Materials with triply degenerate nodal points in their low-energy electronic spectrum produce crystalline-symmetry-enforced threefold fermions, which conceptually lie between the twofold Weyl and fourfold Dirac fermions. Here, we show how a silver-based Dirac semimetal BaAgAs realizes threefold fermions through our first-principles calculations combined with a low-energy effective $\mathbf{k}·\mathbf{p}$ model Hamiltonian analysis. BaAgAs is shown to harbor triply degenerate nodal points, which lie on its $C_3$ rotation axis, and are protected by the $C_{6v}$ ($C_2\otimes C_{3v}$) point-group symmetry in the absence of spin-orbit coupling (SOC) effects. When the SOC is turned on, BaAgAs transitions into a nearly ideal Dirac semimetal state with a pair of Dirac nodes lying on the $C_3$ rotation axis. We show that breaking inversion symmetry in the ${\mathrm{BaAgAs}}_{1-x}{\mathrm{P}}_x$ alloy yields a clean and tunable threefold fermion semimetal. Systematic relaxation of other symmetries in BaAgAs generates a series of other topological phases. BaAgAs materials thus provide another platform for exploring tunable topological properties associated with a variety of different fermionic excitations.

The incubation time preceding nucleation and growth of surface nanostructures is interesting from a fundamental viewpoint but also of practical relevance as it determines statistical properties of nanostructure ensembles such as size homogeneity. Using in situ reflection high-energy electron diffraction, we accurately deduce the incubation times for Ga-assisted GaAs nanowires grown on unpatterned Si(111) substrates by molecular beam epitaxy under different conditions. We develop a nucleation model that explains and fits the data very well. We find that, for a given temperature and Ga flux, the incubation time always increases with decreasing As flux and becomes infinite at a certain minimum flux, which is larger for higher temperature. For given As and Ga fluxes, the incubation time always increases with temperature and rapidly tends to infinity above $640{}^{\circ }\mathrm{C}$ under typical conditions. The strong temperature dependence of the incubation time is reflected in a similar variation of the nanowire number density with temperature. Our analysis provides understanding and guidance for choosing appropriate growth conditions that avoid unnecessary material consumption, long nucleation delays, and highly inhomogeneous ensembles of nanowires. On a more general ground, the existence of a minimum flux and maximum temperature for growing surface nanostructures should be a general phenomenon pertaining to a wide range of material-substrate combinations.

We derive a dielectric-dependent hybrid functional which accurately describes the electronic properties of heterogeneous interfaces and surfaces, as well as those of three- and two-dimensional bulk solids. The functional, which does not contain any adjustable parameter, is a generalization of self-consistent hybrid functionals introduced for homogeneous solids, where the screened Coulomb interaction is defined using a spatially varying, local dielectric function. The latter is determined self-consistently using density functional calculations in finite electric fields. We present results for the band gaps and dielectric constants of 3D and 2D bulk materials, and band offsets for interfaces, showing an accuracy comparable to that of $GW$ calculations.

Accelerating the calculations of finite-temperature thermodynamic properties is a major challenge for rational materials design. Reliable methods can be quite expensive, limiting their applicability in autonomous high-throughput workflows. Here, the three-phonon quasiharmonic approximation (QHA) method is introduced, requiring only three phonon calculations to obtain a thorough characterization of the material. Leveraging a Taylor expansion of the phonon frequencies around the equilibrium volume, the method efficiently resolves the volumetric thermal expansion coefficient, specific heat at constant pressure, the enthalpy, and bulk modulus. Results from the standard QHA and experiments corroborate the procedure, and additional comparisons are made with the recently developed self-consistent QHA. The three approaches—three-phonon, standard, and self-consistent QHAs—are all included within the open-source ab initio framework aflow, allowing the automated determination of properties with various implementations within the same framework.

Following the emergence of two-dimensional (2D) materials, a large amount of work has been dedicated to this class of materials. Numerical simulations have proven to be powerful to analyze and predict structure-properties relationships. However, a recurrent issue that arises in some 2D compounds is the appearance of a dynamical instability as an unstable phonon branch in a small pocket close to the Brillouin zone center. The origin (numerical and/or physical) of this instability is hardly discussed. Here, using a rising 2D material, GaSe, as a case study, this issue is tackled by discussing the numerical techniques that may be used to lift the instability but also by understanding the fundamental origin of it. The interlayer distance is the crucial parameter and the ionicity of the compounds is the key physical property governing the propensity for instability. In many works, this distance is arbitrarily fixed to a value for which the absence of interactions between the periodic images of the layers is assumed. A careful control of the effect of this distance on the stability is required prior to subsequent calculations of physical properties.

We demonstrate first measurements of successful spin generation in crystalline ${\mathrm{Co}}_2\mathrm{FeSi}$/MgO/GaAs hybrid structures grown by molecular-beam epitaxy (MBE) with different MgO interlayer thicknesses. Using nonlocal spin valve and nonlocal Hanle measurement configurations, we determine spin lifetimes of $\tau \approx 100$ ns and spin diffusion lengths of $\lambda \approx 5.6\mu \mathrm{m}$ for different MgO layer thicknesses, proving the high quality of the GaAs transport channel. For an optimized MgO layer thickness, the bias dependence of the spin valve signals indicates the verification of the half-metallic gap (upper edge) of ${\mathrm{Co}}_2\mathrm{FeSi}$ in accordance with first-principles calculations. In addition to that, spin generation efficiencies of up to $18\%$ reveal the high potential of MgO interlayers at the ${\mathrm{Co}}_2\mathrm{FeSi}$/GaAs interface for further device applications.

We report on a detailed investigation of structural and magnetic properties of thin CoFeZr films, produced by combinatorial cosputtering, with compositions in the ranges 30–85 at.% Co, 12–63 at.% Fe, and 4–8 at.% Zr. Extended x-ray absorption fine structure and x-ray diffraction measurements reveal that alloys with a Zr content below 5 at.% are polycrystalline with a bcc structure, while an amorphous morphology is stabilized at Zr contents above 6 at.%. All samples display a growth-induced in-plane uniaxial anisotropy, which is closely related to the Zr concentration gradients across the wafers. A model for the angular dependence of the reduced remanence, including a Gaussian distribution of easy/hard anisotropy axes, is presented and successfully used to fit the data for all samples. The magnetic moments of the polycrystalline films approximately follow the Slater-Pauling curve, and the magnetic moments of the amorphous films follow a similar trend, but with about 20 % lower values. X-ray magnetic circular dichroism measurements show, for the amorphous films, that the Co moments are virtually constant at $1.7\left(2\right){\mu }_{\mathrm{B}}/\mathrm{atom}$.

For ${\left(\mathrm{CoCrFeMn}\right)}_{100-x}\mathrm{A}{\mathrm{l}}_x$ high-entropy alloys, we investigate the phase evolution with increasing Al content ($0\le x\le 20$ at.%). From first-principles theory, aluminum doping drives the alloy structurally from fcc to bcc separated by a narrow two-phase region (fcc+bcc), which is well supported by our experiments. Using KKR-CPA electronic-structure calculations, we highlight the effect of Al doping on the formation enthalpy (alloy stability) and electronic dispersion of ${\left(\mathrm{CoCrFeMn}\right)}_{100-x}\mathrm{A}{\mathrm{l}}_x$ alloys. As chemical short-range order indicates the nascent local order, and entropy changes, as well as expected low-temperature ordering behavior, we use KKR-CPA-based thermodynamic linear response to predict the chemical ordering behavior of arbitrary complex solid-solution alloys—an ideal approach for predictive design of high-entropy alloys. The predictions agree with our present experimental findings and other reported ones.

The deposition of strong molecular electron acceptors onto ZnO induces a substantial work function (ϕ) increase due to electron transfer from the inorganic semiconductor to the molecules. The ϕ increase results from two mechanisms: (i) a change of the surface band bending within ZnO and (ii) an interface dipole between the inorganic surface and the negatively charged acceptors. The molecule adsorption induced upward band bending in ZnO is, however, found to be limited to a few 100 meV, while the ϕ increase is significantly larger (up to 2.8 eV). We elucidate the origin of limited upward band bending by revealing a notable gap state density-of-states (GDOS) using high-sensitivity photoemission spectroscopy. Upon acceptor-induced upward band bending, the GDOS with a wide energy distribution becomes increasingly unoccupied. This, in turn, makes the interface dipole dominant and limits the ZnO surface band bending changes due to a “soft” pinning at the Fermi level.

The quantum kagome antiferromagnets ${\mathrm{YCu}}_3{\left(\mathrm{OH}\right)}_6{\mathrm{O}}_x{\mathrm{Cl}}_{3-x}$ ($x=0,1/3$) are produced using a unified solid state synthesis route for polycrystalline samples. From structural refinements based on neutron diffraction data, we clarify the structure of the ${\mathrm{Y}}_3{\mathrm{Cu}}_9{\left(\mathrm{OH}\right)}_{18}{\mathrm{OCl}}_8$ ($x=1/3$) compound and provide a revised chemical formula. We use muon spin relaxation, as a local probe of magnetism, to investigate the exotic low-temperature properties in the two compounds. In agreement with the low-temperature neutron diffraction data, we find no evidence for long range ordering in both materials but they exhibit distinct ground states: while disordered static magnetism develops in the $x=0$ compound, we conclude on the stabilization of a quantum spin liquid in the $x=1/3$ one, since the local fields remain fully dynamical. Our findings are in contrast to previous reports based on thermodynamical measurements only. We then discuss their origin on the basis of structural details and specific heat measurements. In particular, the $x=1/3$ compound appears to realize an original spatially anisotropic kagome model.

The wide gap perovskite semiconductor ${\mathrm{BaSnO}}_3$ has attracted much interest since the discovery of room temperature electron mobility up to $320{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ in bulk crystals. Motivated by applications in oxide heterostructures, rapid progress has been made with ${\mathrm{BaSnO}}_3$ films, although questions remain regarding transport mechanisms and mobility optimization. Here we report on a detailed study of epitaxial ${\mathrm{BaSnO}}_3$ electric double layer transistors based on ion gel electrolytes, enabling wide-doping-range studies of transport in single films. The work spans an order of magnitude in initial $n$ doping ($\sim {10}^{19}$ to ${10}^{20}{\mathrm{cm}}^{-3}$, with both oxygen vacancies and La), film thicknesses from 10–50 nm, and measurements of resistance, Hall effect, mobility, and magnetoresistance. In contrast with many oxides, electrolyte gating of ${\mathrm{BaSnO}}_3$ is found to be essentially reversible over an exceptional gate voltage window (approaching $\pm 4\mathrm{V}$), even at 300 K, supported by negligible structural modification in operando synchrotron x-ray diffraction. We propose that this occurs due to a special situation in ${\mathrm{BaSnO}}_3$, where electrochemical gating via oxygen vacancies is severely limited by their low diffusivity. Wide-range reversible modulation of transport is thus achieved (in both electron accumulation and depletion modes), spanning strongly localized, weakly localized, and metallic regimes. Two-channel conduction analysis is then combined with self-consistent Schrödinger-Poisson and Thomas-Fermi modeling to extract accumulation layer electron densities and mobilities. Electrostatic electron densities approaching ${10}^{14}{\mathrm{cm}}^{-2}$ are shown to increase room temperature mobility by up to a factor of $\sim 24$. These results lay the groundwork for future studies of electron-density-dependent phenomena in high mobility ${\mathrm{BaSnO}}_3$, and significantly elucidate oxide electrolyte gating mechanisms.

In a first-principles study based on density functional theory and many-body perturbation theory, we investigate the electronic properties and the optical excitations of ${\mathrm{ZrS}}_2$ and ${\mathrm{HfS}}_2$ monolayers and their van der Waals heterostructure. Both materials have an indirect quasiparticle band gap, which amounts to about 2.8 eV in ${\mathrm{ZrS}}_2$ and to 2.6 eV in ${\mathrm{HfS}}_2$. In both systems the valence-band maximum is at $\mathrm{\Gamma }$ and the conduction-band minimum at M. Spin-orbit coupling induces a splitting of about 100 meV at the $\mathrm{\Gamma }$ point in the valence band, while it does not affect the conduction band. The optical absorption spectra are dominated by excitonic peaks with binding energies between 0.6 and 0.8 eV. The ${\mathrm{ZrS}}_2/{\mathrm{HfS}}_2$ heterobilayer exhibits a peculiar type-I level alignment with a large degree of hybridization between the two monolayers in the valence band, while the conduction bands retain either ${\mathrm{ZrS}}_2$ or ${\mathrm{HfS}}_2$ character, respectively. As a consequence, both the electron and the hole components of the first exciton are localized in the ${\mathrm{ZrS}}_2$ monolayer with nonvanishing probability of finding the hole also in the ${\mathrm{HfS}}_2$ sheet.

Despite the importance of thermal conductivity for a range of modern glass applications, its compositional dependence and structural origins in modified oxide glasses remain poorly understood. In particular, the thermal conductivity of oxide glasses with network formers other than silica remain almost unexplored and no thorough connection with structural characteristics of glasses has been made. In this work, we study the thermal conductivity of binary lithium borate glasses using both experiments and classical molecular dynamics (MD) simulations. This glass system is chosen due to the nonmonotonic evolution in the boron coordination number as a function of composition and because glasses may be made in a wide compositional window. Specifically, we show that thermal conductivity exhibits a clear boron anomaly effect, as observed in both experiments and MD simulations. Thermal conduction is thus believed to mainly be promoted by the presence of fourfold coordinated boron. However, simulated vibrational density of states for the studied series suggests that the thermal conductivity is also influenced by the presence of the modifier ions based on an observed overlap between Li and O modes. Overall these results provide insights into the connection between thermal conductivity and structure of modified oxide glasses, which is the first step toward developing a model for predicting the composition dependence of thermal conductivity.

We report direct evidence of the theoretically predicted electron-rich surface of doped perovskite manganites ${\mathrm{La}}_{0.7}{A}_{0.3}{\mathrm{MnO}}_3(A=\text{Ca}$, Sr) by means of surface-enhanced Raman spectroscopy. The required Au nanoparticles were grown on top of thin manganite films by the metalorganic aerosol deposition technique, which provides a stable oxygen atmosphere and prevents deoxygenation effects. The acquired surface-enhanced Raman spectra of rhombohedral ${\mathrm{La}}_{0.7}{\mathrm{Sr}}_{0.3}{\mathrm{MnO}}_3$ thin films reveal the symmetry-forbidden Jahn-Teller stretching modes demonstrating the predicted symmetry breaking at the surface and fallback into the orthorhombic (Pnma) structure. This results in a unique Jahn-Teller reconstructed surface structure, which limits the capabilities of mixed-valence manganites in spintronic applications, but will be highly favorable for catalytic reactions.

Molecules intercalating two-dimensional materials form complex structures that have been characterized primarily by spatially averaged techniques. Here we use aberration-corrected scanning transmission electron microscopy and density-functional-theory (DFT) calculations to study the atomic structure of bilayer graphene (BLG) and few-layer graphene (FLG) intercalated with $\mathrm{FeC}{\mathrm{l}}_3$. In BLG, we discover two distinct intercalated structures that we identify as monolayer $\mathrm{FeC}{\mathrm{l}}_3$ and monolayer $\mathrm{FeC}{\mathrm{l}}_2$. The two structures are separated by atomically sharp boundaries and induce large free-carrier densities on the order of ${10}^{13}\mathrm{c}{\mathrm{m}}^{-2}$ in the graphene layers. In FLG, we observe multiple $\mathrm{FeC}{\mathrm{l}}_3$ layers stacked in a variety of possible configurations with respect to one another. Finally, we find that the microscope's electron beam can convert the $\mathrm{FeC}{\mathrm{l}}_3$ monolayer into FeOCl monolayers in a rectangular lattice. These results reveal the need for a combination of atomically resolved microscopy, spectroscopy, and DFT calculations to identify intercalated structures and study their properties.

We present self flux growth and characterization of single crystalline ${\mathrm{AlMn}}_2{\mathrm{B}}_2$. It is an orthorhombic (space group $Cmmm$), layered material with a platelike morphology. The anisotropic bulk magnetization data, electrical transport, and ${}^{11}\mathrm{B}$ nuclear magnetic resonance (NMR) data revealed an antiferromagnetic (AFM) transition at $313\pm 2\mathrm{K}$. In the magnetization data, there is also a broad local maximum significantly above the AFM transition that could be a signature of low-dimensional magnetic interactions in ${\mathrm{AlMn}}_2{\mathrm{B}}_2$.

We present band structure and optical absorption spectra obtained from density functional theory (DFT) and linear response time-dependent DFT (TDDFT) calculations using a screened range-separated hybrid (SRSH) functional, including spin-orbit coupling, for seven prototypical semiconductors. The results are compared to those obtained from highly converged many-body perturbation theory calculations using the GW approximation and the GW plus Bethe-Salpeter equation (GW-BSE) approaches. We use a single empirical parameter for our SRSH calculations, fit such that the SRSH band gap reproduces the GW band gap at the $\mathrm{\Gamma }$ point. We then find that ground-state generalized Kohn-Sham SRSH eigenvalues accurately reproduce the band structure obtained from GW calculations, typically to within 0.1–0.2 eV, and optical absorption spectra obtained using TDDFT with the SRSH functional agree well with those of GW-BSE, with a mean deviation of 0.03 and 0.11 eV for the location of the first and second absorption peaks, respectively, at a fraction of the computational cost.

The impact of anharmonicity on the vibrational entropy and heat capacity of ${\mathrm{UO}}_2$ has been investigated from 10 to 1200 K using inelastic neutron-scattering measurements of the phonon density of states (PDOS). Small changes in the PDOS are observed from 10 to 295 K, with more noticeable changes appearing in the 750- and 1200-K data. The specific heat determined from the PDOS measurements is in agreement with macroscopic specific heat measurements, and the overall impact of nondilation anharmonicity on the specific heat has been shown to be less than 2%. An analysis of the phonon measurements shows that the softening of acoustic phonons with temperature is consistent with the quasiharmonic approximation. The optical phonons deviate from the quasiharmonic prediction, with the low-energy optical phonons between approximately 20 and 50 meV softening more than expected, while the higher-energy optical phonons between approximately 50 and 80 meV have no appreciable softening over the temperature range measured. The observation of a small anharmonic specific heat contribution has been shown to be the result of relatively large energy-dependent anharmonic effects which have opposite sign, leading to a total contribution near zero.