Density functional theory including a many-body treatment of dispersive forces is used to describe the interplay between structure and electronic properties of two prototypical Br-based hybrid perovskites, namely, ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ and $\mathrm{HC}{\left({\mathrm{NH}}_2\right)}_2{\mathrm{PbBr}}_3$. We find that, like for some of their iodine-based counterparts, the molecules' orientation plays a crucial role in determining the shape of both the conduction and valence bands around the band edges. This is mostly evident in the case of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$, which is a direct band-gap semiconductor when the ${\mathrm{CH}}_3{\mathrm{NH}}_3$ group is oriented along the (111) direction but turns indirect when the orientation is (100). We have constructed a simple dipole model, with parameters all evaluated from ab initio calculations, to describe the molecules' depolarization dynamics. We find that, once the molecules are initially orientated along a given high-symmetry direction, their room-temperature depolarization depends on the specific material investigated. In particular we find that the ratio between the polarization decay constant of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbBr}}_3$ and that of $\mathrm{HC}{\left({\mathrm{NH}}_2\right)}_2{\mathrm{PbBr}}_3$ is about 2 at room temperature. With these results at hand we suggest a simple luminescence decay experiment to prove our findings and establish a correlation between optical activity and the molecules' dynamics in these materials.

A lossy anisotropic epsilon-near-zero (ENZ) medium may lead to a counterintuitive phenomenon of omnidirectional bending-to-normal refraction [S. Feng, Phys. Rev. Lett. 108, 193904 (2012)], which offers a fabulous strategy for energy collimation and energy harvesting. Here, in the scope of effective medium theory, we systematically investigate two simple metamaterial configurations, i.e., metal-dielectric-layered structures and the wire medium, to explore the possibility of fulfilling the conditions of such an anisotropic lossy ENZ medium by playing with materials' parameters. Both realistic metamaterial structures and their effective medium equivalences have been numerically simulated, and the results are in excellent agreement with each other. Our study provides clear guidance and therefore paves the way towards the search for proper designs of anisotropic metamaterials for a decent effect of energy collimation and wave-front manipulation.

Structural phase transitions in ${\mathrm{Sr}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ are currently of interest due to the evidence of strong correlation with their superconductivity. To further obtain additional insight into the thermodynamic properties of the phase transitions, we have performed a study of single-crystalline ${\mathrm{Sr}}_3{\mathrm{Rh}}_4{\mathrm{Sn}}_{13}$ and ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$ by means of the specific-heat and thermal-expansion measurements, mainly focusing on the features around the phase-transition temperature $T^{*}\simeq 138$ and 147 K, respectively. In particular, the specific-heat data have been analyzed in the framework of the critical fluctuation model in addition to a mean-field contribution. Relatively large critical exponents were obtained for ${\mathrm{Sr}}_3{\mathrm{Ir}}_4{\mathrm{Sn}}_{13}$, suggesting a shorter coherence length associated with the phase transition. For each compound, an enhancement in the mean-field jump compared to the BCS value has been quantitatively identified, revealing the strong-coupling characteristics for the observed phase transitions. Furthermore, prominent changes in the coefficient of linear thermal expansion and bulk modulus across $T^{*}$ have been identified, providing new information about the structural phase transitions in the title compounds.

We spectroscopically investigate a series of pyrochlore iridates $R_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ ($R$: rare-earth and Y ions) where the metal-insulator transitions are induced by systematic bandwidth control via chemical substitutions of $R$ ions. We establish the phase diagram of $R_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, as endorsed by the variation of the optical conductivity spectra, in which the competing phases including paramagnetic insulator (PI), paramagnetic metal (PM), and antiferromagnetic insulator (AFI) show up as a function of bandwidth and temperature. For small $R$-ionic radius ($R=$ Y-Sm), i.e., strongly correlated region, pronounced peaks on the edge of the optical gap are discerned below the magnetic transition temperature $T_{\mathrm{N}}$, which is attributable to exciton and magnon sideband absorptions. It turns out that the estimated nearest-neighbor exchange interaction increases as $R$-ionic radius increases, whereas $T_{\mathrm{N}}$ monotonically decreases, indicating that the all-in all-out magnetic order arises from the interplay among several exchange interactions inherent to extended $5d$ orbitals on the frustrated lattice. For larger $R$-ionic radius ($R=$ Sm-Pr), i.e., relatively weakly correlated region, the optical conductivity spectra markedly change below 0.3 eV in the course of PM-AFI transition, implying that the magnetic order induces the insulating state. In particular, we have found distinct electrodynamics in the composition of $R={\mathrm{Nd}}_{0.5}{\mathrm{Pr}}_{0.5}$ which is located on the boundary of the quantum PM-AFI transition, pointing to the possible emergence of unconventional topological electronic phases related possibly to the correlated Weyl electrons.

We present a theoretical study of spectral and magnetic properties of clinoferrosilite ${\mathrm{FeSiO}}_3$. Within the DFT+DMFT method combining local density approximation with dynamical mean-field theory ${\mathrm{FeSiO}}_3$ was investigated in a wide range of pressure and temperature including the lower Earth's mantle conditions. For clinoferrosilite, which crystallizes in a monoclinic crystal structure, we predict two high-spin to low-spin transitions under pressure in the Fe-$3d$ shell with a crossover region at moderate temperatures, which becomes much broader at higher temperatures. An analysis of the Fe electronic configurations reveals that in clinoferrosilite the low- and high-spin states are predominantly involved and coexist in the spin crossover region, while a small amount of the intermediate spin states appears only at very high pressures and can be attributed to the distorted crystal structure of clinoferrosilite ${\mathrm{FeSiO}}_3$.

Recent experiments have produced mounting evidence of Majorana zero modes in nanowire-superconductor hybrids. Signatures of an expected topological phase transition accompanying the onset of these modes nevertheless remain elusive. We investigate a fundamental question concerning this issue: Do well-formed Majorana modes necessarily entail a sharp phase transition in these setups? Assuming reasonable parameters, we argue that finite-size effects can dramatically smooth this putative transition into a crossover, even in systems large enough to support well-localized Majorana modes. We propose overcoming such finite-size effects by examining the behavior of low-lying excited states through tunneling spectroscopy. In particular, the excited-state energies exhibit characteristic field and density dependence, and scaling with system size, that expose an approaching topological phase transition. We suggest several experiments for extracting the predicted behavior. As a useful byproduct, the protocols also allow one to measure the wire's spin-orbit coupling directly in its superconducting environment.

Graphene nanoribbons are prone to in-plane bending even when supported on flat substrates. However, the amount of bending that ribbons can stably withstand remains poorly known. Here, by using molecular dynamics simulations, we study the stability limits of 0.5–1.9-nm-wide armchair and zigzag graphene nanoribbons subject to bending. We observe that the limits for maximum stable curvatures are below $\sim 10\mathrm{deg}/\mathrm{nm}$ in case the bending is externally forced and the limit is caused by buckling instability. Furthermore, it turns out that the limits for maximum stable curvatures are also below $\sim 10\mathrm{deg}/\mathrm{nm}$ in case the bending is not forced and the limit arises only from the corrugated potential-energy landscape due to the substrate. Both of the stability limits lower rapidly when ribbons widen. These results agree with recent experiments and can be understood by means of transparent elasticity models.

Semimetals are characterized by nodal band structures that give rise to exotic electronic properties. The stability of Dirac semimetals, such as graphene in two spatial dimensions, requires the presence of lattice symmetries, while akin to the surface states of topological band insulators, Weyl semimetals in three spatial dimensions are protected by band topology. Here we show that in the bulk of topological band insulators, self-organized topologically protected semimetals can emerge along a grain boundary, a ubiquitous extended lattice defect in any crystalline material. In addition to experimentally accessible electronic transport measurements, these states exhibit a valley anomaly in two dimensions influencing edge spin transport, whereas in three dimensions they appear as graphenelike states that may exhibit an odd-integer quantum Hall effect. The general mechanism underlying these semimetals—the hybridization of spinon modes bound to the grain boundary—suggests that topological semimetals can emerge in any topological material where lattice dislocations bind localized topological modes.

The density and temperature dependence of the shear viscosity of liquid sodium is studied. The stress autocorrelation function is calculated by equilibrium molecular dynamics simulations, which allow us to obtain the value of shear viscosity using the Green-Kubo formula. The Fiolhais potential is used to calculate the interionic interactions, which are validated by comparison between simulation and experimental data along the liquid-gas coexistence curve. The behavior of viscosity over a wide range of the liquid phase of the phase diagram is studied. Along isochoric lines, it presents a minimum, while it monotonically increases along isotherms. An expression is proposed for the viscosity as a function of temperature and density which reproduces our data for liquid sodium at any density in the range [1000–2000 kg ${\mathrm{m}}^{-3}$] and any temperature in the range [700–7000 K]. The validity of the Stokes-Einstein relation over the investigated state points is discussed.

We study negative thermal expansion (NTE) in model lattices with multiple atoms per cell and first- and second-nearest neighbor interactions using the (anharmonic) Morse potential. By exploring the phase space of neighbor distances and thermal expansion rates of the bonds, we determine the conditions under which NTE emerges. By permitting all bond lengths to expand at different rates, we find that NTE is possible without appealing to fully rigid units. Nearly constant, large-amplitude, isotropic NTE is observed up to the melting temperature in a classical molecular dynamics model of a ${\mathrm{ReO}}_3$-like structure when the rigidity of octahedral units is almost completely eliminated. Only weak NTE, changing over to positive expansion, is observed when the corner-linked octahedra are rigid, with flexible second-neighbor bonds between neighboring octahedra permitting easy rotation. We observe similar changes to thermal expansion behavior for the diamond lattice: NTE when second-neighbor interactions are weak to positive thermal expansion when second-neighbor interactions are strong. From these observations, we suggest that the only essential local conditions for NTE are atoms with low coordination numbers along with very low energies for changing bond angles relative to bond-stretching energies.

We have performed magnetic susceptibility, heat capacity, neutron powder diffraction, and muon spin relaxation experiments to investigate the magnetic ground states of the $5d^2$ double perovskites ${\mathrm{Sr}}_2{\mathrm{YReO}}_6$ and ${\mathrm{Sr}}_2{\mathrm{InReO}}_6$. We find that ${\mathrm{Sr}}_2{\mathrm{YReO}}_6$ is a spin glass, while ${\mathrm{Sr}}_2{\mathrm{InReO}}_6$ hosts a nonmagnetic singlet state. By making detailed comparisons with other $5d^2$ double perovskites, we argue that a delicate interplay between spin-orbit coupling, noncubic crystal fields, and exchange interactions plays a key role in the great variation of magnetic ground states observed for this family of materials.

In this paper we analyze the structure of Fe-Ga layers with a Ga content of ∼30 at.% deposited by the sputtering technique under two different regimes. We also studied the correlation between the structure and magnetic behavior of the samples. Keeping the Ar pressure fixed, we modified the flow regime from ballistic to diffusive by increasing the distance between the target and the substrate. X-ray diffraction measurements have shown a lower structural quality when growing in the diffusive flow. We investigated the impact of the growth regime by means of x-ray absorption fine structure (XAFS) measurements and obtained signs of its influence on the local atomic order. Full multiple scattering and finite difference calculations based on XAFS measurements point to a more relevant presence of a disordered $A2$ phase and of orthorhombic Ga clusters on the Fe-Ga alloy deposited under a diffusive regime; however, in the ballistic sample, a higher presence of $D{0}_3/B2$ phases is evidenced. Structural characteristics, from local to long range, seem to determine the magnetic behavior of the layers. Whereas a clear in-plane magnetic anisotropy is observed in the film deposited under ballistic flow, the diffusive sample is magnetically isotropic. Therefore, our experimental results provide evidence of a correlation between flow regime and structural properties and its impact on the magnetic behavior of a rather unexplored compositional region of Fe-Ga compounds.

We examine the lattice thermal conductivities $\left({\kappa }_l\right)$ of $\mathrm{L}{\mathrm{i}}_{2}X(X=\mathrm{O},\mathrm{S},\mathrm{Se},\mathrm{Te})$ using a first-principles Peierls-Boltzmann transport methodology. We find low ${\kappa }_l$ values ranging between 12 and 30 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ despite light Li atoms, a large mass difference between constituent atoms, and tightly bunched acoustic branches, all features that give high ${\kappa }_l$ in other materials including BeSe (630 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$), BeTe (370 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$), and cubic BAs (3170 $\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$). Together these results suggest a missing ingredient in the basic guidelines commonly used to understand and predict ${\kappa }_l$. Unlike typical simple systems (e.g., Si, GaAs, SiC), the dominant resistance to heat-carrying acoustic phonons in $\mathrm{L}{\mathrm{i}}_2\mathrm{Se}$ and $\mathrm{L}{\mathrm{i}}_2\mathrm{Te}$ comes from interactions of these modes with two optic phonons. These interactions require significant bandwidth and dispersion of the optic branches, both present in $\mathrm{L}{\mathrm{i}}_{2}X$ materials. These considerations are important for the discovery and design of new materials for thermal management applications and give a more comprehensive understanding of thermal transport in crystalline solids.

Magnetic properties of thin Co/Pt multilayers have been investigated in order to study the dependence of magnetization $M$, uniaxial anisotropy $K_u$, and Curie temperature $T_{\mathrm{C}}$ on the multilayer thickness, composition, and structure. A comparison between epitaxial submonolayer multilayers and epitaxial fcc ${\mathrm{CoPt}}_3$ alloy films with large perpendicular magnetic anisotropy (PMA) attributed to growth-induced Co clustering reveals significant differences in the temperature dependence of magnetization $M\left(T\right)$, despite the presence of thin planar Co platelets in both cases. Even the thinnest discontinuous multilayered structure shows a Langevin-like $M\left(T\right)$, while the alloy films with PMA show a broadened and enhanced $M\left(T\right)$ indicating a distribution of environments, including monolayer Co platelets separated by only 1–2 layers of Pt. These differences have been reproduced in Monte Carlo simulations, and are shown to be due to different distributions of Co-Co and Co-Pt nearest neighbors. The relatively uniform Co-Co coordination of even a discontinuous rough multilayer produces a Langevin-like $M\left(T\right)$, whereas the broader distribution associated with platelets in the PMA films results in a nearly linear $T$ dependence of $M$.

Stimulated by recent studies of superconductivity and magnetism with local and global broken inversion symmetry, we investigate the superconductivity in magnetic multipole states in locally noncentrosymmetric metals. We consider a one-dimensional zigzag chain with sublattice-dependent antisymmetric spin-orbit coupling and suppose three magnetic multipole orders: monopole order, dipole order, and quadrupole order. It is demonstrated that the Bardeen-Cooper-Schrieffer state, the pair-density wave (PDW) state, and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state are stabilized by these multipole orders, respectively. We show that the PDW state is a topological superconducting state specified by the nontrivial ${\mathbb{Z}}_2$ number and winding number. The origin of the FFLO state without macroscopic magnetic moment is attributed to the asymmetric band structure induced by the magnetic quadrupole order and spin-orbit coupling.

Atomically precise superlattices involving transition-metal oxides provide a unique opportunity to engineer correlated electron physics using strain (modulated by choice of substrate) and quantum confinement (controlled by layer thickness). Here we use the combination of density-functional theory and dynamical mean-field theory (DFT+DMFT) to study $\mathrm{Ni}{E}_{g}d$-orbital polarization in strained ${\mathrm{LaNiO}}_3\text{/}{\mathrm{LaAlO}}_3$ superlattices consisting of four layers of nominally metallic ${\mathrm{NiO}}_2$ and four layers of insulating ${\mathrm{AlO}}_2$ separated by LaO layers. The layer-resolved orbital polarization is calculated as a function of strain and analyzed in terms of structural, quantum confinement, and correlation effects. The effect of strain is determined from the dependence of the results on the Ni-O bond-length ratio and the octahedral rotation angles, quantum confinement is studied by comparison to bulk calculations with similar degrees of strain, and correlation effects are inferred by varying interaction parameters within our DFT+DMFT calculations. The calculated dependence of orbital polarization on strain in superlattices is qualitatively consistent with recent x-ray-absorption spectroscopy and resonant reflectometry data. However, interesting differences of detail are found between theory and experiment. Under tensile strain, the two inequivalent Ni ions display orbital polarization similar to that calculated for strained bulk ${\mathrm{LaNiO}}_3$ and observed in experiment. Compressive strain produces a larger dependence of orbital polarization on Ni position, and even the inner Ni layer exhibits orbital polarization different from that calculated for strained bulk ${\mathrm{LaNiO}}_3$.

Interlayer excitons, comprising an electron in one material bound by Coulomb attraction to a hole in an adjacent material, are composite bosons that can assume a variety of many-body phases. The phase diagram of the bosonic system is largely determined by the dispersion relation of the bosons, which itself arises as a combination of the dispersion relations of the electron and hole separately. Here I show that in situations where either the electron or the hole has a nonmonotonic, “Mexican hat-shaped” dispersion relation, the exciton dispersion relation can have a range of qualitatively different forms, each corresponding to a different many-body phase at low temperature. This diversity suggests a platform for continuously tuning between different quantum phases using an external field.

Cotunneling into Kondo systems, where an electron enters an $f$-electron material via a cotunneling process through the local-moment orbital, has been proposed to explain the characteristic line shapes observed in scanning-tunneling-spectroscopy (STS) experiments. Here we extend the theory of electron cotunneling to Kondo-lattice systems in which the bulk hybridization between conduction and $f$ electrons is odd under inversion, being particularly relevant to Kondo insulators. Compared to the case of even hybridization, we show that the interference between normal tunneling and cotunneling processes is fundamentally altered: it is entirely absent for layered, i.e., quasi-two-dimensional materials, while its energy dependence is strongly modified for three-dimensional materials. We discuss implications for STS experiments.

We investigate the role of short-ranged electron-electron interactions in a paradigmatic model of three-dimensional topological insulators, using dynamical mean-field theory and focusing on nonmagnetically ordered solutions. The noninteracting band structure is controlled by a mass term $M$, whose value discriminates between three different insulating phases, a trivial band insulator and two distinct topologically nontrivial phases. We characterize the evolution of the transitions between the different phases as a function of the local Coulomb repulsion $U$ and find a remarkable dependence of the $U-M$ phase diagram on the value of the local Hund's exchange coupling $J$. However, regardless of the value of $J$, following the evolution of the topological transition line between a trivial band insulator and a topological insulator, we find a critical value of $U$ separating a continuous transition from a first-order one. When the Hund's coupling is significant, a Mott insulator is stabilized at large $U$. In proximity of the Mott transition we observe the emergence of an anomalous “Mott-like” strong topological insulator state.

The correlation energy of the homogeneous electron gas is evaluated by solving the Bethe-Salpeter equation (BSE) beyond the Tamm-Dancoff approximation for the electronic polarization propagator. The BSE is expected to improve on the random-phase approximation, owing to the inclusion of exchange diagrams. For instance, since the BSE reduces in second order to Møller-Plesset perturbation theory, it is self-interaction free in second order. Results for the correlation energy are compared with quantum Monte Carlo benchmarks and excellent agreement is observed. For low densities, however, we find imaginary eigenmodes in the polarization propagator. To avoid the occurrence of imaginary eigenmodes, an approximation to the BSE kernel is proposed that allows us to completely remove this issue in the low-electron-density region. We refer to this approximation as the random-phase approximation with screened exchange (RPAsX). We show that this approximation even slightly improves upon the standard BSE kernel.