The local density approximation (LDA) and generalized gradient approximations (GGA) of density functional theory systematically overestimate the electronic polarizability of materials. We calculate the dielectric constant of NiO by the direct method and find, contrary to previous suggestions, that the $\mathrm{LDA}+U$ method reduces the polarization such that ${\varepsilon }_{\infty }$ decreases monotonically with increasing $U$. We illustrate the existence of a linear term in the effective exchange-correlation potential that counteracts the external electric field, thus demonstrating that the decrease of ${\varepsilon }_{\infty }$ is intrinsic to the $\mathrm{LDA}+U$ correction. The reduction of the polarization is due mostly to reduced orbital mixing between the unoccupied $e_g$ states and the occupied $2p$ states. Our work establishes $\mathrm{LDA}+U$ as a viable method for calculating the dielectric constants of correlated materials.

We propose carbon nanotubes (CNTs) with magnetic impurities as a versatile platform to achieve exciting Kondo physics, where the CNT bath is gapped by the spin-orbit interaction and renormalized by interference effects. While the strong-coupling phase is inaccessible for the special case of half-filled impurities in neutral armchair CNTs, the system in general can undergo quantum phase transitions to the Kondo ground state. The resultant position-specific phase diagrams are investigated upon variation of the CNT radius, chirality, and carrier doping, revealing several striking features, e.g., the existence of a maximal radius for nonarmchair CNTs to realize phase transitions, and an interference-induced suppression of the Kondo screening. We show that by tuning the Fermi energy via electrostatic gating, the quantum critical region can be experimentally accessed.

We study an unconventional two-dimensional, two-component classical plasma on a sphere, with emphasis on detecting signatures of melting transitions. This system is relevant to Ising-type quantum Hall states, and is unconventional in the sense that it features particles interacting via two different two-dimensional Coulomb interactions. One species of particle in the plasma carries charge of both types $(Q_1,Q_2)$, while the other species carries only charge of the second type $(0,-Q_2)$. We find signatures of a freezing transition at $Q_1^2\simeq 140$. This means that the species with charge of both types will form a Wigner crystal, whereas the species with charge of the second type also shows signatures of being a Wigner crystal, due to the attractive intercomponent interaction of the second type. Moreover, there is also a Berezinskii-Kosterlitz-Thouless phase transition at $Q_2^2\simeq 4$, at which the two species of particles bind to form molecules that are neutral with respect to the second Coulomb interaction. These two transitions appear to be independent of each other, giving a rectangular phase diagram. As a special case, $Q_2=0$ describes the (conventional) two-dimensional one-component plasma. Our study is consistent with previous studies of this plasma, and sheds new light on the freezing transition of this system.

Optical investigations of quarter-filled charge ordered insulators $\delta$-(EDT-TTF-CONMe${}_2$)${}_{2}X$, $X={\text{AsF}}_6$, Br are reported. The spectra evidence the one-dimensional nature of the systems at all temperatures. The charge order exists already at room temperature, the optical gap is estimated to be of the order of 550 cm${}^{-1}$ for the $X=\text{Br}$ compound and about 700 cm${}^{-1}$ for the $X={\mathrm{AsF}}_6$ material. The optical response in the direction of the stacks is discussed in terms of domain-wall excitations of one-dimensional quarter-filled Wigner lattice at moderate values of nearest-neighbor electronic repulsion. A similarity between the effects of temperature contraction and chemical pressure is found.

Critical thermodynamics close to a metamagnetic quantum critical end point (QCEP) in a metal is discussed within the framework of spin-fluctuation theory. We analyze the effective potential for the Ising order parameter that is renormalized by spin fluctuations and acquires a characteristic temperature dependence. The resulting magnetic field, $H$, and temperature, $T$, dependence of the magnetization, specific heat, thermal expansion, magnetostriction, susceptibility, and Grüneisen parameter are determined, and the crossover lines in the $(H,T)$ phase diagram are specified. We point out that the metamagnetic QCEP is intrinsically unstable with respect to a magnetoelastic coupling.

Electronic excitations and optical spectra of CdF${}_2$ are calculated up to ultraviolet employing state-of-the-art techniques based on density functional theory and many-body perturbation theory. The GW scheme proposed by Hedin has been used for the electronic self-energy to calculate single-particle excitation properties as energy bands and densities of states. For optical properties many-body effects, treated within the Bethe-Salpeter equation framework, turn out to be crucial. A bound exciton located about 1 eV below the quasiparticle gap is predicted. Within the present scheme the optical absorption spectra and other optical functions show an excellent agreement with experimental data. Moreover, we tested different schemes to obtain the best agreement with experimental data. Among the several schemes, we suggest a self-consistent quasiparticle energy scheme.

The Ag:Si(111)-($\sqrt{3}\times \sqrt{3})R{30}^{\circ }$ surface structure has attracted considerable debate concerning interpretation of scanning tunneling microscope (STM) and noncontact atomic force microscope (NC-AFM) images. In particular, the accepted interpretation of atomic resolution images in NC-AFM has been questioned by theoretical and STM studies. In this paper, we use combined NC-AFM and STM to conclusively show that the inequivalent trimer (IET) configuration best describes the surface ground state. Thermal-averaging effects result in a honeycomb-chained-trimer (HCT) appearance at room temperature, in contrast to studies suggesting that the IET configuration remains stable at higher temperatures [Zhang, Gustafsson, and Johansson, Phys. Rev. B 74, 201304(R) (2006) and J. Phys.: Conf. Ser. 61, 1336 (2007)]. We also comment on results obtained at an intermediate temperature that suggest an intriguing difference between the imaging mechanisms of NC-AFM and STM on structurally fluctuating samples.

This work presents an experimental method to tune the degree of heavy-hole (HH) and light-hole (LH) mixing of the ground state of quantum dots (QDs). A ferroelectric crystal is used to apply reversible anisotropic biaxial stress to thin nanomembranes, containing GaAs/AlGaAs QDs. The stress-induced modification of the QD anisotropy leads to a change of the relative intensity of the two emission lines produced by the recombination of neutral bright excitonic states. Such a change is ascribed to a variation of the degree of HH-LH mixing. At the same time the modified anisotropy produces a change of the excitonic fine structure splitting (FSS). Model calculations provide a qualitative insight into the relation between strain, HH-LH mixing, and the FSS in epitaxial GaAs/AlGaAs QDs.

We propose a nanoscale heat engine that utilizes the physics of resonant tunneling in quantum dots in order to transfer electrons only at specific energies. The nanoengine converts heat into electrical current in a multiterminal geometry which permits one to separate current and heat flows. By putting two quantum dots in series with a hot cavity, electrons that enter one lead are forced to gain a prescribed energy in order to exit the opposite lead, transporting a single electron charge. This condition yields an ideally efficient heat engine. The energy gain is a property of the composite system rather than of the individual dots. It is therefore tunable to optimize the power while keeping a much larger level spacing for the individual quantum dots. Despite the simplicity of the physical model, the optimized rectified current and power is larger than any other candidate nanoengine. The ability to scale the power by putting many such engines into a two-dimensional layered structure gives a paradigmatic system for harvesting thermal energy at the nanoscale. We demonstrate that the high power and efficiency of the layered structure persists even if the quantum dots exhibit some randomness.

We investigate the influence of an external perpendicular magnetic field on the lowest-energy states of an electron bound to a donor which is located near a semiconductor-metal interface. The problem is treated within the effective mass approach and the lowest-energy states are obtained through (1) the “numerically exact” finite element method, and (2) a variational approach using a trial wave function where all image charges that emerge due to the presence of the metallic gate are taken into account. The trial wave functions are constructed such that they reduce to an exponential behavior for sufficiently small magnetic fields and become Gaussian for intermediate and large magnetic fields. The average electron-donor distance can be controlled by the external magnetic field. We find that the size of the $2p_z$ state depends strongly on the magnetic field when the donor is close to the interface, showing a nonmonotonic behavior, in contrast with the ground and the other excited states.

A generalized tight-binding model is employed to study how electrostatic gating influences the magnetoelectronic properties of a Bernal graphene bilayer. With the availability of the Landau wave function, the distribution among its sublattices enables detailed characterization of the Landau levels as well as their optical responses. The different electric potentials on respective layers break the interlayer symmetry, which in turn lifts the intervalley degeneracy. In addition, Landau levels in response to the bias field make direct crossings and anticrossings. The latter are manifestations of the noncrossing theorem: two states not distinguished by unique quantum numbers perform anticrossing with their wave function characteristics interchanged and strongly mixed near the point of anticrossing. Those significant changes are directly reflected in the magneto-optical spectra, including the splitting of absorption peaks and their enhancement or extinction in response to bias strength.

Angle-resolved photoemission spectroscopy (ARPES) is a powerful probe of electron correlations in two-dimensional layered materials. In this article we demonstrate that ARPES can be used to probe the onset of exciton condensation in spatially separated systems of electrons and holes created by gating techniques in either double-layer graphene or topological-insulator thin films.

While the mechanical distortions change the electronic properties of graphene significantly, the effects of electronic manipulation on its mechanical properties have not been known. Using first-principles calculation methods, we show that when graphene expands isotropically under equibiaxial strain, both the electron and hole doping can maintain or improve its ideal strength slightly and enhance the critical breaking strain dramatically. Contrary to the isotropic expansions, the electron doping decreases the ideal strength as well as critical strain of uniaxially strained graphene while the hole doping increases both. Distinct failure mechanisms depending on type of strains are shown to be origins of the different doping induced mechanical stabilities. Our findings may resolve a contradiction between recent experimental and theoretical results on the strength of graphene.

The material KNi${}_2$Se${}_2$ has recently been shown to possess a number of striking physical properties, many of which are apparently related to the mixed valency of this system, in which there is, on average, one quasilocalized electron per every two Ni sites. Remarkably, the material exhibits a charge-density wave (CDW) phase that disappears upon cooling, giving way to a low-temperature coherent phase characterized by an enhanced electron mass, reduced resistivity, and an enlarged unit cell free of structural distortion. Starting from an extended periodic Anderson model and using the slave-boson formulation, we develop a model for this system and study its properties within mean-field theory. We find a reentrant first-order transition from a CDW phase, in which the localized moments form singlet dimers, to a heavy Fermi liquid phase as temperature is lowered. The magnetic susceptibility is Pauli-like in both the high- and low-temperature regions, illustrating the lack of a single-ion Kondo regime and free local moment behavior such as that usually found in heavy-fermion materials.

The spin-orbit interaction strength for electrons in III-V semiconductor heterojunctions and the corresponding in-plane anisotropy are theoretically studied, considering Rashba and Dresselhaus contributions. Starting from a variational solution of Kane's effective Hamiltonian for the Rashba-split subbands, the total spin-orbit splitting at the Fermi level of the two-dimensional electron gas in III-V heterojunctions is calculated analytically, as a function of the electron density and wave-vector direction, by adding the Dresselhaus contribution within quasidegenerate first-order perturbation theory. Available GaAs and InGaAs experimental data are discussed. Effects of the barrier penetration are identified, and the spin-orbit anisotropy is shown to be determined by more than one parameter, even in the small-$k$ limit, contrary to the commonly used $\alpha /\beta$ (where $\alpha$ is the Rashba and $\beta$ the Dresselhaus interaction) single-parameter picture.

We investigate the dynamical purification of maximally entangled electron states by transport through coupled quantum dots. Under resonant ac driving and coherent tunneling, even-parity Bell states perform Rabi oscillations that decouple from the environment, leading to a dark state. The two electrons remain spatially separated, one in each quantum dot. We propose configurations where this effect will prove as antiresonances in transport spectroscopy experiments.

Using a magnetocapacitance technique, we determine the magnetic field of the spin transition, $B^{*}$, at filling factor $\nu =2/3$ in the two-dimensional electron system in GaAs/AlGaAs heterojunctions. The field $B^{*}$ is found to decrease appreciably as the wave function extent controlled by back gate voltage is increased. Our calculations show that the contributions to the shift of $B^{*}$ from the change of the Coulomb energy and the $g$ factor change due to nonparabolicity are approximately the same. The observed relative shift of $B^{*}$ is described with no fitting parameters.

Many-body $GW$ calculations have emerged as a standard for the prediction of band gaps, band structures, and optical properties for main-group semiconductors and insulators, but it is not well established how predictive the $GW$ method is in general for transition metal (TM) compounds. Surveying the series of 3$d$ oxides within a typical $GW$ approach using the random-phase approximation reveals mixed results, including cases where the calculated band gap is either too small or too large, depending on the oxidation states of the TM (e.g., FeO/Fe${}_2$O${}_3$, Cu${}_2$O/CuO). The problem appears to originate mostly from a too high average $d$-orbital energy, whereas the splitting between occupied and unoccupied $d$ symmetries seems to be reasonably accurate. It is shown that augmenting the $GW$ self-energy by an attractive (negative) and occupation-independent on-site potential for the TM $d$ orbitals with a single parameter per TM cation can reconcile the band gaps for different oxide stoichiometries and TM oxidation states. In Cu${}_2$O, which is considered here in more detail, standard $GW$ based on wave functions from initial density or hybrid functional calculations yields an unphysical prediction with an incorrect ordering of the conduction bands, even when the magnitude of the band gap is in apparent agreement with experiment. The correct band ordering is restored either by applying the $d$-state potential or by iterating the wave functions to self-consistency, which both have the effect of lowering the Cu-$d$ orbital energy. While it remains to be determined which improvements over standard $GW$ implementations are needed to achieve an accurate ab initio description for a wide range of transition metal compounds, the application of the empirical on-site potential serves to mitigate the problems specifically related to $d$ states in $GW$ calculations.

The electronic structure and magnetic properties of CrSb${}_2$ have been investigated by ab initio calculations with an emphasis on the role of the magnetic structure for the ground state. The influence of correlation effects has been investigated by performing generalized gradient approximation (GGA) and GGA+U calculations showing their important role for the electronic and magnetic properties. Some details of the electronic structure of CrSb${}_2$ are analyzed by a comparison with those of FeSb${}_2$. The results obtained contribute in particular to the understanding of the temperature dependence of transport and magnetic behavior observed experimentally.