We carried out first-principles slab calculations to investigate the structure, energetics, and electronic properties of the majority (001) surfaces of $\mathrm{NaTa}{\mathrm{O}}_3$ (NTO), a perovskite oxide with excellent photocatalytic properties, and $\mathrm{KTa}{\mathrm{O}}_3$ (KTO), a closely related but somewhat less active compound. Being polar, NTO(001) and KTO (001) require charge compensation to be stabilized. We examine a number of possible structural models for these surfaces by comparing their formation energies to those of the pure NaO/KO and $\mathrm{Ta}{\mathrm{O}}_2$ terminations. Our results show that a “cation-exchange” reconstruction is energetically most favorable for NTO(001) under vacuum conditions, whereas for KTO(001) this reconstruction competes with a “striped” phase with equally exposed KO and $\mathrm{Ta}{\mathrm{O}}_2$ terraces actually observed in recent experiments. NTO is found to exhibit enhanced structural flexibility and more effective charge compensation in comparison to KTO, which is attributed to the significantly smaller size of $\mathrm{N}{\mathrm{a}}^{+}$ relative to ${\mathrm{K}}^{+}$. Upon exposure to water, a (2 × 1) hydroxylated structure is by far most favorable for both NTO and KTO. This structure can thus provide a basis for the mechanistic understanding of photocatalytic processes on NTO and KTO surfaces.

An alternative type of approximation for the exchange and correlation functional in density functional theory is proposed. This approximation depends on a variable $u$ that is able to detect inhomogeneities in the electron density $\rho$ without using derivatives of $\rho$. Instead, $u$ depends on the orbital energies which can also be used to measure how a system differs from the homogeneous electron gas. Starting from the functional of Perdew, Burke, and Ernzerhof (PBE) [Phys. Rev. Lett. 77, 3865 (1996)], a functional depending on $u$ is constructed. Tests on the lattice constant, bulk modulus, and cohesive energy of solids show that this $u$-dependent PBE-like functional is on average as accurate as the original PBE or its solid-state version PBEsol. Since $u$ carries more nonlocality than the reduced density gradient $s$ used in functionals of the generalized gradient approximation (GGA) like PBE and $\alpha$ used in meta-GGAs, it will be certainly useful for the future development of more accurate exchange-correlation functionals.

The large magnetocaloric effect (MCE) observed in Ni-Mn based shape memory Heusler alloys put them forward to use in magnetic refrigeration technology. It is associated with a first-order magnetostructural (martensitic) phase transition. We conducted a comprehensive study of the MCE for the off-stoichiometric Heusler alloy ${\mathrm{Ni}}_{2.2}{\mathrm{Mn}}_{0.8}\mathrm{Ga}$ in the vicinity of its first-order magnetostructural phase transition. We found a reversible MCE under repeated magnetic field cycles. The reversible behavior can be attributed to the small thermal hysteresis of the martensitic phase transition. Based on the analysis of our detailed temperature dependent x-ray diffraction data, we demonstrate the geometric compatibility of the cubic austenite and tetragonal martensite phases. This finding directly relates the reversible MCE behavior to an improved geometric compatibility condition between cubic austenite and tetragonal martensite phases. The approach will help to design shape memory Heusler alloys with a large reversible MCE taking advantage of the first-order martensitic phase transition.

The mobility of vacancies in alloys may limit dislocation climb. Using a combined density functional theory and kinetic Monte Carlo approach we investigate vacancy diffusion in Ni-Re, Ni-W, and Ni-Ta binary alloys up to 10 at.% solute concentration. We introduce an interaction model that takes into account the chemical environment close to the diffusing atom to capture the effect of solute-host and solute-solute interactions on the diffusion barriers. In contrast to an ideal solid solution, it is not only the diffusion barrier of the solute atom that influences the vacancy mobility, but primarily the change in the host diffusion barriers due to the presence of solute atoms. This is evidenced by the fact that the observed vacancy slowdown as a function of solute concentration is larger in Ni-W than in Ni-Re, even though Re is a slower diffuser than W. To model diffusion in complex, nondilute alloys, an explicit treatment of interaction energies is thus unavoidable. In the context of Ni-based superalloys, two conclusions can be drawn from our kinetic Monte Carlo simulations: the observed slowdown in vacancy mobility is not sufficient to be the sole cause for the so-called Re effect, and assuming a direct correlation between vacancy mobility, dislocation climb, and creep strength, the experimentally observed similar effect of W and Re in enhancing creep strength can be confirmed.

Through engineering the interface between $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$ and Pd, we realize a high perpendicular magnetic anisotropy (PMA) and controllable anomalous Hall effect (AHE) in $[\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}\left(t_{\mathrm{CMS}}\right){/\mathrm{Pd}]}_3$ multilayers. The multilayers are characterized by a particulatelike layer morphology following annealing at 573 K with weak B2 crystallographic ordering of the $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$ layer. The largest PMA $\left(K_{\mathrm{eff}}\right)$ of $1.2\times {10}^6\mathrm{erg}/\mathrm{c}{\mathrm{m}}^3$ has been obtained in $[\mathrm{C}{\mathrm{o}}_2{\mathrm{MnSi}\left(1.2\mathrm{nm}\right)/\mathrm{Pd}]}_3$ multilayer annealed at 573 K. The AHE can be tuned, which we attribute to a varying proximity effect at the $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}/\mathrm{Pd}$ interface, by varying the annealing temperature and $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$ thickness. According to the expanded AHE scaling relation, we calculate the different contributions to the origin of AHE. Both skew scattering and side jump decrease monotonically with increasing $\mathrm{C}{\mathrm{o}}_2\mathrm{MnSi}$ thickness, but show maximum values for the multilayer annealed around 573 K with increasing annealing temperature. It is found that skew scattering $\left({\rho }_{\mathrm{ss}}\right)$ and side-jump scattering $\left({\rho }_{\mathrm{sj}}\right)$ exhibit opposite contributions to the AHE; the skew scattering ${\rho }_{\mathrm{ss}}$ is larger than the value of side jump $|{\rho }_{\mathrm{sj}}|$, giving a ${\rho }_{\mathrm{ss}}$ value of approximately 1.34 μΩ cm and the absolute $|{\rho }_{\mathrm{sj}}|$ value of 1.1 μΩ cm at 5 K in $[\mathrm{C}{\mathrm{o}}_2{\mathrm{MnSi}\left(1.2\mathrm{nm}\right)/\mathrm{Pd}]}_3$ multilayer, indicating that the overall AHE originates mostly from the skew scattering.

We employ first-principles density functional theory to investigate the electronic and structural properties of grain boundaries (GBs) in InAs. In particular, we study the energetics and passivation mechanisms of representative low-$\mathrm{\Sigma }$ GBs, including $\mathrm{\Sigma }3\left(111\right),\mathrm{\Sigma }3\left(112\right),\mathrm{\Sigma }5\left(120\right)$, and $\mathrm{\Sigma }5\left(130\right)$, to establish their relative stability and experimental feasibility. We find that the symmetric-tilt twin-boundary $\mathrm{\Sigma }3\left(111\right)$ GB is the most stable GB, in excellent agreement with our experimentally characterized GB structures in InAs. In addition to our theoretically predicted GB structures, we systematically study and analyze different configurations of complex multifold experimentally observed InAs GB structures. We discuss the effect of different passivations and doping mechanisms on the electronic properties of the GBs. Understanding the exact nature of the GB electronic structure and stability, as well as their passivation mechanisms is a key step for the further development of InAs based optoelectronic devices on silicon and other heterogeneous large-area substrates.

We study spatial photoluminescence characteristics of neutral and charged excitons across extended monolayer ${\mathrm{MoS}}_2$ synthesized by chemical vapor deposition. Using two-dimensional hyperspectral photoluminescence mapping at cryogenic temperatures we identify regions with increased emission from charged excitons associated with both spin-orbit split valence subbands. Such regions are attributed to unintentional doping at defect sites with excess charge that bind neutral excitons to form defect-pinned trions. Our findings imply comparable timescales for the formation, relaxation, and radiative decay of $B$ trions, and add defect-localized $A$ and $B$ trions to the realm of photoexcited quasiparticles in layered semiconductors.

We study magnetic and transport properties of an equiatomic quaternary CoFeVSi Heusler alloy. By using a molecular beam epitaxy technique at $400{}^{\circ }\mathrm{C}$, CoFeVSi epitaxial films with stoichiometry on MgO substrate have been grown. The saturation magnetic moment at 10 K and Curie temperature are $\sim 2.1{\mu }_{\mathrm{B}}$/f.u. and $\sim 260$ K, respectively. From electrical and magnetotransport measurements, we find the small temperature dependence of the electrical resistivity, temperature-dependent ambipolar carrier transport, and positive linear magnetoresistance at low temperatures. An anomalous Hall conductivity of $\sim 67$ S/cm at 10 K is slightly deviated from the calculated value from a Berry phase approach. On the basis of the experimental data and first-principles calculations, the origin of the peculiar magnetotransport properties of the CoFeVSi film is discussed.

We report powder neutron diffraction studies in $\mathrm{L}{\mathrm{a}}_{0.95}\mathrm{P}{\mathrm{O}}_4:\mathrm{E}{\mathrm{u}}_{0.05}\left(\mathrm{LaP}{\mathrm{O}}_4:\mathrm{Eu}\right)$ at various temperatures up to 1163 K. From Rietveld refinement of the diffraction data, we identify the presence of water up to 567 K and an irreversible phase transition from hexagonal to monoclinic structure around 900 K involving the coexistence of phases over 700 to 1050 K. We observe from the neutron experiments that the thermal expansion in the hexagonal phase is much smaller than that in the monoclinic phase. We have performed first-principles lattice dynamics calculations in both the phases to identify the anharmonic phonons and thereby understand the thermal expansion behavior. The low-energy translational and librational modes of $\mathrm{P}{\mathrm{O}}_4$ tetrahedral units in the hexagonal phase show negative thermal expansion behavior, which is responsible for the overall smaller positive thermal expansion coefficient in the hexagonal phase in comparison to that in the monoclinic phase.

For two-dimensional superconductors, the high stability in ambient conditions is critical for experiments and applications. Few-layer antimonene can be nondegradative over a couple of months, which is superior to the akin black phosphorus. Based on the anisotropic Migdal-Eliashberg theory and maximally-localized Wannier functions, this work predicts that electron-doping and Ca-intercalation can transform $\beta$-Sb bilayer from a semimetal to a superconductor. However, the stability of antimonene bilayer in air trends to be decreased due to the electron doping. To overcome this drawback, two kinds of protective layers (graphene and $h$-BN) are proposed to enhance the stability. Interestingly, the superconducting transition temperature will also be enhanced to $9.6\mathrm{K}$, making it a promising candidate as nanoscale superconductor.

The stability of $\text{(}{\text{Cr}}_x\text{,}{\text{V}}_{1-x}{\text{)}}_{\text{2}}\text{AlC}$ MAX phases, materials of interest for a variety of magnetic as well as high-temperature applications, has been studied using density-functional-theory first-principles calculations. The enthalpy of mixing predicts these alloys to be unstable towards unmixing at 0 K. The calculations also predict, however, that these phases would be thermally stabilized by configurational entropy at temperatures well below the values used for synthesis. The temperature $T_s$ below which they become unstable is found to be quite sensitive to the presence of magnetic moments on Cr ions, as well as to the material's magnetic order, in addition to chemical order and composition. Allowing for magnetism, the value of $T_s$ for $\text{(}{\text{Cr}}_{\text{0.5}}\text{,}{\text{V}}_{\text{0.5}}{\text{)}}_{\text{2}}\text{AlC}$ with chemically disordered Cr and V atoms is estimated to be between 516 and 645 K depending on the level of theory, while if constrained to spin paired, $T_s$ drops to $\sim 142$ K. Antiferromagnetic spin arrangements are found to be favored. The combination of antiferromagnetic frustration and configurational disorder should give rise to interesting spin textures at low temperatures.

The broadband dielectric response of the uniaxial ferroelectric strontium barium niobate with 35% Sr has been studied from 1 kHz to 30 THz along the polar $c$ axis using high-frequency and low-frequency dielectric spectroscopies, time-domain terahertz spectroscopy, and far-infrared reflectivity, in the wide temperature interval 20–600 K. The mechanisms that contribute to the ferroelectric phase transition and to the dielectric anomaly in this material are the softening of an anharmonic excitation in the THz range related to cation hopping and the slowing down of a relaxation in the GHz range, which saturates below $T_{\mathrm{C}}$ near 1 MHz. In the ferroelectric phase, a relaxation, related to domain-wall dynamics, appears in the sub-GHz range and hardens on cooling up to 10 GHz. The ferroelectric-paraelectric transition, investigated by electron diffraction, has been assessed to the appearance of a supplementary mirror plane perpendicular to the polar axis.

Stable structures of metallic antiperovskites $A{\mathrm{CNi}}_3(A=\mathrm{Mg},\text{Zn},\text{and Cd})$ under epitaxial strain are explored using first-principles lattice dynamics calculations. Although the ground states of $A{\mathrm{CNi}}_3$ are cubic nonpolar phases without structural distortion, transitions to polar phases are predicted to arise in these compounds under compressive strain while maintaining metallic states. In particular, the polar phase of ${\mathrm{MgCNi}}_3$ would be attainable with moderate strain of a few percent. The polar distortions are considered to be generated by the enhancement of the hybridization between C $2p$ and Ni $3d$ states, which is a mechanism analogous to that of the previously discovered polar metal ${\mathrm{CeSiPt}}_3$.

Metallic nanoclusters are functional materials whose unique physical and chemical properties are sensitively controlled by their shapes and structures. An in-depth understanding of their morphological stability is therefore of crucial importance. It has been well documented by transmission electron microscopy (TEM) studies that metallic nanoclusters can interconvert between different isomers. However, the relevant mechanisms remain elusive because the timescales of such shape fluctuations are too short to be resolved experimentally and yet too long for conventional atomistic simulations. By employing massively parallel accelerated molecular dynamics (AMD) simulations reaching timescales of milliseconds, we provide a clear description of the dynamical processes leading to shape fluctuation in metallic nanoclusters of platinum, silver, copper, and gold at different sizes and temperatures. In all these materials, direct transformations between face-centered-cubic and fivefold symmetric structures (decahedron or icosahedron) are observed away from the melting point. These transitions occur following either a slip-mediated twinning mechanism or a surface-reconstruction driven process. The identified pathways are shown to be reversible, allowing for genuine shape fluctuation processes that do not involve melting or other external factors.

Valley photonic crystals can enable nontrivial topology locked to the valley index. Since time-reversal symmetry is preserved, no external magnetic field has to be applied. Therefore, the valley topological insulators can be easily used to construct photonic devices. Here, a reconfigurable valley photonic crystal is proposed, where the inversion symmetry is broken by constructing the unit cell with two different materials. Also, since one of the materials is electro-optic material barium titanate, by simply changing the applied voltage, not only the band-gap frequency will shift, but the topological phase transition can also happen. This is highly beneficial to the design and optimization of integrated photonic devices because a balance between the flexibility of devices and the robustness of topological properties is realized successfully. Moreover, a two-port optical switch as an example of the possible applications is proposed.

Perovskite structure is one of the five symmetry families suitable for exhibiting topological insulator phase. However, none of the halides and oxides stabilizing in this structure exhibit the same. Through density functional calculations on cubic perovskites (${\mathrm{CsSnX}}_3$; X = Cl, Br, and I), we predict a band insulator–Dirac semimetal–topological insulator phase transition with uniform compression. With the aid of a Slater-Koster tight binding Hamiltonian, we show that, apart from the valence electron count, the band topology of these perovksites is determined by five parameters involving electron hopping among the Sn-{$s,p$} orbitals. These parameters monotonically increase with pressure to gradually transform the positive band gap to a negative one and thereby enable the quantum phase transition. The universality of the mechanism of phase transition is established by examining the band topology of Bi based oxide perovskites. Dynamical stability of the halides against pressure strengthens the experimental relevance.