We report on the structure, magnetization, magnetic anisotropy, and domain morphology of ultrathin yttrium iron garnet (YIG)/Pt films with thickness ranging from 3 to 90 nm. We find that the saturation magnetization is close to the bulk value in the thickest films and decreases at low thickness with a strong reduction below 10 nm. We characterize the magnetic anisotropy by measuring the transverse spin Hall magnetoresistance as a function of applied field. Our results reveal strong easy-plane anisotropy fields of the order of 50–100 mT, which add to the demagnetizing field, as well as weaker in-plane uniaxial anisotropy ranging from 10 to 100 $\mu \mathrm{T}$. The in-plane easy-axis direction changes with thickness but presents also significant fluctuations among samples with the same thickness grown on the same substrate. X-ray photoelectron emission microscopy reveals the formation of zigzag magnetic domains in YIG films thicker than 10 nm, which have dimensions larger than several 100 $\mu \mathrm{m}$ and are separated by achiral Néel-type domain walls. Smaller domains characterized by interspersed elongated features are found in YIG films thinner than 10 nm.

We use first-principles density-functional theory calculations to investigate the effect of biaxial strain in the low-temperature structural and ferroelectric properties of [111]-oriented ${\mathrm{SrTiO}}_3$. We find that [111] biaxial strain, achievable by coherent epitaxial growth along the [111] direction, induces structural distortions in ${\mathrm{SrTiO}}_3$ that are not present in either bulk or [001]-oriented ${\mathrm{SrTiO}}_3$. Under [111] biaxial strain, ${\mathrm{SrTiO}}_3$ displays ferroelectricity at tensile strain, and paraelectricity at compressive strain. We compute the phonon spectrum and macroscopic polarization of ${\mathrm{SrTiO}}_3$ as a function of [111] biaxial strain, and relate our results to the predictions of the free-energy phenomenological model of Pertsev, Tagantsev, and Setter [Phys. Rev. B 61, 825 (2000); Phys. Rev. B 65, 219901 (2002)].

We predict high thermoelectric efficiency in the layered perovskite ${\mathrm{La}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$, based on calculations (mostly ab initio) of the electronic structure, transport coefficients, and thermal conductivity in a wide temperature range. The figure of merit $ZT$ computed with a temperature-dependent relaxation time increases monotonically from just above 1 at room temperature to over 2.5 at 1200 K, at an optimal carrier density of around ${10}^{20}{\mathrm{cm}}^{-3}$. The Seebeck thermopower coefficient is between 200 and $300\mu \mathrm{V}$/K at optimal doping, but can reach nearly 1 mV/K at low doping. Much of the potential of this material is due to its lattice thermal conductivity of order 1 W/(K m); using a model based on ab initio anharmonic calculations, we interpret this low value as due to effective phonon confinement within the layered-structure blocks.

We have performed a computational screening of topological two-dimensional (2D) materials from the Computational 2D Materials Database (C2DB) employing density functional theory. A full ab initio scheme for calculating hybrid Wannier functions directly from the Kohn-Sham orbitals has been implemented and the method was used to extract ${\mathbb{Z}}_2$ indices, Chern numbers, and mirror Chern numbers of 3331 2D systems including both experimentally known and hypothetical 2D materials. We have found a total of 48 quantum spin Hall insulators, seven quantum anomalous Hall insulators, and 21 crystalline topological insulators. Roughly 75% are predicted to be dynamically stable and one-third was known prior to the screening. The most interesting of the topological insulators are investigated in more detail. We show that the calculated topological indices of the quantum anomalous Hall insulators are highly sensitive to the approximation used for the exchange-correlation functional and reliable predictions of the topological properties of these materials thus require methods beyond density functional theory. We also performed $GW$ calculations, which yield a gap of 0.65 eV for the quantum spin Hall insulator ${\mathrm{PdSe}}_2$ in the ${\mathrm{MoS}}_2$ crystal structure. This is significantly higher than any known 2D topological insulator and three times larger than the Kohn-Sham gap.

Up-converted photoluminescence (PL), or, in other words, anti-Stokes PL via phonon absorption in semiconductors is a critical property for realizing optical refrigeration. Here, we investigate the anti-Stokes PL from optically thin films and thick crystals of $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$ to assess their potential for laser cooling. Using PL excitation spectroscopy, we determine the excitation photon energy that maximizes the cooling efficiency. The evaluated up-conversion gain for the thick single crystal is close to 70% of that for the thin film, indicating a possibility for laser cooling even in thick crystals. We discuss the competition between anti-Stokes PL and photon reabsorption in $\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3\mathrm{Pb}{\mathrm{I}}_3$.

We investigate the effect of polymer length dispersity on the properties of self-assembled micelles in solution by self-consistent field calculations. Polydispersity stabilizes micelles by raising the free energy barriers of micelle formation and dissolution. Most importantly, it significantly reduces the size fluctuations of micelles: Block copolymers of moderate polydispersity form more uniform particles than their monodisperse counterparts. We attribute this to the fact that the packing of the solvophobic monomers in the core can be optimized if the constituent polymers have different length.

Oxide single-crystal substrates with atomically smooth and chemically uniform surfaces are indispensable for constructing sharp epitaxial heterointerfaces and investigating emergent interfacial physical phenomena. Here, we report a simple method to realize atomically flat and $\mathrm{Al}{\mathrm{O}}_2$-terminated $\mathrm{LaAl}{\mathrm{O}}_3$ (001) [LAO(001)] substrate surfaces. So far, the LAO(001) substrate has been utilized as a structural template for the epitaxial growth of a variety of oxide films. However, well-established methods for achieving atomically flat, singly terminated LAO(001) surfaces have rarely been reported. This is mainly due to the unstable charged surfaces of $\mathrm{La}{\mathrm{O}}^{+}$ or $\mathrm{Al}{{\mathrm{O}}_2}^{-}$, which hinders simultaneous stabilizations of atomic-scale smoothness and single termination. To overcome this problem, we combined thermal annealing and subsequent deionized water leaching to treat the LAO(001) surface. We used atomic force microscopy to investigate the evolution of the LAO(001) surface during the water leaching and confirmed the atomically flat surface of the 120-min-water-leached sample. We further demonstrated the uniform $\mathrm{Al}{\mathrm{O}}_2$ termination of the LAO(001) surface via coaxial impact-collision ion scattering spectroscopy. Using the treated substrates, we are able to grow perovskite oxide films (i.e., $\mathrm{SrRu}{\mathrm{O}}_3$) on the LAO(001) substrate with atomically sharp heterointerfaces. Our paper provides an effective means for controlling the surface and interface of transition-metal oxide heterostructures at the atomic scale.

Recent results in electric-field control of magnetism have paved the way for the design of alternative magnetic and spintronic devices with enhanced functionalities and low power consumption. Among the diversity of reported magnetoelectric effects, the possibility of switching on and off long-range ferromagnetic ordering close to room temperature stands out. Its binary character opens up the avenue for its implementation in magnetoelectric data storage devices. Here we show the possibility to locally switch on superferromagnetism in a wedge-shaped polycrystalline Fe thin film deposited on top of a ferroelectric and ferroelastic ${\mathrm{BaTiO}}_3$ substrate. A superparamagnetic to superferromagnetic transition is observed for confined regions for which a voltage applied to the ferroelectric substrate induces a sizable strain. We argue that electric-field-induced changes of magnetic anisotropy lead to an increase of the critical temperature separating the two regimes so that superparamagnetic regions develop collective long-range superferromagnetic behavior.

In this work, we explore a set of new garnet oxide structures that can be used as an anode, cathode or solid-electrolyte in lithium-ion batteries (LIBs) using high-throughput density functional theory. We tested around 180 combinations of elemental substitutions for the dodecahedral $X$ sites and octahedral $Y$ sites in the ${\mathrm{Li}}_3{X}_3{Y}_2{\mathrm{O}}_{12}$ type garnet structure and identified 19 stable (i.e., on the convex hull) and 11 nearly stable (i.e., within 50 meV/atom of the convex hull) ${\mathrm{Li}}_3$ garnets with respect to decomposition to other stable phases in the Open Quantum Materials Database in the respective four-dimensional $\mathrm{Li}\text{−}X\text{−}Y\text{−}\mathrm{O}$ chemical spaces. Our high-throughput screening strategy allows us to elucidate rules for garnet stability in terms of the ionic radii of the constituent elements. We evaluated the electrochemical window of these new, stable/nearly stable ${\mathrm{Li}}_3$-garnet compounds and classify each for potential applications as an anode, cathode, or solid-state electrolyte to be used in LIBs. Finally, ${\mathrm{Li}}^{+}$ ion diffusivity is calculated for the representative ${\mathrm{Li}}_3{\mathrm{Nd}}_3{\mathrm{W}}_2{\mathrm{O}}_{12}$ model system. The results we present here are expected to serve as a guideline for designing new garnet oxides for Li-ion battery applications.

Some of the strongest polymer-based fibers are obtained by processing a nematic solution of poly-$p$-phenylene terephthalamide (PPTA) in concentrated acid. Molecular dynamics simulations reveal that the lateral density of PPTA chains in the acid solution is inhomogeneous on scales shorter than the persistence length. Hairpin turns, insertions, and other defects are stabilized by chain-chain contacts. These results suggest a mechanism of fiber assembly through coalescence of nanofibrils, and an explanation for the limited strength of PPTA fibers even at high crystallinity.

A singly oriented, single layer of tungsten disulfide (${\mathrm{WS}}_2$) was epitaxially grown on Au(111) and characterized at the nanoscale by combining photoelectron spectroscopy, photoelectron diffraction, and low-energy electron microscopy. Fast x-ray photoelectron spectroscopy revealed that the growth of a single crystalline orientation is triggered by choosing a low W evaporation rate and performing the process with a high temperature of the substrate. Information about the single orientation of the layer was obtained by acquiring x-ray photoelectron diffraction patterns, revealing a 1H polytype for the ${\mathrm{WS}}_2$ layer and, moreover, determining the structural parameters and registry with the substrate. The distribution, size, and orientation of the ${\mathrm{WS}}_2$ layer were further ascertained by low-energy electron microscopy.

Formation and migration energies, elastic dipole, and relaxation volume tensors of nanodefects are the parameters determining the rates of evolution of microstructure under irradiation as well as macroscopic elastic stresses and strains resulting from the accumulation of defects in materials. To find the accurate values of these parameters, we have performed density functional theory simulations of self-interstitial and vacancy defects in all the body-centred cubic metals, including alkaline metals (Li, Na, K, Rb and Cs), alkaline-earth metal (Ba), nonmagnetic transition metals (V, Nb, Mo, Ta and W), and magnetic transition metals (Cr and Fe), correcting the computed values for the effect of finite cell size and periodic boundary conditions. The lowest energy structure of a self-interstitial atom defect is universal to all the nonmagnetic bcc metals, including metals of groups 1 and 2 of the periodic table, and has the $\langle 111\rangle$ symmetry. The only exceptions are the $\langle 110\rangle$ self-interstitial defect configuration in Fe, and a $\langle 11\xi \rangle$ configuration in Cr. We have also computed elastic dipole tensors and relaxation volumes of self-interstitial and vacancy defects in all the bcc metals and explored how elastic relaxation parameters vary along the defect migration pathways.

${\mathrm{Co}}_x{\mathrm{Zn}}_y{\mathrm{Mn}}_z$ ($x+y+z=20$) compounds crystallizing in the chiral $\beta$-Mn crystal structure are known to host skyrmion spin textures even at elevated temperatures. As in other chiral cubic skyrmion hosts, skyrmion lattices in these materials are found at equilibrium in a small pocket just below the magnetic Curie temperature. Remarkably, ${\mathrm{Co}}_x{\mathrm{Zn}}_y{\mathrm{Mn}}_z$ compounds have also been found to host metastable nonequlibrium skyrmion lattices in a broad temperature and field range, including down to zero field and low temperature. This behavior is believed to be related to disorder present in the materials. Here, we use neutron and synchrotron diffraction, density functional theory calculations, and dc and ac magnetic measurements to characterize the atomic and magnetic disorder in these materials. We demonstrate that Co has a strong site preference for the diamondoid $8c$ site in the crystal structure, while Mn tends to share the geometrically frustrated $12d$ site with Zn, due to its ability to develop a large local moment on that site. This magnetism-driven site specificity leads to distinct magnetic behavior for the Co-rich $8c$ sublattice and the Mn on the $12d$ sublattice. The Co-rich sublattice orders at high temperatures (compositionally tunable between 210 and 470 K) with a moment around $1{\mu }_B/\mathrm{atom}$ and maintains this order to low temperature. The Mn-rich sublattice holds larger moments (about $3{\mu }_B$) which remain fluctuating below the Co moment ordering temperature. At lower temperature, the fluctuating Mn moments freeze into a reentrant disordered cluster-glass state with no net moment, while the Co moments maintain order. This two-sublattice behavior allows for the observed coexistence of strong magnetic disorder and ordered magnetic states such as helimagnetism and skyrmion lattices.

We report a facile mechanochemical approach to achieve precisely controlled van der Waals spacing in molybdenum disulfide $\left(\mathrm{Mo}{\mathrm{S}}_2\right)$ nanosheets. This involves mechanochemical, solid-state grinding of bulk $\mathrm{Mo}{\mathrm{S}}_2$ with solid sodium chloride (NaCl), resulting in intercalation of $\mathrm{N}{\mathrm{a}}^{+}$ ions within the $\mathrm{Mo}{\mathrm{S}}_2$ sheets. Consequently, distinct changes to the van der Waals spacing of $\mathrm{Mo}{\mathrm{S}}_2$ is observed and attributed to coordinative adsorption followed by a diffusion-assisted intercalation. In contrast to Li-ion intercalation, the native 2H structure of $\mathrm{Mo}{\mathrm{S}}_2$ is stabilized and completely preserved. Thus the resulting 2H-$\mathrm{Mo}{\mathrm{S}}_2$ layers exhibit expanded van der Waals spacing. Spectroscopic investigations, through time-dependent Raman and X-ray photoelectron spectroscopy, confirm the kinetics of intercalation resulting in precise tunability of van der Waals spacing from 0.61 to 1.25 nm. Such a scalable approach for tunable intercalation of nanosheets presents a route for controlling defect density in $\mathrm{Mo}{\mathrm{S}}_2$, thereby providing directions in defect engineering and heterogeneous catalysis.

Layered solids are ubiquitous in nature: from subnanometer graphene and mica layers, to wood, laminated composites, and paperboard at the centimeter scale, to geologic formations at the kilometer range. And while the similarities between the latter two have been recognized [Budd et al., Philos. Trans. R. Soc. A 370, 1723 (2012)], what has not is that the same physics applies at the atomic-layer scale of crystalline solids. Herein, using a combination of atomistic simulations of graphite and simple instrumented cylindrical indentation experiments on various layered solids—plastic cards, thin steel, and Al sheets—we show that in all cases, confined buckling results in an instability that leads to the nucleation of multiple, oppositely signed ripplocation boundaries that rapidly propagate away from under the indenter in a wavelike manner. Crucially, upon unloading, they disappear, after dissipating considerable amounts of frictional energy. Understanding ripplocation nucleation, self-assembly, and propagation is fundamental to understanding the deformation of most layered solids.

We describe a lattice-based crystallographic approximation for the analysis of distorted crystal structures via electron backscatter diffraction (EBSD) in the scanning electron microscope. EBSD patterns are closely linked to local lattice parameter ratios via Kikuchi bands that indicate geometrical lattice plane projections. Based on the transformation properties of points and lines in the real projective plane, we can obtain continuous estimations of the local lattice distortion based on projectively transformed Kikuchi diffraction simulations for a reference structure. By quantitative image matching to a projective transformation model of the lattice distortion in the full solid angle of possible scattering directions, we enforce a crystallographically consistent approximation in the fitting procedure of distorted simulations to the experimentally observed diffraction patterns. As an application example, we map the locally varying tetragonality in martensite grains of steel.

The emergent ferroelectricity in ${\mathrm{HfO}}_2$-based systems has attracted significant attention as this simple binary high-$k$ dielectric now offers the possibility of nonvolatile function. In this work we employ zero-field and field-dependent pyroelectricity to show that thin films of silicon-doped ${\mathrm{HfO}}_2$ do exhibit a broken-inversion symmetry and are indeed ferroelectric. In addition, the pyroelectric response is found to exhibit a wake-up behavior akin to the wake-up phenomenon observed in the ferroelectric polarization with electric-field cycling. Using polarization-electric field hysteresis measurements, this wake-up phenomenon is attributed to the presence of defect dipoles which explains the measured pyroelectric response. Finally, direct electrocaloric measurements are performed on these silicon-doped ${\mathrm{HfO}}_2$ thin films, revealing an electrocaloric coefficient $\sim 4$ times larger in magnitude than that expected for the measured pyroelectric coefficient. This enhancement is explained using the plausible role played by defect dipoles that contribute to additional configurational or dipolar entropy.

Metal-polymer nanocomposites are hybrid materials combining the superior plasmonic, electrical, and thermal properties of metals with the good elasticity and manufacturability of polymers. This renders metal-polymer nanocomposites promising candidates for conductive filler and coating applications, where mechanical properties are optothermally coupled. Here, we study the interplay of nanostructure, thermoplasmonics, and elastic mechanical properties of silver-polystyrene nanocomposites (AgPS) by transmission electron microscopy, small-angle x-ray scattering, Brillouin light scattering (BLS), and other supplemental techniques. We utilize the well-known particle-brush architecture to ensure a homogeneous and isotropic nanoparticle distribution throughout the hybrid material. The effective longitudinal modulus of the as-prepared samples is found to decrease from 5.7 to 4.8 GPa with increasing Ag content from 0 to 4.4 vol.%. Temperature-dependent BLS measurements reveal the unique contribution of local thermoplasmonic heating that depends on the Ag nanoparticle composition. This thermoplasmonic effect results in a lower apparent glass transition temperature $\left(T_{\mathrm{g}}\right)$ and a stronger laser power dependence of the speed of sound. Exceeding moderate thermal annealing temperatures $\left(>150{}^{\circ }\mathrm{C}\right)$ leads to a strong structural rearrangement within the homogeneous nanocomposite material with a peculiar clustering-redispersion effect, which also translates into altered mechanical properties. The annealing-induced Ag nanoparticle aggregation results in an even stronger thermoplasmonic effect. We validate our experimental findings with complementary thermographic measurements and finite-element modeling. Overall, this work demonstrates the combined effects of composition and (reversible) aggregation on the mechanical and thermoplasmonic properties of metal-polymer nanocomposites. It not only deepens our understanding of the interaction between light, temperature, and mechanical properties in metal-polymer nanocomposites but also provides a guide for customizing AgPS nanocomposites for potential applications.

Doped metal oxide materials are commonly used for applications in energy storage and conversion, such as batteries and solid oxide fuel cells. The knowledge of the electronic properties of dopants and their local environment is essential for understanding the effects of doping on the electrochemical properties. Using a combination of x-ray absorption near-edge structure spectroscopy (XANES) experiment and theoretical modeling we demonstrate that in the dilute ($1\mathrm{at}.\%$) Mn-doped lithium titanate ($\mathrm{L}{\mathrm{i}}_{4/3}\mathrm{T}{\mathrm{i}}_{5/3}{\mathrm{O}}_4$, or LTO) the dopant $\mathrm{M}{\mathrm{n}}^{2+}$ ions reside on tetrahedral ($8a$) sites. First-principles Mn K-edge XANES calculations revealed the spectral signature of the tetrahedrally coordinated Mn as a sharp peak in the middle of the absorption edge rise, caused by the $1s\to 4p$ transition, and it is important to include the effective electron-core hole Coulomb interaction in order to calculate the intensity of this peak accurately. This dopant location can explain the impedance of Li migration through the LTO lattice during the charge-discharge process, and, as a result, the observed remarkable $20\%$ decrease in electrochemical capacity of the $1\%$ Mn-doped LTO compared to pristine LTO.

The influence of surface treatments on the transport properties of epitaxial films of the three-dimensional Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ is investigated. We show that exposure of the surface to a low-energy nitrogen plasma improves carrier mobilities, their temperature dependence, and allows for the observation of the quantum Hall effect in scaled thin films. The results provide insights into surface band bending as a function of the surface chemistry and the relative contributions of surface and bulk states to the measured transport properties.