We report the decomposition of $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Cu}{\mathrm{O}}_4$ into $\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3$ and Cu nanoparticles in ultrahigh vacuum, observed by in situ heating experiments in a transmission electron microscope. The analysis of electron diffraction data reveals that the phase decomposition process starts at about 150 °C and is considerably expedited in the temperature range of 350 °C–450 °C. Two major resultant solid phases are identified as metallic Cu and $\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3$ by electron diffraction, simulation, and electron energy-loss spectroscopy (EELS) analyses. With the aid of calculations, $\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3$ phases are further identified to be derivatives of a fluorite structure—fluorite, pyrochlore, and (distorted) bixbyite—characterized by different oxygen-vacancy order. Additionally, the bulk plasmon energy and the fine structures of the O $K$ and $\mathrm{La}{M}_{4,5}$ EELS edges are reported for these structures, along with simulated O $K$ x-ray absorption near-edge structure. The resultant Cu nanoparticles and $\mathrm{L}{\mathrm{a}}_2{\mathrm{O}}_3$ phases remain unchanged after cooling to room temperature.

A distinguished structural characteristic of martensite in Fe-C steels is its tetragonality originating from carbon atoms occupying only one set of the three available octahedral interstitial sites in the body-centered-cubic (bcc) Fe lattice. Such a body-centered-tetragonal (bct) structure is believed to be thermodynamically stable because of elastic interactions between the interstitial carbon atoms. For such phase stability, however, there has been a lack of direct experimental evidence despite extensive studies of phase transformations in steels over one century. In this Rapid Communication, we report that the martensite formed in a high carbon Fe-8Ni-1.26C (wt%) steel at room temperature induced by applied stress/strain has actually a bcc rather than a bct crystal structure. This finding not only challenges the existing theories on the stability of bcc vs bct martensite in high carbon steels, but also provides insights into the mechanism for martensitic transformation in ferrous alloys.

Crystalline symmetries can generate exotic band-crossing features, which can lead to unconventional fermionic excitations with interesting physical properties. We show how a cubic Dirac point—a fourfold-degenerate band-crossing point with cubic dispersion in a plane and a linear dispersion in the third direction—can be stabilized through the presence of a nonsymmorphic glide mirror symmetry in the space group of the crystal. Notably, the cubic Dirac point in our case appears on a threefold axis, even though it has been believed previously that such a point can only appear on a sixfold axis. We show that a cubic Dirac point involving a threefold axis can be realized close to the Fermi level in the nonferroelectric phase of ${\mathrm{LiOsO}}_3$. Upon lowering temperature, ${\mathrm{LiOsO}}_3$ has been shown experimentally to undergo a structural phase transition from the nonferroelectric phase to the ferroelectric phase with spontaneously broken inversion symmetry. Remarkably, we find that the broken symmetry transforms the cubic Dirac point into three mutually crossed nodal rings. There also exist several linear Dirac points in the low-energy band structure of ${\mathrm{LiOsO}}_3$, each of which is transformed into a single nodal ring across the phase transition.

Disorders and point defects strongly interplay with the phase transition and alter the properties of ferroelastic-martensitic systems. Unusual static and quasistatic behaviors, such as time-dependent phase transitions, are discovered when disorders are introduced. However, the role of disorders on the ferroelastic system in vibrational environments at moderate frequency is rarely known, investigation of which could further shed light on their application as mechanical damping materials. Here we present the emergence of large damping capacity in ferroelastic-martensitic systems [including both the $\mathrm{T}{\mathrm{i}}_{50-x}\mathrm{N}{\mathrm{i}}_{50+x}$ alloy and $\left(\mathrm{C}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\right)\mathrm{Ti}{\mathrm{O}}_3$ ceramics] by introducing disorder (i.e., substitutional Ni and Sr, respectively). As the level disorder increases, the damping capacity of both systems raises and eventually reaches a maximum when long-range-ordered martensite tends to vanish. Moreover, near the disorder-induced phase boundary, we observe a large mechanical damping in ferroelastic ceramics $\left(\mathrm{C}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\right)\mathrm{Ti}{\mathrm{O}}_3$ with a figure of merit $\sim 2\mathrm{GP}{\mathrm{a}}^{1/2}$. Microscopic and dynamic investigations indicate that such damping plateau could result from the competing evolution of density and mobility of domain boundaries when disorder is introduced. Our work provides a degree of freedom to develop ferroelastic damping materials and a potential way to tune domain-boundary-mediated functionalities for other ferroic materials.

Magnetic materials are a cornerstone for developing spintronic devices for the transport of information via magnetic excitations. To date, relatively few materials have been investigated for the purpose of spin transport, mostly due to the paucity of suitable candidates as these materials are often chemically complex and difficult to synthesize. We present the crystal growth and a structure solution on the high-temperature crystal structure of the layered, polar ferrimagnet ${\mathrm{CaBaFe}}_4{\mathrm{O}}_7$, which is a possible new contender for spintronics research. The space group is identified as $P3$ by refinement of single crystal and powder neutron diffraction data. At 400 K, the trigonal lattice parameters are $a=11.0114\left(11\right)\AA$ and $c=10.330\left(3\right)\AA$. The structure is similar to the low-temperature phase with alternating layers of triangular and Kagome-arranged Fe-O tetrahedra. We also present details of the crystal growth by traveling solvent method.

Quantitative correlation between intrinsic coercivity and grain boundaries in three dimensions is critical to further improve the performance of sintered Nd-Fe-B permanent magnets. Here, we quantitatively reveal the local composition variation across and especially along grain boundaries using the powerful atomic-scale analysis technique known as atom probe tomography. We also estimate the saturation magnetization, magnetocrystalline anisotropy constant, and exchange stiffness of the grain boundaries on the basis of the experimentally determined structure and composition. Finally, using micromagnetic simulations, we quantify the intrinsic coercivity degradation caused by inhomogeneous grain boundaries. This approach can be applied to other magnetic materials for the analysis and optimization of magnetic properties.

The electrocaloric effect is investigated systematically in ${\mathrm{Ba}}_{1-x}{\mathrm{Sr}}_x{\mathrm{TiO}}_3$ ferroelectrics using a semiclassical direct computational approach. The data are reported for the technologically important range of Sr concentrations of 0.0–0.6, electric fields up to 1000 kV/cm, and temperatures ranging from 5 to 600 K. A detailed comparison of computational data with experimental data from the literature reveals semiquantitative agreement and suggests the origin of discrepancies. The electrocaloric change in temperature $\mathrm{\Delta }T$ shows strong dependence on Sr concentration which offers a way to tune electrocaloric response. In particular, the maximum electrocaloric $\mathrm{\Delta }T$ is found to decrease with the increase in Sr concentration, whereas the location of the maximum shifts towards lower temperatures following the Curie point of the ferroelectric. Surprisingly, the width of the peak in the dependence of $\mathrm{\Delta }T$ on the initial temperature is independent of the Sr concentration but shows a strong dependence on the applied electric field. Computational data are used to propose a compositionally graded ferroelectric ${\mathrm{Ba}}_{0.70}{\mathrm{Sr}}_{0.30}{\mathrm{TiO}}_3/{\mathrm{Ba}}_{0.55}{\mathrm{Sr}}_{0.45}{\mathrm{TiO}}_3/{\mathrm{Ba}}_{0.50}{\mathrm{Sr}}_{0.50}{\mathrm{TiO}}_3/{\mathrm{Ba}}_{0.45}{\mathrm{Sr}}_{0.55}{\mathrm{TiO}}_3$ whose $\mathrm{\Delta }T$ shows almost no temperature dependence in the technologically important range of temperatures and electric fields. Such a desirable feature could potentially lead to the enhancement of relative cooling power.

Monolayers of transition-metal dichalcogenide semiconductors present spin-valley locked electronic bands, a property with applications in valleytronics and spintronics that is usually believed to be absent in their centrosymmetric (as the bilayer or bulk) counterparts. Here we show that bulk $2\mathrm{H}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ hides a spin-polarized nature of states determining its direct band gap, with the spin sequence of valence and conduction bands expected for its single layer. This relevant finding is attained by investigating the behavior of the binding energy of $A$ and $B$ excitons under high pressure, by means of absorption measurements and density-functional-theory calculations. These results raise an unusual situation in which bright and dark exciton degeneracy is naturally broken in a centrosymmetric material. Additionally, the phonon-assisted scattering process of excitons has been studied by analyzing the pressure dependence of the linewidth of discrete excitons observed at the absorption coefficient edge of $2\mathrm{H}\text{−}\mathrm{Mo}{\mathrm{S}}_2$. Also, the pressure dependence of the indirect optical transitions of bulk $2\mathrm{H}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ has been analyzed by absorption measurements and density-functional-theory calculations. These results reflect a progressive closure of the indirect band gap as pressure increases, indicating that metallization of bulk $\mathrm{Mo}{\mathrm{S}}_2$ may occur at pressures higher than 26 GPa.

Hafnia-based resistive memories technology has come to maturation and acceded to the market of nonvolatile memories. Nevertheless, the physical mechanisms involved in resistive switching are not yet fully understood and the numerous ab initio simulations studies have few many atomic-scale experimental counterparts. In this study we investigate the oxygen migration mechanism from an amorphous ${\mathrm{HfO}}_2$ layer to the Ti cap layer at a local scale before and after a thermal treatment. X-ray absorption spectroscopy at the Ti K edge and Hf ${\mathrm{L}}_{\text{III}}$ edge has been performed on samples as-deposited and annealed in Ar at $400{}^{\circ }\mathrm{C}$ to mimic the back-end-of-line thermal budget (BEOL) of CMOS technology. The short-range Ti and Hf environments have been determined, showing that annealing promotes the migration of O from ${\mathrm{HfO}}_2$ to Ti, the amount of which is quantified. This provokes an expansion and an increase of atomic disorder in the Ti lattice. The nature of the oxygen gettering mechanism by the Ti metal is understood by comparing samples with increasing Ti-capping thickness. We show that the Ti getter effect has to be activated by thermal treatment and that the O diffusion takes place in a region of a few nanometers close to the $\mathrm{Ti}/{\mathrm{HfO}}_2$ interface. Therefore, the thermal budget history and the Ti cap-layer thickness determine the oxygen vacancy content in the ${\mathrm{HfO}}_2$ layer, which in turn controls the electrical properties, especially the forming operation.

Solar energy plays an important role in substituting the ever declining source of fossil fuel energy. Finding novel materials for solar cell applications is an integral part of photovoltaic research. Hybrid lead halide perovskites are one of, if not the most, well sought material in the photovoltaic research community. Its unique intrinsic properties, flexible synthesis techniques, and device fabrication architecture made the community highly buoyant over the past few years. Yet, there are two fundamental issues that still remain a concern, i.e., the stability in external environment and the toxicity due to Pb. This led to a search for alternative materials. More recently, double perovskite $[A_{2}B{B}^{{}^{\prime }}{X}_6(X$=Cl, Br, I)] materials have emerged as a promising choice. Few experimental synthesis and high throughput computational studies have been carried out to check for promising candidates of this class. The main outcome from these studies, however, can essentially be summarized into two categories: (i) either they have an indirect band gap or (ii) a direct but large optical band gap, which is not suitable for solar devices. Here we propose a large set of stable double perovskite materials, ${\mathrm{Cs}}_{2}B{B}^{{}^{\prime }}{X}_6(X$=Cl, Br, I), which show indirect to direct band gap transition via small ${\mathrm{Pb}}^{+2}$ doping at both $B$ and $B^{{}^{\prime }}$ sites. This is done by careful band (orbital) engineering using first-principles calculations. This kind of doping has helped to change the topology of the band structure, triggering an indirect to direct transition that is optically allowed. It also reduces the band gap significantly, bringing it well into the visible region. We also simulated the optical absorption spectra of these systems and found a comparable/higher absorption coefficient and solar efficiency with respect to the state of the art photovoltaic absorber material ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$. A number of materials ${\mathrm{Cs}}_2\left(B_{0.75}{\mathrm{Pb}}_{0.25}\right)\left(B_{0.75}^{{}^{\prime }}{\mathrm{Pb}}_{0.25}\right)X_6$ (for various combinations of $B,B^{{}^{\prime }}$, and $X)$ are found to be promising, some with better stability and solar efficiency than ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, but with much less toxicity. Experimental characterization of one of the materials, ${\mathrm{Cs}}_2\left({\mathrm{Ag}}_{0.75}{\mathrm{Pb}}_{0.25}\right)\left({\mathrm{Bi}}_{0.75}{\mathrm{Pb}}_{0.25}\right){\mathrm{Br}}_6$, is carried out. The measured properties such as band gap and chemical stability agree fairly well with the theoretical predictions. This material is shown to be even more stable than ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$, both under sufficient humidity $(\sim 55$%) and temperature $(T=338$ K), and hence has the potential to become a better candidate than the state of the art materials.

We performed a systematic analysis of the vibrational modes in nanoporous silicon for different values of porosity, separating them into extended modes (diffusons and propagons) and localized vibrations (locons). By calculating the density of states, the participation ratio, and the systems' dispersion curves, the spatial character of each mode as well as the effect of porosity on the thermal conductivity have been investigated. An increase of porosity is shown to promote the existence of increasingly localized modes on one side, and the progressive transformation of propagons to diffusons on the other. Finally, we provide evidence of the sizable contribution of locons to thermal transport found in large porosity samples and discuss the mechanism of energy transfer in terms of mode-mode autocorrelations and cross-correlations.

Quantum coherence and its dynamics in monolayer transition metal dichalcogenides (TMDs) are essential information to fully control valley pseudospin for valleytronics applications. Experimental understanding of coherence dephasing dynamics has been limited for excitons and largely unexplored for trions in monolayer TMDs. Here we use optical two-dimensional coherent spectroscopy to measure the trion coherence dephasing time in monolayer ${\mathrm{MoSe}}_2$ by analyzing the homogeneous linewidth. An intrinsic coherence time of 182 fs is extrapolated from the excitation density and temperature dependence measurement. The results show that trion-trion and trion-phonon interactions strongly affect the coherence dephasing time, while the intrinsic coherence time at zero excitation and zero temperature is primarily limited by the pure dephasing due to defect states. Our experiment also confirms optical two-dimensional coherent spectroscopy as a reliable technique for studying valley quantum dynamics in two-dimensional layered materials.

We present a computer simulation study of the phase behavior of colloidal hard cubic frames, i.e., particles with nonconvex cubic wireframe geometry interacting purely by excluded volume. Despite the propensity of cubic wireframe particles to form cubic phases akin to their convex counterparts, these particles exhibit unusual plastic fluctuations in which a random and dynamic fraction of particles rotate around their lattice positions in the crystal lattice while the remainder of the particles remains fully ordered. We argue that this unexpected effect stems from the nonconvex geometry of the particles in which the faces of a particle can be penetrated by the vertices of the nearest neighbors even at high number densities.

Analytical expressions describing ion-induced sputter or etch processes generally relate the sputter yield to the surface atomic binding energy $\left(U_{\mathrm{sb}}\right)$ for the target material. While straightforward to measure for the crystalline elemental solids, $U_{\mathrm{sb}}$ is more complicated to establish for amorphous and multielement materials due to composition-driven variations and incongruent sublimation. In this regard, we show that for amorphous multielement materials, the ion-driven yield can instead be better understood via a consideration of mechanical rigidity and network topology. We first demonstrate a direct relationship between $U_{\mathrm{sb}}$, bulk modulus, and ion sputter yield for the elements, and then subsequently prove our hypothesis for amorphous multielement compounds by demonstrating that the same relationships exist between the reactive ion etch (RIE) rate and nanoindentation Young's modulus for a series of $a\text{−}\mathrm{Si}{\mathrm{N}}_x$:H and $a\text{−}\mathrm{Si}{\mathrm{O}}_x{\mathrm{C}}_y$:H thin films. The impact of network topology is further revealed via application of the Phillips–Thorpe theory of topological constraints, which directly relates the Young's modulus to the mean atomic coordination $(\langle r\rangle )$ for an amorphous solid. The combined analysis allows the trends and plateaus in the RIE rate to be ultimately reinterpreted in terms of the atomic structure of the target material through a consideration of $\langle r\rangle$. These findings establish the important underlying role of mechanical rigidity and network topology in ion–solid interactions and provide additional considerations for the design and optimization of radiation-hard materials in nuclear and outer space environments.

${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG) is known for its long magnon diffusion length. Although it has the known lowest damping rate, an even longer diffusion distance is still highly desired since it may lead to a much more efficient information transmission and processing. While most of previous works focused on the generation and detection of magnons in YIG, here we demonstrate how to depress the damping rate during the diffusion of magnon. By selectively exciting the spin state transition of the Fe ions in YIG, we successfully increase magnon diffusion length by one order of magnitude, i.e., from the previous reported ∼10 μm up to ∼156 μm (for the sample prepared by liquid phase epitaxy) and ∼180 μm (for the sample prepared by pulsed laser deposition) at room temperature. The diffusion length, determined by nonlocal geometry, is ∼30 μm for the magnons induced by visible light and above 150 μm for the laser of 980 nm. In addition to thermal gradient, light excitation affects the electron configuration of the $\mathrm{F}{\mathrm{e}}^{3+}$ ion in YIG. Long-wavelength laser is more effective since it causes a transition of the $\mathrm{F}{\mathrm{e}}^{3+}$ ions in $\mathrm{Fe}{\mathrm{O}}_6$ octahedron from a high spin to a low spin state and thus causes a magnon softening which favors a long-distance diffusion. The present work paves the way toward an efficient tuning of magnon transport which is crucially important for magnon spintronics.

The beryllides $M{\mathrm{Be}}_{13}(M$ = rare earths and actinides) crystallize in a ${\mathrm{NaZn}}_{13}$-type cubic structure, which can be categorized as a cage-structured compound. In this study, powder x-ray diffraction measurements have been performed on ${\mathrm{LaBe}}_{13},{\mathrm{SmBe}}_{13}$, and ${\mathrm{UBe}}_{13}$ in the temperature range between 7 and 300 K in order to investigate their crystallographic characteristics systematically. They keep the ${\mathrm{NaZn}}_{13}$-type cubic structure down to the lowest temperature. We estimated their Debye temperature to be 600–750 K from analyses of the temperature dependence of a lattice parameter, being in good agreement with the values reported previously. Rietveld refinements on the obtained powder patterns revealed that the $M$ atom in the $8a$ site is located in an almost ideal snub cube formed by $24{\mathrm{Be}}^{\mathrm{II}}$ atoms in the $96i$ site, whose caged structure is unchanged even at the low temperatures. In addition, it is argued from the temperature variation of an isotropic mean-square displacement parameter that the $M{\mathrm{Be}}_{13}$ compounds commonly have a low-energy phonon mode, which can be described by a model assuming an Einstein oscillation of the $M$ atom with a characteristic temperature of $\sim 160$ K.

By using a technique of thermoreflectance that can precisely measure the thermal conductivity of thin films, we found that the thermal conductivity of ${\mathrm{SrVO}}_3\text{−}{\mathrm{SrTiO}}_3$ multilayer thin films normal to the surface was substantially reduced by decreasing the thickness of each layer. This indicates that a large intrinsic thermal resistance exists at the interface between ${\mathrm{SrVO}}_3$ and ${\mathrm{SrTiO}}_3$ in spite of the similar phononic properties for these two compounds.

The combination of topological properties and magnetic order can lead to new quantum states and exotic physical phenomena, such as the quantum anomalous Hall (QAH) effect. The size of the magnetic gap in the topological surface states, key for the robust observation of the QAH state, scales with the magnetic moment of the doped three-dimensional topological insulator (TI). The pioneering transition-metal doped $\left({\mathrm{Sb},\mathrm{Bi})}_2\right({\mathrm{Se},\mathrm{Te})}_3$ thin films only allow for the observation of the QAH effect up to some 100 mK, despite the much higher magnetic ordering temperatures. On the other hand, high magnetic moment materials, such as rare-earth-doped $\left({\mathrm{Sb},\mathrm{Bi})}_2\right({\mathrm{Se},\mathrm{Te})}_3$ thin films, show large moments but no long-range magnetic order. Proximity coupling and interfacial effects, multiplied in artificial heterostructures, allow for the engineering of the electronic and magnetic properties. Here, we show the successful growth of high-quality Dy:${\mathrm{Bi}}_2{\mathrm{Te}}_3$/Cr:${\mathrm{Sb}}_2{\mathrm{Te}}_3$ thin film heterostructures. Using x-ray magnetic spectroscopy we demonstrate that high transition temperature Cr:${\mathrm{Sb}}_2{\mathrm{Te}}_3$ can introduce long-range magnetic order in high-moment Dy:${\mathrm{Bi}}_2{\mathrm{Te}}_3$—up to a temperature of 17 K—in excellent agreement with first-principles calculations, which reveal the origin of the long-range magnetic order in a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. Engineered magnetic TI heterostructures may be an ideal materials platform for observing the QAH effect at liquid He temperatures and above.