${\mathrm{Fe}}_3\mathrm{Si}/\mathrm{Ge}\left(\mathrm{Fe},\mathrm{Si}\right)/{\mathrm{Fe}}_3\mathrm{Si}$ thin-film stacks were grown by a combination of molecular beam epitaxy and solid-phase epitaxy (Ge on ${\mathrm{Fe}}_3\mathrm{Si}$). The stacks were analyzed using electron microscopy, electron diffraction, and synchrotron x-ray diffraction. The Ge(Fe,Si) films crystallize in the well-oriented, layered tetragonal structure ${\mathrm{FeGe}}_2$ with space group $P4mm$. This kind of structure does not exist as a bulk material and is stabilized by the solid-phase epitaxy of Ge on ${\mathrm{Fe}}_3\mathrm{Si}$. We interpret this as an ordering phenomenon induced by minimization of the elastic energy of the epitaxial film.

Energy dissipation in amorphous coatings severely affects high-precision optical and quantum transducers. In order to isolate the source of coating loss, we performed an extensive study of Raman scattering and mechanical loss of a thermally treated sputtered silica coating. Our results show that loss is correlated with the population of three-membered rings of Si-${\mathrm{O}}_4$ tetrahedral units and support the evidence that thermal treatment reduces the density of metastable states separated by a characteristic energy of about 0.5 eV in favor of an increase of the density of states separated by smaller activation energies.

A massively parallel method to build large transition rate matrices from temperature-accelerated molecular dynamics trajectories is presented. Bayesian Markov model analysis is used to estimate the expected residence time in the known state space, providing crucial uncertainty quantification for higher-scale simulation schemes such as kinetic Monte Carlo or cluster dynamics. The estimators are additionally used to optimize where exploration is performed and the degree of temperature acceleration on the fly, giving an autonomous, optimal procedure to explore the state space of complex systems. The method is tested against exactly solvable models and used to explore the dynamics of C15 interstitial defects in iron. Our uncertainty quantification scheme allows for accurate modeling of the evolution of these defects over timescales of several seconds.

The colossal magnetoresistance (CMR) in perovskite manganites and the resistive switching (RS) effect in metal-oxide heterostructures have both attracted intensive attention in the past decades. Up to date, however, there has been surprisingly little effort to study the CMR phenomena by employing a memristive switch or by integrating the CMR and memristive properties in a single RS device. Here, we report a memristive control of the melting of the antiferromagnetic charge ordered (AFM-CO) state in ${\mathrm{La}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_{3-\delta }$ epitaxial films. We show that an in situ electrotailoring of the boundary condition, which results in layers of oxygen vacancies at the metal-oxide interface, can not only suppress the critical magnetic field for the AFM-CO state melting in the interfacial memristive domain, but also promote the one in the common pristine domain of the RS device in the high and low resistive states. Our study thereby highlights the pivotal roles of functional oxygen vacancies and their dynamics in strong correlation physics and electronics.

We report on our attempt to hole-dope the antiferromagnetic semiconductor LaMnAsO by substitution of the ${\mathrm{La}}^{3+}$ site by ${\mathrm{Ca}}^{2+}$. We use neutron and x-ray diffraction, magnetic susceptibility, and transport techniques to characterize polycrystalline $\left({\mathrm{La}}_{1-x}{\mathrm{Ca}}_x\right)\mathrm{MnAsO}$ samples prepared by solid-state reaction and find that the parent compound is highly resistant to substitution with an upper limit $x\le 0.01$. Magnetic susceptibility of the parent and the $x=0.002\left(x_{\mathrm{nom}}=0.04\right)$ compounds indicate a negligible presence of magnetic impurities (i.e., MnO or MnAs). Rietveld analysis of neutron and x-ray diffraction data shows the preservation of both the tetragonal $\left(P4/nmm\right)$ structure upon doping and the antiferromagnetic ordering temperature, $T_{\mathrm{N}}=355\pm 5$ K.

We have successfully synthesized the novel antiferromagnets with ${\mathrm{Ir}}^{4+}$ honeycomb lattice ${\mathrm{ZnIrO}}_3$ and ${\mathrm{MgIrO}}_3$ and investigated their magnetic and thermodynamic properties. The two iridates are isomorphic but exhibit qualitatively different magnetic properties. ${\mathrm{ZnIrO}}_3$ shows antiferromagnetic ordering below 46.6 K, whereas ${\mathrm{MgIrO}}_3$ displays weak ferromagnetic behavior below 31.8 K, owing to formation of a canted antiferromagnetic ordering. The measurement of magnetic susceptibility with using an oriented powder sample revealed the presence of an XY-like magnetic anisotropy and a tilting magnetic structure which is possibly stabilized by the Kitaev interaction. Moreover, magnetization curves of ${\mathrm{ZnIrO}}_3$ and ${\mathrm{MgIrO}}_3$ up to 60 T show different behaviors, demonstrating that each magnetic ground state is different with each other. We discuss the difference in the ground state between ${\mathrm{ZnIrO}}_3$ and ${\mathrm{MgIrO}}_3$ from the viewpoint a magnetic model consisting of the Kitaev and Dzyaloshinskii-Moriya interactions with the spin-frustration effect on the honeycomb lattice.

We explore how anharmonicity, nuclear quantum effects (NQE), many-body dispersion interactions, and Pauli repulsion influence thermal properties of dispersion-bound molecular crystals. Accounting for anharmonicity with ab initio molecular dynamics yields cell parameters accurate to within $2\%$ of experiment for a set of pyridinelike molecular crystals at finite temperatures and pressures. From the experimental thermal expansion curve, we find that pyridine-I has a Debye temperature just above its melting point, indicating sizable NQE across the entire crystalline range of stability. We find that NQE lead to a substantial volume increase in pyridine-I $(\approx 40$% more than classical thermal expansion at 153 K) and attribute this to intermolecular Pauli repulsion promoted by intramolecular quantum fluctuations. When predicting delicate properties such as the thermal expansivity, we show that many-body dispersion interactions and more sophisticated density functional approximations improve the accuracy of the theoretical model.

The development of high quality factor solid-state microcavities with low mode volumes has paved the way towards on-chip cavity quantum electrodynamics experiments and the development of high-performance nanophotonic devices. Here, we report on the implementation of a new kind of solid-state vertical microcavity, which allows for confinement of the electromagnetic field in the lateral direction without deep etching. The confinement originates from a local elongation of the cavity layer imprinted in a shallow etch and epitaxial overgrowth technique. We show that it is possible to improve the quality factor of such microcavities by a specific in-plane bullseye geometry consisting of a set of concentric rings with subwavelength dimensions. This design results in a smooth effective lateral photonic potential and therefore in a reduction of lateral scattering losses, which makes it highly appealing for experiments in the framework of exciton-polariton physics demanding tight spatial confinement.

We study the electronic band structure of three-dimensional ABC-stacked (rhombohedral) graphdiyne, which is a new planar carbon allotrope recently fabricated. Using first-principles calculation, we show that the system is a nodal-line semimetal, in which the conduction band and valence band cross at a closed ring in the momentum space. We derive the minimum tight-binding model and the low-energy effective Hamiltonian in a $4\times 4$ matrix form. The nodal line is protected by a nontrivial winding number, and it ensures the existence of the topological surface state in a finite-thickness slab. The Fermi surface of the doped system exhibits a peculiar, self-intersecting hourglass structure, which is quite different from the torus or pipe shape in the previously proposed nodal semimetals. Despite its simple configuration, three-dimensional graphdiyne offers unique electronic properties distinct from any other carbon allotropes.

Antiphase boundaries (APBs) normally form as a consequence of the initial growth conditions in all spinel ferrite thin films. These boundaries result from the intrinsic nucleation and growth mechanism, and are observed as regions where the periodicity of the crystalline lattice is disrupted. The presence of APBs in epitaxial films of the inverse spinel ${\mathrm{Fe}}_3{\mathrm{O}}_4$ alters their electronic and magnetic properties due to strong antiferromagnetic (AF) interactions across these boundaries. We explore the effect of using in-plane in situ electric-field-assisted growth on the formation of APBs in heteroepitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$(100)/MgO(100) thin films. The electric-field-assisted growth is found to reduce the AF interactions across APBs and, as a consequence, APB-free thin-film-like properties are obtained, which have been probed by electronic, magnetic, and structural characterization. The electric field plays a critical role in controlling the density of APBs during the nucleation process by providing an electrostatic force acting on adatoms and therefore changing their kinetics. This innovative technique can be employed to grow epitaxial spinel thin films with controlled AF interactions across APBs.

We report the structure-property phase diagram of unique single-ion type-1 multiferroic pseudocubic ${\mathrm{Sr}}_{1-x}\mathrm{B}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ perovskites. Employing a specially designed multistep reduction-oxidation synthesis technique, we have synthesized ${\mathrm{Sr}}_{1-x}\mathrm{B}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ compositions in their polycrystalline form with a significantly extended Ba solubility limit that is only rivaled by a very limited number of crystals and thin films grown under nonequilibrium conditions. Understanding the multiferroic interplay with structure in ${\mathrm{Sr}}_{1-x}\mathrm{B}{\mathrm{a}}_x\mathrm{Mn}{\mathrm{O}}_3$ is of great importance as it opens the door wide to the development of newer materials from the parent $\left(A{A}^{\prime }\right)\left(B{B}^{\prime }\right){\mathrm{O}}_3$ system with enhanced properties. To this end, using a combination of time-of-flight neutron and synchrotron x-ray scattering techniques, we determined the exact structures and quantified the Mn and oxygen polar distortions above and below the ferroelectric Curie temperature $T_{\mathrm{C}}$ and the Néel temperature $T_{\mathrm{N}}$. In its ferroelectric state, the system crystalizes in the noncentrosymmetric tetragonal $P4mm$ space group, which gives rise to a large electric dipole moment $P_{\mathrm{s}}$, in the $z$ direction, of 18.4 and $29.5\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$ for $x=0.43$ and 0.45, respectively. The two independently driven ferroelectric and magnetic order parameters are single-handedly accommodated by the Mn sublattice leading to a novel strain-assisted multiferroic behavior in agreement with many theoretical predictions. Our neutron diffraction results demonstrate the large and tunable suppression of the ferroelectric order at the onset of AFM ordering and confirm the coexistence and strong coupling of the two ferroic orders below $T_{\mathrm{N}}$. The refined magnetic moments confirm the strong covalent bonding between Mn and the oxygen anions, which is necessary for stabilizing the ferroelectric phase.

Electric-field tunable indirect-to-direct band gap transitions occur in thin-film silicon and transition metal dichalcogenides; however, they remain challenging to access in three-dimensional transition metal oxides. Very recently, an unusual polar-to-nonpolar phase transition under epitaxial strain was discovered in ${\mathrm{A}}_3{\mathrm{B}}_2{\mathrm{O}}_7$ hybrid improper ferroelectrics (HIFs), which supports controllable dielectric anisotropy and magnetization. Here we examine HIF ${\left(\mathrm{AB}{\mathrm{O}}_3\right)}_1/{\left({\mathrm{A}}^{\prime }\mathrm{B}{\mathrm{O}}_3\right)}_1$ superlattices and ${\mathrm{AA}}^{\prime }{\mathrm{BB}}^{\prime }{\mathrm{O}}_6$ double perovskites and predict a competing nonpolar antiferroelectric phase, demonstrating it is hidden in hybrid improper ferroelectrics exhibiting corner-connected $\mathrm{B}{\mathrm{O}}_6$ octahedra. Furthermore, we show the transition between the polar and nonpolar phases enables an in-plane electric field to control the indirect-to-direct band gap transition at the phase boundary in the ${\left(\mathrm{AB}{\mathrm{O}}_3\right)}_1/{\left({\mathrm{A}}^{\prime }\mathrm{B}{\mathrm{O}}_3\right)}_1$ superlattices and ${\mathrm{AA}}^{\prime }{\mathrm{BB}}^{\prime }{\mathrm{O}}_6$ double perovskites, which may be tuned through static strain or chemical substitution. Our findings establish HIFs as a functional electronics class from which to realize direct gap materials and enables the integration of a broader palette of chemistries and compounds for linear and nonlinear optical applications.

The low-temperature vibrational properties of $\mathrm{B}{\mathrm{a}}_2\mathrm{ZnTe}{\mathrm{O}}_6$ double-perovskite ceramics obtained by the solid-state route were investigated by Raman scattering and Fourier-transform infrared reflectivity. We found that this material undergoes a reversible ferroelastic phase transition at around 140 K, well compatible with a recently proposed rhombohedral-to-monoclinic structural change that would occur below 165 K. Complementary calorimetric measurements showed that the phase transition has a first-order character, with an entropy jump compatible with a displacive mechanism. The vibrational spectra show clearly the splitting of the doubly degenerate $E$ modes into nondegenerate representations of the low-symmetry phase. In particular, the lowest-frequency Raman mode presents soft-mode behavior and splits below the critical temperature, confirming the in-plane ferroelastic deformation in the low-temperature phase.

By combining ab initio simulations including an on-site Coulomb repulsion term and Boltzmann theory, we explore the thermoelectric properties of $({\mathrm{LaNiO}}_3{)}_n$/$({\mathrm{LaAlO}}_3{)}_n$(001) superlattices $\left(n=1,3\right)$ and identify a strong dependence on confinement, spacer thickness, and epitaxial strain. While the system with $n=3$ shows modest values of the Seebeck coefficient and power factor, the simultaneous reduction of the ${\mathrm{LaNiO}}_3$ region and the ${\mathrm{LaAlO}}_3$ spacer thickness to single layers results in a strong enhancement, in particular of the in-plane values. This effect can be further tuned by using epitaxial strain as a control parameter: Under tensile strain corresponding to the lateral lattice constant of ${\mathrm{SrTiO}}_3$ we predict in- and cross-plane Seebeck coefficients of $\pm 600\mu \mathrm{V}$/K and an in-plane power factor of $11\mu \mathrm{W}/{\mathrm{K}}^2\mathrm{cm}$ for an estimated relaxation time of $\tau =4$ fs around room temperature. These values are comparable to some of the best performing oxide systems such as La-doped ${\mathrm{SrTiO}}_3$ or layered cobaltates and are associated with the opening of a small gap (0.29 eV) induced by the concomitant effect of octahedral tilting and Ni-site disproportionation. This establishes oxide superlattices at the verge of a metal-to-insulator transition driven by confinement and strain as promising candidates for thermoelectric materials.

Amorphous metal oxides (AMOs) are important candidate materials for fabricating next-generation thin-film transistors. While much attention has been directed toward the synthesis and electrical properties of AMOs, less is known about growth conditions that allow AMOs to retain their desirable amorphous state when subjected to high operating temperatures. Using in situ x-ray scattering and level-set simulations, we explore the time evolution of the crystallization process for a set of amorphous $\mathrm{I}{\mathrm{n}}_2{\mathrm{O}}_3$ thin films synthesized by pulsed-laser deposition at deposition temperatures ($T_d$) of −50, −25, and 0 °C. The films were annealed isothermally and the degree of crystallinity was determined by a quantitative analysis of the time-evolved x-ray scattering patterns. As expected, for films grown at the same $T_d$, an increase in the annealing temperature $T_A$ led to a shorter delay prior to the onset of crystallization, and a faster crystallization rate. Moreover, when lowering the deposition temperature by 25 °C, a 40 °C increase in annealing temperature is needed to achieve the same time interval for the crystals to grow from 10 to 90% volume fraction of the sample. Films grown at $T_d=0{}^{\circ }\mathrm{C}$ exhibited strong cubic texture after crystallization. A level-set method was employed to quantitatively model the texture that develops in the microstructures and to determine key parameters, such as the interface growth velocity, the nucleation density, and the activation energy. The differences observed in the crystallization processes are attributed to the changes in the atomic structure of the oxide and possible nanocrystalline inclusions that formed during the deposition of the amorphous phase.

Tungsten trioxide $\left({\mathrm{WO}}_3\right)$ is a versatile material with widespread applications ranging from electrochromics and optoelectronics to water splitting and catalysis of chemical reactions. For technological applications, thin films of ${\mathrm{WO}}_3$ are particularly appealing, taking advantage from a high surface-to-volume ratio and tunable physical properties. However, the growth of stoichiometric crystalline thin films is challenging because the deposition conditions are very sensitive to the formation of oxygen vacancies. In this paper, we show how background oxygen pressure during pulsed laser deposition can be used to tune the structural and electronic properties of ${\mathrm{WO}}_3$ thin films. By performing x-ray diffraction and low-temperature electrical transport measurements, we find changes in the ${\mathrm{WO}}_3$ lattice volume of up to 10% concomitantly with a resistivity drop of more than five orders of magnitude at room temperature as a function of increased oxygen deficiency. We use advanced ab initio calculations to describe in detail the properties of the oxygen vacancy defect states and their evolution in terms of excess charge concentration. Our results depict an intriguing scenario where structural, electronic, optical, and transport properties of ${\mathrm{WO}}_3$ single-crystal thin films can all be purposely tuned by controlling the oxygen vacancy formation during growth.

We extract a variable $X$ from electron orbitals ${\mathrm{\Psi }}_{n\mathbf{k}}$ and energies $E_{n\mathbf{k}}$ in the parent high-symmetry structure of a wide range of complex oxides: perovskites, rutiles, pyrochlores, and cristobalites. Even though calculation was done only in the parent structure, with no distortions, we show that $X$ dictates material's true ground-state structure. We propose using Wannier functions to extract concealed variables such as $X$ both for material structure prediction and for high-throughput approaches.

We propose an efficient theoretical scheme for structure prediction on the basis of the idea of combining methods, which optimize theoretical calculation and experimental data simultaneously. In this scheme, we formulate a cost function based on a weighted sum of interatomic potential energies and a penalty function which is defined with partial experimental data totally insufficient for conventional structure analysis. In particular, we define the cost function using “crystallinity” formulated with only peak positions within the small range of the x-ray-diffraction pattern. We apply this method to well-known polymorphs of ${\mathrm{SiO}}_2$ and C with up to 108 atoms in the simulation cell and show that it reproduces the correct structures efficiently with very limited information of diffraction peaks. This scheme opens a new avenue for determining and predicting structures that are difficult to determine by conventional methods.

We report on the discovery of pressure-induced superconductivity in the compensated semimetal pyrite $\mathrm{PtB}{\mathrm{i}}_2$, which exhibits extreme magnetoresistance (XMR) and nontrivial band structure at ambient pressure. The appearance of superconductivity, first observed at $P_{\mathrm{C}}\sim 13$ GPa with an onset critical temperature $T_{\mathrm{C}}$ of $\sim 2.2\mathrm{K}$, is accompanied by a pronounced enhancement of the density of electrons and holes based on Hall-effect measurements. Upon further compression, $T_{\mathrm{C}}$ remains almost unchanged up to 50.0 GPa; remarkably, the perfect electron-hole compensation still holds, while the carrier mobility greatly reduces. No evident trace of structural phase transitions is detected through synchrotron x-ray diffraction over the measured pressure range of 1.5–51.2 GPa. These results highlight a multiband characteristic of the observed superconductivity, making pyrite $\mathrm{PtB}{\mathrm{i}}_2$ unique among the compensated XMR materials where the pressure-induced superconductivity usually links to structural transitions and carrier imbalance.

Nanoporous alumina membranes produced by mild or hard anodization have a controllable pore surface area up to 400 times that of the membrane itself. They exhibit a temperature-independent and almost anhysteretic saturating response to a magnetic field up to temperatures of 300 K or more. The magnetism, which cannot be explained by the ∼1 ppm of transition-metal impurities present in the membranes, increases with the area of the open nanopores, reaching values of 0.6 Bohr magnetons per square nanometer for mild anodization and 8 Bohr magnetons per square nanometer for the faster hard anodization process. Crystallization of the membrane or treatment with salicylic acid can destroy 90% of the magnetism. The effect is therefore linked with the surfaces of the open pores in the amorphous $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$. Possible explanations in terms of electrons associated with oxygen vacancies ($F$ or $F^{+}$ centers) are considered. It is concluded that the phenomenon involved is likely to be saturating giant orbital paramagnetism, rather than any sort of collective ferromagnetic spin order.