The optical and ferroelectric properties of lithium niobate (${\mathrm{LiNbO}}_3$, LN), including the orientation of the ferroelectric domains, i.e., the poling behavior, are decisively influenced by dopants. In this study, we apply density functional theory (DFT) to understand the microscopic origin of the beneficial (and impairing) effects of different Mg-doping concentrations, including nominally undoped stoichiometric and congruent compositions, particularly on the ferroelectric domain inversion. ${\mathrm{Mg}}_{\text{Li}}^{+}$ tends to be the dominant defect for lower doping concentrations, and a combination of ${\mathrm{Mg}}_{\text{Li}}^{+}$ and ${\mathrm{Mg}}_{\text{Nb}}^{3-}$ for intermediate doping concentrations. Single ${\mathrm{Mg}}_{\text{Li}}$ defects are found to be a possible seed point for domain inversion, while the combination of ${\mathrm{Mg}}_{\text{Li}}$ and ${\mathrm{Mg}}_{\text{Nb}}$ defects is suggested to inhibit poling. In this case, the reaction pathways for the inversion are not symmetric and are characterized by different activation energies for each direction, which could promote the crystal to more easily revert to its original polarization.

Thermal expansion and magnetoresistance measurements as a function of temperature (T) and magnetic field (B), respectively, were carried out on Os-substituted ${\mathrm{URu}}_2{\mathrm{Si}}_2$ $\left({\mathrm{URu}}_{2-x}{\mathrm{Os}}_x{\mathrm{Si}}_2\right)$ single crystals with $x=0$ to 0.28. Mean-field-like peaks at temperature $T_0$ in the linear thermal expansion coefficient vs T curves in both the hidden order (HO) phase and the large-moment antiferromagnetic (LMAFM) phase of ${\mathrm{URu}}_{2-x}{\mathrm{Os}}_x{\mathrm{Si}}_2$ exhibit a striking resemblance and a discontinuity in $T_0\left(x\right)$ at $x_{\mathrm{c}}=0.15$ delineating the boundary between the two phases. Anomalies at $x_{\mathrm{c}}$ were also seen in both the calculated effective Grüneisen parameter and the energy gap associated with the HO or LMAFM phase. The hydrostatic pressure derivative of $T_0$ calculated from thermal expansion and specific heat [D. L. Kunwar et al., Phys. Rev. B 105, L041106 (2022)] data agree with the experimental results [C. T. Wolowiec, Proc. Natl Acad. Sci. USA 118, e2026591118 (2021)], whereas the calculated uniaxial pressure derivatives in different crystallographic directions are anisotropic, as shown in the anisotropic linear thermal expansion. This anisotropy was further reflected in the radically different responses of $T_0$ to B applied in different directions: a few percent change in $T_0$ is induced for B parallel to the $c$ axis, whereas $T_0$ barely changes for B applied in the $ab\mathrm{plane}$ of the ${\mathrm{URu}}_{2-x}{\mathrm{Os}}_x{\mathrm{Si}}_2$ single crystals, regardless of whether the crystals are in the HO or LMAFM phase.

Isotopically enriched silicon (Si) can greatly enhance qubit coherence times by minimizing naturally occurring ${}^{29}\mathrm{Si}$ which has a nonzero nuclear spin. Ultrahigh fluence ${}^{28}\mathrm{Si}$ ion implantation of bulk natural Si substrates was recently demonstrated as an attractive technique to ultrahigh ${}^{28}\mathrm{Si}$ isotopic purity. In this work, we apply this ${}^{28}\mathrm{Si}$ enrichment process to produce ${}^{28}\mathrm{Si}$ and ${}^{28}\mathrm{Si}$-on-insulator (SOI) samples. Experimentally, we produced a ${}^{28}\mathrm{Si}$ sample on natural Si substrate with ${}^{29}\mathrm{Si}$ depleted to 7 ppm (limited by measurement noise floor) that is at least 100 nm thick. This is achieved with an ion energy that results in a sputter yield of less than one and an ultrahigh ion fluence, as supported by our improved computational model that is based on fitting a large number of experiments. Further, our model predicts the ${}^{29}\mathrm{Si}$ and ${}^{30}\mathrm{Si}$ depletion in our sample to be less than 1 ppm. In the case of SOI, ion implantation conditions are found to be more stringent than those of bulk natural Si in terms of minimizing threading dislocations upon subsequent solid phase epitaxy annealing. Finally, we do not observe open volume defects in our ${}^{28}\mathrm{SOI}$ and ${}^{28}\mathrm{Si}$ samples after SPE annealing ($620{}^{\circ }\mathrm{C}$, 10 min).

Tantalum pentoxide (${\mathrm{Ta}}_2{\mathrm{O}}_5$) is a component of technologies ranging from photographic lenses to medical implants. Despite more than six decades of interest, however, there is no consensus on the crystal structure of ${\mathrm{Ta}}_2{\mathrm{O}}_5$. Here, we report the results of neutron diffraction experiments on ${\mathrm{Ta}}_2{\mathrm{O}}_5$ powders at temperatures up to $1000{}^{\circ }\mathrm{C}$. Because neutron diffraction is sensitive to the positions of O atoms in the presence of heavy elements, our results provide strong evidence that the orthorhombic ${\mathrm{L}}_{\mathrm{SR}}$ structure can account for experimental observations better than any other proposal in the literature. We also determine the anisotropic crystallographic thermal expansion parameters of ${\mathrm{Ta}}_2{\mathrm{O}}_5$, which are important for modeling the performance of ${\mathrm{Ta}}_2{\mathrm{O}}_5$-bearing technologies.

Several authors have suggested that the surfaces of vanishing potential generated by the electrostatic fields from a periodic distribution of point charges resemble triply-periodic minimal surfaces (TPMS) whose symmetry corresponds to the positions of the point charges. We provide an alternative theory based on the quantum mechanical charge density $\rho$, rather than the classical discrete charges used in previous work. The Boltzmann equation is used to show that surfaces of $\rho =0$ correspond to minimal surfaces, which, for periodic systems, are TPMS. We then use density-functional calculations for elemental materials that differ electronically and structurally, namely, Na, Cu, and Al, to show that surfaces for which $\varrho \to 0$ converge to the TPMS corresponding to the crystalline material.

Defining skyrmion phase boundaries and predicting the phase diagram among various spin textures remain critical challenges in the field of two-dimensional magnets. In this work, we combine atomistic spin simulations with machine learning (ML) to identify skyrmion phase boundaries across multiple parameter spaces, including frustration interaction $\eta$ $(|J_2/J_1|)$, Dzyaloshinskii-Moriya interaction $\left(D\right)$, single-ion anisotropy $\left(K\right)$, magnetic field $\left(B\right)$, and temperature $\left(T\right)$. Using convolutional auto-encoder, t-distributed stochastic neighbor embedding, $K$-means, and Inception-V3 models, we classify three distinct phases (ferromagnet, skyrmion, and mixed phase) with $98.2\%$ accuracy. A Bayesian active learning framework incorporating Gaussian process regression iteratively refines phase boundary predictions, achieving stable errors around $\sim 0.05$. In addition, phase boundary fitting reveals a functional relationship, $\lambda \sim \eta K/D^2$, linking magnetic parameters to skyrmion stability. These findings demonstrate the potential of ML to enhance the efficiency and precision of skyrmion phase boundary analysis, advancing the understanding of complex spintronic systems.

Thin films of polycrystalline $\beta \text{-Mn}$ structure CoZn have been grown on thermally oxidized Si substrates by cosputtering from elemental targets followed by annealing. A range of films grown with variable Co deposition power and fixed Zn deposition power was produced, so as to vary the proportions of the two elements reaching the substrate, which were annealed postgrowth. While all films exhibited a (221) $\beta \text{-Mn}$ structure CoZn texture in x-ray diffraction, transmission electron microscopy showed that the composition with the highest integrated intensity for that Bragg peak contained large voids and was covered by a thick ZnO cap owing to being Co deficient overall. CoZn films deposited at ratios tuned to give the optimal volume fraction of $\beta \text{-Mn}$ were continuous, with crystallites up to 200 nm in size, with a much thinner ZnO cap layer. Magnetic measurements show that such optimal CoZn films have a Curie temperature $T_{\mathrm{C}}\sim 420\mathrm{K}$ and saturation magnetization of $120\mathrm{emu}/{\mathrm{cm}}^3$, properties close to those reported for bulk crystals. Excess Co beyond this point leads to a significant retained ferromagnetism above the Curie point of the CoZn, suggesting Co-rich aggregates. The $\beta \text{-Mn}$ structure is chiral (${P4}_{1}32/{P4}_{3}32$ space group) and is known to give rise to a Dzyaloshinkii-Moriya interaction (DMI) that stabilizes room-temperature skyrmions in the bulk. Our thin films are thus a potential materials platform, compatible with planar processing technology, for magnetic skyrmions arising from a bulk DMI.

The Nowotny chimney ladders (NCL) binary intermetallic compounds have recently attracted particular interest due to their anomalous glasslike thermal conductivity and the partial breakdown of conventional phonon gas behavior for potential thermoelectric applications. A complete understanding of the underlying unusual heat transport mechanisms of these materials is key to further improving their thermoelectric efficiency. In this work, we present a comprehensive study of thermal conductivity in the typical NCL semiconductor, ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$, based on first-principles molecular dynamics calculations and the Wigner thermal transport model. Our investigation uncovers that anticrossings and twisting modes in ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ drive the remarkably low and temperature-independent lattice thermal conductivity. The presence of ultra-low-energy optical phonons, anharmonicity, low group velocity, and pronounced anisotropy in thermal conductivity distinguish ${\mathrm{Fe}}_2{\mathrm{Ge}}_3$ from traditional thermoelectric materials. Furthermore, our analysis reveals that the inclusion of lattice defects, such as grain boundaries, enables the predicted thermal transport to align well with experimental observations. Expanding our findings to lower thermal conductivity NCL materials like ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$, we observe similar physical effects and outcomes, such as twisted phonons, anticrossings, and coherent contributions, which contribute to glasslike thermal transport behaviors. This highlights the broad applicability of our findings and the inherent similarities in thermal transport mechanisms across various NCL compounds. Our study challenges traditional understandings of the correlation between structural indicators $\delta$ and thermal conductivity in NCL materials, providing a fresh perspective on this intricate interplay. Our work contributes to a deeper understanding of heat transport in NCL intermetallic compounds, paving the way for advancements in thermoelectric materials with enhanced performance.

This study demonstrates that thermoelectric power factor (PF) of B20-CoSi with a peculiar electronic band structure (Dirac-like band and heavy hole band) surprisingly gets high regardless of crystal quality. We reveal that the high PF is attributed to energy filtering effect in wavenumber space via electron-phonon interaction or similar scattering processes. The experimental temperature dependences of normalized Seebeck coefficients of epitaxial (fewer defects) and polycrystalline B20-CoSi films (more defects) were in agreement with the calculated ones using the relaxation time for electron-phonon interaction, proving that Seebeck coefficient is robust against crystal defects. Both epitaxial and polycrystalline films exhibited higher electrical conductivities than conventional thermoelectric materials because of pseudogap semimetals. As a result, the B20-CoSi films exhibited relatively high PF of about $20\mu {\mathrm{Wcm}}^{-1}{\mathrm{K}}^{-2}$ at room temperature, which was higher than those of promising silicide thermoelectric films. This study concludes that the robustness of Seebeck coefficient against crystal defects in the material with a peculiar EBS can lead to the realization of high performance thermoelectric materials.

We use the first-principles spin-polarized energy density method (EDM) to calculate the atomic energies in isolated $a_0\left[100\right]\left(010\right)$ edge, $a_0\left[100\right]\left(011\right)$ edge, $\frac{a_0}{2}\left[\overline{1}\overline{1}1\right]\left(1\overline{1}0\right)$ edge, and $\frac{a_0}{2}\left[111\right]\left(1\overline{1}0\right){71}^{\circ }$ mixed dislocations in body-centered cubic (bcc) Fe. The distribution of atomic energies shows the energetic effects of slip, including nonlinear displacements in and near the core, and the elastic field further away. The EDM atomic energies agree well with anisotropic elasticity predictions in the elastic region, while they deviate in the core region due to the failure of linear elasticity, and the energy deviation quantifies the core widths. Dislocation-line energies are obtained by partial sums of the atomic energies within a distance $r$ of the dislocation center; the core energy is extracted from the large-$r$ behavior. We compare our results with an EAM and a GAP potential, showing that while both potentials produce core structures similar to density-functional theory (DFT) predictions, the GAP potential has a closer match with DFT and EDM in core energies.

We used density-functional-theory simulations to examine the structural and electronic properties of the $\mathrm{\Sigma }3{180}^{\circ }\left(100\right)\left[001\right]$ grain boundary in monoclinic ${\mathrm{ZrO}}_2$, which is a very low-energy ($0.06\mathrm{J}{\mathrm{m}}^{-2}$) twin boundary present in experimental oxide texture maps, with suggested special properties. This equilibrium structure was compared with a metastable structure (with a boundary energy of $0.32\mathrm{J}{\mathrm{m}}^{-2}$). The interaction of oxygen vacancies, substitutional Sn and Nb defects (substituting host Zr sites) with both structures—and their effect on the boundary properties—were examined. We found that the equilibrium structure energetically favors ${\mathrm{V}}_{\mathrm{O}}^{2+}$ and ${\mathrm{Nb}}_{\mathrm{Zr}}^{2-}$, whereas the metastable structure favors ${\mathrm{V}}_{\mathrm{O}}^{2+}$ and ${\mathrm{Sn}}_{\mathrm{Zr}}^{2-}$. Tin was further found to bind strongly with oxygen vacancies in both structures and introduce gap states in the band gap of their electronic structure. ${\mathrm{Sn}}_{\mathrm{Zr}}^{2-}$ was, however, found to increase the segregation preference of ${\mathrm{V}}_{\mathrm{O}}^{2+}$ for the metastable structure, which might contribute to increased oxygen and electron transport down this interface, and therefore other less special boundaries, compared to the equilibrium structure of the studied monoclinic twin boundary.

Superionic conductors have recently attracted renewed attention for their use as thermoelectric materials due to their extremely low lattice thermal conductivity. Of central interest is why the superionic conductors exhibit such low thermal conductivity and competing mechanisms have been proposed thus far. In this study, we investigate the effects of Cu and Au substitution for Ag site on the crystal structure and thermal properties of ${\mathrm{AgCrSe}}_2$, which exhibits superionic conduction of Ag ions. We show that Au substitution significantly reduces the lattice thermal conductivity of ${\mathrm{AgCrSe}}_2$. Powder structure analysis using synchrotron x-ray diffraction reveals that Au substitution increases the anisotropic atomic displacement parameter of Ag ions along the $a$ and $b$ axes. This result indicates that the amplitude of in-plane vibrations is enhanced, which is attributed to increased anharmonicity in the potential energy around Ag ions. The enhanced vibrational amplitude also suggests a reduction in the force constants between Ag ions. Consequently, the enhanced anharmonicity not only shortens the phonon lifetime ($\tau$) by increasing phonon-phonon scattering, but also increases the number of low-energy phonons, which further contributes to the reduction of $\tau$. This anharmonicity mechanism is applicable to other superionic conductors exhibiting ultralow thermal conductivity, promoting their widespread use as thermoelectric materials.

Composite materials using carbon nanotubes (CNTs) as a filler and polymers as a host material show great promise in many applications due to exceptionally high electrical and thermal conductivities of CNTs and low density and resistance to mechanical stress of polymer materials. Here, we report a new bulk composite synthesized using commercially available single-walled carbon nanotubes (SWCNTs) and silicone compound. The prepared material demonstrates high electrical conductivity of 20 S/m with nanotube mass fraction of only 2%. The percolation threshold of the synthesized material is 0.3 wt%. Additionally, to analyze the properties of the prepared material we develop an analytical model of electrical conductivity in SWCNT composite materials. Within the proposed model we obtain simple relation for estimating the tunneling conductance of carbon nanotube-polymer interface and discuss the effective shortening of nanotubes in a composite due to buckling and tangling of SWCNTs. The developed model is in excellent agreement with our data as well as other data available in the literature and allows to ananlyze the conductive properties of composites in the wide range of CNT loadings.

Gas hydrates are considered fundamental building blocks of giant icy planets like Neptune and similar exoplanets. The existence of these materials in the interiors of giant icy planets, which are subject to high pressures and temperatures, depends on their stability relative to their constituent components. In this study, we reexamine the structural stability and hydrogen content of hydrogen hydrates, $\left({\mathrm{H}}_2\mathrm{O}\right){\left({\mathrm{H}}_2\right)}_{\mathrm{n}}$, up to 104 GPa, focusing on hydrogen-rich materials. Using synchrotron single-crystal x-ray diffraction, Raman spectroscopy, and first-principles theoretical calculations, we find that the ${\mathrm{C}}_2$-filled ice phase undergoes a transformation to a ${\mathrm{C}}_3$-filled ice phase over a broad pressure range of 47 to 104 GPa at room temperature. The ${\mathrm{C}}_3$ phase contains twice as much molecular ${\mathrm{H}}_2$ as the ${\mathrm{C}}_2$ phase. Heating the ${\mathrm{C}}_2$-filled ice above approximately 1500 K induces the transition to the ${\mathrm{C}}_3$ phase at pressures as low as 47 GPa; upon decompression, this phase remains metastable down to 40 GPa. These findings establish new stability limits for hydrates, with implications for hydrogen storage and the interiors of planetary bodies.