Brownmillerite oxides ($A\mathrm{B}{\mathrm{O}}_{2.5}$) with long-range ordering of oxygen vacancies own a distinct superstructure formed by alternately stacked octahedral $\mathrm{B}{\mathrm{O}}_6$ and tetrahedral $\mathrm{B}{\mathrm{O}}_4$ planes. The one-dimensional oxygen vacancy channels within $\mathrm{B}{\mathrm{O}}_4$ layers usually lead to high ionic conductivity of brownmillerite oxides, demonstrating great application potential in solid-oxide fuel cells, the oxygen separation membrane, and catalyzers. Here, high quality brownmillerite-$\mathrm{SrCo}{\mathrm{O}}_{2.5}$ films have been epitaxially grown on differently oriented substrates by pulsed laser deposition. The anisotropic structural and physical properties of (110)- and (111)-oriented $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ films were systematically investigated. We found, unlike the out-of-plane (001)-oriented $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ films, the $\mathrm{Co}{\mathrm{O}}_6$ and $\mathrm{Co}{\mathrm{O}}_4$ planes would alternately stack along one of the [100] and [010] axes for (110)-oriented films and one of the [100], [010], and [001] axes for (111)-oriented films, forming coexisting crystal domains with different orientations. More importantly, the superstructure of these films could be reversibly tuned by alternately applying an electric field along two orthogonal directions, switching between an ordered and a disordered state. Corresponding to structural anisotropy, strong in-plane electronic anisotropy of the (110)-oriented $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ film was revealed, which was also electrically tunable like the superstructure. This work demonstrates the approaches to modify the ionic conduction channels of brownmillerite oxides, opening avenues towards electrically tunable oxygen separation membrane and catalyzers.

The intermetallic phases $A\mathrm{P}{\mathrm{d}}_3$ $\left(A=\mathrm{Pb},\mathrm{Sn}\right)$ were recently predicted to host an unconventional combination of unique electronic structure features, namely, flat bands near the Fermi energy coexisting with topologically protected surface states at the Γ point. These features each could independently produce alternative electronic states, including electronically or magnetically ordered states coexisting with unconventional edge dominated transport and a significantly large thermopower coexisting with topological characteristics. To investigate these expectations, we report the synthesis, structural/chemical characterization, electrical and thermal transport properties, magnetic torque (up to 45 T), and Fermi surface mapping for single crystals produced using the Czochralski technique. X-ray diffraction and scanning transmission electron microscope measurements establish the absence of defects, while small measured values of the thermopower indicate that the Fermi level is located away from the flat-band region. The electronic properties are further clarified by the topography of the Fermi surfaces, measured through the de Haas–van Alphen effect. We find that the Fermi levels are placed at higher energy values than the original ones resulting from the density functional theory calculations, 54 meV higher for ${\mathrm{PbPd}}_3$ and 68 meV higher for ${\mathrm{SnPd}}_3$. The molten flux method was also used to synthesize ${\mathrm{PbPd}}_3$, yielding nearly identical Fermi surfaces between the specimens grown using different synthesis techniques, indicating the robustness of the Fermi level position. According to the density functional theory calculations, the flat band is mainly formed by the $4d$ bands of Pd. Therefore, we propose monovalent doping on the Pb/Sn site as a viable approach to accessing the flat band while maintaining the unique band structure features of these compounds.

In this work, we revisit the model of PbTe presented by Fradkin et al. [Phys. Rev. Lett. 57, 2967 (1986)]. We show that the low-energy theory of this model corresponds to a (higher-order) topological crystalline insulator in space group $Fm\overline{3}m{1}^{\prime }$, diagnosable by symmetry indicators. We show that the gapless fermions found on antiphase domain walls are the topological boundary modes of the system, due to a nonvanishing mirror Chern number. Furthermore, we show that any symmetric completion of the model must be in this same topological phase. Finally, we comment on the relationship of this model to realistic PbTe, which has recently been predicted to have a phase which realizes same bulk symmetry indicators.

Recent progress in the field of multiferroics led to the discovery of many new materials in which ferroelectricity is induced by cycloidal spiral orders. The direction of the electric polarization is typically constrained by spin anisotropies and magnetic field. Here, we report that the mixed rare-earth manganite, $\mathrm{G}{\mathrm{d}}_{0.5}\mathrm{D}{\mathrm{y}}_{0.5}\mathrm{Mn}{\mathrm{O}}_3$, exhibits a spontaneous electric polarization along a general direction in the crystallographic $ac$ plane, which is suppressed below 10 K but reemerges in an applied magnetic field. Neutron-diffraction measurements show that the polarization direction results from a large tilt of the spiral plane with respect to the crystallographic axes and that the suppression of ferroelectricity is caused by the transformation of a cycloidal spiral into a helical one, a unique property of this rare-earth manganite. The freedom in the orientation of the spiral plane allows for a fine magnetic control of ferroelectricity, i.e., a rotation as well as a strong enhancement of the polarization depending on the magnetic-field direction. We show that this unusual behavior originates from the coupling between the transition-metal and rare-earth magnetic subsystems.

Doping is one of the most common strategies for improving the photocatalytic and solar energy conversion properties of ${\mathrm{TiO}}_2$, hence an accurate theoretical description of the electronic and optical properties of doped ${\mathrm{TiO}}_2$ is of both scientific and practical interest. In this work we use many-body perturbation theory techniques to investigate two typical $n$-type dopants, niobium and hydrogen, in ${\mathrm{TiO}}_2$ rutile. Using the $GW$ approximation to determine band edges and defect energy levels, and the Bethe-Salpeter equation for the calculation of the absorption spectra, we find that the defect energy levels form nondispersive bands lying $\sim 2.2$ eV above the top of the corresponding valence bands $(\sim 0.9$ eV below the conduction bands of the pristine material). The defect states are also responsible for the appearance of low-energy absorption peaks that enhance the solar spectrum absorption of rutile. The spatial distributions of the excitonic wave functions associated with these low-energy excitations are very different for the two dopants, suggesting a larger mobility of photoexcited electrons in Nb-${\mathrm{TiO}}_2$.

Heterogeneous nanosystems offer a robust potential for manipulating various functional material properties, beyond those possible from their individual constituent materials. We demonstrate the formation of a class of materials with a homogeneous lattice but spatially heterogeneous electrical functionality; specifically, we develop epitaxial modulation-doped thin films in which the spatial separation of electronic charge densities is achieved without perturbing the parent crystal's compositional or structural homogeneity. Unlike the previous realizations of modulation doping in crystals, our materials demonstrate periodic layering of spatially segregated, varying electronically donor-doped regions in a single compositionally and structurally homogenous single-crystalline lattice. We demonstrate the formation of “modulation-doped epitaxial crystals” (MoDECs) using alternating layers of doped cadmium oxide, and the ability to spatially confine regions of variable carrier concentration via low potential-energy barriers in a spatially homogeneous, epitaxial crystal with a chemically and structurally homogenous lattice (i.e., no chemical or structural lattice interfaces). The low potential energy that confines electrons within the doped layers coupled with the crystalline nature of the MoDECs and lack of lattice interfaces presents a platform to study the electron thermal boundary resistances at low-energy electronic barriers. We find that the electron interfacial density does not impede thermal conductivity, despite evidence that the doped layers retain their carrier concentrations. Thus, the negligible thermal boundary resistances at the electronic interfaces result in the thermal conductivities of the MoDECs being related to only a series resistance sum of the thermal resistances of each of the individual layers, with no thermal resistances from the electronic boundaries that maintain charge separation. This is in stark contrast with other nanoscale multilayer materials, where thermal boundary resistances at the internal material interfaces reduce the thermal conductivity of the multilayer compared to that of the parent materials. The ability to modulation dope epitaxially grown films with no structural heterogeneity in the lattice will further enable unique platforms for mid-IR photonics, such as hyperbolic metamaterials, optical filters with spatially discrete optical absorption, or energy harvesting based on charge injection across modulation-doped interfaces.

Scanning probe experiments such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) on strongly correlated electronic systems often reveal complex pattern formation on multiple length scales. By studying the universal scaling in these images, we have shown in several distinct correlated electronic systems that the pattern formation is driven by proximity to a disorder-driven critical point, revealing a unification of the pattern formation in these materials. As an alternative approach to this image classification problem of novel materials, here we report an investigation of the machine learning method to determine which underlying physical model is driving pattern formation in a system. Using a neural network architecture, we are able to achieve $97\%$ accuracy on classifying configuration images from three models with Ising symmetry. This investigation also demonstrates that machine learning can capture the implicit universal behavior of a physical system. This broadens our understanding of what machine learning can do, and we expect more synergy between machine learning and condensed matter physics in the future.

The morphotropic phase boundary (MPB) has recently attracted considerable interest in a number of pseudobinary $R{T}_2$ (R is rare earth; T is a 3d-transition metal) Laves phase compounds for achieving enhanced large magnetostriction or near zero magnetostriction. Unlike the MPBs separating a tetragonal and a rhombohedral phase in the literature, here we report that in the $\mathrm{DyC}{\mathrm{o}}_2\text{−}\mathrm{DyF}{\mathrm{e}}_2$ system, the ferromagnetic MPB separating two tetragonal phases of different tetragonality can also lead to a sign-changed-magnetostriction effect. Synchrotron x-ray diffraction data demonstrate the coexistence of two tetragonal phases in the broadening MPB regime, one with $c/a<1$ (T1 phase) and the other with $c/a>1$ (T2 phase). The preferable domain switches of the T1 phase with weaker magnetocrystalline anisotropy under low fields and the subsequent domain switches of the T2 phase with stronger magnetocrystalline anisotropy under high fields give rise to the sign-changed-magnetostriction effect. Further analysis suggests that the MPB effect on the magnetostrictive response is determined by the intrinsic magnetostriction, the magnetocrystalline anisotropy, as well as the magnetic ordering of two end terminals that form the MPB. This work reveals another type of MPB in pseudobinary $R{T}_2$ compounds and provides an effective recipe for designing sign-tunable magnetostrictive materials.

We performed a detailed microscopic analysis of the interlayer magnetic couplings for bilayer ${\mathrm{CrI}}_3$. As a first step toward understanding recent experimental observations and utilizing them for device applications, we estimated the magnetic force response as well as total energy. Various van der Waals functionals unequivocally point to the ferromagnetic ground state for the low-temperature structured bilayer ${\mathrm{CrI}}_3$ which is further confirmed independently by magnetic force response calculations. The calculated orbital-dependent magnetic forces clearly show that the $e_g-t_{2g}$ interaction is the key to stabilize this ferromagnetic order. By suppressing this ferromagnetic interaction and enhancing antiferromagnetic orbital channels of $e_g-e_g$ and $t_{2g}-t_{2g}$, one can realize the desirable antiferromagnetic order. We showed that high-temperature monoclinic stacking can be the case. Our results provide unique information and insight into understanding the magnetism of multilayer ${\mathrm{CrI}}_3$, paving the way to utilize it for applications.

Nickel-based superalloys are widely used in applications requiring high strength and creep and fatigue resistance at elevated temperatures. Such structural properties are controlled by the glide and cross-slip of screw dislocations in the Ni matrix and ${\mathrm{Ni}}_3\mathrm{Al}$ precipitates. The strengthening mechanisms are determined in turn by screw dislocation core structures that are difficult to image with weak-beam transmission electron microscopy. Core structures of two primary superalloy deformation modes, $1/2\langle 110\rangle \mathrm{Ni}$ screw and $\langle 110\rangle$ ${\mathrm{Ni}}_3\mathrm{Al}$ screw superdislocation, are predicted using density functional theory with flexible boundary conditions.

In the context of novel solid electrolytes for solid-state batteries, first-principles calculations are becoming increasingly more popular due to their ability to reproduce and predict accurately the energy, structural, and dynamical properties of fast-ion conductors. To accelerate the discovery of new superionic conductors is convenient to establish meaningful relations between ionic transport and simple materials descriptors. Recently, several experimental studies on lithium fast-ion conductors suggested a correlation between lattice softness and enhanced ionic conductivity due to a concomitant decrease in the activation energy for ion migration $E_a$. In this article, we employ extensive ab initio molecular dynamics simulations based on density functional theory to substantiate the links between ionic transport and lattice dynamics in a number of structurally and chemically distinct lithium superionic conductors. Our first-principles results show no evidence for a direct and general correlation between $E_a$, or the hopping attempt frequency ${\nu }_0$, and lattice softness. However, we find that, in agreement with recent observations, the pre-exponential factor of lithium diffusivity $D_0$, which is proportional to ${\nu }_0$, follows the Meyer-Neldel rule $\propto exp\left(E_a/\langle \omega \rangle \right)$ where $\langle \omega \rangle$ represents an average phonon frequency. Hence, lattice softness can be identified with enhanced lithium diffusivity, but only within families of superionic materials presenting very similar migration activation energies due to an increase in $D_0$ (or, equivalently, in ${\nu }_0$). On the technical side, we show that neglecting temperature effects in the estimation of $E_a$ may lead to huge inaccuracies of $\sim 10\%$. The limitations of zero-temperature harmonic approaches in describing the vibrational properties of lithium-ion conductors are also illustrated.

Glassy carbon (GC) distinguishes itself from other carbon materials by its unique atomic structure and properties. Cold-compressed GC gives rise to new physical properties; however, the atomistic mechanism for the transitions remains elusive. In this study, by combining in situ high-pressure x-ray diffraction with first-principles calculations, we observe pressure-induced disappearance of the initial intermediate range order of GC, followed by formation of local tetrahedral structural domains and ${sp}^3$ bonds. Correspondingly, the resistance of GC increases by four orders of magnitude during compression from $\sim 20$ to $\sim 61\mathrm{GPa}$. Both the structural and resistance transitions are partially reversible upon decompression, with noticeable hysteresis. Our results highlight the central role of layer distortions in inducing the ${sp}^2$-to-${sp}^3$ bonding transition and provide the structural underpining for the various transitions observed in cold-compressed glassy carbon.

${\mathrm{GdCo}}_5$ may be considered as two sublattices—one of Gd and one of Co—whose magnetizations are in antiparallel alignment, forming a ferrimagnet. Substitution of nickel in the cobalt sublattice of ${\mathrm{GdCo}}_5$ has been investigated to gain insight into how the magnetic properties of this prototype rare-earth/transition-metal magnet are affected by changes in the transition-metal sublattice. Polycrystalline samples of ${\mathrm{GdCo}}_{5-x}{\mathrm{Ni}}_x$ for $0\le x\le 5$ were synthesized by arc melting. Structural characterization was carried out by powder x-ray diffraction and optical and scanning electron microscope imagings of metallographic slides, the latter revealing a low concentration of ${\mathrm{Gd}}_2{(\mathrm{Co},\mathrm{Ni})}_7$ lamellae for $x\le 2.5$. Compensation—i.e., the cancellation of the opposing Gd and transition-metal moments—is observed for $1<x<3$ at a temperature which increases with Ni content; for larger $x$, no compensation is observed below 360 K. A peak in the coercivity is seen at $x\approx 1$ at 10 K coinciding with a minimum in the saturation magnetization. Density-functional theory calculations within the disordered local moment picture reproduce the dependence of the magnetization on Ni content and temperature. The calculations also show a peak in the magnetocrystalline anisotropy at similar Ni concentrations to the experimentally observed coercivity maximum.

We report the computational investigation of a series of ternary $X_4{Y}_{2}Z$ and $X_5{Y}_2{Z}_2$ compounds with $X$=Mg, Ca, Sr, Ba; $Y$=P, As, Sb, Bi; and $Z$=S, Se, Te. The compositions for these materials were predicted through a search guided by machine learning, while the structures were resolved using the minima hopping crystal structure prediction method. Based on ab initio calculations, we predict that many of these phases are thermodynamically stable. In particular, 21 of the $X_4{Y}_{2}Z$ compounds crystallize in a tetragonal structure with $I\overline{4}2d$ symmetry, and exhibit band gaps in the range of 0.8 and 1.8 eV, well suited for various energy applications. We show that several candidates (in particular $X_4{Y}_2\mathrm{Te}$ and $X_4{\mathrm{Sb}}_2\mathrm{Se}$) exhibit good photo absorption in the visible range, while others (e.g., ${\mathrm{Ba}}_4{\mathrm{Sb}}_2\mathrm{Se}$) show excellent thermoelectric performance due to high power factors and extremely low lattice thermal conductivities.

We present the electron-phonon averaged via Gaussian process regression (EPA-GPR) method, in which the electron-phonon coupling matrix is represented as a function of two energies and is in turn modeled as a Gaussian process. The EPA-GPR method can be used as an efficient method to estimate thermoelectric properties of materials for fast-screening applications, comparable to the original electron-phonon averaged (EPA) method and the electron-phonon averaged via moving-least-squares (EPA-MLS) method. The EPA-GPR method does not require specification of any open parameter, unlike the other EPA-related methods, since all the hyperparameters in the model can be unambiguously estimated within the type II maximum likelihood (ML-II) approximation. Thus, the EPA-GPR method is a parameter-free estimation method. Additionally, the concept of Gaussian processes in the EPA-GPR method allows us to quantify the uncertainty in estimated properties of thermoelectric materials. One can randomly realize the electron-phonon coupling coefficients from the identified Gaussian process, and those realized samples can be further analyzed in the solution process of the semiclassical Boltzmann transport equation for charge carriers. The results of the semiclassical Boltzmann transport equation provide the statistical properties of the thermoelectric properties of interest. The means, standard deviations, histograms, and confidence intervals of the Seebeck coefficient, the electrical conductivity, and the power factor can be constructed and analyzed. The proposed EPA-GPR method is applied to a $p$-type half-Heusler compound, i.e., HfCoSb, as a case example, the results of which clearly present the advantages of the method.

Metal monochalcogenide compounds offer a large variety of electronic properties depending on chemical composition, number of layers, and stacking order. Among them, the InSe has attracted much attention due to the promise of outstanding electronic properties, attractive quantum physics, and high photoresponse. Precise experimental determination of the electronic structure of InSe is sorely needed for better understanding of potential properties and device applications. Here, combining scanning tunneling spectroscopy (STS) and two-photon photoemission spectroscopy, we demonstrate that InSe exhibits a direct band gap of about 1.25 eV located at the Γ point of the Brillouin zone. STS measurements underline the presence of a finite and almost constant density of states (DOS) near the conduction-band minimum. This particular DOS is generated by a poorly dispersive nature of the top valence band, as shown by angle-resolved photoemission spectroscopy (ARPES) investigation. In fact, a hole effective mass of about $m^{*}/m_0=-0.95$ ($\overline{\mathrm{\Gamma }K}$ direction) was measured. Moreover, using ARPES measurements a spin-orbit splitting of the deeper-lying bands of about 0.35 eV was evidenced. These findings allow a deeper understanding of the InSe electronic properties underlying the potential of III-VI semiconductors for electronic and photonic technologies.

Co-substituted ${\mathrm{Ni}}_{1-x}{\mathrm{Co}}_x{\mathrm{Br}}_2$ ($0\le x\le 1$) single crystals were synthesized using vapor transport. The physical properties of the crystals were characterized by x-ray powder diffraction, magnetization, and specific-heat measurements. Room-temperature x-ray powder diffraction data indicate that a change from the ${\mathrm{CdCl}}_2$ structure type to the ${\mathrm{CdI}}_2$ structure type occurs within $0.56<x<0.76$. ${\mathrm{NiBr}}_2$ has a commensurate antiferromagnetic phase below $T_{\mathrm{N}}\approx 46\mathrm{K}$ and an incommensurate magnetic ground state below $T_{\mathrm{IC}}\approx 20\mathrm{K}$. Both magnetic transitions are affected by cobalt substitution, and the incommensurate phase transition is present up to at least $x=0.56$. The evolution of magnetism has been studied as a function of cobalt content and is summarized in the temperature-composition phase diagram.