We present detailed magnetometry and muon-spin rotation data on polycrystalline samples of overdoped, nonsuperconducting LiFe${}_{1-x}$Ni${}_x$As ($x=0.1,0.2$) and Li${}_{1-y}$Fe${}_{1+y}$As ($0\le y\le 0.04$) as well as superconducting LiFeAs. While LiFe${}_{1-x}$Ni${}_x$As exhibits weak antiferromagnetic fluctuations down to $1.5\mathrm{K}$, Li${}_{1-y}$Fe${}_{1+y}$As samples, which have a much smaller deviation from the $1:1:1$ stoichiometry, show a crossover from ferromagnetic to antiferromagnetic fluctuations on cooling and a freezing of dynamically fluctuating moments at low temperatures. We do not find any signatures of time-reversal symmetry breaking in stoichiometric LiFeAs that would support recent predictions of triplet pairing.

An asymmetric triangular potential well provides the simplest model for the confinement of mobile electrons at the interface between two insulating oxides, such as LaAlO${}_3$ and SrTiO${}_3$ (LAO/STO). These electrons have been recently shown to exhibit a large spin-orbit coupling of the Rashba type, i.e., linear in the in-plane momentum. In this paper we study the intrinsic spin Hall effect due to Rashba coupling in an asymmetric triangular potential well. This is the minimal model that captures the asymmetry of the spin-orbit coupling on opposite sides of the interface. Besides splitting each subband into two branches of opposite chirality, the spin-orbit interaction causes the transverse wave function (i.e., the wave function in the $z$ direction, perpendicular to the plane of the quantum well) to depend on the in-plane wave vector $\mathbf{k}$. At variance with the standard Rashba model, the triangular well supports a nonvanishing intrinsic spin Hall conductivity, which is proportional to the square of the spin-orbit coupling constant and, in the limit of low carrier density, depends only on the effective mass renormalization associated with the $\mathbf{k}$ dependence of the transverse wave functions. The origin of the effects lies in the nonvanishing matrix elements of the spin current between subbands corresponding to different states of quantized motion perpendicular to the plane of the well.

We investigate, with high-resolution angle-resolved photoemission spectroscopy, the spectral function of potassium-doped quasi-free-standing graphene on Au. Angle-dependent x-ray photoemission and density functional theory calculations demonstrate that potassium intercalates into the graphene/Au interface, leading to an upshift of the K-derived electronic band above the Fermi level. This empty band is what makes this system perfectly suited to disentangle the contributions to electron-phonon coupling coming from the $\pi$ band and K-derived bands. From a self-energy analysis we find an anisotropic electron-phonon coupling strength $\lambda$ of 0.1 (0.2) for the $K\Gamma$ ($KM$) high-symmetry directions in momentum space, respectively. Interestingly, the high-energy part of the Eliashberg function which relates to graphene's optical phonons is equal in both directions but only in $KM$ does an additional low-energy part appear.

The growing interest in carbon-based spintronics has stimulated a number of recent theoretical studies on the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in graphene, with the aim of determining the most energetically favorable alignments between embedded magnetic moments. The RKKY interaction in undoped graphene decays faster than expected for conventional two-dimensional materials, and recent studies suggest that the adsorption configurations favored by many transition-metal impurities may lead to even shorter-ranged decays and possible sign-changing oscillations. Here, we show that these features emerge in a mathematically transparent manner when the symmetry of the configurations is included in the calculation. Furthermore, we show that by breaking the symmetry of the graphene lattice, via uniaxial strain, the decay rate, and hence the range, of the RKKY interaction can be significantly altered. Our results suggest that magnetic interactions between adsorbed impurities in graphene can be manipulated by careful strain engineering of such systems.

We study the topological features of noninteracting insulators subject to an antiferromangetic (AFM) Zeeman field, or AFM insulators, the period of which is commensurate with the lattice period. These insulators can be classified by the presence/absence of an emergent antiunitary symmetry: the combined operation of time reversal and a lattice translation by vector $\mathbf{D}$. For AFM insulators that preserve this combined symmetry, regardless of any details in lattice structure or magnetic structure, we show that (i) there is a new type of Kramers' degeneracy protected by the combined symmetry; (ii) a new $Z_2$ index may be defined for three-dimensional (3D) AFM insulators, but not for those in lower dimensions, and (iii) in 3D AFM insulators with a nontrivial $Z_2$ index, there are odd number of gapless surface modes if and only if the surface termination also preserves the combined symmetry, but the dispersion of surface states becomes highly anisotropic if the AFM propagation vector becomes small compared with the reciprocal lattice vectors. We numerically demonstrate the theory by calculating the spectral weight of the surface states of a 3D topological insulator in the presence of AFM fields with different propagation vectors, which may be observed by ARPES in Bi${}_2$Se${}_3$ or Bi${}_2$Te${}_3$ with induced antiferromagnetism.

Knowledge of the structures of Ta that appear at high pressures and high temperatures is critical for addressing the recent controversies regarding the phase diagram and melting temperatures of this refractory metal. Structural searches based on a density functional metadynamics method were therefore employed to obtain candidate structures at high-pressure and finite-temperature conditions. A structural transformation from the initial body-centered cubic phase of Ta stable under ambient conditions to an orthorhombic structure with the space group Pnma is predicted. The Pnma structure is shown to be energetically more favorable and more reasonable than other candidate structures considered previously, since it was also confirmed to be mechanically and dynamically stable by phonon and metadynamics calculations. However, a recently proposed hexagonal-ω phase for dense Ta is found to be mechanically and dynamically unstable when anharmonic effects are characterized by high-temperature, self-consistent phonon calculations.

We present simulation results based on a phase-field model that describes the local migration of recrystallization boundaries into varying deformation energy fields. An important finding from the simulations is that the overall migration rate of the recrystallization front can be considerably affected by the variations in the deformed microstructure, resulting in two regimes. For variations with low amplitude, the overall boundary velocity scales with the average stored deformation energy density. This behavior is in agreement with generally accepted theories of recrystallization. For larger amplitudes, however, the velocity scales with the maximum of the deformation energy density along the variation, resulting in a considerably larger velocity than that obtained from standard recrystallization models. The shape of the migrating grain boundary greatly depends on the local characteristics of the varying stored deformation energy field. For different deformation energy fields, the simulation results are in good qualitative agreement with experiments and add information which cannot be directly derived from experiments.

We investigated rolled-up permalloy microtubes using microwave absorption spectroscopy. The resonance frequencies of the tubes are tuneable by the external magnetic field. Up to four resonant azimuthal modes have been found and were attributed to constructive spin-wave interference in axial and perpendicular magnetization geometry. We show that the degeneracy of the mode can be lifted by applying a magnetic field in specified directions. The resonances of the system are described analytically. Thereby, an effective layer thickness and a demagnetization field are used to account for the multilayer structure and the curved surface, respectively. This modeling approach is supported by micromagnetic simulations.

We show by micromagnetic simulations that a spontaneous skyrmion ground state can exist in Co/Ru/Co nanodisks without the Dzyaloshinsky-Moriya interaction, which can remain stable in the applied magnetic field along the $+$$z$ direction even up to 0.44 T. The guiding center ($R_x$,$R_y$) of skyrmion defined by the moments of the topological density presents a gyration with a star-like trajectory in a pulsed magnetic field and a hexagonal trajectory after the field is switched off, which is different from that of a vortex or bubble. One of the coupled skyrmions could move without an external magnetic field, just induced by the motion of the other one due to strong interlayer magnetostatic interactions. This work sheds light on how skyrmions can be discovered in various (not limited to magnetic) systems with competing energies and contributes to the understanding of the dynamical properties of skyrmions.

We discuss the Josephson effect in strongly spin-polarized ferromagnets where triplet correlations are induced by means of spin-active interface scattering, extending our earlier work [R. Grein, M. Eschrig, G. Metalidis, and G. Schön, Phys. Rev. Lett. 102, 227005 (2009)] by including impurity scattering in the ferromagnetic bulk and the inverse proximity effect in a fully self-consistent way. Our quasiclassical approach accounts for the differences of Fermi momenta and Fermi velocities between the two spin bands of the ferromagnet, and thereby overcomes an important shortcoming of previous work within the framework of Usadel theory. We show that nonmagnetic disorder in conjunction with spin-dependent Fermi velocities may induce a reversal of the spin current as a function of temperature.

We show that an in-plane magnetic field can drive a fully gapped $p\pm ip$ topological superconductor into a gapless phase which supports Majorana flat bands (MFBs). Unlike previous examples, the MFBs in the gapless regime are protected from disorder by a chiral symmetry. In addition, novel unidirectional Majorana edge states (MESs) which propagate in the same direction on opposite edges appear when the chiral symmetry is broken by Rashba terms. Unlike the usual chiral or helical edge states, unidirectional MESs appear only in systems with a gapless bulk. The MFBs and the unidirectional MESs induce nearly quantized zero bias conductance in tunneling experiments.

We present experimental evidences for the dynamical bifurcation behavior of ac-driven magnetic vortex core gyration in a ferromagnetic disk. The dynamical bifurcation, i.e., appearance and disappearance of two stable dynamical states in the vortex gyration, occurring as the amplitude of the driving Oersted field increases to $B_{Oe}>B_{Oe}^{cr}$, manifests itself in a double resonance response in the dependence of homodyne the dc-voltage signal on the frequency $\omega$ of the applied microwave current. We find that the frequency range $\delta \omega$ between the two resonance features strongly increases with the excitation power. Our analysis based on the model of a low dissipative nonlinear oscillator subject to a resonant alternating force is in good agreement with the experimental results. This allows us to determine quantitatively key parameters of magnetic vortex dynamics, i.e., the critical value of the driving Oersted field $B_{Oe}^{cr}$ for nonlinear dynamics to occur, the resonant frequency, and the quality factor as well as damping of the magnetic vortex gyration.

We have measured the inverse spin Hall effect (ISHE) in $n$-Ge at room temperature. The spin current in germanium was generated by spin pumping from a CoFeB/MgO magnetic tunnel junction in order to prevent the impedance mismatch issue. A clear electromotive force was measured in Ge at the ferromagnetic resonance of CoFeB. The same study was then carried out on several test samples, in particular, we have investigated the influence of the MgO tunnel barrier and sample annealing on the ISHE signal. First, the reference CoFeB/MgO bilayer grown on SiO${}_2$ exhibits a clear electromotive force due to anisotropic magnetoresistance and anomalous Hall effect, which is dominated by an asymmetric contribution with respect to the resonance field. We also found that the MgO tunnel barrier is essential to observe ISHE in Ge and that sample annealing systematically leads to an increase of the signal. We propose a theoretical model based on the presence of localized states at the interface between the MgO tunnel barrier and Ge to account for these observations. Finally, all of our results are fully consistent with the observation of ISHE in heavily doped $n$-Ge and we could estimate the spin Hall angle at room temperature to be $\approx$0.001.

We investigate Kondo physics in a host with a strongly diverging density of states. This study is motivated by a recent work on vacancies in the graphene honeycomb lattice, whose density of states is enhanced at low energies due to potential scattering. The generalized quantum impurity model describing the vacancy is shown to support a spin-$\frac{1}{2}$ (doublet) Kondo phase. The special role played by a diverging host density of states is examined in detail, with distinctive signatures associated with the power law Kondo effect shown to appear in thermodynamic quantities and the scattering $t$ matrix, with a strongly enhanced Kondo temperature. Although the effective Kondo model supports a stable phase characterized by strong renormalized particle-hole asymmetry, we find that this phase cannot in fact be accessed in the full Anderson model. In the more realistic case, where the divergence in the host density of states is cut off at low energies, a crossover is generated between pristine power law Kondo physics and a regular Kondo strong coupling state.

This paper describes an all-electron implementation of the self-consistent $GW$ (sc-$GW$) approach—i.e., based on the solution of the Dyson equation—in an all-electron numeric atom-centered orbital basis set. We cast Hedin's equations into a matrix form that is suitable for numerical calculations by means of (i) the resolution-of-identity technique to handle four-center integrals and (ii) a basis representation for the imaginary-frequency dependence of dynamical operators. In contrast to perturbative $G_0{W}_0$, sc-$GW$ provides a consistent framework for ground- and excited-state properties and facilitates an unbiased assessment of the $GW$ approximation. For excited states, we benchmark sc-$GW$ for five molecules relevant for organic photovoltaic applications: thiophene, benzothiazole, 1,2,5-thiadiazole, naphthalene, and tetrathiafulvalene. At self-consistency, the quasiparticle energies are found to be in good agreement with experiment and, on average, more accurate than $G_0{W}_0$ based on Hartree-Fock or density-functional theory with the Perdew-Burke-Ernzerhof exchange-correlation functional. Based on the Galitskii-Migdal total energy, structural properties are investigated for a set of diatomic molecules. For binding energies, bond lengths, and vibrational frequencies sc-$GW$ and $G_0{W}_0$ achieve a comparable performance, which is, however, not as good as that of exact-exchange plus correlation in the random-phase approximation and its advancement to renormalized second-order perturbation theory. Finally, the improved description of dipole moments for a small set of diatomic molecules demonstrates the quality of the sc-$GW$ ground-state density.

Herbertsmithite [ZnCu${}_3$(OH)${}_6$Cl${}_2$] is often discussed as the best realization of the highly frustrated antiferromagnetic kagome lattice known so far. We employ density functional theory (DFT) calculations to determine eight exchange coupling constants of the underlying Heisenberg Hamiltonian. We find the nearest-neighbor coupling $J_1$ to exceed all other couplings by far. However, next-nearest-neighbor kagome layer couplings of $0.019J_1$ and interlayer couplings of up to $-0.035J_1$ slightly modify the perfect antiferromagnetic kagome Hamiltonian. Interestingly, the largest interlayer coupling is ferromagnetic, even without Cu impurities in the Zn layer. In addition, we validate our DFT approach by applying it to kapellasite, a polymorph of herbertsmithite, which is known experimentally to exhibit competing exchange interactions.

By measuring time-dependent optical birefringence with 4-femtosecond (fs) laser pulses, we determine the time scale for photoinduced melting of orbital order in the single-layer manganite La${}_{0.5}$Sr${}_{1.5}$MnO${}_4$. Such high time resolution is required to distinguish atomic motions that control the Jahn-Teller distortion from even faster electronic rearrangements. The experiment reveals an 18 fs bottleneck for the loss of orbital order, corresponding to about one-quarter period of the in-plane Jahn-Teller mode. Furthermore, we observe coherent oscillations of this Jahn-Teller mode. Both their amplitude and the birefringence drop exhibit a threshold in their fluence dependence, indicating cooperativity in the lattice response.

Temperature ($T$) and magnetic field ($H$) dependence of the magnetic properties in metamagnetic UCoAl have been investigated using a soft x-ray magnetic circular dichroism (XMCD). In order to extract element-specific magnetic properties at the U and Co sites, the XMCD experiment has been performed at the U 4$d$-5$f$ ($N_4,5$) and Co 2$p$-3$d$ ($L_2,3$) absorption edges, respectively. Directions of magnetic moments at the U and Co sites have been determined from shapes of the XMCD spectra. The directions of the total magnetic moments at the U and Co sites are parallel to the $H$ direction ($c$ axis), but the direction of the spin magnetic moment at the U site is opposite to that at the Co site. The XMCD intensities at both the U and Co sites at $T=5.5$ K increase steeply at $H=0.77$ T ($H_{\mathrm{m}}$), corresponding to the metamagnetic transition. The XMCD intensities do not saturate, even in the field-induced ferromagnetic state above $H_{\mathrm{m}}$. In addition, the ratio of the increase of the XMCD intensity at the Co site is smaller than that at the U site. From comparison of the $H$ dependence of the XMCD intensities at $T=25$ and 5.5 K, we found that the magnetic behavior of the Co atom has a stronger $T$ dependence than that of the U atom.

We report nuclear magnetic resonance Knight shift data in the heavy-fermion material CeIrIn${}_5$ at fields up to 30 T. The Knight shift of the In displays a strong anomaly, and we analyze the results using two interpretations. We find that the Kondo lattice coherence temperature and the effective mass of the heavy electrons remain largely unaffected by the magnetic field, despite the fact that the Zeeman energy is of the order of the coherence temperature.

The realization of the spin-Hall effect in quantum wells has led to a plethora of studies regarding the properties of the edge states of a two-dimensional topological insulator. These edge states constitute a class of one-dimensional liquids, called the helical liquid, where an electron's spin quantization axis is tied to its momentum. In contrast to one-dimensional conductors, magnetic impurities—below the Kondo temperature—cannot block transport and one expects the current to circumvent the impurity. To study this phenomenon, we consider the single-impurity Anderson model embedded into an edge of a Kane-Mele ribbon with up to $512\times 80$ sites and use the numerically exact continuous time quantum Monte Carlo method to study the Kondo effect. We present results on the temperature dependence of the spectral properties of the impurity and the bulk system that show the behavior of the system in the various regimes of the Anderson model. A view complementary to the single-particle spectral functions can be obtained using the spatial behavior of the spin-spin correlation functions. Here we show the characteristic, algebraic decay in the edge channel near the impurity.