In geometrically frustrated magnetic systems, weak interactions or slight changes to the structure can tip the delicate balance of exchange interactions, sending the system into a different ground state. Brochantite, ${\mathrm{Cu}}_{\text{4}}{\mathrm{SO}}_{\text{4}}{\left(\mathrm{OH}\right)}_{\text{6}}$, has a copper sublattice composed of distorted triangles, making it a likely host for frustrated magnetism, but it exhibits stacking disorder. The lack of synthetic single crystals has limited research on the magnetism in brochantite to powders and natural mineral crystals. We grew crystals which we find to be a new polytype with a tendency toward ABAC stacking and some anion disorder, alongside the expected stacking disorder. Comparison to previous results on natural mineral specimens suggests that cation disorder is more deleterious to the magnetism than anion and stacking disorder. Our specific heat data suggest a double transition on cooling into the magnetically ordered state.

We use density functional theory (DFT) with non-empirically tuned screened range-separated hybrid (SRSH) functionals to calculate the electronic properties of native zinc and oxygen vacancy point defects in ZnO, and we predict their defect levels for thermal and optical transitions in excellent agreement with available experiments and prior calculations that use empirical hybrid functionals. The ability of this non-empirical first-principles framework to accurately predict quantities of relevance to both bulk- and defect-level spectroscopy enables high-accuracy DFT calculations with non-empirical hybrid functionals for defect physics, at a reduced computational cost.

The intensity of red $\mathrm{C}{\mathrm{r}}^{3+}$ photoluminescence (PL) in monoclinic gallium oxide $\left(\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3\right)$ is suppressed by $n$-type conductivity, an effect that has been attributed to a Cr deep acceptor level in the bandgap. In $n$-type material, such an acceptor level would be occupied, resulting in the $\mathrm{C}{\mathrm{r}}^{2+}$ oxidation state and the absence of $\mathrm{C}{\mathrm{r}}^{3+}$ PL. To test this model, $n$-type $\beta \text{−}\mathrm{G}{\mathrm{a}}_2{\mathrm{O}}_3$ crystals co-doped with Cr and Zr (a donor) were grown from the melt. The samples show $\mathrm{C}{\mathrm{r}}^{3+}$ optical absorption bands and a high free-electron concentration of $4\times {10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$. If Cr were an acceptor, then it would be fully compensated and therefore would not exhibit the $\mathrm{C}{\mathrm{r}}^{3+}$ optical signature. Hybrid functional calculations indicate that Cr occupies the substitutional octahedral Ga(II) site and that the $\mathrm{C}{\mathrm{r}}^{2+}$ state is energetically unfavorable, i.e., Cr is not an acceptor. Weak $\mathrm{C}{\mathrm{r}}^{3+}$ PL was observed in the Cr/Zr co-doped samples. The quenching of PL may be caused by a transfer of energy to free electrons, a nonradiative process that would reduce the emission intensity.

We report on a combined structural and magnetotransport study of Hall bar devices of various lateral dimensions patterned side-by-side on epitaxial ${\mathrm{PdCoO}}_2$ thin films. We study the effects of both the thickness of the ${\mathrm{PdCoO}}_2$ film and the width of the channel on the electronic transport and the magnetoresistance properties of the Hall bar devices. All the films with thicknesses down to 4.88 nm are epitaxially oriented, phase-pure, and exhibit metallic behavior. At room temperature, resistivity values as low as 6.85 and $8.17\mu \mathrm{\Omega }\mathrm{cm}$ are achieved in Hall bar devices with channel width $\text{W}=2.5$µm and $\text{W}=10$µm, respectively. For the 4.88 nm thick sample, we find that while the density of the conduction electrons is comparable in both channels, the electrons move about twice as fast in the narrower channel. At low temperatures, for Hall bar devices of channel width $2.5$µm fabricated on epitaxial films of thicknesses 4.88 and 5.21 nm, the electron mobilities of $\approx 65$ and $40{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, respectively, are extracted. For thin-film Hall bar devices of width $10$µm fabricated on the same 4.88 and 5.21 nm thick samples, the mobility values of $\approx 32$ and $18{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ are obtained. The magnetoresistance characteristics of these ${\mathrm{PdCoO}}_2$ films are observed to be temperature- dependent and exhibit a dependency with the orientation of the applied magnetic field. When the applied field is oriented ${90}^{\circ }$ away from the crystal $c$-axis, a persistent negative MR at all temperatures is observed. However when the field is parallel to the $c$-axis, the negative magnetoresistance is suppressed at temperatures above 150 K.

The instantaneous nonlinear optical (NLO) responses of materials are typically limited to subbandgap excitation involving virtual states. However, achieving fast responses for above-bandgap excitation is extremely challenging because of the involvement of real states and associated electron-phonon (e-ph) scattering processes. Here, we investigate the NLO response of ${\mathrm{NiSe}}_2$ nanoparticles (NPs) using femtosecond (120 fs) and nanosecond (7 ns) laser pulses. Our experiments, supported by first-principles calculations, reveal that ${\mathrm{NiSe}}_2$ exhibits reverse saturable absorption (RSA) for femtosecond excitation, enabled by near-instantaneous intraband thermalization within the pulse duration. In contrast, ${\mathrm{CoSe}}_2$ NPs display saturable absorption, likely due to incomplete thermalization. Notably, when the laser interaction time is extended to nanoseconds, RSA emerges in both materials, which concurs with our model. Strikingly, ${\mathrm{NiSe}}_2$ NPs exhibit an unprecedented enhancement in the NLO response for nanosecond excitation, exceeding four orders of magnitude compared to femtosecond excitation, highlighting the role of prolonged excited-state dynamics. Our results provide a frontier in ultrafast optoelectronic device applications for above-bandgap excitations.

Glassy materials yield under large external mechanical solicitations. Under oscillatory shear, yielding shows a well-known rheological fingerprint, common to samples with widely different microstructures. At the microscale, this corresponds to a transition between slow, solidlike dynamics and faster liquidlike dynamics, which can coexist at yielding in a finite range of strain amplitudes. Here we capture this phenomenology in a lattice model with two main parameters: glassiness and disorder, describing the average coupling between adjacent lattice sites, and their variance, respectively. In absence of disorder, our model yields a law of correspondent states equivalent to trajectories on a cusp catastrophe manifold, a well-known class of problems including equilibrium liquid-vapor phase transitions. Introducing a finite disorder in our model entails a qualitative change, to a continuous and rounded transition, whose extent is controlled by the magnitude of the disorder. We show that a spatial correlation length $\xi$ emerges spontaneously from the coupling between disorder and bifurcating dynamics. With vanishing disorder, $\xi$ diverges and yielding becomes discontinuous, suggesting that the abruptness of yielding can be rationalized in terms of a length scale of dynamic heterogeneities.

The deformation behaviors of nanocrystalline high-entropy ceramics are complex and remain poorly understood, despite the critical importance to their performance and applications. Here, we employed molecular dynamics simulations to examine the shear deformation mechanisms of nanocrystalline high-entropy carbide ceramic, $n-\left(\mathrm{T}{\mathrm{i}}_{0.2}\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{N}{\mathrm{b}}_{0.2}\mathrm{H}{\mathrm{f}}_{0.2}\mathrm{T}{\mathrm{a}}_{0.2}\right)\mathrm{C}$, across grain sizes ranging from 4.0 to 12.0 nm. The atomic interactions were described using a machine-learning force field, which was developed based on deep neural networks. The simulation results reveal that the main deformation mechanism of $n-\left(\mathrm{T}{\mathrm{i}}_{0.2}\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{N}{\mathrm{b}}_{0.2}\mathrm{H}{\mathrm{f}}_{0.2}\mathrm{T}{\mathrm{a}}_{0.2}\right)\mathrm{C}$ is grain boundary (GB) sliding, which then leads to the nucleation and growth of cavities and cracks at the GBs, ultimately resulting in intergranular fracture. For larger grains, the reduced soft GB volume fraction inhibits the GB sliding, leading to an inverse Hall-Petch behavior in $n-\left(\mathrm{T}{\mathrm{i}}_{0.2}\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{N}{\mathrm{b}}_{0.2}\mathrm{H}{\mathrm{f}}_{0.2}\mathrm{T}{\mathrm{a}}_{0.2}\right)\mathrm{C}$. Additionally, GB segregation often strengthens GBs, thereby enhancing the strength of $n-\left(\mathrm{T}{\mathrm{i}}_{0.2}\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{N}{\mathrm{b}}_{0.2}\mathrm{H}{\mathrm{f}}_{0.2}\mathrm{T}{\mathrm{a}}_{0.2}\right)\mathrm{C}$ and mitigating intergranular fracture. These findings shed light on the deformation behaviors and underlying mechanisms of nanocrystalline high-entropy carbide ceramics, and they provide important guidance for the design of high-entropy ceramics with superior performance.

Double-halide perovskites are recently attracting significant interest in the field of thermoelectric research due to the possibility of achieving very low thermal conductivity and retaining relatively high Seebeck coefficient and electrical conductivity. Accurate estimates of the transport related properties are, thus, highly desirable and are strictly linked to an extremely detailed characterization of the microscopic mechanism underlying the transport itself. To address this issue, in this study we conduct a comprehensive theoretical investigation of one of the main process-limiting carrier transports, i.e., electron-phonon scattering, in a typical double-halide perovskite, ${\mathrm{Cs}}_2{\mathrm{NaYbCl}}_6$. In order to quantify the specific magnitude of this process, we adopt in this study a comprehensive DFT analysis, with a focus on evaluating the resulting lifetimes (in both hole- and electron-mediated regimes), by solving iteratively the linearized Boltzmann equation. Using the evaluated lifetimes, we found distinct differences in electrical mobilities and Seebeck coefficient as a function of the temperature and carrier concentrations, revealing that the thermoelectric figure-of-merit $ZT$, while quite low, is significantly higher in the case of electron-mediated regime.

Two-dimensional (2D) Janus materials hold a great importance in spintronic and valleytronic applications due to their unique lattice structures and emergent properties. They intrinsically exhibit both in-plane inversion and out-of-plane mirror symmetry breakings, which provide a new degree of freedom to electrons in the material. One of the main limitations in the multifunctional applications of these materials is, however, that they are usually nonmagnetic in nature. Here, using first-principles calculations, we propose to induce magnetic degrees of freedom in nonmagnetic WSTe via substituting transition metal (TM) elements—Mn, Fe, and Co. Further, we comprehensively probe the electronic, spintronic, and valleytronic properties in TM-WSTe systems. Our simulations predict intrinsic Rashba- and Zeeman-type spin splittings in pristine WSTe. The obtained Rashba parameter is $\sim 422\mathrm{meV}\AA$ along the $\mathrm{\Gamma }-K$ direction. The Rashba spin splitting is observed to show a strong dependence on uniaxial and biaxial strains. The electronic structure of TM-WSTe reveals a half-metallic nature for 6.25% and 18.75% of Fe, 25% of Mn, and 18.75% and 25% of Co structures, which leads to a 100% spin polarization. The structural asymmetry, strong spin-orbit coupling, and broken inversion symmetry are found to lead to valley polarization in TM-WSTe systems. Further, our calculations predict a strain-dependent tunability of valley polarization, where we find an increasing(decreasing) trend with uniaxial and biaxial tensile(compressive) strains. We observed a maximum enhancement of $\sim 172\%$ for 6.25% of Fe doping on application of 3% biaxial tensile strain. The ability to control valley degrees of freedom, along with spin and charge, could open new prospects for next-generation spintronic and valleytronic devices.

This work presents a comparative analysis of tin telluride (SnTe) films with and without the protection of a cap layer that was implemented to protect from oxidation the topological surface states on the SnTe. The implementation of the cap layer aims to overcome a major issue that poses a challenge to the implementation of topological insulators for development of operational spintronic devices. Analysis involves a detailed investigation of the Shubnikov–de Haas oscillations and extracting the Berry phases that indeed indicate the presence of Dirac fermions in the capped sample. The introduction of the cap layer was successful in preserving the structural quality of the sample and, at the same time, maintaining the crystalline symmetries responsible for protecting the topological surface states.

The negative magnetization and exchange bias phenomena have been the subject of interest due to their promising applications in spintronic devices. In this study, we have comprehensively investigated these two intertwined magnetic phenomena in $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{P}{\mathrm{r}}_x\mathrm{Cr}{\mathrm{O}}_3$ ($x=0.8-0.9$) compounds. The Cooke's model fit to dc magnetization data infers negative internal magnetic field for $x=0.8$, 0.85, and 0.87 and positive for $x=0.9$. The crossover of internal magnetic field from negative to positive across $x=0.87$ and its dominance over the external applied magnetic field leads to the magnetization switching from negative to positive. The internal magnetic field behavior also explains the anomalous magnetization behavior involving reduced magnetization for $x=0.87$, despite having higher ${Pr}^{3+}$ concentration than that for $x=0.8$ and 0.85. The anomalous magnetization is corroborated well by the neutron depolarization experiments, where no depolarization is observed for $x=0.87$ owing to nearly compensated domain magnetization. Remarkably, a switching of the exchange bias from inverse (with positive $H_{\mathrm{EB}}$) to conventional (with negative $H_{\mathrm{EB}}$) due to a competition of external and internal magnetic fields is found in $x=0.8$ and 0.85 compounds. The $x=0.87$ and 0.9 compounds, on the other hand, show only conventional exchange bias at all measured magnetic fields. Using the Cooke's model, we show that the antiferromagnetic coupling between polarized ${Pr}^{3+}$ and weak ferromagnetic component of canted $\mathrm{C}{\mathrm{r}}^{3+}$ moments explain not only the inverse exchange bias but the conventional exchange bias as well, however, the moment orientations are different for both types of exchange bias. Moreover, our study demonstrates that the ferromagnetic coupling between the two moments can also lead to conventional exchange bias, similar to that observed in interfacial heterostructure systems.

An appropriate description of the lithiation dynamics in biphasic primary cathode particles of Li-ion batteries requires an accurate treatment of the conditions holding at the interface between the particle and the surrounding liquid electrolyte. We propose a phase field model based on the Allen-Cahn approach within which the particle-electrolyte interface is smooth [smoothed boundary method (SBM)], to simulate arbitrarily shaped particles. Surface terms are added to the evolution equations, and SBM calculations are compared with benchmark simulations for which the boundary conditions are explicitly imposed at the borders of the calculation domain. Our findings highlight the necessity of introducing strengthening factors for the surface terms to achieve the desired conditions for the phase and elastic fields, and to enable an accurate reproduction of stress distributions within the particle. This refinement of the SBM is critical for reliable predictions of Li insertion/extraction rates and lithium diffusion behavior in the context of Li-ion batteries. We perform also a simulation under potentiostatic conditions with a full coupling of the different physical processes at play. It illustrates the applicability of our approach and demonstrates the capabilities of the SBM for a simulation of lithiation dynamics with coupled electrochemistry and mechanics.

At very fine grain sizes, grain boundary segregation can deviate from conventional behavior due to triple junction effects. While this issue has been addressed in prior work for substitutional alloys, here we develop a framework that accounts for interstitial sites in the grains, grain boundaries, and triple junctions of model Pd(H) polycrystals. This approach allows computation of interstitial segregation spectra separately at both defect types, which permits an understanding of segregation at all grain sizes via a size-scaling spectral isotherm. The size dependencies of dilute Pd(H) are found to be influenced not only by the triple junction content, but also by grain size–dependent lattice strains; the latter effect is evidenced by size dependencies of individual grain boundary and junction subspectra. The framework proposed here is applicable to interstitial alloys in general and may serve as a basis for interfacial engineering in interstitial nanocrystalline alloys. As an example, we show that using the dilute limit isotherm, hydrogen density can triple in nanocrystalline vis-à-vis microcrystalline Pd due to hydrogen adsorption at intergranular defect sites.

The magnetic ground state of $\mathrm{KT}{\mathrm{b}}_3{\mathrm{F}}_{10}$ was investigated using a combination of magnetization measurements and neutron scattering experiments, carried out on either a polycrystal or a single-crystal sample. Results show that $\mathrm{T}{\mathrm{b}}^{3+}$ remains in a crystalline electric field singlet state down to 30 mK. A susceptibility tensor analysis performed on a comprehensive neutron single-crystal diffraction dataset shows that the induced magnetism observed when applying a magnetic field is highly anisotropic, magnetic moments being confined within local anisotropy planes. These features can be well captured by a detailed analysis of the $\mathrm{T}{\mathrm{b}}^{3+}$ crystal field scheme, combining point charge modeling and inelastic neutron scattering measurements in zero and applied magnetic field. Moreover, the local singlet ground state is protected by a large 2.7 meV gap, which the weak magnetic interactions between Tb moments are too weak to overcome, thereby preventing the formation of a magnetic ground state. Within this approximation, an estimate of the magnetic exchanges was extracted from the magnetic field evolution of the ordered Tb moments, obtained from the analysis of the diffraction data. These results provide a first step toward the understanding of the unusual magneto-optical and low thermal conductivity properties of the singlet system $\mathrm{KT}{\mathrm{b}}_3{\mathrm{F}}_{10}$.

Here we demonstrate the floating zone crystal growth of the $J_{\mathrm{eff}}=1/2$ Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Historically, the growth of iridates from a ternary melt has been precluded by the extreme vapor pressure of the metal oxide species and the difficulty of maintaining the correct oxidation state of Ir at high temperatures. Here, we show that the application of a high-pressure oxygen growth environment stabilizes the ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ phase, leading to the first demonstration of ${\mathrm{cm}}^3$-scale crystals. In contrast with the conventional ${\mathrm{SrCl}}_2$ flux growth method, where poor control over disorder leads to strong sample dependence, the high-pressure floating zone growth enables active control over the homogeneity of the melt. Crystals grown via this technique possess qualitatively similar properties to those grown via flux, with a relatively sharp onset of antiferromagnetic order observed in temperature-dependent magnetization. Furthermore, we demonstrate that by tuning the mixing rate of the melt, we are able to grow natively hole-doped ${\mathrm{Sr}}_2{\mathrm{Ir}}_{1-y}{\mathrm{O}}_4$, which exhibits a strongly modified magnetic and electronic response.

Combining the efficiency of semiempirical potentials with the accuracy of quantum mechanical methods, machine-learning interatomic potentials (MLIPs) have significantly advanced atomistic modeling in computational materials science and chemistry. This necessitates the continual development of MLIP models with improved accuracy and efficiency, which enable long-time scale molecular dynamics simulations to unveil the intricate underlying mechanisms that would otherwise remain elusive. Among various existing MLIP models, the moment tensor potential (MTP) model employs a highly descriptive rotationally covariant moment tensor to describe the local atomic environment, enabling the use of even linear regression for model fitting. Although the current MTP model has achieved state-of-the-art computational performance for similar accuracy, there is still room for optimizing the contraction process of moment tensors. In this paper, we propose an effective genetic algorithm-based optimization scheme that can significantly reduce the number of independent moment tensor components and intermediate tensor components. This leads to a speedup of nearly one order of magnitude in evaluation and also improved accuracy compared to the traditional MTP model for intricate basis sets. We have applied our improved MTP model to predicting the energetic and dynamical properties of various point and planar defects in Ni-Al alloys, showing overall good performance and in general outperforming the semiempirical potentials. This paper paves the way for fast and accurate atomistic modeling of complex systems and provides a useful tool for modeling defects in Ni-Al alloys.