Contact charging between insulators is one of the most basic, yet least well understood, of physical processes. For example we have no clear theory for how insulators recruit enough charge carriers to deposit charge but not enough to discharge. In this paper we note that charging and discharging kinetics may be distinct, and from this observation we develop a mathematical model. The model surprisingly predicts that charging can decrease as contact frequency increases: We confirm this prediction experimentally and propose future steps.

We identify a novel reaction mechanism in the thin film synthesis of compound materials. With the example of the O plasma-assisted molecular beam epitaxy of III-O and IV-O semiconductors—${\mathrm{Ga}}_2{\mathrm{O}}_3, {\mathrm{In}}_2{\mathrm{O}}_3$, and ${\mathrm{SnO}}_2$—we illustrate this mechanism, involving the intermediate formation of a suboxide. This consecutive reaction mechanism, as well as the competing desorption of a subcompound, are the basis for the development of a quantitative growth model parametrized by three material-dependent parameters. It is proposed and justified to be applicable to other III-VI and IV-VI compounds whose constituents exhibit analogous kinetic and thermodynamic properties to those discussed for oxides. We validate this model quantitatively by experimental growth rate and desorption data as a function of all growth parameters for ${\mathrm{Ga}}_2{\mathrm{O}}_3, {\mathrm{In}}_2{\mathrm{O}}_3$, and ${\mathrm{SnO}}_2$. As the first of its kind, our model serves as a basis for more sophisticated growth models, e.g., describing multicomponent materials or including surface diffusion processes, and can be transferred to other growth techniques and thin films that grow via intermediate reaction products.

Defects in two-dimensional (2D) materials are becoming prominent candidates for quantum emitters and scalable optoelectronic applications. However, several physical properties that characterize their behavior, such as charged defect ionization energies, are difficult to simulate with conventional first-principles methods, mainly because of the weak and anisotropic dielectric screening caused by the reduced dimensionality. We establish fundamental principles for accurate and efficient calculations of charged defect ionization energies and electronic structure in ultrathin 2D materials. We propose to use the vacuum level as the reference for defect charge transition levels (CTLs) because it gives robust results insensitive to the level of theory, unlike commonly used band-edge positions. Furthermore, we determine the fraction of Fock exchange in hybrid functionals for accurate band gaps and band-edge positions of 2D materials by enforcing the generalized Koopmans' condition of localized defect states. We found that the obtained fractions of Fock exchange vary significantly from 0.2 for bulk hexagonal boron nitride (h-BN) to 0.4 for monolayer h-BN, whose band gaps are also in good agreement with experimental results and calculated GW results. The combination of these methods allows for the reliable and efficient prediction of defect ionization energies (the difference between CTLs and band-edge positions). We motivate and generalize these findings with several examples including different defects in monolayer to few-layer h-BN, monolayer ${\mathrm{MoS}}_2$, and graphane. Finally, we show that increasing the number of layers of h-BN systematically lowers defect ionization energies, mainly through CTLs shifting towards vacuum, with conduction band minima kept almost unchanged.

We study identical coupled mechanical resonators whose collective dynamics are fully determined by the patterns in which they are arranged. In this work, we call a system topological if (1) boundary resonant modes fully fill all existing spectral gaps whenever the system is halved, and (2) if the boundary spectrum cannot be removed or gapped by any boundary condition. We demonstrate that such topological characteristics can be induced solely through patterning, in a manner entirely independent of the structure of the resonators and the details of the couplings. The existence of such patterns is proven using $K$ theory and exemplified using an experimental platform based on magnetically coupled spinners. Topological metamaterials built on these principles can be easily engineered at any scale, providing a practical platform for applications and devices.

Au adsorption on high-index Si surfaces is known to form quasi-one-dimensional surface reconstructions with interesting physical properties such as Rashba-split bands and ordered arrays of local magnetic moments. Here we report on a novel family of Au-induced chain systems, hosted on Ge(hhk) substrates. Our study includes Ge(553), Ge(557), and Ge(335) for both bare and Au-adsorbed surfaces, respectively. We employ scanning tunneling microscopy and low-energy electron diffraction to characterize the topography and the occurrence of superstructures. Stable bare surfaces with regularly distributed steps are found for the (553) and (335) orientations. For nominal Ge(557) substrates a refaceting to Ge(223) is observed. Addition of Au tends to promote a change in the surface index which creates Ge(221)-Au, Ge(557)-Au, and Ge(335)-Au surfaces. Their STM images show elements that are reminiscent of the well-understood Si(hhk)-Au surfaces but also reveal important differences.

We formulate and study the general boundary conditions dictating the magnetization profile in the vicinity of an interface between magnets with dissimilar properties. Boundary twists in the vicinity of an edge due to Dzyaloshinskii-Moriya interactions have been first discussed by Wilson et al. [Phys. Rev. B 88, 214420 (2013)] and by Rohart and Thiaville [Phys. Rev. B 88, 184422 (2013)]. We show that in general case the boundary conditions lead to the magnetization profile corresponding to the Néel, Bloch, or intermediate twist. We explore how such twists can be utilized for creation of skyrmions and antiskyrmions, e.g., in a view of magnetic memory applications. To this end, we study various scenarios of how skyrmions and antiskyrmions can be created from interface magnetization twists due to local instabilities. We also show that a judicious choice of Dzyaloshinskii-Moriya tensor (hence a carefully designed material) can lead to local instabilities generating certain types of skyrmions or antiskyrmions. The local instabilities are shown to appear in solutions of the Bogoliubov-de-Gennes equations describing ellipticity of magnon modes bound to interfaces. In one considered scenario, a skyrmion-antiskyrmion pair can be created due to instabilities at an interface between materials with properly engineered Dzyaloshinskii-Moriya interactions. We use micromagnetics simulations to confirm our analytical predictions.

Exceptional points (EPs) are responsible for a number of interesting phenomena in non-Hermitian wave systems. Here we show, both theoretically and experimentally, that EPs can be synthesized in non-Hermitian acoustic systems with unbalanced loss factors in different sections. Numerical simulations and measurements confirm that unidirectional zero reflection, one of the hallmarks of EPs, can be realized in such a compact system with controlled Willis coupling and loss. When approached from one direction in the parameter space, the material slab mimics a conventional parity-time (PT) symmetric system. Our findings provide an efficient way for advanced engineering of scattering properties of artificial acoustic materials with EP-related physics.

Magnetic tunnel junctions employing FeCoB as the ferromagnet and MgO as a spacer layer exhibit high performance and are attractive for magnetic random access memory applications. Upon postdeposition annealing, B is observed to diffuse out of the FeCoB layers inducing crystallization of FeCo. It is known that a large proportion of B escapes into the adjacent tantalum underlayer. While diffusion of B into bulk MgO is known to be unfavorable, it is possible that B could diffuse into grain boundaries (GBs) in the polycrystalline MgO layer, affecting its electronic properties. In this paper, density functional theory is used to investigate the stability and electronic properties of oxygen vacancy and B interstitial defects at MgO GBs. We show that both types of defects exhibit increased stability at the GBs, and we introduce electronic states in the gap that could negatively impact performance. These predictions are consistent with recent experimental results, and we discuss further means to confirm the results experimentally using techniques such as x-ray or ultraviolet photoelectron spectroscopy.

A longstanding controversy remains whether $\gamma \text{−}{\mathrm{B}}_{28}$ is intrinsically superhard or not, i.e., $H_v\ge 40\mathrm{GPa}$. Here we perform comprehensive investigations on the mechanical properties of $\gamma \text{−}{\mathrm{B}}_{28}$ to reveal the plasticity and failure mode of $\gamma \text{−}{\mathrm{B}}_{28}$ through the unique combination of microindentation experiment, the ideal strength approach, and the ab initio informed Peierls-Nabarro model. A low load-invariant hardness of ∼30 GPa is found for both polycrystalline and monocrystalline $\gamma \text{−}{\mathrm{B}}_{28}$. By carefully checking the strength anisotropy and strain facilitated phonon instability, a surprising ideal strength of 23.1 GPa is revealed along the (001)[010] slip system for $\gamma \text{−}{\mathrm{B}}_{28}$, together with an inferior Peierls stress of 3.2 GPa, both of which are close to those of ${\mathrm{B}}_6\mathrm{O}$ and $\mathrm{Zr}{\mathrm{B}}_{12}$ yet much lower than those of diamond and c-BN. These results suggest that $\gamma \text{−}{\mathrm{B}}_{28}$ could not be intrinsically superhard. Atomistic simulation and electronic structure analysis uncover an unprecedented plastic flow channel through the specific ultrasoft bonding, which causes a dramatic softening of $\gamma \text{−}{\mathrm{B}}_{28}$. These findings highlight an approach to quantifying the realistic hardness by means of two plasticity descriptors beyond the elastic limit, i.e., the ideal strength approach and the Peierls-Nabarro model.

Recently, an anisotropic Debye model [Dames et al., Phys. Rev. B 87, 125426 (2013)] was proposed for calculations of the interfacial thermal conductance and the minimum thermal conductivity of graphite-like layered materials. Despite successes of the model in explaining heat transport mechanisms in layered materials (e.g., phonon focusing in highly anisotropic materials), the anisotropic Debye model assumes a phonon dispersion with unrealistic speeds of sound especially for the flexural (ZA) phonons and overestimated cutoffs for all phonon branches. The deficiencies lead to substantially underestimated phonon irradiation for low-frequency phonons. Here, we develop an anisotropic model with truncated linear dispersion that resembles the real phonon dispersion, using speeds of sound derived from elastic constants and cutoff frequencies derived from Brillouin zone boundaries. We also employ a piecewise linear function for the ZA phonons. Our model correctly calculates the phonon irradiation over a wide temperature range, verifying the accuracy of our model. We compare calculations of our and the Dames’ models to measurements of thermal conductivity of graphite and thermal conductance of metal/graphite interfaces, and find that the two models differ significantly for heat transport across the basal planes in graphite even at high temperatures. Our work thus provides a convenient analytical tool to study the phonon transport properties in layered materials.

Using first-principles calculations, this paper systematically investigates the ionization potentials (IPs) and electron affinities (EAs) of nonpolar surfaces of binary divalent metal oxides with formally closed-shell electronic structures, namely, BeO, MgO, CaO, SrO, BaO, ZnO, HgO, SnO, and PbO in relevant crystal structures. An emphasis is put on the understanding of the effects of chemical composition, crystal structure, and surface orientation on the surface band positions. Slab models for nonpolar surfaces are automatically generated using a proposed algorithm that provides a set of unique nonpolar surface orientations. A non-self-consistent dielectric-dependent hybrid functional approach is employed that is shown to provide a significant improvement in the band-gap, IP, and EA evaluation over standard density functional theory calculations using the generalized gradient approximation. The valence band maximum, O $2s$ band center, and O-site local potential versus the vacuum level are examined for prototypical rocksalt, wurtzite, and zincblende surfaces of selected systems without atomic relaxation. All of these quantities are found to qualitatively follow the tendency in the Madelung potential at the O site, where decreasing interatomic distance results in deeper energy levels. The valence band maxima versus the vacuum level or the IPs after atomic relaxation also show a similar trend even when other cation species and crystal structures are included in the discussion, while there is much less correlation for EAs. An overall trend indicates that lattice volume reflecting cation species is a determinant factor to the IPs of the divalent metal oxides investigated in this paper although there are surface-specific dipole contributions that cause the surface orientation dependence of the IPs.

We report the successful growth of sizable crystals of the cuprate superconductors ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$ and ${\mathrm{HgBa}}_2{\mathrm{Ca}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{8+\delta }$. These compounds are well known for their high optimal superconducting critical temperatures of $T_{\mathrm{c}}=128$ K and 134 K at ambient pressure, respectively, and for their challenging synthesis. Using a conventional quartz-tube encapsulation method and a two-layer encapsulation method that utilizes custom-built high-pressure furnaces, we are able to grow single crystals with linear dimensions up to several millimeters parallel to the ${\mathrm{CuO}}_2$ planes. Extended postgrowth annealing is shown to lead to sharp superconducting transitions, indicative of high macroscopic homogeneity. X-ray diffraction and polarized Raman spectroscopy are identified as viable nondestructive methods to help separate the two compounds from synthesis products, as the latter often contain intergrowths of the former. Our work helps to remove obstacles toward the study of these model cuprate systems with experimental probes that require sizable high-quality crystals.

Extremely large positive magnetoresistance (XMR) was found in a nonmagnetic semimetal InBi. Using several single crystals with different residual resistivity ratios ($RRR\mathrm{s}$), we revealed that the XMR strongly depended on the $RRR$ (sample quality). Assuming that there were no changes in effective mass $m^{*}$ and carrier concentrations in these single crystals, this dependence was explained by a semiclassical two-carrier model. First-principle calculations including the spin-orbit interactions (SOIs) unveiled that InBi had a compensated carrier balance and SOI-induced “hidden” three-dimensional (3D) Dirac bands at the $M$ and $R$ points. Because the small $m^{*}$ and the large carrier mobilities will be realized, these hidden 3D Dirac bands should play an important role for the XMR in InBi. We suggest that this feature can be employed as a novel strategy for the creation of XMR semimetals.

Using first-principles calculations, we predict a strong coupling of lattice, charge, and spin degrees of freedom in Ruddlesden-Popper phase ${\mathrm{La}}_3{\mathrm{Ni}}_2{\mathrm{O}}_7$, which enables a phase control with epitaxial strain. While the bulk ground state is metallic, moderate compressive strain is found to trigger a Peierls transition to an insulating state in concurrence with a breathing distortion of ${\mathrm{NiO}}_6$ octahedra. The Peierls transition is microscopically interpreted as a band-gap opening arising from lifting and lowering of the Ni $d-e_g$ states due to the octahedral breathing.

We study from first principles the luminescence of ${\mathrm{Lu}}_2{\mathrm{SiO}}_5$:${\mathrm{Ce}}^{3+}$ (LSO:Ce), a scintillator widely used in medical imaging applications, and establish the crucial role of oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in the generated spectrum. The excitation energy, emission energy, and Stokes shift of its luminescent centers are simulated through a constrained density-functional theory method coupled with a $\mathrm{\Delta }\mathrm{SCF}$ analysis of total energies, and compared with experimental spectra. We show that the high-energy emission band comes from a single Ce-based luminescent center, while the large experimental spread of the low-energy emission band originates from a whole set of different $\mathrm{Ce}-{\mathrm{V}}_{\mathrm{O}}$ complexes together with the other Ce-based luminescent center. Further, the luminescence thermal quenching behavior is analyzed. The $4f-5d$ crossover mechanism is found to be very unlikely, with a large crossing energy barrier ($E_{fd}$) in the one-dimensional model. The alternative mechanism usually considered, namely the electron autoionization, is also shown to be unlikely. In this respect, we introduce a new methodology in which the time-consuming accurate computation of the band gap for such models is bypassed. We emphasize the usually overlooked role of the differing geometry relaxation in the excited neutral electronic state ${\mathrm{Ce}}^{3+,*}$ and in the ionized electronic state ${\mathrm{Ce}}^{4+}$. The results indicate that such electron autoionization cannot explain the thermal stability difference between the high- and low-energy emission bands. Finally, a hole autoionization process is proposed as a plausible alternative. With the already well-established excited-state characterization methodology, the approach to color center identification and thermal quenching analysis proposed here can be applied to other luminescent materials in the presence of intrinsic defects.

Polymeric melon shows real promise to be a next generation photocatalyst material for solar hydrogen production. Because it is earth abundant, and highly stable in photoredox reactions. However, a long-standing problem with polymeric melon is that it is inactive for photocatalytic hydrogen evolution without a metal cocatalyst, such as Pt. Sluggish charge transfer kinetics and a high activation energy for proton reduction makes pristine melon overly dependent on a high loading of precious Pt-cocatalyst for hydrogen evolution. While Pt-assisted hydrogen evolution is not considered economic for scale-up, a modulation of intrinsic photophysical and physicochemical properties of melon is therefore crucial to overcome the Pt dependency. However, to our knowledge, no systematic study has been reported to date to make melon independent of Pt for solar hydrogen evolution. Here we show that the limiting factors of Pt-dependent hydrogen production can be overcome by amorphization. The amorphization lowers the band gap, shifts the proton reduction potential close to the origin, and enhances the light harvesting and charge transport. Therefore, it results in a Pt-free hydrogen evolution under visible light irradiation. We here also present a mechanistic understanding of the origin of this cocatalyst-free hydrogen evolution.

We use atomic force microscopy and grand canonical Monte Carlo simulations to study stick-slip friction of tetrahedral amorphous carbon probes sliding against highly oriented pyrolytic graphite (HOPG) at relative humidities ranging from <1% to near saturation. Friction varies with humidity in a nonmonotonic manner such that water acts as a lubricant only above a threshold humidity; below that threshold, water increases friction substantially relative to dry sliding. Adhesion forces also show a similar nonmonotonic behavior. A nonmonotonic dependence of friction and adhesion on humidity for single-asperity interfaces has previously been attributed to the humidity-dependent adhesion force due to the water meniscus that forms at the contact, which is presumed to increase the solid-solid contact area. However, our simulations show no such increase in solid-solid contact area, but do show a small, continuous increase in tip-sample separation as humidity increases. Experimentally, no significant change in lateral stiffness is observed with humidity. All of this contradicts the hypothesis that the friction increase is due solely to capillary adhesion increasing the contact area. We show that water molecules are present between the tip and sample in increasing numbers as the humidity increases. From this, we attribute the nonmonotonic friction trend to the changing quality of the contact between the water and the substrate, quantified by the number of water molecules in the interface and their registry with the HOPG surface atoms, which in simulations also shows a nonmonotonic trend with humidity. Hysteresis observed in the variation of friction with humidity in both experiments and simulations is explained by the larger energy barrier for surface desorption of water molecules compared to adsorption.