We develop a nonlocal-response generalization to the Green's function surface-integral method (GSIM), also known as the boundary-element method. This numerically efficient method can accurately describe the linear hydrodynamic nonlocal response of arbitrarily shaped plasmonic nanowires in arbitrary dielectric backgrounds. All previous general-purpose methods for nonlocal response are bulk methods. We also expand the possible geometries to which the usual local-response GSIM can be applied, by showing how to regularize singularities that occur in the surface integrals when the nanoparticles touch a dielectric substrate. The same regularization works for nonlocal response. Furthermore, an effective theory is developed to explain the numerically observed nonlocal effects. The nonlocal frequency blueshift of a cylindrical nanowire in an inhomogeneous background generally increases as the nanowire radius and the longitudinal wave number become smaller, or when the effective background permittivity or the mode inhomogeneity increase. The inhomogeneity can be expressed in terms of an effective angular momentum of the surface-plasmon mode. We compare local and nonlocal response of freestanding nanowires, and of nanowires close to and on top of planar dielectric substrates. Especially for the latter geometry, considerable differences in extinction cross sections are found for local as compared to nonlocal response, similar to what is found for plasmonic dimer structures.

We present a phenomenological theory of the low-energy moiré minibands of Dirac electrons in graphene placed on an almost commensurate hexagonal underlay with a unit cell approximately three times larger than that of graphene. A slight incommensurability results in a periodically modulated intervalley scattering for electrons in graphene. In contrast to the perfectly commensurate Kekulé distortion of graphene, such superlattice perturbation leaves the zero-energy Dirac cones intact, but is able to open a band gap at the edge of the first moiré subbands, asymmetrically in the conduction and valence bands.

A recent experiment suggested that bismuth forms hexagonal films on the Ni(111) surface, of heights three, five, and seven layers. A quantum size effect based on free electrons was proposed in explanation. To test this idea, we calculate the total energies of Bi on the Ni(111) surface using density functional theory. We find that hexagonal film stabilities disagree with the observed odd layer preferences, and the structures are mechanically destabilized by adding capping atoms which pucker the hexagonal layers. Furthermore, we find that rhombohedral films based on the bulk Bi structure are energetically more favorable than the proposed hexagonal films. These structures also favor odd numbers of layers, but owing to covalent chemical bonding rather than confinement of free electrons. Specifically, a strongly bound adsorbed surface monolayer forms, followed by bulk-like rhombohedral bilayers.

The quantum capacitance of graphene can be negative when the graphene is placed in a strong magnetic field, which is a clear experimental signature of positional correlations between electrons. Here we show that the quantum capacitance of graphene is also strongly affected by its dielectric polarizability, which in a magnetic field is wave-vector dependent. We study this effect both theoretically and experimentally. We develop a theory and numerical procedure for accounting for the graphene dielectric response, and we present measurements of the quantum capacitance of high-quality graphene capacitors on boron nitride. Theory and experiment are found to be in good agreement.

We report inelastic neutron scattering results that reveal an hour-glass magnetic excitation spectrum in La${}_{1.75}$Sr${}_{0.25}$CoO${}_4$. The magnetic spectrum is similar to that observed previously in La${}_{1.67}$Sr${}_{0.33}$CoO${}_4$, but the spectral features are broader. We show that the spectrum of La${}_{1.75}$Sr${}_{0.25}$CoO${}_4$ can be modelled by the spin dynamics of a system with a disordered cluster spin glass ground state. Bulk magnetization measurements are presented that support the proposed glassy ground state. The observations reiterate the importance of quasi-one-dimensional magnetic correlations and disorder for the hour-glass spectrum, and suggest that disordered spin and charge stripes exist at lower doping in La${}_{2-x}$Sr${}_x$CoO${}_4$ than previously thought.

We propose a scheme to obtain a system-dependent fraction of exact exchange ($\alpha$) within the framework of hybrid density functional theory (DFT) that is consistent with the $G_0{W}_0$ approach, where $G_0$ is the noninteracting Green function of the system and $W_0$ the screened Coulomb interaction. We exploit the formally exact condition of exact DFT that the energy of the highest occupied molecular orbital corresponds to the ionization potential of a finite system. We identify the optimal $\alpha$ value for which this statement is obeyed as closely as possible and thereby remove the starting point dependence from the $G_0{W}_0$ method. This combined approach is essential for describing electron transfer (as exemplified by the TTF/TCNQ dimer) and yields the vertical ionization potentials of the G2 benchmark set with a mean absolute percentage error of only $\approx$3%.

We consider the absorption of microwaves by a quantum Hall droplet. We show that the number and velocities of charged edge modes can be directly measured from a droplet of known shape. In contrast to standard transport measurements, different edge equilibration regimes can be accessed in the same device. If there is a quantum point contact in the droplet, then quasiparticle properties, including braiding statistics, can be observed. Their effects are manifested as modulations of the microwave absorption spectrum that are, notably, first-order in the tunneling amplitude at the point contact.

We calculate the transmission coefficient for electrons passing through the helically shaped potential barrier which can be, for example, produced by DNA molecules.

The intercalation of different metals underneath the graphene sheet has opened the possibility of preparing new low-dimensional structures that could present quantum-size effects. Knowing the surface crystallograpy of these low-dimensional systems is mandatory for a complete understanding of their physical and chemical properties. In this work, we present a combined low-energy electron diffraction (LEED), x-ray photoelectron spectroscopy, and first-principles calculations study of the structural properties of graphene-protected Fe films on Ni(111). The results indicate that graphene interacts strongly with the topmost Fe atoms, similar to graphene on Ni(111). For one and two Fe monolayer films, the Fe is isostructurally deposited on Ni (fcc on fcc), and graphene is deposited commensurably with the underlying Fe surface atoms. For one Fe monolayer case, the LEED data indicate the coexistence of two types of crystalline domains, named top-fcc and bridge-top structures. Our first-principles calculations show that for the graphene/Fe/Ni system, the total energies of the the top-fcc and bridge-top structures are nearly degenerate, consistent with the observed bistability.

The microscopic theory of bilayer graphene nonlinear interaction with coherent electromagnetic radiation is developed. The Liouville–von Neumann equation for a single-particle density matrix in the multiphoton resonant approximation is solved. We consider low-energy excitations of a Fermi-Dirac sea with THz pump radiation. The obtained solutions disclose Rabi oscillations and resonant multiphoton excitations of a Fermi-Dirac sea in bilayer graphene. On the basis of numerical simulations we examine the rates of high-harmonic radiation at particle-hole annihilation in the field of a strong pump wave. We show that in the THz domain of frequencies one can achieve efficient generation of high harmonics with pump waves of moderate intensities.

The magnetism of La${}_{0.7}$Ca${}_{0.3}$MnO${}_3$ (LCMO) epitaxial thin films grown on SrTiO${}_3$ (STO) and BaTiO${}_3$ (BTO) substrates is studied using polarized neutron reflectometry (PNR) and ferromagnetic resonance (FMR) techniques. In LCMO/BTO, PNR reveals a strongly suppressed magnetization of 300 kA/m, equivalent to a magnetic moment of 2 μB/Mn, throughout the LCMO layer, amounting to half the expected value. The largest suppression occurs near the interface with BTO, with magnetization values of 50 kA/m, equivalent to 0.3 μB/Mn. FMR is observable at 8.9 GHz only around the [110] crystallographic direction in thin LCMO/BTO. The resonance barely shifts as the applied field is rotated away from [110]. The FMR results are analyzed in terms of magnetoelastic anisotropy and compared to LCMO/STO grown under the same conditions. A two-layer magnetization model is proposed, based on strong out-of-plane anisotropy near the BTO interface and shown to qualitatively explain the main characteristics of the FMR results.

We consider the statistical mechanics of a class of models involving close-packed loops with fugacity $n$ on three-dimensional lattices. The models exhibit phases of two types as a coupling constant is varied: in one, all loops are finite, and in the other, some loops are infinitely extended. We show that the loop models are discretizations of $C{P}^{n-1}$ $\sigma$ models. The finite and infinite loop phases represent, respectively, disordered and ordered phases of the $\sigma$ model, and we discuss the relationship between loop properties and $\sigma$ model correlators. On large scales, loops are Brownian in an ordered phase and have a nontrivial fractal dimension at a critical point. We simulate the models, finding continuous transitions between the two phases for $n=1,2,3$ and first order transitions for $n\ge 4$. We also give a renormalization-group treatment of the $C{P}^{n-1}$ model that shows how a continuous transition can survive for values of $n$ larger than (but close to) 2, despite the presence of a cubic invariant in the Landau-Ginzburg description. The results we obtain are of broader relevance to a variety of problems, including $\mathrm{SU}\left(n\right)$ quantum magnets in (2+1) dimensions, Anderson localization in symmetry class $C$, and the statistics of random curves in three dimensions.

Measurements of the local density of states near impurities can be useful for identifying the superconducting gap structure in alkali-doped iron chalcogenide superconductors K${}_x$Fe${}_{2-y}$Se${}_2$. Here, we study the effects of nonmagnetic and magnetic impurities within a nearest-neighbor $d$-wave and next-nearest-neighbor $s$-wave superconducting state. For both repulsive and attractive nonmagnetic impurities, it is shown that subgap bound states exist only for $d$-wave superconductors with the positions of these bound states depending rather sensitively on the electron doping level. Further, for such disorder, Coulomb interactions can lead to local impurity-induced magnetism in the case of $d$-wave superconductivity. For magnetic impurities, both $s$-wave and $d$-wave superconducting states support subgap bound states. The above results can be explained by a simple analytic model that provides a semiquantitative understanding of the variation of the impurity bound states energies as a function of impurity potential and chemical doping level.

An oxide heterostructure made of manganite bilayers and ferroelectric perovskites is predicted to lead to the full control of magnetism when switching the ferroelectric polarizations. By using asymmetric polar interfaces in the superlattices, more electrons occupy the Mn layer at the $n$-type interface side than at the $p$-type side. This charge disproportionation can be enhanced or suppressed by the ferroelectric polarization. Quantum model and density functional theory calculations reach the same conclusion: a ferromagnetic-ferrimagnetic phase transition with maximal change $>90\%$ of the total magnetization can be achieved by switching the polarization's direction. This function is robust and provides full control of the magnetization's magnitude, not only its direction, via electrical methods.

Using a high-resolution differential technique we have determined the electronic specific heat coefficient $\gamma \left(T\right)=C^{el}/T$ of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$ with $x=0$ to 1.0, at temperatures ($T$) from 2 K to 380 K and in magnetic fields $H=0$ to 13 T. In the normal state ${\gamma }_n(x,T)$ increases strongly with $x$ at low temperature, compatible with a mass renormalization $\sim 12$ at $x=1$, and decreases weakly with $x$ at high temperature. A superconducting transition is seen in all samples from $x=0.2$ to 1, with transition temperatures and condensation energies peaking sharply at $x=0.4$. Superconducting coherence lengths ${\xi }_{ab}\sim 20$ Å and ${\xi }_c\sim 3$ Å are estimated from an analysis of Gaussian fluctuations. For many dopings we see features in the $H$ and $T$ dependencies of ${\gamma }_s(T,H)$ in the superconducting state that suggest superconducting gaps in three distinct bands. A broad “knee” and a sharp mean-field-like peak are typical of two coupled gaps. However, several samples show a shoulder above the sharp peak with an abrupt onset at $T_{c,s}$ and a $T$ dependence ${\gamma }_s\left(T\right)\propto \sqrt{1-T/T_{c,s}}$. We provide strong evidence that the shoulder is not due to doping inhomogeneity and suggest it is a distinct gap with an unconventional $T$ dependence ${\Delta }_s\left(T\right)\propto {(1-T/T_{c,s})}^{0.75}$ near $T_{c,s}$. We estimate band fractions and $T=0$ gaps from three-band $\alpha$-model fits to our data and compare the $x$ dependencies of the band fractions with spectroscopic studies of the Fermi surface.

Modeling the spin-triplet superconductor Sr${}_2$RuO${}_4$ through a three-orbital tight-binding model, we investigate topological properties and edge states assuming chiral $p$-wave pairing. In concordance with experiments, the three Fermi surfaces consist of two electronlike and one holelike surface, corresponding to the $\alpha$, $\beta$, and $\gamma$ band on the level of a two-dimensional system. The quasiparticle spectra and other physical quantities of the superconducting phase are calculated by means of a self-consistent Bogoliubov-de Gennes approach for a ribbon-shaped system. While a full quasiparticle excitation gap is realized in the bulk system, at the edges gapless states appear, some of which have linear and others of which have a nearly flat dispersion around zero energy. This study shows the interplay between spin-orbit-coupling-induced spin currents, chiral edge currents, and correlation-driven surface magnetism. The topological nature of the chiral $p$-wave state manifests itself in the $\gamma$ band characterized by an integer Chern number. As the $\gamma$ band is close to a Lifshitz transition in Sr${}_2$RuO${}_4$, changing the sign of the Chern number, the topological nature may be rather fragile.

The emergence of superconductivity in doped Mott insulators has been debated for decades. In this paper, we report the theoretical discovery of a time-reversal symmetry breaking superconducting ground state in the doped Mott insulator (described by the well known $t$-$J$ model) on honeycomb lattice, based on a recently developed variational method: the Grassmann tensor product state approach. As a benchmark, we use exact diagonalization and density-matrix renormalization methods to check our results on small clusters. We find systematic consistency for the ground-state energy as well as other physical quantities, such as the staggered magnetization. At low doping, the superconductivity coexists with antiferromagnetic ordering.

We present a scheme to calculate exciton dispersions in real materials that is based on the first-principles many-body Bethe-Salpeter equation. We assess its high level of accuracy by comparing our results for LiF with recent inelastic x-ray scattering experimental data on a wide range of energy and momentum transfer. We show its great analysis power by investigating the role of the different electron-hole interactions that determine the exciton band structure and the peculiar “exciton revival” at large momentum transfer. Our calculations for solid argon are a prediction and a suggestion for future experiments. These results demonstrate that the first-principles Bethe-Salpeter equation is able to describe the dispersion of localized and delocalized excitons on equal footing and represent a key step for the ab initio study of the exciton mobility.