We report x-ray diffraction and Raman scattering measurements in combination with density functional theory calculations that reveal the formation of beryllium polyhydride ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ by the laser heating of a Be-${\mathrm{H}}_2$ mixture to above 1700 K at pressures between 5 and 8 GPa. ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ crystallizes in a $P{6}_3/mmc$ structure and consists of the corner-sharing ${\mathrm{BeH}}_4$ tetrahedrons and the ${\mathrm{H}}_2$ molecules that occupy an interstitial site. ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ is stable at least to 14 GPa upon compression and stable down to 4 GPa at room temperature. Our ab initio calculations suggest that ${\mathrm{Be}}_4{\mathrm{H}}_8{\left({\mathrm{H}}_2\right)}_2$ is a metastable phase of the Be-H system.

Structure is the most basic and important property of crystalline solids; it determines directly or indirectly most materials characteristics. However, predicting crystal structure of solids remains a formidable and not fully solved problem. Standard theoretical tools for this task are computationally expensive and at times inaccurate. Here we present an alternative approach utilizing machine learning for crystal structure prediction. We developed a tool called Crystal Structure Prediction Network (CRYSPNet) that can predict the Bravais lattice, space group, and lattice parameters of an inorganic material based only on its chemical composition. CRYSPNet consists of a series of neural network models, using as inputs predictors aggregating the properties of the elements constituting the compound. It was trained and validated on more than 100 000 entries from the Inorganic Crystal Structure Database. The tool demonstrates robust predictive capability and outperforms alternative strategies by a large margin. It can be used both as an independent prediction engine and as a method to generate candidate structures for further computational and/or experimental validation.

The spin-mixing conductance (SMC) is a key quantity determining efficiency of spin transport across interfaces. Thus, knowledge of its precise value is required for accurate measurement of parameters quantifying numerous effects in spintronics, such as spin-orbit torque, spin Hall magnetoresistance, spin Hall effect, and spin pumping. However, the standard expression for SMC, provided by the scattering theory in terms of the reflection probability amplitudes, is inapplicable when strong spin-orbit coupling (SOC) is present directly at the interface. This is the precisely the case of topological-insulator/ferromagnet and heavy-metal/ferromagnet interfaces of great contemporary interest. We introduce an approach where first-principles Hamiltonian of these interfaces, obtained from noncollinear density functional theory (ncDFT) calculations, is combined with charge-conserving Floquet-nonequilibrium-Green-function formalism to compute directly the pumped spin current $I_{\mathrm{L}}^{S_z}$ into semi-infinite left lead of two-terminal heterostructures $\mathrm{Cu}/X/\mathrm{Co}/\mathrm{Cu}$ or $Y/\mathrm{Co}/\mathrm{Cu}$—where $X={\mathrm{Bi}}_2{\mathrm{Se}}_3$ and $Y=\text{Pt}$ or W—due to microwave-driven steadily precessing magnetization of the Co layer. This allows us to extract an effective SMC as a prefactor in $I_{\mathrm{L}}^{S_z}$ versus precession cone angle $\theta$ dependence, as long as it remains the same, $I^{S_z}\propto {sin}^2\theta$, as in the case where SOC is absent. By comparing calculations where SOC in switched off versus switched on in ncDFT calculations, we find that SOC consistently reduces the pumped spin current and therefore the effective SMC.

Co-depositions of immiscible alloy films at specific processing conditions have yielded hierarchical microstructures which consist of distinct features at multiple length scales, often agglomerates and concentration modulations on the submicrometer scale and fine nanoprecipitates in a matrix on the nanoscale. The present work examined a series of immiscible alloy systems: Cu-Mo, Cu-Ag, Cu-Fe, Cu-Ta, Mo-Ag, Cu-Mo-Ag, to determine the kinetic conditions favorable for hierarchical organization and the formation mechanism of such structures. Thin films of six immiscible systems were sputter co-deposited over a range of deposition rates from 0.12 to 2 nm/s and various temperatures from 400 to 800°C. The resulting microstructures indicate that hierarchical structures form with sufficient disparity in kinetic energy between the constituent atoms, one species being highly mobile ($A$) and the other relatively immobile ($B$). This condition arises typically at elevated deposition temperatures and reduced deposition rates but is also alloy dependent. The hierarchical structures form during deposition via phase separation and self-organization processes across the multiple length scales. The adatoms diffuse on the film surface with the highly mobile species swiftly agglomerating into $A$-rich domains within which $B$-rich nanoprecipitates form, often self-organizing into periodic arrays. The smallest $B$-rich nanoprecipitates in the $A$-rich domains are found to be coherent and in a metastable crystal structure ($B$ taking the structure of $A$), but coarser precipitates exhibit the equilibrium structure of the $B$ element. The $A$-rich domains are surrounded by a $B$-rich matrix that phase-separates into a concentration modulated structure. The observations are interpreted via a model incorporating material properties and process parameters.

Graphene intercalation is a novel way to control graphene's band structure and generate two-dimensional quantum materials with unusual spintronic and electronic properties. Despite its importance, information about the intercalation mechanism is lacking, especially the role of low density domain boundaries between regions of graphene of different thickness. With high resolution surface diffraction we have systematically studied Pb intercalation on epi-graphene grown on SiC, with domain boundaries between buffer and single layer graphene. By examining the evolution of different diffraction spots as a function of tempertature, the location of Pb and stability of the intercalated phases underneath were determined.

Heavy transition metal magnets with $J_{eff}=\frac{1}{2}$ electronic ground states have attracted recent interest due to their penchant for hosting new classes of quantum spin liquids and superconductors. Unfortunately, model systems with ideal $J_{eff}=\frac{1}{2}$ states are scarce due to the importance of noncubic local distortions in most candidate materials. In this work, we identify a family of iridium halide systems [i.e., ${\mathrm{K}}_2\mathrm{Ir}{\mathrm{Cl}}_6$, ${\mathrm{K}}_2\mathrm{Ir}{\mathrm{Br}}_6$, ${\left({\mathrm{NH}}_4\right)}_2\mathrm{Ir}{\mathrm{Cl}}_6$, and ${\mathrm{Na}}_2\mathrm{Ir}{\mathrm{Cl}}_6·6\left({\mathrm{H}}_2\mathrm{O}\right)]$ with ${\mathrm{Ir}}^{4+}$ electronic ground states exhibiting extremely small deviations from the ideal $J_{eff}=\frac{1}{2}$ limit. We also find ordered magnetic ground states for the three anhydrous systems, with single-crystal neutron diffraction on ${\mathrm{K}}_2\mathrm{Ir}{\mathrm{Br}}_6$ revealing type-I antiferromagnetism. This spin configuration is consistent with expectations for significant Kitaev exchange in a face-centered-cubic magnet. This work establishes that incorporating isolated $\mathrm{Ir}{X}_6$ octahedra in materials, where $X$ is a halogen ion with a low electronegativity, is an effective design principle for realizing unprecedented proximity to the pure $J_{eff}=\frac{1}{2}$ state. At the same time, we highlight undeniable deviations from this ideal state, even in clean materials with ideal $\mathrm{Ir}{X}_6$ octahedra as inferred from the global cubic crystal structures.

In this work, two organic-ion-intercalated $\mathrm{Sn}{\mathrm{Se}}_2$ superconductors, ${\left(\mathrm{TBA}\right)}_x\mathrm{Sn}{\mathrm{Se}}_2$ $(T_c\sim 6.4\mathrm{K})$ and ${\left(\mathrm{CTA}\right)}_x\mathrm{Sn}{\mathrm{Se}}_2$ $(T_c\sim 7.1\mathrm{K})$, are synthesized by an electrochemical intercalation method. Via the intercalation of organic ions, the interlayer distance is dramatically enlarged from 6.12 Å of pristine $\mathrm{Sn}{\mathrm{Se}}_2$ to 18.62 and 14.74 Å for ${\left(\mathrm{TBA}\right)}_x\mathrm{Sn}{\mathrm{Se}}_2$ and ${\left(\mathrm{CTA}\right)}_x\mathrm{Sn}{\mathrm{Se}}_2$, respectively. Bulk magnetic susceptibility measurements suggest that both superconductors exhibit a strong anisotropic superconducting shielding effect below $T_c$. Further measurements of resistivity, I-V characteristic curves, and magnetoresistance reveal a quasi-two-dimensional (2D) superconductivity in ${\left(\mathrm{CTA}\right)}_x\mathrm{Sn}{\mathrm{Se}}_2$. The present work suggests that the organic-ion-intercalation method can induce quasi-2D superconductivity in $\mathrm{Sn}{\mathrm{Se}}_2$ layers and provides a simple and practical strategy to explore 2D superconductivity as well as other 2D phenomena in layered materials.

A numerical solver based on two-dimensional Poisson and drift-diffusion equations with Gaussian-shaped density of states, the Poole-Frenkel field mobility model, and the Ising model are proposed to analyze carrier transport in the host-guest systems based on organic semiconductors. To investigate the performance of this solver, a series of the electron-only devices and hole-only devices are fabricated with different dopant concentrations. The space-charge-limited current model is applied to the simulation results to calculate the mobility. In addition, experimental results for different host-guest systems are compared to prove the accuracy of the solver. The solver is able to explain the physical mechanisms in different situations. At lower dopant concentrations, the guest states act as traps. Carriers are localized in these guest states, and the carrier mobility decreases dramatically. At the higher dopant concentrations, regions of guest material are able to connect with each other, and carriers have a probability to hop through the guest states. The mobility characteristics gradually approach those of the pure guest material.

We combined scanning tunneling microscopy and x-ray photoelectron spectroscopy experiments with density-functional theory calculations to study dispersed tungsta clusters on anatase $\mathrm{Ti}{\mathrm{O}}_2\left(101\right)$. Following two different preparation methods, we found that monomeric $\mathrm{W}{\mathrm{O}}_3$ species are the most stable configuration rather than $\mathrm{W}{\mathrm{O}}_3$ trimers, ${\left(\mathrm{W}{\mathrm{O}}_3\right)}_3$. The $\mathrm{W}{\mathrm{O}}_3$ monomers form tetrahedral $\mathrm{W}{\mathrm{O}}_4$ structures on anatase $\mathrm{Ti}{\mathrm{O}}_2\left(101\right)$, with one W–O bond and two W–O–Ti linkages per $\mathrm{W}{\mathrm{O}}_3$ monomer. Locally, the $\mathrm{W}{\mathrm{O}}_3$ monomers form well-ordered (2 × 1) structures. The discovered geometric structure of $\mathrm{W}{\mathrm{O}}_3$ on anatase $\mathrm{Ti}{\mathrm{O}}_2\left(101\right)$ opens up numerous opportunities for fundamental studies addressing tungsta and accurate structure-activity studies of $\mathrm{W}{\mathrm{O}}_3/\mathrm{Ti}{\mathrm{O}}_2$ model catalysts.

Among semiconducting materials transparent semiconducting oxides have gained increasing attention within the last decade. While most of these oxides can be only doped $n$-type with room-temperature electron mobilities on the order of $100{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, $p$-type oxides are needed for the realization of $pn$-junction devices but typically suffer from excessively low ($\ll 1$ ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$) hole mobilities. Tin monoxide (SnO) is one of the few $p$-type oxides with higher hole mobility, yet is currently lacking a well-established understanding of its hole transport properties. Moreover, growth of SnO is complicated by its metastability with respect to $\mathrm{Sn}{\mathrm{O}}_2$ and Sn, requiring epitaxy for the realization of single crystalline material typically required for high-end applications. Here, we give a comprehensive account on the epitaxial growth of SnO, its (meta)stability, and its thermoelectric transport properties in the context of the present literature. Textured and single-crystalline, unintentionally doped $p$-type SnO(001) films are grown on ${\mathrm{Al}}_2{\mathrm{O}}_3$(00.1) and ${\mathrm{Y}}_2{\mathrm{O}}_3$-stabilized $\mathrm{Zr}{\mathrm{O}}_2\left(001\right)$, respectively, by plasma-assisted molecular beam epitaxy, and the epitaxial relations are determined. The metastability of this semiconducting oxide is addressed theoretically through an equilibrium phase diagram. Experimentally, the related SnO growth window is rapidly determined by an in situ growth kinetics study as a function of Sn-to-O-plasma flux ratio and growth temperature. The presence of secondary Sn and $\mathrm{Sn}{\mathrm{O}}_x(1<x\le 2)$ phases is comprehensively studied by x-ray diffraction, Raman spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy, indicating the presence of ${\mathrm{Sn}}_3{\mathrm{O}}_4$ or Sn as major secondary phases, as well as a fully oxidized $\mathrm{Sn}{\mathrm{O}}_2$ film surface. The hole transport properties, Seebeck coefficient, and density-of-states effective mass are determined and critically discussed in the context of the present literature on SnO, considering its strongly anisotropic effective hole mass: Hall measurements of our films reveal room-temperature hole concentrations and mobilities in the range of $2\times {10}^{18}$ to ${10}^{19}$ ${\mathrm{cm}}^{-3}$ and 1.0 to 6.0 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively, with consistently higher mobility in the single-crystalline films. Temperature-dependent Hall measurements of the single-crystalline films indicate nondegenerate band transport by free holes (rather than hopping transport) with dominant polar optical phonon scattering at room temperature. Taking into account the impact of acceptor band formation and the apparent activation of the hole concentration by 40–53 meV, we assign tin vacancies rather than their complexes with hydrogen as the unintentional acceptor. The room-temperature Seebeck coefficient in our films confirms $p$-type conductivity by band transport. Its combination with the hole concentration and model scattering parameters allows us to experimentally estimate the density of states effective hole mass to be in the range of 1 to 8 times the free electron mass.

We report the phase diagram of infinite layer ${\mathrm{Pr}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ thin films synthesized via topotactic reduction from the perovskite precursor phase using ${\mathrm{CaH}}_2$. Based on the electrical transport properties, we find a doping-dependent superconducting dome extending between $x=0.12$ and 0.28 with a maximum superconducting transition temperature $T_{\mathrm{c}}$ of 14 K at $x=0.18$, bounded by weakly insulating behavior on both sides. In contrast to the narrower dome observed in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$, a local $T_{\mathrm{c}}$ suppression near $x=0.2$ was not observed for the ${\mathrm{Pr}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$ system. Normal-state Hall effect measurements indicate mixed carrier contributions of both electrons and holes and show a sign change in the Hall coefficient as functions of temperature and $x$, quite similar to that in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x{\mathrm{NiO}}_2$. Also similar is the observation of a minimum in the normal-state resistivity associated with the superconducting compositions. These findings indicate an infinite layer nickelate phase diagram that is relatively insensitive to the rare-earth element but suggest that disorder arising from the variations of the ionic radii on the rare-earth site affects the superconducting dome.

We report preparation, crystal structure, and physical properties of a quasi-one-dimensional Cr-based arsenide hydride ${\mathrm{K}}_{1-\delta }{\mathrm{Cr}}_3{\mathrm{As}}_3{\mathrm{H}}_x$. Through an electrolysis using essentially nonsuperconducting samples as the cathode, additional hydrogen atoms can be successfully intercalated up to $x=0.45$ and, consequently, the in-plane and interplane $\mathrm{Cr}\text{–}\mathrm{Cr}$ bond distances in the chains of face-sharing Cr octahedra increase by 3.7% and 1.5%, respectively. The electrochemically hydrogenized samples show a broad superconducting transition at $T_{\mathrm{c}}=5.8$ K, a record in the K-Cr-As-H system, with nearly full magnetic shielding at 1.8 K. The electronic specific-heat coefficient extracted from the specific-heat measurement is as high as ${\gamma }_{\mathrm{n}}=47 \mathrm{mJ}$ ${\mathrm{K}}^{-2} \mathrm{mol}$ ${\mathrm{Cr}}^{-1}$, suggesting a stronger electron correlation that is likely to be associated with the expansions of $\mathrm{Cr}\text{–}\mathrm{Cr}$ bonds. Meanwhile, the dimensionless specific-heat jump $\mathrm{\Delta }C/\left({\gamma }_{\mathrm{n}}{T}_{\mathrm{c}}\right)$ is only 0.30, about 20% of the expected value in the BCS weak-coupling scenario. Furthermore, the normal-state magnetism is characterized by Curie-Weiss paramagnetism with an enhanced effective localized moment of 1.33 ${\mu }_{\mathrm{B}}/\mathrm{Cr}$, suggesting that a nonsuperconducting phase with localized spins dominates. The ${}^1\mathrm{H}$ nuclear magnetic resonance measurement reveals two different spin-lattice relaxations, corresponding to superconducting and localized-spin phases, respectively. All the results point to phase separation with minority superconducting phase and majority nonsuperconducting phase in the quasi-one-dimensional ${\mathrm{K}}_{1-\delta }{\mathrm{Cr}}_3{\mathrm{As}}_3{\mathrm{H}}_x$ system.

Pulsed laser deposition was used to synthesize vertically aligned nanocomposites, consisting of ultrathin CoPt nanocolumns embedded in a single crystal ${\mathrm{SrTiO}}_3$ thin film. Combining high-resolution transmission electron microscopy, x-ray diffraction, x-ray absorption spectroscopy, and molecular dynamics simulations, we provide an in-depth analysis of the structural and vibrational properties of the CoPt phase over a wide range of concentrations. Our findings show that the CoPt nanoalloys are characterized by a high degree of structural disorder and complete absence of chemical ordering, even after additional sample annealing $(T\simeq 670{}^{\circ }\mathrm{C})$. For a Co concentration exceeding $c\left(\mathrm{Co}\right)\simeq 0.55$, pole figure measurements unravel a transition from a highly textured short-range disordered fcc phase to a disordered hcp phase with $c$-axis oriented out of plane, which is at odds with bulk predictions. These findings, further confirmed by magnetometry measurements, illustrate the profound impact of vertical epitaxy on the structural properties of bimetallic CoPt alloys.

Solid-state diffusion experiments were used to probe the behavior of cation vacancies in the perovskite oxide ${\mathrm{SrTiO}}_3$. Two types of nominally undoped (effectively acceptor-doped) ${\mathrm{SrTiO}}_3$ systems were studied: (1) single crystals and (2) epitaxial thin films with different Sr/Ti stoichiometries produced by pulsed laser deposition. As diffusion sources, thin films of the perovskite oxide ${\mathrm{BaZrO}}_3$ were employed, and diffusion anneals were carried out in air at $1323\le T/\mathrm{K}\le 1523$ for single crystals and at $1073\le T/\mathrm{K}\le 1223$ for thin films. Sample analysis by means of time-of flight secondary ion mass spectrometry (ToF-SIMS) yielded diffusion coefficients of Ba and Zr in ${\mathrm{SrTiO}}_3(D_{\mathrm{Ba}}$ and $D_{\mathrm{Zr}})$. Diffusion profiles in single-crystal samples showed the expected error-function form and yielded $D_{\mathrm{Ba}}\approx D_{\mathrm{Zr}}$ at each temperature, and hence, activation enthalpies of diffusion that are approximately the same, at $\left(3.0\pm 0.4\right)$ eV and $\left(2.8\pm 0.4\right)$ eV. Diffusion profiles in the thin-film samples were unexpectedly complex, showing multiple error-function features. They also yielded $D_{\mathrm{Ba}}\approx D_{\mathrm{Zr}}$ at each temperature, however, but no clear trend was found as a function of Sr/Ti ratio. Comparing results for the two systems, we conclude that the concentration of cation vacancies is orders of magnitude higher in our thin-film samples than in the single crystals. Our results also provide experimental evidence that oxygen vacancies, ${\mathrm{v}}_{\mathrm{O}}^{••}$, can decrease the activation enthalpy of strontium-vacancy migration by forming ${\left({\mathrm{v}}_{\mathrm{O}}{\mathrm{v}}_{\mathrm{Sr}}\right)}^{\times }$ defect associates, and we derive an analytical model for the cation diffusivity as a function of temperature and defect concentrations.

Cadmium (Cd) and its compounds are a family of materials exhibiting diverse physical and chemical properties that find wide-ranging applications in many science and technology fields. In all known Cd compounds, Cd ions behave like a typical $d$-block element, donating electrons and adopting positive $(+2$ or $+1)$ charge states. To explore the possibility of forming compounds with Cd in negative charge states, would greatly expand this material family and enrich understanding of the associated physical and chemical properties and the underlying mechanisms. Here, we report on an exemplary case study via a systematic search and examination of Li-Cd binary compounds over a wide range of stoichiometry. We have employed an unbiased crystal structure search method in conjunction with first-principles energetic calculations to predict viable crystal structures under ambient and high-pressure conditions. Our investigations have identified a series of stable ${\mathrm{Li}}_n\mathrm{Cd}$ compounds with an unusually wide range $(n=1$–6) of stoichiometry at both ambient and high pressures. Strikingly, our results reveal that Cd attracts electrons from Li in forming a rich variety of bonding configurations with the Cd ions adopting a range of negative charge states, producing highly tunable structural and electronic properties. Moreover, calculated elastic parameters show that the mechanical properties of LiCd compound can be enhanced considerably over those of Li and Cd metals and other Li-Cd binary compounds. We analyze the charge transfer from Li to Cd and the resulting electron distribution patterns and find a significant and variable amount of partial occupation of the Cd-$5p$ states in this traditional $d$-block element. The present findings provide insights into fundamental knowledge and practical processes in valence state control for tuning transitions from $d$- to $p$-block behaviors via a proper choice of compound formation and application of high pressure. The results expand conventional definition and understanding of charge states in the compound formation and enrich structure-property relations that have broad implications for Li-rich binary compounds.

Although nanoscale multiferroics with coupled ferroelectricity and magnetism are of great technological and fundamental importance, the ferroic orders are inevitably destroyed in atomic-scale dimensions due to the intrinsic size effect or depolarization field. Here, we propose a strategy for atomic-size multiferroics by engineering the otherwise detrimental point defects in nonmagnetic paraelectric $\mathrm{SrTi}{\mathrm{O}}_3$. Systematic exploration using first-principles and molecular dynamics simulations demonstrates that the Ti antisite $\left({\mathrm{Ti}}_{\mathrm{Sr}}\right)$ defects in $\mathrm{SrTi}{\mathrm{O}}_3$ have a tendency toward clustering with compact closure, which contributes to complex electrical polarization due to the large off-centering displacements of ${\mathrm{Ti}}_{\mathrm{Sr}}$ defects and the resulting separation between negative and positive charges. The ${\mathrm{Ti}}_{\mathrm{Sr}}$ defects also give rise to highly localized magnetic moments attributed to their unpaired spin-polarized electrons. The defect clustering and the associated multiferroic properties thus lead to atomic-size multiferroics with unusual ferroic orders.

Realizing a quantum spin liquid (QSL) ground state in a real material is a leading issue in condensed matter physics research. In this pursuit, it is crucial to fully characterize the structure and influence of defects, as these can significantly affect the fragile QSL physics. Here, we perform a variety of cutting-edge synchrotron x-ray scattering and spectroscopy techniques, and we advance new methodologies for site-specific diffraction and $L$-edge Zn absorption spectroscopy. The experimental results along with our first-principles calculations address outstanding questions about the local and long-range structures of the two leading kagome QSL candidates, Zn-substituted barlowite ${\mathrm{Cu}}_3{\mathrm{Zn}}_x{\mathrm{Cu}}_{1-x}{\left(\mathrm{OH}\right)}_6\mathrm{FBr}$ and herbertsmithite ${\mathrm{Cu}}_3\mathrm{Zn}{\left(\mathrm{OH}\right)}_6{\mathrm{Cl}}_2$. On all length scales probed, there is no evidence that Zn substitutes onto the kagome layers, thereby preserving the QSL physics of the kagome lattice. Our calculations show that antisite disorder is not energetically favorable and is even less favorable in Zn-barlowite compared to herbertsmithite. Site-specific x-ray diffraction measurements of Zn-barlowite reveal that ${\mathrm{Cu}}^{2+}$ and ${\mathrm{Zn}}^{2+}$ selectively occupy distinct interlayer sites, in contrast to herbertsmithite. Using the first measured Zn $L$-edge inelastic x-ray absorption spectra combined with calculations, we discover a systematic correlation between the loss of inversion symmetry from pseudo-octahedral (herbertsmithite) to trigonal prismatic coordination (Zn-barlowite) with the emergence of a new peak. Overall, our measurements suggest that Zn-barlowite has structural advantages over herbertsmithite that make its magnetic properties closer to an ideal QSL candidate: its kagome layers are highly resistant to nonmagnetic defects while the interlayers can accommodate a higher amount of Zn substitution.

We use a picosecond ultrasonic technique to evaluate the photoelastic parameters at the wavelength of 1028 nm in epitaxial vanadium dioxide $\left({\mathrm{VO}}_2\right)$ nanolayers grown on c-cut sapphire substrates. In the experiments, we monitor the picosecond evolution of the reflectivity of ${\mathrm{VO}}_2$ in insulating and metallic phases under the impact of a picosecond longitudinal strain pulse injected into the nanolayer from the side of the substrate. We show that in a 145-nm-thick granular nanolayer, the temporal features of the reflectivity are clearly dependent on the phase state of ${\mathrm{VO}}_2$, showing the change of the photoelastic parameters upon the insulator-metal transition. Analytical consideration and numerical simulations of the optical response to the picosecond strain pulse show that the temporal evolution of the reflectivity strongly depends on the complex photoelastic parameter. The analysis enables us to obtain the values of the photoelastic parameters in the studied nanolayer in both insulating and metallic phases. We find that for a 145-nm film of ${\mathrm{VO}}_2$ in an insulating state the imaginary part of the photoelastic constant is negligible. This means that in the insulating phase the strain does not affect the optical absorption of ${\mathrm{VO}}_2$. In the metallic phase, the photoelastic parameter of ${\mathrm{VO}}_2$ is found to be similar to that typical for metals with positive real and negative imaginary parts. We further show that the optical response to the strain pulse in the layer consisting of disconnected ${\mathrm{VO}}_2$ nanohillocks with an average height of 70 nm is governed by their morphology and is different from what is predicted in the plane ${\mathrm{VO}}_2$ films.

A quantitative understanding of the microscopic constraints, which underlie a well relaxed glassy structure, is the key to developing a microscopic theory of structural evolution and plasticity for the amorphous metallic solid. Here we demonstrate the applicability of one such theory of local bonding constraints developed by D. R. Nelson [Phys. Rev. B 28, 5515 (1983)] for a model binary Lennard-Jones glass structure. By introducing a modified radical Voronoi tessellation, which removes some ambiguity in how nearest-neighbor bonds are enumerated, it is found, that a large proportion $(>95\%)$ of local atomic environments follow the connectivity rules of the SU(2) topology resulting in a dense network of disclination lines characterizing the defect bonds. Furthermore, it is numerically shown that a low-energy glass structure corresponds to a reduced level of bond-length frustration and thus a minimally defected bond-defect network. It is then demonstrated that such a defect network provides a framework to analyze thermally activated structural excitations, revealing those high-energy/low-density/elastically soft regions not following the connectivity constraints are more likely to undergo structural rearrangement that often ends with the creation of new SU(2) local topology content. The work provides a new analysis tool to study the connectivity of developing structural motives characteristic of isotropic undercooled liquids, their transition to a glass, and subsequent glassy structural relaxation.