Recently, a very rich phase diagram has been obtained for an iron-based superconductor Ca${}_4$Al${}_2$O${}_6$Fe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. It has been revealed that nodeless $(x\sim 0)$ and nodal $(x=1)$ superconductivity are separated by an antiferromagnetic phase. Here we study the origin of this peculiar phase diagram using a five orbital model constructed from first-principles band calculation, and applying the fluctuation exchange approximation assuming spin-fluctuation-mediated pairing. At $x=1$, there are three hole Fermi surfaces, but the most inner one around the wave vector $(0,0)$ has strong $d_{X^2-Y^2}$ orbital character, unlike in LaFeAsO, where the most inner Fermi surface has $d_{XZ/YZ}$ character. Since the Fermi surfaces around $(0,0)$, $(\pi ,0)$, and $(\pi ,\pi )$ all have $d_{X^2-Y^2}$ orbital character, the repulsive pairing interaction mediated by the spin fluctuations gives rise to a frustration in momentum space, thereby degrading superconductivity despite the bond angle being close to the regular tetrahedron angle. As $x$ decreases and the bond angle is reduced, the inner hole Fermi surface disappears, but the frustration effect still remains because the top of the band with $d_{X^2-Y^2}$ character lies close to the Fermi level. On the other hand, the loss of the Fermi surface itself gives rise to a very good nesting of the Fermi surface because the number of electron and hole Fermi surfaces are now the same. The pairing interaction frustration and the good nesting combined favors antiferromagnetism over superconductivity. Finally for $x$ close to 0, the band sinks far below the Fermi level, reducing the frustration effect, so that superconductivity is enhanced. There, the Fermi surface nesting is also lost to some extent, once again favoring superconductivity over antiferromagnetism. To see whether the present theoretical scenario is consistent with the actual nature of the competition between superconductivity and antiferromagnetism, we also perform hydrostatic pressure experiment for Ca${}_4$Al${}_2$O${}_6$Fe${}_2$(As${}_{1-x}$P${}_x$)${}_2$. In the intermediate $x$ regime where antiferromagnetism occurs at ambient pressure, applying hydrostatic pressure smears out the antiferromagnetic transition, but superconductivity does not take place. This supports our scenario that superconductivity is suppressed by the momentum space frustration in the intermediate $x$ regime, apart from the presence of the antiferromangnetism.

Muon spin relaxation measurements on Bi${}_2$Ir${}_2$O${}_7$ show that it undergoes a bulk magnetic transition at $1.84\left(3\right)$ K. This is accompanied by increases in the muon spin relaxation rate and the amplitude of the nonrelaxing part of the signal. The magnetic field experienced by muons is estimated to be $0.7$ mT at low temperature, around two orders of magnitude smaller than that in other pyrochlore iridates. These results suggest that the low-temperature state represents static magnetism of exceptionally small magnetic moments, $\sim$$0.01{\mu }_{\mathrm{B}}$/Ir. The relaxation rate increases further below $0.23\left(4\right)$ K, coincident with an upturn in the specific heat, suggesting the existence of a second low-temperature magnetic transition.

We report conventional and time-resolved infrared spectroscopy on LaFeAsO${}_{1-x}$F${}_x$ superconducting thin films. The far-infrared transmission can be quantitatively explained by a two-component model including a conventional $s$-wave superconducting term and a Drude term, suggesting at least one carrier system has a full superconducting gap. Photoinduced studies of excess quasiparticle dynamics reveal a nanosecond effective recombination time and temperature dependence that strongly support a recombination bottleneck in the presence of a full gap. The two experiments provide consistent evidence of a full, nodeless, though not necessarily isotropic, gap for at least one carrier system in LaFeAsO${}_{1-x}$F${}_x$.

The effects of temperature and pressure on phonons in B20 compounds FeSi${}_{1-x}$Al${}_x$ were measured using inelastic neutron scattering and nuclear-resonant inelastic x-ray scattering. The effect of hole doping through Al substitution is compared to results of alloying with Co (electron doping) in Fe${}_{1-x}$Co${}_x$Si. While the temperature dependence of phonons in FeSi is highly anomalous, doping with either type of carriers leads to a recovery of the normal quasiharmonic behavior. Density functional theory (DFT) computations of the electronic band structure and phonons were performed. The anomaly in the temperature dependence of the phonons in undoped FeSi was related to the narrow band gap, and its sensitivity to the effect of thermal disordering by phonons. On the other hand, the pressure dependence of phonons at room temperature in undoped FeSi follows the quasiharmonic behavior and is well reproduced by the DFT calculations.

In this work the magnetization in antiferromagnetic thin films and multilayers with interlayer exchange coupling is simulated using mean-field approximation. Transition-metal oxide antiferromagnets are modeled as multiplane magnetic systems with 1 to 11 planes and the magnetization $M$ is calculated as a function of temperature $T$. The antiferromagnetic films exhibit ferromagnetism when the number of monolayers is odd, i.e., when there is an uncompensated plane, but the net magnetization is lower than that of any single uncompensated plane due to cancellations and finite-size effects. With increasing film thickness the Néel temperature increases monotonically and the magnetic moment near the surface is reduced compared to that of the core, changing the form of the $M\left(T\right)$ curve. When antiferromagnetic films are exchange coupled to each other, as in a multilayer with a nonmagnetic intervening layer, the surface magnetization of each film increases and the ferromagnetism of odd-numbered systems is enhanced. These results are shown to be experimentally testable by comparing magnetometry and neutron diffraction.

Motivated by recent reports [Lüders et al., Phys. Rev. B 80, 241102 (2009)] of room-temperature ferromagnetism in vanadium oxide based superlattices, a single-site dynamical mean-field study of the dependence of the paramagnetic-ferromagnetic phase boundary on superlattice geometry was performed. An examination of variants of the experimentally determined crystal structure indicate that ferromagnetism is found only in a small and probably inaccessible region of the phase diagram. Design criteria for increasing the range over which ferromagnetism might exist are proposed.

The $c$-axis optical response of the underdoped cuprates is qualitatively different from its in-plane counterpart. The features of the pseudogap show themselves more prominently in the $c$-axis than in-plane. We compute both the $c$-axis optical conductivity and the in-plane optical conductivity using the Yang-Rice-Zhang model of underdoped cuprates. This model combined with coherent interlayer tunneling is enough to explain the qualitative differences between the in-plane data and the $c$-axis data. We show how pseudogap features manifest themselves in the infrared and microwave conductivity within this model.

Derivation of low-energy effective models by a partial trace summation of the electronic degrees of freedom far away from the Fermi level, called downfolding, is reexamined. We propose an improved formalism free from the double counting of electron correlation in the low-energy degrees of freedom. In this approach, the exchange-correlation energy in the local-density approximation (LDA) is replaced with the $GW$ self-energy; herewith its low-energy part associated with the double counting is subtracted. Moreover, in our formalism, the frequency dependence of the effective parameter is renormalized into the static one. We apply the formalism to SrVO${}_3$ as well as to two iron-based superconductors, FeSe and FeTe. The resultant bandwidths of the effective models are nearly the same as those of the previous downfolding formalism because of striking cancellations between an increase arising from the exclusion of the low-energy correlation and a shrinking arising from the renormalization of the frequency dependence. In the nondegenerate multiband materials such as FeSe and FeTe, the momentum dependent self-energy effects yield substantial modifications of the band structures and relative shifts of orbital-energy levels of the effective models, which may explain the stability of the bicollinear antiferromagnetic phase in FeTe as well as the experimental absence of the antiferromagnetic phase in FeSe.

By combining modern many-body approaches with a cluster expansion scheme, frequency-dependent dielectric functions including excitonic and local-field effects are computed for wurtzitic group-III nitride alloys with varying composition $x$. The quasiparticle electronic structure required to construct the quasielectron-quasihole pair Hamiltonian for each cluster is approximated using a LDA+$U$+$\Delta$ approach. Two different cluster statistics are employed to perform configurational averages for the frequency-dependent complex dielectric functions. Comparing the resulting composition dependence of peak positions and intensities to experimental data allows conclusions regarding the distribution of the group-III cations in the alloys.

Density-functional theory within the local density approximation $+$ Hubbard $U$ approach was used to study interface electronic structures in stoichiometric and oxygen-deficient LaAlO${}_3$/SrTiO${}_3$ (LAO/STO) superlattices with regularly spaced $n$-type and $p$-type interfaces. Asymmetric behaviors between complementary $n$-type and $p$-type interfaces were revealed in terms of orbital-selective charge transfer. Extra electrons induced by oxygen vacancies at the $p$-type interface easily spread to the $n$-type interface and occupy the Ti $3d_{xy}$orbitals, while those induced by the vacancies at the $n$-type interface are strictly confined and reside in Ti $3d_{x^2-y^2}$ and/or $3d_{3z^2-r^2}$ orbtials. The electronic behavior of oxygen vacancies at the LAO/STO interfaces and the possibility of distinguishing between intrinsic electronic states, which are induced by the polar catastrophe, and extrinsic states due to oxygen vacancies are discussed in detail.

We theoretically investigate dynamic nuclear spin polarization in a self-assembled quantum dot pumped optically by two laser beams. With the assumption that a noncollinear interaction between the hole spin and nuclear spins leads to nuclear spin polarization, we find that both weak and strong nuclear spin polarizations can arise, depending on the intensities and central frequencies of the lasers. For weak nuclear spin polarization, we use a perturbation method to show that the distribution of the nuclear spin Overhauser field may become significantly narrower. Using Monte Carlo simulations to study a single quantum dot, we find that strong nuclear spin polarization can also be generated via appropriate optical pumping.

We study thin strips of three-dimensional topological insulators in the presence of a spin-splitting Zeeman field. We show that under certain conditions the topological edge states at the sides of a strip remain robust against a time-reversal symmetry breaking Zeeman field. For a particle-hole symmetric system with Zeeman field perpendicular to the strip, we strictly prove that the dispersion and the spin-orbital structure of the edge states remains unchanged. When the Zeeman field lies parallel to the strip, the Dirac spectrum becomes flat, but remains intact. Above a critical value of the Zeeman field, a topological flat band appears at the edge. We present numerical calculations for a lattice model of Bi${}_2$Se${}_3$. These calculations show that even though particle-hole symmetry is not strictly fulfilled in this system, these special features are still present. The flat band is tunable by the Zeeman field and can be realistically achieved in Bi${}_2$Se${}_3$-ferromagnet heterosystems at room temperature.

Charge carrier transport in single-layer graphene with one-dimensional charged defects is studied theoretically. Extended charged defects, considered an important factor for mobility degradation in chemically vapor-deposited graphene, are described by a self-consistent Thomas-Fermi potential. A numerical study of electronic transport is performed by means of a time-dependent real-space Kubo approach in honeycomb lattices containing millions of carbon atoms, capturing the linear response of realistic size systems in the highly disordered regime. Our numerical calculations are complemented with a kinetic transport theory describing charge transport in the weak scattering limit. The semiclassical transport lifetimes are obtained by computing scattered amplitudes within the second Born approximation. The transport electron-hole asymmetry found in the semiclassical approach is consistent with the Kubo calculations. In the strong scattering regime, the conductivity is found to be a sublinear function of electronic density and weakly dependent on the Thomas-Fermi screening wavelength. We attribute this atypical behavior to the extended nature of one-dimensional charged defects. Our results are consistent with recent experimental reports.

The electronic conductance of polymer–carbon nanotube multilayered composite films assembled by the spin-spray layer-by-layer method is investigated. Our measurements show that the film conductance per bilayer ${\sigma }_1$ vanishes for film thickness below a critical value, and above this threshold it grows logarithmically with the number of polyelectrolyte bilayers $k_{\mathrm{l}}$. The results of our experiments are interpreted using a stratified quasi-two-dimensional conducting-network model, in which the junction resistance between nanotubes deposited in different bilayers is a function of the interlayer distance. Using scaling arguments and numerical simulations, we show that the linear dependence of the junction resistance on the layer separation leads to the logarithmic behavior ${\sigma }_1\sim \log k_{\mathrm{l}}$ for large $k_{\mathrm{l}}$, as observed in our experiments. Properties of our stratified-network model are investigated, and we show that with proper rescaling, different sets of experimental measurements can be collapsed onto a master curve. The overall shape of the master curve is determined by a single dimensionless parameter characterizing the slope of the junction-resistance function.

The electronic properties of graphene sheets are often understood by starting from a simple phenomenological $\pi$-band tight-binding model. We provide a perspective on these models that is based on a study of ab initio maximally localized Wannier wave functions centered at carbon sites. Hopping processes in graphene can be separated into intersublattice contributions responsible for band dispersion near the Dirac point, and intrasublattice contributions responsible for electron-hole symmetry breaking. Both types of corrections to the simplest near-neighbor model can be experimentally relevant. We find that distant neighbor hopping parameters increase the ratio of the full $\pi$-band width to the Dirac point velocity and flatten bands along the $KM$ Brillouin-zone edge. We propose a five-parameter model which achieves a good compromise between simplicity and accuracy, and an alternate 15-parameter model achieves better accuracy with some loss of simplicity.

We discuss systems which have some, but not all, of the hallmarks of topological phases. These systems’ topological character is not fully captured by a local order parameter, but they are also not fully described at low energies by topological quantum field theories. For such systems, we formulate the concepts of quasi-topological phases (to be contrasted with true topological phases) and symmetry-protected quasi-topological phases. We describe examples of systems in each class and discuss the implications for topological protection of information and operations. We explain why topological phases and quasi-topological phases have greater stability than is sometimes appreciated. In the examples that we discuss, we focus on Ising-type (a.k.a. Majorana) systems particularly relevant to recent theoretical advances and experimental efforts.

In Fe-based superconductors, electron doping is often realized by the substitution of transition-metal atoms for Fe. In order to investigate how the electronic structure of the parent compound is influenced by Zn substitution, which supplies nominally four extra electrons per substituted atom but is expected to induce the strongest impurity potential among the transition-metal atoms, we have performed an angle-resolved photoemission spectroscopy measurement on Ba(Fe${}_{1-x}$Zn${}_x$)${}_2$As${}_2$ (Zn-122). In Zn-122, the temperature dependence of the resistivity shows a kink around $T\sim$ 135 K, indicating antiferromagnetic order below the N$\acute{\mathrm{e}}$el temperature of $T_{\mathrm{N}}\sim$ 135 K. In fact, folded Fermi surfaces (FSs) similar to those of the parent compound have been observed below $T_{\mathrm{N}}$. The hole and electron FS volumes are, therefore, different from those expected from the rigid-band model. The results can be understood if all the extra electrons occupy the Zn 3$d$ state $\sim$10 eV below the Fermi level and do not participate in the formation of the FSs.

Employing scaling analysis of the localization length, we deduce the critical exponent of the metal–topological insulator transitions induced by disorder. The obtained exponent $\nu \sim 2.7$ shows no conspicuous deviation from the value established for metal–ordinary insulator transitions in systems of the symplectic class. We investigate the topological phase diagram upon carrier doping to reveal the nature of the so-called topological Anderson insulator (TAI) region. The critical exponent of the metal–TAI transition is also first estimated, shown to be undistinguishable from the above value within the numerical error. By symmetry considerations we determine the explicit form of Rashba spin-orbit coupling in systems of $C_{4v}$ point group symmetry.