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We explore here the decisive role of film thickness on Casimir and electrostatic forces from topological insulating ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ films. The forces were measured for the Au-${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ system under ambient conditions to simulate realistic applications. It is shown that the electrostatic forces associated with a nonzero contact potential of ${V}_{\mathrm{o}}\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\mathrm{mV}$ played a dominant role, and could not be compensated for thinner films ($\ensuremath{\le}20\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$ thick). However, for thicker films the measured force, after voltage compensation, started to approach the genuine Casimir force in very good agreement with Lifshitz theory. Our results point out that any gradual deviation from theory for thinner films underlies the presence of strong electrostatic effects.

A long-standing question for avant-garde data storage technology concerns the nature of the ultrafast photoinduced phase transformations in the wide class of chalcogenide phase-change materials (PCMs). Overall, a comprehensive understanding of the microstructural evolution and the relevant kinetics mechanisms accompanying the out-of-equilibrium phases is still missing. Here, after overheating a phase-change chalcogenide superlattice by an ultrafast laser pulse, we indirectly track the lattice relaxation by time resolved x-ray absorption spectroscopy (tr-XAS) with a sub-ns time resolution. The approach to the tr-XAS experimental results reported in this work provides an atomistic insight of the mechanism that takes place during the cooling process; meanwhile a first-principles model mimicking the microscopic distortions accounts for a straightforward representation of the observed dynamics. Finally, we envisage that our approach can be applied in future studies addressing the role of dynamical structural strain in PCMs.

Noise measurements were performed to determine the quality factor ($Q$) as a function of gas pressure $P$ for microresonators in cantilever form with systematically modified surfaces. In the free-molecular regime, which is dominated by internal energy losses, $Q$ was substantially decreased by more than an order of magnitude with increasing surface roughness. At higher pressures, within the molecular regime, $Q$ showed the typical inverse linear dependence on pressure $Q\ensuremath{\sim}{P}^{\ensuremath{-}1}$. However, in the molecular regime the $Q$ factor also showed a strong dependence on surface morphology as indicated by surface area calculations using measured roughness data and compared to those obtained from $Q\ensuremath{\sim}{P}^{\ensuremath{-}1}$ plots.

This work demonstrates that instead of paraelectric ${\text{PbTiO}}_{3}$, completely $c$-oriented ferroelectric ${\text{PbTiO}}_{3}$ thin films were directly grown on $(001){\text{-SrTiO}}_{3}$ substrates by pulsed-laser deposition with thickness up to 340 nm at a temperature well above the Curie temperature of bulk ${\text{PbTiO}}_{3}$. The influence of laser-pulse frequency, substrate-surface termination on growth, and functional properties were studied using x-ray diffraction, transmission electron microscopy, and piezoresponse force microscopy. At low growth rates (frequency $l5\text{ }\text{Hz}$) the films were always monodomain. However, at higher growth rates (frequency $g8\text{ }\text{Hz}$) $a$ domains were formed for film thickness above 20--100 nm. Due to coherency strains the Curie temperature $({T}_{c})$ of the monodomain films was increased approximately by $350\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$ with respect to the ${T}_{c}$ of bulk ${\text{PbTiO}}_{3}$ even for 280-nm-thick films. Nonetheless, up to now this type of growth mode has been considered unlikely to occur since the Matthews-Blakeslee (MB) model already predicts strain relaxation for films having a thickness of only $\ensuremath{\sim}10\text{ }\text{nm}$. However, the present work disputes the applicability of the MB model. It clarifies the physical reasons for the large increase in ${T}_{c}$ for thick films, and it is shown that the experimental results are in good agreement with the predictions based on the monodomain model of Pertsev et al. [Phys. Rev. Lett. 80, 1988 (1998)].

Density-functional theory calculations, within the plane-wave-ultrasoft pseudopotential framework, were performed in the ⟨110⟩ projection for MgO and for the coherent {111} $\mathrm{Ag}\ensuremath{-}\mathrm{Mg}\mathrm{O}$ polar interface. First-principles calculations were incorporated in high-resolution transmission electron microscopy (HRTEM) simulations by converting the charge density into electron scattering factors to examine the influence of charge transfer, charge redistribution at the interface, and ionicity on the dynamical electron scattering and on calculated HRTEM images. It is concluded that the ionicity of oxides and the charge redistribution at interfaces play a significant role in HRTEM image simulations. In particular, the calculations show that at oxygen-terminated {111} $\mathrm{Ag}\mathrm{Mg}\mathrm{O}$ interfaces the first oxygen layer at the interface is much brighter than that in calculations with neutral atoms, in agreement with experimental observations.

Monte Carlo (MC) simulations of isothermal phase transformations were performed based on a temperature- and time-dependent nucleation rate and a temperature-dependent and time-independent anisotropic growth rate (linear growth). One- or two-dimensional anisotropic growth in two-dimensional space is considered and nucleation occurs randomly throughout space. The MC simulations show that parallel growth of anisotropically growing transformation products with identical convex shape can be described accurately by the kinetic theory due to Johnson, Mehl, Avrami, and Kolmogorov (JMAK), but nonparallel anisotropic growth, orthogonal in the present work, incorporating blocking up to all relevant orders leads to hard impingement that results in strong deviations from JMAK kinetics. A transparent analytical description extending on, but incorporating the JMAK theory has been developed that turns out to accurately reproduce the numerical results of all present MC simulation, leading to improved understanding of how impingement should be incorporated in JMAK theory.

The phenomenon of positron confinement enables us to investigate the electronic structure of nanoclusters embedded in host matrices. Solid Kr nanoclusters are a very interesting subject of investigation because of the very low predicted value of the positron affinity of bulk Kr. In this work, positron trapping in solid Kr nanoclusters embedded in MgO is investigated. The Kr nanoclusters were created by means of 280 keV Kr ion implantation in single crystals of MgO(100) and subsequent thermal annealing at a temperature of 1100 K. The nanoclusters were observed by cross-sectional transmission electron microscopy in high-resolution mode. The fcc Kr nanoclusters are rectangularly shaped with sizes of 2 to 5 nm and are in a cube-on-cube orientation relationship with the MgO host matrix. From the Moire fringes in high-resolution recordings, the lattice parameter of the solid Kr was deduced and found to vary from 5.3 to 5.8 Angstrom. The corresponding pressures are 0.6-2.5 GPa as found using the Ronchi equation of state. The relationship between lattice parameter and cluster size was investigated and it was found that the lattice parameter increases linearly with increasing nanocluster size. The defect evolution during annealing was monitored by means optical absorption spectroscopy and positron beam analysis. No evidence of positron trapping was found despite the very low positron affinity of solid Kr. Alternative definitions of the positron affinity are proposed for application to insulator materials.