I present first principles calculations of the phonon dispersions of ${\mathrm{TiSe}}_2$ in the $P\overline{3}c1$ phase, which is the currently accepted low-temperature structure of this material. They show weak instabilities in the acoustic branches in the out-of-plane direction, suggesting that this phase may not be the true ground state. To find the lowest energy structure, I study the energetics of all possible distorted structures corresponding to the isotropy subgroups of $P\overline{3}m1$ for the $M_1^{-}$ and $L_1^{-}$ phonon instabilities present in this high-temperature phase at $q=(\frac{1}{2},0,0)$ and $(\frac{1}{2},0,\frac{1}{2})$, respectively. I am able to stabilize ten different structures that are lower in energy relative to the parent $P\overline{3}m1$ phase, including two monoclinic structures more energetically stable than the $P\overline{3}c1$ phase. The lowest energy structure has the space group $C2$ with the order parameter $M_1^{-}(a,0,0)+L_1^{-}(0,b,b)$. This structure lacks inversion symmetry, and its primitive unit cell has 12 atoms.

The recently discovered kagome metals ${\mathrm{KV}}_3{\mathrm{Sb}}_5$, ${\mathrm{RbV}}_3{\mathrm{Sb}}_5$, and ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ exhibit a unique charge density wave state which hosts both superconductivity and a large anomalous Hall response. The microscopic mechanisms that underlie these phenomena have not been fully understood because the structure of the charge density wave order has not been completely determined. Previous theoretical results show that the parent $P6/mmm$ phase of these materials has phonon instabilities at the $M(\frac{1}{2},0,0)$ and $L(\frac{1}{2},0,\frac{1}{2})$ points in their Brillouin zone, but the energetics of all the low-symmetry phases that can arise due to the phonon instabilities was not investigated. Here, I perform such a search for the lowest-energy structure of these materials using first-principles calculations. Group-theoretical analysis shows that there are 17 different distortions that are possible due to the phonon instabilities. I generated these structures for the three compounds and performed full structural relaxations that minimize the atomic forces and lattice stresses. I find that the $Fmmm$ phase with the order parameter $M_1^{+}(a,0,0)+L_2^{-}(0,b,b)$ has the lowest energy among these possibilities in all three compounds. However, the $Fmmm$ exhibits a dynamical instability at its $Z(0,0,1)$ point, which corresponds to a doubly degenerate unstable phonon mode at the $A(0,0,\frac{1}{2})$ point in the parent $P6/mmm$ phase. The $A$ point has only one element in its star, and condensation of the instability at this point leads to a base-centered-orthorhombic structure with the space group $Cmcm$ and $4Q$ order parameter $M_1^{+}(a,0,0)+L_2^{-}(0,b,b)+A_6^{+}(\frac{1}{2}c,\frac{-\sqrt{3}}{2}c)$. A characteristic signature of this charge order is the absence of the mirror symmetry perpendicular to the $b$ axis in individual kagome layers, whose experimental observation below the structural transition temperature would be a strong indication that the $Cmcm$ structure describes the charge density wave state of these materials.

Frank-Kasper phases are complex particle packings known to form in a wide variety of hard and soft materials, including single-component AB diblock copolymer melts. An important open question in the context of this system is why these lower-symmetry packings are selected over the classical, higher-symmetry, body-centered cubic phase. To address this question, we simulated a library of diblock copolymer melts under intermediate-segregation conditions using self-consistent field theory and performed a combination of geometric and thermodynamic analyses. Our findings show that imprinting of the enclosing Voronoi polyhedra onto the micelle core is generally weak, but nonetheless coincides with sharpening of the interface between A and B monomers compared to more spherical cores. The corresponding reduction in enthalpy, which is the dominant contribution to the free energy, drives the bcc-$\sigma$ transition, overcoming increases in stretching penalties and giving way to more polyhedral micelle cores. These results offer insight into the stability and formation of Frank-Kasper phases under experimentally realistic conditions.

As with other transition-metal oxide interfaces, ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ interfaces exhibit interesting electronic properties, such as a two-dimensional electron gas. While the available Mott states and multiple pathways to metallicity in ${\mathrm{LaTiO}}_3$ can lead to a variety of applications in oxide electronics, the origin of the different properties remains uncertain. Utilizing real-time, in situ x-ray diffraction at the synchrotron, robust but thin ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructures were fabricated using oxide molecular beam epitaxy. A thickness of six unit cells was determined to be critical, at which point the ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ heterostructures begin to sustain sharp, well-defined interfaces. The layers of ${\mathrm{LaTiO}}_3$ and ${\mathrm{SrTiO}}_3$ were sufficiently thin to benefit from the finite escape length of electrons at resonant soft x-ray photoemission, and the heterostructures were transferred in vacuo after growth to another chamber for synchrotron-based x-ray photoemission spectroscopy studies. With the x-ray energy tuned to the Ti-2p resonance, the ${\mathrm{LaTiO}}_3/{\mathrm{SrTiO}}_3$ interfaces could be probed with enough penetration depth and selectivity. It is shown that all of the heterointerfaces exhibit the two-dimensional electron gas.

Phonon transport in rutile $\mathrm{Ti}{\mathrm{O}}_2$ between temperatures of 100 and 900 K is investigated using lattice dynamics calculations and the Boltzmann transport equation. Zero- and finite-temperature force constants are extracted from density functional theory calculations to examine the effects of finite temperature on phonon properties and thermal conductivity. Using the zero-temperature force constants leads to thermal conductivity predictions along the $a$ and $c$ axes that are 50% lower than experimental measurements at a temperature of 300 K. The underprediction increases as the temperature is further increased. Using temperature-specific force constants, however, leads to thermal conductivity predictions that fall within the uncertainty bounds of multiple sets of experimental measurements. Phonon mode analysis reveals that the thermal conductivity underprediction from the zero-temperature force constants is a result of an underprediction of phonon lifetimes. The lifetime underprediction results from the temperature dependence of (i) the second-order force constants, which set the phonon frequencies and thus the three-phonon scattering phase space, and (ii) the third-order force constants, which influence the intrinsic scattering rates. Our results emphasize the importance of including finite-temperature effects when studying phonon transport in oxides and inform how nanostructuring impacts the thermal conductivity of rutile $\mathrm{Ti}{\mathrm{O}}_2$.

Frictional contacts lead to the formation of a surface layer called the third body, consisting of wear particles and structures resulting from their agglomerates. Its behavior and properties at the nanoscale control the macroscopic tribological performance. It is known that wear particles and surface topography evolve with time and mutually influence one another. However, the formation of the mature third body is largely uncharted territory, and the properties of its early stages are unknown. Here we show how a third body initially consisting of particles acting as roller bearings transitions into a shear-band-like state by forming adhesive bridges between the particles. Using large-scale atomistic simulations on a brittle model material, we find that this transition is controlled by the growth and increasing disorganization of the particles with increasing sliding distance. Sliding resistance and wear rate are, at first, controlled by the surface roughness, but upon agglomeration wear stagnates and friction becomes solely dependent on the real contact area in accordance with the plasticity theory of contact by Bowden and Tabor.

Comparison of free energies between different phases and different compositions underlies the prediction of alloy phase diagrams. To allow direct comparison, consistent reference points for the energies or enthalpies are required, and the entropy must be placed on an absolute scale, yielding absolute free energies. Here we derive absolute free energies of liquids from ab initio molecular dynamics by combining the directly simulated enthalpies with an entropy derived from simulated densities and pair correlation functions. As an example of the power of this method we calculate the phase diagrams of two binary alkali metal alloys, Li-Na and K-Na, revealing a critical point and liquid-liquid phase separation in the former case, and a deep eutectic in the latter. Good agreement with experimental data demonstrates the power of this simple method.

The synthesis of complex transition metal oxides using molecular beam epitaxy is well established with the exception of gold. Overcoming the oxophobicity of gold is a mandatory step towards an entire new section of complex transition metal oxides. Here, we show that ${\mathrm{Sr}}_5{\mathrm{Au}}_2{\mathrm{O}}_8$ thin films can be epitaxially grown onto ${\mathrm{SrTiO}}_3$ substrates using a high-efficiency radio-frequency-operated atomic oxygen source as an oxidizing agent. Stoichiometric atomic beam fluxes of strontium and gold were controlled by electron-impact emission spectroscopy. ${\mathrm{Sr}}_5{\mathrm{Au}}_2{\mathrm{O}}_8$ thin films were grown at temperatures between 627 and 653 ${}^{\circ }\mathrm{C}$. Owing to the sensitivity of ${\mathrm{Sr}}_5{\mathrm{Au}}_2{\mathrm{O}}_8$ thin films to environmental agents, e.g., water and ${\mathrm{CO}}_2$, we capped them with a 300-nm layer of Se or a 200-nm layer of Au.

The atomic scale computation of dislocation core structures has become an essential tool in the development of models for the plasticity of metals. Competing dislocation core structures are often analyzed at $T=0$ K (with $T$ the temperature), and the dislocation core structure with the lowest energy is assumed to be the structure dictating the dynamics of the individual dislocation at finite temperatures. It is shown here that, for some hexagonal-close-packed (HCP) metals, this approach may be too simplistic. As a prototypical example, $\langle a\rangle$-type screw dislocations within HCP Ti modeled using an empirical interatomic potential are considered. It is shown using molecular dynamics simulations that, at room temperature and above, the core structure of the dislocation is remarkably complex and variable. The implications of this complexity for the dynamics of the dislocations are discussed.

An exactly solvable thermodynamic model for the core structure of $\langle a\rangle$-type screw dislocations in $\alpha$-Ti is introduced and used to explore the role in which polymorphic core spreading contributes to the free energy of the dislocation. Each segment of the dislocation core is assumed to be in one of three possible configurations, and the free energy differences per segment between dislocation core spreadings are taken as parameters. It is shown that thermal fluctuations between the core spreadings should be common, even at room temperature, and that these fluctuations can contribute significantly to the free energy. It is also shown that under some circumstances, the core structures can display morphological switching wherein the stable core spreading changes morphology with a change in temperature. The implications for dislocation dynamics and for modeling finite temperature dislocation core properties using molecular dynamics simulations are considered.

Refractory multiprincipal element alloys (MPEAs) are promising material candidates for high-temperature, high-strength applications. However, their deformation mechanisms, particularly at the dislocation scale, are unusual compared to those of conventional alloys, making it challenging to understand the origin of their high strength. Here, using an atomistically informed phase-field dislocation dynamics model, we study transitions in the morphology and critical stresses of long gliding screw dislocations over extended distances in a refractory MPEA. The model MPEA crystal accounts for the atomic-scale fluctuations in chemical composition across the glide planes via spatially correlated lattice energies and for the differences in glide resistances between screw and edge dislocations of unit length. We show that the dislocation moves in a stop-start motion, alternating between wavy morphology in free flight and nearly recovered straight screw orientation in full arrest. The periods of wavy glide are due to variable kink-pair formation and migration rates along the length, where portions with higher rates glide more quickly. The critical stress to initiate motion corresponds to the stress required to form and migrate a kink pair at the weakest region along the length of the dislocation. Heterogeneity in lattice energy leads to variability in the local stress-strain response and to a strain hardening-like response, in which the critical stress to reactivate glide increases with glide distance. Statistical assessment of hundreds of realizations of dislocations indicates that the amount of hardening directly scales with the dispersion in underlying lattice energy.

Computational materials discovery efforts utilize hundreds or thousands of density functional theory calculations to predict material properties. Historically, such efforts have performed calculations at the generalized gradient approximation (GGA) level of theory due to its efficient compromise between accuracy and computational reliability. However, high-throughput calculations at the higher metaGGA level of theory are becoming feasible. The strongly constrained and appropriately normed (SCAN) metaGGA functional offers superior accuracy to GGA across much of chemical space, making it appealing as a general-purpose metaGGA functional, but it suffers from numerical instabilities that impede its use in high-throughput workflows. The recently developed ${\mathrm{r}}^2\mathrm{SCAN}$ metaGGA functional promises accuracy similar to SCAN in addition to more robust numerical performance. However, its performance compared to SCAN has yet to be evaluated over a large group of solid materials. In this paper, we compared ${\mathrm{r}}^2\mathrm{SCAN}$ and SCAN predictions for key properties of approximately 6000 solid materials using a newly developed high-throughput computational workflow. We find that ${\mathrm{r}}^2\mathrm{SCAN}$ predicts formation energies more accurately than SCAN and PBEsol for both strongly and weakly bound materials and that ${\mathrm{r}}^2\mathrm{SCAN}$ predicts systematically larger lattice constants than SCAN. We also find that ${\mathrm{r}}^2\mathrm{SCAN}$ requires modestly fewer computational resources than SCAN and offers significantly more reliable convergence. Thus, our large-scale benchmark confirms that ${\mathrm{r}}^2\mathrm{SCAN}$ has delivered on its promises of numerical efficiency and accuracy, making it a preferred choice for high-throughput metaGGA calculations.

Two-dimensional (2D) van der Waals (vdW) magnets have garnered considerable attention owing to the existence of magnetic order down to atomic dimensions, flexibility towards interface engineering and unconventional magnetoresistive properties, offering an attractive platform to explore novel phenomena and functionalities, prospective for spintronic or quantum information devices. Among the promising candidates, vdW ferromagnet (FM) ${\mathrm{Fe}}_3{\mathrm{GeTe}}_2$ shows an unusual magnetotransport behavior, tunable by doping at the magnetic (Fe) site, and tentatively arising from complicated underlying spin texture configurations. In this work, we explore an alternative route towards manipulation of magnetotransport properties without directly affecting the magnetic site i.e., by doping at the nonmagnetic (Ge) site of ${\mathrm{Fe}}_3\left(\mathrm{Ge},\mathrm{As}\right){\mathrm{Te}}_2$. Interestingly, doping at the non-magnetic (Ge) site results in an unconventional Hall effect whose strength was considerably modified by increasing As concentration, tentatively attributed to underlying emergent electromagnetic behavior, demonstrating an alternate direction towards tailoring of underlying interactions without perturbing the magnetic (Fe) site in 2D vdW magnetic materials.

Rare-earth transition-metal ferrimagnetic materials are promising candidates for next-generation spintronic devices owing to their small magnetic moments and fast dynamics. However, only a few studies have been conducted thus far on the transport properties of noncollinear ferrimagnetic spin textures. In this study, we investigate the transverse and longitudinal resistances in a ferrimagnet/heavy-metal heterostructure composed of Co-Gd and Pt. The transition-metal sublattice arrangement and magnetic field–temperature phase diagram are obtained based on unconventional hysteresis loops observed near the compensation point. In the collinear regime, the spin Hall magnetoresistance (SMR) and anisotropic magnetoresistance (AMR) exhibit normal oscillation curves. In the noncollinear regime, the SMR and AMR signals are inverted for the existence of a 90° phase shift between magnetization and magnetic fields. Owing to the conical spin textures in the spin-flop state, sharp peaks emerge when the field is perpendicular to the film plane. The abnormal magnetoresistance in our results can be used to analyze complex magnetic structures, which plays an important role in the all-electrical readout of noncollinear spintronics devices.

We determine the anisotropic dielectric functions of rhombohedral $\alpha \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ by far-infrared and infrared generalized spectroscopic ellipsometry and derive all transverse optical and longitudinal optical phonon mode frequencies and broadening parameters. We also determine the high-frequency and static dielectric constants. We perform density functional theory computations and determine the phonon dispersion for all branches in the Brillouin zone, and we derive all phonon mode parameters at the Brillouin zone center including Raman-active, infrared-active, and silent modes. Excellent agreement is obtained between our experimental and computation results as well as among all previously reported partial information from experiment and theory. We also compute the same information for $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$, the binary parent compound for the emerging alloy of $\alpha$-(${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$, and use results from previous investigations [Schubert, Tiwald, and Herzinger, Phys. Rev. B 61, 8187 (2000)] to compare all properties among the two isostructural compounds. From both experimental and theoretical investigations, we compute the frequency shifts of all modes between the two compounds. Additionally, we calculate overlap parameters between phonon mode eigenvectors and discuss the possible evolution of all phonon modes into the ternary alloy system and whether modes may form single-mode or more complex mode behaviors.

Transition metal trichlorides show peculiar and versatile magnetic properties. Whereas ${\mathrm{CrCl}}_3$ is a layered antiferromagnet with potential applications as an ultrathin two-dimensional magnet, $\alpha \text{−}{\mathrm{RuCl}}_3$ may host a spin-liquid state driven by Kitaev interactions. The interest to control their material properties by chemical modifications is immense, both from an application related and from a fundamental point of view. Here, by studying ${\mathrm{CrCl}}_3$, ${\mathrm{Cr}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_3$, and $\alpha \text{−}{\mathrm{RuCl}}_3$ by photoemission and electron energy-loss spectroscopy, we find that transition metal substitution changes the optical properties of the host without compromising its underlying electronic structure. It does so by a Cr–Ru related charge transfer process across the Mott gap effectively opening up a new absorption channel below the principal gap edge of ${\mathrm{CrCl}}_3$. The Cr and Ru valencies as well as the respective valence band density of states remain stable for the mixed ${\mathrm{Cr}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_3$ compound. Our study underlines the potential of transition metal substitution as a means of material engineering of trichlorides.

To elucidate the geometric structure of the putative room-temperature superconductor, carbonaceous sulfur hydride (C-S-H), at high pressure, we present the results of an extensive computational structure search of bulk C-S-H at 250 GPa. Using the minima hopping structure prediction method coupled to the GPU-accelerated sirius library, more than 17 000 local minima with different stoichiometries in large simulation cells were investigated. Only 24 stoichiometries are favorable against elemental decomposition, and all of them are carbon-doped ${\mathrm{H}}_3\mathrm{S}$ crystals. The absence of van Hove singularities or similar peaks in the electronic density of states of more than 3000 candidate phases rules out conventional superconductivity in C-S-H at room temperature.

A deep understanding of the solid-state phase transition processes of zirconia is a mandatory requirement for the development of new zirconia-based materials useful for many industrial applications. For five decades, the monoclinic $\iff$ tetragonal phase transition is described as a martensitic one and it is well known that it is associated with a large unit cell volume variation that promotes the appearance of elastic strains and also microcracking. In the present paper, we study, through in situ high temperature x-ray diffraction experiments, the coupling between strain relaxation and the martensitic phase transition into a pure zirconia bulk polycrystal. Quantitative analysis of the diffraction signal allows us to disentangle, with respect to the temperature variation, the phase transition and the microcracking processes, and we demonstrate that a high temperature postelaboration thermal treatment induces an increase in the stored elastic energy. Finally, we show that in such polycrystals exhibiting a crystal size distribution and in which the crystals are under internal stresses, the tetragonal to monoclinic phase transition process evolves from a first order one to a second order one when the temperature decreases.

The valence and spin-state transition is a very specific transition found in some cobalt oxides which involves a coupled modification in the Co valence and the ${\mathrm{Co}}^{3+}$ spin state. In the phase diagram of these compounds, one can thus expect a competition of this transition with the ferromagnetism associated with ${\mathrm{Co}}^{4+}$ low spin/${\mathrm{Co}}^{3+}$ high spin double exchange. Here, we study the phase diagram of ${\left({\mathrm{Pr}}_{1-y}{\mathrm{Sm}}_y\right)}_{0.7}{\mathrm{Ca}}_3\mathrm{Co}{\mathrm{O}}_3$ and ${\mathrm{Pr}}_{1-x}{\mathrm{Ca}}_x\mathrm{Co}{\mathrm{O}}_3$ to shed some light on the competition between these two phenomena. To this end, we combine results obtained with a wide variety of techniques: magnetization, heat capacity, resistivity, neutron diffraction, x-ray absorption spectroscopy, and x-ray magnetic circular dichroism. The origin of the various transition lines in the phase diagram of ${\left({\mathrm{Pr}}_{1-y}{\mathrm{Sm}}_y\right)}_{0.7}{\mathrm{Ca}}_3\mathrm{Co}{\mathrm{O}}_3$ as well as that of the critical value $y_{\mathrm{crit}}\sim 0.18$ are discussed. Particular attention is paid to the nature of the ground state magnetism which is an issue strongly debated so far in this type of cobaltate.