High-level vibrational calculations have been used to investigate anharmonicity in a wide variety of materials using density functional theory methods. We have developed an efficient approach for describing strongly anharmonic systems using a vibrational self-consistent field method. By far the most computationally expensive part of the calculations is the mapping of an accurate Born-Oppenheimer (BO) energy surface within the region of interest. Here we present an improved method which reduces the computational cost of the mapping. In this approach we use data from a set of energy calculations for different vibrational distortions of the materials and the corresponding forces on the atoms. Results using both energies and forces are presented for the test cases of the hydrogen molecule, solid hydrogen under high pressure including mapping of two-dimensional subspaces of the BO surface, and the bcc phases of the metals Li and Zr. The use of force data speeds up the anharmonic calculations by up to 40%.

Herein we show using theoretical predictions that carbon monosulfide compounds exhibit a variety of layered nanostructures, such as chain arrays, monolayers, and thin films. We show that semiconductor chain arrays are the most stable because they are mainly dimensionality driven by $s{p}^2$ hybridization of the carbon orbitals. In contrast to the thin films, the monolayers are stable at room temperature in a semiconductor phase, which is followed in energy by a metallic phase. Moreover, we study a semiconductor-to-metal phase transition in the carbon monosulfide monolayers by strain engineering to control the conductivity and carrier mobility.

We perform first-principles calculations to predict the morphology of $L{1}_0$ ordered FePt nanoparticles grown on Mg(Ti)O substrates with relevance to application in heat-assisted magnetic recording (HAMR) media. We show how incorporation of Ti into MgO substrates reduces the FePt adhesion energy from $-1.29$ (pure MgO) to $-2.35\mathrm{J}/{\mathrm{m}}^2$ (pure TiO). This effect is due to the formation of strong Fe-Ti bonds at the interface. Consistent with experimental observations, the predicted equilibrium morphology of supported FePt nanoparticles is significantly changed, corresponding to increased wetting. This behavior is undesirable for HAMR media since it promotes grain growth which limits the storage density. We show how passivation of surface Ti atoms (e.g., with MgO) is sufficient to restore the wetting observed for pure MgO substrates offering a viable strategy for optimization of next generation recording media.

Iron tungstate $\left({\mathrm{FeWO}}_4\right)$ and manganese tungstate $\left({\mathrm{MnWO}}_4\right)$ belong to a family of wolframite-type materials that has applications in various areas, including supercapacitors, batteries, and multiferroics. A detailed understanding of bulk properties and defect physics in these transition-metal tungstates has been lacking, however, impeding possible improvement of their functional properties. Here, we report a first-principles study of ${\mathrm{FeWO}}_4$ and ${\mathrm{MnWO}}_4$ using screened hybrid density-functional calculations. We find that in both compounds the electronic structures near the band edges are predominantly the highly localized transition-metal $d$ states, which allows for the formation of both hole polarons at the Fe (Mn) sites and electron polarons at the W sites. The dominant native point defects in ${\mathrm{FeWO}}_4$ $\left({\mathrm{MnWO}}_4\right)$ under realistic synthesis conditions are, however, the hole polarons at the Fe (Mn) sites and negatively charged Fe (Mn) vacancies. The presence of low-energy and highly mobile polarons provides an explanation for the good $p$-type conductivity observed in experiments and the ability of the materials to store energy via a pseudocapacitive mechanism.

Enhanced pinning due to nanoscale strain is unique to the high-$T_{\mathrm{c}}$ cuprates, where pairing may be modified with lattice distortion. Therefore a comprehensive understanding of the defect landscape is required for a broad range of applications. However, determining the type and distribution of defects and their associated strain constitutes a critical task, and for this aim, real-space techniques for atomic resolution characterization are necessary. Here, we use scanning transmission electron microscopy (STEM) to study the atomic structure of individual defects of solution-derived $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_7$ (YBCO) nanocomposites, where the inclusion of incoherent secondary phase nanoparticles within the YBCO matrix dramatically increases the density of ${\mathrm{Y}}_1\mathrm{B}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_4{\mathrm{O}}_8$ (Y124) intergrowths, the commonest defect in YBCO thin films. The formation of the Y124 is found to trigger a concatenation of strain-derived interactions with other defects and the concomitant nucleation of intrinsic defects, which weave a web of randomly distributed nanostrained regions that profoundly transform the vortex-pinning landscape of the YBCO nanocomposite thin films.

We present a cluster model to describe the localization of hot charge carriers on the Si(111)-$7\times 7$ surface, which leads to (nonlocal) desorption of chlorobenzene molecules in scanning tunneling microscope (STM) manipulation experiments. The localized charge carriers are modeled by a small cluster. By means of quantum chemical calculations, this cluster model explains many experimental findings from STM manipulation. We show that the negative charge is mainly localized in the surface, while the positive one also resides on the molecule. Both resonances boost desorption: In the negative resonance the adatom is elevated; in the positive one the chemisorption bond between the silicon surface adatom and chlorobenzene is broken. We find normal modes promoting desorption matching experimental low-temperature activation energies for electron- and hole-induced desorption.

Using scanning tunneling microscopy, we characterize a surface carbide that forms when Dy is deposited on the basal plane of graphite. To form carbide islands on terraces, Dy is first deposited at 650–800 K, which forms large metallic islands. Upon annealing at 1000 K, these clusters convert to carbide. Deposition directly at 1000 K is ineffective because nucleation on terraces is inhibited. Reaction is signaled by the fact that each carbide cluster is partially or totally surrounded by an etch pit. The etch pit is one carbon layer deep for most carbide clusters. Carbide clusters are also identifiable by striations on their surfaces. Based on mass balance, and assuming that only the surface layer of carbon is involved in the reaction, the carbide has stoichiometry $\mathrm{D}{\mathrm{y}}_2\mathrm{C}$. This is Dy-rich compared with the most common bulk carbide $\mathrm{Dy}{\mathrm{C}}_2$, which may reflect limited surface carbon transport to the carbide.

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.