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.

Soft x-ray photoemission electron microscopy with an in situ magnetic field has been used to study the relationship between ferromagnetic and antiferromagnetic spin alignment and the switching/reversal field of epitaxial micromagnetic structures. We investigated a model system consisting of a bilayer of ferromagnetic $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ and antiferromagnetic $\mathrm{LaFe}{\mathrm{O}}_3$ where the spin axes in each layer can be driven from mutually perpendicular (spin-flop) to parallel alignment by varying the temperature between 30 and 300 K. Results show that not only does this spin alignment noticeably influence the bilayer micromagnet coercivity compared to $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_3$ single-layer micromagnets, but the coercivity within this materials system can be tuned over a wide range by careful balance of material properties.

The existence of an intermediate phase of tin oxide was first reported in 1882. However, its stoichiometry and its crystal structure have been dubious and heavily debated ever since, despite a multitude of structural investigations. We show that Raman spectroscopy combined with ab initio theory offers a viable alternative for structure determination in cases where diffraction studies are inconclusive. We unambiguously identify the intermediate phase as ${\mathrm{Sn}}_3{\mathrm{O}}_4$ and rule out the other likely candidate, ${\mathrm{Sn}}_2{\mathrm{O}}_3$. We assign the one-phonon Raman signals of ${\mathrm{Sn}}_3{\mathrm{O}}_4$ to the mode symmetries of the corresponding point group $C_{2h}$ and confirm that $P{2}_1/c$ with 14 atoms per unit cell is the space group of ${\mathrm{Sn}}_3{\mathrm{O}}_4$.

We address the issue of achieving high oxide ion conductivity at low temperatures $\left(\le 500{}^{\circ }\mathrm{C}\right)$ in stable electrolytes for solid oxide fuel cell (SOFC) techonology. We report here a high oxide ion conductivity $(\sim {10}^{-2}\mathrm{S}\mathrm{c}{\mathrm{m}}^{-1})$ in the La substituted yttrium iron garnets $\mathrm{L}{\mathrm{a}}_x{\mathrm{Y}}_{3\text{−}x}\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12+\delta }$ at low temperatures $(\sim 450{}^{\circ }\mathrm{C})$. The judicious substitution of high electropositive $\mathrm{L}{\mathrm{a}}^{3+}$ ions in the yttrium iron garnet $(x\le 0.45)$ affords continuous and easy oxide ion conduction pathways through polyhedral networks even at a low temperature of $\sim 127{}^{\circ }\mathrm{C}$. The conduction pathways have been visualized from the maximum entropy method analyzed soft bond valence sum distribution, obtained from neutron diffraction data. Our study also shows that these iron garnets have high structural stability over a broad range of temperature (at least over $27\text{–}800{}^{\circ }\mathrm{C})$, and compatible thermal expansion coefficient $(\sim 10.6\times {10}^{-6}{}^{\circ }{\mathrm{C}}^{-1})$ with that of the commonly used interconnect and electrode SOFC materials. We reveal that the oxide ion conduction in the present garnet is based on an excess oxide ion concentration mechanism. The observed high oxide-ion conductivity at low temperatures in the present trivalent substituted garnets is a remarkable finding for the development of an efficient SOFC technology.

We study an insulator-metal transition in a ternary chalcogenide glass $({\mathrm{GeSe}}_3{)}_{1-x}{\mathrm{Ag}}_x$ for $x=0.15$ and 0.25. The conducting phase of the glass is obtained by using gap sculpting [Prasai et al., Sci. Rep. 5, 15522 (2015)] and it is observed that the metallic and insulating phases have nearly identical density functional energies but have a conductivity contrast of $\sim {10}^8$. As such, we demonstrate an example of polyamorphism for which energetically close phases exhibit dramatically different optical properties. The transition from insulator to metal involves growth of an Ag-rich phase accompanied by a depletion of tetrahedrally bonded $\mathrm{Ge}{\left({\mathrm{Se}}_{1/2}\right)}_4$ in the host network. The relative fraction of the amorphous ${\mathrm{Ag}}_2\mathrm{Se}$ phase and ${\mathrm{GeSe}}_2$ phase is shown to be a critical determinant of dc conductivity.

Recently, the fabrication of CdSe nanoplatelets became an important research topic. Nanoplatelets are often described as having a similar electronic structure as two-dimensional quantum wells and are promoted as colloidal quantum wells with monolayer precision width. In this paper we show that nanoplatelets are not ideal quantum wells, but cover depending on the size: the strong confinement regime, an intermediate regime, and a Coulomb dominated regime. Thus, nanoplatelets are an ideal platform to study the physics in these regimes. Therefore, the exciton states of the nanoplatelets are numerically calculated by solving the full four-dimensional Schrödinger equation. We compare the results with approximate solutions from semiconductor quantum well and quantum dot theory. The paper can also act as review of these concepts for the colloidal nanoparticle community.

The quasi-two-dimensional electron gas at oxide interfaces provides a platform for investigating quantum phenomena in strongly correlated electronic systems. Here, we study the transport properties at the high-mobility $\left(\mathrm{L}{\mathrm{a}}_{0.3}\mathrm{S}{\mathrm{r}}_{0.7}\right)\left(\mathrm{A}{\mathrm{l}}_{0.65}\mathrm{T}{\mathrm{a}}_{0.35}\right){\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface. Before oxygen annealing, the as-grown interface exhibits a high electron density and electron occupancy of two subbands: higher-mobility electrons (${\mu }_1\approx {10}^4\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 2 K) occupy the lower-energy $3d_{\mathrm{xy}}$ subband, while lower-mobility electrons (${\mu }_1\approx {10}^3\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 2 K) propagate in the higher-energy $3d_{\mathrm{xz}/\mathrm{yz}}$-dominated subband. After removing oxygen vacancies by annealing in oxygen, only a single type of $3d_{\mathrm{xy}}$ electrons remain at the annealed interface, showing tunable Shubnikov–de Haas oscillations below 9 T at 2 K and an effective mass of $0.7m_{\mathrm{e}}$. By contrast, no oscillation is observed at the as-grown interface even when electron mobility is increased to $50000\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ by gating voltage. Our results reveal the important roles of both carrier mobility and subband occupancy in tuning the quantum transport at oxide interfaces.