We report first-principles study of the orbital magnetism and magnetic anisotropy energy (MAE) in ultrathin films of Fe, Co, and FePt disturbed from the ferromagnetic ground state. The deviation of ferromagnetic spin configurations from the easy axis leads to the noncollinearity of the spin and orbital moments. In Fe and Co, this noncollinearity correlates with the magnitude of MAE. In contrast, in FePt a complex interplay of competing processes leads to unusual phenomenon of the orthogonality of the inducing spin and induced orbital moments. Considering the influence of noncollinearity of spin moments we obtain opposite trends in the MAE variation for Fe and Co films. This finding correlates with the absence of universal temperature behavior of MAE in itinerant magnets and is shown to originate in the variation of the electronic structure with the change of spin configuration.

We investigate the nonresonant all-optical switching of magnetization. We treat the inverse Faraday effect (IFE) theoretically in terms of the spin-selective optical Stark effect for linearly or circularly polarized light. In the dilute magnetic semiconductors (Ga,Mn)As, strong laser pulses below the band gap induce effective magnetic fields of several teslas in a direction which depends on the magnetization direction as well as the light polarization and direction. Our theory demonstrates that the polarized light catalyzes the angular momentum transfer between the lattice and the magnetization.

We calculate the effective trigonal crystal field $\Delta$ that splits the $t_{2g}$ levels of effective models for Na${}_x$CoO${}_2$ as the local symmetry around a Co ion is reduced from $O_h$ to $D_{3d}$. To this end, we solve numerically a CoO${}_6$ cluster containing a Co ion with all $3d$ states and their interactions included, and its six nearest-neighbor O atoms, with the geometry of the system, in which the CoO${}_6$ octahedron is compressed along a $C_3$ axis. We obtain $\Delta \approx 130$ meV, with the sign that agrees with previous quantum chemistry calculations but disagrees with first-principles results in the local density approximation (LDA). We find that $\Delta$ is very sensitive to a Coulomb parameter that controls the Hund coupling and charge distribution among the $d$ orbitals. The origin of the discrepancy with LDA results is discussed.

The density functional theory under the generalized gradient approximation has been used to investigate the electronic structure of double perovskite oxides Ba${}_2$YNbO${}_6$ (BYN) and Ba${}_2$YTaO${}_6$ (BYT) synthesized by the solid-state reaction technique. The band gaps of the systems are measured from the uv-visible spectra and compared with the theoretically calculated values. The x-ray photoemission spectra (XPS) of the systems are obtained. The valence-band XPS spectra are analyzed from calculated density of states. The binding energy of Ba $3d_{5/2}$ is found to be less in BYT than in BYN, which has been correlated to the degree of delocalization of $d$ electrons as well as electronegativity of Nb and Ta in the systems. The real and imaginary parts of the optical dielectric function and the refractive index are calculated. The calculated spectra are compared with the experimental results. The interband contribution to the optical properties of these perovskite oxides has been analyzed. The collective vibrational modes of the atoms of the materials are calculated to examine the various vibrational excitations of the systems.

In a previous publication [Betzinger, Friedrich, Görling, and Blügel, Phys. Rev. B 85, 245124 (2012)] we presented a technique to compute accurate all-electron response functions, e.g., the density response function, within the full-potential linearized augmented-plane-wave (FLAPW) method. Response contributions that are not captured (completely) within the finite Hilbert space spanned by the LAPW basis are taken into account by an incomplete-basis-set correction (IBC). The latter is based on a formal response of the basis functions themselves, which is derived by exploiting their dependence on the effective potential. Its construction requires the solution of radial differential equations, having the form of Sternheimer equations, by numerical integration. The approach includes a formally exact treatment of the response contribution from the core states. While we restricted the formalism to spherical perturbations in the previous work, we here generalize the formalism to nonspherical perturbations. The improvements are demonstrated with exact-exchange optimized-effective-potential (EXX-OEP) calculations of antiferromagnetic NiO. It is shown that with the generalized IBC a basis-set convergence is realized that is as fast as in density-functional theory calculations using standard local or semilocal functionals. The EXX-OEP band gap, magnetic moment, and spectral function of NiO are in substantially better agreement with experiment than results obtained from calculations with local and semilocal functionals.

We calculate the conductances of a three-terminal junction setup of spinless Luttinger liquid wires threaded by a magnetic flux, allowing for different interaction strength $g_3\ne g$ in the third wire. We employ the fermionic representation in the scattering state picture, allowing for a direct calculation of the linear response conductances, without the need of introducing contact resistances at the connection points to the outer ideal leads. The matrix of conductances is parametrized by three variables. For these we derive coupled renormalization group (RG) equations, by summing up infinite classes of contributions in perturbation theory. The resulting general structure of the RG equations may be employed to describe junctions with an arbitrary number of wires and arbitrary interaction strength in each wire. The fixed point structure of these equations (for the chiral Y junction) is analyzed in detail. For repulsive interaction ($g,g_3>0$) there is only one stable fixed point, corresponding to the complete separation of the wires. For attractive interaction ($g<0$ and/or $g_3<0$) four fixed points are found, the stability of which depends on the interaction strength. We confirm our previous weak-coupling result of lines of fixed points for special values of the interaction parameters reaching into the strong-coupling domain. We find new fixed points not discussed before, even at the symmetric line $g=g_3$, at variance with the results of M. Oshikawa et al. [J. Stat. Mech. (2006) P02008]. The pair-tunneling phenomenon conjectured by the latter authors is not found by us.

Four-spin exchange interaction has been raising intriguing questions regarding the exotic phase transitions it induces in two-dimensional quantum spin systems. In this context, we investigate the effects of a cyclic four-spin exchange in the quasi-1D limit by considering a general $N$-leg spin ladder. We show by means of a low-energy approach that depending on its sign, this ring exchange interaction can engender either a staggered or a uniform dimerization from the conventional phases of spin ladders. The resulting quantum phase transition is found to be described by the SU(2)${}_N$ conformal field theory. This result, as well as the fractional value of the central charge at the transition, is further confirmed by a large-scale numerical study performed by means of exact diagonalization and density matrix renormalization group approaches for $N\le 4$.

We report the observation of a fully developed fractional quantum Hall state at $\nu =5/2$ in GaAs/Al${}_x$Ga${}_{1-x}$As quantum wells with mobility well below ${10}^7$ cm${}^2$/Vs. This is achieved either by strong illumination or reducing the barrier Al composition without illumination. We explain both results in terms of screening of the ionized remote impurity (RI) potential by nearby neutral shallow donors. Despite the dramatic improvement in the transport features, the energy gap ${\Delta }_{5/2}$ is limited to a rather small value ($\sim 100$ mK), which indicates that once the RI potential is well screened and the $5/2$ state emerges, the size of ${\Delta }_{5/2}$ is limited by the mobility, i.e., by background impurities.

The problem of plasmon resonances in metallic nanoparticles can be formulated as an eigenvalue problem under the condition that the wavelengths of the incident radiation are much larger than the particle dimensions. As the nanoparticle size increases, the quasistatic condition is no longer valid. For this reason, the accuracy of the electrostatic approximation may be compromised and appropriate radiation corrections for the calculation of resonance permittivities and resonance wavelengths are needed. In this paper, we present the radiation corrections in the framework of the eigenvalue method for plasmon mode analysis and demonstrate that the computational results accurately match analytical solutions (for nanospheres) and experimental data (for nanorings and nanocubes). We also demonstrate that the optical spectra of silver nanocube suspensions can be fully assigned to dipole-type resonance modes when radiation corrections are introduced. Finally, our method is used to predict the resonance wavelengths for face-to-face silver nanocube dimers on glass substrates. These results may be useful for the indirect measurements of the gaps in the dimers from extinction cross-section observations.

The lateral propagation of surface ripples on Si, generated by Xe ion irradiation with and without codeposition of Fe surfactant atoms, was investigated by scanning electron microscopy with the help of micron-sized marker structures prepared by focused ion beam milling. For 10-keV Xe ion irradiation of Si at oblique incidence between 62 and 70°, we determine lateral ripple propagation velocities varying from −1.9 to $+$2.9 nm per 10${}^{15}$ Xe ions/cm${}^2$. The propagation direction changes from opposite to the projected direction of the incident ion beam to along the projected beam direction within a narrow angular regime. At 67°, the pattern is almost static. The result is in good agreement with predictions from the theoretical model of Bradley and Harper. For perpendicular incident 5-keV Xe ions and oblique codeposition of Fe surfactant atoms, we find that ripple patterns propagate across the surface with a negative ripple propagation velocity of about −0.7 nm per 10${}^{15}$ Xe ions/cm${}^2$, i.e., opposite to the projected deposition direction of Fe surfactant atoms. The novel experimental method to determine the lateral ripple propagation based on markers set with a focused ion beam system does not require an in situ analysis and can therefore be applied in general to analyze the dynamics of ion beam-induced patterns.

Phase diagram of an Ising-spin Kondo lattice model on a kagome lattice is investigated by a Monte Carlo simulation. We find that the system exhibits a peculiar ferrimagnetic state at a finite temperature, in which each triangle is in a two-up one-down spin configuration but the spin correlation does not develop any superstructure. We call this state the loop liquid, as it is characterized by the emergent degree of freedom, self-avoiding up-spin loops. We elucidate that the system shows phase transitions from the loop liquid to ferrimagnetically ordered states and a crossover to a partially ferromagnetic state by changing the electron density and temperature. These can be viewed as crystallization and cohesion of the loops, respectively. We demonstrate that the loop formation is observed in the optical conductivity as a characteristic resonant peak.

We theoretically investigate the magnitude and range of the photon-mediated interaction between two quantum dots embedded in a photonic crystal waveguide, including fabrication disorder both in the crystal and in the dot positioning. We find that disorder-induced light localization has a drastic effect on the excitation transfer rate—as compared to an ideal structure—and that this rate varies widely among different disorder configurations. Nevertheless, we also find that significant rates of $50\mu \mathrm{eV}$ at a range of $10\mu \mathrm{m}$ can be achieved in realistic systems.

We theoretically consider temperature- and density-dependent electron-phonon interaction induced many-body effects in the two-dimensional (2D) metallic carriers confined on the surface of the three-dimensional topological insulator (e.g., Bi${}_2$Se${}_3$). We calculate the temperature and the carrier density dependence of the real and imaginary parts of the electronic self-energy, the interacting spectral function, and the phonon-induced velocity renormalization, enabling us to obtain a simple density- and temperature-dependent effective dimensionless electron-phonon coupling constant parameter, which increases (decreases) strongly with increasing density (temperature). Our theoretical results can be directly and quantitatively compared with experimental angle-resolved photoemission spectroscopy or scanning tunneling spectroscopy studies of the 2D spectral function of topological insulator surface carriers. In particular, we predict the possible existence of surface superconductivity in Bi${}_2$Se${}_3$ induced by a strong electron-phonon interaction.

Recently, it has been proposed that exotic one-dimensional phases can be realized by gapping out the edge states of a fractional topological insulator. The low-energy edge degrees of freedom are described by a chain of coupled parafermions. We introduce a classification scheme for the phases that can occur in parafermionic chains. We find that the parafermions support both topological symmetry fractionalized phases as well as phases in which the parafermions condense. In the presence of additional symmetries, the phases form a non-Abelian group. As a concrete example of the classification, we consider the effective edge model for a $\nu =1/3$ fractional topological insulator for which we calculate the entanglement spectra numerically and show that all possible predicted phases can be realized.

Two years after the prediction of a giant spin Hall effect for the dilute Cu(Bi) alloy [Gradhand et al., Phys. Rev. B 81, 245109 (2010)], a comparably strong effect was measured in thin films of Cu(Bi) alloys by Niimi et al. [Phys. Rev. Lett. 109, 156602 (2012)]. Both theory and experiment consider the skew-scattering mechanism to be responsible, however they obtain opposite sign for the spin Hall angle. Based on a detailed analysis of the obtained theoretical results, we propose that either the formation of extremely small clusters or the influence of interface roughness and grain boundaries decorated with Bi atoms are responsible for the observed phenomenon.

Fully nonlocal two-projector norm-conserving pseudopotentials are shown to be compatible with a systematic approach to the optimization of convergence with the size of the plane-wave basis. A reformulation of the optimization is developed, including the ability to apply it to positive-energy atomic scattering states and to enforce greater continuity in the pseudopotential. The generalization of norm conservation to multiple projectors is reviewed and recast for the present purposes. Comparisons among the results of all-electron and one- and two-projector norm-conserving pseudopotential calculations of lattice constants and bulk moduli are made for a group of solids chosen to represent a variety of types of bonding and a sampling of the periodic table.

The ultrafast light-induced dynamics in an InAs quantum dot doped with a single Mn atom is studied theoretically. Due to the exchange interaction between the Mn atom and the acceptor hole in the quantum dot, an effective two-level system is formed in the ground-state manifold. We show that this two-level system can be controlled coherently on a picosecond time scale. The control involves an optical manipulation of the quantum dot exciton by using either a series of two ultrafast pulses or a single extended laser pulse. For the excitation with two pulses, we discuss the optical signals that could be used in time-resolved spectroscopy to monitor the dynamics. We compare these results to the spin dynamics in a CdTe quantum dot doped with a single Mn atom.

Thermoelectric effects in transport through a quantum dot coupled to external ferromagnetic leads are investigated theoretically. The basic thermoelectric transport characteristics, such as thermopower, electronic contribution to the heat conductance, and the corresponding figure of merit, are calculated in the linear response regime by means of the density-matrix numerical renormalization group method. The case of a nonzero spin splitting of the electrochemical potential in the electrodes is also considered and the associated spin thermoelectric effects are analyzed. It is shown that the spin-dependent thermoelectric phenomena in the local moment regime depend generally on the exchange field induced by ferromagnetic contacts. In addition, the temperature dependence of the Seebeck coefficient is rather nontrivial, and depends on the spin polarization and spin relaxation in the leads. In the presence of ferromagnetic leads, the thermopower as a function of temperature may change sign more times than the thermopower for nonmagnetic leads. These changes can be thus used to determine the relevant Kondo behavior and Kondo energy scale in the system. Moreover, the effects of external magnetic field and different spin polarization of ferromagnetic leads are also analyzed.

We investigate theoretically the hyperfine-induced dephasing of two-electron-spin states in a double quantum dot with a finite singlet-triplet splitting $J$. In particular, we derive an effective pure dephasing Hamiltonian, which is valid when the hyperfine-induced mixing is suppressed due to the relatively large $J$ and the external magnetic field. Using both a quantum theory based on resummation of ring diagrams and semiclassical methods, we identify the dominant dephasing processes in regimes defined by values of the external magnetic field, the singlet-triplet splitting, and inhomogeneity in the total effective magnetic field. We address both free induction and Hahn echo decay of superposition of singlet and unpolarized triplet states (both cases are relevant for singlet-triplet qubits realized in double quantum dots). We also study hyperfine-induced exchange gate errors for two single-spin qubits. Results for III-V semiconductors as well as silicon-based quantum dots are presented.

The quantum anomalous Hall effect has been predicted in HgMnTe quantum wells with an out-of-plane magnetization of Mn atoms. However, since HgMnTe quantum wells are paramagnetic, an out-of-plane magnetic field is required to polarize magnetic moments of Mn atoms, which inevitably induces Landau levels and makes it difficult to identify the origin of the quantized Hall conductance experimentally. In this work, we study the quantum anomalous Hall effect in the presence of an in-plane magnetic field in Mn-doped HgTe quantum wells. For a small out-of-plane magnetic field, the in-plane magnetic field can drive the system from a normal insulating state to a quantum anomalous Hall state. When the out-of-plane magnetic field is slightly above the transition point, the system shows a reentrant behavior of Hall conductance, varying from $-e^2/h$ to 0 and back to $-e^2/h$, with increasing in-plane magnetic fields. The reentrant quantum anomalous Hall effect originates from the interplay between the exchange coupling of magnetic moments and the direct Zeeman coupling of magnetic fields. The calculation incorporating Landau levels shows that there is no qualitative change of the reentrant behavior.