We perform an ab initio modeling of amorphous copper-doped alumina ($a\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$:Cu), a prospective memory material based on resistance switching, and study the structural origin of electronic conduction in this material. We generate molecular dynamics based models of $a\text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$:Cu at various Cu concentrations and study the structural, electronic, and vibrational properties as a function of Cu concentration. Cu atoms show a strong tendency to cluster in the alumina host, and metallize the system by filling the band gap uniformly for higher Cu concentrations. We also study thermal fluctuations of the HOMO-LUMO energy splitting and observe the time evolution of the size of the band gap, which can be expected to have an important impact on the conductivity. We perform a numerical computation of conduction pathways, and show its explicit dependence on Cu connectivity in the host. We present an analysis of ion dynamics and structural aspects of localization of classical normal modes in our models.

High-entropy alloys (HEAs) are a new class of materials with an attractive combination of tunable mechanical and physicochemical properties. They crystallize mainly in cubic structures, however, for practical applications, HEAs with hexagonal-close-packed (hcp) structure are highly desirable in connection with their, in general, high hardness. Herein, we report the synthesis, structure, and detailed superconducting properties of ${\mathrm{Re}}_{0.56}{\mathrm{Nb}}_{0.11}{\mathrm{Ti}}_{0.11}{\mathrm{Zr}}_{0.11}{\mathrm{Hf}}_{0.11}$—the first hexagonal superconducting HEA composed of five randomly distributed transition metals. A combination of room-temperature precession electron diffraction, precession electron diffraction tomography, and powder x-ray diffraction is utilized to determine the room-temperature crystal structure. Transport, magnetic, and heat capacity measurements show that the material is a type-II superconductor with the bulk superconducting transition at $T_c=4.4$ K, lower critical field $H_{c1}\left(0\right)=2.3$ mT, and upper critical field $H_{c2}\left(0\right)=3.6$ T. Low-temperature specific-heat measurement indicates that ${\mathrm{Re}}_{0.56}{\mathrm{Nb}}_{0.11}{\mathrm{Ti}}_{0.11}{\mathrm{Zr}}_{0.11}{\mathrm{Hf}}_{0.11}$ is a phonon-mediated superconductor in the weak electron-phonon coupling limit with a normalized specific-heat jump $\frac{\mathrm{\Delta }{C}_{el}}{{\gamma }_n{T}_c}=1.32$. Further, hexagonal to cubic structural transition is observed by lowering the valence electron counts and $T_c$ follows crystallinelike behavior.

Grain coarsening behavior in an $\alpha$-phase iron sample is studied in three dimensions using high energy x-ray diffraction microscopy. 4971 grains that are entirely inside the sample are segmented in the initial state and 3905 remain after annealing. A matching procedure was used to track 3299 grains between the two states while the remainder were either consumed by neighbors or the tracking algorithm failed to correlate them. During the single annealing treatment, the average grain volume increased by 13%. Statistical analysis in each state yields subtle changes in the grain size and nearest neighbor number distributions. Correlating topological features with volume changes between states, the average behavior is seen to be consistent with an isotropic model of curvature driven coarsening, but the dispersion of volume changes in each topological class is comparable to the overall trend in the average behaviors. Thus, the topological characterizations used here are not predictive of the behavior of individual grains under the isotropic assumption. Examination of anecdotal cases allows understanding of some outliers but others appear counter to an isotropic theory.

We present a methodology based on ex situ (postgrowth) electrochemistry to control the oxygen concentration in thin films of the superconducting oxide ${\mathrm{La}}_2{\mathrm{CuO}}_{4+y}$ grown epitaxially on substrates of isostructural ${\mathrm{LaSrAlO}}_4$. The superconducting transition temperature, which depends on the oxygen concentration, can be tuned by adjusting the pH level of the base solution used for the electrochemical reaction. As our main finding, we demonstrate that the dopant oxygens can either occupy the interstitial layer in an orientationally disordered state or organize into a crystalline phase via a mechanism in which dopant oxygens are inserted into the substrate, changing the lattice symmetry of both the substrate and the epitaxial film. We discuss this mechanism, and we describe the resulting methodology as a platform to be explored in thin films of other transition-metal oxides.

Electronic transport near the insulator-metal transition is investigated in the molecular beam epitaxy-grown $\mathrm{SrTi}{\mathrm{O}}_3$/$\mathrm{N}{\mathrm{d}}_{1\text{−}x}\mathrm{Ti}{\mathrm{O}}_3$/$\mathrm{SrTi}{\mathrm{O}}_3$ heterostructures using temperature dependent magnetotransport measurements. It was found that Nd vacancies introduce localized electronic states resulting in variable range hopping transport at low temperatures. At a fixed Nd-vacancy concentration, a crossover from Mott to Efros-Shklovskii variable range hopping transport was observed with decreasing temperature. With increasing disorder, a sign reversal of magnetoresistance from positive to negative was observed revealing interplay between intra-state interaction and the energy dependence of the localization length. These findings highlight the important role of stoichiometry when exploring intrinsic effects using heterostructure and interfaces in addition to offering broad opportunities to tailor low temperature transport using nonstoichiometry defects.

Tetragraphene is a theoretically predicted quasi-2D carbon allotrope featuring a combination of square and hexagonal rings. This material is semiconducting and presents highly anisotropic electronic properties. Motivated by the fact that quasi-1D nanocarbon systems can present properties remarkably different from their 2D counterparts, we propose to study tetragraphene-based nanoribbons and investigate their stability and their electronic and magnetic properties using quantum-based computational methods. We show how the electronic structure of these tetragraphene nanoribbons (TGNRs) depends on chirality, width, and the details of edge reconstruction. We predict the existence of different hybridization states at the edges of these systems, thus demonstrating a set of versatile electronic behaviors. In particular, edge hybridization is found to induce band gap modulation, as well as the emergence of magnetic edge states. Our results suggest that TGNRs can be prototypes for future applications in nanoelectronics.

We have investigated the crystal structure and magnetic properties of the synthetic copper mineral engelhauptite, $\mathrm{KC}{\mathrm{u}}_3{\mathrm{V}}_2{\mathrm{O}}_7{\left(\mathrm{OH}\right)}_2\mathrm{Cl}$, which possesses a kagome lattice made of $\mathrm{C}{\mathrm{u}}^{2+}$ ($3d^9$) ions. Unlike the hexagonal structure with a perfect kagome lattice reported in the natural mineral, a synthetic crystal has a monoclinic structure with a distorted kagome lattice. We observe a magnetic transition at 6 K and an anomaly in the magnetization curve around 50 T. Engelhauptite represents a variant of the $S=1/2$ kagome antiferromagnet and we discuss the structure-property relationship by comparing engelhauptite and its relative compound volborthite, $\mathrm{C}{\mathrm{u}}_3{\mathrm{V}}_2{\mathrm{O}}_7{\left(\mathrm{OH}\right)}_2·2{\mathrm{H}}_2\mathrm{O}$.

Applications of nitrogen-vacancy (NV) centers in diamond in quantum technology have attracted considerable attention in recent years. Deterministic generation of ensembles of NV centers can advance the research on quantum sensing, many-body quantum systems, multipartite entanglement, and so on. Here we report the complete process of controlled generation of NV centers in diamond as well as their characterization: growing diamond films through chemical vapor deposition (CVD), ion implantation, and spectroscopic characterization of the defect centers using a confocal microscope. A microwave-assisted CVD setup is presented which we constructed for the preparation of single-crystalline homoepitaxial diamond films. The films were prepared with minimized nitrogen concentration, which is confirmed through photoluminescence measurements. We demonstrate an in situ ultrahigh vacuum (UHV) implantation and heating process for creation of NV centers using a novel experimental setup. For the first time hot implantation has been shown which prevents surface charging effects. We do not observe graphitization due to UHV heating. By optimizing the implantation parameters it has been possible to implant NV centers in a precise way. We present large area mapping of the samples to determine the distribution of the centers and describe the characterization of the centers by spectroscopic techniques. Reducing the decoherence caused by environmental noise is of primary importance for many applications in quantum technology. We demonstrate improvement on coherence time $T_2$ of the NV spins by suppression of their interaction with the surrounding spin bath using robust dynamical decoupling sequences.

Hyperbolic (or indefinite) materials that have been extensively studied in artificial electromagnetic metamaterials are attracting increasing interest due to their unusual optical properties, which enable useful functionalities in emerging devices based on light. Natural hyperbolic materials have many obvious advantages over metamaterials, most of which are inorganic. Here, using first-principles calculations, we demonstrated a metal-organic framework in the form of a natural hyperbolic material, Cu-benzenehexathial (Cu-BHT), which has been synthesized and proved to be a superconductor in recent experiments. Cu-BHT exhibits hyperbolic dispersion, i.e., all-angle negative refraction over a broad frequency from near-IR (∼1.4 μm) to the visible regime (∼750 nm). The operating hyperbolic window can be effectively tuned by applying uniaxial strain along the optical direction, constructing van der Waals heterostructures with BN or graphene or electron doping. This work is expected to offer a promising platform for the design of high-performance natural hyperbolic materials with great potential for applications.

Whereas thin films of topological insulators grown by molecular beam epitaxy often display regular, triangular features, $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$ films grown onto yttrium iron garnet (YIG) display much greater disorder. Here, we present observations of various types of disorder present in these films using atomic force microscopy and scanning transmission electron microscopy. The investigation reveals the presence of an amorphous metal oxide layer between the substrate and the film, which appears to smooth out the nanometer-scale undulations in the YIG surface. It also shows the existence of quasiordered arrays of heavy atoms in some interfacial regions, as well as rotations and tilting between adjacent grains and basal twinning at various heights in the film. Using density functional theory, we explore the impact of these prominent basal twins on the electronic structure of the film.

The field of two-dimensional (2D) materials has expanded to multilayered systems in which electronic, optical, and mechanical properties change—often dramatically—with stacking order, thickness, twist, and interlayer spacing. For transition metal dichalcogenides (TMDs), bond coordination within a single van der Waals layer changes the out-of-plane symmetry that can cause metal-insulator transitions or emergent quantum behavior. Discerning these structural order parameters is often difficult using real-space measurements; however, we show that 2D materials have distinct, conspicuous three-dimensional (3D) structure in reciprocal space described by nearly infinite oscillating Bragg rods. Combining electron diffraction and specimen tilt we probe Bragg rods in all three dimensions to identify multilayer structure with subangstrom precision across several 2D materials—including TMDs (${\mathrm{MoS}}_2$, ${\mathrm{TaSe}}_2$, ${\mathrm{TaS}}_2$) and multilayer graphene. We demonstrate quantitative determination of key structural parameters such as surface roughness, inter- and intralayer spacings, stacking order, and interlayer twist using a rudimentary transmission electron microscope. We accurately characterize the full interlayer stacking order of multilayer graphene (1, 2, 6, 12 layers) as well the intralayer structure of ${\mathrm{MoS}}_2$ and extract a chalcogen-chalcogen layer spacing of $3.07\pm 0.11$ Å. Furthermore, we demonstrate quick identification of multilayer rhombohedral graphene.

Using first-principles calculations we examine the band structures of ferromagnetic hexagonal manganites ${\text{YXO}}_3$ ($\text{X}=\text{V}$, Cr, Mn, Fe, and Co) in the nonpolar nonsymmorphic $P{6}_3/mmc$ space group. For ${\mathrm{YVO}}_3$ and ${\mathrm{YCrO}}_3$ we find a band inversion near the Fermi energy that generates a nodal ring in the $k_z=0$ mirror plane. We perform a more detailed analysis for these compounds and predict the existence of the topological “drumhead” surface states. Finally, we briefly discuss the low-symmetry polar phases (space group $P{6}_{3}cm$) of these systems, and show they can undergo a $P{6}_3/mmc\to P{6}_{3}cm$ transition by condensation of soft $K_3$ and ${\mathrm{\Gamma }}_2^{-}$ phonons. Based on our findings, stabilizing these compounds in the hexagonal phase could offer a promising platform for studying the interplay of topology and multiferroicity, and the coexistence of real-space and reciprocal-space topological protection in the same phase.

The projection of acoustic images is of relevance for a wide range of applications, including acoustic communication and medical imaging. Various acoustic projection devices that use holograms have been proposed; however, they inevitably involve multiple basic elements and lack reconfigurability. In this paper, we demonstrate that an acoustic coding metasurface that consists of a single type of asymmetric acoustic unit (AAU) can be used to project the desired acoustic images. The AAU is constructed using a perforated panel and coating unit cell, which can be tunable in terms of the transmission coefficient and reflection phase asymmetry. Patterned in regular and inverse forms, the AAU can function as the coding elements “0” and “1,” respectively. We demonstrate, both numerically and experimentally, the efficient projection of complex acoustic images such as the letters “N,” “J,” and “U.” More importantly, we develop a simple composite metasurface system to achieve the controllable encryption and decryption of acoustic images. Our proposed approach provides a design methodology for the coding metasurface and may have applications in acoustic security, information storage, and wave manipulation.

Diffusion-induced grain boundary migration is a phenomenon in which a grain boundary moves in response to the driving forces generated by diffusing solute species. For example, diffusing solute species change the atomic volume in the host material, either by filling a vacancy with a misfitting solute atom or by expanding host lattices through interstitial diffusion. These volume changes are inhomogeneous and are stored as elastic energy in the material that drives grain boundaries. In this paper, we introduce our previously developed Cahn-Hilliard–phase-field-crystal model (CH-PFC) as a computational tool to investigate diffusion-induced grain boundary migration in crystalline materials. This multiscale phase-field model couples the composition field of a diffusing species with the crystallographic texture of a host material. We apply the CH-PFC model to battery electrodes and investigate whether interstitial solute diffusion induces grain growth in ${\mathrm{FePO}}_4/{\mathrm{LiFePO}}_4$ electrodes. To this end, we compute grain growth in 60 ${\mathrm{FePO}}_4$ electrodes by conducting two parallel trials: In the first trial, we cycle the electrode and calculate diffusion-induced grain growth. In the second trial, we do not cycle the electrode and calculate curvature-driven grain growth. We find a statistically significant grain growth in the cycled electrodes and negligible grain growth in the noncycled electrodes. Overall, we show that the CH-PFC model not only predicts electrode microstructures as a function of the Li composition, but also predicts the crystallographic features of an electrode during battery operation.

We report the synthesis of millimeter-sized freestanding single-crystal $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-x}$ heterostructure membranes with nanoscale thickness. On a $\mathrm{SrTi}{\mathrm{O}}_3$ substrate, a heteroepitaxial water-soluble sacrificial layer of $\mathrm{S}{\mathrm{r}}_3\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_6$ followed by an encapsulated $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-x}$ film is grown by pulsed laser deposition. A freestanding $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-x}$ heterostructure membrane is obtained by etching the sacrificial layer. Its high crystalline quality is demonstrated via x-ray diffraction, electrical measurements, and scanning transmission electron microscopy.

We report the effects of nitrogen diffusion on exchange bias in MnN/CoFeB heterostructures as a function of MnN thickness and field-annealing temperature. We find that competing effects occur in which high-temperature annealing enhances exchange bias in heterostructures with thick MnN through improved crystallinity, but in thinner samples this annealing ultimately eliminates the exchange bias due to nitrogen deficiency. Using polarized neutron reflectometry and magnetic x-ray spectroscopy, we directly observe increasing amounts of nitrogen migration from MnN into the underlying Ta seed layer with increased annealing temperature. In heterostructures with thin MnN layers, the resulting nitrogen deficiency becomes significant enough to alter the antiferromagnetic state before the Ta seed layer is nitrogen saturated. Furthermore, we observe intermixing at the MnN/CoFeB interface which is attributed to the nitrogen deficiency creating vacancies in the MnN layer after annealing in a field. This intermixing of Mn with Co and Fe is not believed to be the cause for loss of exchange bias when the MnN layer is too thin but is instead a secondary effect due to increased vacancies after nitrogen migration.

We apply density functional theory for superconductors (SCDFT) to doped tungsten oxide in three forms: electrostatically doped ${\mathrm{WO}}_3$, perovskite ${\mathrm{WO}}_{3-x}{\mathrm{F}}_x$, and hexagonal ${\mathrm{Cs}}_x{\mathrm{WO}}_3$. We achieve a consistent picture in which the experimental superconducting transition temperature $T_c$ is reproduced, and superconductivity is understood as a weak-coupling state sustained by soft vibrational modes of the ${\mathrm{WO}}_6$ octahedra. SCDFT simulations of ${\mathrm{Cs}}_x{\mathrm{WO}}_3$ allow us to explain the anomalous $T_c$ behavior observed in most tungsten bronzes, where $T_c$ decreases with increasing carrier density. Here, the opening of structural channels to host Cs atoms induces a softening of strongly coupled W-O modes. By increasing the Cs content, these modes are screened and $T_c$ is strongly reduced.

${\mathrm{SrTiO}}_3$ is a model perovskite compound with unique properties and technological relevance. At 105 K it undergoes a transition from a cubic to a tetragonal phase with characteristic antiferrodistortive rotations of the ${\mathrm{TiO}}_6$ octahedra. Here we study systematically the effect of different exchange-correlation functionals on the structural, electronic, and optical properties of cubic and tetragonal STO by comparing the recently implemented strongly constrained and appropriately normed (SCAN) meta-GGA functional with the generalized gradient approximation (PBE96 and PBEsol) and the hybrid functional (HSE06). SCAN is found to significantly improve the description of the structural properties, in particular the rotational angle of the tetragonal phase, comparable to HSE06 at a computational cost similar to GGA. The addition of a Hubbard $U$ term ($\mathrm{SCAN}+U$, $U=7.45$ eV) allows us to achieve the experimental band gap of 3.25 eV with a moderate increase in the lattice constant, whereas within $\mathrm{GGA}+U$ the gap is underestimated even for high $U$ values. The effect of the exchange-correlation functional on the optical properties is progressively reduced from 1.5 eV variance in the onset of the spectrum in the independent particle picture to 0.3 eV upon inclusion of many-body effects within the framework of the $GW$ approximation (single-shot $G_0{W}_0$) and excitonic corrections by solving the Bethe-Salpeter equation (BSE). Moreover, a model BSE approach is shown to reproduce the main features of the optical spectrum at a lower cost compared to $G_0{W}_0+\text{BSE}$. Strong excitonic effects are found in agreement with previous results and their origin is analyzed based on the contributing interband transitions. Last but not least, the effect of the tetragonal distortion on the optical spectrum is discussed and compared to available experimental data.

The electronic and structural properties of a Ge-Pt alloy film on Ge(001) were studied with low-energy electron microscopy. Well above 1000 K, a bulklike eutectic film forms, followed by the emergence of an ordered ${\mathrm{Ge}}_3\mathrm{Pt}$ alloy upon solidification, which sets in at 1004 K. This eight-layers-thick ${\mathrm{Ge}}_3\mathrm{Pt}$ phase is remarkably robust as it is stabilized by quantum size effects. Further cooling results in the evolution of ultrathin Pt-induced nanowires around 650 K. The process is reversible upon temperature variation around the eutectic point. The results are considered representative of an important class of surface alloys.

Motivated by suggestions that hybrid nanomaterials from nanoporous metal and aqueous electrolyte can be used as actuators, we study the impact of an external load on the actuation behavior of nanoporous gold impregnated with aqueous electrolyte. At no load, we observe the well-documented trend for a more positive electrode potential prompting elongation of the nanoporous body. For purely capacitive electrode processes we confirm that the elastic response to external load is simply superimposed on the potential-induced elongation, so that the strain per electric charge is invariant with the load. The observations so far are consistent with the expectation for surface-stress driven actuation in a linear elastic materials system. Surprisingly, however, actuation in the regime of oxygen electrosorption responds strongly to loading: as the load is increased, the strain per charge gradually drops to zero and even inverts its direction. In other words, the actuator moves backward when asked to do work against a substantial external load. Furthermore, we demonstrate that the length change in response to lifting the oxygen adsorbate layer depends on the load that was present at the instant of oxysorption. This “load memory effect” has analogies to shape-memory behavior in massive alloys. Yet, contrary to shape-memory alloys, the microscopic origin is here a surface phase transition. We argue that the observation is a signature of the reorientation of local surface domains with anisotropic surface stress, and that the required atomic transport process acts only while mobile adatoms are supplied during the deposition or lifting of the oxygen adsorbate layer.