We present the successful synthesis of single-atom-thick borophene nanoribbons (BNRs) by self-assembly of boron on Ag(110) surface. The scanning tunneling microscopy (STM) studies reveal high quality BNRs: all the ribbons are along the $[-110]$ direction of Ag(110), and can run across the steps on the surface. The width of ribbons is distributed in a narrow range around $10.3\pm 0.2\mathrm{nm}$. High resolution STM images revealed four ordered surface structures in BNRs. Combined with DFT calculations, we found that all four structures of boron nanoribbons consist of the boron chains with different widths, separated by hexagonal hole arrays. The successful synthesis of BNRs enriches the low dimensional allotrope of boron and may promote further applications of borophene.

We present results on the growth of CdTe-HgTe core-shell nanowires, a realization of a quasi-one-dimensional heterostructure of the topological insulator HgTe. The growth is a two step process consisting of the growth of single crystalline zinc blende CdTe nanowires with the vapor-liquid-solid method and the overgrowth of these wires with HgTe such that a closed shell is formed around the CdTe core structure. The CdTe wire growth is monitored by RHEED allowing us to infer information on the crystal properties from the electron diffraction pattern. This information is used to find and control the optimal growth temperature. High quality single crystal CdTe nanowires grow with a preferred orientation. For the growth of the conductive HgTe shell structure we find that the supplied Hg:Te ratio is the crucial parameter to facilitate growth on all surface facets.

Flux growth of single crystals is normally performed in a vertical configuration with an upright refractory container holding the flux melt. At high temperatures, flux dissolves the charge, forming a homogeneous solution before nucleation and growth of crystals takes place under proper supersaturation generated by cooling or evaporating the flux. In this work, we report flux growth in a horizontal configuration with a temperature gradient along the horizontal axis: a liquid transport growth analogous to the vapor transport technique. In a typical liquid transport growth, the charge is kept at the hot end of the refractory container and the flux melt dissolves the charge and transfers it to the cold end. Once the concentration of charge is above the solubility limit at the cold end, the thermodynamically stable phase nucleates and grows. Compared to the vertical flux growth, the liquid transport growth can provide a large quantity of crystals in a single growth since the charge/flux ratio is not limited by the solubility limit at the growth temperature. This technique is complementary to the vertical flux growth and can be considered when a large amount of crystals is needed but the yield from the conventional vertical flux growth is limited. We applied this technique to the growth of ${\mathrm{IrSb}}_3$, ${\mathrm{Mo}}_3{\mathrm{Sb}}_7$, and MnBi from self-flux, and the growth of FeSe, ${\mathrm{CrTe}}_3$, ${\mathrm{NiPSe}}_3$, ${\mathrm{FePSe}}_3$, ${\mathrm{CuInP}}_2{\mathrm{S}}_6$, ${\mathrm{RuCl}}_3$, and ${\mathrm{OsCl}}_4$ from a halide flux.

${\mathrm{BaBiO}}_3$ is a mixed-valence perovskite, which escapes the metallic state through a Bi valence (and Bi-O bond) disproportionation or CDW distortion, resulting in a semiconductor with a gap of 0.8 eV at zero pressure. The evolution of structural and electronic properties at high pressure is, however, largely unknown. Pressure, one might have hoped, could reduce the disproportionation, making the two Bi ions equivalent and bringing the system closer to metallicity or even to superconductivity, such as is attained at ambient pressure upon metal doping. We address the high-pressure phase diagram of pristine ${\mathrm{BaBiO}}_3$ by ab initio DFT calculations based on GGA and hybrid functionals in combination with crystal structure prediction methods based on evolutionary algorithms, molecular dynamics, and metadynamics. The calculated phase diagram from 0 to 50 GPa indicates that pristine ${\mathrm{BaBiO}}_3$ resists metallization under pressure, undergoing instead at room temperature structural phase transitions from monoclinic I2/m to nearly tetragonal P-1 at 7 GPa, possibly to monoclinic C2/m at 27 GPa, and to triclinic P1 at 43 GPa. Remarkably, all these phases sustain and in fact increase the inequivalence of two Bi neighboring sites and of their Bi-O bonds and, in all cases except semimetallic C2/m, the associated insulating character. We then present high-pressure resistivity data, which generally corroborate these results, and show that the insulating character persists at least up to 80 GPa, suggesting that the C2/m phase is probably an artifact of the small computational cell.

The energetics and atomic structures of $\mathrm{\Sigma }3\left[1\overline{1}0\right]\left(111\right)$ grain boundary (GB) of body-centered cubic (bcc) Ti-Mo and Ti-V alloys are investigated using density-functional-theory calculations and virtual crystal approximation. The electron density in bcc structure and the atomic displacements and excess energy of the GB are correlated to bcc-$\omega$ phase stability. Model calculations based on pairwise interplanar interactions successfully reproduce the chemical part of GB energy. The chemical GB energy can be expressed as a sum of excess pairwise interactions between bcc (111) layers, which are obtained from Gaussian elimination of the total energies of a number of periodic structures. The energy associated with the relaxation near the GB is solved by numerical minimization using the derivatives of the excess interactions. Anharmonic interlayer interactions are necessary for obtaining accurate relaxation energy and excess GB volume from model calculations. The effect of GB on vibrational spectrum is also investigated. Segregation energies of B and Y to a substitutional site on the GB plane are calculated. Preliminary results suggest that Y tends to segregate, while B tends to antisegregate.

We have used soft x-ray photoemission electron microscopy to image the magnetization of single-domain $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ nanoislands arranged in geometrically frustrated configurations such as square ice and kagome ice geometries. Upon thermal randomization, ensembles of nanoislands with strong interisland magnetic coupling relax towards low-energy configurations. Statistical analysis shows that the likelihood of ensembles falling into low-energy configurations depends strongly on the annealing temperature. Annealing to just below the Curie temperature of the ferromagnetic film ($T_{\mathrm{C}}=338\mathrm{K}$) allows for a much greater probability of achieving low-energy configurations as compared to annealing above the Curie temperature. At this thermally active temperature of 325 K, the ensemble of ferromagnetic nanoislands explore their energy landscape over time and eventually transition to lower energy states as compared to the frozen-in configurations obtained upon cooling from above the Curie temperature. Thus, this materials system allows for a facile method to systematically study thermal evolution of artificial spin ice arrays of nanoislands at temperatures modestly above room temperature.

In the present work we suggest a recipe for finding tetragonal Heusler compounds with perpendicular magnetic anisotropy (PMA) that also exhibit large tunneling magnetoresistance (TMR) when used as electrodes in magnetic tunnel junction devices with suitable tunneling barrier materials. We performed density-functional theory calculations for 286 Heusler compounds and identified 116 stable tetragonal compounds. Ten of these compounds are predicted to have strong PMA and, simultaneously, exponentially increasing TMR with increasing tunneling barrier thickness due to the so-called Brillouin zone spin filtering effect. Experimental measurements performed for 25 Heusler compounds theoretically identified as tetragonal show that ten of these compounds indeed have tetragonal structure with PMA.

The zone-center phonons are calculated for yellow phase ${\mathrm{CsSnI}}_3$ using density functional perturbation theory in a plane-wave pseudopotential method. The infrared absorption and reflection spectra are simulated and show that the absorption has a strong contribution from the LO as well as TO modes. The polarization-dependent Raman spectra for various configurations are simulated. The results for the Raman and the infrared reflection spectra are found to be in good agreement with the available experimental data when averaging over directions. The Born effective charges and high- and low-frequency dielectric tensors are calculated. The ratio of the static to the high-frequency dielectric constants is high in the $y$ direction.

We present a detailed characterization of the growth dynamics of Ga(Al)As(111)A surfaces. We develop a theoretical growth model that well describes the observed behavior on the growth parameters and underlines the Ehrlich-Schwöbel barrier as a leading factor that determines the growth dynamics. On such basis we analyze the factors that lead to the huge observed roughness on such surface orientations and we identify the growth conditions that drive the typical three-dimensional growth of Ga(Al)As(111)A towards an atomically flat surface. ${\mathrm{GaAs}/\mathrm{Al}}_{0.30}{\mathrm{Ga}}_{0.70}\mathrm{As}$ quantum wells realized on an optimized surface ($<0.2$ nm roughness) show a record low emission linewidth of 4.5 meV.

Shifting the Fermi energy in solids by doping, defect formation, or gating generally results in changes in the charge density distribution, which reflect the ability of the bonding pattern in solids to adjust to such external perturbations. In the traditional chemistry textbook, such changes are often described by the formal oxidation states (FOS) whereby a single atom type is presumed to absorb the full burden of the perturbation (change in charge) of the whole compound. In the present paper, we analyze the changes in the position-dependence charge density due to shifts of the Fermi energy on a general physical basis, comparing with the view of the FOS picture. We use the halide perovskites $\mathrm{CsSn}{\mathrm{X}}_3$ ($\mathrm{X}=\mathrm{F}$, Cl, Br, I) as examples for studying the general principle. When the solar absorber $\mathrm{CsSn}{\mathrm{I}}_3$ (termed 113) loses $50\%$ of its Sn atoms, thereby forming the ordered vacancy compound $\mathrm{C}{\mathrm{s}}_2\mathrm{Sn}{\mathrm{I}}_6$ (termed 216), the Sn is said in the FOS picture to change from Sn(II) to Sn(IV). To understand the electronic properties of these two groups we studied the 113 and 216 compound pairs $\mathrm{CsSnC}{\mathrm{l}}_3$ and $\mathrm{C}{\mathrm{s}}_2\mathrm{SnC}{\mathrm{l}}_6, \mathrm{CsSnB}{\mathrm{r}}_3$ and $\mathrm{C}{\mathrm{s}}_2\mathrm{SnB}{\mathrm{r}}_6$, and $\mathrm{CsSn}{\mathrm{I}}_3$ and $\mathrm{C}{\mathrm{s}}_2\mathrm{Sn}{\mathrm{I}}_6$, complementing them by $\mathrm{CsSn}{\mathrm{F}}_3$ and $\mathrm{C}{\mathrm{s}}_2\mathrm{Sn}{\mathrm{F}}_6$ in the hypothetical cubic structure for completing the chemical trends. These materials were also synthesized by chemical routes and characterized by x-ray diffraction, ${}^{119}\mathrm{Sn}$-Mössbauer spectroscopy, and $K$-edge x-ray absorption spectroscopy. We find that indeed in going from 113 to 216 (equivalent to the introduction of two holes per unit) there is a decrease in the $s$ charge on Sn, in agreement with the FOS picture. However, at the same time, we observe an increase of the $p$ charge via downshift of the otherwise unoccupied $p$ level, an effect that tends to replenish much of the lost $s$ charge. At the end, the change in the charge on the Sn site as a result of adding two holes to the unit cell is rather small. This effect is theoretically explained as a “self-regulating response” [Raebiger, Lany, and Zunger, Nature (London) 453, 763 (2008)] whereby the system rehybridizes to minimize the effect of the charge perturbation created by vacancy formation. Rather than having a single preselected atom (here Sn) absorb the full brunt of the perturbation producing two holes, we find that the holes are distributed in a complex pattern throughout the octahedral systems of ${\mathrm{X}}_6$ ligands, forming hole orbitals with some specific symmetries. This clarifies the relation between FOS and charge transfer that can be applied to a wide variety of materials.

A microscopic understanding of fast ionic transport is fundamental to design novel solid-state electrolytes. We address the role of correlations in these systems and study in detail the tracer and charge diffusion coefficients, deriving a novel inequality between these two quantities. We investigate the failure of the Nernst-Einstein and the physical consequences of a nontrivial Haven ratio with extensive first-principles molecular dynamics in the fast ion conductor ${\text{Li}}_{10}{\text{GeP}}_2{\text{S}}_{12}$. Last, we show that the approximate tracer diffusion still provides accurate activation free energies.

Recent advances in active soft structures envision the large deformations resulting from mechanical instabilities as routes for functional shape morphing. Numerous such examples exist for filamentary and plate systems. However, examples with double-curved shells are rarer, with progress hampered by challenges in fabrication and the complexities involved in analyzing their underlying geometrical nonlinearities. We show that on-demand patterning of hemispherical shells can be achieved through constrained buckling. Their postbuckling response is stabilized by an inner rigid mandrel. Through a combination of experiments, simulations, and scaling analyses, our investigation focuses on the nucleation and evolution of the buckling patterns into a reticulated network of sharp ridges. The geometry of the system, namely, the shell radius and the gap between the shell and the mandrel, is found to be the primary ingredient to set the surface morphology. This prominence of geometry suggests a robust, scalable, and tunable mechanism for reversible shape morphing of elastic shells.

Atomic sized two-level systems (TLSs) in amorphous dielectrics are known as a major source of loss in superconducting devices. In addition, individual TLSs are known to induce large frequency shifts due to strong coupling to the devices. However, in the presence of a broad ensemble of TLSs these shifts are symmetrically canceled out and not observed in a typical single-tone spectroscopy experiment. We introduce a two-tone spectroscopy on the normal modes of a pair of coupled superconducting coplanar waveguide resonators to reveal this effect. Together with an appropriate saturation model this enables us to extract the average single-photon Rabi frequency of dominant TLSs to be ${\mathrm{\Omega }}_0/2\pi \approx 79$ kHz. At high photon numbers we observe an enhanced frequency shift due to nonlinear kinetic inductance when using the two-tone method and estimate the value of the nonlinear coefficient as $K/2\pi \approx -1\times {10}^{-4}$ Hz/photon. Furthermore, the lifetime of each resonance can be controlled (increased) by pumping of the other mode as demonstrated both experimentally and theoretically.

We present a comprehensive study of the complex interface between perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) and the (111) surfaces of the three coinage metals. The specific structural, electronic, and chemical properties of the interface rendered by the different substrate reactivities are monitored with low-energy electron diffraction (LEED), x-ray standing waves (XSW), and ultraviolet and x-ray photelectron spectroscopy (UPS and XPS). In particular, the balance between molecule-substrate and molecule-molecule interactions is considered when interpreting the core-level spectra of the different interfaces. By presenting additional adsorption distances of the unsubstituted perylene, we show that the molecular functionalization via end groups with acceptor character facilitates the charge transfer from the substrate but it is not directly responsible for the associated short adsorption distances, demonstrating that this frequently assumed correlation is not necessarily correct.

We compute the structural energies, elastic constants, and stacking fault energies, and investigate the phase stability of monoborides with different compositions $\left(X_{1-x}^1{X}_x^2\right)\text{B}$ $\left(X=\text{Ti/Fe/Mo/Nb/V}\right)$ using density functional theory in order to search for Ti monoborides with improved mechanical properties. Our computed Young's modulus and Pugh's modulus ratio, which correlate with stiffness and toughness, agree well with predictions from Vegard's law with the exceptions of mixed monoborides containing Mo and Fe. Among all the monoborides considered in this paper, TiB has the smallest Pugh's ratio, which suggests that the addition of solutes can improve the toughness of a Ti matrix. When $X^1\mathrm{B}$ and $X^2\mathrm{B}$ are respectively most stable in the $B_{27}$ and $B_f$ structures, the mixed monoborides $\left(X_{1-x}^1{X}_x^2\right)\text{B}$ have a lower or similar stacking fault energy than TiB and could therefore improve the ductility of the Ti matrix. Among all $\left(X_{0.5}^1{X}_{0.5}^2\right)\text{B}$, mixed $\left({\mathrm{Ti}}_{0.5}{\mathrm{Mo}}_{0.5}\right)\text{B}$ and mixed $\left({\mathrm{Ti}}_{0.5}{\mathrm{V}}_{0.5}\right)\text{B}$ have a higher Young's modulus, a higher Pugh's ratio, and a smaller stacking fault energy than TiB. We also construct phase diagrams and find large solubility limits for solid solutions containing Ti compared to those containing Fe.

Few layer bismuth nanofilms with (111) orientation have shown striking electronic properties, especially as building blocks of novel two-dimensional heterostructures. In this paper we present state-of-the-art first principles calculations, based on both density functional theory and maximally localized Wannier functions, that encompass electronic and structural properties of free-standing Bi(111) nanofilms. We accurately evaluate both the in-plane lattice constant and, by including the van der Waals interaction between bismuth bilayers, the intra/interlayer distances. Interestingly and somehow unexpectedly, the in-plane lattice constant is predicted to shrink by about 5% going from the thickest investigated nanofilm $\left(\sim 80\AA \right)$ to single bilayer Bi(111), entailing a thickness dependent lattice mismatch in complex heterostructures involving ultrathin Bi(111). Moreover, quantum confinement effects, that would be expected to rule the electronic structure at this size range, compete with surface states that appear close to and across the Fermi level. The implication is that not only all but the thinnest films have a metallic band structure but also that such surface states might play a role in either the formation of interfaces with other materials or for sensing applications. Finally, the calculated electronic structure compares extremely well with ARPES measurements.