High-precision simulations are used to generate the $\mathit{2}\mathit{D}$ Raman mode of graphene under a range of screening conditions and laser energies $E_L$. We reproduce the decreasing trend of the $\mathit{2}\mathit{D}$ mode FWHM vs $E_L$ and the nearly linearly increasing dispersion $\partial {\omega }_{\mathit{2}\mathit{D}}/\partial E_L$ seen experimentally in freestanding (unscreened) graphene, and propose relations between these experimentally accessible quantities and the local, two-dimensional gradients $\left|\nabla \right|$ of the electronic and TO phonon bands. In light of state-of-the-art electronic structure calculations that acutely treat the long-range $e$-$e$ interactions of isolated graphene and its experimentally observed $\mathit{2}\mathit{D}$ Raman mode, our calculations determine a 40% greater slope of the TO phonons about $\mathit{K}$ than given by explicit phonon measurements performed in graphite or $GW$ phonon calculations in graphene. We also deduce the variation of the broadening energy $\gamma \left[E_L\right]$ for freestanding graphene and find a nominal value $\gamma \sim 140$ meV, showing a gradually increasing trend for the range of frequencies available experimentally.

We calculate the density of states for an interacting quantum wire in the presence of two impurities of arbitrary potential strength. To perform this calculation, we describe the Coulomb interactions in the wire within the Tomonaga-Luttinger liquid theory. After establishing and solving the Dyson equation for the fermionic retarded Green's functions, we study how the profile of the local density of states is affected by the interactions in the entire range of impurity potentials. Same as in the noninteracting case, when increasing the impurity strength, the central part of the wire becomes more and more disconnected from the semi-infinite leads, and discrete localized states begin to form; the width and the periodicity of the corresponding peaks in the spectrum depends on the interaction strength. As expected from the Luttinger liquid theory, impurities also induce a reduction of the local density of states at small energies. Two other important aspects are highlighted: the appearance of an extra modulation in the density of states at nonzero Fermi momentum when interactions are present, and the fact that forward scattering must be taken into account in order to recover the Coulomb-blockade regime for strong impurities.

The magnetic properties of antiferromagnetic (AFM) spins were investigated using x-ray magnetic linear dichroism (XMLD) in epitaxial NiO/CoO/MgO(001) films as a function of film thickness, temperature, and interface modulation. We found that the NiO AFM spins undergo a spin reorientation transition (SRT) from the in-plane orientation to the out-of-plane orientation at a critical NiO thickness. The NiO AFM SRT can be attributed to the competition between the out-of-plane anisotropy of the NiO AFM spins and the AFM interfacial exchange coupling with the CoO in-plane spins. The NiO SRT thickness increases with the CoO thickness and also with the interfacial coupling strength. The temperature-dependent XMLD measurement indicates that the exchange coupling at the NiO/CoO interface can greatly enhance the Néel temperature of the CoO layer.

Recent studies reveal a pressure induced transition from a paramagnetic tetragonal phase (T) to a collapsed tetragonal phase (CT) in ${\text{CaFe}}_2{\text{As}}_2$, which was found to be superconducting with pressure at low temperature. We have investigated the effects of electron correlation and a local fluctuating moment in both tetragonal and collapsed tetragonal phases of the paramagnetic ${\text{CaFe}}_2{\text{As}}_2$ using self-consistent DFT-DMFT with continuous time quantum Monte Carlo as the impurity solver. From the computed optical conductivity, we find a gain in the optical kinetic energy due to the loss in Hund's rule coupling energy in the CT phase. We find that the transition from T to CT turns ${\text{CaFe}}_2{\text{As}}_2$ from a bad metal into a good metal. Computed mass enhancement and local moments also show a significant decrease in the CT phase, which confirms the suppression of the electron correlation in the CT phase of ${\text{CaFe}}_2{\text{As}}_2$.

While multiple time scales generally arise in the dynamics of disordered systems, we find multiple time scales in the absence of disorder in a simple model with hard local constraints. The dynamics of the model, which consists of local collective rearrangements of various scales, is not determined by the smallest scale but by a length $l^{*}$ that grows at low energies. In real space we find a hierarchy of fast and slow regions: Each slow region is geometrically insulated from all faster degrees of freedom, which are localized in fast pockets below percolation thresholds. A tentative analogy with structural glasses is given, which attributes the slowing down of the dynamics to the growing size of mobile elementary excitations, rather than to the size of some domains.

We generalize the Kitaev's spin-1/2 model on the honeycomb by introducing a two-dimensional $Z_3$ clock model on the triangular lattice with three-body interaction. We discuss various properties of this model and show that the low energy theory of the $Z_3$ generalized Kitaev model (GKM) is described by a single $Z_3$ parafermion per lattice site coupled to a $Z_3$ gauge field. We also introduce a slave-fermion approach for this GKM, treat the resulting fermionic Hamiltonian at the mean-field level, solve the mean-field parameters self-consistently, and obtain the low energy effective Chern-Simons (CS) gauge theory. The resulting CS gauge theory is identical to that of a $\left(221\right)$ fractional quantum Hall state. We then go beyond the mean-field approximation and demonstrate that fluctuations generate a uniform interlayer pairing for the dual $\left(221\right)$ bilayer state. We argue that this perturbed system can undergo a phase transition to the Fibonacci phase by tuning the interlayer pairing strength.

It was proposed that a flat silver layer could be used to form a subdiffraction-limited image when illuminated near its surface plasmon resonance frequency [J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000)]. In this paper, we study the possibility of obtaining sub diffraction resolution using a different mechanism, with no surface plasmons involved. Instead, by taking into account the nonlocal response of a thin silver slab, we show that longitudinal modes contribute to the formation of a subdiffraction-limited image in a frequency regime above the plasma frequency. The differences between these two distinct mechanisms are studied and explained.

Meta-atoms, nanoantennas, plasmonic particles, and other small scatterers are commonly modeled in terms of their modes. However these modal solutions are seldom determined explicitly, due to the conceptual and numerical difficulties in solving eigenvalue problems for open systems with strong radiative losses. Here these modes are directly calculated from Maxwell's equations expressed in integral operator form, by finding the complex frequencies which yield a homogeneous solution. This gives a clear physical interpretation of the modes, and enables their conduction or polarization current distribution to be calculated numerically for particles of arbitrary shape. By combining the modal current distribution with a scalar impedance function, simple yet accurate models of scatterers are constructed which describe their response to an arbitrary incident field over a broad bandwidth. These models generalize both equivalent-dipole and and equivalent-circuit models to finite-sized structures with multiple modes. They are applied here to explain the frequency-splitting for a pair of coupled split rings, and the accompanying change in radiative losses. The approach presented in this paper is made available in an open-source code.

Using first-principles calculations, we investigate the electronic and vibrational properties of monolayer $T$-phase ${\mathrm{TaS}}_2$. We demonstrate that a charge-density wave is energetically favorable at low temperature, similar to bulk $1T\text{−}{\mathrm{TaS}}_2$. Electron-phonon coupling is found to be essential for the lattice reconstruction. The charge-density wave results in a strong localization of the electronic states near the Fermi level and consequently in spin polarization, transforming the material into a magnetic semiconductor with enhanced electronic correlations. The combination of inherent spin polarization with a semiconducting nature distinguishes the monolayer fundamentally from the bulk compound as well as from other two-dimensional transition metal dichalcogenides. Monolayer $T$-phase ${\mathrm{TaS}}_2$ therefore has the potential to enable two-dimensional spintronics.

Fractional Chern insulators are novel realizations of fractional quantum Hall states in lattice systems without orbital magnetic field. These states can be mapped onto conventional fractional quantum Hall states through the Wannier state representation [Qi, Phys. Rev. Lett. 107, 126803 (2011)]. In this paper, we use the Wannier state representation to construct the pseudopotential Hamiltonians for fractional Chern insulators, which are interaction Hamiltonians with certain ideal model wave functions as exact ground states. We show that these pseudopotential Hamiltonians can be approximated by short-ranged interactions in fractional Chern insulators, and that their range will be minimized by an optimal gauge choice for the Wannier states. As illustrative examples, we explicitly write down the form of the lowest pseudopotential for several fractional Chern insulator models like the lattice Dirac model, the checkerboard model with Chern number 1, the $d$-wave model, and the triangular lattice model with Chern number 2. The proposed pseudopotential Hamiltonians have the $1/3$ Laughlin state as their ground state when the Chern number $C_1=1$, and a topological nematic $\left(330\right)$ state as their ground state when $C_1=2$. Also included are the results of an interpolation between the $d$-wave model and two decoupled layers of lattice Dirac models, which explicitly demonstrate the relation between $C_1=2$ fractional Chern insulators and bilayer fractional quantum Hall states. The proposed states can be verified by future numerical works and, in particular, provide a model Hamiltonian for the topological nematic states that have not been realized numerically.

Nanoscale electronic transport is of intense technological interest, with applications ranging from semiconducting devices and molecular junctions to charge migration in biological systems. Most explicit theoretical approaches treat transport using a combination of density functional theory (DFT) and nonequilibrium Green's functions. This is a static formalism, with dynamic response properties accommodated only through complicated extensions. To circumvent this limitation, the carrier density may be propagated using real-time time-dependent DFT (RT-TDDFT), with boundary conditions corresponding to an open quantum system. Complex absorbing potentials can emulate outgoing particles at the simulation boundary, although these do not account for introduction of charge density. It is demonstrated that the desired positive particle flux is afforded by a class of $\mathcal{PT}$-symmetric generating potentials that are characterized by anisotropic transmission resonances. These potentials add density every time a particle traverses the cell boundary, and may be used to engineer a continuous pulse train for incident packets. This is a first step toward developing a complete transport formalism unique to RT-TDDFT.

By employing a configuration interaction approach, we study the spin polarization induced by an electric field applied to graphene nanodots. The evolution of both the ground and excited states of the graphene nanodot with the applied electric field is calculated systematically. Three clearly separated field regimes have been identified for the spin gap in the model system, namely, (a) nearly constant in the nonmagnetic phase, (b) rapidly decreasing in the antiferromagnetic phase, and (c) exponential decay in the ferromagnetic phase. It is therefore demonstrated that ferromagnetism can be induced purely by applying an electric field to a nonmagnetic graphene nanodot. Electron-electron interactions which are greatly modified by the applied electric field are believed to account for the two consecutive magnetic phase transitions.

There is a clear need for a practical and mathematically rigorous description of local structure in inorganic compounds so that structures and chemistries can be easily compared across large data sets. Here a method for decomposing crystal structures into substructures is given, and a similarity function between those substructures is defined. The similarity function is based on both geometric and chemical similarity. This construction allows for large-scale data mining of substructural properties, and the analysis of substructures and void spaces within crystal structures. The method is validated via the prediction of Li-ion intercalation sites for the oxides. Tested on databases of known Li-ion-containing oxides, the method reproduces all Li-ion sites in an oxide with a maximum of 4 incorrect guesses 80% of the time.

CuNb nanocomposites have proven stable under light-ion irradiation, although their response to heavy-ion bombardment and subsequent loading is still uncertain. In this paper we analyze the change in mechanical properties of model CuNb nanocomposites in vacancy supersaturated environments, mimicking certain effects introduced by heavy-ion irradiation. We have performed compression tests using molecular dynamics for different defect contents. The presence of defects substantially modifies the system response upon external loading. The dislocation nucleation mechanism changes with the interface atomic density. Stacking fault tetrahedra in the Cu layer may act as dislocation sources to lower significantly the yield stress and the same applies for voids present in the Nb layer. We have analyzed in detail the dislocation-interface interaction mechanisms under different conditions, showing how dislocations react with the misfit dislocations present at the interface to modify the atomic structure of the boundary, which suggests that the interface could be designed to optimize its defect healing properties. We conclude that heavy-ion irradiation softens the multilayer nanocomposites and that interfaces help in recovering these materials as they absorb the dislocations created from irradiation-induced defects.

We analyze the resonant x-ray scattering (RIXS) spectra from ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ in an itinerant-electron approach. Employing a multiorbital tight-binding model on the square lattice, we calculate the one-electron energy band within the Hartree-Fock approximation, which leads to an antiferromagnetic ground state. We then evaluate the two-particle Green's functions for the particle-hole pair excitations within the random phase approximation, which are connected to the RIXS spectra within the fast collision approximation. The calculated RIXS spectra exhibit a two-peak structure with slightly different energies in the low-energy region, which originate from the bound states in the two-particle Green's function. They may be interpreted as the split modes of a magnon. We also obtain several $\delta$-function peaks, which arise from the bound states around the bottom of the energy continuum. They may be called exciton modes. These characteristics are in qualitative agreement with the RIXS experiment, demonstrating that the weak coupling theory could explain both the magnon and the exciton peaks in the RIXS spectra on an equal footing.

Magnetic properties were investigated in ${\mathrm{La}}_3{\mathrm{Re}}_2{\mathrm{O}}_{10}$ with ${\mathrm{Re}}_2$ clusters. An ${\mathrm{Re}}_2$ cluster has three $5d$ electrons in nondegenerated cluster orbitals, resulting in the construction of $S=1/2$ spin per ${\mathrm{Re}}_2$ cluster. ${\mathrm{La}}_3{\mathrm{Re}}_2{\mathrm{O}}_{10}$ has a Weiss temperature of approximately $-110$ K and exhibits a broad maximum in the temperature dependence of magnetic susceptibility, suggesting the development of short-range ordering among cluster spins at low temperatures. ${\mathrm{La}}_3{\mathrm{Re}}_2{\mathrm{O}}_{10}$ shows an antiferromangetic ordering at $T_N=18.7$ K, and below $T_N$, ${\mathrm{La}}_3{\mathrm{Re}}_2{\mathrm{O}}_{10}$ shows a strong field dependence of magnetization, which indicates several phase transitions. The magnetic properties of ${\mathrm{La}}_3{\mathrm{Re}}_2{\mathrm{O}}_{10}$ are summarized as a magnetic field-temperature phase diagram, which suggests an unconventional multicritical point. Such a phenomenon cannot be interpreted only by the magnetic interactions owing to phase rule, suggesting the existence of other degrees of freedom for making an exotic phase transition.

The bulk band structure of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ has been determined by angle-resolved photoemission spectroscopy and compared to first-principles calculations. We have performed calculations using the local density approximation (LDA) of density functional theory and the one-shot $GW$ approximation within the all-electron full-potential linearized augmented-plane-wave (FLAPW) formalism, fully taking into account spin-orbit coupling. Quasiparticle effects produce significant changes in the band structure of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ when compared to LDA. Experimental and calculated results are compared in the spectral regions where distinct differences between the LDA and $GW$ results are present. Overall a superior agreement with $GW$ is found, highlighting the importance of many-body effects in the band structure of this family of topological insulators.