Selective area growth (SAG) of semiconductors is a scalable method for fabricating gate-controlled quantum platforms. This letter reports on the adatom diffusion, incorporation, and desorption mechanisms that govern the growth rates of SAG nanowire (NW) arrays. We propose a model for the crystal growth rates that considers two parameter groups: the crystal growth control parameters and the design parameters. Using GaAs and InGaAs SAG NWs as a platform, we show how the design parameters such as NW pitch, width, and orientation have an impact on the growth rates. We demonstrate that by varying the control parameters (i.e., substrate temperature and beam fluxes) source, balance, and sink growth modes may exist in the SAG selectivity window. Using this model, we show that inhomogeneous growth rates can be compensated by tuning the design parameters.

Structural, electronic, and magnetic properties of a crystalline material formed by parallel chains of silver atoms inside the tunnels of a manganese dioxide host with the hollandite structure, ${\mathrm{Ag}}_x{\mathrm{Mn}}_8{\mathrm{O}}_{16}$, are studied with density functional theory (DFT) calculations. More magnetic structures were studied, including ferromagnetic (FM) and C2 antiferromagnetic (C2-AFM) orderings on Mn atoms of pristine hollandite ${\mathrm{Mn}}_8{\mathrm{O}}_{16}$ and the FM state of ${\mathrm{Ag}}_x{\mathrm{Mn}}_8{\mathrm{O}}_{16}$. Thereby, Hubbard corrections and different values for the Ag content, $x\le 2$, were considered. The results show that hollandite ${\mathrm{Mn}}_8{\mathrm{O}}_{16}$ is a semiconducting oxide both in the FM and in the AFM states, but with the C2-AFM configuration as the ground state. Interestingly, for the FM state of ${\mathrm{Ag}}_x{\mathrm{Mn}}_8{\mathrm{O}}_{16}$ for high silver content, $x=2$, the system is a perfect and isotropic half-metal, but when not all tunnels are filled with Ag chains, i.e., for $x<2$, it behaves as a quasi-one-dimensional system retaining the perfect half-metallicity. The results demonstrate that the incorporation of the monoatomic Ag chains inside the hollandite ${\mathrm{Mn}}_8{\mathrm{O}}_{16}$ host leads to a stable system that can be used as a spin filter for instance for spintronics devices.

Two-dimensional (2D) transition metal carbides and nitrides, called as MXenes, displaying astonishing properties are emerged as a new class of 2D layered materials. In this study, we have studied the structural, electronic, and magnetic properties of 2D bare $M_2\mathrm{C}$ and Janus Cr-based ${MM}^{\prime }\mathrm{C}$ metal carbides $(M\ne M^{\prime }=\mathrm{Cr}, \mathrm{Ti}, \mathrm{Sc}, \mathrm{V})$ based on density functional theory. We found that 2D Janus ${MM}^{\prime }\mathrm{C}$ MXenes are dynamically and thermally stable and show unique electronic and magnetic properties which are not seen in their individual bare cases. Our calculated electronic band structures indicate that the Janus ${MM}^{\prime }\mathrm{C}$ MXenes have Dirac-type band dispersion as seen in stanene with tiny band gaps. In addition, all considered bare $M_2\mathrm{C}$ MXenes show metallic property. However, Janus CrScC has dilute magnetic semiconducting property, CrVC shows half-metallicity, and CrTiC shows metallic property which can easily turn to half-metallicity by external effects. Our extensive density functional theory and Monte Carlo calculations indicate that Janus ${MM}^{\prime }\mathrm{C}$ MXenes also show different magnetic properties than their bare cases. While ${\mathrm{Cr}}_2\mathrm{C}$ and ${\mathrm{Sc}}_2\mathrm{C}$ have ferromagnetic spin orientations with $T_C=2000$ and 226 K Curie temperatures, respectively, CrScC shows ferrimagnetic character with antiferromagnetic Néel spin orientation and has $T_N=1120$ K magnetic phase transition temperature between ferrimagnetic and paramagnetic phases. Finally, external magnetic field effects on each crystal have also been elucidated in detail. The numerical outcomes reveal that Janus Cr-based ${MM}^{\prime }\mathrm{C}$ metal carbides have some unusual and interesting hysteresis characters. Our investigations presented here are not only interesting from a theoretical perspective, but also they show that Janus ${MM}^{\prime }\mathrm{C}$ MXenes are promising candidates for future spintronic applications, which should encourage their synthesis.

At ambient pressure tin transforms from its ground state, the semimetal $\alpha$-Sn (diamond structure), to metallic $\beta$-Sn at $13{}^{\circ }\mathrm{C}$ (286 K). There may be a further transition to a simple hexagonal phase, $\gamma$-Sn, above 450 K. These relatively low transition temperatures are due to the small energy differences between the structures, $\approx 20$ meV/atom between $\alpha$- and $\beta$-Sn, which makes tin an exceptionally sensitive test of the accuracy of density functionals and computational methods used in calculating electronic and vibrational energy, including zero-point energy. Here we use the high-throughput automatic-flow (AFLOW) method to study the energetics of tin in multiple structures using a variety of density functionals and examine the vibrational contributions to the free energy with the AFLOW Automatic Phonon Library (APL). We look at the successes and deficiencies of each functional. We also discuss the necessity of testing high-throughput calculations for convergence of systems with small energy differences.

Using a fast and accurate neural network potential, we are able to systematically explore the energy landscape of large unit cells of bulk magnesium oxide with the minima hopping method. The potential is trained with a focus on the near-stoichiometric compositions, in particular on suboxides, i.e., ${\mathrm{Mg}}_x{\mathrm{O}}_{1-x}$ with $0.50<x<0.60$. Our extensive exploration demonstrates that for bulk stoichiometric compounds, there are several new low-energy rock-salt-like structures in which Mg atoms are octahedrally six-coordinated and form trigonal prismatic motifs with different stacking sequences. Furthermore, we find a dense spectrum of novel nonstoichiometric crystal phases of ${\mathrm{Mg}}_x{\mathrm{O}}_{1-x}$ for each composition of $x$. These structures are mostly similar to the rock-salt structure with octahedral coordination and five-coordinated Mg atoms. Due to the removal of one oxygen atom, the energy landscape becomes more glasslike with oxygen-vacancy type structures that all lie very close to each other energetically. For the same number of magnesium and oxygen atoms, our oxygen-deficient structures are lower in energy if the vacancies are aligned along lines or planes than rock-salt structures with randomly distributed oxygen vacancies. We also found the putative global minima configurations for each composition of the nonstoichiometric suboxide structures. These structures are predominantly composed of $\mathrm{MgO}\left(111\right)$ layers of the rock-salt structure which are terminated with Mg atoms at the top and bottom and are stacked in different sequences along the $z$ direction. Like for other materials, these Magnéli-type phases have properties that differ considerably from their stoichiometric counterparts such as high electrical conductivity.

From first principles, we calculate the Heisenberg interactions between neighboring dimers in several compounds within the ${\mathrm{Et}}_n{\mathrm{Me}}_{4-n}X{\left[{\mathrm{Pd}\left(\mathrm{dmit}\right)}_2\right]}_2$ (Et = ethyl, Me = methyl, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate) family using an atomistic approach, with broken-symmetry density functional theory. In all materials, we find a scalene triangular model where the strongest exchange coupling along one crystallographic axis is up to three times larger than the others and where the frustration further enhances this quasi-one-dimensionality. We calculate the Néel ordering temperature via the chain random phase approximation. We show that the difference in the frustrated interchain couplings is equivalent to a single bipartite interchain coupling, favoring long-range magnetic order. We find that the Néel ordering temperatures are the same order of magnitude as the experimentally measured values for most compounds.

The “two-step” growth technique has been used to grow atomically uniform Ag films on $7\times 7$ Si(111) and $8\times 8 \beta {\text{-Si}}_3{\text{N}}_4$(0001)/Si(111) surfaces. Angle-resolved photoemission spectroscopy reveals the formation of $sp$ quantum well states in the Ag films with distinct properties in the two cases. It is shown that the valence electrons in silver can be confined in the fundamental gap of a less than 1-nm-thin nitride layer, effectively decoupling the Ag and Si states.

The electronic structure of layered van der Waals ferromagnet ${\mathrm{CrI}}_3$ is studied with self-consistent diagrammatic approaches beyond GW approximation. Considerable improvement in the calculated band gap as compared to the non-self-consistent $G_0{W}_0$ results has been found. Certain spectral features in the valence bands discovered recently by angle-resolved photoemission spectroscopy are reproduced better when we use full frequency-dependent self-energy. Density-functional theory and quasiparticle self-consistent GW method which are based on frequency-independent self-energy are unable to resolve these features. Nonlocality effects in the diagrams beyond GW approximation are large for both polarizability and self-energy. This finding can potentially have an impact on the development of methods like GW$+$dynamical mean field theory.

We present atomic force microscopy experiments of wearless sliding between nanoscale tips and both bulk and monolayer $\mathrm{Mo}{\mathrm{S}}_2$ in ultrahigh vacuum across a wide range of temperatures (150–450 K) and scanning speeds (5 nm/s to 500 $\mu \mathrm{m}/\mathrm{s}$). Atomic lattice stick-slip behavior is consistently resolved. However, a bifurcation of behavior is seen, with some measurements showing a strong decrease in friction with increasing temperature and others showing athermal and low friction under nominally identical conditions. The difference between thermal and athermal behavior is attributed to a change in the corrugation of the potential energy surface, possibly due to trace amounts of adsorbed contaminants. While the speed dependence at a given temperature is consistent with the thermal Prandtl-Tomlinson model for atomic-scale friction, that is not the case for the temperature dependence (when it is present), nor can the temperature dependence be described by other existing models. We discuss the limitations of these models considering the measured results.

The ordered tetragonal intermetallic ${\mathrm{Cr}}_2\mathrm{Al}$ forms the same structure type as ${\mathrm{Mn}}_2\mathrm{Au}$, and the latter has been heavily investigated for its potential in antiferromagnetic spintronics due to its degenerate in-plane Néel vector. We present the single-crystal flux growth of ${\mathrm{Cr}}_2\mathrm{Al}$ and orientation-dependent magnetic properties. Powder neutron diffraction of ${\mathrm{Cr}}_2\mathrm{Al}$ and first-principles simulations reveal that the magnetic ordering is likely in-plane and therefore identical to ${\mathrm{Mn}}_2\mathrm{Au}$, providing a second material candidate in the ${\mathrm{MoSi}}_2$ structure type to evaluate the fundamental interactions that govern spintronic effects. The single ordering transition seen in thermal analysis and resistivity indicates that canting of the moments along the $c$ axis is unlikely. Magnetometry, resistivity, and differential scanning calorimetry measurements confirm the Néel temperature to be $634\pm 2$ K. First-principles simulations indicate that the system has a small density of states at the Fermi energy and confirm the lowest-energy magnetic ground-state ordering, while Monte Carlo simulations match the experimental Néel temperature.

Local chemical ordering in CoCrNi medium-entropy alloy (MEA) was directly observed by the use of atom probe tomography. It was found that the densities of Cr, Co, and Ni were almost the same along the [111] direction, while those along the [001] direction were modulated to take a slightly enhanced and depleted value alternately within approximately 10 atomic layers corresponding to about 2 nm. The degree of modulation of Co and Ni was stronger than that of Cr. It was suggested that Cr-rich {001} atomic layers and (Ni + Co)-rich {001} layers tended to align mutually in the face-centered-cubic CoCrNi solid solution alloy. The mechanical properties of the MEA was found not to be affected by the presence of the local chemical ordering.

We report on the correlation between the boson peak and the structural changes associated with thermally treating amorphous indium-oxide films. In this process, the resistance of a given sample may decrease by a considerable margin whereas its amorphous structure is preserved. In the present paper, we focus on the changes that result from the heat treatment by employing electron-microscopy, x-ray, and Raman spectroscopy. These techniques were used on films with different stoichiometry and, thus, different carrier concentrations. The main effect of heat treatment is material densification, which presumably results from elimination of microvoids. The densified system presents better wave-function overlap and more efficient connectivity for the current flow. X-ray and electron-beam diffraction experiments indicate that the heat-treated samples show significantly less spatial heterogeneity with only a moderate change in the radial-distribution function metrics. These results are consistent with the changes that occur in the boson-peak characteristics due to annealing as observed in their Raman spectra.

Phonons are important carriers of energy and information in many cryogenic devices used for quantum information science and in fundamental physics experiments such as dark matter detectors. In these systems phonon behaviors can be dominated by interfaces and their atomic structures; hence, there is increasing demand for a more detailed understanding of interfacial phonon transport in relevant material systems. Previous studies have focused on understanding thermal transport over the entire phonon spectrum at and above room temperature. At ultralow temperatures, however, knowledge is missing regarding athermal phonon behavior due to the challenge in modeling the extreme conditions in microscale, heterogeneous cryogenic systems, as well as extracting single-phonon information from a large ensemble. In this paper, we delineate the effects of interfacial atomic structures on phonon transport using a combination of classical molecular dynamics (MD) and phonon wave-packet simulations, to illustrate the consistency and differences between the ensemble- and single-phonon dynamics. We consider three single-crystal Si surface reconstructions—$(1\times 1), (\sqrt{3}\times \sqrt{3})$ and $(7\times 7)$—and model both experimentally observed $\mathrm{Si}(1\times 1)/\mathrm{Al}$ interfaces and hypothesized $\mathrm{Si}(\sqrt{3}\times \sqrt{3})$/Al and $\mathrm{Si}(7\times 7)/\mathrm{Al}$ interfaces. The overall interfacial thermal conductance calculated from non-equilibrium MD shows that for the $\mathrm{Si}(1\times 1)/\mathrm{Al}$ system, the presence of Al twin boundaries can hinder phonon transport and reduce thermal conductance by 2–12% relative to single-crystal Al; whereas the Si $(\sqrt{3}\times \sqrt{3})$ and $(7\times 7)$ reconstructions can enhance it by 6–19%. Normal mode decomposition reveals that both the increase and decrease in conductance are related to inelastic phonon scattering. Single-phonon wave-packet simulations predict phonon transport properties consistent with non-equilibrium MD, while further suggesting that phonon polarization conversion is significant even when elastic transmission dominates, and that the interfacial structures have anisotropic impacts on atomic vibrations along different lattice directions. Our findings suggest avenues for achieving selective phonon transport via controlling interfacial structures of materials using atomically precise fabrication techniques, and that the phonon wave-packet formalism is a potentially powerful method for developing a detailed understanding of non-equilibrium phenomena in the low-temperature limit.

Various industrial/commercial applications use Al-Mg alloys, yet the Mg added to Al materials, to improve strength, is susceptible to surface segregation and oxidation, leaving behind a softer and Al-enriched bulk alloy. To better understand this process and provide a systematic methodology for investigating dopants that can mitigate corrosion, we have developed a robust atomistic deep neural net potential (DNP) using a dataset generated with first-principles density-functional theory (DFT). The potential, validated systematically against DFT values, has been shown to have a high fidelity in calculating different elemental and intermetallic Al-Mg systems’ properties. Our calculations predict a linear trend in the formation energy of the Al-Mg alloy and its density as a function of temperature, consistent with experimental literature. Employing the DNP within a hybrid Monte Carlo and molecular dynamics (MC/MD) approach, we predict anisotropic surface segregation for Al-Mg alloys such that (111)<(100)<(110), with (111) surfaces displaying the lowest segregation enthalpies and Mg enrichment. Furthermore, we model the segregation tendencies by adapting a recently introduced isotherm model for grain boundary segregation. Our results show that this model describes the MC/MD segregation profiles with higher fidelity than the McLean and Fowler-Guggenheim isotherm models.

Starting with twisted bilayer graphene, graphene-based moiré materials have recently been established as a new platform for studying strong electron correlations. In this paper, we study twisted graphene monolayers on trilayer graphene and demonstrate that this system can host flat bands when the twist angle is close to the magic angle of $1.{16}^{\circ }$. When monolayer graphene is twisted on ABA trilayer graphene, the flat bands are not isolated, but are intersected by a Dirac cone with a large Fermi velocity. In contrast, graphene twisted on ABC trilayer graphene (denoted AtABC) exhibits a gap between flat and remote bands. Since ABC trilayer graphene and twisted bilayer graphene are known to host broken-symmetry phases, we further investigate the ostensibly similar magic-angle AtABC system. We study the effect of electron-electron interactions in AtABC using both Hartree theory and an atomic Hubbard theory to calculate the magnetic phase diagram as a function of doping, twist angle, and perpendicular electric field. Our analysis reveals a rich variety of magnetic orderings, including ferromagnetism and ferrimagnetism, and demonstrates that a perpendicular electric field makes AtABC more susceptible to magnetic ordering.

Magnetic field dependent unpolarized small-angle neutron scattering (SANS) experiments demonstrate that high-pressure torsion (HPT) straining induces spin misalignments in pure Ni, which persist in magnetic fields up to 4 T. The spin-misalignment scattering patterns are elongated perpendicular to the applied magnetic field due to an unusual predominant longitudinal $\mathrm{si}{\mathrm{n}}^2\theta$-type angular anisotropy. Such a contribution cannot be explained by the conventional second-order (in spin-misalignment amplitude) micromagnetic SANS theory in the approach-to-saturation regime, nor can its magnitude relative to the other features of the cross sections be explained by the third-order micromagnetic SANS theory. This indicates that the high density of crystal defects induced via HPT straining in Ni makes such higher-order effects in the micromagnetic SANS cross sections observable.

$\mathrm{SrV}{\mathrm{O}}_3$ thin films with a high figure of merit for applications as transparent conductors were crystallized from amorphous layers using solid phase epitaxy (SPE). Epitaxial $\mathrm{SrV}{\mathrm{O}}_3$ films crystallized on $\mathrm{SrTi}{\mathrm{O}}_3$ using SPE exhibit room-temperature resistivities as low as $5.2\times {10}^{–5}$ and $2.5\times {10}^{–5}\mathrm{\Omega }\mathrm{cm}$, residual resistivity ratios of 2.0 and 3.8, and visible light transmission maxima of 0.89 and 0.52 for film thicknesses of 16 and 60 nm, respectively. $\mathrm{SrV}{\mathrm{O}}_3$ layers were deposited at room temperature using radiofrequency sputtering in an amorphous form and subsequently crystallized by heating in a controlled gas environment. The lattice parameters and mosaic angular width of x-ray reflections from the crystallized films are consistent with partial relaxation of the strain resulting from the epitaxial mismatch between $\mathrm{SrV}{\mathrm{O}}_3$ and $\mathrm{SrTi}{\mathrm{O}}_3$. A reflection high-energy electron diffraction study of the kinetics of SPE indicates that crystallization occurs via the thermally activated propagation of the crystalline/amorphous interface, like SPE phenomena in other perovskite oxides. Thermodynamic calculations based on density functional theory predict the temperature and oxygen partial pressure conditions required to produce the $\mathrm{SrV}{\mathrm{O}}_3$ phase and are consistent with the experiments. The separate control of deposition and crystallization conditions in SPE presents possibilities for the crystallization of transparent conductors in complex geometries and over large areas.

We studied electronic states of borophene in a crystal of rare-earth aluminum/chromium boride and the hydrogenated sheet, borophane, successfully prepared by ion exchange and liquid exfoliation of the crystal. The layer has a boron network with five-membered and seven-membered rings. Electronic structures of all the boron-derived layers are found to be gapless by soft x-ray absorption and emission spectroscopy measurements at the B K-shell absorption edge and density functional theory calculations. The present results support the existence of a Dirac nodal loop at the Fermi level in the borophane layer with five- and seven-membered rings, predicted recently by topological calculations.

Growth, structural, magnetic, and spin transport properties of epitaxial scandium-substituted terbium iron garnet (TbScIG) thin films on gadolinium gallium garnet [GGG, (111) orientation] substrates are reported as a function of Sc content. The films are epitaxial on GGG without strain relaxation up to a thickness of 90 nm and exhibit perpendicular magnetic anisotropy. Sc, a nonmagnetic cation, occupies up to 40% of the octahedral Fe sites. Increasing Sc lowers the compensation temperature and Curie temperature and increases the room-temperature saturation magnetization from 37 kA/m for ${\mathrm{Tb}}_{2.8}{\mathrm{Fe}}_{5.2}{\mathrm{O}}_{12}$ to 59 kA/m for ${\mathrm{Tb}}_{2.8}{\mathrm{Sc}}_{0.8}{\mathrm{Fe}}_{4.4}{\mathrm{O}}_{12}$. Anomalous Hall effect-like spin Hall magnetoresistance measurements were performed to determine the effect of scandium content on spin mixing conductance of the TbScIG|Pt interface. The spin mixing conductance increased significantly with an increase in Sc content, consistent with a dependence of spin transport on the net Fe magnetization instead of the total magnetization of the garnet.