ultimately informs best practices.

Many political processes consist of a series of theoretically meaningful transitions across discrete phases that occur through time. Yet political scientists are often theoretically interested in studying not just individual transitions between phases, but also the duration that subjects spend within phases, as well as the effect of covariates on subjects’ trajectories through the process's multiple phases. We introduce the multistate survival model to political scientists, which is capable of modeling precisely this type of situation. The model is appealing because of its ability to accommodate multiple forms of causal complexity that unfold over time. In particular, we highlight three attractive features of multistate models: transition-specific baseline hazards, transition-specific covariate effects, and the ability to estimate transition probabilities. We provide two applications to illustrate these features.

Changes in landscape composition and structure may impact the conservation and management of protected areas. Species that depend on specific habitats are at risk of extinction when these habitats are degraded or lost. Designing robust methods to evaluate landscape composition will assist decision- and policy-making in emerging landscapes. This paper describes a rapid assessment methodology aimed at evaluating land-cover quality for birds, plants, butterflies and bees around seven UK Natura 2000 sites. An expert panel assigned quality values to standard Coordination of Information on the Environment (CORINE) land-cover classes for each taxonomic group. Quality was assessed based on historical (1950, 1990), current (2000) and future (2030) land-cover data, the last projected using three alternative scenarios: a growth-applied strategy (GRAS), a business-as-might-be-usual (BAMBU) scenario, and sustainable European development goal (SEDG) scenario. A quantitative quality index weighted the area of each land-cover parcel with a taxa-specific quality measure. Land parcels with high quality for all taxonomic groups were evaluated for temporal changes in area, size and adjacency. For all sites and taxonomic groups, the rate of deterioration of land-cover quality was greater between 1950 and 1990 than current rates or as modelled using the alternative future scenarios (2000–2030). Model predictions indicated land-cover quality stabilized over time under the GRAS scenario, and was close to stable for the BAMBU scenario. The SEDG scenario suggested an ongoing loss of quality, though this was lower than the historical rate of c. 1% loss per decade. None of the future scenarios showed accelerated fragmentation, but rather increases in the area, adjacency and diversity of high quality land parcels in the landscape.

In standard diffraction experiments, ensembles of objects are characterized yielding averaged, statistical properties (meaningful only if the ensemble is monodisperse). Focused x-ray beams are used here to localize single nanostructures, identifying and probing individual objects one by one. In a scanning mode, a 2-dimensional image of the sample is recorded, which allows the reproducible alignment of a specific nanostructure for analysis. The x-ray scattered signal is analyzed and modelled, to give access to the shape, strain and composition inside the single object with sub-micron resolution. Combination of x-ray microdiffraction technique with other micro-probe experiments on the very same individual object (simultaneous coupling of x-ray diffraction measurements with atomic force microscopy (AFM)) is also shown; we prove the possibility to interact with the objects and to address elastic properties for individual nano-structures out of an ensemble.

Ordering of Co nanoparticles (∼11 nm in diameter) into 2-D and 3-D arrays on Si/Si3N4 substrates in external magnetic field and without field is reported. Arrays of particles were studied by TEM, SEM and GISAXS. The GISAXS measurements were performed at the wavelengths 0.155 nm and 0.336 nm and the spectra were simulated using distorted wave Born approximation approach. From results it follows that 2-D ordered monolayers of particles are composed of hexagonal close-packed mosaic blocks. 3-D arrays – rods are formed along magnetic field direction, being parallel or perpendicular to the substrate surface, when the colloid was more concentrated. Distribution of particles in rods was analyzed only by GISAXS and it was described by close packing of hard spheres. Their effective diameter was 14.7 nm.

In a grazing incidence x-ray diffuse scattering study of defects in boron implanted and annealed silicon we have discovered narrow rods of intensity along [111] directions. These diffuse rods of intensity arise from stacking faults formed in the early stages of annealing in the range around 1000°C. From the width of the stacking fault induced rods we can estimate their size, while the integrated intensity is a measure of the total stacking fault area in the implanted layer. Surprisingly we find that these faults initially grow in size and density and after reaching a maximum begin to dissolve and ultimately are totally annihilated. The intensity rods are distinct from the point defect or point defect cluster scattering in the tails of the Bragg peaks referred to as Huang Scattering. From the nature of the q dependence of the diffuse scattering in the Bragg tails we find unequivocal evidence for the presence of clusters in our annealed samples. The average effective size seems remarkably independent of annealing temperature. These observations will be discussed in the context of the enhanced diffusion of boron over its bulk value.

Titanium-induced structural modifications in thin waveguide layers of lithium niobate have been investigated by grazing incidence diffraction and complementary thin film techniques. The study was focused on the high-temperature phase transformation in this system and its influence on the lattice parameter changes, depending on the annealing time.

To characterize the point defects and point defect clusters introduced by ion implantation and annealing, we have used grazing incidence x-rays to measure the diffuse scattering in the tails of Bragg peaks (Huang Scattering). An analysis of the diffuse scattered intensity will allow us to characterize the nature of point defects or defect clusters introduced by ion implantation. We have also observed unexpected satellite peaks in the diffuse scattered tails. Possible causes for the occurrence of the peaks will be discussed.

We have developed a method to determine the relationship between strain and lateral size of coherent self-organized quantum dots. In our approach, X-ray grazing incidence diffraction is used to collect information on strain and shape effects in the vicinity of a prominent surface reflection. We demonstrate that for highly strained nano-scale islands it is possible to separate strain-induced and form factor-induced scattering without comparing different reflections. Experimental data from InAs on GaAs(100) quantum dots is discussed with respect to this model. Reciprocal space mapping around the (220) surface reflection shows a linear relationship between relaxation from the substrate lattice parameter and the outer perimeter of the dot. In addition, the functional form of the gradient of relaxation is found to be nonmonotonous and rapidly increasing towards the tip of the dot.

AlxGa1-xN films were grown on c-plane sapphire by plasma induced molecular beam epitaxy with 0 ≤ x ≤ 1. The composition and purity of the AlxGa1-xN layers was determined by elastic recoil detection analysis with a relative error of 5% for the Al content. Both X-ray diffraction and atomic force microscopy indicate only a slight decrease in epitaxial quality of the AlxGa1-xN films with increasing Al content up to x = 0.65. X-ray diffraction is used to separate the effects of thermally induced biaxial compressive stress and the alloy composition on the shift of interplanar spacings by measuring both lattice constants. The deviation of the c/a ratio from that of fully relaxed films is a quantitative measure of the biaxial compressive stress leading to a distortion of the unit cell. Values up to 0.5 GPa were observed. By the method proposed, the determination of alloy composition can be corrected for this effect. The results obtained by this method are in very good agreement with the elastic recoil detection measurements substantiating the validity of Vegard's law. These results, compared with optical measurements, indicate that the bowing parameter of the optical bandgap is 1.3 eV within the experimental error.

The asymmetrical forward versus reverse - bias DC electrical conductivity (macroscopic and also nanoscopic) through Langmuir-Blodgett multilayers and monolayers of γ-(nhexadecyl) quinolinum tricyanoquinodimethanide, C16H33Q-3CNQ (1) is attributable to rectification of electrical current by a single molecule.

The GaN on LGO system is the near perfect template (due to extremely high etch selectivity) for developing a viable thin film/compliant GaN substrate. Herein, we report on our efforts to grow GaN on LGO, including improvement of the microscopic surface morphology using pre-growthpretreatments. We also report on the first transferred thin film GaN substrate grown on LGO, transferred off of LGO, and mounted on GaAs. With this approach, (InAl)GaN alloys can be grown on thin GaN films, implementing a truly “compliant” substrate for the nitride alloy system. In addition, the flexibility of bonding to low cost Si, metal or standard ceramic IC packages is an attractive alternative to SiC and HVPE GaN substrates for optimizing cost verses thermal conductivity concerns. We have demonstratedhigh quality growth of GaN on LGO. X-Ray rocking curves of 145 arc-seconds are obtained with only a 0.28 μm thick film. We present data on the out of plane crystalline quality of GaN/LGO material. Likewise, we show 2 orders of magnitude improvement in residual doping concentration and factors of 4 improvement in electron mobility as compared to the only previously reported electrical data. We show substantial vendor to vendor and intra-vendor LGO material quality variations. We have also quantified the desorption of Ga and Li from the surface of LGO at typical growth temperatures using in situ desorption mass spectroscopy and XPS.

AIN thin films were grown on c-plane sapphire by metalorganic chemical vapor deposition from tritertiarybutylaluminum and ammonia at 1050°C. These films exhibit a full width at half maximum of the 002 X-ray rocking curve below 200 arcsec indicating high epitaxial quality. By measuring asymmetric reflections, a structural disorder of the lattice mainly due to edge dislocations can be observed. For further investigations, atomic force microscopy and photothermal deflection spectroscopy were performed. In order to study the effect of increasing AIN layer thickness on the optical and structural properties of GaN in an AIN/GaN heterostructure, AIN thin films with increasing thickness ranging from 0.02 to 0.36 μm were used as sublayers for the deposition of 0.75 μm thick GaN layers. Photoluminescence, micro- Raman and X-ray diffraction measurements confirm the relaxation of biaxial compressive stress in the GaN layers due to different thermal expansion coefficients by increasing AIN layer thickness. The pressure dependence of the band gap shift was determined as 24 meV/GPa for biaxial compressive stress. Our results indicate that the growth of AIN with metal organic chemical vapor deposition from tritertiarybutylaluminum and ammonia is a promising method for obtaining high quality epitaxial films.

We present measurements of specular and nonspecular x-ray scattering from interfacial roughness in ND/Al2O3 multilayers grown by rf-sputtering on a sapphire substrate at different Ar pressures. The rms-roughness of the interfaces is found to increase drastically if the Ar pessure is raised above a critical value corresponding to the thermalization of the impinging atoms. At the same time the height-height correlation function of the interfaces changes from a non-self-affine to a self-affine form. This result can be deduced from the structure factor of the diffuse scattering measured over a wide range in parallel momentum transfer.