We develop a first-principles procedure for the individual evaluation of the intrinsic, side-jump, and skew-scattering contributions to the anomalous Hall conductivity ${\sigma }_{xy}$. This method is based on the different microscopic conductive processes of each origin of ${\sigma }_{xy}$ in the Kubo-Bastin formula. We also present an approach for implementing this scheme in the tight-binding linear muffin-tin orbital (TB-LMTO) method with the coherent potential approximation (CPA). The validity of this calculation method is demonstrated for disordered FePt and FePd alloys. We find that the estimated value of each origin of ${\sigma }_{xy}$ exhibits reasonable dependencies on the electron scattering in these disordered alloys.

In this paper, we use simulations to study current-induced domain wall dynamics by simultaneously resolving the spin transport and micromagnetic equations for a three-dimensional ferromagnetic strip. In contrast to local approaches, which neglect mutual interaction between spins and magnetic moments, our approach recalculates the spin distribution at each time step using the generalized drift diffusion model, which takes the transverse spin absorption phenomenon into account. We quantified the differences between a local approach and treatment based on a self-consistent method by plotting the domain wall velocity as a function of the domain wall width. We also characterized the domain wall velocity and the Walker breakdown condition as a function of the transverse spin absorption length $l_{\perp }$, which plays a crucial role in domain wall dynamics.

Small angle neutron scattering measurements on a bulk single crystal of the doped chiral magnet ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\mathrm{Si}$ with $x=0.3$ reveal a pronounced effect of the magnetic history and cooling rates on the magnetic phase diagram. The extracted phase diagrams are qualitatively different for zero and field cooling and reveal a metastable skyrmion lattice phase outside the $A$ phase for the latter case. These thermodynamically metastable skyrmion lattice correlations coexist with the conical phase and can be enhanced by increasing the cooling rate. They appear in a wide region of the phase diagram at temperatures below the $A$ phase but also at fields considerably smaller or higher than the fields required to stabilize the $A$ phase.

Laser-initiated switching of magnetization direction in ferrimagnetic rare-earth–transition-metal (RE-TM) alloys—whether laser induced or photothermal via compensation point—is being vigorously pursued owing to the promise of extending operating frequencies of magnetic devices into the terahertz regime. Despite intense interest, however, the effects of repeated laser exposure on the film structure and subsequent switching behavior have yet to be investigated. In order to better understand the correlated effects of femtosecond-laser irradiation on both the magnetic response and photoinduced morphological variations of RE-TM alloys, we performed in situ Fresnel transmission electron microscopy (TEM) on $\mathrm{T}{\mathrm{b}}_{23}\mathrm{C}{\mathrm{o}}_{77}$ thin films with Ta protecting layers. Via optical access to the specimen in a modified TEM, we irradiated the thin films in situ with both individual and series of femtosecond optical pulses, and correlated laser-induced changes in magnetic domain-wall formation and growth with photothermal crystal formation and accompanying pinned magnetic sites. We find that, for a range of applied laser fluences and numbers of individual pulses, several distinct regions are formed displaying varied magnetic behavior (switchable, nonswitchable, demagnetized) and morphological features (small-to-large crystal-grain variations). Through a series of systematic studies, we quantified these linked magnetic and morphological properties as a function of laser fluence, number of pulse-train cycles, and number of individual femtosecond-laser pulses and the duration between each. Our results show how the sensitive connection between magnetic behavior and morphological structure can emerge in magneto-optic experiments across several parameters, thus illustrating the need for rigorous characterization so that potential operating regimes may be universally identified.

The magnetic phase diagram and low-energy magnon excitations of structurally and magnetically chiral ${\text{MnSb}}_2{\text{O}}_6$ are reported. The specific heat and the static magnetization are investigated in magnetic fields up to 9 and 30 T, respectively, while the dynamic magnetic properties are probed by X-band as well as tunable high-frequency electron spin-resonance spectroscopy. Below $T_{\mathrm{N}}=11.5$ K, we observe antiferromagnetic resonance modes which imply small but finite planar anisotropy showing up in a zero-field splitting of 20 GHz. The data are well described by means of an easy-plane two-sublattice model with the anisotropy field $B_A=0.02$ T. The exchange field $B_{\mathrm{E}}=13$ T is obtained from the saturation field derived from the pulsed-field magnetization. A crucial role of the small anisotropy for the spin structure is reflected by competing antiferromagnetic phases appearing, at $T=2$ K, in small magnetic fields at $B_{\mathrm{C}1} \approx 0.5$ T and $B_{\mathrm{C}2}=0.9$ T. We discuss the results in terms of spin reorientation and of small magnetic fields favoring helical spin structure over the cycloidal ground state which, at $B=0$, is stabilized by the planar anisotropy. Above $T_{\mathrm{N}}$, short-range magnetic correlations up to $\gtrsim 60$ K and magnetic entropy changes well above $T_{\mathrm{N}}$ reflect the frustrated triangular arrangement of ${\mathrm{Mn}}^{2+}$ ions in ${\text{MnSb}}_2{\text{O}}_6$.

We propose a general scheme to probe the compatibility of arbitrary pairing states with a given normal state Hamiltonian by the introduction of a concept called superconducting fitness. This quantity gives a direct measure of the suppression of the superconducting critical temperature in the presence of key symmetry-breaking fields. A merit of the superconducting fitness is that it can be used as a tool to identify nontrivial mechanisms to suppress superconductivity under various external influences, in particular, magnetic fields or distortions, even in complex multiorbital systems. In the light of this concept we analyze the multiband superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and propose a new mechanism for the suppression of superconductivity in multiorbital systems, which we call interorbital effect, as a possible explanation for the unusual limiting feature observed in the upper critical field of this material.

We propose a model for the theoretical description of a weak-link Josephson junction, in which the weak link is spin-polarized due to proximity to a ferromagnetic metal [S-($\mathrm{F}|\mathrm{S}$)-S, where S is a superconductor and F is a ferromagnetic metal]. Employing Usadel transport theory appropriate for diffusive systems, we show that the weak link is described within the framework of Andreev circuit theory by an effective self-energy resulting from the implementation of spin-dependent boundary conditions. This leads to a considerable simplification of the model, and allows for an efficient numerical treatment. As an application of our model, we show numerical calculations of important physical observables such as the local density of states, proximity-induced minigaps, spin-magnetization, and the phase and temperature dependence of Josephson currents of the S-($\mathrm{F}|\mathrm{S}$)-S system. We discuss multivalued current-phase relationships at low temperatures as well as their crossover to sinusoidal form at high temperatures. Additionally, we numerically treat (S-F-S) systems that exhibit a magnetic domain wall in the F region and calculate the temperature-dependence of the critical currents.

We present the first-principles determination of electronic stopping power for protons and $\alpha$ particles in a semiconductor material of great technological interest: silicon carbide. The calculations are based on nonequilibrium simulations of the electronic response to swift ions using real-time, time-dependent density functional theory (RT-TDDFT). We compare the results from this first-principles approach to those of the widely used linear response formalism and determine the ion velocity regime within which linear response treatments are appropriate. We also use the nonequilibrium electron densities in our simulations to quantitatively address the longstanding question of the velocity-dependent effective charge state of projectile ions in a material, due to its importance in linear response theory. We further examine the validity of the recently proposed centroid path approximation for reducing the computational cost of acquiring stopping power curves from RT-TDDFT simulations.

Fractional occupation numbers can be used in density functional theory to create a symmetric Kohn-Sham potential, resulting in orbitals with degenerate eigenvalues. We develop the corresponding perturbation theory and apply it to a system of $N_d$ degenerate electrons in a harmonic oscillator potential. The order-by-order expansions of both the fractional occupation numbers and unitary transformations within the degenerate subspace are determined by the requirement that a differentiable map exists connecting the initial and perturbed states. Using the $\mathrm{X}\alpha$ exchange-correlation (XC) functional, we find an analytic solution for the first-order density and first- through third-order energies as a function of $\alpha$, with and without a self-interaction correction. The fact that the XC Hessian is not positive definite plays an important role in the behavior of the occupation numbers.

We have studied the temperature dependence of $\mathrm{TbM}{\mathrm{n}}_2{\mathrm{O}}_5$ Raman-active phonons and its $\mathrm{T}{\mathrm{b}}^{3+}$ crystal field (CF) excitations. Interestingly, the Raman intensities of some phonons are significantly reduced below $\sim 180\mathrm{K}$. Such behaviors are also observed in $\mathrm{HoM}{\mathrm{n}}_2{\mathrm{O}}_5$ and $\mathrm{YM}{\mathrm{n}}_2{\mathrm{O}}_5$ systems. A connection between the Raman intensities and the nearest-neighbor mean-square relative displacement ${\sigma }^2$ is established. Also, some of the $\mathrm{T}{\mathrm{b}}^{3+}$ and $\mathrm{H}{\mathrm{o}}^{3+}$ CF excitations become broader below $\sim 180\mathrm{K}$. These results are discussed in terms of the disorder induced by the Tb-O bond splitting.

We present a dynamical mechanism leading to dissipationless conductance, whose quantized value is controllable in a (3+1)-dimensional electronic system. The mechanism is exemplified by a theory of Weyl fermions coupled to a Higgs field, also known as an axion insulator. We show that the insertion of an axial gauge flux can induce vortex lines in the Higgs field, similar to the development of vortices in a superconductor upon the insertion of magnetic flux. We further show that the necessary axial gauge flux can be generated using Rashba spin-orbit coupling or a magnetic field. Vortex lines in the Higgs field are known to bind chiral fermionic modes, each of which serves as a one-way channel for electric charge with conductance $e^2/h$. Combining these elements, we present a physical picture, the “topological coaxial cable,” illustrating how the value of the quantized conductance could be controlled in such an axion insulator.

The crystal-field ground-state wave functions of the tetragonal, magnetically ordering Kondo lattice materials $\mathrm{Ce}M{\mathrm{Al}}_4{\mathrm{Si}}_2$ $(M=\text{Rh}$, Ir, and Pt) are determined with low-temperature linearly polarized soft-x-ray absorption spectroscopy, and estimates for the crystal-field splittings are given from the temperature evolution of the linear dichroism. Values for the dominant exchange field in the magnetically ordered phases can be obtained from fitting the influence of magnetic order on the linear dichroism. The direction of the required exchange field is $\parallel c$ for the antiferromagnetic Rh and Ir compounds, with the corresponding strength of the order of ${\lambda }_{\text{ex}}\approx 6\text{meV}$ (65 K). Furthermore, the presence of Kondo screening in the Rh and Ir compound is demonstrated on the basis of the absorption due to $f^0$ in the initial state.

We theoretically demonstrate that graphene-insulator-graphene tunnel structures can serve as plasmonic gain media due to the possibility of stimulated electron tunneling accompanied by emission of plasmons under application of interlayer voltage. The probability of plasmon-assisted tunneling is resonantly large at certain values of frequency and interlayer voltage corresponding to the transitions between chiral electron states with collinear momenta, which is a feature unique to the linear bands of graphene. The plasmon dispersion develops an anticrossing with the resonances in tunnel conductivity and demonstrates negative group velocity in several frequency ranges.

We propose a direct approach for calculating individual energy of polar semiconductor interfaces using density functional theory calculations. This approach is applied to polar interfaces between group-III nitrides (AlN and GaN) and SiC and clarifies the interplay of chemical bonding and charge neutrality at the interface, which is crucial for the stability and polarity of group-III nitrides on SiC substrates. The ideal interface is stabilized among various atomic arrangements over the wide range of the chemical potential on Si-face SiC, whereas those with intermixing are favorable on C-face SiC. The stabilization of the ideal interfaces resulting in Ga-polar GaN and Al-polar AlN films on Si-face SiC is consistent with experiments, suggesting that our approach is versatile to evaluate various polar heterojunction interfaces as well as group-III nitrides on semiconductor substrates.

Nonlinear phenomena in optically excited semiconductor structures are of high interest. Here we develop a model capable of studying the dynamics of the photoexcited carriers, including Coulomb interaction on a Hartree-Fock level, on the same footing as the dynamics of the light field impinging on an arbitrary photonic structure. Applying this method to calculate the third-harmonic generation in a semiconductor quantum well embedded in a Bragg mirror structure, we find that the power-law exponent of the intensity dependence of the third-harmonic generation depends on the frequency of the exciting pulse. Off-resonant pulses follow the expected cubic dependence, while the exponent is smaller for resonant pulses due to saturation effects in the induced carrier density. Our study provides a detailed understanding of the carrier and light field dynamics during nonlinear processes.

The electrical properties of donor-doped $\mathrm{SrTi}{\mathrm{O}}_3$ $(n$-STO) are profoundly affected by an oxidation-induced metal-insulator transition (MIT). Here we employ dynamical numerical simulations to examine the high-temperature MIT of $n$-STO over a large range of time and length scales. The simulations are based on the Nernst-Planck equations, the continuity equations, and the Poisson equation, in combination with surface lattice disorder equilibria serving as time-dependent boundary conditions. The simulations reveal that $n$-STO, upon oxidation, develops a kinetic space charge region (SCR) in the near-surface region. The surface concentrations of the variously mobile defects (electrons, Sr vacancies, and O vacancies) are found to vary over time and to differ considerably from the values of the new equilibrium. The formation of the SCR in which electrons are strongly depleted occurs within nanoseconds, i.e., it yields a fast MIT in the near-surface region during the oxidation process. As a result of charge (over-)compensation by Sr vacancies incorporated at the surface of $n$-STO, this SCR is much more pronounced than conventionally expected. In addition, we find an anomalous increase of O vacancy concentration at the surface upon oxidation caused by the SCR. Our simulations show that the SCR fades in the long term as a result of the slow in-diffusion of Sr vacancies. We discuss implications for the electrical conductivity of $n$-STO crystals used as substrates for epitaxial oxide thin films, of $n$-STO thin films and interfaces, and of polycrystalline $n$-STO with various functionalities.

The electron states in the field of a charged impurity in graphene in a magnetic field are studied numerically. It is shown that a charged impurity removes the degeneracy of Landau levels converting them into bandlike structures. As the charge of impurity grows, the repulsion of sublevels of different Landau levels with the same value of orbital momentum takes place leading to the redistribution of the wave function profiles of these sublevels near the impurity. By studying the polarization effects, it is shown in agreement with the recent experiments that the effective charge of impurity can be very effectively tuned by chemical potential. If the chemical potential is situated inside a Landau level, then the charge of impurity is strongly diminished. In addition, the polarization function in this case has a peak at zero momentum, which leads to the sign-changing oscillations of the screened potential as a function of distance. If the chemical potential lies between the Landau levels, then the screened potential does not change sign, the screening is minimal, and the charged impurity can strongly affect the electron spectrum.

We propose a constitutive parameter retrieval approach in which all electromagnetic parameters of the medium are obtained taking spatial dispersion into account. Moreover, the constraint on nonmagnetic metal/dielectric metamaterials is relaxed. This procedure is applied to metal/dielectric stacks in order to address the effects of the layer thickness, layer number, and material choice on the spatial dispersion. The results demonstrate that the investigated metal/dielectric stacks have a clear magnetic response, particularly for thicker layers. Moreover, this magnetic response is also a function of the magnitude of the $|k_x/k_0|$ ratio, where $k_x$ is the wave vector parallel to the interface planes and $k_0$ is the free-space wave number. We demonstrate that the real part of the dispersion curve flattens out (with a corresponding large imaginary part being present) as a result of the absence of propagating modes inside the metamaterial. This flat region is strongly dependent on the thickness of the layers and is a direct manifestation of spatial dispersion. Using this parameter retrieval method we calculate the Purcell factor for Rb atoms 10 nm above the surface of a ${\mathrm{Ag}/\mathrm{TiO}}_2$ stack with two filling factors $\rho$ $[\rho =l/(l+d)$ with $l$ and $d$ as the metal and dielectric layer thicknesses, respectively], $\rho =0.3$ and $\rho =0.5$, and having $N=13$ layers, for the emission wavelengths of 435 nm and 785 nm. Results are then compared with three different approaches, and we show that if spatial dispersion is not properly taken into account, then the Purcell factor is overestimated. Our approach shows excellent agreement with Purcell factors obtained from precise and accurate numerical calculations of the corresponding nonhomogenized structures.

Topological insulators are attracting considerable interest due to their potential for technological applications and as platforms for exploring wide-ranging fundamental science questions. In order to exploit, fine-tune, control, and manipulate the topological surface states, spectroscopic tools which can effectively probe their properties are of key importance. Here, we demonstrate that positrons provide a sensitive probe for topological states and that the associated annihilation spectrum provides a technique for characterizing these states. Firm experimental evidence for the existence of a positron surface state near ${\mathrm{Bi}}_2{\mathrm{Te}}_2\mathrm{Se}$ with a binding energy of $E_b=2.7\pm 0.2\mathrm{eV}$ is presented and is confirmed by first-principles calculations. Additionally, the simulations predict a significant signal originating from annihilation with the topological surface states and show the feasibility to detect their spin texture through the use of spin-polarized positron beams.