The band connectivity as imposed by the compatibility relations between the irreducible representations of little groups can give rise to the exotic hourglasslike shape composed of four branches of bands and five band crossings (BCs). Such an hourglass band connectivity could enforce the emergence of nontrivial excitations like Weyl fermion, Dirac fermion, or even beyond them. On the other hand, the bosons, like phonons, magnons, and photons, were also shown to possess nontrivial topology and a comprehensive symmetry classification of the hourglass bosonic excitations would be of great significance to both materials design and device applications. Here we first list all concrete positions and representations of little groups in the Brillouin zone (BZ) related to the hourglass bosonic excitations in all 1651 magnetic space groups and 528 magnetic layer groups, applicable to three dimensional (3D) and two dimensional (2D) systems, respectively. 255 (42) MSGs (MLGs) are found to essentially host such hourglass BCs: Here “essentially” means that the bosonic hourglass BC exists definitely as long as the studied system is crystallized in the corresponding MSG/MLG. We also perform first-principles calculations on hundreds of 3D nonmagnetic materials, essentially hosting hourglass phonons and propose that the 2D material AlI can host hourglass phonons. We choose AuX (X = Br and I) as illustrative examples to demonstrate that two essential hourglass band structures can coexist in the phonon spectra for both materials while for AuBr, an accidental band crossing sticking two hourglasses is found, interestingly. Our results of symmetry conditions for hourglass bosonic excitations can provide a useful guide of designing artificial structures with hourglass bosonic excitations.

The effects of chemical pressure on the structural and magnetic properties of the triple perovskite ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ are investigated by substituting ${\mathrm{Sr}}^{2+}$ ions for ${\mathrm{Ba}}^{2+}$ ions. Two ${\mathrm{Ba}}_{3-x}{\mathrm{Sr}}_x\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ phases could be stabilized via a solid-state reaction at ambient pressure (AP) in air. The $6H$ with ${\mathrm{Sb}}_2{\mathrm{O}}_9$ pairs $\left(x=0\right)\to 6H$ with $\mathrm{NiSb}{\mathrm{O}}_9$ pairs $\left(x=0.5\right)\to 3C$ (cubic with corner-sharing octahedral, $x=1.25$) sequence of structural phases occurs with increasing Sr content, i.e., chemical pressure, which is like that previously reported for pure samples of ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ obtained under increasing high physical pressure (HP). For the $6H {\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9 \left(x=0.5\right)$ phase, using combined Rietveld refinements of powder x-ray and neutron diffraction patterns, precession electron diffraction tomography data collected on thin crystals, aberration-corrected high-angle annular dark field scanning transmission electron microscopy coupled to energy dispersive x-ray spectroscopy mapping, we reach the conclusion that the structure features corner-sharing $\mathrm{Sb}{\mathrm{O}}_6$ octahedra and $\mathrm{NiSb}{\mathrm{O}}_9$ pairs of face-shared octahedra (or Ni-Sb dumbbells) with either a random orientation of the Ni-Sb dumbbells or nanosized chemical correlations for the dumbbell arrangement. As observed in HP ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ produced through synthesis at 9 GPa, AP ${\mathrm{Ba}}_{1.75}{\mathrm{Sr}}_{1.25}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9 \left(x=1.25\right)$ crystallizes in a $3C$ double perovskite $A_{2}B{B}^{\prime }{\mathrm{O}}_6$ cubic structure where $A, B$, and $B^{\prime }$ sites are occupied by (Ba + Sr), Sb, and $\left(\frac{2}{3}\mathrm{Ni}+\frac{1}{3}\mathrm{Sb}\right)$ atoms, respectively. The $B^{\prime }$ sites, which are randomly occupied by spin-1 ${\mathrm{Ni}}^{2+}$ and diamagnetic ${\mathrm{Sb}}^{5+}$, form a face-centered-cubic (FCC) sublattice where the ${\mathrm{Ni}}^{2+}$ amount stays above the site percolation threshold. Weiss temperatures ($\approx -65$ and $\approx -213$ K for ${\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ and ${\mathrm{Ba}}_{1.75}{\mathrm{Sr}}_{1.25}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$, respectively) indicate that dominant magnetic interactions between ${\mathrm{Ni}}^{2+}$ spins are antiferromagnetic with magnitudes like those observed in the corresponding HP phases of pure ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$. As for the $6H$ HP ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ compound, in $6H {\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$, muon spin relaxation $\left(\mu \mathrm{SR}\right)$ measurements identify a dynamic magnetic state down to the base temperature (95 mK), consistent with a previously published inelastic neutron scattering study. For $3C {\mathrm{Ba}}_{1.75}{\mathrm{Sr}}_{1.25}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9, \mu \mathrm{SR}$ and ${}^{121}\mathrm{Sb}$ nuclear magnetic resonance measurements both indicate the presence of a transition to a static magnetic state below $11\left(1\right)$ K with a significant amount of disorder in this frozen state, in contrast to the spin-liquid state previously suggested for the $3C$ HP phase of ${\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$. Consistently, a broad maximum is observed in the specific heat at the same temperature. Building on the structural data, the magnetic properties of HP $6H {\mathrm{Ba}}_3\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ and AP $6H {\mathrm{Ba}}_{2.5}{\mathrm{Sr}}_{0.5}\mathrm{Ni}{\mathrm{Sb}}_2{\mathrm{O}}_9$ are discussed in light of recent works on triangular and $J_1\text{−}{J}_2$ honeycomb systems with or without quenched disorder. We are led to the conclusion that the driving force toward a spin-liquid-like state is quenched disorder which needs to be incorporated in $J_1\text{−}{J}_2$ honeycomb models. Our evidence of a magnetic transition to a frozen magnetic ground state for the AP Sr-doped $3C$ phase is in line with models for geometrically frustrated FCC antiferromagnets. This calls for a better experimental and possibly theoretical understanding of the HP $3C$ phase.

The quasi-one-dimensional Chevrel phases, $A_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ ($A=\mathrm{Tl}$, In, K, Rb, Cs), are of interest due to their atypical electronic properties. The Tl and In analogs undergo a superconducting transition whereas the alkali metal analogs show charge gapping of another, not well understood type. We report the results of inelastic neutron scattering on polycrystalline ${\mathrm{In}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ ($T_{\mathrm{c}}=2.85$ K) and ${\mathrm{Rb}}_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ (nonsuperconducting) samples, which reveal a column of intensity with linear dispersion from [0 0 1/2] to [0 0 1] in both compounds. The observed temperature and $\left|Q\right|$ independence together suggest the presence of unconventional carriers with a spin contribution to the excitation. This is contrary to the prevailing model for these materials, which is that they are nonmagnetic. The excitation has similar dispersion and $S\left(Q,\mathrm{E},T\right)$ behavior as one observed in the structurally related superconducting compounds $A_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ and $A_2{\mathrm{Mo}}_3{\mathrm{As}}_3$ ($A=\mathrm{K}$, Rb, Cs), which has been interpreted as magnetic in origin and related to Fermi surface nesting. The connection is unexpected because the calculated Fermi surface of the arsenides differs substantially from the $A_2{\mathrm{Mo}}_6{\mathrm{Se}}_6$ compounds, and many consider them distinct classes of materials. This observation suggests a hidden link in the physics between both classes of superconductors, perhaps originating from their quasi-low-dimensional character.

Materials with one-dimensional (1D) chains of magnetic ions with reduced spin provide a platform for enhanced quantum behavior. Coordination polymers, or equivalently magnetic metal-organic frameworks (MOFs), with both organic and inorganic building blocks offer the ability to design these tailored structural motifs due to the versatility and predictability of organic chemistry. Here we focus on a series of compounds $M{\left({\mathrm{N}}_2{\mathrm{H}}_5\right)}_2{\left({\mathrm{SO}}_4\right)}_2$, where $M$ is the transition metal ion ${\mathrm{Cu}}^{2+}$, ${\mathrm{Co}}^{2+}$, or ${\mathrm{Mn}}^{2+}$, that forms isolated 1D spin chains within the bulk crystalline lattice. The behavior of the S = 1/2 Cu compound is compared with higher spin Mn and Co based materials through low temperature elastic and inelastic neutron scattering measurements. These provide insights into the magnetic interactions and structure in these materials. In addition, polarized neutron powder diffraction measurements were utilized to determine the site susceptibility tensor in all compounds that provides further insight into the magnetic interactions.

Antiferromagnetic antiperovskites, where magnetically active $3d$ metal cations are placed in the octahedral corners of a perovskite structure, are in the spotlight due to their intertwined magnetic structure and topological properties. Especially their anomalous Hall conductivity, which can be controlled by applied strain and/or electric field, makes them highly attractive in different electronic applications. Here, we present the study and theoretical understanding of an antiperovskite compound that can offer enormous opportunities in a broad set of applications. Using first-principles calculations, we investigated the structure, lattice dynamics, noncollinear magnetic ordering, and electronic behavior in the vanadium-based antiperovskite ${\mathrm{V}}_3\mathrm{AuN}$. We found an antiperovskite structure centered on N similar to the ${\mathrm{Mn}}_{3}A\mathrm{N}$ family as the structural ground state. In such a phase, a $Pm\overline{3}m$ ground state was found in contrast to the Cmcm post-antiperovskite layered structure, as in the ${\mathrm{V}}_{3}A\mathrm{N}$, $A=\mathrm{Ga}$, Ge, As, and P. We studied the lattice dynamics and electronic properties, demonstrating its vibrational stability in the cubic structure and a chiral antiferromagnetic noncollinear ordering as a magnetic ground state. Finally, we found that the anomalous Hall conductivity, associated with the topological features induced by the magnetic symmetry, is ${\sigma }_{xy}=-291\mathrm{S}{\mathrm{cm}}^{-1}$ $({\sigma }_{111}=-504\mathrm{S}{\mathrm{cm}}^{-1})$. The latter is the largest reported in the antiferromagnetic antiperovskite family of compounds.

We demonstrate that surface morphogenesis in compressed thin films may result from spatially correlated plastic activity. A soft glassy film strongly adhering to a smooth and rigid substrate and subjected to uniaxial compression, indeed, does not undergo any global elastic pattern-forming instability, but responds plastically via localized burst events that self-organize, leading to the emergence of a series of parallel ridges transverse to the compression axis. This phenomenon has been completely overlooked, but results from common features of the plastic response of glasses hence should be highly generic for compressed glassy thin films.

We report on the magnetic, electrical transport, and thermal properties of ${\mathrm{Ho}}_2{\mathrm{Re}}_3{\mathrm{Si}}_5$ single crystal by means of magnetization, electrical resistivity, and heat capacity measurements in the temperature range 2–300 K. Magnetic susceptibility, heat capacity, and electrical resistivity reveal two closely spaced antiferromagnetic transitions at $T_{\mathrm{N}1}\sim$ 3.8 K and $T_{\mathrm{N}2}\sim$ 4.2 K. Three field-induced metamagnetic transitions at 4.5, 10, and 15 kOe, in the magnetically ordered state at 2 K, have been observed, in the isothermal magnetization, for ${\mu }_{0}H\parallel c$, and their first-order nature is evident from the magnetic hysteresis. The hysteresis disappears and metamagnetic transitions become feeble at around 3.5 K. These metamagnetic transitions are consistent with field-induced magnetic phases as observed in the magnetic susceptibility, electrical resistivity, and heat capacity. The magnetization for ${\mu }_{0}H\parallel a$ is linear without any signature of metamagnetic transition for fields up to 70 kOe suggesting the hard axis of magnetization. From the temperature dependence of the isothermal magnetization, we have constructed a magnetic phase diagram, which signals a complex magnetic structure in this compound. These results suggest that magnetic and transport properties are highly sensitive to the external magnetic field.

Crosslinked polymer materials provide tremendous opportunities for delivering unique material properties for a wide variety of applications such as shape-memory and self-healing materials, aerospace materials, and biomedical systems. Most crosslinked polymer networks investigated to date are designed based on covalent or noncovalent crosslinkers that include two-ended connections to the backbone polymer base. In this paper, we investigate the topic of multi-arm crosslinking of polymer materials using a computational platform. We take advantage of molecular dynamics simulations and graph theory to define computational models that describe potential architectures for multi-arm crosslinked polymer networks. We also discuss feasible experimental routes for implementation of these approaches. We investigate how the polymer network architecture affects the mechanical and self-healing behavior of the material. Our results show that the angular stiffness of multi-arm crosslinkers can be adjusted to control the mechanical strength of the overall polymer network. Additionally, using graph theory, we find that the complex connections that exist between the crosslinkers and the backbone polymer chains define the overall connectivity of the polymer network, which in turn dictates the mechanical behavior of the material. Furthermore, we find that increasing the number of crosslinking arms may improve the self-healing behavior, but it can reduce the overall mechanical strength of the polymer network. The findings of this paper can motivate future experimental and data-driven studies to realize more sophisticated crosslinked polymer materials with a large number of degrees of freedom to deliver a variety of tunable properties.

We have investigated the As-doping effect in the newly discovered kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ via measurements of x-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat. Our results show that partial substitution of Sb with As leads to shrinkage of the $c$ lattice, which effectively applies a chemical pressure in the system. The solubility of As atoms is about $2.3\%$ for single crystals grown by the self-flux method. In the As-doped single crystal ${\mathrm{CsV}}_3{\left({\mathrm{Sb}}_{0.977}{\mathrm{As}}_{0.023}\right)}_5$, the charge-density-wave (CDW) transition temperature $T_{\mathrm{CDW}}$ is suppressed from 94 to 83 K, and the superconducting transition temperature $T_{\mathrm{c}}$ is enhanced from 2.5 to 3.6 K. Furthermore, the residual resistivity ratio is significantly reduced, and the magnetoresistance with magnetic field along the $c$ axis decreases by nearly one order of magnitude, indicating substantial disorder scattering induced by the As doping. The anomalous Hall effect (AHE) is only observed below $T_{\mathrm{CDW}}$, indicating the intimate relationship between CDW and AHE. We found that the As-doping effect on $T_{\mathrm{c}}$ and $T_{\mathrm{CDW}}$ is basically equivalent to the hydrostatic pressure effect at 0.2 GPa.

Understanding the interplay of structural and electronic symmetry breaking in iron-based high-temperature superconductors remains of high interest. In this work we grow strain-patterned multilayer FeSe thin films in a range of thicknesses using molecular beam epitaxy. We study the formation of electronic nematic domains and spatially varying strain using scanning tunneling microscopy and spectroscopy. We directly visualize the formation of edge dislocations that give rise to a two-dimensional edge dislocation network in the films. Interestingly, we observe a ${45}^{\circ }$ in-plane rotation of the dislocation network as a function of film thickness, yielding antisymmetric strain along different directions. This results in different coupling ratios between electronic nematic domains and antisymmetric strain. Lastly, we are able to distinguish between different orthogonal nematic domains by revealing a small energy-dependent difference in differential conductance maps between the two regions. This could be explained by orbital-selective tip-sample tunneling. Our observations bring new insights into the dislocation network formation in epitaxial thin films and provide another nanoscale tool to explore electronic nematicity in iron-based superconductors.

On the basis of purely their electronic properties, semiconducting alkali and alkaline earth metal based Heusler alloys have been identified as potentially promising thermoelectrics. Their thermal transport properties are decisive and help to further gauge this potential. This motivates us to systematically investigate lattice properties of three eight valence electron Heusler alloys, namely, ${\mathrm{Li}}_2\mathrm{NaSb}, {\mathrm{Na}}_2\mathrm{KSb}$, and ${\mathrm{K}}_2\mathrm{CsSb}$, using first-principles approaches. On the basis of both qualitative and quantitative descriptors (assessing covalent bonding, the atomic displacement parameter, the phonon density of states, the Grüneisen parameter, the phonon group velocity, and the phonon lifetime) we analyze in detail the lattice degrees of freedom and their contribution to thermal transport. Out of the three materials, ${\mathrm{Na}}_2\mathrm{KSb}$ and ${\mathrm{K}}_2\mathrm{CsSb}$ exhibit surprisingly low lattice thermal conductivity, which we can attribute to the anharmonic rattling of the loosely bound alkali atom in the crystal lattice. ${\mathrm{K}}_2\mathrm{CsSb}$ has a lattice thermal conductivity two times lower than that of SnSe, a material with a record figure of merit.

Using activation-relaxation technique nouveau (artn), we search for two-level systems (TLSs) in models of amorphous silicon ($a$-Si). The TLSs are mechanisms related to internal mechanical dissipation and represent the main source of noise in the most sensitive frequency range of the largest gravitational wave detectors as well as one of the main sources of decoherence in many quantum computers. We show that in $a$-Si, the majority of the TLSs of interest fall into two main categories: bond-defect hopping, where neighbors exchange a topological defect, and Wooten-Winer-Weaire bond exchange. The distribution of these categories depends heavily on the preparation schedule of the $a$-Si. We use our results to compute the mechanical loss in amorphous silicon, leading to a loss angle of ${10}^{-3}$ at room temperature, decreasing to ${10}^{-4}$ at 150 K in some configurations. Our modeling results indicate that multiple classes of events can cause experimentally relevant TLSs in disordered materials and therefore multiple attenuation strategies might be needed to reduce their impact.

Machine learning (ML) methods are extraordinarily successful at denoising photographic images. The application of such denoising methods to scientific images is, however, often complicated by the difficulty in experimentally obtaining a suitable expected result as an input to training the ML network. Here, we propose and demonstrate a simulation-based approach to address this challenge for denoising atomic-scale scanning tunneling microscopy (STM) images, which consists of training a convolutional neural network on STM images simulated based on a tight-binding electronic structure model. As model materials, we consider graphite and its mono- and few-layer counterpart, graphene. With the goal of applying it to any experimental STM image obtained on graphitic systems, the network was trained on a set of simulated images with varying characteristics such as tip height, sample bias, atomic-scale defects, and nonlinear background. Denoising of both simulated and experimental images with this approach is compared to that of commonly used filters, revealing a superior outcome of the ML method in the removal of noise as well as scanning artifacts—including on features not simulated in the training set. An extension to larger STM images is further discussed, along with intrinsic limitations arising from training set biases that discourage application to fundamentally unknown surface features. The approach demonstrated here provides an effective way to remove noise and artifacts from typical STM images, yielding the basis for further feature discernment and automated processing.

Thermal decomposition of vicinal $6H$-SiC(0001) surfaces with miscut angles toward the $\left[1\overline{1}00\right]$ direction results in the appearance of pairs of (0001) macroterraces and $\left(1\overline{1}0n\right)$ macrofacets covered with graphene, as follows. A carpetlike carbon layer grows on the surface, covering both the macroterraces and macrofacets; it forms a $(6\sqrt{3}\times 6\sqrt{3})$ buffer layer on the former ones, whereas its partial periodic bonding with the SiC steps on the latter ones generates a pseudographene nanoribbon (pseudo-GNR) array. The nanoribbons have a width of 1.7–1.8 nm and are aligned in the $\left[11\overline{2}0\right]$ direction with a spatial periodicity of 3.3 nm. Scanning tunneling spectroscopy at a nanoribbon indicated a 0.4–0.5 eV energy gap and the Raman spectroscopy analysis of the pseudo-GNR array showed the absence of the 2$D$ peak and the polarization dependence of the $G$ and $D$ peaks, which is typical of the armchair-edge nanoribbon.

One of the hallmarks of disordered matter is the large amplitude of the anelastic deformation, i.e., the fraction of reversible deformation that is not instantaneously recovered after the release of load but is delayed in time. In this paper, this delayed elasticity is studied for the glass-forming ${\mathrm{Zr}}_{46.25}{\mathrm{Ti}}_{8.25}{\mathrm{Cu}}_{7.5}{\mathrm{Ni}}_{10}{\mathrm{Be}}_{27.5}$ alloy by means of stress step and recovery experiments. Even at high temperatures, not far from the glass transition, the delayed elasticity can recover an important fraction of the deformation and endure for a long time. Analyzing the effects of loading time and waiting time on the strain evolution, we reveal the presence of an anelastic response with a timescale dependent on loading time and an invariant shape, which indicates the presence of a distribution of reversible relaxation modes following a ${\tau }^{-n}$ law with exponent $n$ between 0.5 and 1. The underlying distribution of energy barriers activated at different temperatures is accordingly shape invariant. Moreover, we found that a distribution of reversible modes corresponding to the high-frequency side of the $\alpha \text{−}\mathrm{relaxation}$ peak can reproduce the experimental results. The results establish a direct link between the dynamical spectrum and the distribution of activation energies, revealing the origin of the transient creep and anelastic recovery behaviors of metallic glasses.

Motivated by observations of extreme magnetoresistance (XMR) in bulk crystals of rare-earth monopnictide (RE-V) compounds and emerging applications in novel spintronic and plasmonic devices based on thin-film semimetals, we have investigated the electronic band structure and transport behavior of epitaxial GdSb thin films grown on III-V semiconductor surfaces. The ${\mathrm{Gd}}^{3+}$ ion in GdSb has a high spin $S=7/2$ and no orbital angular momentum, serving as a model system for studying the effects of antiferromagnetic order and strong exchange coupling on the resulting Fermi surface and magnetotransport properties of RE-Vs. We present a surface and structural characterization study mapping the optimal synthesis window of thin epitaxial GdSb films grown on III-V lattice-matched buffer layers via molecular-beam epitaxy. To determine the factors limiting XMR in RE-V thin films and provide a benchmark for band-structure predictions of topological phases of RE-Vs, the electronic band structure of GdSb thin films is studied, comparing carrier densities extracted from magnetotransport, angle-resolved photoemission spectroscopy (ARPES), and density-functional theory (DFT) calculations. ARPES shows a hole-carrier rich, topologically trivial, semimetallic band structure close to complete electron-hole compensation, with quantum confinement effects in the thin films observed through the presence of quantum-well states. DFT-predicted Fermi wave vectors are in excellent agreement with values obtained from quantum oscillations observed in magnetic field-dependent resistivity measurements. An electron-rich Hall coefficient is measured despite the higher hole-carrier density, attributed to the higher electron Hall mobility. The carrier mobilities are limited by surface and interface scattering, resulting in lower magnetoresistance than that measured for bulk crystals.

Incomplete cancellation of collinear antiparallel spins gives rise to ferrimagnetism. Even if the oppositely polarized spins are owing to the equal number of a single magnetic element having the same valence state, in principle, a ferrimagnetic state can still arise from the crystallographic inequivalence of the host ions. However, experimental identification of such a state as “ferrimagnetic” is not straightforward because of the often tiny magnitude expected for $M$ and the requirement for a sophisticated technique to differentiate similar magnetic sites. We report a synchrotron-based resonant x-ray investigation at the Fe $L_{2,3}$ edges on an epitaxial film of $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{O}}_4$, which exhibits two magnetic phases with similar energies. We find that while one phase of $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{O}}_4$ is antiferromagnetic, the other one is ferrimagnetic with an antiparallel arrangement of an equal number of spins between two distinct crystallographic sites with very similar local coordination environments. Our results further indicate two distinct origins of an overall minute $M$; one is intrinsic, from distinct ${\mathrm{Fe}}^{3+}$ sites, and the other one is extrinsic, arising from defective ${\mathrm{Fe}}^{2+}$ likely forming weakly coupled ferrimagnetic clusters. These two origins are uncorrelated and have very different coercive fields. Hence, this work provides a direct experimental demonstration of ferrimagnetism solely due to crystallographic inequivalence of the ${\mathrm{Fe}}^{3+}$ as the origin of the weak $M$ of $\mathrm{Ca}{\mathrm{Fe}}_2{\mathrm{O}}_4$.

Superalloys are a special class of heavy-duty materials with excellent strength retention and chemical stability at very high temperatures. Nickel-based superalloys are used commercially in aircraft turbines, power plants, and space launch vehicles. The optimization of mechanical properties of alloys has been traditionally carried out using experimental approaches, which demand massive costs in terms of time and infrastructure for testing. In this paper, we propose a method for mechanical property prediction of Ni-based superalloys by learning from past experimental results using machine learning (ML). Five highly accurate ML models are developed to predict yield strength (YS), ultimate tensile strength (UTS), creep rupture life, fatigue life with stress, and strain values. We have developed an extensive database containing mechanical properties of over 1500 Ni-based superalloys. Basic material parameters such as the composition of the alloy, annealing conditions, and testing conditions are also collected and used as features for developing the ML models. The prediction root mean squared errors for the YS, UTS, creep, and fatigue life models are 0.11, 0.06, 0.19, 0.22, which are minimal, leading to a highly accurate estimation of the target values. These ML models are highly transferable and require a minimum number of input features. In addition, feature analysis performed by SHapley Additive exPlanations (SHAP) for individual properties reveals the relative significance of each descriptor in deciding the target property. We demonstrate that a unified and highly accurate ML framework can be developed using common features for all mechanical properties. The models are developed on experimental data, making them directly applicable for industries.

The propagation of excitons in transition metal dichalcogenide (TMD) monolayers has been intensively studied revealing interesting many-particle effects, such as halo formation and nonclassical diffusion. Initial studies have investigated how exciton transport changes in twisted TMD bilayers, including Coulomb repulsion and Hubbard-like exciton hopping. In this work, we investigate the twist-angle dependent transition of the hopping regime to the dispersive regime of effectively free excitons. Based on a microscopic approach for excitons in the presence of a moiré potential, we show that the hopping regime occurs up to an angle of approximately $2^{\circ }$ and is well described by a moiré intercell tunneling model. At large angles, however, this nearest-neighbor approximation fails due to increasingly delocalized exciton states. Here, the quantum-mechanical dispersion of free particles with an effective mass determines the propagation of excitons. Overall, our work provides microscopic insights into the character of exciton propagation in twisted van der Waals heterostructures.