Defects in halide perovskites play an essential role in determining the efficiency and stability of the optoelectronic devices based on these materials. We present a systematic study of intrinsic point defects in six primary metal halide perovskites, ${\mathrm{MAPbI}}_3, {\mathrm{MAPbBr}}_3, {\mathrm{MAPbCl}}_3, {\mathrm{FAPbI}}_3, {\mathrm{CsPbI}}_3$, and ${\mathrm{MASnI}}_3$ (where MA denotes methylammonium and FA denotes formamidinium), based upon density functional theory calculations. Within a single computational scheme, using the $\text{SCAN}+\text{rVV10}$ functional, we compare the impact of changing anions and cations on the defect formation energies and the charge state transition levels in the six compounds, and identify the physical origins underlying the observed trends. Dominant defects in the lead iodide compounds are the $A^{+}$ cation interstitials ($A=\text{Cs}$, MA, FA), charge-compensated by ${\mathrm{I}}^{-}$ interstitials or lead $\left(2-\right)$ vacancies. In the lead bromide and lead chloride compounds, halide interstitials are most prominent, and for ${\mathrm{MAPbCl}}_3$, the chlorine vacancy also becomes important. These trends can be explained in terms of the changes in electrostatic interactions and chemical bonding upon replacing cations and anions. Defect physics in ${\mathrm{MASnI}}_3$ is strongly dominated by tin $\left(2-\right)$ vacancies, promoted by the easy oxidation of the tin. Intrinsically, all compounds are mildly $p$ doped, except for ${\mathrm{MASnI}}_3$, which is strongly $p$ doped. All acceptor levels created by defects in the six perovskites are shallow. Some defects, halide vacancies and Pb or Sn interstitials in particular, can create deep donor traps. Although such traps might hamper the electronic behavior of ${\mathrm{MAPbCl}}_3$, in bromine- and iodine-based perovskites their equilibrium concentrations are too small to affect the materials' properties.

An elegant approach to overcome the intrinsic limitations of magnetically doped topological insulators is to bring a topological insulator in direct contact with a magnetic material. The aspiration is to realize the quantum anomalous Hall effect at high temperatures where the symmetry-breaking magnetic field is provided by a proximity-induced magnetization at the interface. Hence, a detailed understanding of the interfacial magnetism in such heterostructures is crucial, yet its distinction from structural and magnetic background effects is a rather nontrivial task. Here, we combine several magnetic characterization techniques to investigate the magnetic ordering in $\mathrm{MnTe}/{\mathrm{Bi}}_2{\mathrm{Te}}_3$ heterostructures. A magnetization profile of the layer stack is obtained using depth-sensitive polarized neutron reflectometry. The magnetic constituents are characterized in more detail using element-sensitive magnetic x-ray spectroscopy. Magnetotransport measurements provide additional information about the magnetic transitions. We find that the supposedly antiferromagnetic MnTe layer does not exhibit an x-ray magnetic linear dichroic signal, raising doubt that it is in its antiferromagnetic state. Instead, Mn seems to penetrate into the surface region of the ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ layer. Furthermore, the interface between MnTe and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ is not abrupt, but extending over $\sim 2.2$ nm. These conditions are the likely reason that we do not observe proximity-induced magnetization at the interface. Our findings illustrate the importance of not solely relying on one single technique as proof for proximity-induced magnetism at interfaces. We demonstrate that a holistic, multitechnique approach is essential to gain a more complete picture of the magnetic structure in which the interface is embedded.

The Berry phase is an important concept in solids, correlated to the band topology, axion electrodynamics, and potential applications of topological materials. Here, we investigate the magnetotransport and Berry phase of rare earth element Gd-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ $(\mathrm{Gd}:{\mathrm{Bi}}_2{\mathrm{Se}}_3)$ topological insulators (TIs) at low temperatures and high magnetic fields. $\mathrm{Gd}:{\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystals show Shubnikov–de Haas (SdH) oscillations with nontrivial Berry phase, while ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ single crystals show zero Berry phase in SdH oscillations. A fitting of the temperature-dependent magnetization curves using the Curie-Weiss law reveals that the Gd dopants in the crystals show paramagnetism in the 3–300 K region, indicating that the origin of the Berry phase is not long-range magnetic ordering. Moreover, Gd doping has limited influence on the quantum oscillation parameters (e.g., frequency of oscillation, area of Fermi surface, effective electron mass, and Fermi wave vectors) but has a significant impact on the Hall mobility, carrier density, and band topology. Our results demonstrate that Gd doping can tune the Berry phase of TIs effectively, which may pave the way for the future realization of many predicted exotic transport phenomena of topological origin.

Polycrystalline ${\mathrm{In}}_2{\mathrm{O}}_3$ films, which are solid-phase crystallized (spc) from amorphous films doped with hydrogen (H) and transition metals (TMs), exhibit remarkably high mobilities $(100–160{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1})$; moreover, their specific resistance is equivalent to $2–3\times {10}^{-4}\mathrm{\Omega }\mathrm{cm}$ or less, even when the carrier density is reduced to one-fifth $(1–4\times {10}^{20}{\mathrm{cm}}^{-3})$ that of conventional transparent conductive oxide (TCO) films. The high mobility and low carrier density significantly reduce free-carrier absorption, and the transparent region of the TCO films extends across the visible and near-infrared regions. In this study, we experimentally demonstrate that spc-${\mathrm{In}}_2{\mathrm{O}}_3$:H and ${\mathrm{In}}_2{\mathrm{O}}_3$:TM,H films with an impurity concentration of less than a few at. % exhibit superior mobility compared to that of optimized ${\mathrm{In}}_2{\mathrm{O}}_3$:Sn,H films $(\sim 70{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1})$ with similar Sn and close-carrier concentrations. Photoelectron spectroscopy measurements revealed that the electronic state of the spc films, including the valence of TM impurities, changed with varying In/O ratios of the amorphous films. Furthermore, the In, O, TM, and H compositions in the amorphous films were found to be critical for effective activation of ${\mathrm{TM}}_{\mathrm{In}}^{+}$, ${\mathrm{H}}_{\mathrm{O}}^{+}$, and ${{\mathrm{H}}_{\mathrm{i}}}^{+}$ donors after crystallization. To determine the origin of the high electron mobility, the effective mass and relaxation time of electrons in these films were evaluated by spectroscopic ellipsometry. The results showed that the high mobility could be primarily attributed to the long relaxation time instead of the small effective mass. Additionally, the dispersion of conduction bands near the Fermi energy was found to be almost independent of the type of metallic impurity (Sn, H, Ce, Zr, and W) for the investigated impurity concentrations. Moreover, the increase in relaxation time by H and TM doping was examined.

The promises of spintronics have longed for the realization of fully compensated states and full spin polarization. Half-metallic/half-semiconducting (HM/HS) and fully compensated materials are suggested as candidates. We use density functional theory to report that the experimentally realized rhombohedral ${\mathrm{Cr}}_2{\mathrm{S}}_3$ is a promising candidate as a bipolar magnetic semiconducting (BMS)/HS fully compensated ferrimagnetic (FCFiM) material. Our calculations demonstrate that the uniquely layered structure of ${\mathrm{Cr}}_2{\mathrm{S}}_3$ produces different interlayer and intralayer $d\text{−}p\text{−}d$ hybridization schemes between Cr atoms. Strong interlayer and weak intralayer antiferromagnetic coupling between different Cr sites make the overall magnetic state a fully compensated structure. The origin of BMS/HS and fully compensated state has been explained by a structural analysis, where magnetic exchange interaction between Cr sites is dependent on bond distance and bond angle of each Cr-centered octahedron. Furthermore, by applying strains perpendicular to the basal plane, distortion of Cr octahedron sites and Cr–Cr distance is altered, resulting in the phase transition of the material both electronically and magnetically from BMS-FCFiM to HS-FCFiM to ferrimagnetic (FiM). These studies enable us to rediscover ${\mathrm{Cr}}_2{\mathrm{S}}_3$ as a novel class of BMS materials.

The development of high-strength steels requires detailed understanding of the effect of solute elements on γ-Fe grain boundaries (GBs). In this study, first-principles computational tensile tests (FPCTT) were conducted on γ-Fe GBs to elucidate the mechanism of GB embrittlement due to Zn segregation. The paramagnetic γ-Fe GB was simulated by the $\mathrm{\Sigma }5\left(310\right)$ GB in the antiferromagnetic double-layer (AFMD) configuration. The FPCTTs revealed that the fracture stress and fracture energy of the γ-Fe GB were reduced by Zn segregation, which is consistent with experimental results. Crystal orbital Hamilton population analysis was also performed to investigate the change in electronic states during the tensile process, and the enhancement of GB fracture by Zn segregation is caused by the breaking of the covalent-like bonds between Fe and Zn at a relatively small strain compared to the Fe–Fe bonds. This behavior is attributed to the localized nature of the $3d$ orbitals of Zn in γ-Fe. The FPCTTs of γ-Fe GBs using the AFMD properly consider the effect of magnetism in paramagnetic γ-Fe under tensile strain and is useful for investigating the effects of various solute elements on GB fracture and for the development of high-strength steels.

The stability of $(R,\mathrm{Zr}){(\mathrm{Fe}, \mathrm{Co}, \mathrm{Ti})}_{12}$ with a ${\mathrm{ThMn}}_{12}$ structure is investigated using first-principles calculations. We consider energetic competition with multiple phases that have the ${\mathrm{Th}}_2{\mathrm{Zn}}_{17}$ structure and the unary phases of $R$, Zr, Fe, Co, and Ti simultaneously by constructing a quinary energy convex hull. From the analysis, we list the stable phases at zero temperature, and show possible stable and metastable ${\mathrm{ThMn}}_{12}$ phases.

Transition metal dichalcogenides host a wide variety of lattice and electronic structures, as well as corresponding exotic physical properties, especially under certain tuning conditions. Here, we report the observation of pressure-induced three superconducting transitions in ${\mathrm{TaTe}}_2$, a charge density wave (CDW)–bearing layered transition-metal dichalcogenide that is metallic but not superconducting at ambient pressure. We find that its CDW state can be easily suppressed upon increasing pressure up to ∼1 GPa. A superconducting state then emerges from the suppressed CDW state and persists to the pressure about 7 GPa. Unexpectedly, another superconducting state appears at ∼11 GPa within the same monoclinic (M) structure of its ambient-pressure one. Upon further compression to 21 GPa, a third superconducting state with higher $T_c$ appears from a high-pressure (HP) phase. Our experimental results suggest that the pressure-induced three superconducting transitions in ${\mathrm{TaTe}}_2$ are, respectively, driven by the suppression of the CDW state, the change of the β angle in the M phase and the transition of M-to-HP phase. These results demonstrate not only the versatile nature of this correlated electron system, but also the first experimental example that shows the pressure-induced evolution from a CDW state to three superconducting states driven by different mechanisms.

This paper aims to perform a systematic density functional study on alkali-metal-based half-Heusler compounds, namely $A\mathrm{Cr}Z$ ($A=\mathrm{Li}$, Na, and K; $Z=\mathrm{As}$, Sb, and P), to identify the optimal half-metal (HM) for practical applications. Unlike most HMs proposed so far, the majority of ACrZ compounds in our study exhibit a wide band gap (1.60–2.38 eV) and retain robust half-metallicity even at the surface. Furthermore, the half-metallicity is robust under severe strain, up to 10%. Because of their stability, robust half-metallicity at the surface and under strain, and good lattice mismatch with zinc-blende semiconductors, we propose $\mathrm{LiCr}Z$ and $\mathrm{NaCr}Z$ ($Z=\mathrm{As}$ and Sb) as promising compounds for practical applications to spintronics.

Topological electronic structures are key to the topological classification of quantum materials and play an important role in their physical properties and applications. Recently, SnSe has attracted great research interest due to its superior thermoelectric performance. However, its topological nature has long been ignored. In this work, by combining synchrotron-based angle-resolved photoemission spectroscopy and ab initio calculations, we systematically investigate the topological electronic structure of orthorhombic SnSe. By identifying the continuous gap in the valence bands due to the band inversion and the topological surface states on its (001) surface, we establish SnSe as a strong topological insulator. Furthermore, we study the evolution of the topological electronic structure and propose the topological phase diagram in ${\mathrm{SnSe}}_{1\text{–}x}{\mathrm{Te}}_x$. Our work reveals the topological nontrivial nature of SnSe and provides understandings of its intriguing transport properties.

Ferroelectric oxides based on ${\mathrm{HfO}}_2$ show tremendous promise for the next generation of memory and logic devices. The ferroelectric polymorph is one of several that can be derived from the high symmetry cubic fluorite structure of ${\mathrm{HfO}}_2$. A single grain of ${\mathrm{HfO}}_2$ may consist of a coherent mixture of multiple orientational and translational variants of different polymorphs. Here, we use symmetry-adapted strain-order parameters to elucidate the relationship between the different ${\mathrm{HfO}}_2$ polymorphs and their symmetrically equivalent variants. We use first-principles electronic structure methods to identify minimum energy pathways and map them in subspaces of the symmetry-adapted strain order parameters. We next investigate the atomic structure of domain boundaries that separate coexisting variants of ferroelectric ${\mathrm{HfO}}_2$. We rely on Gibbsian excess quantities and a precise specification of mechanical boundary conditions to describe the thermodynamic properties of domain boundaries. Our first-principles calculations show that the O and Hf shuffle arrangement within a domain boundary is closely related to the intermediate shuffle patterns of the homogeneous pathways between ferroelectric variants. Furthermore, the preferred structure within a boundary is very sensitive to local strain constraints imposed by the adjacent ferroelectric variants, leading to highly anisotropic domain boundary energies.

We present first-principles calculations of the dynamic susceptibility in strained and doped ferromagnetic MnBi using time-dependent density functional theory. In spite of being a metal, MnBi exhibits signatures of strong correlation and a proper description in the framework of density functional theory requires Hubbard corrections to the $\mathrm{Mn}d$ orbitals. To permit calculations of the dynamic susceptibility with Hubbard corrections applied to the ground-state electronic structure, we use a consistent rescaling of the exchange-correlation kernel maintaining the delicate balance between the magnon dispersion and the Stoner continuum. We find excellent agreement with the experimentally observed magnon dispersion for pristine MnBi and show that the material undergoes a phase transition to helical order under application of either doping or strain. The presented methodology paves the way for future linear response time-dependent density functional theory studies of magnetic phase transitions, also for the wide range of materials with pronounced static correlation effects that are not accounted for at the local density approximation level.

Banded spherulitic growth of crystals is observed in some materials with spherically symmetric growth front and periodic radial variation of birefringence. This variation of birefringence in quasi-two-dimensional geometry produces concentric interference color bands when viewed through crossed polarizers. In most materials, the banded spherulites are found to be formed by radially oriented periodically twisted fibrillar crystallites. Here, we report the formation of banded spherulites due to the rhythmic growth of concentric crystallite-rich and crystallite-poor bands for a pure compound consisting of strongly polar rodlike molecules. The compound exhibits coexistence of untwisted fibrillar crystallites and an amorphous phase in its most stable solid state. On sufficient supercooling of the sample from its melting point, the banded spherulites are formed with a periodic variation of composition of untwisted radially aligned fibrillar crystallites and an amorphous solid phase. We have developed a time-dependent Ginzburg-Landau model to account for the observed banded spherulitic growth.

Many nonmetals and metal dioxides $M{\mathrm{O}}_2$, including the dense form of ${\mathrm{SiO}}_2$ stishovite, crystalize in a rutile structure at low pressure and transform to a denser ${\mathrm{CaCl}}_2$ structure under high pressure. Structures and transformations in $M{\mathrm{O}}_2$ dioxides hence serve as an archetype for applications in materials science and inside the Earth and terrestrial planets. Despite its significance, however, the deformation behavior of $M{\mathrm{O}}_2$ compounds in the ${\mathrm{CaCl}}_2$ structure is very poorly constrained. Here we use radial x-ray diffraction in a diamond-anvil cell and study ${\mathrm{MnO}}_2$ as a representative system of the $M{\mathrm{O}}_2$ family. We identify the dominant slip systems and constrain texture evolution in ${\mathrm{CaCl}}_2$-structured phases. After phase transition to a ${\mathrm{CaCl}}_2$ structure above 3.5 GPa, the dominant (010)[100] and secondary {110}[001] and {011}[0-11] slip systems induce a 121 texture in compression. Further compression increases the activity of the $\left\{011\right\}\langle 0\text{−}11\rangle$ slip system, with an enhanced 001 texture at $\sim 50\mathrm{GPa}$. During pressure release, the 001 texture becomes dominant over the original 121 texture. This clearly demonstrates the effect of pressure on the deformation behavior and slip systems of ${\mathrm{CaCl}}_2$-structured dioxides. Finally, ${\mathrm{MnO}}_2$ transforms back to a rutile structure upon pressure release, with a significant orientation memory, highlighting the martensitic nature of the ${\mathrm{CaCl}}_2$ to rutile structural transformation. These findings provide key guidance regarding the plasticity of ${\mathrm{CaCl}}_2$-structured dioxides, with implications in materials and Earth and planetary science.

Application of isotropic pressure or uniaxial strain alters the elastic properties of materials; sufficiently large strains can drive structural transformations. Linear elasticity describes stability against infinitesimal strains, while nonlinear elasticity describes the response to finite deformations. It was previously shown that uniaxial strain along [100] drives refractory metals and alloys towards mechanical instabilities. These include an extensional instability, and a symmetry-breaking orthorhombic distortion caused by a Jahn-Teller-Peierls instability that splays the cubic lattice vectors. Here we analyze these transitions in depth. Eigenvalues and eigenvectors of the Wallace tensor identify and classify linear instabilities in the presence of strain. We show that both instabilities are discontinuous, leading to discrete jumps in the lattice parameters. We provide physical intuition for the instabilities by analyzing the changes in first-principles energy, stress, bond lengths, and angles upon application of strain. Electronic band structure calculations show differential occupation of bonding and antibonding orbitals, driven by the changing bond lengths and leading to the structural transformations. Strain thresholds for these instabilities depend on the valence electron count.

In this work we perform a neutron Bragg edge tomography of stainless steel 316L additive manufacturing samples, one as built via standard laser powder bed fusion, and one using the novel three-dimensional (3D) laser shock peening technique. First, we consider conventional attenuation tomography of the two samples by integrating the signal for neutron wavelengths beyond the last Bragg edge, to analyze the bulk density properties of the material. This is used to map defects, such as porosities or cracks, which yield a lower density. Second, we obtain strain maps for each of the tomography projections by tracking the wavelength of the strongest Bragg edge corresponding to the {111} lattice plane family. Algebraic reconstruction techniques are used to obtain volumetric 3D maps of the strain in the bulk of the samples. It is found that not only the volume of the sample where the shock peening treatment was carried out yields a higher bulk density, but also a deep and remarkable compressive strain region. Finally, the analysis of the Bragg edge heights as a function of the projection angle is used to describe qualitatively crystallographic texture properties of the samples.

Multicaloric effect refers to a thermal response of materials driven by more than one external field. In this work, using a self-designed multicaloric effect characterization setup, we directly measured the adiabatic temperature change ($\mathrm{\Delta }{T}_{\mathrm{ad}}$) under uniaxial stress and magnetic field for a spin-lattice synergic coupled system of Ni-Mn-Ga-In. Both stress and magnetic field favor the martensitic transformation. A large $\mathrm{\Delta }{T}_{\mathrm{ad}}$ of −6.7 K was achieved in the ${\mathrm{Ni}}_{57}{\mathrm{Mn}}_{18}{\mathrm{Ga}}_{21.45}{\mathrm{In}}_{3.55}$ alloy when simultaneously applying a moderate dual-external-field (uniaxial stress of 95 MPa and magnetic field of 2 T). Such a multicaloric $\mathrm{\Delta }{T}_{\mathrm{ad}}$ is 72% higher than the single elastocaloric cooling, which is ascribed to the larger transformation volume fractions obtained under dual fields. Additionally, benefiting from the reduction in critical stress by a bias magnetic field, ${\mathrm{Ni}}_{57}{\mathrm{Mn}}_{18}{\mathrm{Ga}}_{21.45}{\mathrm{In}}_{3.55}$ exhibits a good fatigue resistance with a relatively stable $\mathrm{\Delta }{T}_{\mathrm{ad}}$ of −5.7 K upon dual-field cycles of about 600 times. Our experimental results reveal that the multicaloric strategy based on the synergic magnetostructural transformation is feasible and promising for solid-state cooling under relatively lower driven fields.

Topological superconductors are expected to host non-Abelian Majorana zero mode which is of fundamental importance for topological quantum computing. However, the discovered topological superconductor candidates are still rare. Here, we systematically studied the topological semimetal antimony Sb through first-principles calculations, and found that a moderate pressure could induce the superconductivity coexisting with topological surface states. The superconducting critical transition temperature ($T_c$) reaches $\sim 0.3\mathrm{K}$ under $\sim 8\mathrm{GPa}$ and the topological surface states can be well preserved. The $T_c$ can be much enhanced to $\sim 2\mathrm{K}$ through further electron doping. Elementary substance antimony may provide a simple material platform to detect the Majorana boundary states and develop the topological quantum computation.

With molecular beam epitaxy and a topotactic chemical reaction, we prepared NdFeAs(O,H) epitaxial thin films with various hydrogen concentrations on 5° vicinal-cut MgO substrates. By measuring the resistivities along the longitudinal and transversal directions, the $ab$-plane and the $c$-axis resistivities (${\rho }_{ab}$ and ${\rho }_c$) were obtained. The resistivity anisotropy ${\gamma }_{\rho }\equiv {\rho }_c/{\rho }_{ab}$ of NdFeAs(O,H) with various hydrogen concentrations was compared with that of NdFeAs(O,F). At the H concentrations which led to superconducting transition temperatures $T_{\mathrm{c}}$ over 40 K, ${\gamma }_{\rho }$ recorded $\sim 100–150$ at 50 K. On the other hand, a low ${\gamma }_{\rho }$ value of 9 was observed with the highest-doped sample. The exponent β of the $ab$-plane resistivity obtained by fitting a power-law expression ${\rho }_{ab}\left(T\right)={\rho }_0+A{T}^{\beta }$ to the data was close to unity down to a low temperature in the vicinity where the second antiferromagnetic phase locates, which may be related to the quantum critical point discussed at the overdoped side of the phase diagram.

Sesquioxides are an important class of compounds which are used in various applications such as optical glasses, high-power electronics, and gate dielectrics. Using density functional theory, we investigated bixbyite ${\mathrm{La}}_2{\mathrm{O}}_3, {\mathrm{In}}_2{\mathrm{O}}_3$, and ${\mathrm{Ga}}_2{\mathrm{O}}_3$; hexagonal ${\mathrm{La}}_2{\mathrm{O}}_3$; corundum ${\mathrm{In}}_2{\mathrm{O}}_3$; and monoclinic ${\mathrm{Ga}}_2{\mathrm{O}}_3$. We compared the predicted structural properties for each material using the local density approximation (LDA) and the generalized-gradient approximation [GGA PBE (Perdew-Burke-Ernzerhof)]. We found that LDA predicts the correct ground state structure for all three sesquioxides; however, GGA PBE predicts the incorrect ground state structure for ${\mathrm{La}}_2{\mathrm{O}}_3$. To gain better insight on why this happens, we calculated the phonon properties and the free energy for all structures to determine phase transition temperatures. Additionally, we determined the transition pressure for ${\mathrm{In}}_2{\mathrm{O}}_3$. We also studied several surface terminations for each compound and determined the lowest-energy surface for each structure as well as the interfacial energy for ${\mathrm{La}}_2{\mathrm{O}}_3$.