Ionic control of magnetic properties, dubbed magneto-ionics, has gained much attention in recent years due to the sizable effects that can be induced by electrically controlled ion motion. Here we assess the mechanism by which hydrogen affects magnetic anisotropy in representative ferromagnetic/nonmagnetic metal layers. We take Co/Pd film as a model system that is widely used in spintronics. First-principles calculations demonstrate that the magnetic moment can be switched by ${90}^{\circ }$ via hydrogen insertion at the Co/Pd interface. This control results from hydrogen-induced changes in magnetic anisotropy originating from modifications to the electronic structure. Accumulation of hydrogen at the Co/Pd interface affects the hybridization between neighboring Co and Pd layers, leading to a decrease of the perpendicular anisotropy component, and eventually changes the net magnetic anisotropy to in-plane. Hydrogen penetration into the interior Co layers has the opposite effect, promoting perpendicular magnetic anisotropy. These changes are governed by competing contributions of the $d_{xy};$ $d_{x^2+y^2}$ and the $3d_{z^2};$ $3d_{zy}$ states, which are mainly responsible for the perpendicular and the in-plane magnetocrystalline anisotropy, respectively. By using this understanding, we predict that hydrogen accumulation at Fe/V interfacial layers causes the opposite spin reorientation effect, promoting perpendicular magnetic anisotropy.

In this paper, we present pressure dependent structural parameters and electronic structure of the ThFeAsN superconductor. There are no anomalies in the structural parameters as well as elastic constants with hydrostatic pressure which is consistent with the experiments. We study the electronic structure of this compound at different external pressures in terms of density of states, band structure, and Fermi surface. Density of states at the Fermi level, coming from $\mathrm{Fe}\text{−}d$ orbitals, follows the same trend as that of the superconducting transition temperature $\left(T_c\right)$ as a function of hydrostatic pressure. We also observe a pressure induced orbital selective Lifshitz transition in ThFeAsN compound which is quite different from the Lifshitz transitions observed in the other families of Fe-based superconductors. Fermi surfaces of ThFeAsN especially holelike Fermi surfaces at the $\mathrm{\Gamma }$ point are altered significantly with pressure. This modification of Fermi surface topology with pressure seems to play a major role in the reduction of $T_c$ with pressure in ThFeAsN superconductor. Spin-orbit coupling does not affect the Lifshitz transition but it modifies the energy ordering of bands near the $\mathrm{\Gamma }$ point at higher pressure.

${\mathrm{Yb}}^{3+}$-doped inorganic metal-halide perovskites $(\mathrm{Y}{\mathrm{b}}^{3+}:\mathrm{CsPb}{X}_3$, $X=\mathrm{Cl},\mathrm{Br})$ have recently been discovered to display highly efficient quantum cutting, in which the energy from individual blue or UV photons absorbed by the material is reemitted in the form of pairs of near-infrared photons by ${\mathrm{Yb}}^{3+}$ dopants. Experimental photoluminescence quantum yields approaching 200% have been reported. As the first quantum-cutting materials that combine such high-photoluminescence quantum yields with strong, broadband absorption in the visible, these materials offer unique opportunities for enhancing the efficiencies of solar technologies. Little is known about the fundamental origins of this quantum cutting, however. Here, we describe variable-temperature and time-resolved photoluminescence studies of ${\mathrm{Yb}}^{3+}:\mathrm{CsPb}{\mathrm{Cl}}_3$ in two disparate forms–colloidal nanocrystals and macroscopic single crystals. Both forms show very similar spectroscopic properties, demonstrating that quantum cutting is an intrinsic property of the ${\mathrm{Yb}}^{3+}:\mathrm{CsPb}{X}_3$ composition itself. Diverse ${\mathrm{Yb}}^{3+}$ speciation is observed in both forms by low-temperature photoluminescence spectroscopy, but remarkably, quantum cutting is dominated by the same specific ${\mathrm{Yb}}^{3+}$ species in both cases. Time-resolved photoluminescence measurements provide direct evidence of the previously hypothesized intermediate state in the quantum-cutting mechanism. This intermediate state mediates relaxation from the photogenerated excited state of the perovskite to the emissive excited state of ${\mathrm{Yb}}^{3+}$, and hence is of critical mechanistic importance. At room temperature, this intermediate state is populated within a few picoseconds and has a decay time of only ∼7 ns in both nanocrystalline and single-crystal ${\mathrm{Yb}}^{3+}:\mathrm{CsPb}{\mathrm{Cl}}_3$. The mechanistic implications of these observations are discussed. These results provide valuable information about characteristics of this unique quantum cutter that will aid its optimization and application in solar technologies.

Prediction of the glass-forming ability (GFA) of alloys remains a major challenge. We are not yet able to predict the composition dependence of the GFA of even binary alloys. To investigate the effect of each element's propensity to form particular crystal structures on glass formation, we focus on binary alloys composed of elements with the same size but different atomic symmetries using the patchy-particle model. For mixtures with atomic symmetries that promote different crystal structures, the minimum critical cooling rate $R_c$ is only a factor of 5 lower than that for the pure substances. For mixtures with different atomic symmetries that promote local crystalline and icosahedral order, the minimum $R_c$ is more than three orders of magnitude lower than that for pure substances. Results for $R_c$ for the patchy-particle model are consistent with those from embedded atom method simulations and sputtering experiments of NiCu, TiAl, and high entropy alloys.

In this work, ferroelastic domain walls inside ${\mathrm{BaTiO}}_3$ (BTO) tetragonal nanocrystals are distinguished by Bragg peak position and studied with Bragg coherent x-ray diffraction imaging (BCDI). Convergence-related features of the BCDI method for strongly phased objects are reported. A ferroelastic domain wall inside a BTO crystal has been tracked and imaged across the tetragonal-cubic phase transition and proves to be reversible. The linear relationship of relative displacement between two twin domains with temperature is measured and shows a different slope for heating and cooling, while the tetragonality reproduces well over temperature changes in both directions. An edge dislocation is also observed and found to annihilate when heating the crystal close to the phase transition temperature.

The near equiatomic NiTi alloy is the most successful shape memory alloy by a large margin. It is widely and increasingly used in biomedical devices. Yet, despite having a repeatable superelastic effect and excellent shape-memory, NiTi is very far from satisfying the conditions that characterize the most reversible phase-transforming materials. Thus, the scientific reasons underlying its vast success present an enigma. In this paper, we perform rigorous mathematical derivations and accurate density-functional theory calculations of transformation mechanisms to seek previously unrecognized twinlike defects that we term involution domains, and we observe them in real space in NiTi by aberration-corrected scanning transmission electron microscopy. Involution domains lead to an additional 216 compatible interfaces between phases in NiTi, and we theorize that this feature contributes importantly to its reliability. They are expected to arise in other transformations and to alter the conventional interpretation of the mechanism of the martensitic transformation.

High entropy alloys (HEAs) exhibit an unusual combination of high fracture strength and ductility. However, atomic mechanisms responsible for crack propagation in HEAs are still not clear, which limits further improving the damage tolerance. Here we investigate effect of crystal orientation on the crack-tip behaviors in single-crystal HEA CrMnFeCoNi using atomic simulations to explore fracture micromechanism. The formation of deformation twinning and activation of multislip systems are observed during the propagation crack with the $\left(001\right)\langle 110\rangle$ orientation, consistent with the previous experiments. Under the $\left(\overline{1}10\right)\langle 110\rangle$ orientation, the amorphous region takes place throughout the crack growth, and is difficult to occur in traditional metal materials. Dissimilarly, for the $\left(1\overline{1}\overline{1}\right)\langle 110\rangle$ orientation, the blunting and slip bands occur at the front of the crack tip by switching the slip mode from the planar to wavy slip, observed in recent transmission electron microscopy experiments. The chemical disorder leads to the obvious fluctuation of flow stress, but hardly affects the deformation mechanism at the crack tip. Compared to traditional metals and alloys, the high local stress concentration induced by coupling effect of severe lattice distortion and tension strain leads to the structure transformation from crystallization to amorphization at the crack tip in HEA. While the presented atomic simulations and the associated conclusions are based on CrMnFeCoNi HEA, it is believed that the current deformation mechanism at crack tip could also be applied to other face-centered-cubic HEA.

We studied metastable $\alpha \text{−}\mathrm{Fe}{\mathrm{Ge}}_2$, a layered tetragonal material, embedded as a spacer layer in spin valve structures with ferromagnetic ${\mathrm{Fe}}_3\mathrm{Si}$ and ${\mathrm{Co}}_2\mathrm{Fe}\mathrm{Si}$ electrodes. For both types of electrodes, spin valve operation is demonstrated with A metallic transport behavior of the $\alpha \text{−}\mathrm{Fe}{\mathrm{Ge}}_2$ spacer layer. The spin valve signals are found to increase with both temperature and spacer thickness, which is discussed in terms of a decreasing magnetic coupling strength between the ferromagnetic bottom and top electrodes. The temperature-dependent resistances of the spin valve structures exhibit characteristic features, which are explained by ferromagnetic phase transitions between 55 and 110 K. The metallic transport characteristics as well as the low-temperature ferromagnetism are found to be consistent with the results of first-principles calculations.

Hyperbolic media usually possess large dissipation induced by the inherent imaginary part in the material dispersion of their metallic components. Here, we theoretically and experimentally demonstrate a route toward hyperbolic media with low dissipation: using structural dispersions. In a planar microwave waveguide filled with anisotropic dielectrics, we insert an array of metal wires and transform the original hybrid modes into pure transverse-electric modes. Interestingly, by engineering the structural dispersions such that waves are evanescent in one direction and propagating in the orthogonal direction, hyperbolic media with negligible loss can be achieved. Conical radiation pattern and negative refraction, which are characteristic phenomena of the hyperbolic media, have been numerically or experimentally demonstrated in microwaves. Our work opens an approach to utilize structural dispersions instead of material dispersions in realizing hyperbolic media with negligible loss.

Antimony selenide (${\mathrm{Sb}}_2{\mathrm{Se}}_3$) consists of one-dimensional ribbons that are van der Waals bonded to each other. Due to its favorable optoelectronic properties, it is a promising material for solar energy conversion. Owing to its narrow direct band gap and its high absorption coefficient it is an efficient absorber for solar light. Besides, it is cheap, stable in harsh environments, and abundant with low toxicity. For its use in solar devices, efficient transport of photoexcited charge carriers towards the active interface and long lifetimes at sufficiently high energies are mandatory. Here time-resolved two-photon photoemission experiments on the (100) surface of ${\mathrm{Sb}}_2{\mathrm{Se}}_3$ are presented. Data were taken along and perpendicular to the direction of the one-dimensional ribbons in order to unravel the anisotropic lifetimes and relaxation dynamics of photoexcited charge carriers. In both cases, various conduction bands could be populated and within the first picosecond ultrafast scattering towards the lowest conduction band was observed. Moreover, long lifetimes of up to 35 ps in the conduction band bottom at the surface were obtained and no sign of trapping in defect states was found. These measurements shed light on the relaxation and scattering of free carriers in ${\mathrm{Sb}}_2{\mathrm{Se}}_3$, which is the key for the design of ${\mathrm{Sb}}_2{\mathrm{Se}}_3$-based electrodes.

We report the crystal structures and magnetic properties of two pseudopolymorphs of the $S=1/2 {\mathrm{Ti}}^{3+}$ coordination framework, $\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$. Single-crystal x-ray and powder neutron diffraction measurements on $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$ confirm its structure in the tetragonal $I4/mcm$ space group with a square planar arrangement of ${\mathrm{Ti}}^{3+}$ ions. Magnetometry and specific heat measurements reveal weak antiferromagnetic interactions, with $J_1\approx 7$ K and $J_2/J_1=0.11$ indicating a slight frustration of nearest- and next-nearest-neighbor interactions. Below 1.8 K, $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$ undergoes a transition to G-type antiferromagnetic order with magnetic moments aligned along the $c$ axis of the tetragonal structure. The estimated ordered moment of ${\mathrm{Ti}}^{3+}$ in $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$ is suppressed from its spin-only value to $0.62\left(3\right){\mu }_B$, thus verifying the two-dimensional nature of the magnetic interactions within the system. $\beta -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·2{\mathrm{H}}_2\mathrm{O}$, on the other hand, realizes a three-dimensional diamondlike magnetic network of ${\mathrm{Ti}}^{3+}$ moments within a hexagonal $P{6}_{2}22$ structure. An antiferromagnetic exchange coupling of $J\approx 54$ K—an order of magnitude larger than in $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$—is extracted from magnetometry and specific heat data. $\beta -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·2{\mathrm{H}}_2\mathrm{O}$ undergoes Néel ordering at $T_N=28$ K, with the magnetic moments aligned within the $ab$ plane and a slightly reduced ordered moment of $0.79{\mu }_B$ per ${\mathrm{Ti}}^{3+}$. Through density-functional theory calculations, we address the origin of the large difference in the exchange parameters between the $\alpha$ and $\beta$ pseudopolymorphs. Given their observed magnetic behaviors, we propose $\alpha -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·x{\mathrm{H}}_2\mathrm{O}$ and $\beta -\mathrm{K}\mathrm{Ti}{\left({\mathrm{C}}_2{\mathrm{O}}_4\right)}_2·2{\mathrm{H}}_2\mathrm{O}$ as close to ideal model $S=1/2$ Heisenberg square and diamond lattice antiferromagnets, respectively.

In the search for an oxide-based 2D electron system with a large concentration of highly mobile electrons, a promising strategy is to introduce electrons through donor doping while spatially separating electrons and donors to prevent scattering. In $\mathrm{SrTi}{\mathrm{O}}_3$, this can be achieved by tailoring the oxygen vacancy profile through reduction, e.g., by creating an interface with an oxygen scavenging layer. Through reduction, oxygen atoms are removed close to the interface, leaving behind oxygen vacancies in the $\mathrm{SrTi}{\mathrm{O}}_3$ lattice and mobile electrons in the $\mathrm{SrTi}{\mathrm{O}}_3$ conduction band. The commonly assumed picture is that the oxygen vacancies then remain confined close to the interface while the electrons leak a few nanometers into the bulk, resulting in an electron-defect separation and a highly mobile, oxide-based 2D electron system. So far it has remained unclear how the confinement and electron-defect separation develop over time. Here, we present transient finite element simulations that consider three driving forces acting on the oxygen vacancy distribution: diffusion due to the concentration gradient, drift due to the intrinsic electric field, and an oxygen vacancy trapping energy that holds oxygen vacancies at the interface. Our simulations show that at room temperature, three distinct regions are formed in $\mathrm{SrTi}{\mathrm{O}}_3$ within days: (1) Oxygen vacancies are partially held at the interface due to the oxygen vacancy trapping energy. (2) The accompanying positive space charge causes an oxygen vacancy depletion layer with large electron concentration and high mobility just below the interface. This electron-defect separation, indeed, leads to a highly conductive region. (3) While we are able to describe measured conductivity data with an oxygen vacancy trapping energy of $-0.2$ eV, this value does not prevent oxygen vacancy diffusion into the bulk: A diffusion front progresses into the bulk and leads to significant conductivity arising over the first micrometer within a couple of months. An enhanced oxygen vacancy trapping energy of $-0.5$ eV or below would suppress this loss of confinement, leading to a static and pronounced electron-defect separation. Consequently, our results highlight the importance of oxygen vacancy redistribution and suggest the trapping energy of oxygen vacancies at the interface as an important design parameter for oxygen-vacancy-based 2D electron systems.

We performed ab initio density functional theory simulations of $\frac{1}{2}\langle 111\rangle$ interstitial dislocation loops, closed and open vacancy loops, $\langle 100\rangle$ interstitial loops, and voids in tungsten, using simulation cells involving from 2000 to 2700 atoms. The size of the loops transcends the microscopic scale and reaches the mesoscopic scale where asymptotic elasticity treatment applies. Comparing the formation energies of dislocation vacancy loops and voids, we conclude that a void remains the most energetically favorable vacancy defect over the entire range of sizes investigated here. A closed $\frac{1}{2}\langle 111\rangle$ vacancy loop is more stable than an open loop if the number of vacancies in the loop is greater than $\sim 45$, corresponding to the diameter of a loop of approximately 1.8 nm. We have also computed elastic dipole tensors and relaxation volumes of loops and voids, representing the source terms in continuum models for radiation induced stresses and strains in the material. A detailed analysis of metastable configurations of closed and open vacancy loops performed using molecular statics simulations shows that vacancy loop configurations are not unique, and significant fluctuations of defect structures may occur in the course of microstructural evolution under irradiation.

The search for the stable polymeric forms of solid nitrogen is of great importance in view of its potential application as a high-energy-density material. Using first-principles calculations combined with an effective crystal structure search method, we demonstrate that nitrogen can react with methane under relatively moderate pressures. The structures and the stability strongly depend on the composition. ${\mathrm{CN}}_2{\mathrm{H}}_4$ and ${\mathrm{CN}}_4{\mathrm{H}}_4$ compounds become stable in $Pca{2}_1$ and $I\text{−}42d$ phases under pressures of 11 and 41 GPa, respectively. Especially, ${\mathrm{CN}}_4{\mathrm{H}}_4$ is recoverable as a metastable high energy material at ambient condition, and can release enormous amount of energy ($6.43\mathrm{kJ}{\mathrm{g}}^{-1}$) while decomposing to molecular nitrogen and methane.

Apart from a handful of exceptions, all known complex oxide two-dimensional electron gases (2DEGs) are formed in ${\mathrm{SrTiO}}_3$-based heterostructures, and microscopic information about non-${\mathrm{SrTiO}}_3$ 2DEGs systems is scarce. Here, we report on the realization of metallic conductance in a ${\mathrm{CaTiO}}_3$-based system, ${\mathrm{CaTiO}}_3$/${\mathrm{LaTiO}}_3$ superlattices, epitaxially grown in a layer-by-layer way on a ${\mathrm{NdGaO}}_3$(110) substrate by pulsed laser deposition. The high quality of the crystal and electronic structures is characterized by in situ reflection high-energy electron diffraction, x-ray diffraction, scanning transmission electron microscopy, and x-ray photoemission spectroscopy. Electrical transport confirms the formation of metallic interfaces in the ${\mathrm{CaTiO}}_3$/${\mathrm{LaTiO}}_3$ superlattice. In addition, Hall measurements reveal that in the ${\mathrm{CaTiO}}_3$/${\mathrm{LaTiO}}_3$ superlattice the room-temperature carrier mobility is nearly three times higher than that of the ${\mathrm{CaTiO}}_3$/${\mathrm{YTiO}}_3$ superlattice, implying the importance of ${\mathrm{TiO}}_6$ octahedral tilts and rotations on the carrier mobility of a 2DEG. Since doped ${\mathrm{CaTiO}}_3$ is an A-site polar metal, our results provide a materials system for designing synthetic two-dimensional polar metals.

Dielectric and elastic properties of aluminate-sodalite-type oxides, ${\left({\mathrm{Ca}}_{1-x}{\mathrm{Sr}}_x\right)}_8{\left[\mathrm{Al}{\mathrm{O}}_2\right]}_{12}{\left(\mathrm{Mo}{\mathrm{O}}_4\right)}_2$ with $x=1/16$ and 1/4, are investigated in the present study. A well-defined improper ferroelectric phase transition is demonstrated in the composition of $x=1/16$ by the onset of spontaneous strain and sharp anomalies in the dielectric and elastic responses around 600 K. As the Sr concentration increases to $x=1/4$, strong frequency dispersion appears in the dielectric and elastic responses, indicating that the system changes into a short-range ordered state. The dielectric and elastic anomalies are found to follow a single Vogel-Fulcher equation over a wide frequency range from ${10}^{-1}$ to ${10}^6\mathrm{Hz}$. The present results suggest an improper relaxor state at the composition of $x=1/4$, in which polar nanoregions form under spatially heterogeneous strain due to differences in the local coordination environments around Ca and Sr.

Pulsed laser deposition is widely used to synthesize complex oxide thin films with advanced functional properties because of the versatility of the process, although the actual processes and mechanisms that take place are highly dynamic and nontrivial. Detailed understanding of the plume dynamics is required to achieve enhanced control of the growth of complex oxide thin films. However, analytical models of the plasma plume proposed in previous studies, the so-called shockwave model, drag model, and adiabatic thermalization model, only apply to a specific pressure regime and only focus on the propagation of the plasma front. Numerical modeling of the plasma dynamics has previously been pursued for the one-dimensional propagation of a Si plasma plume in a noble background gas. Here we have explored a more generalized numerical model for the dynamics of a ${\mathrm{TiO}}_2$ plasma plume by extending to three dimensions, multiple elements in the plasma plume, and including chemical reactions in an oxygen environment without the use of adaptive parameters, other than used for the description of the initial plume shape. Comparison between the simulations and the self-emission measurements of the ${\mathrm{TiO}}_2$ plasma plume shows good agreement. Our model enables detailed simulation of the oxidation state of the arriving particles (Ti, TiO, and ${\mathrm{TiO}}_2$) on the substrate surface depending on oxygen background pressure. Interestingly, low pressures of about 0.02 mbar result in fast oxygen particles arriving prior to the slower titanium particles, which could significantly influence the oxidation state of the initial substrate surface. The enhanced number of collisions for higher pressures of about 0.1 mbar leads to a large amount of the low-mass oxygen particles coming to a standstill 1–2 cm away from the ablated target. Furthermore, the three-dimensional nature of our model has enabled simulation of the lateral variations in composition of the deposited particles on the substrate surface. Although negligible variations in the Ti:TiO and Ti:${\mathrm{TiO}}_2$ ratios are present for high pressures over deposition areas with diameters up to 5 cm, significant variations can be observed for low pressures. These insights could play an important role in upscaling pulsed laser deposition from a scientific laboratory-based scale to an industrial large-area scale.

We present a systematic study of electrical resistivity, Hall coefficient, magneto-optical imaging, magnetization, and scanning transmission electron microscopy (STEM) analyses of $\mathrm{K}{\mathrm{Ca}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ single crystals. Sharp diamagnetic transition and magneto-optical imaging reveal homogeneity of single crystal and prominent Bean-like penetrations of vortices. Large anisotropy of electrical resistivity, with ${\rho }_c/{\rho }_{ab}>100$, and semiconductorlike ${\rho }_c$ suggest that the electronic state is quasi-two-dimensional. Hall effect measurements indicate that $\mathrm{K}{\mathrm{Ca}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ is a multiband system with holes as main carriers. Magnetization measurements reveal significantly larger $J_{\mathrm{c}}$ compared with that in other iron-based superconductors with different values of $J_{\mathrm{c}}$ depending on the direction of magnetic field. The origin of these $J_{\mathrm{c}}$ characteristics is discussed based on microstructural observations using STEM. In addition, further enhancement of $J_{\mathrm{c}}$ in $\mathrm{K}{\mathrm{Ca}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ for future high-field application is demonstrated in terms of heavy-ion irradiation.

The threading dislocation density (TDD) in plastically relaxed Ge/Si(001) heteroepitaxial films is commonly observed to decrease progressively with their thickness due to mutual annihilation. However, there exists a saturation limit, known as the geometrical limit, beyond which a further decrease of the TDD in the Ge film is hindered. Here, we show that such a limit can be overcome in SiGe/Ge/Si heterostructures thanks to the beneficial role of the second interface. Indeed, we show that ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/\mathrm{Ge}/\mathrm{Si}\left(001\right)$ films display a TDD remarkably lower than the saturation limit of Ge/Si(001). Such a result is interpreted with the help of dislocation dynamics simulations. The reduction of TDD is attributed to the enhanced mobility acquired by preexisting threading dislocations after bending at the new interface to release the strain in the upper layer. Importantly, we demonstrate that the low TDD achieved in ${\mathrm{Si}}_{0.06}{\mathrm{Ge}}_{0.94}/\mathrm{Ge}/\mathrm{Si}$ layers is preserved also when a second, relaxed Ge layer is subsequently deposited. This makes the present reverse-grading technique of direct interest also for achieving a low TDD in pure-Ge films.

Raising the Curie temperature $T_{\mathrm{C}}$ of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of $T_{\mathrm{C}}$ with free carrier density $n$ and the surprisingly low dopant activation $p$, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether $T_{\mathrm{C}}$ could be considerably enhanced by increasing $p$. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation ($p$) values of up to 67%, effectively more than doubling $p$ as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating $n, p$, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the $T_{\mathrm{C}}$ of these samples allows us to identify several different mechanisms influencing $T_{\mathrm{C}}$ and causing an experimental maximum in $T_{\mathrm{C}}$. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.