Nanoscale complex-oxide thin films prepared by well-established growth techniques, such as pulsed-laser deposition or molecular-beam epitaxy, often exhibit compositions that deviate from the ideal stoichiometry. Even small variations in composition can lead to substantial changes in the technologically relevant electronic, magnetic, and optical properties of these materials. To assess the reasons behind this variability, and ultimately to allow tuning the properties of oxide films with precise control of the deposition parameters, high-resolution detection of the nonstoichiometry introduced during growth is needed. The resolution of current techniques, such as x-ray diffraction, fluorescence, or spectroscopy, is limited to estimating composition differences in the percent level, which is often insufficient for electronic-device quality. We develop an unconventional approach based on scanning tunneling microscopy for enabling the determination of cation imbalance introduced in thin films with exceptionally small detection limit. We take advantage of the well-controlled surface reconstructions on ${\mathrm{SrTiO}}_3$(110), and use the established relation between those reconstructions and the surface composition to assess the cation excess deposited in pulsed-laser grown ${\mathrm{SrTiO}}_3$(110) films. We demonstrate that a <0.1% change in cation nonstoichiometry is detectable by our approach. Furthermore, we show that, for thin films that accommodate all the nonstoichiometry at the surface, this method has no fundamental detection limit.

For several decades, the striking contradiction between the Huang diffuse scattering experiments, resistivity recovery data, and predictions derived from density functional theory (DFT) remained one of the mysteries of defect physics in molybdenum. Since the 1970s, observations of Huang x-ray diffuse scattering appeared to indicate that a self-interstitial atom (SIA) defect in Mo adopts a $\langle 110\rangle$ dumbbell configuration. However, the low temperature defect diffusion data supported the DFT prediction of a different, highly mobile $\langle 111\rangle$ SIA defect structure in the same metal. Using DFT simulations, we show that an SIA adopts a symmetry-broken configuration in all the group 6 metals: chromium, molybdenum, and tungsten. The symmetry-broken defect structure, a $\langle 11\xi \rangle$ dumbbell, where $\xi$ is an irrational number, agrees with nudged elastic band analyses of $\langle 110\rangle$ to $\langle 111\rangle$ transformations. Direct simulations of Huang diffuse scattering by symmetry-broken defect configurations predicted by DFT explain why no zero intensity lines were observed in experiment and resolve the long outstanding question about the structure of defects in Mo and similar metals. A $\langle 11\xi \rangle$ defect migrates on average one dimensionally through a sequence of three-dimensional nonplanar $\left[11\xi \right]$ to $\left[\xi 11\right]$ or $\left[1\xi 1\right]$ transitions. Barriers for defect migration in nonmagnetic Cr, antiferromagnetic Cr, Mo, and W derived from DFT calculations, 0.052, 0.075, 0.064, and 0.040 eV are well correlated with the onset of defect migration temperatures observed experimentally.

Cellulose nanocrystals (CNCs) are biosourced chiral nanorods that can form stable colloidal suspensions able to spontaneously assemble above a critical concentration into a cholesteric liquid crystal, with a cholesteric pitch usually in the micron range. When these suspensions are dried on a substrate, solid films with a pitch of the order of few hundreds of nanometers can be produced, leading to intense reflection in the visible range. However, the resulting cholesteric nanostructure is usually not homogeneous within a sample and comports important variations of the cholesteric domain orientation and pitch, which affect the photonic properties. In this work, we first propose a model accounting for the formation of the photonic structure from the vertical compression of the cholesteric suspension upon solvent evaporation, starting at the onset of the kinetic arrest of the drying suspension and ending when solvent evaporation is complete. From that assumption, various structural features of the films can be derived, such as the variation of the cholesteric pitch with the domain tilt, the orientation distribution density of the final cholesteric domains and the distortion of the helix from the unperturbed cholesteric case. The angular-resolved optical response of such films is then derived, including the iridescence and the generation of higher-order reflection bands, and a simulation of the angular optical response is provided, including its tailoring under external magnetic fields. Second, we conducted an experimental investigation of CNC films covering a structural and optical analysis of the films. The macroscopic appearance of the films is discussed and complemented with angular-resolved optical spectroscopy, optical and electron microscopy, and our quantitative analysis shows an excellent agreement with the proposed model. This allows us to access the precise composition and the pitch of the suspension when it transited into a kinetically arrested phase directly from the optical analysis of the film. This work highlights the key role that the anisotropic compression of the kinetically arrested state plays in the formation of CNC films and is relevant to the broader case of structure formation in cast dispersions and colloidal self-assembly upon solvent evaporation.

We have studied the barocaloric properties associated with the martensitic transition of a shape memory Heusler alloy ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5}$ which is composed of all-$d$-metal elements. The composition of the sample has been tailored to avoid long-range ferromagnetic order in both austenite and martensite. The lack of ferromagnetism results in a weak magnetic contribution to the total entropy change, thereby leading to a large transition entropy change. The combination of such a large entropy change and a relatively large volume change at the martensitic transition gives rise to giant barocaloric properties in this alloy. When compared to other shape memory Heusler alloys, our material exhibits values for adiabatic temperature and isothermal entropy changes significantly larger than values reported so far for this class of materials. Furthermore, our ${\mathrm{Ni}}_{50}{\mathrm{Mn}}_{31.5}{\mathrm{Ti}}_{18.5}$ also compares favorably to the best state-of-the-art magnetic barocaloric materials.

We study the electronic structure of delafossite ${\mathrm{PtCoO}}_2$ to elucidate its extremely small resistivity and high mobility. The band exhibits steep dispersion near the Fermi level despite the fact that it is formed mainly by Pt $d$ orbitals that are typically localized. We propose a picture based on two hidden kagome-lattice-like electronic structures: one originating from Pt $s+p_x/p_y$ orbitals, and the other from Pt $d_{3z^2-r^2}+d_{xy}/d_{x^2-y^2}$ orbitals, each placed on the bonds of the triangular lattice. In particular, we find that the underlying Pt $s+p_x/p_y$ bands actually determine the steepness of the original dispersion, so that the large Fermi velocity can be attributed to the large width of the Pt $s+p_x/p_y$ band. In addition, the kagome-like electronic structure gives rise to “orbital-momentum locking” on the Fermi surface, which reduces the electron scattering by impurities. We conclude that the combination of the large Fermi velocity and the orbital-momentum locking is likely to be the origin of the extremely small resistivity in ${\mathrm{PtCoO}}_2$.

We derive surface polarity at and near ferroelastic domain boundaries from molecular dynamics simulations based on an ionic spring model. Interatomic gradient forces lead to flexoelectricity which, in turn, generates polarity at the surface and in twin boundaries. We then derive generic properties of electron scattering spectra equivalent to those observed in low-energy electron microscopy (LEEM) and mirror electron microscopy (MEM) experiments. Negatively (positively) charged surfaces reflect (attract) incident electrons with low kinetic energy. The electron images reveal the valley and ridge surface structures near the intersection of the twin boundary and the surface. Polarity in surface layers is predicted to be visible in LEEM and MEM spectra at neutral surfaces, but much less when surfaces are charged. Inward polarity reflects electrons similar to negative surface charges, and outward polarity backscatters electrons like positive surface charges. Both the polarity in the twin boundary and the physical topography scatter electrons, consistent with experimental LEEM and MEM experiments on $\mathrm{CaTi}{\mathrm{O}}_3$ with (001) and (111) surface terminations.

Electron-defect ($e-\mathrm{d}$) interactions govern charge carrier dynamics at low temperature, where they limit the carrier mobility and give rise to phenomena of broad relevance in condensed matter physics. Ab initio calculations of $e-\mathrm{d}$ interactions are still in their infancy, mainly because they require large supercells and computationally expensive workflows. Here we develop an efficient ab initio approach for computing elastic $e-\mathrm{d}$ interactions, their associated $e-\mathrm{d}$ relaxation times (RTs), and the low-temperature defect-limited carrier mobility. The method is applied to silicon with simple neutral defects, such as vacancies and interstitials. Contrary to conventional wisdom, the computed $e-\mathrm{d}$ RTs depend strongly on carrier energy and defect type, and the defect-limited mobility is temperature dependent. These results highlight the shortcomings of widely employed heuristic models of $e-\mathrm{d}$ interactions in materials. Our method opens avenues for studying $e-\mathrm{d}$ scattering and low-temperature charge transport from first principles.

Edge dislocations are frequently found in epitaxial ${\mathrm{BiFeO}}_3$ multiferroic thin films and are expected to exhibit distinctive and localized magnetoelectric properties. However, an exhaustive characterization of these dislocations at the atomic level has to date been largely overlooked. Here, we use a combination of scanning transmission electron microscopy techniques, atomistic simulations obtained from classical molecular dynamics calculations, and real-space multiple-scattering theory to explore the chemical properties and the bonding characteristics of the atoms located at and near the dislocation cores. We find that in addition to Bi, small amounts of Fe atoms are present in the ${\mathrm{BiFeO}}_3$ dislocation cores which result in uncompensated Fe spins along the dislocations and give rise to a magnetic signal. Our results suggest that edge dislocations in ${\mathrm{BiFeO}}_3$ films could be efficiently used for realizing ${\mathrm{BiFeO}}_3$-based magnetic devices.

(001)-oriented thin films of the three-dimensional Dirac semimetal cadmium arsenide can realize a quantum spin Hall insulator and other kinds of topological physics, all within the flexible architecture of epitaxial heterostructures. Here, we report a method for growing (001) cadmium arsenide films using molecular beam epitaxy. The introduction of a thin indium arsenide wetting layer improves surface morphology and structural characteristics, as measured by x-ray diffraction and reflectivity, atomic force microscopy, and scanning transmission electron microscopy. The electron mobility of 50-nm-thick films is found to be $9300\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$ at 2 K, comparable to the highest-quality films grown in the conventional (112) orientation. This work demonstrates a simple experimental framework for exploring topological phases that are predicted to exist in proximity to the three-dimensional Dirac semimetal phase.

We report on atom probe tomography studies of variable chemical decorations at extended defects in Cu-poor and Zn-rich $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ thin films. For a precursor film, which was co-evaporated at $320{}^{\circ }\mathrm{C}$, grain boundaries and dislocations are found enriched with Cu. Furthermore, Na out-diffusion from the soda-lime glass substrate occurs even at such a low temperature, resulting in Na segregation at defects. In contrast, stacking faults in the precursor film show clear Zn enrichment as well as Cu and Sn depletion. After an annealing step at $500{}^{\circ }\mathrm{C}$, we detect changes in the chemical composition of grain boundaries as compared to the precursor. Moreover, we measure an increase in the grain boundary excess of Na by one order of magnitude. We show that grain boundaries and dislocations in the annealed $\mathrm{C}{\mathrm{u}}_2\mathrm{ZnSnS}{\mathrm{e}}_4$ film exhibit no or only slight variations in composition of the matrix elements. Thus, the effect of annealing is a homogenization of the chemical composition.

X-ray absorption spectroscopy is a premier element-specific technique for materials characterization. Specifically, the x-ray absorption near-edge structure (XANES) encodes important information about the local chemical environment of an absorbing atom, including coordination number, symmetry, and oxidation state. Interpreting XANES spectra is a key step towards understanding the structural and electronic properties of materials, and as such, extracting structural and electronic descriptors from XANES spectra is akin to solving a challenging inverse problem. Existing methods rely on empirical fingerprints, which are often qualitative or semiquantitative and not transferable. In this paper, we present a machine learning-based approach, which is capable of classifying the local coordination environments of the absorbing atom from simulated $K$-edge XANES spectra. The machine learning classifiers can learn important spectral features in a broad energy range without human bias and once trained, can make predictions on the fly. The robustness and fidelity of the machine learning method are demonstrated by an average 86% accuracy across the wide chemical space of oxides in eight $3d$ transition-metal families. We found that spectral features beyond the preedge region play an important role in the local structure classification problem especially for the late $3d$ transition-metal elements.

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.