We investigate the effect of surface tunneling on charge distributions of two-dimensional hole gases (2DHGs) in undoped Ge/GeSi heterostructures. As in the electron channel case, the 2DHG density saturates at a high gate voltage. As the channel depth of 2DHGs increases, a crossover of charge distributions in the system from equilibrium to nonequilibrium is observed at a depth of $\sim 50$ nm. A surface tunneling model is proposed to explain the density crossover. Magnetotransport analysis is performed to investigate the limiting scattering mechanisms. The power law dependence of mobility on density suggests that the dominant scattering mechanisms for the shallow- and deep-channel 2DHGs are remote impurity and background impurity scattering, respectively. Clear quantum Hall plateaus and vanishing longitudinal magnetoresistance are observed in the 2DHG device of channels as shallow as 9 nm.

Half-metallic ferromagnetism (HMFM) occurs rarely in materials and yet offers great potential for spintronic devices. Recent experiments suggest a class of compounds with the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ (122) structure (isostructural and containing elements common with Fe pnictide-based superconductors) can exhibit HMFM. Here, we use ab initio density functional theory calculations to understand the onset of half-metallicity in this family of materials and explain the appearance of ferromagnetism at a quantum critical point. We also predict new candidate materials with HMFM and potentially high Curie temperatures through $A$-site alloying.

We present a detailed study of the electronic structure of the layered semiconductor InSe. We calculate the band structure of the monolayer and bulk material using density functional theory, hybrid functionals, and $GW$. The band gap of the monolayer InSe is calculated to be 2.4 eV in screened exchange hybrid functional, close to the experimental photoluminescence gap. The electron affinities and band offsets are calculated for vertical stacked-layer heterostructures, and are found to be suitable for tunnel field effect transistors (TFETs) in combination with $\mathrm{WS}{\mathrm{e}}_2$ or similar. The valence-band edge of InSe is calculated to lie 5.2 eV below the vacuum level, similar to that for the closed shell systems $\mathrm{HfS}{\mathrm{e}}_2$ or $\mathrm{SnS}{\mathrm{e}}_2$. Hence InSe would be suitable to act as a $p$-type drain in the TFET. The intrinsic defects are calculated. For Se-rich layers, the Se adatom (interstitial) is found to be the most stable defect, whereas for In-rich layers, the Se vacancy is the most stable for the neutral state. Antisites tend to have energies just above those of vacancies. The Se antisite distorts towards a bond-breaking distortion as in the EL2 center of GaAs. Both substitutional donors and acceptors are calculated to be shallow, and effective dopants. They do not reconstruct to form nondoping configurations as occurs in black phosphorus. Finally, the Schottky barriers of metals on InSe are found to be strongly pinned by metal induced gap states (MIGS) at $\sim 0.5\mathrm{eV}$ above the valence-band edge. Any interfacial defects would lead to a stronger pinning at a similar energy. Overall, InSe is an effective semiconductor combining the good features of 2D (lack of dangling bonds, etc.) with the good features of 3D (effective doping), which few others achieve.

Carbon solubility in face-centered-cubic Ni-W alloys and the phase diagrams of Ni-W and C-Ni-W are investigated by means of first principles calculations and semigrand canonical Monte Carlo simulations. With density functional theory total energies as fitting data, we build a superatom model for efficient simulation. Multihistogram analysis is utilized to predict free energies for different compositions and temperatures. By comparing free energies of competing phases, we are able to predict carbon solubility and phase diagrams of C-Ni-W at different temperatures. A simple ideal mixing approximation gives qualitatively similar predictions.

Preparation of single-atom-thick layers of ordinary metals has been a challenging task since their closely packed atoms lack layered structure with highly anisotropic bonding. Using computational modeling based on density functional theory we showed that graphene/${\mathrm{MoS}}_2$ heterostructures can be used as suitable templates to grow stable two-dimensional (2D) clusters, as well as extended monoatomic layers of metals with nonlayered structure in the bulk. Considering gold and lithium as two metals with markedly different properties, we found that Li intercalants strengthen coupling between graphene (G) and ${\mathrm{MoS}}_2$, mainly due to electrostatic attraction of 2D materials with positively charged Li atoms. However, intercalation with large Au atoms gives rise to a significant increase in the distance between G and ${\mathrm{MoS}}_2$ and thus, weakens their interaction. In addition to strong preference for 2D growth, we demonstrated that Au intercalants weakly interact with both G and ${\mathrm{MoS}}_2$, and hence $\mathrm{G}/{\mathrm{MoS}}_2$ vertical heterostructures could be a promising framework to prepare gold 2D structures with electronic properties closely resembling those of the hypothetical free-standing hexagonal gold monolayer.

Networks of superconducting ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$ (Bi-2212) nanowires were fabricated by the electrospinning technique. The nanowires have a diameter of the order of 150–200 nm and lengths up to the micrometer range and form a nonwoven, fabric-like network with numerous interconnections enabling a current flow between the nanowires. The porosity of this nanowire network is 0.9928. Therefore, this material represents a novel class of ultraporous high-temperature superconductors. The magnetizations of the nanowire networks $\left[M\right(T)$ and $M\left(H\right)]$ were recorded by SQUID magnetometry. The magnetic properties were analyzed using the extended critical state model (ECSM). It is supposed that the averaged diameter of the nanowires rules the magnetic field dependence of the critical current density of the nanowire network. Single nanowires have remarkably high values of the critical current density of $1.04\times {10}^7\mathrm{A}/{\mathrm{cm}}^2$ at 5 K. The macroscopic critical current density less than $\sim 0.05\mathrm{A}/{\mathrm{cm}}^2$ at 5 K is fine for this lightweight material. Using ECSM, several important magnetic parameters could be determined including the penetration field $H_p$, the irreversibility fields $H_{\mathrm{irr}}$, the upper critical field $H_{c2}$, and the flux pinning forces. Applications for this material class may be found in the direction of sensors, thin shielding layers, or nanoporous bulks.

A systematic study of the superconducting properties in a series of arc-melted Nb-B samples close to the 1:1 composition was carried out. Powder x-ray diffraction (XRD) shows that all samples are both nonstoichiometric and composed of two crystal phases: a majority orthorhombic NbB-type phase and traces of a minor body-centered-cubic Nb-rich phase ${\mathrm{Nb}}_{ss}$ with stoichiometry close to ${\mathrm{Nb}}_{0.98}{\mathrm{B}}_{0.02}$. The emergence of superconductivity near $T_c\sim 9.0\mathrm{K}$ was inferred from magnetization data in chunk and powder samples. However, the very small superconducting volume fractions are inconsistent with superconductivity arising from the major NbB phase. On the other hand, micrographs of selected samples clearly show that the minority ${\mathrm{Nb}}_{ss}$ forms a three-dimensional network of filaments that meander around the grains of the majority phase, forming a percolation path. Here we report the superconductivity of the ${\mathrm{Nb}}_{ss}$ phase and argue that the low superconducting volume fraction of nonstoichiometric NbB and zero resistance are due to the filaments of the minority phase. The electronic contribution to the entropy of the superconducting state, yielded from an analysis using the $\alpha$ model for single-band systems, indicates that the Sommerfeld constant of the arc-melted samples is close to the values found in nonsuperconducting NbB. Micrograph, XRD, and bulk measurements of magnetization, electrical resistivity, and specific heat suggest that the superconducting state in the NbB samples bearing some ${\mathrm{Nb}}_{ss}$ minority phase is due to the latter.

We have synthesized ten iron oxyarsenides, $\mathrm{K}L{n}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{O}}_2$ $(Ln=\text{Gd},\text{Tb},\text{Dy},\text{and Ho})$ and $\mathrm{Cs}L{n}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{O}}_2$ $(Ln\text{=}\text{Nd},\text{Sm},\text{Gd},\text{Tb},\text{Dy},\text{and Ho})$, with the aid of the lattice-match approach. The resultant compounds possess hole-doped conducting double FeAs layers ${\left[A{\mathrm{Fe}}_4{\mathrm{As}}_4\right]}^{2-}$ that are separated by the insulating ${\left[L{n}_2{\mathrm{O}}_2\right]}^{2+}$ slabs. Measurements of electrical resistivity and dc magnetic susceptibility demonstrate bulk superconductivity at $T_{\mathrm{c}}=33$–37 K. We find that $T_{\mathrm{c}}$ correlates with the axial ratio $c/a$ for all 12442-type superconductors discovered. Also, $T_{\mathrm{c}}$ tends to increase with the lattice mismatch, implying that lattice instability plays a role in the enhancement of superconductivity.

The growth of pentacene thin films on a sapphire $\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3$(0001) surface was investigated in situ using differential reflectance spectroscopy (DRS). Two different film structures are observed depending on the substrate temperature. If pentacene is deposited at room temperature, a wetting layer consisting of flat-lying molecules is formed after which upright-standing molecular layers with a herringbone structure start to grow. At low substrate temperature of 100 K, the long molecular axis of the pentacene molecules remains parallel to the surface plane throughout the entire growth regime up to rather large thicknesses. Heating thin films deposited at 100 K to room temperature causes the pentacene molecules beyond the wetting layer to stand up and assemble into a herringbone structure. Another interesting observation is the dewetting of the first flat-lying monolayer upon exposure to air, leading to the condensation of islands consisting of upright-standing molecules. Our results emphasize the interplay between growth kinetics and thermodynamics and its influence on the molecular orientation in organic thin films.

We introduce a rigorous, physically appealing, and practical way to measure distances between exchange-only correlations of interacting many-electron systems, which works regardless of their size and inhomogeneity. We show that this distance captures fundamental physical features such as the periodicity of atomic elements, and that it can be used to effectively and efficiently analyze the performance of density functional approximations. We suggest that this metric can find useful applications in high-throughput materials design.

MXenes are a comparatively young class of 2D materials, composed of transition-metal carbides/nitrides of the general formula ${\mathrm{M}}_{n+1}{\mathrm{X}}_n{\mathrm{T}}_x$, where T represents surface terminations, typically O, OH, and/or F. Recently, a new type of MXene with vacancy ordering was discovered, $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_x$, with conduction and capacitance superior to the MXene counterpart without vacancies, $\mathrm{M}{\mathrm{o}}_2\mathrm{C}{\mathrm{T}}_x$. We here present a theoretical evaluation of $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_x$ based on first-principles calculations, where $x=2$ and T is O, F, OH, or a mixture thereof. In addition to structural evaluation upon vacancy formation, we identify preferred terminations as well as termination sites, and resulting dynamical stability and electronic properties. For mixed terminations, the mixing energy is evaluated. We show that while $\mathrm{M}{\mathrm{o}}_2\mathrm{C}$ is typically O terminated, mixed terminations with a high F content are suggested for $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_x$, which in turn gives the highest metallicity out of all the configurations investigated. In addition, the results indicate a strong tuning potential of the band gap through choice of terminations, with an electronic structure changing between insulating and metallic depending on termination(s) and their configuration. We also performed x-ray photoelectron spectroscopy to identify and quantify the terminating species on the MXene, as well as their respective binding energies. The experimental results are consistent with the theoretical analysis, and combined they suggest an explanation to the MXene chemistry as well as the reported high conductivity of $\mathrm{M}{\mathrm{o}}_{1.33}\mathrm{C}{\mathrm{T}}_x$.

Interaction of sodium with graphene (Gr) on Ir(111) was studied with the aim to resolve the issue of Na adsorption/intercalation kinetics. The system Na/Gr/Ir(111) was studied by means of angle-resolved photoemission spectroscopy, low-energy electron diffraction, and ab initio density functional theory (DFT) calculation. It has been found that at room temperature (RT) and low concentrations Na is dominantly adsorbed on graphene. At higher concentrations, an intercalation process sets in so that it is possible to observe the coexistence of these two states. Eventually, all Na atoms are found in the intercalated state as determined by exposure to oxygen. While adsorption of Na on graphene already intercalated by Na [Na/Gr/Na/Ir(111) system] at RT was not possible, we could observe Li adsorption through the increase of Dirac point binding energy. Li coadsorption strongly affects the binding energy of the iridium surface state as well. This finding was supported by DFT calculations of adsorption energy of Na and Li on bare and fully Na intercalated graphene.

Since the discovery of graphene and its outstanding chemical, optical, and mechanical properties, other layered materials have been fiercely hunted for throughout various techniques. Thanks to their van der Waals interaction, acting as weak glue, different types of layered materials with mismatched lattices can be stacked with high quality interfaces. The properties of the resulting multilayer structures can be tuned by choice of the materials, layer thicknesses, and sequence in which they are arranged. This opens the possibility for a large array of applications across many different fields. Here we present a systematic study with two-dimensional stacked layered materials, where their properties are tailored monolayer by monolayer. By arranging ${\mathrm{WSe}}_2,{\mathrm{MoSe}}_2,{\mathrm{WS}}_2$, and ${\mathrm{MoS}}_2$ monolayers in predetermined sequences, that are predicted to have a ladder band alignment in both the conduction and valence bands, we separate electrons and holes between the two utmost layers by monolayer increments. The samples studied are a ${\mathrm{WSe}}_2$ monolayer, a ${\mathrm{WSe}}_2\text{−}{\mathrm{MoSe}}_2$ bilayer, a ${\mathrm{WSe}}_2\text{−}{\mathrm{MoSe}}_2\text{−}{\mathrm{WS}}_2$ trilayer, and a ${\mathrm{WSe}}_2\text{−}{\mathrm{MoSe}}_2\text{−}{\mathrm{WS}}_2\text{−}{\mathrm{MoS}}_2$ four-layer. We observe an increase in absorbance, a decrease in photoluminescence, a variation in interlayer charge transfer, and photocarrier lifetimes that are extended up to a few nanoseconds as additional layers were added. With these results, we demonstrate that van der Waals stacked two-dimensional materials can form effective complex stacks and are promising platforms for fabricating ultrathin and flexible optoelectronics.

Based on their formation mechanisms, Dirac points in three-dimensional systems can be classified as accidental or essential. The former can be further distinguished into type I and type II, depending on whether the Dirac cone spectrum is completely tipped over along a certain direction. Here we predict the coexistence of all three kinds of Dirac points in the low-energy band structure of CaAgBi-family materials with a stuffed wurtzite structure. Two pairs of accidental Dirac points reside on the rotational axis, with one pair being type I and the other pair type II; while another essential Dirac point is pinned at the high symmetry point on the Brillouin zone boundary. Due to broken inversion symmetry, the band degeneracy around accidental Dirac points is completely lifted except along the rotational axis, realizing a kind of birefringent Dirac fermions, which may enable the splitting of chiral carriers at a ballistic $p\text{−}n$ junction with a double negative refraction effect. We clarify their symmetry protections, and find both the Dirac cone and Fermi arc topological surface states.

We present resistance versus temperature data for a series of boron-doped nanocrystalline diamond films. Upon extracting the fluctuation conductivity near the critical temperature we observe three distinct scaling regions—three-dimensional (3D) intragrain, quasi-0D, and 3D intergrain—in confirmation of the prediction of Lerner, Varlamov, and Vinokur. The location of the dimensional crossovers between these scaling regions allows us to determine the tunneling energy and the Thouless energy directly. This is a demonstration of the use of fluctuation spectroscopy to determine the properties of a superconducting granular system.

Polarization-induced renormalization of the frontier energy levels of interacting molecules and surfaces can cause significant shifts in the excitation and transport behavior of electrons. This phenomenon is crucial in determining the oxidative stability of nonaqueous electrolytes in high-energy density electrochemical systems such as the Li-${\mathrm{O}}_2$ battery. On the basis of partially self-consistent first-principles $\mathrm{Sc}G{W}_0$ calculations, we systematically study how the electronic energy levels of four commonly used solvent molecules, namely, dimethylsulfoxide (DMSO), dimethoxyethane (DME), tetrahydrofuran (THF), and acetonitrile (ACN), renormalize when physisorbed on the different stable surfaces of ${\mathrm{Li}}_2{\mathrm{O}}_2$, the main discharge product. Using band level alignment arguments, we propose that the difference between the solvent's highest occupied molecular orbital (HOMO) level and the surface's valence-band maximum (VBM) is a refined metric of oxidative stability. This metric and a previously used descriptor, solvent's gas phase HOMO level, agree quite well for physisorbed cases on pristine surfaces where ACN is oxidatively most stable followed by DME, THF, and DMSO. However, this effect is intrinsically linked to the surface chemistry of the solvent's interaction with the surface states and defects, and depends strongly on their nature. We conclusively show that the propensity of solvent molecules to oxidize will be significantly higher on ${\mathrm{Li}}_2{\mathrm{O}}_2$ surfaces with defects as compared to pristine surfaces. This suggests that the oxidative stability of a solvent is dynamic and is a strong function of surface electronic properties. Thus, while gas phase HOMO levels could be used for preliminary solvent candidate screening, a more refined picture of solvent stability requires mapping out the solvent stability as a function of the state of the surface under operating conditions.