${\mathrm{Cu}}_2{\mathrm{ZnSnS}}_4$ (CZTS) is a promising nontoxic and widely available solar absorber. In a recent experimental study of CZTS with the Zn(O, S) buffer layer, the best-performing cell has a Zn/Sn ratio of 0.98 before annealing, different from the conventional tin-poor condition [L. Huang, et al., Sol. RRL 4, 2000010 (2020)]. A common belief is that a normal Sn content favors the formation of a ${\mathrm{Sn}}_{Zn}$ antisite, which is detrimental as it produces a deep gap state. Experimentally, it is challenging to investigate the interface electronic environment with different defect profiles. We constructed a CZTS(112)-ZnS(111) interface and simulated a low concentration of oxygen by introducing it as a dopant in pure ZnS near the interface. We discovered that the ${\mathrm{O}}_{\mathrm{S}}$ isovalent center strongly couples with the ${\mathrm{Sn}}_{Zn}$ antisite near the interface and forms various stable codoping configurations with clean gaps. As a result, a growth condition with stoichiometric Sn concentration can be restored.

Using $ab$ $initio$ density functional theory (DFT) calculations, we investigate the electronic structure, phase stability, and magnetic properties of equiatomic binary alloys between Al and $3d$ magnetic transition elements (Cr, Mn, Fe, Co, and Ni). Thermodynamically, all five binary aluminides are more stable in the ordered B2 phase than in the disordered body-centered-cubic phase, and Co is found to be the strongest B2 forming element with Al. The AlCo and AlNi compounds with B2 structure are verified to be nonmagnetic, whereas AlFe turns out to be weakly magnetic, which is consistent with other DFT calculations employing similar exchange-correlation approximations. Magnetic simulations based on the Heisenberg Hamiltonian predict an antiferromagnetic ground state for the hypothetical B2 AlCr, which is also confirmed by direct DFT calculations. Doping AlCr with Co leads to an antiferromagnetic-to-ferromagnetic transition, where ferromagnetism is to a large extent attributed to Cr atoms. The phase stability and magnetic trends are explained using electronic structure arguments. The present findings contribute to a deeper understanding of the phase stability and magnetic properties of AlX binary alloys, providing insights into the formation mechanisms of the B2 structure with $3d$ magnetic transition metals.

Granular materials are ubiquitous in nature and are used extensively in daily life and in industry. The modeling of these materials remains challenging; therefore, finding models with acceptable predictive accuracy that at the same time also reflect the complexity of the granular dynamics is a central research theme in the field. Soft particle packings present additional modeling challenges, as it has become clear that soft particles also have particle-level relaxation timescales that affect the packing behavior. We construct a simple one-dimensional, one-timescale model that replicates much of the essence of compressed hydrogel packing mechanics. We verify the model performance against both 3D and 2D packings of hydrogel particles, under both controlled strain and stress deformation conditions. We find that the modification of a standard linear solid model with a strain-dependent prefactor for the relaxation captures the time-history and rate dependence, as well as the necessary absence of cohesion effectively. We also indicate some directions of future improvement of the modeling.

In the pursuit of identifying improved amorphous oxide coatings for gravitational wave detector test masses, 44% Ti-doped ${\mathrm{GeO}}_2$ has emerged as a potential candidate due to its low mechanical loss [Vajente et al. Phys. Rev. Lett. 127, 071101 (2021)]. It has been proposed and experimentally demonstrated that a reduction in mechanical loss in amorphous oxides is associated with medium-range order (MRO) dominated by corner-sharing links in the amorphous network. In this work, we describe a combined experimental and theoretical approach to investigate the MRO of Ti-doped ${\mathrm{GeO}}_2$. Using atomistic modeling and simulations, we calculate the Raman spectrum of 44% Ti-doped ${\mathrm{GeO}}_2$, and we show that it predicts the main features observed in the experiments and further provides insight into the MRO, which would otherwise be hidden. It is shown that the amorphous oxide mixture exhibits a distribution of predominantly large ($>4$ member) rings formed by corner-shared tetrahedral Ge-O-Ti connections. It is also shown that the addition of Ti to $a\text{−}{\mathrm{GeO}}_2$ does not largely compromise corner-sharing connections, as the smaller populations of Ti-O-Ti and Ge-O-Ge links in the network are predominantly corner-shared. The mainly covalently bonded representation of the MRO of Ti-doped ${\mathrm{GeO}}_2$ revealed by this analysis validates the importance of the MRO network organization in influencing mechanical loss in amorphous oxides.

Antimony (Sb), an element with strong spin-orbit coupling, is predicted to undergo a topological phase transition from a topological semimetal to a topological insulator as its dimensionality approaches the two-dimensional limit, driven by the quantum confinement effect. In this study, we investigate this transition in Sb thin films grown by molecular beam epitaxy, employing electrical transport measurements and angle-resolved photoemission spectroscopy (ARPES). Electrical transport measurements revealed signatures of a modified electronic band structure, including a Hall response with multiple carrier types, a decreasing carrier concentration, and a transition in the curvature of the longitudinal resistance from quadratic to linear with decreasing film thickness. Temperature-dependent magnetoresistance further showed weak antilocalization below 16 K, indicating strong spin-orbit coupling and suggesting the presence of nontrivial topological states. Analysis of the WAL characteristics revealed a single coherent conducting channel and a thickness-dependent change in the phase decoherence mechanism. Complementary ARPES measurements confirmed that reducing the film thickness lifts the conduction band at the M point, consistent with the emergence of a band gap. These findings support theoretical predictions of a thickness-dependent band structure evolution driven by the quantum confinement effect, providing a foundation for further exploration of topological phase transitions in Sb as well as $\mathrm{B}{\mathrm{i}}_{1–x}\mathrm{S}{\mathrm{b}}_x$. The realization of an elemental topological material with simplified stoichiometry and semiconductor compatibility presents a promising avenue for next-generation hybrid systems and applications in spintronics and quantum technologies.

We present a controlled mechanical exfoliation technique for bulk three-dimensional topological insulators that yields atomically clean ultrathin flakes, enabling quantum well states (QWS) of Dirac electrons to be clearly resolved. Achieving reliable fabrication of pristine, high-quality two-dimensional layers suitable for atomic-scale spectroscopy remains a central experimental challenge in uncovering their emergent quantum states and realizing device-relevant functionalities. Atomically resolved scanning probe microscopy and micro-Raman spectroscopy reveal a strong correlation between Raman intensity and film thickness, enabling rapid identification of (${\mathrm{Bi}}_{0.1}{\mathrm{Sb}}_{0.9}{)}_2{\mathrm{Te}}_3$ flakes with desired thickness. High-resolution scanning tunneling spectroscopy on exfoliated flakes with atomically flat terraces reveals QWS, driven by quantum confinement of Dirac electrons. This effect is rarely observed due to the electrons resistance to electrostatic confinement caused by Klein tunneling. The standard phase accumulation model accurately captures the characteristics of QWS and extracts the electronic band dispersion, showing excellent agreement with density-functional-theory calculations. Band-structure calculation reveals that with increasing quantum-layer thickness, the interlayer coupling enhances the electronic dispersion, progressively reducing subband splitting and giving rise to bulklike continuous bands. Spatially resolved spectroscopy around surface defects further confirms that QWS of Dirac electrons in topological insulators remains robust against defect scattering. This work paves the way for exploring diverse quantum phenomena and device applications through quantum confinement, surface-state engineering, and tunable topological phases.

Large databases of crystalline materials and associated properties, typically computed using density functional theory (DFT), have become widely available and used for training machine learning (ML) models. Thermodynamic stability, band gap, and elastic mechanical properties are now available in large volumes with sufficient accuracy to train effective models. Magnetic properties, however, require special attention to model using DFT and are only currently available in datasets that are orders of magnitude smaller in size. ML models can therefore be used to screen candidate materials for magnetic properties if their effectiveness can be demonstrated in lower data regimes. In this work, we compare the ability of multiple model architectures to predict saturation polarization, testing a prevailing assumption that knowledge of crystal structure is critical to predict magnetic properties. While a prototypical crystal-structure-based model (CGCNN) did on average outperform a model based on composition only [Representation learning from Stoichiometry (RooSt)], we found that RooSt performed nearly as well. For example, in predicting saturation polarization, we observed a mean absolute error from RooSt models (0.15 T) only worse than from CGCNN (0.13 T): still sufficiently low for useful screening and with the significant benefit of not requiring crystal structure to be known a priori. Further, we found that models trained on DFT-computed values of saturation polarization are also usefully predictive of experimentally tested values, as confirmed in samples fabricated by arc melting. This suggests composition-based models can enable screening of novel candidate material spaces without prior knowledge of crystal structure, which would drastically accelerate screening of materials with promising magnetic properties.

We report the synthesis and properties of a new layered material, $\mathrm{CrG}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{e}}_{3+y}$. The crystal structure was determined by using single crystal x-ray diffraction and transmission electron microscopy. $\mathrm{CrG}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{e}}_{3+y}$ crystallizes with a space group $R-3m$, featuring a double kagome layer of chromium atoms, sandwiched between disordered Ge-Se layers. This compound displays antiferromagnetic order below 50 K, unlike the ferromagnetic behavior in $\mathrm{CrGeT}{\mathrm{e}}_3$. Chromium displays an effective moment of $\sim 3.9{\mu }_{\mathrm{B}}/\mathrm{Cr}$, consistent with $\mathrm{C}{\mathrm{r}}^{3+}$ oxidation. The presence of this phase limits the possibility of a ferromagnetic $\mathrm{CrGeS}{\mathrm{e}}_3$ analogous to $\mathrm{CrGeT}{\mathrm{e}}_3$.

We present polarization-resolved optical absorption measurements on ${\mathrm{Er}}^{3+}$ ions in ${\mathrm{CaWO}}_4$ under vector magnetic fields, focusing on charge-compensated sites arising from local ${\mathrm{Ca}}^{2+}$ vacancies. While the known axial ${\mathrm{Er}}^{3+}$ site displays a single symmetric Zeeman-split transition pattern consistent with ${\mathrm{S}}_4$ symmetry, two additional sites exhibit more complex spectral behavior, including sets of transitions that interchange under ${90}^{\circ }$ crystal rotations-evidence of reduced, rhombiclike symmetry. From these polarization- and temperature-dependent spectra, we extract effective $g$ factors. Our findings are corroborated by electron paramagnetic resonance measurements and support a model of multiple inequivalent ${\mathrm{Ca}}^{2+}$ vacancies around ${\mathrm{Er}}^{3+}$ sites in the host lattice. This detailed characterization contributes to understanding defect-engineered rare-earth sites for quantum information applications.

Intercalation provides an effective approach to reducing the dimensionality and modifying the electronic structure of layered materials. Here, we report the intercalation of organic molecules into the van der Waals materials ${\mathrm{HfTiTe}}_4$ and ${\mathrm{TaRhTe}}_4$, which are theoretically predicted to be two-dimensional topological insulators in their monolayer forms. In ${\mathrm{HfTiTe}}_4$, intercalation resulted in an overall increase of resistivity, while preserving the metallic temperature dependence of the pristine compound. The magnetoresistance exhibited a weak antilocalization behavior, indicative of reduced dimensionality. Band structure calculations revealed that the band topology of intercalated ${\mathrm{HfTiTe}}_4$ corresponds to that of a weak topological insulator, although there is a band that crosses the Fermi level, which explains the observed metallic behavior. For ${\mathrm{TaRhTe}}_4$, on the other hand, the temperature dependence of resistivity changed from metallic to insulating by the intercalation. Moreover, magnetotransport measurements provided evidence of topological edge states, supporting the prediction that monolayer ${\mathrm{TaRhTe}}_4$ is a two-dimensional topological insulator. The band calculations indicated that the intercalation caused a bulk gap opening at the Fermi level and the electronic structure is very similar to that of a monolayer ${\mathrm{TaRhTe}}_4$, in agreement with the transport properties.

Breaking of the inversion symmetry of the crystal structure plays a crucial role in nonlinear responses. Although this concept should be invalid for amorphous materials, there are few experimental studies on nonlinear transport responses for highly disordered noncentrosymmetric materials. Here, we employ nonreciprocal magnetoresistance to trace the structural changes of low-symmetry ${\mathrm{WTe}}_2$ with heavy-ion irradiation. The irradiated ${\mathrm{WTe}}_2$ specimens were mostly amorphized and exhibited semimetallic transport with completely different carrier densities and mobilities from those of single crystals. We showed that the nonreciprocal magnetoresistance is remarkably suppressed in the irradiated samples. This suppression of nonlinear transport is attributed to the disordered atomic arrangement by amorphization rather than carrier doping effects by ion irradiation, corroborated by structural analysis using transmission electron microscopy and extended x-ray absorption fine structure analysis.

We performed inelastic neutron scattering experiments on a polycrystalline sample of the two-dimensional $d$-wave altermagnetic candidate ${\mathrm{La}}_2{\mathrm{O}}_3{\mathrm{Mn}}_2{\mathrm{Se}}_2$, which features a Lieb lattice geometry. The magnetic-excitation spectrum is well reproduced by linear spin-wave theory. The analysis yields a first-neighbor exchange interaction of $J_1=2.34\left(3\right)\text{meV}$ and two distinct second-neighbor altermagnetic exchange interactions of $J_2=0.56\left(2\right)\text{meV}$ and $J_2^{\prime }=0.41\left(4\right)\text{meV}$, establishing that the material is a two-dimensional $d$-wave altermagnet. These interaction values are consistent with first-principles calculations. This demonstration of a $d$-wave altermagnet that allows magnon spin currents underscores the promise of altermagnets for magnonics.

Molecular dynamics techniques are used to investigate $\left\{10\overline{1}2\right\}$ and $\left\{11\overline{2}1\right\}$ twin thickening processes through spontaneous formation of twinning disconnections (TD) under applied shear deformation with a constant strain rate, based on an embedded atom method potential model of Zr. The observed stress responses and twin boundary structure changes verify that both twins thicken through TD formation. However, the thickening processes of $\left\{10\overline{1}2\right\}$ and $\left\{11\overline{2}1\right\}$ twins show significant differences. While $\left\{10\overline{1}2\right\}$ TD loops were observed to form and expand during the twin thickening process, the $\left\{11\overline{2}1\right\}$ twin boundary exhibits a more structurally disordered interface throughout the simulation, and no sharp boundary corresponding to a $\left\{11\overline{2}1\right\}$ TD ‘loop’ can be identified. Moreover, the $\left\{11\overline{2}1\right\}$ twin requires a much lower critical shear stress for twin thickening, approximately 10% of that for the $\left\{10\overline{1}2\right\}$ twin. The low critical shear stress of $\left\{11\overline{2}1\right\}$ twin thickening also indicates that these twins should grow easily once nucleated. To the extent that these results apply to hexagonal-close-packed (HCP) materials more generally, promotion of $\left\{11\overline{2}1\right\}$ twin nucleation may be an effective way to enhance twinning deformation and twin boundary-related work hardening in HCP materials, with the potential to manipulate the balance between strength and ductility.

Quantum materials with layered structures offer an excellent platform for exploring emerging electronic, mechanical, and optical properties with potential for technological applications. Trigonally structured $t\text{−}{\mathrm{PtBi}}_2$ is such a material. When a femtosecond laser pulse is directed onto $t\text{−}{\mathrm{PtBi}}_2$, the resulting transient reflectivity reveals resonances of coherent acoustic phonons, arising from the interference between the diffracted light and light reflected at the $t\text{−}{\mathrm{PtBi}}_2$/${\mathrm{SiO}}_2$ interface. The resonance frequency is found to decrease with increasing the sample thickness from 71.48 GHz for the 20-nm-thick sample to 2.62 GHz for the 276-nm-thick sample. By combining the resonance frequency and mass density, we determine the out-of-plane elastic constant ${\mathrm{C}}_{33}$, which decreases from 84 GPa for the 20-nm-thick sample to 27 GPa for the 276-nm-thick sample, indicating high sensitivity to the sample thickness. However, the theoretical ${\mathrm{C}}_{33}$ value of 60 GPa obtained from first-principles calculations is higher than the experimentally determined saturated value for bulk. The discrepancy may be explained by considering the temperature difference and possible point defects in $t\text{−}{\mathrm{PtBi}}_2$, which can cause softening through wave sliding. Our combined experimental and theoretical investigation demonstrates that the elastic properties of layered materials can be obtained via light-matter interaction, which is particularly powerful for $t\text{−}{\mathrm{PtBi}}_2$ due to its low thermal conductivity and high optical absorption.