Ultradense domain walls are increasingly important for many devices but their microscopic properties are so far not fully understood. Here we use molecular dynamic simulations to study the domain wall stability in the prototypical ferroelectric ${\mathrm{BaTiO}}_3$ combining core-shell pair potentials and a coarse-grained effective Hamiltonian. We transfer the discussion of the field-driven nucleation and motion of domain walls to thermally induced modifications of the wall without an external driving force. Our simulations show that domain wall dynamics and stability depend crucially on microscopic thermal fluctuations. Enhanced fluctuations at domain walls may result in the formation of critical nuclei for the permanent shift of the domain wall. If two domain walls are close—put in other words, when domains are small—thermal fluctuations can be sufficient to bring domain walls into contact and lead to the annihilation of small domains. This is even true well below the Curie temperature and when domain walls are initially as far apart as six unit cells. Such small domains are, thus, not stable and limit the maximum achievable domain wall density in nanoelectronic devices.

The magnetic van der Waals crystals ${\left(\mathrm{Mn}{X}_2{\mathrm{Te}}_4\right)}_m{\left(X_2{\mathrm{Te}}_3\right)}_n$ $\left(X=\mathrm{Sb},\mathrm{Bi}\right)$ have drawn significant attention due to their rich topological properties and tenability by external magnetic field. In this letter, we report on the discovery of superconductivity in magnetic topological insulator candidate ${\mathrm{MnSb}}_4{\mathrm{Te}}_7$ ($m=1$, $n=1$) via the application of high pressure. The antiferromagnetic ordering is robust to pressure until 8 GPa and then fully suppressed. The carrier type converts from hole to electron accompanied with structural phase transition at $\sim 16\mathrm{GPa}$. Superconductivity emerges near the critical pressure 30 GPa where ${\mathrm{MnSb}}_4{\mathrm{Te}}_7$ converts to phase III. Interestingly, ${\mathrm{MnSb}}_4{\mathrm{Te}}_7$ shows a domelike phase diagram with a maximum $T_{\mathrm{c}}$ of 2.2 K at 50.7 GPa.

We find that in ${\mathrm{BaTiO}}_3$ the phonon angular momentum is dominantly pointing in directions perpendicular to the electrical polarization. Therefore, the external electric field in ferroelectric ${\mathrm{BaTiO}}_3$ does not control only the direction of electrical polarization but also the direction of the phonon angular momentum. This finding opens up the possibility of electric field control of physical phenomena that depend on phonon angular momentum. We construct an intuitive model, based on our first-principles calculations, that captures the origin of the relationship between phonon angular momentum and electric polarization.

Time-resolved photoluminescence (PL) experiments on ZnO crystals have been conducted at room temperature to elucidate the origins of a near-band edge (NBE) emission. A temporal profile of the PL spectra exhibits a two-curve structure with different decaying rates and is reproduced reasonably by a biexponential function, indicating that there are at least two origins with different decay times in the NBE emission. By taking free exciton (FX)–longitudinal optical (LO) phonon interactions into account in the analysis of the PL spectra, the faster and slower decaying emissions are found to be associated with stronger and weaker FX-LO phonon coupling, respectively. We propose that the NBE emissions with different decay times are related to crystal imperfections; the fast-decaying emission originates from the region with a higher density of defects or impurities, while the slower one is from the regions with better crystallinity.

Ruddlesden-Popper (RP) oxides ($A_{n+1}{B}_n{\mathrm{O}}_{3n+1}$) comprised of perovskite ${\left(AB{\mathrm{O}}_3\right)}_n$ slabs can host a wider variety of structural distortions than their perovskite counterparts. This makes an accurate structural determination of RP oxides more challenging. In this study, we investigate the structural phase diagram of $n=1$ RP ${\mathrm{Sr}}_2{\mathrm{SnO}}_4$, one of alkaline earth stannates that are promising for opto-electronic applications by using group theory based symmetry analysis and first-principles calculations. We explore the symmetry breaking effects of different dynamical instabilities, predict the energies of phases they lead to, and take into account different (biaxial strain and hydrostatic pressure) boundary conditions. We also address the effect of structural changes on the electronic structure and find that compressive biaxial strain drives ${\mathrm{Sr}}_2{\mathrm{SnO}}_4$ into a regime with wider band gap and lower electron effective mass.

Density functional theory (DFT) has been highly successful in supporting experimental materials science; however, a correct electronic ground state is required to realize the full theoretical capacity of DFT. The uranium oxides, $\alpha -{\mathrm{U}}_3{\mathrm{O}}_8$ in particular, are simultaneously technologically important materials and theoretically challenging for DFT because the uranium magnetic ground state is not obvious. This is true for both experiment and theory—magnetic susceptibility measurements indicate an antiferromagnetic (AFM) ground state with transitions near 4.2 and 8.0 K, but the ordering itself is not known. Theoretical literature reports are in contradiction, with independent studies finding paramagnetic, ferromagnetic (FM), and AFM states as the lowest energy configuration. However, recent inelastic neutron scattering experiments suggested an uninvestigated magnetic structure with ordering along the [0.5 1 1] plane, motivating a theoretical reinvestigation. Using this insight, we calculated the relative energy of FM and AFM orderings along [0.5 1 1], [0.5 0 0], [0 1 0], and [0 0 1] using noncollinear DFT calculations with spin-orbital coupling. We found that the [0.5 1 1] AFM structure is lower in energy than FM or AFM orderings along the low Miller index directions. We also investigated polarization of the magnetic moment along each lattice vector and found that polarization along the out-of-plane direction is the energetically preferred orientation for the AFM structures. Additionally, we found in all calculations that moments initially pointing along the in-plane lattice vectors significantly relax until they point along the coordinate between the two crystallographically distinct uranium sites with complex noncollinear magnetic configurations. The new [0.5 1 1] AFM magnetic structure provides an additional path forward toward understanding the electronic structure of α-${\mathrm{U}}_3{\mathrm{O}}_8$ and lends theoretical credibility to recent neutron scattering results.

Two benchmark problems for a quantitative assessment of anisotropic curvature driving force in a phase-field method were developed and introduced. Both benchmarks contained an anisotropically shrinking grain in a homogeneous matrix. The first benchmark was a shrinking Wulff shape and in the second, such inclination dependence of the kinetic coefficient was added so that the shrinkage was isotropic. In both cases, the match to the expected shape was quantified by means of Hausdorff distance and the shrinkage rate was analytically expressed. Three different ways of interface energy anisotropy inclusion in a multi-phase-field model were compared. Because their performance was comparable, they were tested in an additional benchmark problem, which concerned the direct measurement of equilibrium triple junction angles. Based on this benchmark, only one of the three strategies to include anisotropy was reliable in strongly anisotropic systems.

Two-dimensional (2D) Cairo pentagon materials have attracted tremendous interest due to their exotic physical properties. Based on first-principles calculations, we predict that monolayer penta-${\mathrm{BP}}_2$ is a promising Cairo pentagon-based 2D material simultaneously harboring a negative Poisson ratio and intrinsic superconductivity. The Poisson ratio exhibits strong anisotropy with alternating positive-negative signs, and its value along the diagonal direction is more negative than that of pentagraphene. The medium-strength coupling between the Fermi-surface electrons and a phonon mode majorly originating from the ${\mathrm{P}}_2$ dimers leads to a superconducting transition estimated to occur at $T_{\mathrm{c}}\sim 7.7\mathrm{K}$. Moreover, we find that fluorination/chlorination can efficiently engineer the superconductivity and mechanical properties of penta-${\mathrm{BP}}_2$: achieving higher $T_{\mathrm{c}}$ of 9.6/10.5 K, turning Poisson's ratio from negative to positive and enhancing Young's modulus. These results render penta-${\mathrm{BP}}_2$ an appealing platform to investigate unique superconductivity and mechanical properties in the Cairo pentagon lattice.

We investigate tunable electric polarization and electronic structure of quasi-two-dimensional (quasi-2D) allotropes of selenium, which are formed from their constituent one-dimensional (1D) structures through an interchain interaction facilitated by the multivalence nature of Se. Our ab initio calculations reveal that different quasi-2D Se allotropes display different types of electric polarization, including ferroelectric (FE) polarization normal to the chain direction in $\alpha$ and $\delta$ allotropes, noncollinear ferrielectric (FiE) polarization along the chain axis in $\tau$-Se, and anti-ferroelectric (AFE) polarization in $\eta$-Se. The magnitude and direction of the polarization can be changed by a previously unexplored rotation of the constituent chains. In that case, an in-plane polarization direction may change to out-of-plane in $\alpha$-Se and $\delta$-Se, flip its direction, and even disappear in $\tau$-Se. Also, the band gap may be reduced and changed from indirect to direct by rotating the constituent chains about their axes in these quasi-2D Se allotropes.

Structural and thermodynamic properties of single-crystalline ${\mathrm{UNi}}_{1-x}{\mathrm{Ge}}_2$ with $x=0.66$ have been investigated by measuring magnetization, specific heat, and thermal expansion over a wide range of temperatures and magnetic fields. The measurements revealed the emergence of a long-range antiferromagnetic ordering of uranium magnetic moments below the Néel temperature $T_{\mathrm{N}}=45.5\left(1\right)\mathrm{K}$ and the existence of two easy axes in the studied compound, namely, $b$ and $c$, which correspond to the plane of the uranium zigzag chains. Magnetic field applied along these two crystallographic directions induces in the system a first-order metamagnetic phase transition (from antiferromagnetism to field-polarized paramagnetism), and the width of the magnetic hysteresis associated with that transition reaches as much as about 40 kOe at the lowest temperatures. A magnetic phase diagram developed from the experimental data showed that the metastable region associated with that magnetic hysteresis forms a funnel that narrows toward the Néel point in a zero magnetic field. The four-layer Ising model has successfully predicted the collinear antiferromagnetic structure in ${\mathrm{UNi}}_{0.34}{\mathrm{Ge}}_2$ (known from earlier reports), its magnetic phase diagram, and temperature and field variations of its magnetization. Moreover, it suggests that the first-order phase transition extends down to a zero magnetic field, although it is barely detectable in the experiments performed in low magnetic fields. According to this model, the second-order phase transition occurs in the compound only in a zero field.

MXenes are a family of two-dimensional materials of great interest due to their unique properties, e.g., adjustability based on changes in their composition, structure, and surface functionality, which grant MXenes a variety of applications. One way of changing the catalytic effect of MXenes consists of adsorbing isolated metallic elements, such as transition metals (TMs), onto their surface, leading to the formation of single-atom catalysts. Herewith, the adsorption behavior of 31 TMs on the surface of two titanium carbide MXenes, viz. ${\mathrm{Ti}}_2\mathrm{C}$ and ${\mathrm{Ti}}_2{\mathrm{CO}}_2$, is analyzed by means of density-functional theory (DFT) calculations. We find that the oxygen surface termination causes most of the TM atoms to adsorb on a hollow site above a carbon atom, whereas on bare ${\mathrm{Ti}}_2\mathrm{C}$, the adsorption preference follows a pattern related to groups of the periodic table. The interaction between the TM atoms and the surface of both ${\mathrm{Ti}}_2\mathrm{C}$ and ${\mathrm{Ti}}_2{\mathrm{CO}}_2$ is strong, as demonstrated by the calculated adsorption energies, which range between about −1 and −9 eV on either surface. Upon adsorption on ${\mathrm{Ti}}_2{\mathrm{CO}}_2$, electrons are transferred from the adatom to the MXene surface, whereas on ${\mathrm{Ti}}_2\mathrm{C}$, the only TM atoms for which this happens are the ones in group 3 of the periodic table. All the other transition metal atoms become negatively charged after adsorption on ${\mathrm{Ti}}_2\mathrm{C}$. On the oxygen-covered MXene, stronger adsorptions are accompanied by higher charge transfers. The energy barriers for TM adatom diffusion on ${\mathrm{Ti}}_2\mathrm{C}$ are very small, meaning that the adatoms can move rather freely along it. On ${\mathrm{Ti}}_2{\mathrm{CO}}_2$, however, higher diffusion barriers were found, many being above 1 eV, which suggests that the oxygen termination layer blocks the diffusion. On both surfaces, the highest diffusion barriers were found to correspond to the TM elements which adsorb most strongly.

${R\mathrm{V}}_6{\mathrm{Sn}}_6$ ($R$ = rare earth) compounds are appealing materials platforms for exploring the interplay between $R$-site magnetism and nontrivial band topology associated with the nonmagnetic vanadium-based kagome network. Here we present the synthesis and characterization of the kagome metal ${\mathrm{TbV}}_6{\mathrm{Sn}}_6$ via single crystal x-ray diffraction, magnetization, transport, and heat capacity measurements. Magnetization measurements reveal strong, uniaxial magnetic anisotropy rooted in the alignment of ${\mathrm{Tb}}^{3+}$ moments in the interplane direction below 4.3(2) K. ${\mathrm{TbV}}_6{\mathrm{Sn}}_6$ exhibits multiband transport behavior with high mobilities of charge carriers, and our measurements suggest ${\mathrm{TbV}}_6{\mathrm{Sn}}_6$ is a promising candidate for hosting Chern gaps driven via the interplay between Tb-site magnetic order and the band topology of the V-site kagome network.

The voltage-controlled magnetic anisotropy (VCMA) effect is the basis for ultralow-power spin manipulation in spintronic devices such as voltage-controlled magnetoresistive random-access memory (VC-MRAM). A larger VCMA coefficient $\xi$ is required to develop VC-MRAM with a higher density. Although $\xi$ is theoretically proportional to the dielectric constant ${\varepsilon }_{\mathrm{r}}$ of the tunnel barrier material, this relationship has not been experimentally examined. We systematically investigated the relationship between $\xi$ and ${\varepsilon }_{\mathrm{r}}$ in a ${\mathrm{Fe}}_{80}{\mathrm{B}}_{20}/\mathrm{MgO}/{\mathrm{HfO}}_2$ structure prepared by sputtering-based deposition. We successfully controlled ${\varepsilon }_{\mathrm{r}}$ while maintaining perpendicular magnetic anisotropy at the ${\mathrm{Fe}}_{80}{\mathrm{B}}_{20}/\mathrm{MgO}$ interface. Both ${\varepsilon }_{\mathrm{r}}$ and $\xi$ were observed to monotonically increase with ${\mathrm{HfO}}_2$, showing a positive correlation between $\xi$ and ${\varepsilon }_{\mathrm{r}}$. The maximum $\xi$ reached −92 ± 4 fJ/Vm with ${\varepsilon }_{\mathrm{r}}=19.6\pm 0.3$. This result experimentally demonstrated that the VCMA effect could be enhanced by increasing ${\varepsilon }_{\mathrm{r}}$.

Using in situ x-ray pair distribution analysis in magnetic field, we study the strong spin-lattice coupling in archetypal magnetocaloric ${\mathrm{Gd}}_5{\left(\mathrm{Ge},\mathrm{Si}\right)}_4$ alloys manifested by the presence of a first order paramagnetic (PM)-to-ferromagnetic (FM) phase transition that can be triggered by either decreasing temperature in zero magnetic field or increasing magnetic field at constant temperature. We find that the coupling arises from the tendency of Gd-(Si,Ge) slabs in the alloys to reversibly slide against each other to both pack closely and couple ferromagnetically. We also find that, locally, the packing of slabs in FM phases induced by decreasing temperature in zero field is different from that in FM phases of the same chemical composition induced by increasing magnetic field isothermally, indicating that temperature and magnetic field are coupled but not necessarily equivalent control variables for triggering phase transitions in strongly correlated systems.

The accurate prediction of the band gaps of halide perovskites within density functional theory is known to be challenging. The recently developed Wannier-localized optimally tuned screened range-separated hybrid functional was shown to be highly accurate for fundamental band gaps of standard semiconductors and insulators. This was achieved by selecting the parameters of the functional to satisfy an ansatz that generalizes the ionization potential theorem to the removal of charge from a state that corresponds to a Wannier function. Here, we present applications of the method to the band gaps of typical halide perovskites. We find a satisfyingly small formal mean absolute error of $\sim$0.1 eV with respect to experimental band gaps and very good agreement with previous many-body perturbation theory calculations.