Here, we employ a combination of advanced synchrotron-based scattering characterization techniques to understand and unravel the atomic origins of the colossal stored energy release in neutron-irradiated silicon carbide. The quantification of the neutron irradiation-induced defects and their impact on the structure-property relationship are important for the design and safe operation of advanced fission and fusion reactors. Our experimental results show that the atomic structure in the as-irradiated samples is significantly perturbed due to a large fraction of vacancy- and interstitial-type defects that lead to complex microstructures and additional components in the x-ray diffraction and pair distribution function (PDF) results. We directly correlate the stored energy release to the recovery of the sublattices with PDF analysis, highlighting that the carbon interstitial- and vacancy-type defects contribute to stored energy more than those of silicon. We find these results to be striking and believe our discoveries to be timely and noteworthy given the technological importance of silicon carbide to the nuclear fission and fusion communities.

We investigate the native point defects in delafossite $\mathrm{Cu}M{\mathrm{O}}_2$ ($M=\mathrm{Al}$, Ga, In) using first-principles calculations based on the Heyd-Scuseria-Ernzerhof (HSE06) hybrid functional approach. The Cu vacancies in all the systems show low formation energies and form relatively shallow acceptor levels, which would contribute mainly to the $p$-type conductivity. The hole compensation by the donor-type native defects does not essentially limit the $p$-type doping in all of $\mathrm{Cu}M{\mathrm{O}}_2$ under controlled growth conditions. In contrast, the acceptor-type native defects, especially the Cu vacancies, show low or even negative formation energies at high Fermi level positions in $\mathrm{Cu}\mathrm{Al}{\mathrm{O}}_2$ and $\mathrm{Cu}\mathrm{Ga}{\mathrm{O}}_2$, thereby compensating carrier electrons to limit the $n$-type doping. The neutral Cu vacancy forms an in-gap state with hole localization to the neighboring Cu atoms in each of $\mathrm{Cu}M{\mathrm{O}}_2$, whereas the neutral Cu-on-Al antisite in $\mathrm{Cu}\mathrm{Al}{\mathrm{O}}_2$ and the Cu-on-Ga antisite in $\mathrm{Cu}\mathrm{Ga}{\mathrm{O}}_2$ form in-gap states with hole localization to themselves. In the framework of the HSE06 hybrid functional, the generalized Koopmans' theorem is almost satisfied for the Cu-on-Al and Cu-on-Ga antisites, but not for the Cu vacancies in all of $\mathrm{Cu}M{\mathrm{O}}_2$. However, the absolute positions of the acceptor levels of the Cu vacancies are almost constant regardless of the convex/concave behavior of the hybrid functional controlled by the Fock-exchange parameter, suggesting that the determination of the valence band maximum is mostly relevant to accurate prediction of the acceptor level position. The $n$-type doping limits, namely the upper limits of the Fermi level in thermodynamic equilibrium, determined by the spontaneous formation of the Cu vacancies, are almost common to all of $\mathrm{Cu}M{\mathrm{O}}_2$ in the band alignment with respect to the vacuum level. In contrast, the conduction band minimum significantly depends on the system, which suggests, along with the Fermi level restriction by the Cu-vacancy formation, that strong compensation of carrier electrons is avoidable only in $\mathrm{Cu}\mathrm{In}{\mathrm{O}}_2$. This finding indicates that the position of the conduction band minimum is an important indicator for discussing and designing $n$-type doping of $\mathrm{Cu}M{\mathrm{O}}_2$ as proposed previously.

We demonstrate that the interfacial dipole associated with bonding across the $\mathrm{SrTi}{\mathrm{O}}_3/\mathrm{Si}$ heterojunction can be tuned through space charge, thereby enabling the band alignment to be altered via doping. Oxygen impurities in Si act as donors that create space charge by transferring electrons across the interface into $\mathrm{SrTi}{\mathrm{O}}_3$. The space charge induces an electric field that modifies the interfacial dipole, thereby tuning the band alignment from type II to III. The transferred charge, accompanying built-in electric fields, and change in band alignment are manifested in electrical transport and hard x-ray photoelectron spectroscopy measurements. Ab initio models reveal the interplay between polarization and band offsets. We find that band offsets can be tuned by modulating the density of space charge across the interface. Modulating the interface dipole to enable electrostatic altering of band alignment opens additional pathways to realize functional behavior in semiconducting hybrid heterojunctions.

Hybridization of electronic states and orbital symmetry in transition metal oxides are generally considered key ingredients in the description of both their electronic and magnetic properties. In the prototypical case of ${\mathrm{La}}_{0.65}{\mathrm{Sr}}_{0.35}{\mathrm{MnO}}_3$ (LSMO), a landmark system for spintronics applications, a description based solely on Mn $3d$ and O $2p$ electronic states is reductive. We thus analyzed elemental and orbital distributions in the LSMO valence band through a comparison between density functional theory calculations and experimental photoelectron spectra in a photon energy range from soft to hard x rays. We reveal a number of hidden contributions, arising specifically from La $5p$, Mn $4s$, and O $2s$ orbitals, considered negligible in previous analyses; our results demonstrate that all these contributions are significant for a correct description of the valence band of LSMO and of transition metal oxides in general.

We report electrical transport properties and electronic structure of a nonmagnetic metal ${\mathrm{Cu}}_2\mathrm{Sb}$ single crystal. ${\mathrm{Cu}}_2\mathrm{Sb}$ was found to be a compensated metal with high carrier density $\sim {10}^{22}\mathrm{c}{\mathrm{m}}^{-3}$ and high carrier mobility $\ge {10}^3\mathrm{c}{\mathrm{m}}^2/\mathrm{Vs}$ for both electron and hole carriers. The current-in-plane magnetoresistance at 2 K and 9 T was 730%, while the current-perpendicular-to-plane magnetoresistance at 2 K and 9 T was 2700% without the saturation. Angle-resolved photoemission spectroscopy throughout the three-dimensional (3D) bulk Brillouin zone signified a quasi-two-dimensional (2D) electron pocket axially centered along the M-A line and a 3D hole pocket at the Γ point, in accordance with the electron-hole compensated nature. The presence of quasi-2D open Fermi surface, in line with the first-principles band-structure calculations, is likely responsible for the observed nonsaturating current-in-plane magnetoresistance. The present result lays the foundation for realizing large magnetoresistance via Fermiology engineering in compensated metals.

As the characteristic dimensions of perovskite devices shrink to the nanoscale, operating voltages of a few volts lead to huge field strengths and, consequently, to the possibility of field-enhanced ion mobility. In this paper, the electrochemical mobility of $X$ anions ($u_X$) along $\langle 100\rangle$ in the $AB{X}_3$ perovskite structure was investigated as a function of electric field strength $E$ and temperature $T$ by means of classical molecular dynamics simulations. Two different cases were examined: one representative of inorganic perovskites, oxide-ion mobility ($u_{\text{O}}$) in cubic ${\mathrm{SrTiO}}_3$; and the other representative of hybrid inorganic-organic perovskites, iodide-ion mobility ($u_{\mathrm{I}}$) in cubic ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$. In both cases, isothermal mobilities are, as expected, independent of field at low values ($E<{10}^0$ MV ${\mathrm{cm}}^{-1}$) but become field dependent at higher values. Data obtained for $u_{\text{O}}(E,T)$ can be described quantitatively with an analytical treatment incorporating a modified Haven ratio for a dilute solution. In contrast, $u_{\text{I}}(E,T)$ displays complex behavior. At high fields, the degree of field enhancement is underestimated by the analytical treatment, while in the field-independent regime, the data imply that moderate fields decrease $u_{\mathrm{I}}$. Our study thus demonstrates that for cubic, inorganic $AB{X}_3$ perovskites $u_X(E,T)$ along $\langle 100\rangle$ can now be predicted quantitatively, but for hybrid perovskites substantially more complex models are required.

Magnetoelastic coupling, structural, magnetic, electronic transport, and magnetotransport properties of ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_6$ have been studied by a combination of macroscopic [magnetization, electrical resistivity, and magnetoresistance (MR)] and microscopic temperature- and magnetic-field-dependent x-ray powder diffraction measurements. The itinerant-electron system ${\mathrm{La}}_{0.85}{\mathrm{Ce}}_{0.15}{\mathrm{Fe}}_{12}{\mathrm{B}}_6$ exhibits an antiferromagnetic (AFM) ground state and multiple magnetic transitions, AFM-ferromagnetic (FM) and FM-paramagnetic (PM), triggered by changes in both temperature and magnetic fields. At low temperatures, the field-induced first-order AFM-FM metamagnetic phase transition is discontinuous, manifesting itself by extremely sharp steps in magnetization as well as in MR and is accompanied by large magnetic hysteresis. A remarkably large negative MR of −73% was discovered. In addition, the time evolution of the electrical resistivity displays a colossal spontaneous jump when both the applied magnetic field and temperature are constant. Diffraction data reveal a magnetic-field-induced structural phase transition associated with the AFM-FM and PM-FM transformations. The lattice distortion is driven by magnetoelastic coupling and converts the crystal structure from rhombohedral ($R\overline{3}m$) to monoclinic $\left(C2/m\right)$. The AFM and PM states are related to the rhombohedral structure, whereas the FM order develops in the monoclinic symmetry. A huge volume magnetostriction of ∼0.9% accompanies this symmetry-lowering lattice distortion. Meanwhile, a highly anisotropic thermal expansion involving giant negative thermal expansion with an average volumetric thermal expansion coefficient ${\alpha }_V=-195\times {10}^{-6}{\mathrm{K}}^{-1}$ was observed. The consistency seen in these different experimental data constitutes direct evidence of the strong correlations between charge, magnetic, and crystallographic degrees of freedom in this material.

We present a first-principles investigation of the structural, electronic, and magnetic properties of pyrolusite ($\beta \text{−}\mathrm{Mn}{\mathrm{O}}_2$) using conventional and extended Hubbard-corrected density-functional theory ($\mathrm{DFT}+U$ and $\mathrm{DFT}+U+V$). The onsite $U$ and intersite $V$ Hubbard parameters are computed using linear-response theory in the framework of density-functional perturbation theory. We show that while the inclusion of the onsite $U$ is crucial to describe the localized nature of the $\mathrm{Mn}\left(3d\right)$ states, the intersite $V$ is key to capture accurately the strong hybridization between neighboring $\mathrm{Mn}\left(3d\right)$ and $\mathrm{O}\left(2p\right)$ states. In this framework, we stabilize the simplified collinear antiferromagnetic (AFM) ordering (suggested by the Goodenough-Kanamori rule) that is commonly used as an approximation to the experimentally-observed noncollinear screw-type spiral magnetic ordering. A detailed investigation of the ferromagnetic and of other three collinear AFM spin configurations is also presented. The findings from Hubbard-corrected DFT are discussed using two kinds of Hubbard manifolds—nonorthogonalized and orthogonalized atomic orbitals—showing that special attention must be given to the choice of the Hubbard projectors, with orthogonalized manifolds providing more accurate results than nonorthogonalized ones within $\mathrm{DFT}+U+V$. This paper paves the way for future studies of complex transition-metal compounds containing strongly localized electrons in the presence of pronounced covalent interactions.

The rates of oxygen incorporation into, and oxygen diffusion in, cubic, nominally undoped ${\mathrm{LaAlO}}_3$ were investigated by means of isotope exchange experiments and subsequent time-of-flight secondary ion mass spectrometry analysis. ${}^{18}\mathrm{O}$ diffusion profiles were introduced into single-crystal samples by annealing in isotope-enriched molecular oxygen (${}^{18}\mathrm{O}_2$) at various temperatures in the range $873\text{K}\le T\le 1173\text{K}$ at an oxygen activity of $a{\mathrm{O}}_2=0.2$ or at various oxygen activities $2\times {10}^{-3}\le a{\mathrm{O}}_2\le 1$ at $T=983\mathrm{K}$. All measured isotope profiles consisted of a sharp drop at the surface, attributed to hindered diffusion through a depletion space-charge zone, followed by a shallow decrease, attributed to faster diffusion in a uniform bulk phase. A numerical description of an entire isotope profile with a single solution of the diffusion equation yielded the tracer diffusion coefficient $D^{*}$, the surface exchange coefficient $k^{*}$, and the surface space-charge potential ${\mathrm{\Phi }}_0$. The relatively high activation enthalpy of oxygen diffusion of $\mathrm{\Delta }{H}_{D^{*}}=(0.90\pm 0.06)\mathrm{eV}$ was attributed to oxygen vacancies interacting with cation vacancies. The activation enthalpy for oxygen surface exchange was surprisingly low ($\mathrm{\Delta }{H}_{k^{*}}<3\mathrm{eV}$) in view of the large band gap of ${\mathrm{LaAlO}}_3$ ($\approx 5.6\mathrm{eV}$).

For the application of GaN to power transistors, we theoretically search for the interface between GaN and amorphous ${\mathrm{SiO}}_2$ $(a\text{−}{\mathrm{SiO}}_2)$ that meets the following requirements: (1) both the conduction band minimum and valence band maximum originate from the GaN (straddling band gap) and (2) the formation of states in the band gap of the GaN is minimized. To this end, we first show that an epitaxial silica bilayer on $m$-plane GaN $\left(m\text{−}\mathrm{GaN}\right)$ meets these requirements when the GaN surface is doped with zinc and oxygen. By using this epitaxial layer as a seed, we make a structure model of the $m\text{−}\mathrm{GaN}/a\text{−}{\mathrm{SiO}}_2$ interface with a straddling gap without in-gap states. Our findings show that a key determinant of the nature of $m\text{−}\mathrm{GaN}/a\text{−}{\mathrm{SiO}}_2$ interface band structure is what we call the “contact structure,” a region of just a few atomic layers thickness at the very interface. We propose that the complex problem of designing semiconductor/insulator interfaces can be reduced to the simpler problem of designing ultrathin epitaxial insulators on the semiconductor surface.

We study the stabilities and geometric and electronic properties of hexagonal boron nitride $\left(h\text{−}\mathrm{BN}\right)$ trilayers by using first-principles electronic-structure calculations within the framework of the density functional theory. From the results of total-energy calculations, we reveal the relative stabilities for various stacking sequences of $h$-BN trilayers. We also show that energy-band structures as well as spatial distributions of wave functions at the valence-band maximum (VBM) and the conduction-band minimum (CBM) strongly depend on the stacking sequences of the $h$-BN trilayers. We further investigate the effects of substitutional doping of a carbon atom on the electronic properties of the $h$-BN trilayers. In several stacking sequences of the C-doped $h$-BN trilayers, we find that the C-atom dopant can be spatially separated from the carrier transport layers associated with the VBM or the CBM, suggesting the possibility of realizing conduction channels only weakly disturbed by the C-atom impurity in $h$-BN trilayers. Interestingly, these donor states spatially separated from the CBM state are found to become rather shallow. This theoretical finding of “atomically thin modulation doping” using the $h$-BN layers may open an important way to design future layered electronic device materials.

Iron-based hexaferrites are critical-element-free permanent magnet components of magnetic devices. Of particular interest is electron-doped M-type hexaferrite i.e., ${\mathrm{LaFe}}_{12}{\mathrm{O}}_{19}$ (LaM) in which extra electrons introduced by lanthanum substitution of barium/strontium play a key role in uplifting the magnetocrystalline anisotropy. We investigate the electronic structure of lanthanum hexaferrite using a density functional theory with localized charge density, which reproduces semiconducting behavior and identifies the origin of the very large magnetocrystalline anisotropy. Localized charge transfer from lanthanum to the iron at the crystal's $2a$ site produces a narrow $3d_{z^2}$ valence band strongly locking the magnetization along the $c$ axis. The calculated uniaxial magnetic anisotropy energies from fully self-consistent calculations are nearly double the single-shot values, and agree well with available experiments. The chemical similarity of lanthanum to other rare earths suggests that LaM can host other rare earths possessing nontrivial $4f$ electronic states for, e.g., microwave-optical quantum transduction.

The competition between Ruderman-Kittel-Kasuya-Yosida (RKKY), crystal electric field (CEF), and Kondo energy scales has recently emerged at the heart of complex magnetism in several Ce- or Yb-based intermetallics. Hard axis magnetic order has been observed in a handful of these compounds, independent of the crystal symmetry, size of the ordered moment, or the relative scale of the Kondo and magnetic ordering temperatures. This raises the question of the role of each energy scale in driving the ground state properties. In focusing on a single class of compounds, the rhombohedral R${\mathrm{Rh}}_3{\mathrm{Si}}_7$, we compare the anisotropy and magnetic ground states in members of this series with only RKKY interactions (R = Gd), or RKKY and CEF effects (R = Tb-Tm), with the behavior of the R = Yb compound, where all three energy scales (RKKY, CEF, Kondo) are at play. Moreover, we extend the comparison to two other isostructural Kondo systems ${\mathrm{YbIr}}_3{\mathrm{Si}}_7$ and ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$, where hard axis magnetic order is also observed. The non-Kondo compounds R${\mathrm{Rh}}_3{\mathrm{Si}}_7$ (R = Tb-Tm) lack the complexity of magnetic order along the hard CEF axis, pointing to the dominant role of the Kondo effect in driving this magnetic order. However, the CEF-RKKY competition is still responsible for complex magnetic ground states, and it appears that the electronic and magnetic degrees of freedom are entangled in all magnetic members of this series of compounds.

The discovery of the 1144 phase, e.g., ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$, creates opportunities to build novel intermetallics with alternative stacking of two parent compounds. Here we formalize the idea by defining a class of bulk crystalline solids with $A\text{−}B$ stacking (including 1144 phases and beyond), which is a generalization of heterostructures from few-layer or thin-film semiconductors to bulk intermetallics. Theoretically, four families of phosphides $\mathit{AB}$(${\mathrm{TM})}_4{\mathrm{P}}_4$ (TM=Fe, Ru, Co, Ni) are investigated by first-principles calculations, wherein configurational, vibrational, and electronic degrees of freedom are considered. It predicts a variety of stable 1144 phases (especially Ru- and Fe-phosphides). Stability rules are found and structural/electronic properties are discussed. Experimentally, we synthesize high-purity ${\mathrm{CaKRu}}_4{\mathrm{P}}_4$ as a proof of principle example. The synthetic method is simple and easily applied. Moreover, it alludes to a strategy to explore complex multicomponent compounds, facilitated by a phase diagram coordinated by collective descriptors.

We study the carbon-dimer defect in a hexagonal boron-nitride $\left(h\text{−}\mathrm{BN}\right)$ monolayer using the $GW$ and Bethe-Salpeter many-body perturbation theories within a finite-size cluster approach. While quasiparticle energies converge very slowly with system size due to missing long-range polarization effects, optical excitations converge much faster, with a $1/R^3$ scaling law with respect to cluster average radius. We obtain a luminescence zero-phonon energy of 4.36 eV, including significant 0.13 eV zero-point vibrational energy and 0.15 eV reorganization energy contributions. Interlayer screening decreases further the emission energy by about 0.3 eV. These results bring support to the recent identification of the substitutional carbon dimer as the likely source of the zero-phonon 4.1 eV luminescence line. Finally, the $GW$ quasiparticle energies are extrapolated to the infinite $h$-BN monolayer limit, leading to a predicted defect highest occupied molecular orbital to lowest unoccupied molecular orbital photoemission gap of 7.6 eV. Comparison with the optical gap yields a very large excitonic binding energy of 3 eV for the associated localized Frenkel exciton.

A detailed study of the structural phase transition in ${\mathrm{Eu}}_{1\text{–}x}{\mathrm{La}}_x{\mathrm{Fe}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ mixed crystals as a function of composition is reported. By analyzing a frequency shift of an electronic f-f transition in high-resolution optical spectra of ${\mathrm{Eu}}^{3+}$ ions, we detected a decrease in the phase transition temperature $T_{\mathrm{s}}$ from 87.05 to 12.2 K (upon cooling) and a simultaneous increase in thermal hysteresis $\mathrm{\Delta }{T}_{\mathrm{s}}$ from 0.29 to 4.7 K with increasing $x$ from $x=0$ to $x=0.12$. A rectangular hysteresis loop was observed. The experimental $T_{\mathrm{s}}\left(x\right)$ and $\mathrm{\Delta }{T}_{\mathrm{s}}\left(x\right)$ dependences are described within the developed analytical model utilizing linear decrease in $T_{\mathrm{s}}$ with $x$ and treating the increase in $\mathrm{\Delta }{T}_{\mathrm{s}}$ in terms of the impurity-related decrease in the interaction between some local order parameters. We argue that $R_{1-x}{R}_x^{\prime }{\mathrm{Fe}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ solid solutions, where $R$ and $R$′ are different rare-earth elements, can be used to implement optical storage devices and switches operating at any chosen temperature between 0 and 450 K. It is found that the changes in the composition and, correspondingly, structural phase transition parameters do not affect the magnetic phase transformation. ${\mathrm{Eu}}_{0.88}{\mathrm{La}}_{0.12}{\mathrm{Fe}}_3{\left(\mathrm{B}{\mathrm{O}}_3\right)}_4$ demonstrates the structural phase transition at about 12 K, well below the Néel temperature $T_{\mathrm{N}}=32\mathrm{K}$.

The value of angle-dependent magnetoresistance (MR) synergistically and simultaneously depends on the magnitudes of magnetoresistance (MR) and magnetocrystalline anisotropy energy (MAE). In a magnetic material, the concurrence of gigantic angle-dependent MR and MR signals is rather difficult due to weak spin-lattice coupling and small MAE. Here we report the considerable magnetotransport effect in layered A-type antiferromagnetic (AFM) $\mathrm{NaCr}{\mathrm{Te}}_2$ by realigning the spin configurations. Above 3 (8) T, the antiparallel spins of adjacent layers experience a spin-flip transition to a parallel alignment along the $c$ axis ($ab$ plane). Theoretical calculations reveal that the energy band gap narrows from 0.39 to 0.11 eV, accompanying a transition from semiconductor (high-$R$ state) and half semiconductor (low-$R$ state), respectively. Thus, a gigantic negative MR ratio of −90% is obtained at 10 K. More importantly, the decrement of $R$ along $H\parallel c$ is far quicker than that of $H\parallel ab$ because the MAE of the Ising-like ferromagnetic (FM) state is $1017\mu \mathrm{eV}/{\mathrm{Cr}}^{3+}$ lower than that of XY-like FM. The distinct trends result in the angle-dependent MR ratio of 732% at 10 K. These findings unravel the intrinsic origin of magnetoresistance in $\mathrm{NaCr}{\mathrm{Te}}_2$ and will stimulate us to explore the $H$-sensitive transport property in more AFM materials.

Geometrically frustrated systems populated with large spin-orbit coupled ions are an ideal setting for the exploration of novel exotic states of matter. Here we present an example of iridium on a mixed B-site spinel oxide structure: $\mathrm{Cu}\left[{\mathrm{Ir}}_{1.498\left(2\right)}{\mathrm{Cu}}_{0.502\left(2\right)}\right]{\mathrm{O}}_4$. Synchrotron XRD refinements reveal a face-centered-cubic structure with space group $Fd\overline{3}m$ and mixed Cu-Ir site disorder within the ${\mathrm{B}}_2{\mathrm{O}}_4$ rocksalt substructure. Electrical properties reveal a metallic state within the 50–600-K range with a Kondo effect at $T<50\mathrm{K}$. X-ray absorption spectroscopy (XAS) measurements show a mixed ${\mathrm{Cu}}^{1+/2+}$ and ${\mathrm{Ir}}^{3+/4+}$ charge partitioned picture, which suggests a metallic/band description with reduced on-site Coulomb interactions. Spin-glass-like freezing is seen at $T_{\mathrm{g}}=49\mathrm{K}$, and the hysteresis behavior for $T>T_{\mathrm{g}}$ resembles that of a strongly frustrated magnet. DFT calculations show sizable hybridization between the Cu $3d$ and Ir $5d$ states with an effective mixed ${\mathrm{Ir}}^{3+/4+}$ charge partitioned picture, supporting the electronic and XAS results.