In twist-bend nematic ($N_{\mathrm{TB}}$) liquid crystals (LCs), the director (mean molecular orientation) exhibits heliconical structure with nanoscale periodicity. On the mesoscopic scale, $N_{\mathrm{TB}}$ resembles layered systems (like smectics) without a true mass density wave, where the helical pitch is equivalent to a “pseudolayer.” We study rheological properties of a $N_{\mathrm{TB}}$ phase and compare the results with those of a usual smectic-$A$ phase. Analyzing the shear response and adapting a simplified physical model for the rheology of defect-mediated lamellar systems, we measure the pseudolayer compression elastic constant $B_{\mathrm{eff}}$ of the $N_{\mathrm{TB}}$ phase from the measurements of the dynamic modulus $G^{*}\left(\omega \right)$. It is found that $B_{\mathrm{eff}}$ of the $N_{\mathrm{TB}}$ phase is in the range of ${10}^3$–${10}^6$ Pa and it follows a temperature dependence, $B_{\mathrm{eff}}\sim {(T_{\mathrm{TB}}-T)}^2$, as predicted by the recent coarse-grained elastic theory. Our results show that the structural rheology of $N_{\mathrm{TB}}$ LCs is strikingly similar to that of the usual smectic LCs, although the temperature dependence of $B_{\mathrm{eff}}$ is much faster than that of smectic LCs as predicted by the coarse-grained models.

Using finite-temperature phonon calculations and the Lyddane-Sachs-Teller relations, we calculate ab initio the static dielectric constants of 78 semiconducting oxides and fluorides with cubic perovskite structures at 1000 K. We first compare our method with experimental measurements, and we find that it succeeds in describing the temperature dependence and the relative ordering of the static dielectric constant ${\epsilon }_{DC}$ in the series of oxides ${\mathrm{BaTiO}}_3$, ${\mathrm{SrTiO}}_3$, ${\mathrm{KTaO}}_3$. We show that the effects of anharmonicity on the ion-clamped dielectric constant, on Born charges, and on phonon lifetimes, can be neglected in the framework of our high-throughput study. Based on the high-temperature phonon spectra, we find that the dispersion of ${\epsilon }_{DC}$ is one order of magnitude larger among oxides than fluorides at 1000 K. We display the correlograms of the dielectric constants with simple structural descriptors, and we point out that ${\epsilon }_{DC}$ is actually well correlated with the infinite-frequency dielectric constant ${\epsilon }_{\infty }$, even in those materials with phase transitions in which ${\epsilon }_{DC}$ is strongly temperature dependent.

Gate-tunable spin-dependent properties could be induced in graphene at room temperature through the magnetic proximity effect by placing it in contact with a metallic ferromagnet. Because strong chemical bonding with the metallic substrate makes gating ineffective, an intervening passivation layer is needed. Previously considered passivation layers result in a large shift of the Dirac point away from the Fermi level, so that unrealistically large gate fields are required to tune the spin polarization in graphene (Gr). We show that a monolayer of Au or Pt used as the passivation layer between Co and graphene brings the Dirac point closer to the Fermi level. In the $\text{Co}/\text{Pt}/\text{Gr}$ system the proximity-induced spin polarization in graphene and its gate control are strongly enhanced by the presence of a surface band near the Fermi level. Furthermore, the shift of the Dirac point could be eliminated entirely by selecting submonolayer coverage in the passivation layer. Our findings open a path towards experimental realization of an optimized two-dimensional system with gate-tunable spin-dependent properties.

The structure of a 180 ° uncharged rotational domain wall in a multiaxial ferroelectric film is studied in the framework of an analytical Landau-Ginzburg-Devonshire (LGD) approach. Finite element modeling (FEM) is used to solve numerically the system of the coupled nonlinear Euler-Lagrange (EL) second-order differential equations for two components of polarization. We show that the structure of the domain wall and corresponding metastable or stable phase of the film are controlled by a single parameter—the dimensionless ferroelectric anisotropy factor $\mu$. We fitted the static profile of a solitary domain wall, calculated by FEM, with kinklike functions for polarization components, and extracted the five $\mu$-dependent parameters from the fitting to FEM curves. The surprisingly high accuracy of the fitting results for two polarization components in the entire $\mu$ range allows us to conclude that the analytical functions, which are trial functions in the direct variational method, can be treated as a high-accuracy variational solution of the static EL equations. We further derive exact two-component analytical solutions of the static EL equations for a polydomain 180 ° domain structure in a multiaxial ferroelectric film. Using these, we derive analytical expressions for the system free energy and analyze its dependence on the film thickness and boundary conditions at the film surfaces. The single-domain state is ground for zero polarization derivative at the surfaces, while the polydomain states minimize the system energy for zero polarization at the surfaces. Counterintuitively, the energy of the polydomain states split into two levels, “0” and “1,” for zero polarization at the surfaces, and each of the levels contains a large number of close-energy sublevels, whose structure is characterized by a different number and type of domain walls. The analytical solutions can become a useful tool for Bayesian analysis of high-resolution scanning transmission electron microscopy images in ferroelectric films.

The newfound two-dimensional antiferromagnetic $\mathrm{Gd}{\mathrm{Te}}_3$ has great potential in novel magnetic twistronic and spintronic devices because it has the highest carrier mobility among all known layered magnetic materials. Here, we used high-resolution angle-resolved photoemission spectroscopy to investigate its Fermi-surface topology and low-lying electronic band structure. The Fermi surface is partially gapped by charge-density waves below the transition temperature. Very steep and nearly linear band dispersion near the Fermi energy contributes to the high carrier mobility in $\mathrm{Gd}{\mathrm{Te}}_3$. We find that the scattering rate of the quasiparticle increases linearly as a function of binding energy within a wide energy range, indicating that $\mathrm{Gd}{\mathrm{Te}}_3$ is a non-Fermi-liquid metal. Our results in this paper provide a fundamental understanding of this layered antiferromagnetic material to guide future studies on it.

We report the growth of the intrinsic magnetic topological system $\mathrm{MnTe}{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ by molecular beam epitaxy. By mapping the temperature and the Bi:Mn flux ratio, it is shown that there is a narrow growth window for the $n=1$ phase $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ with $2.0<\mathrm{Bi}:\mathrm{Mn}<2.6$ at 225 °C. Here the films are stoichiometric and excess Bi and Te is not incorporated. At higher flux ratios (Bi:Mn≥4.5) it is found that the $n=2\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$ phase is stabilized. Transport measurements indicate that the $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ and $\mathrm{Mn}{\mathrm{Bi}}_4{\mathrm{Te}}_7$ undergo magnetic transitions around 25 and 10 K, respectively, consistent with antiferromagnetic phases found in the bulk. Further, for Mn-rich conditions (Bi:Mn<2), ferromagnetism emerges that exhibits a clear hysteretic state in the Hall effect, which likely indicates Mn-doped $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. Understanding how to grow ternary chalcogenide phases is the key to synthesizing new materials and to interface magnetism and topology, which together are routes to realize and control exotic quantum phenomena.

${\mathrm{Yb}}_{14}\mathrm{Mn}{\mathrm{Sb}}_{11}$ is a magnetic Zintl compound as well as being one of the best high temperature $p$-type thermoelectric materials. According to the Zintl formalism, which defines intermetallic phases where cations and anions are valence satisfied, this structure type is nominally made up of 14 ${\mathrm{Yb}}^{2+}$, 1 ${\mathrm{MnSb}}_4^{9-}$, 1 ${\mathrm{Sb}}_3^{7-}$, and 4 ${\mathrm{Sb}}^{3-}$ atoms. When Mn is replaced by Mg or Zn, the Zintl defined motifs become 13 ${\mathrm{Yb}}^{2+}$, 1 ${\mathrm{Yb}}^{3+}$, 1 (Mg, Zn)${\mathrm{Sb}}_4^{10-}$, 1 ${\mathrm{Sb}}_3^{7-}$, and 4 ${\mathrm{Sb}}^{3-}$. The predicted existence of ${\mathrm{Yb}}^{3+}$ based on simple electron counting rules of the Zintl formalism calls the Yb valence of these compounds into question. X-ray absorption near-edge structure, magnetic susceptibility, and specific heat measurements on single crystals of the three analogs show signatures of intermediate valence Yb behavior and in particular, reveal the heavy fermion nature of ${\mathrm{Yb}}_{14}{\mathrm{MgSb}}_{11}$. In these isostructural compounds, Yb can exhibit a variety of electronic configurations from intermediate ($M=\mathrm{Zn}$), mostly 2+ ($M=\mathrm{Mn}$), to 3+ ($M=\mathrm{Mg}$). In all cases, there is a small amount of intermediate valency at the lowest temperatures. The amount of intermediate valency is constant for $M=\mathrm{Mn}$, Mg and temperature dependent for $M=\mathrm{Zn}$. The evolution of the Yb valence correlated to the transport properties of these phases is highlighted. The presence of Yb in this structure type allows for fine tuning of the carrier concentration and thereby the possibility of optimized thermoelectric properties along with unique magnetic phenomena.

A ferroelectric ${\mathrm{BaTiO}}_3$ thin film grown on a ${\mathrm{NdScO}}_3$ substrate was studied using x-ray photon correlation spectroscopy (XPCS) to characterize thermal fluctuations near the $a/b$ to $a/c$ domain structure transformation present in this low-strain material, which is absent in the bulk. XPCS studies provide a direct comparison of the role of domain fluctuations in first- and second-order phase transformations. The $a/b$ to $a/c$ domain transformation is accompanied by a decrease in fluctuation timescales, and an increase in intensity and correlation length. Surprisingly, domain fluctuations are observed up to 25 °C above the transformation, concomitant with the growth of $a/c$ domains and coexistence of both domain types. After a small window of stability, as the Curie temperature is approached, $a/c$ domain fluctuations are observed, albeit slower, potentially due to the structural transformation associated with the ferroelectric to paraelectric transformation. The observed time evolution and reconfiguration of domain patterns highlight the role played by phase coexistence and elastic boundary conditions in altering fluctuation timescales in ferroelectric thin films.

The crystal structure of a compound plays an important role in determining its properties. Here we analyze over 4000 intermetallic compounds, and we identify a hierarchical relationship between their crystal structures. By considering each intermetallic compound as a particular chemical ordering over the sites of a higher symmetry crystal structure, we determine that most intermetallic compounds can be derived from a small number of parent crystal structures. While many compounds are chemical orderings over the sites of simple crystal structures such as body-centered-cubic (bcc) and face-centered-cubic (fcc) structures, the majority map onto more complex parent crystal structures. Surprisingly, many intermetallic compounds map onto parent crystal structures that differ from those of their elemental constituents. We find that the occurrence of several of the more complex parent crystal structures, such as the Laves phases, can be understood in terms of simple descriptors such as the ratio of atomic radii and electronegativity differences.

Over the past decades, investigations of the anomalous low-energy electronic properties of $\mathrm{Zr}{\mathrm{Te}}_5$ have reached a wide array of conclusions. An open question is the growth method's impact on the stoichiometry of $\mathrm{Zr}{\mathrm{Te}}_5$ samples, especially given the very small density of states near its chemical potential. Here we report on high-resolution scanning tunneling microscopy and spectroscopy measurements performed on samples grown via different methods. Using density functional theory calculations, we identify the most prevalent types of atomic defects on the surface of $\mathrm{Zr}{\mathrm{Te}}_5$, namely, Te vacancies and intercalated Zr atoms. Finally, we precisely quantify their density and outline their role as ionized defects in the anomalous resistivity of this material.

Faceted grain boundaries can migrate in interesting and unexpected ways. For example, faceted Σ11 $\langle 110\rangle$ tilt grain boundaries were observed to exhibit mobility values that could be strongly dependent on the direction of migration. In order to understand whether this directionally anisotropic mobility is a general phenomenon and to isolate mechanistic explanations for this behavior, molecular dynamics simulations of bicrystals evolved under an artificial driving force are used to study interface migration for a range of boundary plane inclination angles and temperatures in multiple face-centered-cubic metals (Al, Ni, and Cu). We find that directionally anisotropic mobility is active in a large fraction of these boundaries in Ni and Cu and should therefore impact the coarsening of polycrystalline materials. On the other hand, no such anisotropy is observed in any of the Al boundaries, showing that this behavior is material dependent. Migration of the faceted boundaries is accomplished through transformation events at facet nodes and incommensurate boundary plane facets, which are termed shuffling modes. Three major shuffling modes have been identified, namely, Shockley shuffling, slip plane shuffling, and disordered shuffling. A shift from the first two ordered modes to the third disordered mode is found to be responsible for reducing or removing directionally anisotropic mobility, especially at the highest temperatures studied.

The chemistry (composition and bonding information) of metallic glasses (MGs) is at least as important as structural topology for understanding their properties and production/processing peculiarities. This paper reports a machine learning (ML)-based approach that brings an unprecedented “big picture” view of chemical bond strengths in MGs of a prototypical alloy system. The connection between electronic structure and chemical bonding is given by crystal orbital Hamilton population (COHP) analysis; within the framework of density functional theory (DFT). The stated comprehensive overview is made possible through a combination of: efficient quantitative estimate of bond strengths supplied by COHP analysis, representative statistics regarding structure in terms of atomic configurations achieved with classical molecular dynamics simulations, and the smooth overlap of atomic positions (SOAP) descriptor. The study is supplemented by an application of that ML model under the scope of mechanical loading in which the resulting overview of chemical bond strengths revealed a chemical/structural heterogeneity that is in line with the tendency to bond exchange verified for atomic local environments. The encouraging results pave the way towards alternative approaches applicable in plenty of other contexts in which atom categorization (from the perspective of chemical bonds) plays a key role.

The atomic order and its effect on the electronic structure is a key issue in the application of half-metallic Heusler alloys as spin-polarized electron sources. In this study, we investigated the atomic ordering and electronic structures of $\mathrm{C}{\mathrm{o}}_2\mathrm{Fe}\left(\mathrm{G}{\mathrm{e}}_{0.5}\mathrm{G}{\mathrm{a}}_{0.5}\right)$ (CFGG) Heusler alloy thin films at various annealing temperatures using anomalous x-ray diffraction (AXRD) and hard x-ray photoelectron spectroscopy (HAXPES). AXRD measurements clearly showed that the Co-Fe disorder is large in the as-deposited state, which is reduced to almost zero by annealing at $T_{\mathrm{an}}=500{}^{\circ }\mathrm{C}$ for 30 min. The reduction of the Co-Fe disorder explains the observed increase in the spin polarization of currents estimated by ordinary and anisotropic magnetoresistance measurements. The photoelectron spectra of the valence band in CFGG thin films with different $T_{\mathrm{an}}$ agree well with the first-principles calculations that considers the atomic disorder. We also found that the characteristic peaks appearing in the valence band HAXPES spectra shifted to higher binding energies, compared to the calculated density of states for stoichiometric $\mathrm{L}{2}_1$-ordered CFGG. This indicates that the Fermi level is shifted and located in the vicinity of the conduction band edge in the present CFGG thin film due to electron doping by a mildly Co-rich chemical composition. Our first-principles calculations of partial density of state of $sp$ electron predicted that the high spin-polarization is obtainable in a wider energy region near Fermi level in the $\mathrm{L}{2}_1$-ordered structure than the B2 structure, which explains the enhancement of spin-polarization by promoting $\mathrm{L}{2}_1$-ordering in the present CFGG film.

Recently, lead chloride perovskites represented by $\mathrm{CsPb}{\mathrm{Cl}}_3$ have been theoretically predicted to be ideal $p$-type transparent conductors and hence have attracted a lot of attention. However, experimentally, these materials have long been known to be insulators that can hardly be converted to $p$-type conductors by extrinsic doping. In this work, we systematically reevaluate the $p$-type dopability of lead chloride perovskites by density functional theory calculations. We find that the previously predicted dopability is due to an overestimation caused by the functional employed that gives an unreasonable high-lying valence band maximum. The hybrid functional with an optimized mixing parameter and the inclusion of spin-orbit coupling gives a suitable description of the band edge positions and thus a better assessment of the dopability. Our defect calculations suggest that lead chloride perovskites are intrinsically insulating and can hardly be converted to $p$-type conductors due to the lack of effective dopants, in agreement with the experimental observations. Our results highlight the importance of the suitable description of band edge positions on the prediction of defect properties and dopability of semiconductors.

Topological insulators (TIs) house two-dimensional Dirac plasmons on their surfaces. In TI thin films, plasmons on the top and bottom surfaces couple electrostatically, giving rise to an acoustic and an optical plasmon mode. By extension, a superlattice comprising TI layers and trivially insulating layers could house multiple complex plasmon modes which could be used to create a new type of Dirac metamaterial. In this paper, we synthesize TI superlattices, fabricate them into stripe arrays, characterize their optical and plasmonic properties, and model the samples using transfer matrices. We excite plasmon modes that couple to the phonons in the superlattice, resulting in hybrid plasmon-phonon polaritons. These modes are modeled using an analytical Fano resonance model and the extracted resonant positions are reproduced by transfer matrix modeling. We also excite an epsilon near-zero mode in the top dielectric material ${\left({\mathrm{Bi}}_{0.5}{\mathrm{In}}_{0.5}\right)}_2{\mathrm{Se}}_3$. Understanding the behavior of the polariton modes in this complex system will lend insight into the many-body interaction in low-dimensional systems.

We investigate at the theoretical (time-dependent) density functional theory level the influence of the nature of the anchoring group on the electronic and optical properties of oligothiophene dyes when adsorbed on ${\mathrm{Ti}\mathrm{O}}_2$ surface. The computed electronic structures point to a strong orbital hybridization between the dye and the substrate in the presence of the carboxylic acid and thiocarboxylic acid group, leading to a pronounced pinning effect of the lowest unoccupied molecular orbital (LUMO) level and faster electron injection. In contrast, phosphonic acid and catechol promote a weak electronic coupling between the two components and hence slower injection times. The simulated absorption spectra demonstrate that carboxylic and thiocarboxylic anchoring groups can induce a large redshift of the lowest optical transition of the dye upon adsorption due to a strong stabilization of the LUMO level triggered by the pinning effect while a small redshift prevails for phosphonic and catechol dyes. When pinning is active, the chain-size evolution of the lowest optical transition is also less sensitive to the conjugation length compared to the free dyes.

We have developed a method that can analyze large random grain boundary (GB) models with the accuracy of density functional theory (DFT) calculations using active learning. It is assumed that the atomic energy is represented by the linear regression of the atomic structural descriptor. The atomic energy is obtained through DFT calculations using a small cell extracted from a huge GB model, called replica DFT atomic energy. The uncertainty reduction (UR) approach in active learning is used to efficiently collect the training data for the atomic energy. In this approach, atomic energy is not required to search for candidate points; therefore, sequential DFT calculations are not required. This approach is suitable for massively parallel computers that can execute a large number of jobs simultaneously. In this study, we demonstrate the prediction of the atomic energy of a Fe random GB model containing one million atoms using the UR approach, and we show that the prediction error decreases more rapidly compared with random sampling. We conclude that the UR approach with replica DFT atomic energy is useful for modeling huge GBs and will be essential for modeling other structural defects.

Amorphous ${\mathrm{TiO}}_2$ (a-${\mathrm{TiO}}_2$) is widely used in many fields, ranging from photoelectrochemistry to bioengineering, hence detailed knowledge of its atomic structure is of scientific and technological interest. Here we use an ab initio-based deep neural network potential (DP) to simulate large scale atomic models of crystalline and disordered ${\mathrm{TiO}}_2$ with molecular dynamics. Our DP reproduces the structural properties of all 11 ${\mathrm{TiO}}_2$ crystalline phases, predicts the densities and structure factors of molten and amorphous ${\mathrm{TiO}}_2$ with only a few percent deviation from experiments, and describes the pressure dependence of the amorphous structure in agreement with recent observations. It can be extended to model additional structures and compositions, and can be thus of great value in the study of ${\mathrm{TiO}}_2$-based nanomaterials.