Density functional theory (DFT) is widely used to predict materials properties, but the local density approximation (LDA) and generalized gradient approximation (GGA) exchange-correlation functionals are known to poorly predict the energetics of reactions involving molecular species. In this paper, we obtain corrections for the O${}_2$, H${}_2$, N${}_2$, F${}_2$, and Cl${}_2$ molecules within the Perdew-Burke-Enzerhof GGA, Perdew-Wang GGA, and Perdew-Zunger LDA exchange-correlation functionals by comparing DFT-calculated formation energies of oxides, hydrides, nitrides, fluorides, and chlorides to experimental values. We also show that the choice of compounds used to obtain the correction is significant, and we use a leave-one-out cross-validation approach to rigorously determine the proper fit set. We report confidence intervals with our correction values, which quantifies the variation caused by the choice of fit set after outlier removal. The remaining variation in the correction values is of the order of 1 kcal/mol, which indicates that chemical accuracy is a realistic goal for these systems.

We study Anderson localization in disordered helical conductors that are obtained from one-dimensional conductors with spin-orbit interaction and a magnetic field, or from equivalent systems. We call such conductors “quasihelical” because the spins of the counterpropagating modes are not perfectly antiparallel and have a small spin-wave-function overlap that is tunable by the magnetic field. Due to the overlap, disorder backscattering is possible and allows a localization transition. A conductor can pass through two localization transitions with increasing field, one from the conventionally localized system to the quasihelical conductor (with localization length exceeding the system length), and one at a higher field again to a localized state, due now, however, to backscattering below the magnetic-field induced pseudogap. We investigate these transitions using a unified two-step renormalization group approach.

We discuss the influence of geometry and type of meta-atom resonance on important parameters for strong and ultrastrong light-matter coupling experiments based on localized plasmons in planar metamaterials and intersubband plasmons in quantum wells. An analytic model based on the density matrix formalism is used to extract the radiative linewidth, the achievable coupling rate, and its dependence on distance between a single quantum well and a metasurface for five different types of meta-atoms. Depending on these values, the optical response of the coupled system gives rise to different regimes ranging from the weak-coupling regime over the hybridization-induced transparency regime to the strong-coupling regime. As a result of our investigation, we show that the achievable coupling rates exhibit only a weak dependence on the actual geometry and thus the physical nature of the metamaterial resonance. In all cases, the coupling rate can be made large enough to reach the strong-coupling regime by using multiple quantum wells.

We investigate the Schottky barrier and effective work function (EWF) at TiN/HfO${}_2$ interface through density functional calculations. For different interfaces that consist of either Ti-O or N-Hf interface bonds, the intrinsic metal-induced gap states are nearly independent of the interface structure, with similar decay lengths into the oxide. Due to the weak Fermi-level pinning, the EWF is more sensitive to the extrinsic effect of interface bonding. As N-rich interface bonds are replaced by O-rich bonds, the EWF decreases by up to 0.36 eV, which is attributed to the formation of opposing interface dipoles. To improve the band gap and EWF, we perform both hybrid functional and quasiparticle (QP) calculations. In the $G{W}_0$ approximation, in which the Green's function is self-consistently calculated by updating only QP energies and the full frequency-dependent dielectric function is used, the agreement of the EWF with experiment is greatly improved, while QP calculations at the $G_0{W}_0$ level or using the plasmon-pole dielectric function tend to overestimate the EWF. In the self-consistent $GW$ approach, in which both QP energies and wave functions are updated in iterations, the band gap is overestimated, resulting in the lower EWF. On the other hand, the EWF is severely underestimated with the hybrid functional because of the larger shift of the valence band edge level of HfO${}_2$.

We studied structure of a Bi(001)/Bi${}_2$Te${}_3$(001) heteroepitaxial film grown on Si(111) by means of surface x-ray diffraction. We revealed that on top of the Bi${}_2$Te${}_3$(001) film the $\sim 7$-bilayer-thick Bi(001) film is contracted in the surface plane by $\sim 3\%$ and is expanded along the vertical direction by $\sim 4\%$ compared with the bulk values. The epitaxial strain supports the realization of the topological phase transition of the Bi film [Hirahara et al., Phys. Rev. Lett. 109, 227401 (2012)]. We also revealed structural properties of the Bi${}_2$Te${}_3$(001)/Si(111) interface. The electron density profile in the vertical direction, obtained by the combination of the holographic method and phase retrieval methods, exhibits a wetting layer at the interface. It is also indicated that the bottom two Bi${}_2$Te${}_3$ layers act as a buffer layer for further growth of the crystalline film.

In this paper, a detailed density functional theory analysis of oxygen binding to Mg(0001) and subsequent clustering is presented. Oxygen monomer adsorption to Mg(0001) is demonstrated to be subsurface. It is shown that magnesium mediates an attractive oxygen-oxygen interaction which ultimately leads to the formation of hexagonal clusters of O${}^{*}$ in the tetrahedral-1 site. The structure, work function, and binding properties of oxygen chemisorbed structures are compared with experiment, which allows the unique identification of the tetrahedral-1 site as the low coverage oxygen binding site and the construction of a picture of the early stages of oxide nuclei formation over magnesium. A model of oxide growth at O${}^{*}$/Mg(0001) is proposed. First-principles thermodynamics analysis is used to describe the surface oxide structures and reveals that surface oxides of intermediate oxygen coverage undergo spinodal decomposition. The thermodynamics of an underlying spinodal create an energetic driving force for decomposition of an oxide surface and renewal of a reactive metal interface that may be important in understanding magnesium corrosion. The implications of the findings are that magnesium unalloyed for oxide behavior will always be highly vulnerable to corrosion.

We study the electronic structure of a heterojunction made of two monolayers of MoS${}_2$ and WS${}_2$. Our first-principles density functional calculations show that, unlike in the homogeneous bilayers, the heterojunction has an optically active band gap, smaller than the ones of MoS${}_2$ and WS${}_2$ single layers. We find that the optically active states of the maximum valence and minimum conduction bands are localized on opposite monolayers, and thus the lowest energy electron-holes pairs are spatially separated. Our findings portray the MoS${}_2$-WS${}_2$ bilayer as a prototypical example for band-gap engineering of atomically thin two-dimensional semiconducting heterostructures.

We present a theoretical study of the thermodynamical properties of a fcc-Cu(001) substrate covered by an iron-cobalt monolayer as well as by an incomplete iron layer. The effective two-dimensional Heisenberg Hamiltonian is constructed from first principles and the properties of exchange interactions are investigated. The Curie temperatures are estimated using the Monte Carlo (MC) simulations and compared with a simplified approach using the random-phase approximation (RPA) in connection with the virtual-crystal approach (VCA) to treat randomness in exchange integrals. Calculations indicate a weak maximum of the Curie temperature as a function of composition of the iron-cobalt overlayer. While a good quantitative agreement between RPA-VCA and MC was found for the iron-cobalt monolayer, the RPA-VCA approach fails quantitatively for low coverage due to the magnetic percolation effect. We also present a study of the effect of alloy disorder on the shape of magnon spectra of random overlayers.

Two two-dimensional carbon allotropes comprised of octagons and pentagons are proposed based on the first-principles calculations. The two carbon allotropes, named OPG-L and OPG-Z, are found to have distinct properties. OPG-L is metallic, while OPG-Z is a gapless semimetal. Remarkably, OPG-Z exhibits pronounced electronic anisotropy with highly anisotropic Dirac points at the Fermi level. A tight-binding model is suggested to describe the low-energy quasiparticles, which clarifies the origin of the anisotropic Dirac points. The electronic anisotropy of OPG-Z is expected to have interesting potential applications in electronic devices.

We study the disorder potential induced by random Coulomb impurities at the surface of a topological insulator (TI). We use a simple model in which positive and negative impurities are distributed uniformly throughout the bulk of the TI, and we derive the magnitude of the disorder potential at the TI surface using a self-consistent theory based on the Thomas-Fermi approximation for screening by the Dirac mode. Simple formulas are presented for the mean squared potential both at the Dirac point and far from it, as well as for the characteristic size of electron/hole puddles at the Dirac point and the total concentration of electrons/holes that they contain. We also derive an expression for the autocorrelation function for the potential at the surface and show that it has an unusually slow decay, which can be used to verify the bulk origin of disorder. The implications of our model for the electron conductivity of the surface are also presented.

We have systematically introduced charged impurity scatterers in the form of Mg adsorbates to exfoliated single-layer graphene and observe little variation of the spin relaxation times despite pronounced changes in the charge transport behavior. All measurements are performed on nonlocal graphene tunneling spin valves exposed in situ to Mg adatoms, thus systematically introducing atomic-scale charged impurity scattering. While charge transport properties exhibit decreased mobility and decreased momentum scattering times, the observed spin lifetimes are not significantly affected, indicating that charged impurity scattering is inconsequential in the present regime of spin relaxation times ($\sim$1 ns).

We have found persistent and bidirectional x-ray photoinduced phase transition (PIPT) between the charge-orbital ordered and the ferromagnetic phase in a few percent Fe-doped layered manganite La${}_{0.5}$Sr${}_{1.5}$Mn${}_{1-x}$Fe${}_x$O${}_4$. Although partial x-ray PIPT has been observed in undoped manganite as well, the photoinduced effect has been confirmed to be much enhanced by impurity doping. The partial substitution of Mn ions with Fe ions was found to suppress the long-range charge-orbital order and to form a phase-separated ground state between the charge-orbital ordered and the ferromagnetic phases. Such bi-stability arising from competing phases plays an essential role in persistent and reversible PIPT phenomena.

Entanglement properties, including the Renyi $\alpha$ entropies and scaling laws, are becoming increasingly important in condensed-matter physics because they can be used to determine physical properties and the “fingerprint” of quantum phases. In this work we use variational quantum Monte Carlo to compute the Renyi $\alpha$ entropies, their scaling laws, and the relationship between different $\alpha$ entropies for one of the most important phases in condensed matter, the interacting Fermi liquid. We also investigate the relationship between the scaling laws and the discontinuity in the momentum distribution at the Fermi surface. Contrary to recent theoretical predictions, we find that interactions increase the prefactor for the $\alpha$-entropy scaling laws for all particle interaction strengths and forms. We also show that a theory of these scaling laws for the interacting systems may be developed by extending the free theory to incorporate properties of the momentum distribution.

Bulk magnetoresistance quantum oscillations are observed in high quality single crystal samples of BiTeI. This compound shows an extremely large internal spin-orbit coupling, associated with the polarity of the alternating Bi, Te, and I layers perpendicular to the $c$ axis. The corresponding areas of the inner and outer Fermi surfaces around the A point show good agreement with theoretical calculations, demonstrating that the intrinsic bulk Rashba-type splitting is nearly 360 meV, comparable to the largest spin-orbit coupling generated in heterostructures and at surfaces.

We study spontaneous symmetry breaking in a system of spinless fermions in the honeycomb lattice paying special emphasis to the role of an enlarged unit cell on time reversal symmetry broken phases. We use a tight-binding model with nearest-neighbor hopping $t$ and Hubbard interaction $V_1$ and $V_2$ and extract the phase diagram as a function of electron density and interaction within a mean-field variational approach. The analysis completes the previous work done in Phys. Rev. Lett. 107, 106402 (2011) where phases with nontrivial topological properties were found with only a nearest-neighbor interaction $V_1$ in the absence of charge decouplings. We see that the topological phases are suppressed by the presence of metallic charge density fluctuations. The addition of next to nearest-neighbor interaction $V_2$ restores the topological nontrivial phases.

We study the nonequilibrium dynamics of the Luttinger model after suddenly turning on and off the bare Coulomb interaction between the fermions. We analyze several correlation functions such as the one particle density matrix and vertex correlations, its finite time dynamics, and the stationary state limit. Correlations exhibit a nonlinear light-cone effect: The spreading of the initial signal accelerates as a consequence of the quantum nature of the excitations, whose peculiar dispersion of plasmonic type in one dimension (1D) gives rise to a logarithmic divergence in the group velocity at $q=0$. In addition, we show that both the static and dynamic stationary state correlations can be reproduced with a simple generalized Gibbs ensemble despite the long-range character of the interactions which precludes the application of the Lieb-Robinson bounds. We propose a suitable experimental setup in which these effect can be observed based on ultracold ions loaded on linear traps.

We compute three-point correlators between the stress-energy tensor and the conserved currents of conformal field theories (CFTs) in 2+1 dimensions. We first compute the correlators in the large-flavor-number expansion of conformal gauge theories and then perform the computation using holography. In the holographic approach, the correlators are computed from an effective action on (3+1)-dimensional anti-de Sitter space (${\mathrm{AdS}}_4$) and depend upon the coefficient $\gamma$ of a four-derivative term in the action. We find a precise match between the CFT and the holographic results, thus, fixing the values of $\gamma$. The CFTs of free fermions and bosons take the values $\gamma =1/12,-1/12$, respectively, and so saturate the bound $\left|\gamma \right|\le 1/12$ obtained earlier from the holographic theory; the correlator of the conserved gauge flux of $U\left(1\right)$ gauge theories takes intermediate values of $\gamma$. The value of $\gamma$ also controls the frequency dependence of the conductivity and other properties of quantum critical transport at nonzero temperatures. Our results for the values of $\gamma$ lead to an appealing physical interpretation of particlelike or vortexlike transport near quantum phase transitions of interest in condensed-matter physics. This paper includes Appendices reviewing key features of the AdS-CFT correspondence for condensed-matter physicists.

The ac admittance of solar cells under illumination is investigated under open-circuit conditions. Open-circuit conditions are imposed by inserting a probe capacitor into the circuit. The capacitance and conductance of the cells are investigated as function of frequency and continuous illumination intensity. Results are compared with numerical and analytical modeling of charge recombination and transport. In bulk heterojunction solar cells with [6,6]-Phenyl-C${}_{61}$(C${}_{71}$)-butyric acid methyl ester as acceptor and poly(3-hexylthiophene) or poly[2-methoxy-5-(2${}^{\prime }$-ethylhexyloxy)-p-phenylene vinylene] as electron donor, the high-frequency capacitance $C$ and conductance $G$ follow a power-law dependence on intensity of white light $I$, with $G$($I$) $\propto$ $I$${}^{3/4}$ and $C$($I$) $\propto$ $I$${}^{1/4}$. The modeling shows that these dependencies can be explained in terms of space-charge-limited current in combination with Langevin type recombination of carriers. For poly[2,1,3-benzothiadiazole-4,7-diyl[4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b${}^{\prime }$]dithiophene-2,6-diyl]] the capacitance shows a weaker dependence on intensity, indicating fast recombination of photogenerated carriers. Results indicate that the fill factor of relatively well performing polymer solar cells can still be limited by space charge effects and can be improved by enhancing the charge carrier mobility or by reducing the bimolecular Langevin recombination.

Density functional calculations are presented for a (4$\times$4) reconstruction of the polar ZnO(0001) surface which is, in agreement with previous knowledge, characterized by one ZnO double-layer high triangular islands or pits. Its structure and electronic properties are compared to those of other reconstructions, the (1$\times$1) and the (2$\times$2) reconstructions with a Zn vacancy, and a (4$\times$4) reconstruction with triangular islands and pits that satisfies the electron counting rule. Among the four calculated reconstructions, the model surface is verified to be energetically preferable for a large range of oxygen chemical potential. This justifies its further exploration. Its layer and wave-vector-resolved projected density of states (PDOS) of the O atoms reveals highly surface-localized valence states well below the Fermi level. The PDOS around the Fermi level, however, is very small, which is compatible with the states not being observed by photoemission. Qualitatively experimental surface core-level shifts by Lahiri et al. [Phys. Rev. B 78, 155414 (2008)] compare favorably with the calculated electrostatic potential variations close to the ZnO(0001) surface. An ionic model with everywhere equally charged Zn and O ions turns out not to explain the potential variation obtained from density functional theory. This points towards an additional charge transfer at the surface not covered by the ionic model.