Motivated by the recent experiments indicating a spin-unpolarized $\nu =0$ quantum Hall state in graphene, we theoretically investigate the ground state based on the many-body problem projected onto the $n=0$ Landau level. For an effective model with the on-site Coulomb repulsion and antiferromagnetic exchange couplings, we show that the ground state is a doubly degenerate spin-resolved chiral condensate in which all the zero-energy states with up spin are condensed into one chirality, while those with down spin to the other. This can be exactly shown for an Ising-type exchange interaction. The charge gap due to the on-site repulsion in the ground state is shown to grow linearly with the magnetic field, in qualitative agreement with the experiments.

A mechanism for electronic phase separation in iron pnictides is proposed. It is based on the competition between commensurate and incommensurate spin-density-wave phases in a system with an imperfect doping-dependent nesting of a multisheeted Fermi surface. We model the Fermi surface by two elliptical electron pockets and three circular hole pockets. The interaction between a charge carrier in a hole band and a carrier in an electron band leads to the formation of spin-density-wave order. The commensurate spin density wave in the parent compound transforms to the incommensurate phase when doping is introduced. We show that, for certain parameter values, the uniform state is unstable with respect to phase separation. The resulting inhomogeneous state consists of regions of commensurate and incommensurate spin-density-wave phases. Our results are in qualitative agreement with recent observations of incommensurate spin density waves and electronic inhomogeneity in iron pnictides.

We investigate the electronic structure of the Sr${}_{n+1}$Ru${}_n$O${}_{3n+1}$ family $(n=1,2,3)$ in the vicinity the Fermi level by means of polarization-dependent resonant O 1$s$ x-ray emission spectroscopy. Using both energy window and polarization analysis we disentangle the contribution of apical and planar oxygen, and compare it with first-principles calculations. Our results provide a new insight on the nature of the oxygen bonds and the role of Sr $3d$ states.

We present a biwaveguide paradigm composed of joined positive-index-material (PIM)/negative-index-material (NIM) slabs, demonstrating ultraslow-light propagation stemming from the competing propagation disposition in the PIM and NIM regions. We report for the first time a mesoscopic extended electromagnetic (EM) enhancement covering regions of the order of the free-space wavelength, enabled by the slow-light mode in our system. Our dynamic numerical results are consistent with our developed theoretical model, predicting an EM energy accumulation reminiscent of a charging capacitor. Our analysis reveals that spatial compression is not a requirement for EM enhancement in slow-light systems and stresses the merits of a high coupling efficiency, strong temporal compression, monomodality, and modal index bandwidth—all present in our proposed paradigm. Furthermore, we show that the heterostructure waveguide mode is an extraordinary entity with a unique energy velocity, which is opposite to the Poynting vector in one of the participant waveguides. We believe that these results will inspire new slow-light platforms relevant to the collective harvesting of strong light-matter interactions.

We investigate the details of the electronic structure in the neighborhoods of a carbon atom vacancy in graphene by employing magnetization-constrained density-functional theory on periodic slabs, and spin-exact, multireference, second-order perturbation theory on a finite cluster. The picture that emerges is that of two local magnetic moments (one $\pi$-like and one $\sigma$-like) decoupled from the $\pi$ band and coupled to each other. We find that the ground state is a triplet with a planar equilibrium geometry where an apical C atom opposes a pentagonal ring. This state lies $\sim$0.2 eV lower in energy than the open-shell singlet with one spin flipped, which is a bistable system with two equivalent equilibrium lattice configurations (for the apical C atom above or below the lattice plane) and a barrier $\sim$0.1 eV high separating them. Accordingly, a bare carbon atom vacancy is predicted to be a spin-1 paramagnetic species, but spin-$\frac{1}{2}$ paramagnetism can be accommodated if binding to foreign species, ripples, coupling to a substrate, or doping are taken into account.

We study transport through a double quantum dot system in which each quantum dot is coupled to a phonon mode. Such a system can be realized, e.g., using a suspended carbon nanotube. We find that the interplay between strong electron-phonon coupling and interdot tunneling can lead to a negative differential conductance at bias voltages exceeding the phonon frequency. Various transport properties are discussed, and we explain the physics of the occurrence of negative differential conductance in this system.

We study analytically and numerically the magnetotransport of strained graphene in a Corbino geometry gating in the presence of an external perpendicular magnetic field. The conductance of the Corbino disc of deformed graphene with a uniaxial and an inhomogeneous strain is calculated by using the Landauer-Büttiker method. We show that the oscillation period of the conductance as a function of the magnetic flux depends on uniaxial strain and the conductance sharply drops along the direction of graphene stretching. The conductance amplitude, on the other hand, can be manipulated by induced pseudomagnetic flux. A valley polarized regime, caused by the inhomogeneous strain, is obtained and in addition we find a wide energy interval in which the system is fully valley polarized.

We have studied the incommensurate moiré structure of epitaxial graphene grown on iridium(111) by dynamic low-energy electron diffraction [LEED $I$($V$)] and noncontact atomic force microscopy (AFM) with a CO-terminated tip. Our LEED $I$($V$) results yield the average positions of all the atoms in the surface unit cell and are in qualitative agreement with the structure obtained from density functional theory. The AFM experiments reveal local variations of the moiré structure: The corrugation varies smoothly over several moiré unit cells between 42 and 56 pm. We attribute these variations to the varying registry between the moiré symmetry sites and the underlying substrate. We also observe isolated outliers, where the moiré top sites can be offset by an additional 10 pm. This study demonstrates that AFM imaging can be used to directly yield the local surface topography with pm accuracy even on incommensurate two-dimensional structures with varying chemical reactivity.

The nonequilibrium dynamics of integrable systems are highly constrained by the conservation of certain charges. There is substantial evidence that after a quantum quench they do not thermalize but their asymptotic steady state can be described by a generalized Gibbs ensemble (GGE) built from the conserved charges. Most of the studies on the GGE so far have focused on models that can be mapped to quadratic systems, while analytic treatment in nonquadratic systems remained elusive. We obtain results on interaction quenches in a nonquadratic continuum system, the one-dimensional (1D) Bose gas described by the integrable Lieb-Liniger model. The direct implementation of the GGE prescription is prohibited by the divergence of the conserved charges, which we conjecture to be endemic to any continuum integrable systems with contact interactions undergoing a sudden quench. We compute local correlators for a noninteracting initial state and arbitrary final interactions as well as two-point functions for quenches to the Tonks-Girardeau regime. We show that in the long time limit integrability leads to significant deviations from the predictions of the grand canonical ensemble, allowing for an experimental verification in cold-atom systems.

This study proposes a method to control the frequency-dependent scattering spectra from plasmonic spheres via the conservation of incident orbital angular momentum (OAM) in classical light scattering. By providing controllable distributions of OAM content, fractional vortex beams allow selective tailoring of Fano features present in coherent scattering processes. The applicability of this control methodology is briefly discussed in the context of plasmonic crystals that recent studies have shown possess modes described by a well-defined OAM content.

We report angle-resolved photoemission spectroscopy results of $A$-site ordered perovskite CaCu${}_3$Ti${}_4$O${}_{12}$. We have observed the clear band dispersions, which are shifted to the higher energy by 1.7 eV and show the band narrowing around 2 eV in comparison with the local density approximation calculations. In addition, the high-energy multiplet structures of Cu 3$d^8$ final states have been found around 8–13 eV. These results reveal that CaCu${}_3$Ti${}_4$O${}_{12}$ is a Mott-type insulator caused by the strong correlation effects of the Cu 3$d$ electrons well hybridized with O 2$p$ states. Unexpectedly, there exists a very small spectral weight at the Fermi level in the insulator phase, indicating the existence of isolated metallic states.

Experimental results for the carrier capture and relaxation dynamics in self-organized semiconductor quantum dots are analyzed using a microscopic theory. Time-resolved differential transmission changes of the quantum-dot transitions after ultrafast optical excitation of the barrier states are studied in a wide range of carrier temperatures and excitation densities. The measurements can be explained by quantum-dot polaron scattering and their excitation-dependent renormalization due to additional Coulomb scattering processes. Results of configuration-picture and single-particle-picture descriptions, both with nonperturbative transition rates, show good agreement with the experiments while Boltzmann scattering rates lead to a different excitation density and temperature dependence.

Coulomb interaction among electrons is found to have profound effects on the electronic properties of anisotropic quantum dots in a perpendicular external magnetic field and in the presence of the Rashba spin-orbit interaction. This is more evident in optical transitions, which we find in this system to be highly anisotropic and superintense, in particular, for large values of the anisotropy parameter.

We present a first-principles investigation of the electronic and magnetic properties of an adatom on bilayer graphene within the framework of density functional theory. In particular, we study the influence of an applied gate voltage which modifies the electronic states of the bilayer graphene as well as shifts the adatom energy states relative to that of the graphene energy states. Our study carried out for a choice of three different adatoms, Na, Cu, and Fe, shows that the nature of adatom-graphene bonding evolves from ionic to covalent, in moving from an alkali metal, Na, to a transition metal, Cu or Fe. This leads to the formation of magnetic moments in the latter cases (Cu, Fe) and the absence of a magnetic moment in the former (Na). Application of an external electric field to bilayer graphene completely changes the scenario, switching on a magnetic moment for the Na adatom, and switching off the magnetic moments for Cu and Fe adatoms. Our results have important implications for fundamental studies of controlled adatom magnetism and spintronics applications in nanotechnology.

For correct height measurements by noncontact atomic force microscopy a precise knowledge of the interacting tip-surface forces is required. Preceding studies have shown the necessity to compensate the electrostatic forces between tip and surface with Kelvin probe force microscopy to obtain realistic topographic height values for samples out of multiple components. In the present study we demonstrate, however, that a simple Kelvin compensation can be insufficient using the example of poly(3-dodecylthiophene) on highly oriented pyrolytic graphite (HOPG). The measured polymer height on HOPG can be strongly falsified due to the uncompensated long-range (LR) van der Waals (vdW) forces. We conclude that in order to minimize the perturbing influence of these LR-vdW forces on the height measurement, imaging should be performed in the regime where the fast-decaying short-range forces are dominant.

Ferroelectric nanodomains can be created by the application of a bias voltage to the sharp conducting tip of a scanning force microscope (SFM) contacting the sample surface. Since an inhomogeneous electric field created by an SFM tip has maximum intensity along the surface normal, in multiaxial ferroelectrics the polarization inside these domains also tends to orient perpendicularly to the surface. Here we show theoretically that unusual domains can be created in uniaxial ferroelectrics when the SFM tip is applied to the crystal surface parallel to the polar axis. These 180° nanodomains have polarization directed along the surface and should appear in LiNbO${}_3$ and LiTaO${}_3$ crystals at moderate tip voltages well below 100 V. Calculations of equilibrium domain dimensions demonstrate that subsurface domains have the shape of a needle oriented along the polar axis.

Using first-principles calculations, we elucidate microscopic mechanisms of perpendicular magnetic anisotropy (PMA) in Fe/MgO magnetic tunnel junctions through evaluation of orbital and layer resolved contributions into the total anisotropy value. It is demonstrated that the origin of the large PMA values is far beyond simply considering the hybridization between Fe-$3d$ and O-$2p$ orbitals at the interface between the metal and the insulator. Onsite projected analysis shows that the anisotropy energy is not localized at the interface but it rather propagates into the bulk showing an attenuating oscillatory behavior which depends on orbital character of contributing states and interfacial conditions. Furthermore, it is found in most situations that states with $d_{yz\left(xz\right)}$ and $d_{z^2}$ character tend always to maintain the PMA while those with $d_{xy}$ and $d_{x^2-y^2}$ character tend to favor the in-plane anisotropy. It is also found that while MgO thickness has no influence on PMA, the calculated perpendicular magnetic anisotropy oscillates as a function of Fe thickness with a period of 2 ML and reaches a maximum value of 3.6 mJ/m${}^2$.

Inspired by recent conflicting views on the order of the phase transition from an antiferromagnetic Néel state to a valence bond solid, we use the functional renormalization group to study the underlying quantum critical field theory which couples two complex matter fields to a noncompact gauge field. In our functional renormalization group approach, we only expand in covariant derivatives of the fields and use a truncation in which the full field dependence of all wave-function renormalization functions is kept. While we do find critical exponents which agree well with some quantum Monte Carlo studies and support the scenario of deconfined criticality, we also obtain an irrelevant eigenvalue of small magnitude, leading to strong corrections to scaling and slow convergence in related numerical studies.

Traditional spintronics relies on spin transport by charge carriers, such as electrons in semiconductor crystals. The challenges for the realization of long-range electron spin transport include rapid spin relaxation due to electron scattering. Scattering and, in turn, spin relaxation can be effectively suppressed in excitonic devices where the spin currents are carried by electrically neutral bosonic quasiparticles: excitons or exciton-polaritons. They can form coherent quantum liquids that carry spins over macroscopic distances. The price to pay is a finite lifetime of the bosonic spin carriers. We present the theory of exciton ballistic spin transport which may be applied to a range of systems supporting bosonic spin transport, in particular to indirect excitons in coupled quantum wells. We describe the effect of spin-orbit interaction for the electron and the hole on the exciton spin, account for the Zeeman effect induced by external magnetic fields and long-range and short-range exchange splittings of the exciton resonances. We also consider exciton transport in the nonlinear regime and discuss the definitions of the exciton spin current, polarization current, and spin conductivity.

The interaction between electrons and plasmons in trilayer graphene is investigated within the Overhauser approach resulting in the “plasmaron” quasiparticle. This interaction is cast into a field theoretical problem, and its effect on the energy spectrum is calculated using improved Wigner-Brillouin perturbation theory. The plasmaron spectrum is shifted with respect to the bare electron spectrum by $\Delta E\left(\mathbf{k}\right)\sim 150-200\mathrm{meV}$ for ABC stacked trilayer graphene and for ABA trilayer graphene by $\Delta E\left(\mathbf{k}\right)\sim 30-150\mathrm{meV}$ [$\Delta E\left(\mathbf{k}\right)\sim 1-5\mathrm{meV}$] for the hyperbolic (linear) part of the spectrum. The shift in general increases with the electron concentration $n_e$ and electron momentum. The dispersion of plasmarons is more pronounced in ABC stacked than in ABA stacked trilayer graphene, because of the different energy band structure and their different plasmon dispersion.