The recent reports of superconductivity in ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ heterostructures have reinvigorated interest in potential superconductivity of low-oxidation state nickelates. Synthesis of ${\mathrm{Ni}}^{1+}$-containing compounds is notoriously difficult. In the current work, a combined sol-gel combustion and high-pressure annealing technique was employed to prepare polycrystalline perovskite ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ $(x=0, 0.1, \mathrm{and} 0.2)$. Metal nitrates and metal acetates were used as starting materials, and the latter were found to be superior to the former in terms of safety and reactivity. The ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$ compounds were subsequently reduced to ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$ using calcium hydride in a sealed, evacuated quartz tube. To understand the synthesis pathway, the evolution from $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ to $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_2$ was monitored using in situ synchrotron x-ray diffraction during the reduction process. Electrical transport properties were consistent with an insulator-metal transition occurring between $x=0$ and 0.1 for ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_3$. Superconductivity was not observed in our bulk samples of ${\mathrm{Nd}}_{1-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{O}}_2$. Neutron diffraction experiments at 3 and 300 K were performed on ${\mathrm{Nd}}_{0.9}{\mathrm{Sr}}_{0.1}\mathrm{Ni}{\mathrm{O}}_2$, in which no magnetic Bragg reflections were observed, and the results of structural Rietveld refinement are provided.

As-grown $\mathrm{Ag}{\mathrm{F}}_2$ has an electronic structure remarkably similar to insulating cuprates, but it is extremely electronegative, which makes it hard to handle and dope. Furthermore, buckling of layers reduces magnetic interactions and enhances unwanted self-trapping lattice effects. We argue that epitaxial engineering can solve all these problems. By using a high-throughput approach and first-principle computations, we find a set of candidate substrates which can sustain the chemical aggressiveness of $\mathrm{Ag}{\mathrm{F}}_2$ and at the same time have good lattice parameter matching for heteroepitaxy, enhancing $\mathrm{Ag}{\mathrm{F}}_2$ magnetic and transport properties, and opening the possibility of field-effect carrier injection to achieve a unique generation of high-$T_c$ superconductors. Assuming a magnetic mechanism and extrapolating from cuprates we predict that the superconducting critical temperature of a single layer can reach 195 K.

Electrophoretic deposition (EPD) is an industrially relevant and scalable technique used to form particle deposits from colloidal suspensions. Highly concentrated particle suspensions generally prevent real-time in situ microscopy observations which limit the characterization of EPD films to ex situ, or postprocessed, laboratory techniques. For dynamic systems, such as tunable amorphous photonic crystals (APCs), only reversible deposits are formed during the EPD process. Since reversible deposits cannot be characterized with standard ex situ methods, the particle-particle and particle-field interactions that govern the displayed color and crystallinity of these systems are not well understood. Here, we present in situ small-angle x-ray scattering and UV-Vis techniques for measuring both the structural and optical response of an APC under applied electric fields. We also develop a computational model based on colloidal interactions to explain the observed change in the interparticle spacing of APCs due to the applied electric field which correlates to displayed color. Ultimately, this work provides a new in situ characterization method that could be expanded for other dynamic, tunable colloidal systems.

This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices.

Borides of group-IB and group-IIB metals normally do not exist in the bulk form due to the small electronegativity difference and large mismatch in their atomic sizes. Contrary to these established rules, several metastable two-dimensional structures of copper boride were predicted from ab initio evolutionary searches, identifying that two structures are metallic whereas another one is strikingly a nodal line semimetal. The reduced dimensionality and intralayer coupling are responsible for the emergence of copper borides. This makes copper borides an attractive material for potential electronics applications.

We present an interacting model for the electronic and magnetic behaviors of a strained $\left(001\right)$ atomic layer of ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6,$ which shows room-temperature ferrimagnetism and magnetoresistance with potential spintronics application in the bulk. We find that the strong spin-orbit coupling in the molybdenum $4d$ shell gives rise to a robust ferrimagnetic state with an emergent spin-polarized electronic structure consisting of flatbands and four massive or massless Dirac dispersions. Based on the spin-wave theory, we demonstrate that the magnetic order remains intact for a wide range of doping, leading to the possibility of exploring flatband physics, such as Wigner crystallization in electron-doped ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{FeMoO}}_6.$

This paper is a contribution to the joint Physical Review Applied and Physical Review Materials collection titled Two-Dimensional Materials and Devices.

${\mathrm{Fe}}_{5-x}{\mathrm{GeTe}}_2$ is a van der Waals material with one of the highest reported bulk Curie temperatures, $T_{\mathrm{C}}\approx 310\mathrm{K}$. In this study, theoretical calculations and experiments are utilized to demonstrate that the magnetic ground state is highly sensitive to local atomic arrangements and the interlayer stacking. Cobalt substitution is found to be an effective way to manipulate the magnetic properties while also increasing the ordering temperature. In particular, cobalt substitution up to $\approx 30\%$ enhances $T_{\mathrm{C}}$ and changes the magnetic anisotropy, while $\approx 50\%$ cobalt substitution yields an antiferromagnetic state. Single crystal x-ray diffraction evidences a structural change upon increasing the cobalt concentration, with a rhombohedral cell observed in the parent material and a primitive cell observed for $\approx 46\%$ cobalt content relative to iron. First-principles calculations demonstrate that it is a combination of high cobalt content and the concomitant change to primitive layer stacking that produces antiferromagnetic order. These results illustrate the sensitivity of magnetism in ${\mathrm{Fe}}_{5-x}{\mathrm{GeTe}}_2$ to composition and structure, and emphasize the important role of local structural order-disorder and layer stacking in cleavable magnetic materials.

While it is known that alloy components can segregate to grain boundaries (GBs) and the atomic mobility in GBs greatly exceeds the atomic mobility in the lattice, little is known about the effect of GB segregation on GB diffusion. Atomistic computer simulations offer a means of gaining insights into the segregation-diffusion relationship by computing the GB diffusion coefficients of the alloy components as a function of their segregated amounts. In such simulations, thermodynamically equilibrium GB segregation is prepared by a semigrand canonical Monte Carlo method, followed by calculation of the diffusion coefficients of all alloy components by molecular dynamics. As a demonstration, the proposed methodology is applied to a GB is the Cu-Ag system. The GB diffusivities obtained exhibit nontrivial composition dependencies that can be explained by site blocking, site competition, and the onset of GB disordering due to the premelting effect.

The magnetic order and the spin dynamics in the antiferromagnetic entropy-stabilized oxide (${\mathrm{Mg}}_{0.2}{\mathrm{Co}}_{0.2}{\mathrm{Ni}}_{0.2}{\mathrm{Cu}}_{0.2}{\mathrm{Zn}}_{0.2}$)O (MgO-ESO) have been studied using muon spin relaxation ($\mu \mathrm{SR}$) and inelastic neutron scattering. We find that antiferromagnetic order develops gradually in the sample volume as it is cooled below 140 K, becoming fully ordered around 100 K. The spin dynamics show a critical slowing down in the vicinity of the transition, and the magnetic order parameter grows continuously in the ordered state. These results indicate that the antiferromagnetic transition is continuous but proceeds with a Gaussian distribution of ordering temperatures. The magnetic contribution to the specific heat determined from inelastic neutron scattering likewise shows a broad feature centered around 120 K. High-resolution inelastic neutron scattering further reveals an initially gapped spectrum at low temperature which sees an increase in a quasielastic contribution upon heating until the ordering temperature.

Recent discoveries of two-dimensional (2D) superconductors have uncovered various aspects of physical properties including vortex matter. In this paper, we report transport properties and a dynamical phase diagram at zero magnetic field in ion-gated $\mathrm{Mo}{\mathrm{S}}_2$. In addition to the universal jump in the current-voltage characteristic showing unambiguous evidence of the Berezinskii-Kosterlitz-Thouless transition, we observed multiple peaks in the temperature and current derivative of the electrical resistance, based on which a dynamical phase diagram in the current-temperature plane was constructed. We found current-induced dynamical states of vortex-antivortex pairs, containing that with the phase slip line. Also, we present a global phase diagram of vortices in gated $\mathrm{Mo}{\mathrm{S}}_2$ which captures the nature of vortex matter of clean 2D superconductors.

Two-dimensional (2D) Rashba systems have been intensively studied in the last decade due to their unconventional physics, tunability capabilities, and potential for spin-charge interconversion when compared to conventional heavy metals. With the advent of a new generation of spin-based logic and memory devices, the search for Rashba systems with more robust and larger conversion efficiencies is expanding. Conventionally, demanding techniques such as angle- and spin-resolved photoemission spectroscopy are required to determine the Rashba parameter ${\alpha }_R$ that characterizes these systems. Here, we introduce a simple method that allows a quantitative extraction of ${\alpha }_R$, through the analysis of the bilinear response of angle-dependent magnetotransport experiments. This method is based on the modulation of the Rashba-split bands under a rotating in-plane magnetic field. We show that our method is able to correctly yield the value of ${\alpha }_R$ for a wide range of Fermi energies in the 2D electron gas at the ${\mathrm{LaAlO}}_3/{\mathrm{SrTiO}}_3$ interface. By applying a gate voltage, we observe a maximum ${\alpha }_R$ in the region of the band structure where interband effects maximize the Rashba effect, consistent with theoretical predictions.

An analytical model invoking variations in the charge-carrier density is used to generate magnetoresistance curves that are almost indistinguishable from those produced by sophisticated numerical models. This demonstrates that, though disorder is pivotal in causing linear magnetoresistance, the form of the magnetoresistance thus generated is insensitive to details of the disorder. Taken in conjunction with the temperature ($T$) dependence of the zero-field resistivity, realistic levels of disorder are shown to be sufficient to explain the linear magnetoresistance and field-$T$ resistance scaling observed in high-temperature pnictide and cuprate superconductors. Hence, though the $T$-linear zero-field resistance is a definite signature of the “strange metal” state of high-temperature superconductors, their linear magnetoresistance and its scaling is unlikely to be so.

Single-phase concentrated solid-solution alloys (CSA), i.e., alloys without a principle alloying element but one randomly populated crystal structure, exhibit attractive material properties such as very high ductility at cryogenic temperatures, a gentle decrease of strength with temperature, or an unexpectedly high resistance against irradiation. For clarification of those observations assessment of atomic transport mechanisms including formation and migration of equilibrium point defects is indispensable. Positron annihilation lifetime spectroscopy measurements are performed to quantify the concentration of quenched-in thermal vacancies in fcc CSAs after quenching from temperatures close to their onset of melting. For various alloy compositions the concentration of quenched-in vacancies decreases with increasing entropy of mixing $\mathrm{\Delta }{S}_{\mathrm{mix}}$. Whereas alloys with three constituents in nonequimolar fractions (CrFeNi) exhibit vacancy concentrations in the ${10}^{-5}$ range, the studied alloys with four (CoCrFeNi) and five constituents (CoCrFeMnNi, AlCoCrFeNi) do not show a vacancy-specific positron lifetime. Therefore, the concentration of quenched-in vacancies must be in the range of ${10}^{-6}$ or less. It can be concluded that there is either only a vanishingly small fraction of vacancies present at temperatures near the onset of melting or the generated vacancies are inherently unstable.

In the $A{B}_4{Q}_8$ lacunar spinels, the electronic structure is described on the basis of inter- and intracluster interactions of tetrahedral $B_4$ clusters, and tuning these can lead to myriad fascinating electronic and magnetic ground states. In this work, we employ magnetic measurements, synchrotron x-ray and neutron scattering, and first-principles electronic structure calculations to examine the coupling between structural and magnetic phase evolution in ${\mathrm{GaMo}}_4{\mathrm{Se}}_8$, including the emergence of a skyrmionic regime in the magnetic phase diagram. We show that the competition between two distinct Jahn-Teller distortions of the room temperature cubic $F\overline{4}3m$ structure leads to the coexistence of the ground-state $R3m$ phase and a metastable $Imm2$ phase. The magnetic properties of these two phases are computationally shown to be very different, with the $Imm2$ phase exhibiting uniaxial ferromagnetism and the $R3m$ phase hosting a complex magnetic phase diagram including equilibrium Néel-type skyrmions stable from nearly $T$ = 28 K down to $T$ = 2 K, the lowest measured temperature. The large change in magnetic behavior induced by a small structural distortion reveals that ${\mathrm{GaMo}}_4{\mathrm{Se}}_8$ is an exciting candidate material for tuning unconventional magnetic properties via mechanical means.

By means of temperature and wavelength-dependent small-angle neutron scattering (SANS) experiments on sintered isotropic and textured Nd-Fe-B magnets we provide evidence for the existence of an anisometric structure in the microstructure of the textured magnets. This conclusion is reached by observing a characteristic cross-shaped angular anisotropy in the total unpolarized SANS cross section at temperatures well above the Curie temperature. Comparison of the experimental SANS data to a microstructural model based on the superquadrics form factor allows us to estimate the shape and lower bounds for the size of the structure. Subtraction of the scattering cross section in the paramagnetic regime from data taken at room temperature provides the magnetic SANS cross section. Surprisingly, the anisotropy of the magnetic scattering is very similar to the nuclear SANS signal, suggesting that the nuclear structure is decorated by the magnetic moments via spin-orbit coupling. Based on the computation of the two-dimensional correlation function we estimate lower bounds for the longitudinal and transversal magnetic correlation lengths.

The influence of spatial confinement on the thermally excited stochastic cation dynamics of the room-temperature ionic liquid 1-N-butylpyridinium bis-[(trifluoromethyl)sulfonyl]imide ([BuPy][${\mathrm{Tf}}_2\mathrm{N}$]) inside porous carbide-derived carbons with various pore sizes in the sub- to a few nanometer range is investigated by quasielastic neutron spectroscopy. Using the potential of fixed window scans, i.e., scanning a sample parameter, while observing solely one specific energy transfer value, an overview of the dynamic landscape within a wide temperature range is obtained. It is shown that already these data provide a quite comprehensive understanding of the confinement-induced alteration of the molecular mobility in comparison to the bulk. A complementary, more detailed analysis of full energy transfer spectra at selected temperatures reveals two translational diffusive processes on different time scales. Both are considerably slower than in the bulk liquid and show a decrease of the respective self-diffusion coefficients with decreasing nanopore size. Different thermal activation energies for molecular self-diffusion in nanoporous carbons with similar pore size indicate the importance of pore morphology on the molecular mobility, beyond the pure degree of confinement. In spite of the dynamic slowing down we can show that the temperature range of the liquid state upon nanoconfinement is remarkably extended to much lower temperatures, which is beneficial for potential technical applications of such systems.

In ferroelectrics, domain walls have significant effects on ferroic properties. The mechanical behavior of domain walls has been investigated because of its scientific and technological importance. Numerous experiments and simulations have examined domain walls and their motion under mechanical strain. However, since nano- and micromechanical testing is challenging, previous studies have mainly involved indirect or intermittent observations, and there have been few real-time in situ observations. As a result, the mechanism by which mechanical loads induce wall motion remains elusive. Here, we directly observe stress-induced domain wall motion in real time by using rationally designed specimens and transmission electron microscopy (TEM). We imaged the domain walls directly as they moved and disappeared depending on the mechanical conditions during the experiments. Furthermore, we experimentally determined the mechanical criteria for wall motion. Our results not only provide fundamental knowledge about ferroelectrics but also provide information on how to control their dynamics from an engineering standpoint.

We report on the evolution of the average and depth-dependent magnetic order in thin-film samples of biaxially stressed and electron-doped ${\mathrm{EuTiO}}_3$ for samples across a doping range $<0.1$ to $7.8\times {10}^{20}{\mathrm{cm}}^{-3}$. Under an applied in-plane magnetic field, the $G$-type antiferromagnetic ground state undergoes a continuous spin-flop phase transition into in-plane, field-polarized ferromagnetism. The critical field for ferromagnetism slightly decreases with an increasing number of free carriers, yet the field evolution of the spin-flop transition is qualitatively similar across the doping range. Unexpectedly, we observe interfacial ferromagnetism with saturated ${\mathrm{Eu}}^{2+}$ moments at the substrate interface at low fields preceding ferromagnetic saturation throughout the bulk of the degenerate semiconductor film. We discuss the implications of these findings for the unusual magnetotransport properties of this compound.

We have synthesized a monoclinic phase of IrSi by means of high-pressure and high-temperature treatment above 25 GPa, whereas the theoretically predicted high-pressure phase (i.e., FeSi-type structure) was not identified up to 48 GPa. Magnetotransport measurements clarify the semimetallic nature (carrier density of $\sim 6-9\times {10}^{19}/\mathrm{c}{\mathrm{m}}^3$) of monoclinic IrSi, in agreement with our band structure calculations. Furthermore, the surface electronic bands are predicted to show the large Rashba spin splitting, offering potential spintronic applications of this material.

Designing and understanding functional electronic and magnetic properties in perovskite oxides requires controlling and tuning the underlying crystal lattice. Here we report the structure, including oxygen and cation positions, of a single-crystal, entropy stabilized perovskite oxide film of $\mathrm{La}\left(\mathrm{C}{\mathrm{r}}_{0.2}\mathrm{M}{\mathrm{n}}_{0.2}\mathrm{F}{\mathrm{e}}_{0.2}\mathrm{C}{\mathrm{o}}_{0.2}\mathrm{N}{\mathrm{i}}_{0.2}\right){\mathrm{O}}_3$ grown on $\mathrm{SrTi}{\mathrm{O}}_3$ (001). The parent materials range from orthorhombic ($\mathrm{LaCr}{\mathrm{O}}_3, \mathrm{LaMn}{\mathrm{O}}_3$, and $\mathrm{LaFe}{\mathrm{O}}_3$) to rhombohedral ($\mathrm{LaCo}{\mathrm{O}}_3$ and $\mathrm{LaNi}{\mathrm{O}}_3$), and first principles calculations indicate that these structural motifs are nearly degenerate in energy and should be highly distorted site to site. Despite this extraordinary local configurational disorder on the $B$-site sublattice, we find a structure with unexpected macroscopic crystalline homogeneity with a clear orthorhombic unit cell, whose orientation is demonstrated to be controlled by the strain and crystal structure of the substrate for films grown on $\left(\mathrm{L}{\mathrm{a}}_{0.3}\mathrm{S}{\mathrm{r}}_{0.7}\right)\left(\mathrm{A}{\mathrm{l}}_{0.65}\mathrm{T}{\mathrm{a}}_{0.35}\right){\mathrm{O}}_3$ and $\mathrm{NdGa}{\mathrm{O}}_3$ (110). Furthermore, quantification of the atom positions within the unit cell reveals that the orthorhombic distortions are small, close to $\mathrm{LaCr}{\mathrm{O}}_3$, which may be driven by a combination of disorder averaging and the average ionic radii. This is a step toward understanding the rules for designing crystal motifs and tuning functional properties through controlled configurational complexity.

Understanding the physics of associative polymers is often limited by our inability to directly measure bond dissociation under deformation. In this work, we developed a rheo-fluorescence technique and applied it to characterize the nonlinear shear response of linear side-functionalized polymer chains cross-linked via nickel-terpyridine complexation. As the network was sheared, the fraction of dissociated bonds was quantitatively measured based upon a change in fluorescence with metal dissociation. Shear thinning of the gel was accompanied by only a small increase in the fraction of dissociated bonds. Comparison with several transient network models shows that the shear thinning within the constraint of the measured fraction of dissociated bonds cannot be explained by classical theories that include retraction of dangling chains alone; the rheological response likely involves alternative modes of stress relaxation.