We show that the insulating character of the iron selenide $\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$ can be explained in terms of Mott localization in sharp contrast with the metallic behavior of FeSe and other parent compounds of iron superconductors. We demonstrate that the key ingredient that makes $\mathrm{La}{}_2\mathrm{O}{}_3\mathrm{Fe}{}_2\mathrm{Se}{}_2$ a Mott insulator, rather than a correlated metal dominated by the Hund's coupling, is the enhanced crystal-field splitting, accompanied by a smaller orbital-resolved kinetic energy. The strong deviation from orbital degeneracy introduced by the crystal-field splitting also pushes this material close to an orbital-selective Mott transition. We predict that either doping or uniaxial external pressure can drive the material into an orbital-selective Mott state, where only one or a few orbitals are metallized while the others remain insulating.

We investigate the properties of a two-dimensional quasicrystal in the presence of a uniform magnetic field. In this configuration, the density of states (DOS) displays a Hofstadter-butterfly-like structure when it is represented as a function of the magnetic flux per tile. We show that the low-DOS regions of the energy spectrum are associated with chiral edge states, in direct analogy with the Chern insulators realized with periodic lattices. We establish the topological nature of the edge states by computing the topological Chern number associated with the bulk of the quasicrystal. This topological characterization of the nonperiodic lattice is achieved through a local (real-space) topological marker. This work opens a route for the exploration of topological-insulating materials in a wide range of nonperiodic lattice systems, including photonic crystals and cold atoms in optical lattices.

We quantitatively obtain the quantum ground-state phases of a Fermi system with on-site and dipole-dipole interactions in one-dimensional lattice chains within the density matrix renormalization group. We show, at a given spin polarization, the existence of six phases in the phase diagram and find that the phases are highly dependent on the spin degree of freedom. These phases can be constructed using available experimental techniques.

We study the nonequilibrium formation of a spin screening cloud that accompanies the quenching of a local magnetic moment immersed in a Fermi sea at zero temperature. Based on high-precision density matrix renormalization-group results for the interacting single-impurity Anderson model, we discuss the real-time evolution after a quantum quench in the impurity-reservoir hybridization using time-evolving block decimation. We report emergent length and time scales in the spatiotemporal structure of nonlocal correlation functions in the spin and the charge density channel. At equilibrium, our data for the correlation functions and the extracted length scales show good agreement with existing results, as do local time-dependent observables at the impurity. In the time-dependent data, we identify a major signal which defines a “light cone” moving at the Fermi velocity and a ferromagnetic component in its wake. Inside the light cone we find that the structure of the nonequilibrium correlation functions emerges on two time scales. Initially, the qualitative structure of the correlation functions develops rapidly at the lattice Fermi velocity. Subsequently the spin correlations converge to the equilibrium results on a much larger time scale. This process sets a dynamic energy scale, which we identify to be proportional to the Kondo temperature. Outside the light cone we observe two different power-law decays of the correlation functions in space, with time- and interaction-strength-independent exponents.

We report the magnetic and electronic properties of ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$, which has a structure based on a triangular bilayer of Mn, rather than the ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$ structure commonly encountered for 122 compounds in intermetallic systems. ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ has an antiferromagnetic ground state, with a ${\mathrm{T}}_N$ of 150 K, and for a 250 K temperature range above ${\mathrm{T}}_N$ does not exhibit Curie-Weiss behavior, indicating the presence of strong magnetic correlations at high temperatures. Resistivity measurements show that ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ exhibits semiconducting properties at low temperatures, with an energy gap of only 62 meV, indicating it to be a very narrow band gap semiconductor. The electronic structure of ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$, examined via ab-initio electronic structure calculations, indicates that Mn 3d orbital hybridization is essential for the formation of the band gap, suggesting that ${\mathrm{CaMn}}_2{\mathrm{Bi}}_2$ may be a hybridization-gap semiconductor.

The electronic structure of pure and Ta-doped $\mathrm{ZrSi}{\mathrm{O}}_4$ in the tetragonal $I{4}_1/amd$ phase with and without defects has been studied using the ab initio full-potential linear augmented plane wave plus local orbitals method. From the determined charge densities, the electric field gradient tensor at native Zr sites and at Ta impurities localized on cation sites of $\mathrm{ZrSi}{\mathrm{O}}_4$ were derived and compared to experimental data obtained using hyperfine techniques. The effects of the Ta probe atom, including its different charge states on the lattice, are investigated. In addition, different types of defects, such as O or Si vacancies, Ta replacing Si, and Ta enclosed in microstructures of $\mathrm{Si}{\mathrm{O}}_2$ phases, are examined. The combination of experiments and theory enables us to identify the different interactions observed in Ta-doped $\mathrm{ZrSi}{\mathrm{O}}_4$ and to elucidate the role played by different defects.

The thermoelectric properties of PbSe have significantly improved in recent years reaching figures of merit $ZT\sim 1.6$. The transport properties of the hole-doped high temperature thermoelectric material PbSe are particularly interesting and play a key role in this. Here, they were analyzed over wide temperature and hole concentration ranges. The special features observed in the variation of the experimental Seebeck coefficient and Hall coefficient can be accounted for within the framework of a two-band model. Two valence bands separated by a temperature dependent energy offset are considered. The extremum of the light-hole band has a density of states mass $\sim 0.27m_{\mathrm{o}}$ at room temperature. It is nonparabolic and anisotropic and can be described by the Kane model. The extremum of the heavy-hole band is isotropic and parabolic with a much larger density of states mass $\sim 2.5m_{\mathrm{o}}$. We find that for heavily doped compositions, the high mass band contributes the Seebeck coefficient, even at room temperature. With rising temperature, holes are transferred from the light-hole to the heavy-hole branch, giving rise to the anomalous temperature dependent Hall coefficient, which is found peaked near $\sim 650\mathrm{K}$. For Na-doped samples $\mathrm{P}{\mathrm{b}}_{1-x}\mathrm{N}{\mathrm{a}}_{\mathrm{x}}\mathrm{Se}$ for $0.01\le x\le 0.03$, the high thermopower values of $200-300\mu \mathrm{V}/\mathrm{K}$ at 900 K arise from the heavy-hole band, which are responsible for the excellent thermoelectric performance of PbSe.

We study the thermoelectric transport of a small metallic island weakly coupled to two electrodes by tunnel junctions. In the Coulomb blockade regime, in the case when the ground state of the system corresponds to an even number of electrons on the island, the main mechanism of electron transport at the lowest temperatures is elastic cotunneling. In this regime, the transport coefficients strongly depend on the realization of the random impurity potential or the shape of the island. Using random-matrix theory, we calculate the thermopower and the thermoelectric kinetic coefficient and study the statistics of their mesoscopic fluctuations in the elastic cotunneling regime. The fluctuations of the thermopower turn out to be much larger than the average value.

Several scientific issues concerning the latest generation read heads for magnetic storage devices, based on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs) are known to be controversial, including such fundamental questions as to the behavior and the role of B in optimizing the physical properties of these devices. Quantitatively establishing the internal structures of several such devices with different annealing conditions using hard x-ray photoelectron spectroscopy, we resolve these controversies and establish that the B diffusion is controlled by the capping Ta layer, though Ta is physically separated from the layer with B by several nanometers. While explaining this unusual phenomenon, we also provide insight into why the tunneling magnetoresistance (TMR) is optimized at an intermediate annealing temperature, relating it to B diffusion, coupled with our studies based on x-ray diffraction and magnetic studies.

We consider electron states in $n\text{−}p$ double quantum dots defined in a semiconducting carbon nanotube (CNT) by an external potential. We describe formation of extended single-electron orbitals originating from the conduction and valence bands confined in a minimum and a maximum of the external potential, respectively. We solve the problem of a confined electron pair using an exact diagonalization method within the tight-binding approach, which allows for a straightforward treatment of the conduction- and valence-band states, keeping an exact account for the intervalley scattering mediated by the atomic defects and the electron-electron interaction. The exchange interaction, which in the unipolar double dots is nearly independent of the axial magnetic field ($B$) and forms singletlike and tripletlike states, in the $n\text{−}p$ system appears only for selected states and narrow intervals of $B$. In particular, the ground-state energy level of a $n\text{−}p$ double dot is not split by the exchange interaction and remains fourfold degenerate at zero magnetic field also for a strong tunnel coupling between the dots.

A theory for the electron (and hole) $g$ factor in multivalley lead-salt IV-VI semiconductor quantum wells (QWs) is presented. An effective Hamiltonian for the QW electronic states in the presence of an external magnetic field is introduced within the envelope-function approximation, based on the multiband $kp$ Dimmock model for the bulk. The mesoscopic spin-orbit (Rashba-type) and Zeeman interactions are taken into account on an equal footing and the effective $g$ factor in symmetric quantum wells ($g_{\mathrm{QW}}^{*}$) is calculated analytically for each nonequivalent conduction-band (and valence-band) valley, and for QWs grown along different crystallographic directions.

We study the quantum anomalous Hall effect in a strip of stripes model coupled to a magnetic skyrmion texture with zero total magnetization and in the presence of strong electron-electron interactions. A helical magnetization along the stripes and a spin-selective coupling between the stripes gives rise to a bulk gap and chiral edge modes. Depending on the ratio between the period of the magnetic structure and the Fermi wavelength, the system can exhibit the integer or fractional quantum anomalous Hall effect. In the fractional regime, the quasiparticles have fractional charges and nontrivial Abelian braid statistics.

Mesoscopic fluctuations and correlations of the local density of states are studied near metal-insulator transitions in disordered interacting electronic systems. We show that the multifractal behavior of the local density of states survives in the presence of Coulomb interaction. We calculate the spectrum of multifractal exponents in $2+\epsilon$ spatial dimensions for symmetry classes characterized by broken (partially or fully) spin-rotation invariance and show that it differs from that in the absence of interaction. We also estimate the multifractal exponents at the Anderson metal-insulator transition in 2D systems with preserved spin-rotation invariance. Our results for multifractal correlations of the local density of states are in qualitative agreement with recent experimental findings.

Dopants positioned near edges in nanostructured graphene behave differently from bulk dopants. Most notable, the amount of charge transferred to delocalized states (i.e., doping efficiency) depends on position as well as edge chirality. We apply a self-consistent tight-binding model to analyze this problem focusing on substitutional nitrogen and boron doping. Using a Green's-function technique, very large structures can be studied, and artificial interactions between dopants in periodically repeated simulations cells are avoided. We find pronounced signatures of edges in the local impurity density of states. Importantly, the doping efficiency is found to oscillate with sublattice position, in particular, for dopants near zigzag edges. Finally, to assess the effect of electron-electron interactions, we compute the self-energy corrected Green's function.

Motivated by a recent experiment that reported the successful synthesis of hexagonal $\left(h\right)$ AlN [Tsipas et al., Appl. Phys. Lett. 103, 251605 (2013)], we investigate structural, electronic, and vibrational properties of bulk, bilayer, and monolayer structures of $h$-AlN by using first-principles calculations. We show that the hexagonal phase of the bulk $h$-AlN is a stable direct-band-gap semiconductor. The calculated phonon spectrum displays a rigid-layer shear mode at $274 {\text{cm}}^{-1}$ and an $E_g$ mode at $703 {\text{cm}}^{-1}$, which are observable by Raman measurements. In addition, single-layer $h$-AlN is an indirect-band-gap semiconductor with a nonmagnetic ground state. For the bilayer structure, $A{A}^{\prime }$-type stacking is found to be the most favorable one, and interlayer interaction is strong. While $N$-layered $h$-AlN is an indirect-band-gap semiconductor for $N=1-9$, we predict that thicker structures $\left(N\ge 10\right)$ have a direct band gap at the $\Gamma$ point. The number-of-layer-dependent band-gap transitions in $h$-AlN is interesting in that it is significantly different from the indirect-to-direct crossover obtained in the transition-metal dichalcogenides.

We study the thermoelectric properties of rectangular graphene rings connected symmetrically or asymmetrically to the leads. A side-gate voltage applied across the ring allows for the precise control of the electric current flowing through the system. The transmission coefficient of the rings manifest Breit-Wigner line shapes and/or Fano line shapes, depending on the connection configuration, the width of nanoribbons forming the ring and the side-gate voltage. We find that the thermopower and the figure of merit are greatly enhanced when the chemical potential is tuned close to resonances. Such enhancement is even more pronounced in the vicinity of Fano-like antiresonances which can be induced by a side-gate voltage independently of the geometry. This opens a possibility to use the proposed device as a tunable thermoelectric generator.

We study spin-dependent transport in a suspended carbon nanotube quantum dot in contact with two ferromagnetic leads and with the dot's spin coupled to the flexural mechanical modes. The spin-vibration interaction induces spin-flip processes between the two energy levels of the dot. This interaction arises from the spin-orbit coupling or a magnetic field gradient. The inelastic vibration-assisted spin flips give rise to a mechanical damping and, for an applied bias voltage, to a steady nonequilibrium occupation of the harmonic oscillator. We analyze these effects as function of the energy-level separation of the dot and the magnetic polarization of the leads. Depending on the magnetic configuration and the bias-voltage polarity, we can strongly cool a single mode or pump energy into it. In the latter case, we find that within our approximation, the system approaches eventually a regime of mechanical instability. Furthermore, owing to the sensitivity of the electron transport to the spin orientation, we find signatures of the nanomechanical motion in the current-voltage characteristic. Hence, the vibrational state can be read out in transport measurements.

An important aim of current research on plasmonics is to develop compact components to manipulate surface plasmon polaritons (SPPs) and specifically to develop efficient SPP couplers. The commonly used metallic resonators are inefficient to couple free-space waves to SPPs and metallic gratings require oblique incidence for achieving unidirectional propagation. In this article, we propose to use nanoscale nonuniform arrays of dielectric resonator antennas (DRAs) to realize unidirectional launching of SPPs. DRAs are made of low-loss high-permittivity nanostructures operating on a metal surface. The applications of metallodielectric nanostructures can produce resonances mainly in the low-loss dielectric parts and hence the power dissipated through oscillating current in metal can be reduced. Similar to metallic resonators, DRAs operating near resonance can provide phase control when coupling incident waves into SPPs, adding degrees of freedom in controlling propagation direction. The theoretical analysis in this article, with numerical validation, shows efficient SPPs launching by nonuniform array of cylindrical DRAs into a predesigned direction. Furthermore, with proper patterning, optimal launching can be achieved by avoiding power leakage via deflection into free space. The SPP launching condition and the influence of propagation loss are also mathematically analyzed from the viewpoint of antenna array theory. The SPPs launchers based on DRAs have a potential for applications in highly efficient integrated optics and optical waveguides.

Using real-space density functional theory, electronic structure and equilibrium geometries of sodium clusters in the size range of 2–20 atoms have been calculated as a function of confinement. We have examined the evolution of the five lowest isomers as a function of volume for six different compressions. The minimum volume considered is about $1/15$ to $1/10$ of the free-space box volume. We observe a strong tendency for isomeric transitions in many cases, with the higher isomers evolving into the ground state under confinement. In general the clusters tend to become more spherical. The changes in the total energies and the geometries are not significant until the volume gets reduced beyond the 1/3 of the original volume. In this sense, the clusters are not easy to compress. Once the critical volume is reached, the changes in the total energies and geometries are rapid. It turns out that the increase in the total energy is mainly due to the ion-ion and Hartree energies of electrons. We also address how anisotropic confinement affects the geometry of clusters. We further show that geometries obtained with anisotropic confinement are strongly supported by the simulation of clusters inside a carbon nanotube using a hybrid quantum-mechanical and molecular-mechanics approach.