We use the exact strong-interaction limit of the Hohenberg-Kohn energy density functional to construct an approximation for the exchange-correlation term of the Kohn-Sham approach. The resulting exchange-correlation potential is able to capture the features of the strongly correlated regime without breaking the spin or any other symmetry. In particular, it shows “bumps” (or barriers) that give rise to charge localization at low densities and that are a well-known key feature of the exact Kohn-Sham potential for strongly correlated systems. Here, we illustrate this approach for the study of both weakly and strongly correlated model quantum wires, comparing our results with those obtained with the configuration interaction method and with the usual Kohn-Sham local density approximation.

By means of optical spectroscopy, Kelvin probe, and conductivity measurements, we study the $p$-type doping of the donor polymer poly(3-hexylthiophene), P3HT, with the molecular acceptor tetrafluorotetracyanoquinodimethane, F${}_4$TCNQ, covering a broad range of molar doping ratios from the ppm to the percent regime. Thorough quantitative analysis of the specific near-infrared absorption bands of ionized F${}_4$TCNQ reveals that almost every F${}_4$TCNQ dopant undergoes integer charge transfer with a P3HT site. However, only about 5$\%$ of these charge carrier pairs are found to dissociate and contribute a free hole for electrical conduction. The nonlinear behavior of the conductivity on doping ratio is rationalized by a numerical mobility model that accounts for the broadening of the energetic distribution of transport sites by the Coulomb potentials of ionized F${}_4$TCNQ dopants.

The potassium-induced missing row reconstruction of Ag(110) is used to selectively modify the local chemical interaction between the functional anhydride groups of 3,4,9,10-perylene-tetracarboxylic-dianhydride (PTCDA) and Ag(110). We find a significant upward shift of the anhydride groups, while the adsorption height of the perylene core is essentially preserved. This demonstrates an attractive perylene/substrate interaction for PTCDA/K:Ag(110), elucidating also the bonding situation for the potassium-free system.

Symmetry-protected topological phases of one-dimensional spin systems have been classified using group cohomology. In this paper, we revisit this problem for general spin chains which are invariant under a continuous onsite symmetry group $G$. We evaluate the relevant cohomology groups and find that the topological phases are in one-to-one correspondence with the elements of the fundamental group of $G$ if $G$ is compact, simple, and connected and if no additional symmetries are imposed. For spin chains with symmetry $PSU\left(N\right)=SU\left(N\right)/{\mathbb{Z}}_N$, our analysis implies the existence of $N$ distinct topological phases. For symmetry groups of orthogonal, symplectic, or exceptional type, we find up to four different phases. Our work suggests a natural generalization of Haldane's conjecture beyond $SU\left(2\right)$.

We investigate the behavior of the periodic Anderson model in the presence of $d$–$f$ Coulomb interaction ($U_{df}$) using mean-field theory, variational calculation, and exact diagonalization of finite chains. The variational approach based on the Gutzwiller trial wave function gives a critical value of $U_{df}$ and two quantum critical points (QCPs), where the valence susceptibility diverges. We derive the critical exponent for the valence susceptibility and investigate how the position of the QCP depends on the other parameters of the Hamiltonian. For larger values of $U_{df}$, the Kondo regime is bounded by two first-order transitions. These first-order transitions merge into a triple point at a certain value of $U_{df}$. For even larger $U_{df}$ valence skipping occurs. Although the other methods do not give a critical point, they support this scenario.

Motivated by the potential chiral spin liquid in the metallic spin ice Pr${}_2$Ir${}_2$O${}_7$, we consider how such a chiral state might be selected from the spin-ice manifold. We propose that chiral fluctuations of the conducting Ir moments promote ferrochiral couplings between the local Pr moments, as a chiral analog of the magnetic RKKY effect. Pr${}_2$Ir${}_2$O${}_7$ provides an ideal setting to explore such a chiral RKKY effect, given the inherent chirality of the spin-ice manifold. We use a slave-rotor calculation on the pyrochlore lattice to estimate the sign and magnitude of the chiral coupling, and find it can easily explain the 1.5 K transition to a ferrochiral state.

Elastic properties of cubic TiN are studied theoretically in a wide temperature interval. First-principles simulations are based on ab initio molecular dynamics (AIMD). Computational efficiency of the method is greatly enhanced by a careful preparation of the initial state of the simulation cell that minimizes or completely removes a need for equilibration and therefore allows for parallel AIMD calculations. Elastic constants $C_{11}$, $C_{12}$, and $C_{44}$ are calculated. A strong dependence on the temperature is predicted, with $C_{11}$ decreasing by more than 29$\%$ at 1800 K as compared to its value obtained at $T=0$ K. Furthermore, we analyze the effect of temperature on the elastic properties of polycrystalline TiN in terms of the bulk and shear moduli, the Young's modulus and Poisson ratio. We construct sound velocity anisotropy maps, investigate the temperature dependence of elastic anisotropy of TiN, and observe that the material becomes substantially more isotropic at high temperatures. Our results unambiguously demonstrate the importance of taking into account finite temperature effects in theoretical calculations of elastic properties of materials intended for high-temperature applications.

In coherent diffractive imaging (CDI), conventional image-forming optics are replaced by the solution of an inverse scattering problem which obtains the exit surface wave from the diffraction pattern. To date CDI implementations typically use nonlinear iterative solutions, often using more than one diffraction pattern, and have been mainly implemented in the optical and x-ray regimes. Here we present single-exposure CDI reconstructions of a region of a cerium dioxide nanocrystal, illuminated by a coherent electron probe, in which the positions of both cerium and the lighter oxygen atoms are recovered simultaneously. We employ a fast deterministic algorithm based on the iterative solution of a set of linear equations with input data obtained from the inverse Fourier transform of the diffraction pattern. The solution of the linear equations proceeds using a conjugate gradient least-squares method based on fast Fourier transforms. This allows regularization of the inversion problem by iteration number, making it robust to experimental noise. This approach is successful where standard nonlinear phase retrieval techniques for the same data set are not.

Single crystals of MgCNi${}_3$, with areas sized up to 1 mm${}^2$, were grown by the self-flux method using a cubic anvil high-pressure technique. In low applied fields, the dc magnetization exhibited a very narrow transition into the superconducting state, demonstrating good quality of the grown crystals. The first critical field H${}_{c1}$, determined from a zero-temperature extrapolation, is around 18 mT. Using the tunnel-diode resonator technique, the London penetration depth was measured with no applied dc field and the Campbell penetration depth was measured with the external dc fields up to 9 T for two different sample orientations with respect to the direction of applied magnetic field. The absolute value of the London penetration depth, $\lambda \left(0\right)=245\pm 10$ nm, was determined from the thermodynamic Rutgers formula. The superfluid density, ${\rho }_s={[\lambda \left(0\right)/\lambda \left(T\right)]}^2$, was found to follow the clean isotropic $s$-wave behavior predicted by the weak-coupling BCS theory in the whole temperature range. The low-temperature behavior of the London penetration depth fits the BCS analytic form as well and produces a value close to the weak coupling one of $\Delta \left(0\right)/\left(k_B{T}_c\right)=1.71$. The temperature dependence of the upper critical field $H_{c2}$ was found to be isotropic with a slope at T${}_c$ of $-$2.6 T/K and H${}_{c2}$(0) $\approx$ 12.3 T at zero temperature. The Campbell penetration depth probes the vortex lattice response in the mixed state and is sensitive to the details of the pinning potential. For MgCNi${}_3$, an irreversible feature has been observed in the TDR response when the sample is field cooled and warmed versus zero-field cooled and warmed. This feature possesses a nonmonotonic field dependence and has commonly been referred to as the peak effect. It is most likely related to a field-dependent nonparabolic pinning potential.

We consider a theoretical problem of the upper critical magnetic field parallel to a conducting axis of a quasi-one-dimensional layered superconductor. We show that the orbital effects against superconductivity in a magnetic field are capable of destroying the superconducting phase at low temperatures if the interplane distance is less than the corresponding coherence length. Applications of our results to recent experiments, performed in the superconductor Li${}_{0.9}$Mo${}_6$O${}_{17}$ [J.-F. Mercure et al., Phys. Rev. Lett. 108, 187003 (2012)], provide strong arguments in favor of a triplet superconducting pairing in this quasi-one-dimensional layered conductor.

The crystal structure of sodium bismuth titanate (NBT) and related compounds is of great interest, as these may form part of a new generation of ferroelectric materials used in a multitude of piezoelectric applications. This work examines the short- and long-range structure of sodium bismuth titanate in different states of synthesis using x-ray and neutron pair distribution function studies. The average structure of NBT was modeled using the monoclinic $Cc$ space group through a combined structural refinement of x-ray and neutron diffraction data via the Rietveld method. A small box approach was used to model the local structure based on the average structure of the material, as determined from the Rietveld structural refinement, and rule out the presence of local $A$-site ordering in NBT. A “box-car fitting” method used to analyze the neutron pair distribution function showed that bond environments change when averaged over different length scales and the calculated bond valence of Bi${}^{3+}$, in particular, is different locally from its average value. A model calculated using the reverse Monte Carlo method allowed the positions of Na${}^{+}$ and Bi${}^{3+}$ to move independently, allowing the determination of their distinctive bonding environments with O${}^{2-}$. This method revealed that Na${}^{+}$ and Bi${}^{3+}$ have slightly different atomic positions, an effect that may be the origin of the large atomic displacement parameters calculated for the $A$ site from the average structure model. The local structure described here is discussed in comparison with published long-range structure models.

The phonon dispersion was measured at room temperature along ($0,0,L$) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Spin-polarized first-principles calculations imposing various types of antiferromagnetic order are in better agreement with the experimental results than nonmagnetic calculations, although the measurements were made well above the magnetic ordering temperature, $T_N$. Splitting observed between two $A_{1g}$ phonon modes at 22 and 26 meV is only reproduced in spin-polarized calculations. Magnetostructural effects similar to those observed in the $A$Fe${}_2$As${}_2$ ($A=\text{Ca}$,Sr,Ba,Eu) materials are present in LaFeAsO. The inclusion of Fe spin is necessary to find reasonable agreement of the calculations with the measured spectrum well above $T_N$. On-site Fe and As force constants show significant softening compared to nonmagnetic calculations, however an investigation of the real-space force constants associates the magnetoelastic coupling with a complex renormalization instead of softening of a specific pairwise force.

The structural and electronic properties of a Pb monolayer (ML) grown on Au(111) are investigated by scanning tunneling microscopy (STM) and angle resolved photoelectron spectroscopy (ARPES). We find an incommensurate Moiré structure with an approximate $(5.77\times 5.77)$ R21.5${}^{\circ }$ unit cell, and two rotational domains. We observe three Pb-derived bands of $p$ orbital character. The symmetry properties and sharpness of their Fermi surfaces are remarkably different. They reflect the different degrees of hybridization of these bands with the Au(111) bulk continuum.

We analyze a possibility of gain-induced compensation of losses in a one-dimensional layered structure, consisting of alternating layers of metal and dielectric with gain. Using an exact approach without effective medium approximations, we demonstrate that losses can be completely compensated for a certain value of the longitudinal component of the wave vector and partially reduced for other wave numbers. In the systems with larger values of the gain coefficient, convective instabilities can appear. At certain frequencies, losses can be greatly reduced for a broad range of the incidence angles. We find that in the canalization regime when the isofrequency contours are close to straight lines, the loss compensation becomes possible for a wide spatial spectrum being useful for improvement of the subwavelength imaging capability.

V${}_6$O${}_{11}$ and V${}_7$O${}_{13}$ are two members of the V${}_n$O${}_{2n-1}$ Magnéli series ($n=3–9$). At ambient pressure, V${}_6$O${}_{11}$ manifests a metal to insulator (MI) transition near $T_{\mathrm{MI}}=170$ K and V${}_7$O${}_{13}$ (the exception in the series that does not become insulating at ambient pressure) manifests an antiferromagnetic (AFM) transition with spin density wave character at $T_{\mathrm{N}}=43$ K. Temperature-dependent resistivity data for V${}_6$O${}_{11}$ and V${}_7$O${}_{13}$ were measured under pressures up to 7.52 and 6.40 GPa, respectively with critical pressures of $P_c^{\mathrm{MI}}=3.8$ GPa for V${}_6$O${}_{11}$ and $P_c^{\mathrm{AFM}}=3.5$ GPa for V${}_7$O${}_{13}$. As the MI transition for V${}_6$O${}_{11}$ is suppressed no features associated with an AFM transition in the resistivity are seen. Near the critical pressure for V${}_6$O${}_{11}$ where the first-order MI transition disappears, a $T^2$ dependence of the low-temperature resistance can be found. On the other hand, in V${}_7$O${}_{13}$ as the second-order, antiferromagnetic transition is brought towards $T=0$, the resistivity shows a vanishing low-temperature region of Fermi-liquid-like behavior, consistent with proximity to a quantum critical point. Improved hydrostaticity of the pressurized sample space enhances the divergence of the $T^2$ coefficient for V${}_7$O${}_{13}$ near the AFM critical pressure, 3.5 GPa.

Using the noncommutative Kubo formula for aperiodic solids and a recently developed numerical implementation, we study the conductivity $\sigma$ and resistivity $\rho$ tensors as functions of Fermi level $E_F$ and temperature $T$ for models of strongly disordered Chern insulators. The formalism enabled us to converge the transport coefficients at temperatures low enough to enter the quantum critical regime at the Chern-to-trivial insulator transition. We find that the ${\rho }_{xx}$ curves at different temperatures intersect each other at one single critical point, and that they obey a single-parameter scaling law with an exponent close to the universally accepted value for the unitary symmetry class. However, when compared with the established experimental facts on the plateau-insulator transition in the integer quantum Hall effect, we find a universal critical conductance ${\sigma }_{xx}^c$ twice as large, an ellipse rather than a semicircle law, and absence of the quantized Hall insulator phase.

We have investigated the spectral properties of Fe impurities in a Cs host, for both surface and bulk systems, by means of a combination of density functional theory in the local density approximation and dynamical mean-field theory (LDA + DMFT). The effective impurity model arising in LDA + DMFT was solved via two different techniques, i.e., the Hubbard I approximation and the exact diagonalization. It is shown that noticeable differences can be seen in the unoccupied part of the spectrum for different positions of Fe atoms in the host, despite the fact that hybridization between Fe $d$-states and Cs is low. Our calculations show good agreement with the experimental photoemission spectra reported by Carbone et al. [Carbone, Veronese, Moras, Gardonio, Grazioli, Zhou, Rader, Varykhalov, Krull, Balashov, Mugarza, Gambardella, Lebègue, Eriksson, Katsnelson, and Lichtenstein, Phys. Rev. Lett. 104, 117601 (2010)].

An elastic analog of graphene is introduced and analyzed. The system consists of a honeycomb arrangement of spring-mass resonators attached to a thin elastic layer, and the propagation properties of flexural waves along it is studied. The band-structure calculation shows the presence of Dirac points near the $K$ point of the Brillouin zone. Analytical expressions are found for both Dirac frequency and velocity as a function of the resonator's parameters. Finally, the bounded modes of infinitely long ribbons of this honeycomb arrangement are analyzed. The presence of edge states, which are studied using multiple scattering theory, is shown. It is concluded that these structures can be used to control the propagation of flexural waves in thin plates.

We simulate the Shastry-Sutherland model in two dimensions by means of infinite projected entangled-pair states (iPEPS)—a variational tensor network method where the accuracy can be systematically controlled by the so-called bond dimension. Besides the well-established dimer and Néel phase iPEPS confirms the presence of an intermediate phase with plaquette long-range order, and we determine its phase boundaries with high accuracy. The first-order phase transition for $J=0.675\left(2\right)$ between dimer and plaquette phase is compatible with previous series expansion results. iPEPS predicts a weak first-order phase transition between plaquette and Néel phase occurring for $J=0.765\left(15\right)$. We do not find a stable intermediate columnar-dimer phase, even when we bias the state towards this order.

As alternatives to the mixed-anion Cu${}_2$ZnSn(S,Se)${}_4$ alloys, the mixed-cation Cu${}_2$Zn(Sn,Ge)Se${}_4$ and Cu${}_2$Zn(Sn,Si)Se${}_4$ alloys can also span a band gap range that fits the requirement of the solar cell light absorber. However, material properties of these alloys as functions of alloy composition $x$ are not well known. In this paper, using the first-principles calculations, we study the structural and electronic properties of these alloys. We find that (i) the Cu${}_2$Zn(Sn,Ge)Se${}_4$ alloys are highly miscible with low formation enthalpies, while the Cu${}_2$Zn(Sn,Si)Se${}_4$ alloys are less miscible; (ii) the band gap of Cu${}_2$Zn(Sn,Ge)Se${}_4$ increases almost linearly from 1.0 eV to 1.5 eV as the Ge composition $x$ increases from 0 to 1, whereas the band gap of Cu${}_2$Zn(Sn,Si)Se${}_4$ spans a larger range from 1.0 eV to 2.4 eV and shows a slightly larger bowing; and (iii) the calculated band offsets shows that the band gap increase of the alloys with the addition of Ge or Si results primarily from the conduction band upshift, whereas the valence band shift is less than 0.2 eV. Based on these results, we expect that the component-uniform and band-gap-tunable Cu${}_2$Zn(Sn,Ge)Se${}_4$ and Cu${}_2$Zn(Sn,Si)Se${}_4$ alloys can be synthesized and have an improved photovoltaic efficiency.