The local structure of ${\mathrm{K}}_{0.5}{\mathrm{Na}}_{0.5}\mathrm{Nb}{\mathrm{O}}_3$ is investigated using first-principles methods with an optimized special quasirandom structure (SQS). Through a comparison of the computed pair distribution functions with those from neutron powder diffraction data, the SQS approach demonstrates its ability to accurately capture the local structure patterns derived from the random distribution of K and Na on the perovskite $A$-site. Using these structures, local variations in Na-O interactions are suggested to be the driving force behind the $R3c$ to $Pm$ phase transition. A comparison between the SQS and a rocksalt structure shows the inability of the latter to account for the local variability present in a random solid solution. As such, the predictive nature of the SQS demonstrated here suggests that this approach may provide insight in understanding the properties of a wide range of bulk oxide alloys or solid solutions.

The method of in situ high-pressure neutron diffraction is used to investigate the structure of ${\mathrm{B}}_2{\mathrm{O}}_3$ glass on compression in the range from ambient to 17.5(5) GPa. The experimental results are supplemented by molecular dynamics simulations made using a newly developed aspherical ion model. The results tie together those obtained from other experimental techniques to reveal three densification regimes. In the first, ${\mathrm{BO}}_3$ triangles are the predominant structural motifs as the pressure is increased from ambient to 6.3(5) GPa, but there is an alteration to the intermediate range order which is associated with the dissolution of boroxol rings. In the second, ${\mathrm{BO}}_4$ motifs replace ${\mathrm{BO}}_3$ triangles at pressures beyond 6.3 GPa and the dissolution of boroxol rings continues until it is completed at 11–14 GPa. In the third, the B-O coordination number continues to increase with pressure to give a predominantly tetrahedral glass, a process that is completed at a pressure in excess of 22.5 GPa. On recovery of the glass to ambient from a pressure of 8.2 GPa, triangular ${\mathrm{BO}}_3$ motifs are recovered but, relative to the uncompressed material, there is a change to the intermediate range order. The comparison between experiment and simulation shows that the aspherical ion model is able to provide results of unprecedented accuracy at pressures up to at least 10 GPa.

Considering that various investigations identified a correlation between the interstitial electron density in crystalline metals and some ground-state properties, including the compressibility, we propose a procedure to estimate the interstitial electron density in liquid metals starting from the experimental static structure factor. From the calculated electron density, starting from the standard approximation, which describes a liquid metal as made up of a homogeneous classic ion plasma with Coulomb interaction and a homogeneous interacting electron gas, we determine the ion collective mode velocity. The so-derived collective mode velocity is compared to the experimental data and a coherent view in different metallic systems at the melting point is obtained. Some guess about the collective mode damping is also presented because of the connection to the local static fluctuations of the interstitial electron density.

We apply a homogenization process to the acoustic velocity potential wave equation. The study of various examples shows that the resulting effective properties are different from those of the homogenized pressure wave equation for the same underlying acoustic parameters. A careful analysis reveals that a given set of inhomogeneous parameters represents an entirely different physical system depending on the considered equation. Our findings unveil a different way of tailoring acoustic properties through gradients of the static pressure. In contrast to standard metafluids based on isobaric composites, this alternative kind of metafluid is suitable for the implementation of transformational devices designed via the velocity potential equation. This includes acoustic systems in a moving background or arising from general space-time transformations. As an example, we design a device able to cloak the acoustic velocity potential.

We introduce a one-dimensional plaquette orbital model with a topology of a ladder and alternating interactions between $x$ and $z$ pseudospin components both along the ladder legs and on the rungs. We show by an exact transformation that it is equivalent to an effective spin model in a magnetic field, with spin dimers that replace plaquettes and are coupled along the chain by three-spin interactions in the Hilbert space reduced by a factor of 2. Using perturbative treatment and mean-field approaches with dimer correlations we study the ground state spin configuration and its defects in the lowest excited states. By the exact diagonalization approach we establish that the quantum effects are of purely short-range nature in the model and we find estimated values of the ground state energy and the gap in the thermodynamic limit from the system sizes up to $L=12$ dimers. Finally, we study a class of excited states with classical-like defects accumulated in the central region of the chain to find that in this region the quantum entanglement measured by the mutual information of neighboring dimers is locally increased and coincides with disorder and frustration. Such islands of entanglement in an otherwise rather classical system may be of interest in the context of quantum computing devices.

We study scattering of charge and spin excitations in a system of interacting electrons in one dimension. At low densities, electrons form a one-dimensional Wigner crystal. To a first approximation, the charge excitations are the phonons in the Wigner crystal, and the spin excitations are described by the Heisenberg model with nearest-neighbor exchange coupling. This model is integrable and thus incapable of describing some important phenomena, such as scattering of excitations off each other and the resulting equilibration of the system. We obtain the leading corrections to this model, including charge-spin coupling and the next-nearest-neighbor exchange in the spin subsystem. We apply the results to the problem of equilibration of the one-dimensional Wigner crystal and find that the leading contribution to the equilibration rate arises from scattering of spin excitations off each other. We discuss the implications of our results for the conductance of quantum wires at low electron densities.

We unveil nontrivial topological properties of zero-temperature magnetization plateau states in periodically modulated quantum spin chains under a uniform magnetic field. As positions of plateaus are uniquely determined by the modulation period of exchange couplings, we find that the topologically nontrivial plateau state can be characterized by a nonzero integer Chern number and has nontrivial edge excitations. Our study clarifies the topological origin of the well-known phenomena of quantized magnetization plateaus in one-dimensional quantum spin systems and relates the plateau state to the correlated topological insulator.

Defects in the rutile $\mathrm{Ti}{\mathrm{O}}_2$ structures have been extensively studied, but the intrinsic defects of the oxygen-deficient ${\mathrm{Ti}}_n{\mathrm{O}}_{2n-1}$ phases have not been given the same amount of consideration. Those structures, known as Magnéli phases, are characterized by the presence of ordered planes of oxygen vacancies, also known as shear planes, and it has been shown that they form conducting channels inside TiO-based memristor devices. Memristors are excellent candidates for a new generation of memory devices in the electronics industry. In this paper we present density-functional-theory–based electronic structure calculations for ${\mathrm{Ti}}_n{\mathrm{O}}_{2n-1}$ Magnéli structures using $\text{PBESol}+U$ ($0\le U\le 5$ eV) and Heyd-Scuseria-Ernzerhof functionals, showing that intrinsic defects present in these structures are responsible for the appearance of states inside the band gap, which can act as intrinsic dopants for the enhanced conductivity of $\mathrm{Ti}{\mathrm{O}}_2$ memristive devices.

We present a flexible model for a generic straight transport barrier in graphene in the specular limit. The specular limit applies for scattering of carriers sufficiently close to the Fermi level in three quarters of the transport barriers. Using the Lippmann-Schwinger equation, we obtain the wave function, from which we derive the reflection and transmission probabilities as a function of the angle of incidence. The results, which are compared to those from exact multichannel tight-binding quantum transport calculations, show that some barriers exhibit a broader absence of reflection across a wide range of incident angles, whereas other barriers are highly reflective. The power of our model is that it could be used even when the precise atomistic detail of the transport barriers is unknown.

We have controlled the amount of charged defects in MgO thin films grown on Nb-doped ${\mathrm{SrTiO}}_3$ $(\text{Nb}:{\mathrm{SrTiO}}_3)$ substrate by pulsed laser deposition at room temperature. The work function of $\text{MgO}/\text{Nb}:{\mathrm{SrTiO}}_3$ varies from ∼2 to ∼5 eV depending on the partial oxygen pressure during film growth. The increase in the work function under the oxygen-rich deposition condition is consistent with the dipole moment formation at the interface, which is given by negatively charged ${\mathrm{V}}^{-}$ centers in MgO thin films and the corresponding image charges in the conductive substrate. That the formation of charged defects in an insulating capping layer can induce a strong modulation of work function as large as ∼3 eV implies that controlling charged defects could be an effective method to design the electronic properties of oxide surfaces.

Depth-sensitive magnetic, structural, and chemical characterization is important in the understanding and optimization of physical phenomena emerging at the interfaces of transition metal oxide heterostructures. In a simultaneous approach we have used polarized neutron and resonant x-ray reflectometry to determine the magnetic profile across atomically sharp interfaces of ferromagnetic ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$/multiferroic ${\mathrm{BiFeO}}_3$ bilayers with subnanometer resolution. In particular, the x-ray resonant magnetic reflectivity measurements at the Fe and Mn resonance edges allowed us to determine the element-specific depth profile of the ferromagnetic moments in both the ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ and ${\mathrm{BiFeO}}_3$ layers. Our measurements indicate a magnetically diluted interface layer within the ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}{\mathrm{MnO}}_3$ layer, in contrast to previous observations on inversely deposited layers [P. Yu et al., Phys. Rev. Lett. 105, 027201 (2010)]. Additional resonant x-ray reflection measurements indicate a region of altered Mn and O content at the interface, with a thickness matching that of the magnetic diluted layer, as the origin of the reduction of the magnetic moment.

We measured the lifetime of optically created valley polarization in single-layer ${\mathrm{WS}}_2$ using transient absorption spectroscopy. The electron valley relaxation is very short ($<1$ ps). However, the hole valley lifetime is at least two orders of magnitude longer and exhibits a temperature dependence that cannot be explained by single-carrier spin/valley relaxation mechanisms. Our theoretical analysis suggests that a collective contribution of two potential processes may explain the valley relaxation in single-layer ${\mathrm{WS}}_2$. One process involves direct scattering of excitons from $K$ to $K^{\prime }$ valleys with a spin flip-flop interaction. The other mechanism involves scattering through the spin-degenerate $\Gamma$ valley. This second process is thermally activated with an Arrhenius behavior due to the energy barrier between the $\Gamma$ and $K$ valleys.

We introduce a theoretical framework for computations of anisotropic multipolar exchange interactions found in many spin-orbit coupled magnetic systems and propose a method to extract these coupling constants using a density functional total energy calculation. This method is developed using a multipolar expansion of local density matrices for correlated orbitals that are responsible for magnetic degrees of freedom. Within the mean-field approximation, we show that each coupling constant can be recovered from a series of total energy calculations via what we call the “pair-flip” technique. This technique flips the relative phase of a pair of multipoles and computes the corresponding total energy cost associated with the given exchange constant. To test it, we apply our method to uranium dioxide, which is a system known to have pseudospin $J=1$ superexchange induced dipolar, and superexchange plus spin-lattice induced quadrupolar orderings. Our calculation reveals that the superexchange and spin-lattice contributions to the quadrupolar exchange interactions are about the same order with ferro- and antiferromagnetic contributions, respectively. This highlights a competition rather than a cooperation between them. Our method could be a promising tool to explore magnetic properties of rare-earth compounds and hidden-order materials.

We study the response of topological insulator films to strong magnetic and electric fields beyond the linear response theory. As a model, we use a three-dimensional lattice Wilson-Dirac Hamiltonian where we simultaneously introduce both magnetic field through the Peierls substitution and electric field as a potential energy depending on lattice coordinate. We compute the electron energy spectrum by numerically diagonalizing this Hamiltonian and obtain quantized magnetoelectric polarizability. In addition, we find that the magnetoelectric effect vanishes as the film width decreases due to the hybridization of surface wave functions. Furthermore, applying a gate voltage between the surfaces, we observe several quantized plateaus of $\theta$ term, which are mainly determined by the Landau level structures on the top and bottom surfaces.

We study how the energy and momentum resolution of angle-resolved photoemission spectroscopy (ARPES) affects the linewidth, Fermi crossing, velocity, and curvature of the measured band structure. Based on the fact that the resolution smoothes out the spectra, acting as a low-pass filter, we develop an iterative simulation scheme that compensates for resolution effects and allows the fundamental physical parameters to be accurately extracted. By simulating a parabolic band structure of Fermi-liquid quasiparticles, we show that this method works for an energy resolution up to 100 meV and a momentum resolution equal to twice the energy resolution scaled by the Fermi velocity. Our analysis acquires particular relevance in the hard and soft x-ray regimes, where a degraded resolution limits the accuracy of the extracted physical parameters, making it possible to study how the electronic excitations are modified when the ARPES probing depth increases beyond the surface.

Almost periodic particle chains exhibit peculiar propagation properties that are not observed in perfectly periodic ones. Furthermore, since they inherently support nonnegligible long-range interactions and radiation through the surrounding free space, nearest-neighbor approximations cannot be invoked. Hence the governing operator is fundamentally different from that used in traditional analysis of almost periodic structures, e.g., Harper's model and almost Mathieu difference equations. We present a mathematical framework for the analysis of almost periodic particle chains, and study their electrodynamic properties. We show that they support guided modes that exhibit a complex interaction mechanism with the light cone. These modes possess a two-dimensional fractal-like structure in the frequency–wave number space, such that a modal phase velocity cannot be uniquely defined. However, a well-defined group velocity is revealed due to the fractal's inner structure.

We investigate a mechanism to establish ground-state ferromagnetism in flat-band Hubbard systems based on a kind of order-from-disorder effect driven by dispersion. As a paradigm we consider a frustrated diamond chain, where for ideal diamond geometry the lowest one-electron band is flat, but the ground state remains paramagnetic for arbitrary on-site repulsion $U$. We focus on half-filling of the flat band. By using numerical and analytical arguments we present the ground-state phase diagram for a distorted diamond chain, i.e., the former flat band becomes dispersive. Driven by the interplay of dispersion and interaction the ground state is ferromagnetic if the interaction exceeds a critical value $U_c$.