Dimensionality reduction induced metal-insulator transitions in oxide heterostructures are usually coupled with structural and magnetic phase transitions, which complicate the interpretation of the underlying physics. Therefore, achieving isostructural MIT is of great importance for fundamental physics and even more for applications. Here, we report an isostructural metal-insulator transition driven by dimensional-crossover in spin-orbital coupled ${\mathrm{SrIrO}}_3$ films. By using in situ pulsed laser deposition and angle-resolved photoemission spectroscopy, we synthesized and investigated the electronic structure of ${\mathrm{SrIrO}}_3$ ultrathin films with atomic-layer precision. Through inserting orthorhombic ${\mathrm{CaTiO}}_3$ buffer layers, combined low-energy electron diffraction measurements and first-principles density functional theory calculations show that the crystal structure of ${\mathrm{SrIrO}}_3$ films remains bulklike with similar oxygen octahedra rotation and tilting when approaching the ultrathin limit. We observe that a dimensional-crossover metal-insulator transition occurs in isostructural ${\mathrm{SrIrO}}_3$ films. Intriguingly, we find the bandwidth of $J_{\mathrm{eff}}=3/2$ states reduces with lowering the dimensionality and drives the metal-insulator transition. Our results establish a bandwidth controlled metal-insulator transition in the isostructural ${\mathrm{SrIrO}}_3$ thin films.

The superior piezoelectric (PE) performance of devices based on thin-films of wurtzite (w) w-${\mathrm{Sc}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ alloys has motivated research efforts to determine the material properties of other alloys of aluminum nitride and rare-earth nitride (w-${\mathrm{RE}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$). The longitudinal PE, elastic and dielectric properties of substitutional w-${\mathrm{RE}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ alloys have been calculated with density-functional perturbation theory. The rare-earth elements in the simulations stand for scandium, yttrium and seven lanthanides: lanthanum, neodymium, gadolinium, dysprosium, erbium, ytterbium, and lutetium. The structure of the investigated alloys is modeled with special quasirandom $2\times 2\times 2$ supercells at rare-earth nitride fractions $x$ of 0.0625 and 0.25. The $4f$ electrons of the lanthanides are incorporated in the pseudopotential core, an approach supported by the standard model of lanthanide chemistry. An increase of the PE stress coefficient $e_{33}$ and a softening of the stiffness coefficient $c_{33}^E$ are found for all investigated alloys for $x$ = 0.25, when compared to w-AlN. This leads to significant improvements of the PE strain effective coefficient $d_{33,f}$ of films of these alloys. For w-${\mathrm{Lu}}_{0.25}{\mathrm{Al}}_{0.75}\mathrm{N}$, the $d_{33,f}$ increases by $\sim 100\%$ compared to pure w-AlN. Increased displacement-response internal-strains and Born effective charges are demonstrated for all alloys at $x$ = 0.25, in a manner consistent with the behavior of w-${\mathrm{Sc}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ alloys, suggesting shared intrinsic properties for the entire w-${\mathrm{RE}}_x{\mathrm{Al}}_{1-x}\mathrm{N}$ alloy family. Distortions of the alloy structure, correlated with the ionic radius of the rare earths, negatively affect their PE properties. At a small fraction $x$ = 0.0625, the alloys incorporating Sc, Y, Er, Yb, and Lu possess PE coupling coefficients $k_{33,f}^2$ greater than for w-AlN.

Recently, GaN-on-BAs has been synthesized and demonstrated as a promising architecture for efficient thermal management in GaN based high-power electronic devices with remarkably reduced thermal boundary resistance compared to that of GaN-on-diamond. In this paper, we report studies on ideal strengths and band alignments for polar BAs/GaN heterojunctions and superlattices using first-principles calculations. The results show that under normal compression, all BAs/GaN interface configurations show much higher compressive stiffness compared to that in bulk GaN [0001] direction, with the GaN softening during its structural transformation under compression markedly suppressed, which improves protection of the electronic properties under external impacts. The natural band alignments of the mismatched BAs/GaN heterojunctions are calculated by a three-step approach. Most of the heterojunction and all the superlattice interfaces show type-II staggered band offsets. Large polarization built-in electric fields are predicated in superlattice with repeatedly positive- and negative-charged N-As and Ga-B interfaces, producing a saw-tooth like dipole potential, which can effectively separate electrons and holes to different interfaces, desirable for the photocatalytic processes. Our research shows that BAs/GaN heterojunction can not only provide a much-needed alternative to GaN-on-diamond heat dissipation system in future designing of high-power electronic devices, but also offers possibilities of applications in photovoltaic and photocatalytic devices as a type-II semiconductor heterojunction or superlattice.

We performed an optical spectroscopy study of single crystals of ${\mathrm{Pr}}_{0.85}{\mathrm{LaCe}}_{0.15}{\mathrm{CuO}}_{4-\delta }$ (PLCCO) to revisit the electron-hole asymmetry, which has been understood as a fundamental property of cuprates. Four differently annealed samples—as-grown, reduced, optimally oxygenated, and overoxygenated samples—were prepared, which have superconducting transition temperatures $T_c=0$, 15, 24, and 18 K, respectively. We observed that the low-energy quasiparticle spectral weights of all the PLCCO samples are significantly small in comparison with those of other electron-doped cuprate families. Instead, they are rather close to those of their hole-doped counterpart ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$. Accordingly, estimated effective carrier numbers $N_{\mathrm{eff}}$ per Cu atom of superconducting samples are also very small, despite their relatively high critical temperatures. A complementary photoemission study reveals that the low-energy quasiparticle spectral weight of PLCCO is much smaller than that of ${\mathrm{Nd}}_{1.85}{\mathrm{Ce}}_{0.15}{\mathrm{CuO}}_{4-\delta }$, consistent with the optical results. Our observations demonstrate that PLCCO provides the electron-hole symmetry in the quasiparticle spectral weight and highlight the importance of Cu $3d$–O $2p$ hybridization to understand the low-energy spectral weight transfer in doped cuprates.

The formation of multilayer graphene on a nickel substrate is usually unavoidable due to the precipitation of carbon upon cooling after the growth of the single layer of graphene. We found that the growth rate of graphene on a Ni(110) surface was markedly reduced when the surface was covered by sulfur atoms arranged in a c($2\times 2$) periodicity. An atomistic survey by scanning tunneling microscopy revealed a compression of the sulfur superstructure as a result of the growth of graphene-covered regions. Photoelectron emission microscopy showed that the growth rate of graphene is linear with time, which indicates that carbon atoms are supplied only at the growth front of graphene islands. Density functional theory calculations rationalized these experimental results and suggested a mechanism for suppressing the growth of graphene. Sulfur atoms might extend the utility of nickel substrates for the production of single-layer graphene or nanographenes.

In the strong light-matter coupling regime realized, e.g., by integrating semiconductors into optical microcavities, polaritons as new hybrid light-matter quasiparticles are formed. The corresponding change in the dispersion relation has a large impact on optics, dynamics, and transport behavior of semiconductors. In this paper, we investigate the strong-coupling regime in hBN-encapsulated ${\mathrm{MoSe}}_2$ monolayers focusing on exciton-polariton diffusion. Applying a microscopic approach based on the exciton density matrix formalism combined with the Hopfield approach, we predict a drastic increase of the diffusion coefficients by two to three orders of magnitude in the strong-coupling regime. We explain this behavior by the much larger polariton group velocity and suppressed polariton-phonon scattering channels with respect to the case of bare excitons. Our study contributes to a better microscopic understanding of polariton diffusion in atomically thin semiconductors.

Hybrid structures consisting of a topological insulator brought in proximity to an $s$-wave superconductor are one of the most promising routes to topological superconductivity and Majorana quasiparticles. A key challenge is the superconductor/topological insulator interface, which must be transparent to Cooper pairs while also preserving high-mobility topological states. Here, we investigate Hall bar devices fabricated on the topological insulator-like surfaces of (001) cadmium arsenide films, which we partially cover with different superconductors. We show that, depending on the magnitude of a perpendicular magnetic field, devices with sputter-deposited Nb exhibit superconductivity and the quantum Hall effect from the topological surface states, respectively. By contrast, devices with thermally evaporated Sn do not show superconductivity, indicating poor interface transparency. We discuss the coupling between the superconductor and the surface states.

Selective area growth (SAG) of nanowires and networks promise a route toward scalable electronics, photonics, and quantum devices based on III-V semiconductor materials. The potential of high-mobility SAG nanowires however is not yet fully realised, since interfacial roughness, misfit dislocations at the nanowire/substrate interface and nonuniform composition due to material intermixing all scatter electrons. Here, we explore SAG of highly lattice-mismatched InAs nanowires on insulating GaAs(001) substrates and address these key challenges. Atomically smooth nanowire/substrate interfaces are achieved with the use of atomic hydrogen (a-H) as an alternative to conventional thermal annealing for the native oxide removal. The problem of high lattice mismatch is addressed through an ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ buffer layer introduced between the InAs transport channel and the GaAs substrate. The Ga-In material intermixing observed in both the buffer layer and the channel is inhibited via careful tuning of the growth temperature. Performing scanning transmission electron microscopy and x-ray diffraction analysis along with low-temperature transport measurements we show that optimized In-rich buffer layers promote high-quality InAs transport channels with the field-effect electron mobility over 10 000 ${\mathrm{cm}}^2$ ${\mathrm{V}}^{-1}$ ${\mathrm{s}}^{-1}$. This is twice as high as for nonoptimized samples and among the highest reported for InAs selective area grown nanostructures.

We report spectroscopic measurements of optical transmission, ellipsometry, and second-harmonic generation (SHG) from as-grown vapor-deposited $\alpha {\text{-In}}_2{\text{Se}}_3$ nanoflakes ranging in thickness from a single quintuple layer (QL) to bulk material. We compare these measurements with ab initio calculations of structural and optical properties. Linear optical measurements yield thickness-dependent band gaps and dielectric functions, while SHG diagnoses microscopic film structure and ferroelectric polarization. The rotational anisotropy of SHG for $s$-polarized incident photons of energy $1.4\mathrm{e}\mathrm{V}$ reveals crystalline symmetry and orientation of individual QLs. Peak SHG intensity increases from one to three QLs, then decreases for larger numbers, tracking thickness-dependent trends in the second-order susceptibility components ${\chi }^{xxx}$ and ${\chi }^{zxx}$. Comparison of measured and calculated SHG spectra for $s$-polarized photons of energies between 1.2 and 1.7 eV incident upon two QL samples discriminates among candidate stacking arrangements, and favors an arrangement with electric polarization vectors of the two QLs pointing outward in opposite directions.

We report on the polarity-dependent excitonic emission at the interface in MgZnO/ZnO heterostructures grown on both polar and nonpolar ZnO single-crystal substrates. Structural and morphological analyses confirm that the heterostructures grow homoepitaxially on nonpolar $m$-plane and $a$-plane crystal substrates. The nonpolar heterostructures are investigated by depth-resolved cathodoluminescence spectroscopy revealing an excitonic interface emission centered at 3.12 eV, identified as the $H$-band signature of indirect excitons. These results reveal the formation of robust indirect excitons confined at the MgZnO/ZnO interface due to the presence of an epitaxially strained interfacial layer near the interface in the MgZnO, which is confirmed by high-order x-ray diffraction. It is also found that the $H$-band emission originates from the self-absorption of the ZnO free exciton emission in the proximity of the interface. Temperature-dependent studies of the $H$ band yields a thermal activation energy of 47 meV for the indirect exciton in the nonpolar heterojunctions. This activation energy is the largest among all free and bound excitons in ZnO and is much higher than the values found in AlGaN/GaN and MgZnO/ZnO/sapphire hetrostructures. Our findings indicate that the optical properties of nonpolar heterostructures are strongly influenced by the piezoelectric effect.

The three-dimensional microstructure of Ni, observed after five annealing intervals, was compared to simulations of grain growth using the threshold dynamics method with the assumption of capillarity as the only driving force. A grain-by-grain comparison made it possible to identify the sources of differences between the simulation and experiment. The most significant difference was for grains of the smallest sizes, which the simulation predicted would lose volume and disappear at a greater rate than observed in the experiment. The loss of grains created errors in the numbers of neighbors of the remaining grains, and it was found that errors in the simulated grain volume were correlated to errors in the number of near neighbors. While anisotropic grain boundary properties likely play a role in the differences, the size dependence of the errors suggest that it might be necessary to include a size dependence in the model for grain boundary migration kinetics.

We report the heteroepitaxial growth of single-crystalline bixbyite ${\left({\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\right)}_2{\mathrm{O}}_3$ films on (111)-oriented yttria-stabilized zirconia substrates using plasma-assisted molecular beam epitaxy. A pure ${\mathrm{In}}_2{\mathrm{O}}_3$ buffer layer between the substrate and ${\left({\mathrm{In}}_{1-x}{\mathrm{Ga}}_x\right)}_2{\mathrm{O}}_3$ alloy is shown to result in smoother film surfaces and significantly improved crystallinity. Transmission electron microscopy confirms the single crystallinity up to $x=0.18$ and only slight film quality deterioration with increasing Ga content. X-ray diffraction demonstrates partially relaxed layers with lattice parameters fitting well to Vegard's law. However, the Ga cations are not evenly distributed within the films containing nominally $x>0.11$: inclusions with high Ga density up to $x=0.50$ are observed within a “matrix” with $x\approx 0.08$. The cubic bixbyite phase is preserved in both the matrix and the inclusions, which is in contrast to previous work reporting secondary, Ga-rich monoclinic or hexagonal phases forming beyond the solubility limit (of $x\approx 0.10$) of Ga in bixbyite ${\mathrm{In}}_2{\mathrm{O}}_3$. Moreover, for $x\ge 0.11$, both the Raman phonon lines and the optical absorption onset remain nearly constant. Hard x-ray photoelectron spectroscopy measurements also indicate a widening of the band gap and exhibit similar saturation of the Ga $2p$ core level position for high Ga contents. This saturation behavior of the spectroscopic properties largely reflects the properties of the matrix of the film, while the results of x-ray diffraction are related only to the average (matrix and inclusions) film composition.

The complex band structure, large spin-orbit induced band splitting, and heavy effective mass of two-dimensional (2D) hole systems hosted in GaAs quantum wells render them rich platforms to study many-body physics and ballistic transport phenomena. Here we report ultra-high-quality (001) GaAs 2D hole systems, fabricated using molecular beam epitaxy and modulation doping, with mobility values as high as $5.8\times {10}^6{\mathrm{cm}}^2$/(V s) at a hole density of $p=1.3\times {10}^{11}$/${\mathrm{cm}}^2$, implying a mean free path of $\simeq 27\mu \mathrm{m}$. In the low-temperature magnetoresistance trace of this sample, we observe high-order fractional quantum Hall states up to the Landau level filling $\nu =12/25$ near $\nu =1/2$. Furthermore, we see a deep minimum develop at $\nu =1/5$ in the magnetoresistance of a sample with a much lower hole density of $p=4.0\times {10}^{10}$/${\mathrm{cm}}^2$ where we measure a mobility of $3.6\times {10}^6{\mathrm{cm}}^2$/(V s). These improvements in sample quality were achieved by the reduction of residual impurities both in the GaAs channel and in the AlGaAs barrier material, as well as optimization in the design of the sample structure.

Topological nodal lines are open or closed one-dimensional manifolds formed by symmetry-protected band crossings in momentum space. Here we propose one hybrid type of closed nodal lines, i.e., the intersecting nodal rings, which exhibit hourglass dispersions guaranteed by two glide mirror symmetries. By performing first-principles calculations, we identify that type-I and type-II topological phonons coexist in a realistic material ${\mathrm{AgAlO}}_2$, forming hybrid-type nodal rings. Moreover, the hybrid nodal-ring phonons display unique saddlelike instead of drumheadlike surface states due to their exotic frequency dispersion. Our findings trigger the study of topological materials with hybrid topological features.

An approach using combined results, obtained by different experimental variants of positron annihilation spectroscopy and from kinetic Monte Carlo simulations, made it possible to determine the fundamental mechanisms by which vacancies assist solute atoms to form clusters during the earliest stages of precipitation kinetics in age-hardenable Al-Cu and Al-Cu-Mg alloys. The investigations were performed on the conventional precipitation-hardening system Al-1.74Cu (at. %) alloy aged at 293 and 342 K; these temperatures were chosen since they are low enough to avoid the formation of more stable nanostructures. To understand the influence of a minor alloying element on the solute clustering, the ternary Al-1.74Cu-0.35Mg (at. %) alloy was also studied. Interpreted in terms of the so-called vacancy pump model of solute aggregation, the results obtained made it possible to give a detailed and precise description concerning the role of the solute-vacancy exchanges in the solute clustering dynamics and the energetic stabilization of the formed clusters. It deserves to be pointed out that the results of our simulations for the ternary alloy indicate that, when aging proceeds, the solute atoms transported by vacancies progressively form Cu-Mg coclusters containing different amounts of nonmixed Cu or Mg solute atoms.

There has been a recent surge of interest in using machine learning to approximate density functional theory in materials science. However, many of the most performant models are evaluated on large databases of computed properties of, primarily, materials with precise atomic coordinates available, and which have been experimentally synthesized, i.e., which are thermodynamically stable or metastable. These aspects provide challenges when applying such models on theoretical candidate materials, for example for materials discovery, where the coordinates are not known. To extend the scope of this methodology, we investigate the performance of the crystal graph convolutional neural network on a data set of theoretical structures in three related ternary phase diagrams (Ti,Zr,Hf)-Zn-N, which thus include many highly unstable structures. We then investigate the impact on the performance of using atomic positions that are only partially relaxed into local energy minima. We also explore options for improving the performance in these scenarios by transfer learning, either from models trained on a large database of mostly stable systems, or a different but related phase diagram. Models pretrained on stable materials do not significantly improve performance, but models trained on similar data transfer very well. We demonstrate how our findings can be utilized to generate phase diagrams with a major reduction in computational effort.

In recent years many attempts have been made to discover new types of two-dimensional (2D) nanostructures with novel properties beyond the hexagonal crystals. The prediction of pentagraphene has sparked a great deal of research interest to investigate 2D pentagonal systems. In line with these efforts, in this paper, we propose a 2D ternary pentagonal monolayer of BNSi (penta-BNSi) and systematically investigate its structural, vibrational, mechanical, piezoelectric, electronic, photocatalytic, and optical properties by performing first-principles methods. We verify the stability of the penta-BNSi monolayer from the dynamical, thermal, and mechanical aspects based on phonon dispersion, ab initio molecular dynamics, and elastic tensor analysis, respectively. The mechanical properties are examined by calculating in-plane stiffness $\left(Y_{2\mathrm{D}}\right)$, Poisson's ratio $\left(\nu \right)$, and ultimate tensile strength and penta-BNSi is found to be soft and ductile. The electronic structure and electronic transport calculations indicate that the penta-BNSi monolayer possesses a quasidirect band gap and anisotropic, potentially high carrier mobility. Due to the noncentral symmetric character and semiconducting feature, an intrinsic piezoelectric response emerges in the structure. In addition, penta-BNSi has a suitable band gap as well as proper band edge positions for photocatalytic water splitting within practical pH levels. The analysis of optical properties, including many-body effects, points out strong exciton binding and high light absorption in the visible and near-UV parts of the spectrum. Our findings not only expand the family of 2D pentagonal materials but also uncover an ideal ultrathin material for photocatalytic applications.

Finding the ground-state configuration of point defects in semiconductors is in general a high-dimensional optimization problem. However, their adiabatic potential energy surfaces (APES) can be described with few effective coordinates by exploiting symmetry arguments within the Jahn-Teller (JT) theoretical framework. In this paper, we propose a general framework, that combines group theory and ab initio calculations, to build a full picture of common defects in diamond lattices (vacancies and substitutional impurities) as instances of the general T $\otimes (e+t_2)$ APES. Starting from the original tetrahedral symmetry, we consider all possible JT symmetries and we numerically investigate the shape of the APES by targeting the minimum energy path between equivalent JT distortions taking as examples the vacancy (V) and the metallic impurities Pd, Pt, and Au in crystalline silicon.

Gadolinium molybdate is a classical example of an improper ferroelectric and ferroelastic material. It is established that the spontaneous polarization arises as a secondary effect, induced by a structural instability in the paraelectric phase, which leads to a unit cell doubling and the formation of a polar axis. However, previous x-ray diffraction (XRD) studies on gadolinium molybdate have been restricted by the limited ability to include a wide $2\theta$ range in the analysis, and thus, at atomic scale, much remains to be explored. By applying temperature-dependent XRD, we observe the transition from the paraelectric tetragonal phase to the orthorhombic ferroelectric phase. The ferroelastic strain is calculated based on the thermal evolution of the lattice parameters, and Rietveld refinement of the temperature-dependent data reveals that the displacement of different cations follows different critical behavior, providing insight into the structural changes that drive the improper ferroelectricity in gadolinium molybdate.