${\mathrm{Bi}}_2{\mathrm{O}}_2\mathrm{Se}$ belongs to a group of quasi-2D semiconductors that can replace silicon in future high-speed/low-power electronics. However, the correlation between crystal/band structure and other physical properties still eludes understanding: carrier mobility increases non-intuitively with carrier concentration; the observed ${\mathrm{T}}^2$ temperature dependence of resistivity lacks explanation. Moreover, a very high relative out-of-plane permittivity of about 150 has been reported in the literature. A proper explanation for such a high permittivity is still lacking. We have performed infrared (IR) reflectivity and Raman scattering experiments on a large perfect single crystal with defined mosaicity, carrier concentration, and mobility. Five of the eight phonons allowed by factor group theory have been observed and their symmetries determined. The IR spectra show that the permittivity measured in the tetragonal plane is as high as ${\varepsilon }_r\approx 500$, and this high value is due to a strong polar phonon with a low frequency of ∼34 ${\mathrm{cm}}^{-1}$ (∼1 THz). Such an unusually high permittivity allows the screening of charge defects, leading to the observation of high electron mobility at low temperatures. It also allows effective modulation doping providing a platform for high-performance 2D electronics. DFT calculations suggest the existence of a very low-frequency acoustic phonon ∼14 ${\mathrm{cm}}^{-1}$ (∼0.4 THz). Both the low-frequency phonons cause anomalous phonon DOS, which is reflected in the unconventional temperature dependence of the heat capacity, $C_p\approx {\mathrm{T}}^{3.5}$. The temperature-dependent, two-component group velocity is proposed to explain the unusual temperature dependence of the thermal conductivity, $\kappa \approx {\mathrm{T}}^{1.5}$.

Accurate predictions of the properties of transition-metal oxides using density functional theory (DFT) calculations are essential for the computational design of energy materials. In this work, we investigate the anomalous reversal of the stability of structural distortions (where distorted structures go from being energetically favorable to sharply unfavorable relative to undistorted ones) induced by $\mathrm{DFT}+U_{\text{eff}}$ on Mo d orbitals in layered $A{\mathrm{MoO}}_2$ $(A=\mathrm{Li}, \mathrm{Na}, \mathrm{K})$ and rutilelike ${\mathrm{MoO}}_2$. We highlight the significant impact of varying $U_{\text{eff}}$ values on the structural stability, convex hull, and thermodynamic stability predictions, noting that deviations can reach up to the order of 100 meV/atom across these energetic quantities. We find the transitions in stability are coincident with changes in the electron localization, magnetic behavior, and volume. The anomalous reversal persists across PBE, ${\mathrm{r}}^2\mathrm{SCAN}$ functionals, and also with vdW-dispersion energy corrections $(\mathrm{PBE}+\mathrm{D}3)$. In Mo-containing oxide systems, high $U_{\text{eff}}$ leads to inaccurate descriptions of physical quantities and structural relaxations under artificial symmetry constraints, as demonstrated by the phonon band structures, the Heyd-Scuseria-Ernzerhof hybrid functional results, and comparisons with experimental structural data. We conclude that high $U_{\text{eff}}$ values (around 4 eV and above, depending on the specific structures and compositions) might be unsuitable for energetic predictions in A-Mo-O chemical spaces. Our results suggest that the common practice of applying $\mathrm{DFT}+U_{\text{eff}}$ to convex hull constructions, especially with high $U_{\text{eff}}$ values derived from fittings, should be carefully evaluated to ensure that ground states are correctly reproduced, with careful consideration of dynamic stability and possible energetically favorable distortions.

Pyrite ${\mathrm{FeS}}_2$ is a low-cost, sustainable, nontoxic, $\sim 1$-eV-band gap semiconductor with unrealized potential in several application arenas, including photovoltaics. From the fundamental perspective, issues such as surface conduction and the deep-donor nature of S vacancies have hindered the study of low-temperature electronic phenomena in pyrite, including the insulator-metal transition (IMT). Here, we leverage a recently developed ${\mathrm{CoS}}_2$-based contact scheme in tandem with wide-range doping via shallow Co donors to directly access low-temperature bulk ${\mathrm{FeS}}_2$ transport properties and thus probe the IMT. Thoroughly characterized ${\mathrm{FeS}}_2$:Co single crystals are studied over broad ranges of temperature $(0.4-400 \mathrm{K})$ and Hall electron density $(8.6\times {10}^{16}-2.0\times {10}^{20}\mathrm{c}{\mathrm{m}}^{-3})$ through resistivity, Hall effect, and magnetoresistance measurements. The IMT is found to occur near $2\times {10}^{17}\mathrm{c}{\mathrm{m}}^{-3}$, with Efros-Shklovskii variable-range hopping below this, weak-localization-corrected metallic conductivity above this, and the onset of magnetic effects at the highest doping levels. Most significantly, unexpected additional phenomena are found near the IMT, including a nonlinear Hall effect with nonmonotonic temperature and doping dependence, and a nonsaturating, linear positive magnetoresistance at low temperatures. Quantitative analysis points to unusually strong disorder effects in the vicinity of the IMT, further elucidating the electronic behavior of this unique semiconductor.

We present a systematic experimental study of the topological phase transition in a series of $\sim 10\mathrm{nm}$ thick ${\mathrm{Hg}}_{1-x}{\mathrm{Cd}}_x\mathrm{Te}$ quantum wells by tuning the Cd content $x$. We provide detailed structural and magneto-optical spectroscopic characterization measurements and establish a comprehensive picture of the alloy structural and energetic properties. Using these as input, we employ $\mathbf{\text{k}}·\mathbf{\text{p}}$ modeling to establish the topological phase diagram of ${\mathrm{Hg}}_{1-x}{\mathrm{Cd}}_x\mathrm{Te}$ in dependence on the Cd content and quantum well thickness for thin films which are pseudomorphically strained to the lattice constant of pure CdTe.

The coupling between the organic $\mathrm{C}{\mathrm{H}}_3{\mathrm{NH}}_3^{+}$ cations and the inorganic perovskite ${\mathrm{PbI}}_3^{-}$ lattice framework was studied using photoabsorption and neutron diffraction. Four incommensurate (ICM) lattice phases were found, marked $\mathrm{IC}{\mathrm{M}}_{\mathrm{LT}}, \mathrm{IC}{\mathrm{M}}_{\mathrm{IT}}, \mathrm{IC}{\mathrm{M}}_{\mathrm{AT}}$, and $\mathrm{IC}{\mathrm{M}}_{\mathrm{HT}}$, from low to high temperatures of 40 to 500 K, prior to the melting of the corner-sharing $\mathrm{Pb}{\mathrm{I}}_6$ octahedra. Six photoabsorption bands, all in the infrared region, were detected at 20 K. Two of these disappeared upon transition from the $\mathrm{IC}{\mathrm{M}}_{\mathrm{LT}}$ phase into the $\mathrm{IC}{\mathrm{M}}_{\mathrm{IT}}$ phase at 120 K; the other two disappeared upon transition from the $\mathrm{IC}{\mathrm{M}}_{\mathrm{IT}}$ phase into the $\mathrm{IC}{\mathrm{M}}_{\mathrm{AT}}$ phase at 220 K. There was a progressive decrease in the widths of the Bragg reflections from the ${\mathrm{PbI}}_3^{-}$ host lattice and the ICM $\mathrm{C}{\mathrm{H}}_3{\mathrm{NH}}_3^{+}$ sublattice upon warming in the $\mathrm{IC}{\mathrm{M}}_{\mathrm{AT}}$ and $\mathrm{IC}{\mathrm{M}}_{\mathrm{HT}}$ phases. The observed behaviors can be understood as due to the asymmetric structure of the ($\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3-\mathrm{I}$) ions, together with the indirect interactions between the ($\mathrm{C}{\mathrm{H}}_3\mathrm{N}{\mathrm{H}}_3-\mathrm{I}$) ions, mediated through the I ions located between them.

Using first principles calculations, $\left\{111\right\}$ intrinsic stacking fault (ISF) energies in groups IVB, VB, and VIB high-entropy transition metal carbides (HETMCs) are shown to be predictable from an optimized rule of mixtures based on the atomic arrangement near the stacking fault. A composition-independent linear relationship is demonstrated between the ISF energies and the unstable stacking fault (USF) energies along the $\langle 11\overline{2}\rangle \left\{111\right\}$ γ surface slip path. This relationship represents a new application of the Evans-Polanyi-Semenov principle by treating the ISF and USF energies as analogous to the heat of reaction and transition state barrier in chemical reactions. Further, a full defect energy distribution can be obtained from the predicted ISF energies for each early transition metal HETMC. Balancing the elastic repulsion between partial dislocations with the distribution of ISF energies, we show that Shockley partial edge dislocations should remain bound for HETMCs with valence electron concentration up to 9.6, even when the average stacking fault energy is negative.

Two-dimensional (2D) materials combined with surface nearly free electrons (NFE) have been considered quite interesting platforms to be exploited for the development of 2D electronic devices. Further incorporation of foreign elements adds a new degree of freedom to engineer both the electronic and magnetic properties of 2D materials. Here, we have performed an ab initio study of ${\mathrm{Ca}}_2\mathrm{N}$ electrenes fully (i.e., both sides) adsorbed by fluorine ($\mathrm{F}/{\mathrm{Ca}}_2\mathrm{N}/\mathrm{F}$) atoms. The NFE states are suppressed in these systems, followed by the appearance of a net magnetic moment localized in the nitrogen atoms intercalated by the fluorinated calcium layers. In the sequence, we have proposed lateral heterostructures combining $\mathrm{F}/{\mathrm{Ca}}_2\mathrm{N}/\mathrm{F}$ regions with pristine ${\mathrm{Ca}}_2\mathrm{N}$ electrenes $\left[\left({\mathrm{Ca}}_2\mathrm{N}\right)(\mathrm{F}/{\mathrm{Ca}}_2\mathrm{N}/\mathrm{F})\right]$. We found that the magnetic moment of the fluorinated regions promotes the emergence of spin-polarized NFE states confined along the pristine (${\mathrm{Ca}}_2\mathrm{N}$) stripes. Further electronic transport calculations reveal that the ($\mathrm{F}/{\mathrm{Ca}}_2\mathrm{N}/\mathrm{F}$) regions act as spin-dependent scattering centers, or spin filter. We believe that these findings make an important contribution to the development of spintronic devices based on 2D electrides.

Recently, time-reversal symmetry broken magnetic Weyl semimetals (WSMs) have attracted extensive attention and have provided an intriguing platform for exploring fundamental physical phenomena. The study of chromium telluride-based systems has also drawn significant interest towards spintronics applications owing to their high Curie temperatures. Here, using ab initio calculations, we propose the emergence of multiple Weyl points (WPs) near the Fermi level in such an intrinsic ferromagnetic system, ${\mathrm{Cr}}_3{\mathrm{Te}}_4$. The large, well-separated, nontrivial Fermi arcs and surface states, suggest that the WPs are highly robust and resilient to perturbations. A substantial Berry curvature contribution in the vicinity of the Fermi energy not only serves as the origin of large conventional anomalous Hall conductivity (AHC), but also produces unconventional parallel AHC in this material, owing to the low structural symmetry. In addition to the charge Hall conductivity, we also find significant anomalous Nernst conductivities originating from the Berry curvature. Alongside our theoretical predictions, we present complementary experimental results, including x-ray diffraction (XRD) analysis and an examination of the magnetic properties, which demonstrate a Curie temperature of 327 K. Our study advances the understanding of magnetic WSMs, and also encourages further studies in the context of topological properties of our proposed material.

Machine learning (ML) has impacted numerous areas of materials science, most prominently improving molecular simulations, where force fields were trained on previously relaxed structures. One natural next step is to predict material properties beyond structure. In this work, we investigate the applicability and explainability of ML methods in the use case of estimating the critical temperature $\left(T_{\text{c}}\right)$ for magnetic Heusler alloys calculated using ab initio methods determined materials-specific magnetic interactions and a subsequent Monte Carlo (MC) approach. We compare the performance of regression and classification models to predict the range of the $T_{\text{c}}$ of given compounds without performing the MC calculations. Since the MC calculation requires computational resources in the same order of magnitude as the density functional theory (DFT) calculation, it would be advantageous to replace either step with a less computationally intensive method such as ML. We discuss the necessity to generate the magnetic ab initio results to make a quantitative prediction of the $T_{\text{c}}$. We used state-of-the-art explainable artificial intelligence (XAI) methods to extract physical relations and deepen our understanding of patterns learned by our models from the examined data.

Shape-memory alloys exhibit the remarkable ability to recover from a deformed state through heating, strain, or a combination of both. Their industrial applications are expanding, with a growing demand for materials capable of operating at higher temperatures. The martensitic phase transition is well-established as the mechanism underlying high-temperature shape-memory (HTSM) behavior. In this work, we investigate the martensitic phase transition in a promising HTSM alloy, NbRu, using first-principles molecular dynamics. By accurately incorporating temperature effects, we reproduce the transition temperature and thermoelastic properties of NbRu, achieving good agreement with experimental observations. Furthermore, we uncover the significant impact of chemical disorder on the mechanical stability of NbRu, which can suppress the martensitic transition and compromise shape-memory functionality. Our findings suggest that such disorder effects could be a general feature of HTSM alloys when disorder is present near the martensitic transition temperature. We propose that this factor should be carefully considered in the design and discovery of new HTSM alloys.

Applying both compressive and tensile strain, we explore via first-principles calculations how applied strain affects the band gap and charge density distribution within a class of recently synthesized topological polyacene polymers. Our findings reveal a strikingly nonmonotonic behavior of the semiconducting band gap, characterized by several closures and reopenings at critical strain points, which can be rationalized by the presence of multiple topological phase transitions. Additionally, we also observe transitions between aromatic and quinonoid configurations, revealing qualitative structural changes as strain continuously tunes these systems across the rich landscape of trivial to nontrivial topological transitions, which includes at least two quasimetallic phases. This highlights the pivotal role of strain in tuning the electronic properties of novel conjugated polymers, with promising implications for leveraging the flexible, nontoxic properties of organic materials with topological electronic effects for advanced electronic and optoelectronic devices.

Bulk two-dimensional (2D) superconductivity has gained considerable attention due to its intricate interplay between symmetry breaking, nontrivial topology, 2D phase fluctuations, and unconventional superconductivity. However, certain intercalated layered superconductors, despite their short $c$-axis superconducting coherence length, have been misclassified as anisotropic three-dimensional (3D) superconductors. Here, we investigate (Li,Fe)OHFeSe superconductors with varying degrees of interlayer misalignment, revealing sample-dependent superconducting dimensionality while consistently observing Berezinskii–Kosterlitz–Thouless (BKT) transitions. To resolve this discrepancy, we develop an extended 2D Tinkham model that quantitatively captures the blurring effects induced by interlayer misalignment. We further demonstrate the validity of this model in both (Li,Fe)OHFeSe and cetyltrimethyl ammonium (${\mathrm{CTA}}^{+}$)-intercalated ${\left(\mathrm{CTA}\right)}_{0.5}{\mathrm{SnSe}}_2$ superconductors, highlighting its broad applicability. This work provides valuable insights into bulk 2D superconductivity and establishes an extended 2D Tinkham model for quantitatively extracting intrinsic superconducting properties in intercalated layered superconductors, particularly those exhibiting significant interlayer misalignments.

We present the experimental and theoretical studies of lattice dynamics, electronic structure, and magnetodielectric relaxation in the ferrimagnetic (FiM) ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. This system exhibits a distorted honeycomb lattice with the space group $Pbcn$ (no. 60) and undergoes FiM ordering below 76 K (${\mathrm{T}}_{FN}$), accompanied by a magnetic compensation at 33 K (${\mathrm{T}}_{\mathrm{COM}}$). By employing density-functional theory ($\mathrm{DFT}+\mathrm{U}$) calculations and vibrational spectroscopy, we study all the allowed Raman and infrared phonon modes in ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Temperature-dependent analysis of Raman-active phonon frequencies provides evidence of spin-phonon coupling in ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Interestingly, we observe a nonlinear mode softening phenomenon in both frequencies $\omega \left(\mathrm{T}\right)$ and their linewidths $\delta {\omega }_F\left(\mathrm{T}\right)$, which can be described using three-phonon anharmonic processes. Contrary to the behavior of the remaining modes, 510 ${\mathrm{cm}}^{-1}$ mode shows a blue shift and effectively acts as a breathing mode for Ni atoms of two sublattices, and hence plays a crucial role in stabilizing the FiM ground state, thus explaining the strong sensitivity of this ${\mathrm{A}}_g$ mode to temperature. Our investigation of the temperature- and field-dependent relative dielectric permittivity and loss-tangent data reveals the absence of magnetodielectric coupling and the presence of dipole relaxation behavior in ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$. Additionally, the theoretically determined indirect band gap is about 2.7 eV. Overall, our study helps to fill the existing knowledge gap and provides valuable insights into the electronic, vibrational, and magnetodielectric properties of ${\mathrm{Ni}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$, shedding light on its potential applications in dielectric-ceramic resonators and electrochemical sensors.

Materials with high thermal conductivity are at the forefront of research in advancing thermal management, as benchmarked by the recent discovery of cubic BAs. In this study, we utilized BAs as a prototype material to assess the predictive capabilities of a machine learning approach for thermal transport, particularly in scenarios where high-order phonon anharmonicity plays a crucial role. We developed a training methodology for the moment tensor potential based on ab initio molecular dynamics, which provides accurate predictions of atomic energies, forces, stresses, phonon dispersion relations, elastic modulus, and thermal expansion coefficients. Our approach yields quantitative predictions of thermal conductivity and phonon mean free paths, closely matching first-principles calculations and experimental measurements under varied conditions and size confinements. The predictions of pressure-dependent thermal conductivity, taking into account complex interactions from phonon anharmonicity, isotope scattering, and defect scattering, reveal intrinsic behavior resulting from competing 3-phonon and 4-phonon processes in high-quality BAs, while also showing weak pressure dependence in samples dominated by defects. This study explores the feasibility of using machine learning for simulating high-order phonon scattering and demonstrates its potential as a high-throughput computational approach in advancing thermal management solutions.

We reinvestigate the mechanism of near-field heat-transfer rectification between two Weyl semimetal nanoparticles and a planar Weyl semimetal substrate via the coupling to nonreciprocal surface modes. We first show that the previously predicted rectification ratio of 2673 is incorrect and should rather be 1502 for the same parameters. In addition, we show that the explanation for this enhancement was wrong and must be understood by the coupling to the surface modes in the Weyl semimetal. Furthermore, we provide a detailed study of the enhancement effect as function of different parameters as the distance between the nanoparticles, the Fermi energy, the number of Weyl points, and the temperature. We show that depending on the distance between the nanoparticles there can be a much more efficient heat-flux rectification with ratios of about 6000 than previously predicted. Furthermore, we identify a previously overlooked range of forward rectification and a range of strong backward rectification with rectification ratios larger than 8000 for relatively small Weyl-node separations by using a more precise parameter scan. We investigate the mechanism behind this large heat-flux rectification and study its sensitivity with respect to certain material parameters and temperature showing that even larger rectification ratios up to 15 000 are possible, highlighting that certain Weyl semimetals are strong candidates for highly efficient heat-flux rectification not only for low temperatures but also for room temperatures.