${\mathrm{Cr}}_2{\mathrm{O}}_3$ is the archetypal magnetoelectric (ME) material, which has a linear coupling between electric and magnetic polarizations. Quadratic ME effects are forbidden for the magnetic point group of ${\mathrm{Cr}}_2{\mathrm{O}}_3$, due to space-time inversion symmetry. In ${\mathrm{Cr}}_2{\mathrm{O}}_3$ films grown by sputtering, we find a signature of a quadratic ME effect that is not found in bulk single crystals. We use Raman spectroscopy and magnetization measurements to deduce the removal of space-time symmetry and corroborate the emergence of the quadratic ME effect. We propose that metastable site-selective trace dopants remove the space, time, and space-time inversion symmetries from the original magnetic point group of bulk ${\mathrm{Cr}}_2{\mathrm{O}}_3$. We include the quadratic ME effect in a model describing the switching process during ME field cooling and estimate the effective quadratic susceptibility value. The quadratic magnetoelectric effect in a uniaxial antiferromagnet is promising for multifunctional antiferromagnetic and magnetoelectric devices that can incorporate optical, strain-induced, and multiferroic effects.

Polar and chiral ${\mathrm{Ni}}_3{\mathrm{TeO}}_6$ was found to exhibit a colossal magnetoelectric (ME) effect associated with collinear antiferromagnetic order. We have investigated the evolution of its magnetic state with the substitution of the ${\mathrm{Ni}}^{2+}$ spins by isotropic ${\mathrm{Mn}}^{2+}$ spins, namely ${(\mathrm{Ni},\mathrm{Mn})}_3{\mathrm{TeO}}_6$ (NMTO). The ground state of NMTO maintains an Ising nature, but a new phase with XY-type magnetic anisotropy is discovered at an intermediate temperature range. Neutron powder diffraction experiments reveal that (1) Mn ions tend to occupy a specific transition-metal site, consistent with the first-principle calculation, and (2) the intermediate phase is an incommensurate (IC) helical magnetic state propagating along the $c$ axis with ${\vec{q}}_{\text{IC}}=(0,0,1.5\pm \delta )$. We also found that the balance between these two magnetic states can be readily manipulated with magnetic fields, which results in significant ME effects. This selective site occupancy of NMTO allows for a unique competition between collinear and helical magnetic structures with minimal chemical disorder. Thus, NMTO serves as a model system to study site-specific chemical control of noncollinear magnetism and ME coupling.

We use a 30-nm x-ray beam to study the spatially resolved properties of a ${\mathrm{SmNiO}}_3$-based nanodevice that is doped with protons. The x-ray absorption spectra supported by density-functional theory simulations show partial reduction of nickel valence in the region with high proton concentration, which leads to the insulating behavior. Concurrently, x-ray diffraction reveals only a small lattice distortion in the doped regions. Together, our results directly show that the knob which proton doping modifies is the electronic valency and not the crystal lattice. The studies are relevant to ongoing efforts to disentangle structural and electronic effects across metal-insulator phase transitions in correlated oxides.

The effect of electron doping of $\mathrm{Eu}{\mathrm{O}}_{1\text{−}\delta }$ thin films through oxygen vacancies ($\delta =0$, 0.025, and 0.09) upon the magnetocaloric response is presented. The films each showed a paramagnetic to ferromagnetic transition around 65 K, with an additional magnetic ordering transition at higher temperatures in the oxygen deficient samples. All transitions are observed to be of second order. A maximum magnetic entropy change of 6.4 J/kg K over a field change of 2 T with a refrigerant capacity of 223 J/kg was found in the sample with $\delta =0$, and in all cases the refrigerant capacities of the thin films under study were found to exceed that reported for bulk EuO. Adjusting the oxygen content was shown to produce tablelike magnetocaloric effects, desirable for ideal Ericsson-cycle magnetic refrigeration. These films are thus excellent candidates for small-scale magnetic cooling technology in the liquid nitrogen temperature range.

${\mathrm{BaFe}}_2{\mathrm{S}}_3$ is a quasi-one-dimensional antiferromagnetic insulator that becomes superconducting under hydrostatic pressure. The magnetic ordering temperature $T_N$, as well as the presence of superconductivity have been found to be sample dependent. It has been argued that the Fe content may play a decisive role, with the use of 5 mol% ($\delta =0.1$) excess Fe being reportedly required during the synthesis to optimize the magnetic ordering temperature and the superconducting properties. However, it is yet unclear whether a Fe off-stoichiometry is actually present in the samples, and how it affects the structural, magnetic, and transport properties. Here, we present a systematic study of compositional, structural, and physical properties of ${\mathrm{BaFe}}_{2+\delta }{\mathrm{S}}_3$ as a function of the nominal Fe excess $\delta$. As $\delta$ increases, we observe the presence of an increasing fraction of secondary phases but no systematic change in the average composition or crystal structure of the main phase. Magnetic susceptibility curves are influenced by the presence of magnetic secondary phases. While a small excess Fe (2.5 mol%, i.e., $\delta =0.05$) can slightly increase $T_N$ and decrease the temperature of the resistivity anomaly at $T^{*}$, a range of $T_N^{\prime }\mathrm{s}/T^{*}$'s is observed within each batch. This result strongly contrasts with the previously reported maximum of $T_N$ at $\delta =0.1$. Rather than with the value of $\delta$, $T_N$ and $T^{*}$ seem to correlate with the broadening in the logarithmic derivative of the resistivity around $T_N$ that could be an indicator of the level of disorder in the samples. Finally, we show that crystals free of ferromagnetic secondary phases can be obtained by remelting samples with nominal $\delta =0.05$ in a Bridgman-like process based on optical heating.

Oxygen packaging in transition metal oxides determines the metal-oxygen hybridization and electronic occupation at metal orbitals. Strontium vanadate $\left(\mathrm{SrV}{\mathrm{O}}_3\right)$, having a single electron in a $3d$ orbital, is thought to be the simplest example of strongly correlated metallic oxides. Here, we determine the effects of epitaxial strain on the electronic properties of $\mathrm{SrV}{\mathrm{O}}_3$ thin films, where the metal-oxide sublattice is corner connected. Using x-ray absorption and x-ray linear dichroism at the $\mathrm{V}{L}_{2,3}$ and O $K$ edges, it is observed that tensile or compressive epitaxial strain change the hierarchy of orbitals within the $t_{2g}$ and $e_g$ manifolds. Data show a remarkable $2p-3d$ hybridization, as well as a strain-induced reordering of the $\mathrm{V}3d\left(t_{2g},e_g\right)$ orbitals. The latter is itself accompanied by a consequent change of hybridization that modulates the hybrid ${\pi }^{*}$ and ${\sigma }^{*}$ orbitals and the carrier population at the metal ions, challenging a rigid band picture.

We present results of a state-of-the-art computational study of the atomic and electronic structure of ${\mathrm{Cs}}_{2n+2}{M}_n{M}_n^{{}^{\prime }}{X}_{6n+2}$ ($M=\mathrm{Cu}$, Ag; $M^{\prime }=\mathrm{In}$, Bi; $X=\mathrm{Cl}$, Br, I) layers with up to three-unit-cell thickness ($n=3$) as well as their bulk counterparts in the search for economical and stable halide double perovskites (HDPs) with a direct band gap and strong light absorption. Among the 24 layers we have studied, seven are found to be suitable for light emitting diodes, whereas one is proposed for solar cell applications. Our results on bulk ${\mathrm{Cs}}_2\mathrm{Ag}M{}^{\prime }{X}_6$ ($M^{\prime }=\mathrm{In}$, Bi; $X=\mathrm{Cl}$, Br) HDPs agree with previous studies. Interestingly, we find that HDP layers as well as bulk are more stable than their respective lead-halide perovskites. The Cu-based HDPs are cost effective compared with those based on Ag and are found to be equally suitable as absorber materials with a lower band gap. However, bulk ${\mathrm{Cs}}_2\mathrm{CuIn}{X}_6$ ($X=\mathrm{Cl}$, Br) HDPs are found to be dynamically unstable. Our calculations show a combined effect of confinement and spin-orbit coupling in deciding the nature of the band gap for one (two)-unit-cell thick layers of ${\mathrm{Cs}}_4\mathrm{AgBi}{X}_8$ (${\mathrm{Cs}}_8{\mathrm{Cu}}_2{\mathrm{Bi}}_2{X}_{14}$). Furthermore, strong confinement effects are found to be crucial in making the thinnest layers of ${\mathrm{Cs}}_4\mathrm{CuBi}{X}_8$ ($X=\mathrm{Cl}$, Br) direct band gap semiconductors. The reduction in the dimension from three to two dimensions introduces distortions in the layers of the HDPs, and it is more in the case of the Cu-based layers. The nature of the band gap becomes direct for very thin layers of ${\mathrm{Cs}}_{2n+2}{\mathrm{Ag}}_n{\mathrm{Bi}}_n{X}_{6n+2}$ ($n=1$) and ${\mathrm{Cs}}_{2n+2}{\mathrm{Cu}}_n{\mathrm{Bi}}_n{X}_{6n+2}$ ($n=1$, 2) compared with indirect band gap in bulk. The HDP layers with a direct band gap and enhanced stability also have a strong absorption coefficient of the order of ${10}^5\mathrm{c}{\mathrm{m}}^{\text{–}1}$, making them interesting for solar cell applications. The effects of surfaces on the electronic structure of direct band gap layers of ${\mathrm{Cs}}_{2n+2}{\mathrm{Cu}}_n{\mathrm{Bi}}_n{\mathrm{Br}}_{6n+2}$ ($n=1$, 2) and ${\mathrm{Cs}}_{2n+2}{\mathrm{Ag}}_n{\mathrm{In}}_n{\mathrm{Cl}}_{6n+2}$ ($n=3$) are also shown with surface states. We hope that our results on HDPs will stimulate further research on these materials.

Here we present an ab initio prediction of an order-disorder transition (ODT) from a $I\overline{4}2d$-type to ${\mathrm{Th}}_3{\mathrm{P}}_4$-type phase in the cation sublattices of ${\mathrm{Mg}}_2\mathrm{Ge}{\mathrm{O}}_4$, a post-post-perovskite phase. This uncommon type of prediction is achieved by carrying out a high-throughput sampling of atomic configurations in a 56-atom supercell followed by a Boltzmann ensemble statistics calculation. ${\mathrm{Mg}}_2\mathrm{Ge}{\mathrm{O}}_4$ is a low-pressure analog of $I\overline{4}2d$-type ${\mathrm{Mg}}_2\mathrm{Si}{\mathrm{O}}_4$, a predicted major planet-forming phase of super-Earths' mantles. Therefore, a similar ODT is anticipated in $I\overline{4}2d$-type ${\mathrm{Mg}}_2\mathrm{Si}{\mathrm{O}}_4$ as well, which should impact the internal structure and dynamics of these planets. The prediction of this ${\mathrm{Th}}_3{\mathrm{P}}_4$-type phase in ${\mathrm{Mg}}_2\mathrm{Ge}{\mathrm{O}}_4$ further enhances the relationship between the crystal structures of Earth/planet-forming silicates and oxides at extreme pressures and those of rare-earth sesquisulfides at low pressures.

Y-rich yttrium iron garnet (Y-YIG) thin films were grown on gadolinium gallium garnet substrates using pulsed laser deposition from a ${\mathrm{YFeO}}_3$ target. The films are epitaxial on the substrate, and the unit cell of (001)- and (111)-oriented Y-YIG undergoes a tetragonal or rhombohedral distortion, respectively. The Y-YIG has lower room temperature magnetization but higher low-temperature magnetization than bulk YIG, and its Curie temperature was in the range of 330–400 K, depending on the growth conditions. First-principles calculations predict that ${\mathrm{Y}}_{\mathrm{Fe}}$ antisite defects are more stable in octahedral than tetrahedral Fe sites. Temperature-dependent magnetization measurements together with a superexchange dilution model and composition determined from x-ray photoelectron spectroscopy indicate that nonmagnetic species (${\mathrm{Y}}^{3+}$ and vacancies) are predominantly accommodated in the octahedral sites, but there is a significant occupancy of tetrahedral sites.

The flexibility of two-dimensional (2D) materials enables static and dynamic ripples that are known to cause lateral contraction, shrinking of the material boundary. However, the limits of 2D materials' lateral expansion are unknown. Therefore, here we discuss the limits of the intrinsic lateral expansion of 2D materials that are modified by compressive line defects. Using thin sheet elasticity theory and sequential multiscale modeling, we find that the lateral expansion is inevitably limited by the onset of rippling. The maximum lateral expansion ${\chi }_{\text{max}}\approx 2.1t^2{\sigma }_d$, governed by the elastic thickness $t$ and the defect density ${\sigma }_d$, remains typically well below 1%. In addition to providing insight to the limits of 2D materials' mechanical limits and applications, the results highlight the potential of line defects in strain engineering, since for graphene they suggest giant pseudomagnetic fields that can exceed 1000 T.

The low fracture toughness of oxide glasses is a key limitation for many of their applications. Inducing and controlling nanoscale phase separation in oxide glasses has been proposed as a potential toughening strategy, as, unlike many alternative extrinsic toughening approaches, it allows one to retain the optical transparency. Using molecular dynamics simulations, we here investigate the toughening mechanism in soda-lime-silica glasses with embedded glassy nanoscale silica droplets. This system is chosen as a model for the experimental structure of phase-separated soda-lime-silica glass, which is attractive considering its existing commercial use and the ease of inducing phase separation. We calculate the fracture toughness of glass structures containing nanodroplets of varying sizes and with different precrack positions, revealing that the glassy silica droplets toughen the material. The simulations show that crack propagation is impeded by crack arrest, crack deflection and diversion, and stress field alteration, ultimately increasing the fracture toughness. Our findings thus shed light on the toughening mechanism due to phase separation, with important implications for the experimental design of oxide glasses with controlled nanoscale phase separation.

Recently, we have established that, when loaded in compression, edge-on, atomic layers in layered solid can fail by buckling. The resulting structure is termed a ripplocation. When more than one layer buckles, they outline standing waves with boundaries that we labeled ripplocation boundaries that are nearly fully recoverable. In this paper, we examine buckling of layers at the centimeter level to explore continuum buckling theory and its applicability to atomic layers. Specifically, we examine buckling by confining and cyclically loading thin steel sheets, edge-on, determining that increasing confining pressure, sheet thickness, and/or decreasing the number of layers increases the buckling load. Concomitantly, the resulting wavelengths and amplitudes are reduced. A nonlinear, folding mechanics model, which accounts for frictional bending and foundation energies, is adapted and verified on our experimental results. We also demonstrate that Coulombic friction between the layers can account for the energy dissipated per cycle. The predicted values of buckling nucleation loads and number of modes from the model are in good agreement—at low levels of confinement—with continuum and atomistic scale results. The wavelength estimates from the model correlate surprisingly well with the continuum buckling results; however, likely due to the complex mechanics at the lower length scales and limiting theoretical assumptions in the derivation, the accuracy decreases at the atomistic scale and at higher confining pressures.

WC-Co cemented carbides have a unique combination of high hardness and good toughness, making them ideal as tool materials in applications such as metal machining or rock drilling. Dopants are commonly added to retard grain growth and thereby creating a harder material. Thin films with cubic structure have been observed experimentally at phase boundaries between hexagonal WC and fcc Co-rich binder when doping with, e.g., Ti, V, or Cr. These films are generally considered to play a crucial role in the grain growth inhibition effect. Therefore, the thermodynamics of these thin cubic films is important to understand. Here, we construct, using ab initio calculations and modeling, an interfacial phase diagram for thin cubic films in Ti-doped WC-Co. We consider $\mathrm{C}\leftrightarrow \mathrm{vacancy}$ and $\mathrm{W}\leftrightarrow \mathrm{Ti}$ substitutions by constructing alloy cluster expansions and use Monte Carlo simulations to calculate the configurational free energy. Furthermore, force-constant fitting is used to extract the harmonic free energy for the ground-state structures. Additionally, we use information from thermodynamic databases to couple our atomic-scale calculations to overall compositions of typical WC-Co materials. We predict that Ti segregates to WC/Co phase boundaries to form thin cubic films of two metallic layer thickness, both at solid-state and liquid-phase sintering temperatures. Furthermore, we predict that these films are stable also for low doping concentrations when no Ti-containing carbide phase precipitates in the material. We show that Ti essentially only segregates to the inner layer of the thin cubic film leaving an almost pure W layer towards Co, an ordering which has been observed in recent experimental high-resolution transmission electron microscopy studies.

${\mathrm{Cd}}_3{\mathrm{As}}_2$ is one of the prototypical topological Dirac semimetals. Here, we manipulate the band inversion responsible for the emergence of Dirac nodes by alloying ${\mathrm{Cd}}_3{\mathrm{As}}_2$ with topologically trivial ${\mathrm{Zn}}_3{\mathrm{As}}_2$. The carrier density monotonically decreases and the resistivity is enhanced as $x$ is increased. For larger $x$, the thermoelectric figure of merit exhibits comparably large values exceeding 0.3 at room temperature, due to the combined effects of a strong enhancement of the thermopower, an only moderate increase in the resistivity, and a suppression of the thermal conductivity. Complementary quantum-oscillation data and optical-conductivity measurements allow us to infer that the Dirac nodes are gapped out, and a band structure with a partly flat dispersion likely emerges in the higher-$x$ region in ${\mathrm{Cd}}_{3-x}{\mathrm{Zn}}_x{\mathrm{As}}_2$.

Recent interest in honeycomb lattice materials has focused on their potential to host quantum spin liquid (QSL) states. Variations in bond angles and spin allow a range of interesting behaviors on this lattice, from the predicted QSL ground state of the Kitaev model to exotic magnetic orders. Here we report the physical properties of two compounds with rare earths on an approximate honeycomb lattice. The isostructural compounds ${\mathrm{Nd}}_2{\mathrm{S}}_5\mathrm{Sn}$ ($J=\frac{9}{2}$) and ${\mathrm{Pr}}_2{\mathrm{S}}_5\mathrm{Sn}$ ($J=4$) permit a direct comparison of half-integer versus integer spins on this lattice. We find strikingly different magnetic properties for the two compounds. ${\mathrm{Nd}}_2{\mathrm{S}}_5\mathrm{Sn}$ orders antiferromagnetically at $T_N\approx 2.5$ K and undergoes several magnetic transitions to other ordered states under applied field. ${\mathrm{Pr}}_2{\mathrm{S}}_5\mathrm{Sn}$ displays no magnetic ordering transition above $T=0.41$ K, and may be proximate to a spin liquid state.

Magnetic properties of $A_2{BB}^{\prime }{\mathrm{O}}_6$ ($A=\mathrm{rare}$ or alkaline-earth ions; $B,B^{\prime }=\mathrm{transition}\text{−}\mathrm{metal}$ ions) double perovskites are of great interest due to their potential spintronic applications. Particularly fascinating is the zero-field-cooled exchange bias effect observed for the hole-doped ${\mathrm{La}}_{2-x}{A}_x{\mathrm{CoMnO}}_6$ polycrystalline samples. In this paper we synthesize ${\mathrm{La}}_2{\mathrm{CoMnO}}_6,{\mathrm{La}}_{1.5}{\mathrm{Ca}}_{0.5}{\mathrm{CoMnO}}_6$, and ${\mathrm{La}}_{1.5}{\mathrm{Sr}}_{0.5}{\mathrm{CoMnO}}_6$ single crystals by the floating zone method and study their magnetic behavior. The three materials are ferromagnetic. Surprisingly, we observe no zero or even conventional exchange bias effect for the Ca- and Sr-doped single crystals, in sharp contrast to polycrystalline samples. This absence indicates that the lack of grain boundaries and spin-glass-like behavior, not observed in our samples, might be key ingredients for the spontaneous exchange bias phenomena seen in polycrystalline samples.

ScN has attracted great attention for its electronic properties and its ability to enhance polarization of AlN; however, its sister compounds, YN and LaN, remain much less studied. Here, we use first-principles calculations to evaluate YN and LaN in their cubic and hexagonal phases. Rocksalt YN and LaN are semiconductors, although we show that LaN differs from ScN and YN in having a direct band gap, which we attribute to its weaker $p\text{−}p$ coupling. Both have low electron effective masses. In addition to their rocksalt structures, we evaluate the layered hexagonal and wurtzite phases of YN and LaN. For YN, the wurtzite phase cannot be stabilized, and hexagonal YN is higher in energy than rocksalt YN. In contrast, for LaN, the wurtzite phase is favored, and it is comparable in energy to rocksalt LaN. Wurtzite LaN has a polarization of $0.608\mathrm{C}/{\mathrm{m}}^2$ (referenced to the centrosymmetric layered hexagonal phase), and a high piezoelectric coefficient $e_{33}=1.78\mathrm{C}/{\mathrm{m}}^2$. Interestingly, we find that the polarization of wurtzite LaN may be reversible; we find a relatively small switching barrier of 0.06 eV per formula unit, offering the potential for its use as a ferroelectric. Since wurtzite LaN is closely lattice matched to InP, we investigate a heterostructure between (0001) wurtzite LaN and (111) zinc-blende InP, and find the polarization discontinuity would yield a bound charge of $1.3\times {10}^{14}e/{\mathrm{cm}}^2$, offering the potential for novel electronic applications such as tunnel junctions. Our results compare and contrast ScN, YN, and LaN, and highlight the potential of these materials for adoption in electronic and ferroelectric devices.

Polymers assembled using computationally designed coiled coil bundlemers display tunable stiffness via control of interbundlemer covalent connectivity as confirmed using small-angle neutron scattering. Neutron spin echo spectroscopy reveals that rigid rod polymers show a decay rate $\mathrm{\Gamma }\sim Q^2$ ($Q$ is the scattering vector) expected of straight cylinders. Semirigid polymers assembled using bundlemers linked via 4-armed organic linker show flexible segmental dynamics at mid-$Q$ and $\mathrm{\Gamma }\sim Q^2$ behavior at high Q. The results give insight into linker flexibility-dependent interbundlemer dynamics in the hybrid polymers.

We report new compositions of CoNi-based superalloys that have low mass densities (7.84–8.20 g/cc), negative $\gamma /{\gamma }^{\prime }$ lattice misfit, and high ${\gamma }^{\prime }$ solvus temperature up to 1094 °C with promising high-temperature mechanical properties. The $\gamma /{\gamma }^{\prime }$ in the base Co-30Ni-10Al-5Cr alloy was stabilized with Nb and further developed by varying Ti and Cr. The γ′ precipitates in these alloys are richer in Ni than the γ matrix phase, unlike for ${\gamma }^{\prime }$ in other CoNi-based superalloys. The atomic-scale compositional analysis indicates that the ${\gamma }^{\prime }$ has a stoichiometry of ${\left(\mathrm{Ni},\mathrm{Co}\right)}_3\left(\mathrm{Al},\mathrm{Nb},\mathrm{Ti},\mathrm{Cr}\right)$. In these alloys, the ${\gamma }^{\prime }$ microstructure is stable at 900 °C up to 1000 h. In an alloy with a ${\gamma }^{\prime }$ solvus of 1089 °C, the coarsening kinetics of γ′ at 900 °C follows the Lifshitz-Slyozov-Wagner model with a rate constant of $6.6\pm 0.7\mathrm{n}{\mathrm{m}}^3/\mathrm{s}$, comparable with high-density Co-Al-W-based superalloys. The alloys also show an anomalous increase in strength, with temperature having a peak of 0.2% proof stress (PS) and specific PS (SPS) of ∼710 MPa and $\sim 91\mathrm{MPa}{\mathrm{g}}^{–1}\mathrm{cc}$, respectively, at 770 °C. The low density and high strength make this class of alloys a potential and cost-effective alternative to the other Co-based superalloys and comparable with Ni-based superalloys.