The interest in ferroelectric van der Waals crystals arises from the potential to realize ultrathin ferroic systems owing to the reduced surface energy of these materials and the layered structure that allows for exfoliation. Here, we quantitatively unravel giant negative electrostriction of van der Waals layered copper indium thiophosphate (CIPS), which exhibits an electrostrictive coefficient $Q_{33}$ as high as $-3.2{\mathrm{m}}^4/{\mathrm{C}}^2$ and a resulting bulk piezoelectric coefficient $d_{33}$ up to −85 pm/V. As a result, the electromechanical response of CIPS is comparable in magnitude to established perovskite ferroelectrics despite possessing a much smaller spontaneous polarization of only a few $\mu \mathrm{C}/\mathrm{c}{\mathrm{m}}^2$. In the paraelectric state, readily accessible owing to low transition temperatures, CIPS exhibits large dielectric tunability, similar to widely used barium strontium titanate, and as a result both giant and continuously tunable electromechanical response. The persistence of electrostrictive and tunable responses in the paraelectric state indicates that even few-layer films or nanoparticles will sustain significant electromechanical functionality, offsetting the inevitable suppression of ferroelectric properties in the nanoscale limit. These findings can likely be extended to other ferroelectric transition metal thiophosphates and (quasi-) two-dimensional materials, and might facilitate the quest toward alternative ultrathin functional devices incorporating electromechanical response.

Developing steels with high strength and ductility is needed in order to improve the mechanical reliability and environmental performance of engineering products. The addition of Mn is a key technology for developing next-generation high-strength steels. However, the addition of Mn leads to a serious side effect, grain boundary (GB) embrittlement, which decreases the mechanical toughness of steels. Understanding the mechanism of GB embrittlement due to Mn is an essential process for improving the toughness of steels containing Mn. In this work, in order to reveal the fundamental mechanism of GB embrittlement by Mn, the effect of Mn on the cleavage fracture of bcc-Fe GBs, especially the influence of the difference in the magnetic coupling state between Mn and Fe, is investigated using uniaxial tensile simulations of the bcc-Fe $\mathrm{\Sigma }3\left(111\right)$ GB with and without Mn segregation using the first-principles density functional theory (DFT). The uniaxial tensile simulations demonstrate that Mn decreases the cleavage-fracture energy of the GB. In particular, the ferromagnetically coupled Mn substantially decreases the cleavage-fracture energy of the GB, promoting cleavage fracture. When ferromagnetically coupled Mn is present in the bcc-Fe GBs, the electrons contributing to the bonds between Mn and the surrounding Fe atoms easily localize to the Mn atom with increasing stress, and the bonding between Mn and the surrounding Fe atoms rapidly weakens, leading to a cleavage fracture of the GBs at a lower stress and strain. This unusual behavior is derived from the stability of the nonbonding Mn as a result of its half-filled d shell. These results show that the local magnetic state in GBs is one of the factors determining the macroscopic mechanical properties of steels containing Mn.

${\mathrm{BaFe}}_2{\mathrm{S}}_3$ is a special iron superconductor with a two-leg ladder structure, which can help to unravel the role played by the electronic correlations in high-$T_c$ superconductivity. At zero pressure, it is insulating with stripe antiferromagnetic (AF) order and it has been claimed to be a Mott insulator. Superconductivity emerges under pressure. To analyze the strength of the local correlations in ${\mathrm{BaFe}}_2{\mathrm{S}}_3$, we apply the slave-spin technique to a model, which includes the five Fe $d$ orbitals. We find that at zero pressure, ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ is strongly correlated. The correlations are particularly severe for the two orbitals with the largest weight at the Fermi level, which show mass enhancements $m^{*}\sim 12–15$. Such strong correlations invalidate nesting as the mechanism for AF. We find that the system is not a Mott insulator at zero temperature. Nevertheless these two orbitals will become incoherent at higher temperatures. At the pressure at which the superconductivity appears, the electronic correlations in ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ are strongly reduced and similar to the ones measured in other iron superconductors. Different from what happens in other iron superconductors, at both pressures, the Fermi surface is reconstructed by the electronic correlations.

We study the surface stability of the layered bismuth-oxide ${\mathrm{Bi}}_2{\mathrm{WO}}_6$ and ${\mathrm{Bi}}_2{\mathrm{MoO}}_6$ photocatalysts, which belong to the series of Aurivillius (${\mathrm{Bi}}_2{\mathrm{A}}_{n-1}{\mathrm{B}}_n{\mathrm{O}}_{3n+3}$) perovskites and have been proposed as efficient visible-light absorbers, due to favorable electronic hybridization induced by the bismuth 6s and 6p orbitals. We present a Newton-Raphson optimization of the charge distribution at the semiconductor-solution interface using the self-consistent continuum solvation (SCCS) model to describe the influence of the aqueous environment. Our analysis provides a description of the charged interface under controlled pH and applied voltage and offers a molecular interpretation of the competing structural and electrical factors that underlie the facet-dependent photocatalytic activity of layered ${\mathrm{Bi}}_2{\mathrm{A}}_{n-1}{\mathrm{B}}_n{\mathrm{O}}_{3n+3}$ compounds.

The angular dependence of spin-orbit torque in a disordered Co/Pt bilayer is calculated using a first-principles nonequilibrium Green's function formalism with an explicit supercell averaging over Anderson disorder. In addition to the usual dampinglike and fieldlike terms, the odd torque contains a sizable planar Hall-like term $(\mathbf{m}·\mathbf{E})\mathbf{m}\times (\mathbf{z}\times \mathbf{m})$ whose contribution to current-induced damping is consistent with experimental observations. The dampinglike and planar Hall-like torquances depend weakly on disorder strength, while the fieldlike torquance declines with increasing disorder. The torques that contribute to damping are almost entirely due to spin-orbit coupling on the Pt atoms, but the fieldlike torque does not require it.

A giant planar Hall effect (PHE) and anisotropic magnetoresistance (AMR) are observed in TaP, a nonmagnetic Weyl semimetal with ultrahigh mobility. The perpendicular resistivity (i.e., the planar magnetic field applied normal to the current) far exceeds the zero-field resistivity, which thus rules out the possible origin of negative longitudinal magnetoresistance. The giant PHE and AMR are finally attributed to the large anisotropic orbital magnetoresistance that stems from the ultrahigh mobility. Furthermore, the mobility-enhanced current jetting effects are found to strongly deform the line shape of the curves, and their evolution with the changing magnetic field and temperature is also studied. Although the giant PHE and AMR suggest promising applications in spintronics, the enhanced current jetting shows the other side of the coin, which needs to be considered in future device design.

Ferroelectric-field switching in $\mathrm{BaTi}{\mathrm{O}}_3$ generates “Barkhausen noise” when domain walls are displaced. We show by acoustic emission spectroscopy that electric-field switching of ${90}^{\circ }$ boundaries generates large strain fields, which emit acoustic phonons during ferroelectric hysteresis measurements. We use highly sensitive receivers (microphones) to measure the time sequences of noise in close analogy to noise patterns in ferroelastic and magnetic materials. Domain-wall interactions and jamming generate the “crackling noise” that follows scale invariant avalanche dynamics: the avalanche energy and amplitude probability distribution functions follow power laws with exponents $\varepsilon =1.65$ (energy) and τ′ = 2.25 (amplitudes). Aftershocks are very common and follow Omori law with probability $\sim t^{-p}$ where $t$ is the time elapsed after the main shock and $p$ is the Omori exponent $p\sim 1$. The interevent times follow a double power-law distribution with exponents 0.9 for small times and 2.2 for the larger times. The scaling behavior is consistent with predictions of mean field theory.

We realize classical Hall bars, made of chain-mail-inspired cubic-symmetry 3D metamaterials composed of interlinked hollow tori, contacted to printed circuit boards. This step is a prerequisite for refined experiments compared to our previous probe-station based measurements as well as for potential applications of 3D Hall-effect metamaterials in general. On this basis, we systematically study the dependence of the Hall voltage on the relative orientation of the 3D metamaterial crystallographic axes with respect to the applied static magnetic field vector and the imposed current flow direction. Our findings are consistent with an isotropic sign-reversed Hall coefficient, which has been predicted by homogenization theory.

Efficient thermoelectric materials require a rare and contraindicated combination of materials properties: large electrical conductivity, large Seebeck coefficient, and low thermal conductivity. One strategy to achieve the first two properties is via low-energy electronic bands containing both flat and dispersive parts in different regions of crystal momentum space, known as a pudding-mold band structure. Here, we illustrate that ${\mathrm{BaPdS}}_2$ successfully achieves the pudding-mold band structure for the valence band, contributing to a large $p$-type thermoelectric power factor, due to its anisotropic crystal structure containing zigzag chains of edge-sharing square planar ${\mathrm{PdS}}_4$ units; large power factor is achieved for $n$-type doping as well due to band convergence. In addition, ${\mathrm{BaPdS}}_2$ exhibits ultralow lattice thermal conductivity, and thus also achieves the third property, due to extremely soft and anharmonic interactions in its transverse acoustic phonon branch. We predict a remarkably large thermoelectric figure of merit, with peak values between 2 and 3 for two of the three crystallographic directions, suggesting ${\mathrm{BaPdS}}_2$ warrants experimental investigation.

A two-dimensional silicon boride phase, “siliborophene,” was formed on a ${\mathrm{ZrB}}_2$(0001) single-crystal surface, which coincided with the $2\times 2$ periodicity of the substrate. The phonon dispersion relations were measured using high-resolution electron energy-loss spectroscopy and were compared with theoretical predictions based on first-principles density functional theory. Among many theoretically derived surface models, only one structure made of ${\mathrm{Si}}_3{\mathrm{B}}_6$ can reproduce details of the measured phonon dispersion. That model consists of a cyclic boron ring ($c{\mathrm{B}}_6$) capped by a Si atom, ${\mathrm{SiB}}_6$ group, and $s{p}^2$-like Si atoms connecting them. This structure is much more robust than that of silicene: it shows no order–disorder transition until 1300 K; above that temperature, Si is desorbed gradually. Electron band calculation of the isolated film of ${\mathrm{Si}}_3{\mathrm{B}}_6$ displays a band crossing at the $\overline{\mathrm{K}}$ point, which makes this material promising for use with Fermi-level engineering.

A wedge bilayer of epitaxial 4–8 nm $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Co}{\mathrm{O}}_3\left(\mathrm{LSCO}\right)$/6-nm $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3\left(\mathrm{LSMO}\right)$, hard and soft ferromagnets (FM) respectively, was studied using soft x-ray magnetic spectroscopy to elucidate the interplay between the magnetic and electronic properties of the constituent layers. This system exhibits magnetic and electronic segregation within the LSCO layer characterized by soft FM $\mathrm{C}{\mathrm{o}}^{2+}$ ions at the interface (s-LSCO), which is biased by the underlying hard LSCO layer (h-LSCO), characterized by predominantly $\mathrm{C}{\mathrm{o}}^{3+}/\mathrm{C}{\mathrm{o}}^{4+}$ FM ions. For a 5.1-nm layer, the interfacial $\mathrm{C}{\mathrm{o}}^{2+}$ ion concentration is maximized and the h-LSCO coercivity is minimized, while the opposite is true for a 7.8-nm LSCO layer. These dual trends in $\mathrm{C}{\mathrm{o}}^{2+}$ ion concentration and h-LSCO coercivity within the LSCO serve to independently affect the exchange bias and coercivity of the soft FM layer, resulting in inverse trends between these properties that typically vary simultaneously.

We investigate magnetic excitations in iridium insulators with pentavalent ${\mathrm{Ir}}^{5+}$ ($5d^4$) ions with strong spin-orbit coupling. We obtain a microscopic model based on the local ${\mathrm{Ir}}^{5+}$ multiplets involving $J=0$ (singlet), $J=1$ (triplet), and $J=2$ (quintet) spin-orbital states. We get effective interactions between these multiplets on square and face-centered-cubic (fcc) structures of magnetic ions in the layered-perovskites and the double-perovskites, in particular ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$. Further, we derive an effective spin-orbital Hamiltonian in terms of bond bosons, and we explore possible instabilities toward magnetic and quadrupole orderings. Additionally, we study charge excitations with the help of the variational cluster perturbation theory, and we calculate the electronic charge gap as a function of hopping and Coulomb interactions. Based on both electronic and magnetic phase diagrams, we verify the possibility of excitonic magnetism due to condensation of spin-orbital excitons in ${\mathrm{Ir}}^{5+}$ iridates.

$\mathrm{B}{\mathrm{a}}_4\mathrm{NbI}{\mathrm{r}}_3{\mathrm{O}}_{12}$, a material with a triangular planar geometry of $\mathrm{I}{\mathrm{r}}_3{\mathrm{O}}_{12}$ trimers, is described. Magnetic susceptibility measurements show it to be paramagnetic with no magnetic ordering down to 1.8 K despite the Curie-Weiss temperature $\left({\theta }_{\mathrm{CW}}\right)$ of −13 K. The material has a very low effective magnetic moment $\left({\mu }_{\mathrm{eff}}\right)$ of 0.80 ${\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ To look at the lower-temperature behavior, the specific heat $\left(C_p\right)$ was measured down to 0.35 K; it shows no indication of magnetic ordering and fitting a power law to $C_p$ vs $T$ below 2 K yields the power $\alpha =3/4$. Comparison to the trimer compound made with the $4d$ element Rh in place of the $5d$ element Ir, $\mathrm{B}{\mathrm{a}}_4\mathrm{NbR}{\mathrm{h}}_3{\mathrm{O}}_{12}$, which is also paramagnetic (${\mu }_{\mathrm{eff}}=1.48{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}., {\theta }_{\mathrm{CW}}=-23\mathrm{K}$), is presented. The analysis suggests that $\mathrm{B}{\mathrm{a}}_4\mathrm{NbI}{\mathrm{r}}_3{\mathrm{O}}_{12}$ is a candidate spin liquid material.

Materials with extreme photonic properties such as maximum diffuse reflectance, high albedo, or tunable band gaps are essential in many current and future photonic devices and coatings. While photonic crystals, periodic anisotropic structures, are well established, their disordered counterparts, photonic glasses (PGs), are less understood despite their most interesting isotropic photonic properties. Here, we introduce a controlled high index model PG system. It is made of monodisperse spherical ${\mathrm{TiO}}_2$ colloids to exploit strongly resonant Mie scattering for optimal turbidity. We report spectrally resolved combined measurements of turbidity and light energy velocity from large monolithic crack-free samples. This material class reveals pronounced resonances enabled by the possibility to tune both the refractive index of the extremely low polydisperse constituents and their radius. All our results are rationalized by a model based on the energy coherent potential approximation, which is free of any fitting parameter. Surprisingly good quantitative agreement is found even at high index and elevated packing fraction. This class of PGs may be the key to optimized tunable photonic materials and also central to understand fundamental questions such as isotropic structural colors, random lasing or strong light localization in 3D.

Atomically thin hexagonal boron nitride (h-BN) exhibits a wide band gap, as well as excellent thermal and chemical stability, and thus has been used in ultraviolet light emission and as building blocks for two-dimensional (2D) heterostructures. Large-area h-BN films for technical applications can now be produced by chemical vapor deposition (CVD). Unfortunately, grain boundaries (GBs) are ubiquitously introduced as a result of the coalescence of grains with different crystallographic orientations. It is well known that the properties of materials largely depend on GB structures. Here, we carried out a systematic study on the GB structures in CVD-grown polycrystalline h-BN monolayer films with a transmission electron microscope. Interestingly, most of these GBs are revealed to be formed via overlapping between neighboring grains, which are distinct from the covalently bonded GBs as commonly observed in other 2D materials. Further density functional theory calculations show that hydrogen plays an essential role in overlapping GB formation. This work provides an in-depth understanding of the microstructures and formation mechanisms of GBs in CVD-grown h-BN films, which should be informative in guiding the precisely controlled synthesis of large-area single-crystalline h-BN and other 2D materials.

The ab initio Korringa-Kohn-Rostoker method combined with the coherent potential approximation (CPA) was employed to investigate the electronic, magnetic, and transport properties of medium-entropy face-centered-cubic (fcc) NiCoMn solid solution alloys. By comparing the CPA electronic structure with that from supercell calculations, we uncovered an unconventional CPA ground state, which correctly distinguishes two equally populated Mn CPA components—with large spin moments but opposite orientations. Using the spin spiral calculations, we further demonstrated that this ground state is most energetically favorable in the presence of spin noncollinearity, and no significant longitudinal spin fluctuation is observed, justifying the applicability of the Heisenberg model. The finite-temperature magnetism was further studied using different approximations based on the Heisenberg model, and we found the Mn moments to be fully disordered at low temperature due to a small net effective Weiss field on Mn. In addition, the magnetic effect on the electron scattering at finite temperatures was evaluated and compared with other scattering mechanisms. Since the magnetization-induced electron scattering is almost saturated in the ground state, (full) spin disorder only yields a small addition to the resistivity, whereas the thermal displacements increase it modestly. Finally, we elucidate the role of hydrostatic pressure on the magnetic and transport properties. These findings reflect the importance of the magnetic signatures on the physical properties of alloys, and they provide a window into magnetism-controlled electronic structure and energy dissipation.

Theoretical studies have predicted the existence of topological magnons in honeycomb compounds with stripy or zigzag antiferromagnetic (AFM) order. Here we report the discovery of AFM order in the layered and noncentrosymmetric honeycomb nickelate $\mathrm{N}{\mathrm{i}}_2\mathrm{M}{\mathrm{o}}_3{\mathrm{O}}_8$ through a combination of magnetization, specific heat, x-ray and neutron diffraction, and electron paramagnetic resonance measurements. The AFM order is complex, with a mixture of stripy and zigzag character on an integer spin noncentrosymmetric honeycomb lattice ($P{6}_{3}mc$). Further, each of the two sublattices of the bipartite honeycomb lattice is comprised of a different crystal field environment, i.e., octahedral and tetrahedral $\mathrm{N}{\mathrm{i}}^{2+}$, respectively, enabling independent substitution on each. Replacement of Ni by Mg on the octahedral site suppresses the long-range magnetic order and results in a weakly ferromagnetic state. Conversely, substitution of Fe for Ni enhances the strength of the AFM exchange and increases the ordering temperature. Thus, $\mathrm{N}{\mathrm{i}}_2\mathrm{M}{\mathrm{o}}_3{\mathrm{O}}_8$ provides a platform on which to explore the rich physics of $S=1$ on the honeycomb lattice in the presence of competing magnetic interactions with a noncentrosymmetric, formally piezopolar, crystal structure.

We predict the structures of neutral gas phase gold clusters $(\mathrm{A}{\mathrm{u}}_n,n=5-13)$ at finite temperatures based on free-energy calculations obtained by replica-exchange ab initio molecular dynamics. The structures of neutral $\mathrm{A}{\mathrm{u}}_5-\mathrm{A}{\mathrm{u}}_{13}$ clusters are assigned at 100 K based on a comparison of experimental far-infrared multiple photon dissociation spectra performed on Kr-tagged gold clusters with theoretical anharmonic IR spectra and free-energy calculations. The critical gold cluster size at which the most stable isomer changes from planar to nonplanar is $\mathrm{A}{\mathrm{u}}_{11}$ (capped-trigonal prism, ${\mathrm{D}}_{3h}$) at 100 K. However, at 300 K (i.e., room temperature), planar and nonplanar isomers may coexist even for $\mathrm{A}{\mathrm{u}}_8$, $\mathrm{A}{\mathrm{u}}_9$, and $\mathrm{A}{\mathrm{u}}_{10}$ clusters. Density-functional theory exchange-correlation functionals within the generalized gradient or hybrid approximation must be corrected for long-range van der Waals interactions to accurately predict relative gold cluster isomer stabilities. Our paper gives insight into the stable structures of gas phase gold clusters by highlighting the impact of temperature, and therefore the importance of free energy over total-energy studies, and long-range van der Waals interactions on gold cluster stability.