We study a quantum Hall bilayer system of bosons at total filling factor $\nu =1$, and study the phase that results from short-ranged pair tunneling combined with short-ranged interlayer interactions. We introduce two exactly solvable model Hamiltonians which both yield the coupled Moore-Read state [Phys. Rev. Lett. 108, 256809 (2012)] as a ground state, when projected onto fixed particle numbers in each layer. One of these Hamiltonians describes a gapped topological phase, while the other is gapless. However, on introduction of a pair-tunneling term, the second system becomes gapped and develops the same topological order as the gapped Hamiltonian. Supported by the exact solution of the full zero-energy quasihole spectrum and a conformal field-theory approach, we develop an intuitive picture of this system as two coupled composite fermion superconductors. In this language, pair tunneling provides a Josephson coupling of the superconducting phases of the two layers, and gaps out the Goldstone mode associated with particle transport between the layers. In particular, this implies that quasiparticles are confined between the layers. In the bulk, the resulting phase has the topological order of the Halperin 220 phase with U(1)${}_2\times \mathrm{U}$(1)${}_2$ topological order, but it is realized in the symmetric/antisymmetric basis of the layer index. Consequently, the edge spectrum at a fixed particle number reveals an unexpected U(1)${}_4\times \mathrm{U}$(1) structure.

We study the influence of the pulse energy and fluence on the thermalization of photodoped Mott insulators. If the Mott gap is smaller than the width of the Hubbard bands, the kinetic energy of individual carriers can be large enough to produce additional doublon-hole pairs via a process analogous to impact ionization. The thermalization dynamics, which involves an adjustment of the doublon and hole densities, thus changes as a function of the energy of the photo-doped carriers and exhibits two time scales: a fast relaxation related to the impact ionization of high-energy carriers and a slower time scale associated with higher-order scattering processes. The slow dynamics depends more strongly on the gap size and the photodoping concentration.

We have formulated and implemented a fully charge self-consistent density functional theory plus dynamical mean-field theory methodology which enables an efficient calculation of the total energy of realistic correlated electron systems. The density functional portion of the calculation uses a planewave basis set within the projector augmented wave method enabling study of systems with large, complex unit cells. The dynamical mean-field portion of the calculation is formulated using maximally localized Wannier functions, enabling a convenient implementation which is independent of the basis set used in the density functional portion of the calculation. The importance of using a correct double-counting term is demonstrated. A generalized form of the standard double-counting correction, which we refer to as the $U^{\prime }$ form, is described in detail and used. For comparison, the density functional plus $U$ method is implemented within the same framework including the generalized double counting. The formalism is validated via a calculation of the metal-insulator and structural phase diagrams of the rare-earth nickelate perovskites as functions of applied pressure and A-site rare-earth ions. The calculated density functional plus dynamical mean-field results are found to be consistent with experiment. The density functional plus $U$ method is shown to grossly overestimate the tendency for bond disproportionation and insulating behavior.

Measurements of thermoelectric power $S\left(T\right)$ on the Kondo disorder system ${\mathrm{CePt}}_4{\mathrm{Ge}}_{12-x}{\mathrm{Sb}}_x$ are remarkably sensitive to the distribution of Kondo temperatures $P\left(T_K\right)$, which develops out of a single Kondo temperature $T_K$ in ${\mathrm{CePt}}_4{\mathrm{Ge}}_{12}$ and is governed by the degree to which hybridization is disordered. Three distinct $S\left(T\right)$ behaviors are observed in concentration regions which coincide with the boundaries of Fermi-liquid, non-Fermi-liquid, and antiferromagnetically ordered ground states in this system. In the non-Fermi-liquid region, $S\left(T\right)/T$ diverges logarithmically with decreasing temperature over nearly one decade in the case of $x=0.84$.

We consider the collective, long-wavelength charge excitations in correlated media in presence of short- and long-range forces. As an example for the case of a short-range interaction, we examine the two-dimensional Hubbard model within dynamical mean-field theory (DMFT). It is shown explicitly that the DMFT susceptibility including vertex corrections respects the Ward identity and yields a manifestly gauge-invariant response in finite dimensions. For computing the susceptibility, we use a different expression and establish its formal equivalence to the standard DMFT formula. It allows for a more stable analytical continuation. We find a zero-sound mode expected for short-range forces. The relation between the vertex corrections, gauge invariance, and the appearance of the collective modes is discussed. Long-range forces are treated within extended dynamical mean-field theory. In order to obtain a gauge-invariant response, it is necessary to additionally incorporate some nonlocal vertex corrections into the polarization. In doing so, we obtain plasmons in the three-dimensional Hubbard model. The plasma frequency is determined by the (single-particle) density distribution as a consequence of gauge invariance. We compare this result with the plasma frequency extracted from the analytical continuation of the susceptibility. It is in good agreement with the prediction from the gauge-invariance condition.

A microscopic understanding of the formation of polar-on-nonpolar interfaces is a prerequisite for well-defined heteroepitaxial preparation of III-V compounds on (100) silicon for next-generation high-performance devices. Energetically and kinetically driven Si(100) step formations result in majority domains of monohydride-terminated Si dimers oriented either parallel or perpendicular to the step edges. Here, the intentional variation of the Si(100) surface reconstruction controls the sublattice orientation of the heteroepitaxial GaP film, as observed by in situ reflection anisotropy spectroscopy (RAS) in chemical vapor ambient and confirmed by benchmarking to surface science analytics in ultrahigh vacuum. Ab initio density functional calculations of both abrupt and compensated interfaces are carried out. For P-rich chemical potentials at abrupt interfaces, Si-P bonds are energetically favored over Si-Ga bonds, in agreement with in situ RAS experiments. The energetically most favorable interface is compensated with an intermixed interfacial layer. In situ RAS reveals that the GaP sublattice orientation depends on the P chemical potential during nucleation, which agrees with a kinetically limited formation of abrupt interfaces.

By performing first-principles electronic structure calculations in frames of density functional theory we study the dependence of the valence band shape on the thickness of few-layer III-VI crystals (GaS, GaSe, and InSe). We estimate the critical thickness of transition from the bulklike parabolic to the ring-shaped valence band. Direct supercell calculations show that the ring-shaped extremum of the valence band appears in $\beta$-GaS and $\beta$-GaSe at a thickness below 6 tetralayers $(\sim 4.6\mathrm{nm})$ and 8 tetralayers $(\sim 6.4\mathrm{nm})$, respectively. Zone-folding calculations estimate the $\beta$-InSe critical thickness to be equal to 28 tetralayers $(\sim 24.0\mathrm{nm})$. The origin of the ring-shaped valence band maximum can be understood in terms of $\text{k}·\text{p}$ theory, which provides a link between the curvature of the energy bands and the distance between them. We explain the dependence of the band shape on the thickness, as well as the transition between two types of extremes, by the $k$-dependent orbital composition of the topmost valence band. We show that in the vicinity of critical thickness the effective mass of holes in III-VI compounds depends strongly on the number of tetralayers.

The characteristics of shallow hydrogenlike muonium (Mu) states in nominally undoped ZnO and CdS (0001) crystals have been studied close to the surface at depths in the range of 10–180 nm by using low-energy muons and in the bulk using conventional $\mu \mathrm{SR}$. The muon implantation depths are adjusted by tuning the energy of the low-energy muons between 2.5 and 30 keV. We find that the bulk ionization energy $E_i$ of the shallow donorlike Mu state is lowered by about 10 meV at a depth of 100 nm, and continuously decreasing on approaching the surface. At a depth of about 10 nm, $E_i$ is further reduced by 25–30 meV compared to its bulk value. We attribute this change to the presence of electric fields due to band bending close to the surface, and we determine the depth profile of the electric field within a simple one-dimensional model.

A dipolariton is a voltage-controlled mixture of direct and indirect excitons in asymmetric double quantum wells coupled by resonant carrier tunneling, and a microcavity photon. We calculate the voltage dependence of the spin-singlet and spin-triplet interaction parameters ${\alpha }_1$ and ${\alpha }_2$. Both parameters can reach values 1 order of magnitude larger than that of exciton-polaritons thanks to the strong interaction between indirect excitons. We show that the variation of the indirect exciton fraction of the dipolaritons induces a change of sign of ${\alpha }_2$: the interaction passes from attractive to repulsive with increasing voltage. For large enough voltage, ${\alpha }_2$ becomes larger than ${\alpha }_1$, which in principle can lead to the formation of a circularly polarized dipolariton condensate. We propose an application of the ${\alpha }_2$ dependence to a voltage-controlled dipolaritonic optical parametric oscillator. The change of sign of ${\alpha }_2$ allows on-site control of the linear polarization degree of the optical signal and its on-demand inversion.

We introduce a distinct approach to engineer a topologically protected surface state of a topological insulator. By covering the surface of a topological insulator, ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$, with a Bi monolayer film, the original surface state is completely removed and three new spin helical surface states, originating from the Bi film, emerge with different dispersion and spin polarization, through a strong electron hybridization. These new states play the role of topological surface states keeping the bulk topological nature intact. This mechanism provides a way to create various different types of topologically protected electron channels on top of a single topological insulator, possibly with tailored properties for various applications.

We calculate the differential, total, and transport cross-sections for scattering of two-dimensional massless Dirac electrons by a circular barrier. For scatterer of a small radius, the cross-sections are dominated by quantum effects such as resonant scattering that can be computed using the partial-wave series. Scattering by larger size barriers is better described within the classical picture of reflection and refraction of rays, which leads to phenomena of caustics, rainbow, and critical scattering. Refraction can be negative if the potential of the scatterer is repulsive, so that a $p-n$ junction forms at its boundary. Qualitative differences of this case from the $n-N$ doping case are examined. Quantum interference effects beyond the classical ray picture are also considered, such as normal and anomalous diffraction, and also whispering-gallery resonances. Implications of these results for transport and scanned-probe experiments in graphene and topological insulators are discussed.

Intrinsic bilayer graphene is a gapless semimetal. Under the application of a bias field it becomes a semiconductor with a direct band gap that is proportional to the applied field. Under a layer-asymmetric strain (where the upper layer undergoes compression and the lower layer tension or vice versa) we find that the band gap of a biased bilayer graphene ribbon becomes indirect and, for higher strains, becomes negative, returning the material to its original semimetal state. As a result, the conductivity of the ribbon increases and can be almost an order of magnitude larger than that of the intrinsic unbiased material—a change that can be induced with a strain of only $\approx 2–3\%$. The conductivity is proportional to the applied strain and the magnitude of the effect is tunable with the bias field. Such layer-asymmetric strains can be achieved by bending, with forces on the order of $\approx 1\mathrm{nN}$, resulting in a layer-asymmetric strain of $\approx 1\%$. This electromechanical effect has a wide potential for application in the areas of nanoforce microscopy and pressure sensing on the atomic scale.

Highly efficient spin-flip Raman scattering of the resident electron spin is found in singly charged (In,Ga)As/GaAs quantum dots. The applied magnetic field induces a symmetry reduction for the negatively charged exciton, which serves as intermediate scattering state, thus making the spin-flip Raman scattering of the resident electron allowed. Electron-electron exchange interaction mediates the electron spin-flip. Above a threshold magnetic field that depends on the dot size and experiment geometry, the efficiency of the scattering cross section is spectrally shifted with increasing field. This shift, which follows the electron cyclotron energy, is assigned to a hybridization of s-shell singlet and p-shell triplet states of the negatively charged exciton.

Atomic structure of interfaces between oxide layers and semiconductors is usually challenging to probe because of its buried nature. Here, we present a synchrotron photoemission approach to unveil the interface structure of BaO/Si(100), a prototype model of crystalline-oxide/semiconductor junctions, and demonstrate that such interface outspreads over four Si atomic planes and contains five different crystal sites for Si atoms, including three Si-O bonding sites. An atomic model is suggested for this system. Heating enhances Si diffusion and oxidation, but the junction still remains crystalline at 500 °C, promising potential for integration of III-V films and Si(100).

We show that a one-dimensional quantum wire with 23 channels of interacting fermions has a perfect metal phase in which all weak perturbations that could destabilize this phase are irrelevant. Consequently, weak disorder does not localize it, a weak periodic potential does not open a gap, and contact with a superconductor also fails to open a gap. Similar phases occur for $N\ge 24$ channels of fermions, except for $N=25$, and for $8k$ channels of interacting bosons, with $k\ge 3$. Arrays of perfect metallic wires form higher-dimensional fermionic or bosonic perfect metals, albeit highly anisotropic ones.

Interferences in open systems embedded in a continuum can lead to states that are bound within the continuum itself. An electromagnetic state that naturally decays becomes bound at a unique point in phase space. We demonstrate the striking occurrence of multiple such peculiar states in coupled deep subwavelength resonators. The bound states in the continuum originate from the control of multipolar radiation and their symmetries. The architectures investigated here, using all-dielectric resonators, constitute a flexible and readily achievable platform for applications requiring strong light-matter interaction and light localization.

Based on luminescence studies and density functional theory calculations we identify the origin of the unusually strong luminescence of $a$-type screw dislocations in GaN. In contrast to previous models where only a localization of the holes was considered, density functional theory calculations show a localization of both electrons and holes in the dislocation strain field. This strain field causes a mixing of the $s$-type state at the conduction band minimum with the next highest state that has $p$ character and is thus susceptible to the shear strain induced by the dislocation.

A quantum theory of third-harmonic generation in graphene is presented. An analytical formula for the nonlinear conductivity tensor ${\sigma }_{\alpha \beta \gamma \delta }^{\left(3\right)}(\omega ,\omega ,\omega )$ is derived. Resonant maxima of the third harmonic are shown to exist at low frequencies $\omega \ll E_F/\hslash$, as well as around the frequency $\omega =2E_F/\hslash$, where $E_F$ is the Fermi energy in graphene. At an input power of a ${\mathrm{CO}}_2$ laser $(\lambda \approx 10$ $\mu \mathrm{m})$ of about $1\mathrm{MW}/{\mathrm{cm}}^2$, the output power of the third harmonic $(\lambda \approx 3.3$ $\mu \mathrm{m})$ is expected to be $\simeq 50$ $\mathrm{W}/{\mathrm{cm}}^2$.

Man-made artificial graphene has attracted significant attention in the past few years due to the possibilities to construct designer Dirac fermions with unexpected topological properties and applications in nanoelectronics. Here we use a first-principles approach within density-functional theory to study molecular graphene similarly to the experiment by Gomes et al. [Nature (London) 483, 306 (2012)]. The system comprises carbon monoxide molecules arranged on a copper (111) surface in such a way that a honeycomb lattice is obtained with the characteristic electronic properties of graphene. Our results show in detail how carbon monoxide molecules modify the copper surface (and regions beneath) and create a honeycomb lattice of accumulated electrons between the adsorbate molecules. We also demonstrate how the properties of the formed Dirac fermions change as the CO density is tuned, and provide a direct comparison with experimental scanning tunneling microscope images.