Significant insights for critical current density ($J_{\mathrm{c}}$) improvement in Bi-2212 superconductor wires can be obtained by an accurate analysis of the structural and microstructural properties evolving during the so-called partial-melt process, a heat treatment needed to improve grain connectivity and therefore gain high $J_{\mathrm{c}}$. Here, we report an in situ analysis by means of synchrotron x-ray and neutron diffraction performed, for the first time, during the heat treatment carried out with the very same temperature profile and reacting oxygen atmosphere in which the Bi-2212 wires are usually treated for practical applications. The obtained results show the thermal evolution of the Bi-2212 structure, focusing in particular on texturing and secondary phases formation. The role of the oxygen is discussed as well. Hence, the present investigation marks a significant advance for the comprehension of the phenomena involved in the wire fabrication process and provides useful insights for the process optimization as well.

In this study, we perform a comparative theoretical study on the thermoelectric performance of materials with $\mathrm{Cu}C{h}_4$ ($Ch=$ S, Se) tetrahedra, including famous thermoelectric materials BiCuSeO and tetrahedrite ${\mathrm{Cu}}_{12}{\mathrm{Sb}}_4{\mathrm{S}}_{13}$, by means of first-principles calculations. By comparing these electronic band structures, we find that many of these materials possess a Cu-$t_{2g}$ band structure consisting of quasi-one-dimensional band dispersions and the isotropic (two-dimensional for layered compounds) band dispersion near the valence-band edge. Therefore, the key factors for the thermoelectric performance are the anisotropy of the former band dispersion and the degeneracy of these two kinds of band dispersions. We also find that a large extension of the chalcogen orbitals often improves their thermoelectric performance by improving these two factors or by going beyond such a basic band structure through a large alternation of its shape. Such a large extension of the chalcogen orbitals might partially originate from the anisotropic Cu-$Ch$ bond geometry of a tetrahedron. Our study reveals interesting similarities and differences of materials with $\mathrm{Cu}C{h}_4$, which provides important knowledge for a future search of high-performance thermoelectric materials.

The crystalline strain properties of nanoparticles have broad implications in a number of emerging fields, including quantum and biological sensing in which heterogeneous internal strain fields are detrimental to performance. Here we used synchrotron-based Bragg coherent x-ray diffraction imaging (BCDI) to measure three-dimensional lattice strain fields within individual 3C-SiC nanoparticles, a candidate host material for quantum sensing, as a function of temperature during and after annealing up to ${900}^{\circ }\mathrm{C}$. We observed pronounced homogenization of the initial strain field at temperatures above ${500}^{\circ }\mathrm{C}$, and we find that the surface layers and central volumes of the nanoparticles reduce strain at similar rates, suggesting a uniform healing mechanism. Thus, we attribute the observed strain homogenization to activation of mobile point defects that annihilate and improve the overall quality of the crystal lattice. This work also establishes the feasibility of performing BCDI at high temperatures (up to ${900}^{\circ }\mathrm{C}$) to map structural hystereses relevant to the processing of quantum nanomaterials.

We set up an evolutionary algorithm combined with density functional tight-binding calculations to investigate hydrogen adsorption on flat graphene and graphene monolayers curved over substrate steps. During the evolution, candidates for the new generations are created by adsorption of an additional hydrogen atom to the stable configurations of the previous generation, where a mutation mechanism is also incorporated. Afterwards a two-stage selection procedure is employed. Selected candidates act as the parents of the next generation. The evolutionary algorithm predicts formation of lines of hydrogen atoms on flat graphene. In curved graphene, the evolution follows a similar path except for a new mechanism, which aligns hydrogen atoms on the line of minimum curvature. The mechanism is due to the increased chemical reactivity of graphene along the minimum radius of curvature line (MRCL) and to ${\mathrm{sp}}^3$ bond angles being commensurate with the kinked geometry of hydrogenated graphene at the substrate edge. As a result, the reaction barrier is reduced considerably along the MRCL and hydrogenation continues like a mechanical chain reaction. This growth mechanism enables lines of hydrogen atoms along the MRCL, which has the potential to overcome substrate or rippling effects and could make it possible to define edges or nanoribbons without actually cutting the material.

We report the synthesis of $\mathrm{SrMn}{\mathrm{O}}_{3\text{−}\delta }{\mathrm{F}}_{\gamma }$ perovskite oxyfluoride thin films using a vapor transport method to fluorinate as-grown $\mathrm{SrMn}{\mathrm{O}}_{2.5}$ epitaxial thin films. The influence of the fluoropolymer, which acts as a fluorine vapor source, was investigated by utilizing polyvinyl fluoride (PVF), polyvinylidene difluoride (PVDF), and polytetrafluoroethylene (PTFE) in the reaction. The same process was carried out with polyethylene to isolate the role of carbon in the vapor transport process. The F distribution was probed by x-ray photoemission spectroscopy, which confirmed the incorporation of F into the films and revealed higher F concentrations in films exposed to PVF and PVDF compared to PTFE. The $c$-axis parameter expands after fluorination, a result consistent with density functional theory calculations that attribute the volume expansion to elongated Mn-F bonds compared to shorter Mn-O bonds. Using x-ray absorption spectroscopy, we show that the fluorination process reduces the nominal Mn oxidation state suggesting that F substitutes on O sites in the lattice as opposed to filling anion vacancy sites, a finding further supported by calculated formation energies of different F site occupancies. These results provide insights into topochemical fluorination of perovskite oxides, which should enable future synthesis and design efforts focused on oxyfluoride heterostructures.

Ti-Ta-X (X = Al, Sn, Zr) compounds are emerging candidates as high-temperature shape memory alloys (HTSMAs). The stability of the one-way shape memory effect (1WE), the exploitable pseudoelastic (PE) strain intervals, as well as the transformation temperature in these alloys depend strongly on composition, resulting in a trade-off between a stable shape memory effect and a high transformation temperature. In this work, experimental measurements and first-principles calculations are combined to rationalize the effect of alloying a third component to Ti-Ta–based HTSMAs. Most notably, an increase in the transformation temperature with increasing Al content is detected experimentally in Ti-Ta-Al for low Ta concentrations, in contrast to the generally observed dependence of the transformation temperature on composition in Ti-Ta-X. This inversion of trend is confirmed by the ab initio calculations. Furthermore, a simple analytical model based on the ab initio data is derived. The model can not only explain the unusual composition dependence of the transformation temperature in Ti-Ta-Al but also provide a fast and elegant tool for a qualitative evaluation of other ternary systems. This is exemplified by predicting the trend of the transformation temperature of Ti-Ta-Sn and Ti-Ta-Zr alloys, yielding a remarkable agreement with available experimental data.

The magnetic properties and structural stability of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}X$ ($X=\mathrm{B}$, C, N, O, F) are theoretically studied by first-principles calculations focusing on the role of $X$. We find that B reduces the magnetic moment (per formula unit) and magnetization (per volume) in ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$. The crystal-field parameter $A_2^0\langle r^2\rangle$ of Nd is not enhanced either, suggesting that B has minor roles in the uniaxial magnetocrystalline anisotropy of Nd. These findings are in contrast to the long-held belief that B works positively for the magnetic properties of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$. As $X$ changes from B to C, N, O, and F, both the magnetic properties and stability vary significantly. The formation energies of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}X$ and $\alpha$-Fe relative to that of ${\mathrm{Nd}}_2{\mathrm{Fe}}_{17}X$ are negative for $X=\mathrm{B}$ and C, whereas they are positive when $X$ = N, O, and F. This indicates that B plays an important role in stabilizing the ${\mathrm{Nd}}_2{\mathrm{Fe}}_{14}\mathrm{B}$ phase.

The dielectric response of ceramics of co-doped rutile $\mathrm{T}{\mathrm{i}}_{1\text{−}x}{\left(\mathrm{N}{\mathrm{b}}_{0.5}\mathrm{I}{\mathrm{n}}_{0.5}\right)}_x{\mathrm{O}}_2$ has been measured via a combination of impedance, high-frequency coaxial, terahertz transmission, and infrared reflectivity spectroscopies spanning 15 decades of frequency between 0.1 Hz and 240 THz. It is argued that the colossal dielectric permittivity reported by Hu et al. [Hu et al., Nat. Mater. 12, 821 (2013)] has the same explanation as in the original colossal permittivity material $\mathrm{CaC}{\mathrm{u}}_3\mathrm{Ti}{\mathrm{O}}_4{\mathrm{O}}_{12}$ (CCTO), namely, a combination of the internal barrier layer capacitor (IBLC) and the surface barrier layer capacitor (SBLC) effects. The IBLC effect is caused by a microstructure consisting of insulating grain boundaries (thickness ∼1 nm and conductivity of $\sim {10}^{-6}\mathrm{S}/\mathrm{cm})$ surrounding interior regions of bulk conductivity approximately equal to ${10}^{-1}\mathrm{S}/\mathrm{cm}$. The SBLC effect is the result of a depletion layer adjacent to the contacts approximately 100 nm thick with conductivity of $5\times {10}^{-7}\mathrm{S}/\mathrm{cm}$. The SBLC and IBLC effects give rise to dielectric relaxations in low-frequency and radiofrequency regions, respectively. The temperature dependence of both relaxation rates measured down to 20 K is thermally activated. No separate absorption process has been observed that could be linked to giant defect dipoles. Infrared spectroscopy has revealed four weak defect-related modes at frequencies close to the Raman mode frequencies of rutile, activated in the infrared through a symmetry-breaking process. Co-doping produces a significant loss in the partial spectral weight between 75 and $350\mathrm{c}{\mathrm{m}}^{-1}$ associated with the lowest transverse optical mode, which softens on cooling. The loss in its spectral weight corresponds to the gain in near-infrared spectral weight, previously assigned to small polaron absorption.

We study the optical properties of the Ruddlesden-Popper series of iridates ${\mathrm{Sr}}_{n+1}{\mathrm{Ir}}_n{\mathrm{O}}_{3n+1}(n=1$, 2, and $\infty )$ by solving the Bethe-Salpeter equation (BSE), where the quasiparticle (QP) energies and screened interactions $W$ are obtained by the $GW$ approximation including spin-orbit coupling. The computed optical conductivity spectra show strong excitonic effects and reproduce very well the experimentally observed double-peak structure, in particular for the spin-orbital Mott insulators ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$. However, $GW$ does not account well for the correlated metallic state of ${\mathrm{SrIrO}}_3$ owing to a much too small band renormalization, and this affects the overall quality of the optical conductivity. Our analysis describes well the progressive redshift of the main optical peaks as a function of dimensionality $\left(n\right)$, which is correlated with the gradual decrease of the electronic correlation (quantified by the constrained random phase approximation) towards the metallic $n=\infty$ limit. We have also assessed the quality of a computationally cheaper BSE approach that is based on a model dielectric function and conducted on top of DFT+$U$ one-electron energies. Unfortunately, this model BSE approach does not accurately reproduce the outcome of the full $GW$+BSE method and leads to larger deviations to the measured spectra.

We determine the tip growth velocities of equiaxed dendrites in an Al-24 at. % Ge alloy using in situ x-ray radiography. The tip growth velocity is a main characteristic of dendritic growth and allows for an accurate comparison against theories and simulations describing the growth of a single dendrite tip or an array of dendrites. The disk-shaped samples are 200 or 350 $\mu \mathrm{m}$ thin and are positioned horizontally in a near-isothermal furnace to suppress buoyancy and gravity-driven melt flows. We obtain a large data set of free dendritic tip growth velocities of Al dendrites over one order of magnitude (4–140 $\mu \mathrm{m}{\mathrm{s}}^{-1}$) and over a decade of global undercooling (1–10 K). No indications for a significant deviation from three-dimensional growth kinetics due to the sample confinement are found.

Two-dimensional $\beta \text{−}{\mathrm{MoCl}}_4$ is an attractive material from the perspectives of magnetism in $4d$ transition metal compounds, geometrically frustrated lattices, and magnetic van der Waals layered materials, but the magnetism in this compound has not been particularly well studied to date. Here the magnetic properties and crystal structure of ${\mathrm{MoCl}}_4$ are revisited, and results of ac and dc magnetic measurements and single crystal x-ray diffraction are reported. Crystals grow as well-formed and easily cleaved hexagonal plates that are unstable in air. The revised structural model comprises ${\mathrm{CdCl}}_2$-type layers with 50% Mo vacancies distributed over the sites of the triangular cation net. Interestingly, a structural ambiguity regarding the vacancy distribution is identified in the analysis of the diffraction data. The orbital moment is not expected to be quenched in this $4d^2$ compound. Accordingly, magnetization measurements indicate an effective moment that is about 20% lower than the spin-only value. The magnetic data reveal an anomaly near 5 K, below which a divergence of field-cooled and zero-field-cooled dc magnetization, a slow relaxation of thermoremanent magnetization, and enhanced frequency dependence of ac magnetization are observed. Thus, $\beta \text{−}{\mathrm{MoCl}}_4$ represents an uncommon example of a cleavable spin-glass system.

We studied hollandite titanates ${\mathrm{Ba}}_x{\mathrm{Ti}}_8{\mathrm{O}}_{16+\delta }$, which have double chains of edge-sharing ${\mathrm{TiO}}_6$ octahedra with various numbers of $d$ electrons per Ti $n=(x-\delta )/4$. Anomalies at $\sim 200\mathrm{K}$ were observed in the temperature dependence of various quantities over a wide range of $n$, which were caused by the charge ordering of the $d$ electrons. It was found that charge ordering is stable when $d$ electrons exist at every fifth site along the Ti chain. A structural phase transition from tetragonal to monoclinic occurs as a secondary effect of the charge ordering. When $n$ is close to 0.4, the anomalies at $\sim 200\mathrm{K}$ disappear, and the crystal structure remains monoclinic from 300 K down to the lowest temperatures. It was also found that this series of compounds exhibits negative Seebeck coefficients with large absolute values at room temperature and that the figure of merit for thermoelectric materials ($ZT$) was maximized at $n\sim 0.22$ (0.05 at room temperature).

We present a comprehensive study of the critical current densities and the superconducting vortex phase diagram in the stoichiometric superconductor ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ which has a critical temperature of $\sim 35\mathrm{K}$. We performed detailed magnetization measurements both of high quality single crystals for different orientations in an applied magnetic field up to 16 T and for a powder sample. We find an extremely large critical current density, $J_c$, up to ${10}^8\mathrm{A}/{\mathrm{cm}}^2$ for single crystals when $H\parallel \left(ab\right)$ at $5\mathrm{K}$, which remains robust in fields up to $16\mathrm{T}$, being the largest of any other iron-based superconductor. The critical current density is reduced by a factor 10 in single crystals when $H\parallel c$ at 5 K and significantly suppressed by the presence of grain boundaries in the powder sample. We also observe the presence of the fishtail effect in the magnetic hysteresis loops of single crystals when $H\parallel c$. The flux pinning force density and the pinning parameters suggest that the large critical current could be linked to the existence of point core and surface pinning. Based on the vortex phase diagram and the large critical current densities, ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ is now established as a potential iron-based superconductor candidate for practical applications.

Hexagonal boron nitride (hBN) is an essential component in van der Waals heterostructures as it provides high-quality and weakly interacting interfaces that preserve the electronic properties of adjacent materials. While exfoliated flakes of hBN have been extensively studied using electron transport and optical probes, detailed experimental measurements of the energy- and momentum-dependent electronic excitation spectrum are lacking. Here, we directly determine the full valence-band (VB) electronic structure of micrometer-sized exfoliated flakes of hBN using angle-resolved photoemission spectroscopy with micrometer spatial resolution. We identify the $\pi$- and $\sigma$-band dispersions, the hBN stacking order, and determine a total VB bandwidth of 19.4 eV. We compare these results with electronic structure data for epitaxial hBN on graphene on silicon carbide grown in situ using a borazine precursor. The epitaxial growth and electronic properties are investigated using photoemission electron microscopy. Our measurements show that the fundamental electronic properties of hBN are highly dependent on the fabrication strategy.

Materials with strong spin-orbit interactions are presently a main target in the search for systems with novel magnetic properties. Magnetic anisotropies can be very large in such compounds, ranging from strongly frustrated Kitaev exchange and the associated spin-liquid states in honeycomb iridates to robust antisymmetric couplings in square-lattice ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Here we predict from ab initio quantum chemistry calculations that another highly unusual regime is realized in pyrochlore iridium oxides: the isotropic nearest-neighbor Heisenberg term can vanish while the antisymmetric Dzyaloshinskii-Moriya interaction reaches values as large as 5 meV, a result which challenges common notions and existing phenomenological models of magnetic superexchange. The resulting spin-excitation spectra reveal a very flat magnon dispersion in the Nd- and Tb-based pyrochlore iridates, suggesting the possibility of using these modes to store magnetic information. Indeed, the magnetization dynamics indicates that these modes are unable to propagate out of the excitation region. Although most of the results presented here are predictions of exotic magnetic states based on first-principles theory, we make connections to observations and establish the accuracy of our approach by reproducing experimental data for ${\mathrm{Sm}}_2{\mathrm{Ir}}_2{\mathrm{O}}_4$.

The van der Waals (vdW) bond is traditionally believed to be orders of magnitude lower than the typical chemical bond (i.e., ionic, metallic, or covalent), and hence the effects caused by a vdW interface are thought to be trivial. In this paper, by investigating the vdW epitaxial growth of a mechanically soft perovskite on the vdW substrate, we obtained the solid proof of strong non-negligible vdW interaction. The experimental results illustrate the formation of cracks and holes for the relaxation of the vdW strain as well as a lattice-constant-dependent epitaxial angle evolution and a pronounced band-structure change. The first-principles calculations indicate that the contribution of the vdW interaction energy at the epitaxial interface reaches up to more than a quarter of the overall interaction energy far exceeding the traditionally recognized vdW bonding strength. Both the experimental and the theoretical work show that given the appropriate material system, the vdW interaction could be strong enough to significantly manipulate material properties.

Understanding the response of functional oxides to extrinsic ion insertion is important for technological applications including electrochemical energy storage and conversion, corrosion, and electronic materials in neuromorphic computing devices. Decoupling the complicated chemical and mechanical effects of ion insertion is difficult experimentally. In this work, we assessed the effect of lithium incorporation in zirconium oxide as a model system, by performing first-principles based calculations. The chemical effect of lithium is to change the equilibria of charged defects. Lithium exists in $\mathrm{Zr}{\mathrm{O}}_2$ as a positively charged interstitial defect, and raises the concentration of free electrons, negatively charged oxygen interstitials, and zirconium vacancies. As a result, oxygen diffusion becomes faster by five orders of magnitude, and the total electronic conduction increases by up to five orders of magnitude in the low oxygen partial pressure regime. In the context of Zr metal oxidation, this effect accelerates oxide growth kinetics. In the context of electronic materials, it has implications for resistance modulations via ion incorporation. The mechanical effect of lithium is in changing the volume and equilibrium phase of the oxide. Lithium interstitials together with zirconium vacancies shrink the volume of the oxide matrix, release the compressive stress that is needed for stabilizing the tetragonal phase $\mathrm{Zr}{\mathrm{O}}_2$ at low temperature, and promote tetragonal-to-monoclinic phase transformation. By identifying these factors, we are able to mechanistically interpret experimental results in the literature for zirconium alloy corrosion in different alkali-metal hydroxide solutions. These results provide a mechanistic and quantitative understanding of lithium-accelerated corrosion of zirconium alloy, as well as, and more broadly, show the importance of considering coupled electro-chemo-mechanical effects of cation insertion in functional oxides.

Hydrogen is known to be present as an impurity in amorphous oxide semiconductors at the 0.1% level. The behavior of hydrogen and oxygen vacancies in amorphous In-Ga-Zn-O is studied using density functional supercell calculations. We show that the hydrogens pair up at oxygen vacancies in the amorphous network, where they form metal-H-metal bridge bonds. These bonds are shown to create filled defect gap states lying just above the valence band edge, infrared modes at about 1400 and $1520\mathrm{c}{\mathrm{m}}^{-1}$, and they are shown to give a consistent mechanism to explain the negative bias illumination stress instability found in oxide semiconductors such as In-Ga-Zn-O (IGZO).