The effect of Mn substitution, acting as a magnetic impurity for Fe, on the Dirac cone was investigated in Ba(Fe${}_{1-x}$Mn${}_x$As)${}_2$. Both magnetoresistance and Hall resistivity studies clearly indicate that the cyclotron effective mass ($m^{*}$) of the Dirac cone is anomalously enhanced at low temperatures by the impurity, although its evolution as a function of carrier number proceeds in a conventional manner at higher temperatures. Kondo-like band renormalization induced by the magnetic impurity scattering is suggested as the most plausible explanation for this, and the anomalous mass enhancement of the Dirac fermions is discussed.

We consider a quantum particle in tilted two-dimensional lattices in the tight-binding approximations. We show that for certain lattice geometries the particle can freely move across the lattice in the direction perpendicular to the vector of the static force. This effect is argued to be analog of the photon-induced tunneling in driven one-dimensional lattices. We calculate the particle dispersion relation by using a method based on the Bogoliubov-Mitropolskii averaging technique from the theory of dynamical systems. This dispersion relation draws the analogy with driven one-dimensional lattices further by eventually showing band collapses when a control parameter is varied.

We study the stability against disorder of surface states of weak ${\mathbb{Z}}_2$ topological insulators (superconductors) which are stacks of strong ${\mathbb{Z}}_2$ topological insulators (superconductors), considering representative Dirac Hamiltonians in the Altland-Zirnbauer symmetry classes in various spatial dimensions. We show that, in the absence of disorder, surface Dirac fermions of weak ${\mathbb{Z}}_2$ topological insulators (superconductors) can be gapped out by a Dirac mass term which couples surface Dirac cones and leads to breaking of a translation symmetry (dimerization). The dimerization mass is a unique Dirac mass term in the surface Dirac Hamiltonian, and the two dimerized gapped phases which differ in the sign of the Dirac mass are distinguished by a ${\mathbb{Z}}_2$ index. In other words the dimerized surfaces can be regarded as a strong ${\mathbb{Z}}_2$ topological insulator (superconductor). We argue that the surface states are not localized by disorder when the ensemble average of the Dirac mass term vanishes.

We report the growth of ferromagnetic epitaxial CoFe${}_2$O${}_4$ (CFO) thin films on 011-oriented tetragonal-structured 0.62Pb(Mg,Nb)O${}_3$-0.38PbTiO${}_3$ single crystal substrates. In the substrates, electric field-induced polarization reorientation resulted in maximum strains of −1.3% and 0.64% along in-plane 100 and along in-plane $0\overline{1}1$, respectively. As a consequence, in the CFO films, irreversible magnetic coercive field changes of 580, −510, and −220 Oe were observed along in-plane 100 and $0\overline{1}1$ and out-of-plane 011, respectively, after electric field poling. After annealing at 470 K, the substrate depoled, and both the magnetic coercive fields and the remnant magnetizations along the three directions recovered their original values.

We show that a finite Hall effect in zero applied magnetic field occurs for partially filled bands in certain time-reversal violating states with zero net flux per unit cell. These states are the Magneto-chiral states with parameters in the effective one-particle Hamiltonian such that they do not satisfy the Haldane-type constraints for topological electronic states. The results extend an earlier discussion of the Kerr effect observed in the cuprates but may be applicable to other experimental situations.

We present a scalable method for calculating the electronic properties of a $\delta$-doped phosphorus layer in silicon and germanium. Our calculations are based on an $s{p}^3{d}^5{s}^{*}$ tight-binding model and the Thomas-Fermi-Dirac approximation. The energy shift in the lowest conduction band states of the Ge band structure is characterized and a comparison is made to a $\delta$-doped P layer in Si. The results for the $\delta$-doped Si:P layer themselves compare well to the predictions of more “resource intensive” computational models. The Thomas-Fermi method presented herein scales easily to large system sizes. Efficient scaling is important for the calculation of quantum transport properties in $\delta$-doped semiconductors that are currently of experimental interest.

CdSe nanoplatelets show perfectly quantized thicknesses of a few monolayers. They present a situation of extreme, yet well defined quantum confinement. Due to a large dielectric contrast between the semiconductor and its ligand environment, interaction between carriers and their dielectric images strongly renormalize bare single particle states. We discuss the electronic properties of this original system in an advanced tight-binding model, and show that Coulomb interactions, including self-energy corrections and enhanced electron-hole interaction, lead to exciton binding energies up to several hundred meV.

Is the plasmon description within the nonlocal correlation of the van der Waals density functional by Dion and coworkers (vdW-DF) robust enough to describe all exchange-correlation components? To address this question, we design an exchange functional based on this plasmon description as well as recent analysis on exchange in the large-$s$ regime. In the regime with reduced gradients $s=\left|\nabla n\right|/2n{k}_{\mathrm{F}}\left(n\right)$ smaller than $\approx 2.5$, dominating the nonlocal correlation part of the binding energy, the enhancement factor $F_x\left(s\right)$ closely resembles the Langreth-Vosko screened exchange. In the $s$ regime beyond, dominated by exchange, $F_x\left(s\right)$ passes smoothly over to the revised Perdew-Wang-86 form. We term the specific exchange functional LV-PW86r, wheras the full van der Waals functional version emphasizing consistent handling of exchange is termed vdW-DF-cx. Our tests indicate that vdW-DF-cx produces accurate separations and binding energies of the S22 data set of molecular dimers as well as accurate lattice constants and bulk moduli of layered materials and tightly bound solids. These results suggest that the plasmon description within vdW-DF gives a good description of both exchange and correlation effects in the low-to-moderate $s$ regime.

We present a scattering theory description for the inelastic current noise in the presence of electron-vibration interactions. In this description, we specify elastic and inelastic scattering contributions to the shot noise by examining charge transfers between scattering states and energy exchange between electrons and vibrations. The elastic and inelastic scattering processes are further decomposed into current correlations of electrons at the same energy and those of electrons at different energies. Focusing on the inelastic noise signals defined as steps in the voltage derivative of the shot noise, we show that single-channel systems have two ranges of transmission at which the inelastic noise signals exhibit the crossover between positive and negative signs. In a high-transmission regime, even and odd vibrational modes of mirror-symmetric systems provide upper and lower bounds to the ratio of the inelastic noise signal to the conductance step. This can be a theoretical justification for models used to understand the recent noise experiment [Phys. Rev. Lett. 108, 146602 (2012)] and numerical calculations on gold atomic chains [Phys. Rev. B 86, 155411 (2012)].

The recent production of synthetic magnetic fields acting on electroneutral particles, such as atoms or photons, has boosted interest in the quantum Hall physics of bosons. Adding pseudospin 1/2 to the bosons greatly enriches the scenario, as it allows them to form an interacting integer quantum Hall (IQH) phase with no fermionic counterpart. Here we show that, for a small two-component Bose gas on a disk, the complete strongly correlated regime, extending from the integer phase at filling factor $\nu =2$ to the Halperin phase at filling factor $\nu =2/3$, is well described by composite fermionization of the bosons. Moreover we study the edge excitations of the IQH state, which, in agreement with expectations from topological field theory, are found to consist of forward-moving charge excitations and backward-moving spin excitations. Finally, we demonstrate how pair-correlation functions allow one to experimentally distinguish the IQH state from competing states, such as non-Abelian spin singlet (NASS) states.

Using the real-space Hartree-Fock approximation, the magnetic phase diagram of a five-orbital Hubbard model for the iron-based superconductors is studied varying the electronic density $n$ in the range from five to seven electrons per transition metal atom. The Hubbard interaction $U$ is also varied, at a fixed Hund coupling $J/U=0.25$. Several qualitative trends and a variety of competing magnetic states are observed. At $n=5$, a robust G-type antiferromagnetic insulator is found, in agreement with experimental results for BaMn${}_2$As${}_2$. As $n$ increases away from 5, magnetic states with an increasing number of nearest-neighbors ferromagnetic links become energetically stable. This includes the well-known C-type antiferromagnetic state at $n=6$, the E-phase known to exist in FeTe, and also a variety of novel states not found yet experimentally, some of them involving blocks of ferromagnetically oriented spins. Regions of phase separation, as in Mn oxides, have also been detected. Comparison to previous theoretical investigations indicate that these qualitative trends may be generic characteristics of phase diagrams of multi-orbital Hubbard models.

The ab initio calculation of charged defect properties in solids is not straightforward because of the delicate interplay between the long-range Coulomb interaction and the periodic boundary conditions. We derive the projector augmented-wave (PAW) energy and Hamiltonian with special care taken on the potentials from the Coulomb interaction. By explicitly treating the background compensation charge, we find additional terms in the total energy of the charged cells and in the potential. We show that these background terms are needed to accurately reproduce all-electron calculations of the formation energy of a charged defect. In particular, the previous PAW expressions were spuriously sensitive to the pseudization conditions and this artifact is removed by the background term. This PAW derivation also provides insights into the norm-conserving pseudopotential framework. We propose then an alternative definition for the total energy of charged cells and for the Kohn-Sham potential within this framework that better approximates the all-electron results.

Motivated by recent experiments on $\beta$-Li${}_2$IrO${}_3$, we study the phase diagram of the Heisenberg-Kitaev model on a three-dimensional lattice of tricoordinated Ir${}^{4+}$, dubbed the hyperhoneycomb lattice. The lattice geometry of this material, along with Ir${}^{4+}$ ions carrying $J_{\mathrm{eff}}=1/2$ moments, suggests that the Heisenberg-Kitaev model may effectively capture the low-energy spin-physics of the system in the strong-coupling limit. Using a combination of semiclassical analysis, exact solution, and slave-fermion mean-field theory, we find, in addition to the spin liquid, four different magnetically ordered phases depending on the parameter regime. All four magnetic phases—the Néel, the polarized ferromagnet, the skew-stripy, and the skew-zig-zag—have collinear spin ordering. The three-dimensional $Z_2$ spin liquid, which extends over an extended parameter regime around the exactly solvable Kitaev point, has a gapless Majorana mode with a deformed Fermi circle (codimensions, $d_c=2$). We discuss the effect of the magnetic field and finite temperature on different phases that may be relevant for future experiments.

The time evolution of a local spin excitation in a (001)-confined two-dimensional electron gas subjected to Rashba and Dresselhaus spin-orbit interactions of similar strength is investigated theoretically and compared with experimental data. Specifically, the consequences of the finite spatial extension of the initial spin polarization are studied for nonbalanced Rashba and Dresselhaus terms and for finite cubic Dresselhaus spin-orbit interaction. We show that the initial out-of-plane spin polarization evolves into a helical spin pattern with a wave number that gradually approaches the value $q_0$ of the persistent spin helix mode. In addition to an exponential decay of the spin polarization that is proportional to both the spin-orbit imbalance and the cubic Dresselhaus term, the finite width $w$ of the spin excitation reduces the spin polarization by a factor that approaches $exp(-q_0^2{w}^2/2)$ at longer times.

The electronic transport properties of ordered and disordered nitrogen-doped metallic carbon nanotubes with long-range correlation are studied numerically with a tight-binding model. Doping with both translational (axial) and screw symmetry are considered. In periodic defective systems, when axial doping is considered, two classes of electronic transport responses are obtained. One quantum conductance plateau settles down around the defect energy only when the period of the structure is a multiple of the Fermi wavelength 3$d_0$ ($d_0$ being the period of the perfect armchair host nanotube), because the Bloch-like propagating modes survive. Otherwise, a conduction gap is predicted. For the screw doping configuration, the same resonant electronic transport is observed when the rotational angle fulfills one of the rotational symmetries of the perfect nanotube. Furthermore, for disordered systems, the conventional Anderson localization is partially prohibited around the defect energy for particular axial and circular random doping configurations. These conclusions are valid for both armchair and chiral metallic nanotubes and should remain true for other modifications of the nanotube by covalent or noncovalent doping.

We show that the scattering interaction between a high energy electron and a photon can be strongly enhanced by different types of localized plasmons in a nontrivial way. The scattering interaction is predicted by an eigen-response theory, numerically verified by finite-difference-time-domain simulation, and experimentally verified by cathodoluminescence spectroscopy. We find that the scattering interaction associated with dark plasmons can be as strong as that of bright plasmons. Such a strong interaction may offer new opportunities to improve single-plasmon detection and high-resolution characterization techniques for high quality plasmonic materials.

We derive a general expression for the electron nonequilibrium (NE) distribution function in the context of steady state quantum transport through a two-terminal nanodevice with interactions. The central idea for the use of NE distributions for open quantum systems is that both the NE and many-body (MB) effects are taken into account in the statistics of the finite size system connected to reservoirs. We develop an alternative scheme to calculate the NE steady state properties of such systems. The method, using NE distribution and spectral functions, presents several advantages, and is equivalent to conventional steady state NE Green's function (NEGF) calculations when the same level of approximation for the MB interaction is used. The advantages of our method resides in the fact that the NE distribution and spectral functions have better analytic behavior for numerical calculations. Furthermore, our approach offers the possibility of introducing further approximations, not only at the level of the MB interaction as in NEGF, but also at the level of the functional form used for the NE distributions. For the single-level model with electron-phonon coupling we have considered, such approximations provide a good representation of the exact results, for either the NE distributions themselves or the transport properties. We also derive the formal extensions of our method for systems consisting of several electronic levels and several vibration modes.

We construct a hierarchy of exactly solvable spin-1/2 chains with $so{\left(N\right)}_1$ critical points. Our construction is based on the framework of condensate-induced transitions between topological phases. We employ this framework to construct a Hamiltonian term that couples $N$ transverse field Ising chains such that the resulting theory is critical and described by the $so{\left(N\right)}_1$ conformal field theory. By employing spin duality transformations, we then cast these spin chains for arbitrary $N$ into translationally invariant forms that all allow exact solution by the means of a Jordan-Wigner transformation. For odd $N$ our models generalize the phase diagram of the transverse field Ising chain, the simplest model in our hierarchy. For even $N$ the models can be viewed as longer ranger generalizations of the $XY$ chain, the next model in the hierarchy. We also demonstrate that our method of constructing spin chains with given critical points goes beyond exactly solvable models. Applying the same strategy to the Blume-Capel model, a spin-1 generalization of the Ising chain in a generic magnetic field, we construct another critical spin-1 chain with the predicted conformal field theory (CFT) describing the criticality.

Unconventional superconducting states of matter are realized in the presence of strong spin-orbit coupling. In particular, nondegenerate bands can support odd-parity superconductivity with rich topological content. Here we study whether this is the case for Weyl semimetals. These are systems whose low-energy sector, in the absence of interactions, is described by linearly dispersing chiral fermions in three dimensions. The energy spectrum has nodes at an even number of points in the Brillouin zone. Consequently both intranodal finite momentum pairing and internodal BCS superconductivity are allowed. For local attractive interaction the finite momentum pairing state with chiral $p$-wave symmetry is found to be most favorable at finite chemical potential. The state is an analog of the superfluid ${}^3$He $A$ phase, with Cooper pairs having finite center-of-mass momentum. For chemical potential at the node the state is preempted by a fully gapped charge density wave. For nonlocal attraction the BCS state wins out for all values of the chemical potential.

We theoretically study the superconducting proximity effect in silicene, which features massive Dirac fermions with a tunable mass (band gap), and compute the conductance across a normal-superconductor (N-S) silicene junction, the nonlocal conductance of an N-S-N junction, and the supercurrent flowing in an S-N-S junction. It is demonstrated that the transport processes consisting of local and nonlocal Andreev reflection may be efficiently controlled via an external electric field owing to the buckled structure of silicene. In particular, we demonstrate that it is possible to obtain a fully spin-valley-polarized crossed Andreev reflection process without any contamination of elastic cotunneling or local Andreev reflection, in stark contrast to ordinary metals. It is also shown that the supercurrent flowing in the S-N-S junction can be fully spin-valley polarized and that it is controllable by an external electric field.