A novel feature of the iron arsenides is the magnetoelastic coupling between the long-wavelength in-plane strains of the lattice and the collective spin fluctuations of the electrons near the magnetic ordering wave vectors. Here, we study its microscopic origin from an electronic model with nested Fermi pockets and a nominal interaction. We find that the couplings diverge with a power law as the system is tuned to perfect nesting. Furthermore, the theory reveals how nematicity is boosted by nesting. These results are relevant for other systems with nesting-driven density wave transitions.

We develop a continuous-time quantum Monte Carlo (CTQMC) method for quantum impurities coupled to interacting quantum wires described by a Tomonaga-Luttinger liquid. The method is negative-sign free for any values of the Tomonaga-Luttinger parameter, which is rigorously proved, and, thus, efficient low-temperature calculations are possible. Duality between electrons and bosons in one-dimensional systems allows us to construct a simple formula for the CTQMC algorithm in these systems. We show that the CTQMC for Tomonaga-Luttinger liquids can be implemented with only minor modifications of previous CTQMC codes developed for impurities coupled to noninteracting fermions. We apply this method to the Kane-Fisher model of a potential scatterer in a spin-less quantum wire and to a single spin coupled with the edge state of a two-dimensional topological insulator assuming an anisotropic XXZ coupling. Various dynamical response functions such as the electron Green's function and spin-spin correlation functions are calculated numerically and their scaling properties are discussed.

At ferroelectric longitudinal domain walls there is an uncompensated charge, which could form a two-dimensional electron gas in the insulator. However, the uncompensated charges can be accommodated by, e.g., defects or localized states that split off from the conduction band. We carried out density functional theory calculations to study these scenarios in ${\mathrm{PbTiO}}_3$ with and without consideration of strong correlation effects simulated via inclusion of a Hubbard parameter $U$. The optimized structure and electronic structure depend on the choice of this parameter: For vanishing $U$, a broad, conducting domain wall is obtained, while increasing $U$ leads to localized Ti $3d$ states and an insulating, sharp domain wall. We also investigated the effects of varying the ferroelectric polarization on the electronic structure of these domain walls.

We present a framework to carry out highly accurate $\mathrm{GGA}+U$ thermochemistry calculations by deriving effective $U$ values from experimental data. The $U$ values predicted in this approach are applied to metal cations, and depend not only on (i) the chemical identity and the band to which the $U$ correction is applied, but also on the local environment of the metal described by (ii) its oxidation state and (iii) the surrounding ligand. We predict such local environment dependent (LD) $U$ values for the common oxidation states of $3d$ metals $M=$ Ti, V, Cr, Mn, Fe, Co, and Ni in their oxides and fluorides. We implement the GGA/$\mathrm{GGA}+U$ mixing method [Jain et al. Phys. Rev. B 84, 045115 (2011)] to establish the total energy compatibility among the $\mathrm{GGA}+U$ calculations involving $M$ treated with different LD-$U$ values. Using the presented framework, formation enthalpies of 52 transition metal bearing oxides (which are not used during the LD-$U$ parametrization) are predicted with a remarkably small mean absolute error of $\sim 19$ meV/atom, which is on the order of the experimental chemical accuracy. In addition, we present applications of the method in redox processes of important $3d$-metal oxide and fluoride systems such as ${\mathrm{Li}}_x{\mathrm{CoO}}_2,{\mathrm{Li}}_x{\mathrm{V}}_6{\mathrm{O}}_{13},{\mathrm{Li}}_x{\mathrm{FeF}}_3$, and ${\mathrm{VO}}_{1.5+x}$, and show that LD-$\mathrm{GGA}+U$ can overcome several drawbacks of using constant-$U$ values in conventional $\mathrm{GGA}+U$.

Electrical conductivity, thermopower, and lattice strain were investigated in the tetragonal tungsten bronze structured $({\mathrm{Sr}}_x,{\mathrm{Ba}}_{1-x}){\mathrm{Nb}}_2{\mathrm{O}}_{6-\delta }$ system for $0.7>x>0.4$ with large values of δ. These materials show attractive thermoelectric characteristics, especially in single-crystal form. Here, the Sr/Ba ratio was changed in order to vary the material between a normal ferroelectric with long-range polarization to relaxor behavior with short-range order and dynamic polarization. The influence of this on the electrical conduction mechanisms was then investigated. The temperature dependence of both the thermopower and differential activation energy for conduction suggests that the electronic conduction is controlled by an impurity band with a mobility edge separating localized and delocalized states. Conduction is controlled via hopping at low temperatures, and as temperature rises electrons are activated above the mobility edge, resulting in a large increase in electrical conductivity. For relaxor ferroelectric-based compositions, when dynamic short-range order polarization is present in the system, trends in the differential activation energy and thermopower show deviations from this conduction mechanism. The results are consistent with the polarization acting as a source of disorder that affects the location of the mobility edge and, therefore, the activation energy for conduction.

Recent calculations show that the band gaps of the nonisovalent random alloys such as ${\mathrm{Zn}}_{0.5}{\mathrm{Sn}}_{0.5}\mathrm{P}$ are much smaller than those of their ordered phases; that is, the band gap of the random alloy is not the ensemble averaged value of the ordered structures, in contrast to the trend observed in most isovalent semiconductor alloys and predicted by the cluster expansion theory. We show that this abnormal behavior is caused by the strong wave-function localization of the band-edge states in the nonisovalent alloys. Moreover, we show that although the disordered phase of the isovalent alloys is similar to the random phase, for the nonisovalent alloy, the disordered phase deviates significantly from the random phase and the fully random phase is not achievable under the equilibrium growth conditions.

We consider spin-polarized transport in a quantum spin Hall antidot system coupled to normal leads. Due to the helical nature of the conducting edge states, the screening potential at the dot region becomes spin dependent without external magnetic fields nor ferromagnetic contacts. Therefore, the electric current due to voltage or temperature differences becomes spin polarized, its degree of polarization being tuned with the dot level position or the base temperature. This spin-filter effect arises in the nonlinear transport regime only and has a purely interaction origin. Likewise, we find a spin polarization of the heat current, which is asymmetric with respect to the bias direction. Interestingly, our results show that a pure spin current can be generated by thermoelectric means: when a temperature gradient is applied, the created thermovoltage (Seebeck effect) induces a spin-polarized current for vanishingly small charge current. An analogous effect can be observed for the heat transport: a pure spin heat flows in response to a voltage shift even if the thermal current is zero.

An optical vortex is an inhomogeneous light beam having a phase singularity at its axis, where the intensity of the electric and/or magnetic field may vanish. Already well studied are the paraxial beams, which may carry well-defined values of spin (polarization $\sigma$) and orbital angular momenta; the orbital angular momentum per photon is given by the topological charge $\ell$ times the Planck constant. Here we study the light hole–to–conduction band transitions in a semiconductor quantum dot induced by a highly focused beam originating from a $\ell =1$ paraxial optical vortex. We find that at normal incidence the pulse will produce two distinct types of electron-hole pairs, depending on the relative signs of $\sigma$ and $\ell$. When sgn($\sigma$) $=$ sgn($\ell$), the pulse will create electron-hole pairs with band+spin and envelope angular momenta both equal to 1. In contrast, for sgn($\sigma$) $\ne$ sgn($\ell$), the electron-hole pairs will have neither band+spin nor envelope angular momenta. A tightly focused optical-vortex beam thus makes possible the creation of pairs that cannot be produced with plane waves at normal incidence. With the addition of co-propagating plane waves or switching techniques to change the charge $\ell$ both the band+spin and the envelope angular momenta of the pair wave function can be precisely controlled. We discuss possible applications in the field of spintronics that open up.

The creation of van der Waals heterostructures based on a graphene monolayer and other two-dimensional crystals has attracted great interest because the atomic registry of the two-dimensional crystals can modify the electronic spectra and properties of graphene. A twisted graphene bilayer can be viewed as a special van der Waals structure composed of two mutually misoriented graphene layers, where the sublayer graphene not only plays the role of a substrate, but also acts in an equivalent role as the top graphene layer in the structure. Here we report the electronic spectra of slightly twisted graphene bilayers studied by scanning tunneling microscopy and spectroscopy. Our experiment demonstrates that twist-induced van Hove singularities are ubiquitously present for rotation angles θ of less than about 3.5°, corresponding to moiré-pattern periods $D$ longer than 4 nm. However, they totally vanish for θ>5.5° $(D<2.5\text{nm})$. Such a behavior indicates that the continuum models, which capture moiré-pattern periodicity more accurately at small rotation angles, are no longer applicable at large rotation angles.

The diffusion of energy that is locally deposited into two-dimensional electron gases by Joule heating generates transverse voltages across devices with broken symmetry. For mesoscopic structures characterized by device dimensions comparable to the energy diffusion length, the resulting thermopower strongly depends on details of the potential profile defined by electric gates. We discuss these mesoscopic features within a diffusion thermopower model and propose schemes to measure the energy diffusion length and its dependence on gate voltage.

Far-infrared reflection and ellipsometry measurements on multiferroic ${\mathrm{PrFe}}_3\left({\mathrm{BO}}_3\right){}_4$ and ${\mathrm{SmFe}}_3\left({\mathrm{BO}}_3\right){}_4$ single crystals were used to investigate the interaction between lattice phonons and electronic excitations associated with $4f$ crystal-field transitions. A temperature-dependent interference between two types of excitations was observed in ${\mathrm{PrFe}}_3\left({\mathrm{BO}}_3\right){}_4$ in which the frequency of $4f$ crystal-field electronic excitation of ${\mathrm{Pr}}^{3+}$ falls into the TO-LO frequency interval of the optical phonon mode near 50 ${\mathrm{cm}}^{-1}$ (1.5 THz). Experimental data were explained on the basis of a theoretical model of coupled electron-phonon modes. The fitting procedure revealed the value 14.8 ${\mathrm{cm}}^{-1}$ for the electron-phonon coupling constant. This rather large value points to an essential role played by the electron-phonon interaction in the physics of multiferroics.

We consider a natural generalization of the lattice model for a periodic array of two layers, $A$ and $B$, of spinless electrons proposed by Fu [Phys. Rev. Lett. 106, 106802 (2011)] as a prototype for a crystalline insulator. This model has time-reversal symmetry and broken inversion symmetry. We show that when the intralayer next-nearest-neighbor hoppings $t_2^a,a=A,B$ vanish, this model supports a Weyl semimetal phase for a wide range of the remaining model parameters. When the effect of $t_2^a$ is considered, topological crystalline insulating phases take place within the Weyl semimetal one. By mapping to an effective Weyl Hamiltonian we derive some analytical results for the phase diagram as well as for the structure of the nodes in the spectrum of the Weyl semimetal.

The Gutzwiller projector technique has long been known as a method to include correlations in electronic structure calculations. We describe a model implementation for a $\mathrm{Gutzwiller}+\mathrm{LDA}$ calculation in a localized-orbital restricted basis framework, emphasizing the protocol step by step and illustrating our specific procedure for this and future applications. We demonstrate the method with a classic problem, the ferromagnetism of bulk bcc Fe, whose nature is attracting fresh interest. In the conventional Stoner-Wohlfarth model, and in spin-polarized LDA calculations, the ferromagnetic ordering of iron sets in so that the electrons can reduce their mutual Coulomb repulsion, at the cost of some increase of electron kinetic energy. This balance may, however, be altered by correlations, which are strong for localized $d$ orbitals. The present localized basis $\mathrm{Gutzwiller}+\mathrm{LDA}$ calculation demonstrates how the ferromagnetic ordering of Fe may, in fact, entrain a decrease of kinetic energy at the cost of some increase of potential energy. This happens because, as foreshadowed long ago by Goodenough and others and more recently supported by LDA-DMFT calculations, correlations cause $e_g$ and $t_{2g}$ $d$ orbitals to behave differently, with the weakly propagating $e_g$ states fully spin polarized and almost localized, and only $t_{2g}$ states forming a broad partly filled itinerant band. Owing to an intra-atomic Hund's rule exchange that aligns $e_g$ and $t_{2g}$ spins, the propagation of itinerant $t_{2g}$ holes is favored when different atomic spins are ferromagnetically aligned. This suggests a strong analogy with double exchange in iron ferromagnetism.

We follow the formation of a spin-lattice polaron after a quantum quench that simulates absorption of the pump pulse in the time-resolved experiments. We discover a two-stage relaxation where spin and lattice degrees of freedom represent an integral part of the relaxation mechanism. In the first stage, the kinetic energy of the spin-lattice polaron relaxes towards its ground-state value. In the second, typically much longer stage, a subsequent energy transfer between lattice and spin degrees of freedom via the charge carrier emerges. The multistage relaxation is generic and occurs even when carriers couple to different subsystems with comparable strengths.

We investigate optically-pumped nuclear magnetic resonance (OPNMR) in GaAs/AlGaAs multiple quantum wells. The photon energy dependence of the ${}^{69}\text{Ga}$ OPNMR shows a sign change in the signal at the light-hole transition for both ${\sigma }^{+}$ and ${\sigma }^{-}$ circular polarized irradiation. The energy for this transition and those of other Landau levels can be predicted by an eight-band $\vec{k}·\vec{p}$ theory used to calculate the conduction band spin polarization. Comparison of the spin polarization with the OPNMR measurements shows the origin of many features. In addition, we find features associated with the bulk GaAs substrate. In an external magnetic field, there are regions of low quantum well absorptivity between the Landau levels and in this region, OPNMR signals arising from the bulk GaAs substrate can be observed. The discrete nature of transitions between Landau levels in two-dimensional electron systems compared to the Landau levels in three-dimensional systems leads to the change in the phase of the OPNMR signal at the light-hole transitions.

We describe dynamics of spin and valley transitions driven by alternating electric fields in quantum dots defined electrostatically within semiconducting carbon nanotubes (CNT). We use the tight-binding approach to describe the states localized within a quantum dot taking into account the circumferential spin-orbit interaction due to the $s\text{−}p$ hybridization and external fields. The basis of eigenstates localized in the quantum dot is used in the solution of the time-dependent Schrödinger equation for description of spin flips and intervalley transitions that are driven by periodic perturbation in the presence of coupling between the spin, valley, and orbital degrees of freedom. Aside from the first-order transitions, we find also fractional resonances. We discuss the transition rates with selection rules that are lifted by atomic disorder and the bend of the tube. We demonstrate that the electric field component perpendicular to the axis of the CNT activates spin transitions which are otherwise absent and that the resonant spin-flip time scales with the inverse of the electric field.

We derive the frequency shifts and the broadening of $\Gamma$-point longitudinal optical (LO) and transverse optical (TO) phonon modes, due to electron-phonon interaction, in graphene under uniaxial strain as a function of the electron density and the disorder amount. We show that, in the absence of a shear strain component, such interaction gives rise to a lifting of the degeneracy of the LO and TO modes which contributes to the splitting of the G Raman band. The anisotropy of the electronic spectrum, induced by the strain, results in a polarization dependence of the LO and TO modes. This dependence is in agreement with the experimental results showing a periodic modulation of the Raman intensity of the split G peak. Moreover, the anomalous behavior of the frequency shift reported in undeformed graphene is found to be robust under strain.

A theoretical framework is proposed to derive a dynamic equation motion for rectilinear dislocations within isotropic continuum elastodynamics. The theory relies on a recent dynamic extension of the Peierls-Nabarro equation, so as to account for core-width generalized stacking-fault energy effects. The degrees of freedom of the solution of the latter equation are reduced by means of the collective-variable method, well known in soliton theory, which we reformulate in a way suitable to the problem at hand. Through these means, two coupled governing equations for the dislocation position and core width are obtained, which are combined into one single complex-valued equation of motion, of compact form. The latter equation embodies the history dependence of dislocation inertia. It is employed to investigate the motion of an edge dislocation under uniform time-dependent loading, with focus on the subsonic/transonic transition. Except in the steady-state supersonic range of velocities—which the equation does not address—our results are in good agreement with atomistic simulations on tungsten. In particular, we provide an explanation for the transition, showing that it is governed by a loading-dependent dynamic critical stress. The transition has the character of a delayed bifurcation. Moreover, various quantitative predictions are made, that could be tested in atomistic simulations. Overall, this work demonstrates the crucial role played by core-width variations in dynamic dislocation motion.

Using two experimental techniques, we studied single crystals of the 122-FeAs family with almost the same critical temperature, $T_c$. We investigated the temperature dependence of the lower critical field $H_{c1}\left(T\right)$ of a ${\mathrm{Ca}}_{0.32}{\mathrm{Na}}_{0.68}{\mathrm{Fe}}_2{\mathrm{As}}_2$ $(T_c\approx 34\text{K})$ single crystal under static magnetic fields $H$ parallel to the $c$ axis. The temperature dependence of the London penetration depth can be described equally well either by a single anisotropic $s$-wave-like gap or by a two-gap model, while a $d$-wave approach cannot be used to fit the London penetration depth data. Intrinsic multiple Andreev reflection effect spectroscopy was used to detect bulk gap values in single crystals of the intimate compound ${\mathrm{Ba}}_{0.65}{\mathrm{K}}_{0.35}{\mathrm{Fe}}_2{\mathrm{As}}_2$, with the same $T_c$. We estimated the range of the large gap value ${\Delta }_L=6–8$ meV (depending on small variation of $T_c)$ and its a $k$ space anisotropy of about 30%, and the small gap ${\Delta }_S\approx 1.7\pm 0.3$ meV. This clearly indicates that the gap structure of our investigated systems more likely corresponds to a nodeless $s$-wave two gaps.