We show how the one-loop “poor man's scaling” equations for the Kondo model with arbitrary impurity spin can be obtained within the framework of the functional renormalization group approach for quantum spin systems recently developed by Krieg and Kopietz (arXiv:1807.02524). We argue that our method supersedes the poor man's scaling approach and can also be used to study the strong coupling regime of the Kondo model.

We provide a definition of the quantum distances of correlated many-fermion wave functions in terms of the expectation values of certain operators that we call exchange operators. We prove that the distances satisfy the triangle inequalities. We apply our formalism to the one-dimensional $t-V$ model, which we solve numerically by exact diagonalization. We compute the distance matrix and illustrate that it shows clear signatures of the metal-insulator transition.

We introduce a computational scheme for calculating the electronic structure of random alloys that includes electronic correlations within the framework of the combined density functional and dynamical mean-field theory. By making use of the particularly simple parametrization of the electron Green's function within the linearized muffin-tin orbitals method, we show that it is possible to greatly simplify the embedding of the self-energy. This in turn facilitates the implementation of the coherent potential approximation, which is used to model the substitutional disorder. The computational technique is tested on the Cu-Pd binary alloy system, and for disordered Mn-Ni interchange in the half-metallic NiMnSb.

We report the magnetic field and temperature dependences of magnetoconductance (MC) and magnetoelectroluminescence (MEL) in organic light emitting diodes (OLED) made of homopolymer organic layers. We show that in these OLEDs a spin mixing mechanism due to the anisotropy of the organic layer $g$ factor has a significant effect in the measured MC($B$) and MEL($B$) at fields of order of $\sim 1\mathrm{T}$. We discuss the model of spin mixing among polaron pairs due to the anisotropy of the $g$ factor and show that it can be described as an angle dependent $\mathrm{\Delta }g$ mechanism that is known to have significant magnetoresponse effects. We show that the internal structure of the polaron pair is material dependent rationalizing thus the differences in their electrical mobility.

Based on an Anderson-like model including the electron-lattice interaction and a Hubbard term, the ground-state properties of metal/organic-ferromagnet heterojunctions are investigated. Both the nonmagnetic and ferromagnetic metals are considered. The results reveal a picture of electron injection and the induced excitations in the organic ferromagnets, which is divided into two regimes with the increase of injected electrons: the first solitonlike interfacial state and the subsequent coexistence of the interfacial state and polaron states. The interfacial state is further explored by the shallow states and localized wave functions with the analysis of the molecular self-consistent Hamiltonian. In the case of the ferromagnetic metal, the effect of the magnetization orientation alignment between the metal and the organic ferromagnet is discussed, where the lowest injection barrier is found when the magnetization orientations of the two components are parallel. An injection-induced degradation of the magnetism of the organic ferromagnet is also obtained. This paper provides a basic understanding of the metal/organic-ferromagnet interface, which is essential for further spintronic study in organic ferromagnetic devices.

Thermal radiation from bulk disorderly placed nonresonant emitters is incoherent broadband and isotropic. In an external magnetic field the thermal radiation from any source is circularly polarized. Here we propose a thermal radiation source which emits circularly polarized radiation and which is not placed in a magnetic field. The thermal source consists of a slab waveguide with etched chiral metasurface of fourfold rotational symmetry. Using the Fourier modal method we analyze the eigenmodes of the structure and the emissivity spectra. We demonstrate that due to the absence of a mirror symmetry of the metasurface its eigenmodes as well as the thermally generated electromagnetic waves become circularly polarized. In this paper we discuss the origin of this phenomenon in detail. We demonstrate that the degree of circular polarization in an optimized structure can be as high as 0.87.

With analytical (generalized Mie scattering) and numerical (integral-equation-based) considerations we show the existence of strong resonances in the scattering response of small spheres with a lossless impedance boundary. With increasing size, these multipolar resonances are damped and shifted with respect to the magnitude of the surface impedance. The electric-type resonances are inductive and the magnetic ones capacitive. Interestingly, these subwavelength resonances resemble plasmonic resonances in small negative-permittivity scatterers but occur in two families unlike the only-electric multipoles in the plasmonic case. The fundamental dipolar mode is also analyzed from the point of view of surface currents and the effect of the change of the shape into a nonspherical geometry. For dissipative impedance scatterers, maximum absorption is shown to result in the matched-impedance case.

The spin-$\frac{1}{2}$ $J_1-J_2$ Heisenberg model on square lattices is investigated via the finite projected entangled pair states (PEPS) method. Using the recently developed gradient optimization method combined with Monte Carlo sampling techniques, we are able to obtain the ground state energies that are competitive with the best results. The calculations show that there is no Néel order, dimer order, or plaquette order in the region of $0.42\lesssim J_2/J_1\lesssim 0.6$, suggesting a single spin liquid phase in the intermediate region. Furthermore, the calculated staggered spin, dimer, and plaquette correlation functions all have power law decay behaviors, which provide strong evidence that the intermediate nonmagnetic phase is a single gapless spin liquid state.

In contrast to the well-defined lattices of various two-dimensional (2D) systems, the atomic structure of borophene is sensitive to growth conditions and type of the substrate which results in rich polymorphism. By employing ab initio methods, we reveal a thermodynamically stable borophene polymorph without vacancies which is a semiconductor unlike the other known boron sheets, in the form of an asymmetric centered-washboard structure. Our results indicate that asymmetric distortion is induced due to Peierls instability which transforms a symmetric metallic system into a semiconductor. We also show that applying uniaxial or biaxial strain gradually lowers the obtained band gap and the symmetric configuration is restored following the closure of the band gap. Furthermore, while the Poisson's ratio is calculated to be high and positive in the semiconducting regime, it switches to negative once the metallicity is retrieved. The realization of semiconducting borophene polymorphs without defects and tunability of its electronic and mechanical response can extend the usage of boron sheets in a variety of nanoelectronic applications.

Matrix product representation provides a useful formalism to study not only entangled states but also entangled operators in one dimension. In this paper, we focus on unitary transformations and show that matrix product operators that are unitary provide a necessary and sufficient representation of one-dimensional (1D) unitaries that preserve locality. That is, we show that matrix product operators that are unitary are guaranteed to preserve locality by mapping local operators to local operators, while at the same time all locality-preserving unitaries can be represented in a matrix product way. Moreover, we show that matrix product representation gives a straightforward way to extract the index defined by Gross, Nesme, Vogts, and Werner in [D. Gross et al., Commun. Math. Phys. 310, 419 (2012)] for classifying 1D locality-preserving unitaries. The key to our discussion is a set of “fixed-point” conditions which characterize the form of the matrix product unitary operators after blocking sites. Finally, we show that if the unitary condition is only required for certain system sizes, then matrix product formalism allows more possibilities. In particular, we give an example of a simple matrix product operator which is unitary only for odd system sizes, does not preserve locality, and carries a “fractional” index.

We introduce a Maxwell-Bloch simulation approach which self-consistently combines a microscopic description of the carrier and polarization dynamics of a transition-metal-dichalcogenide (TMDC) monolayer with a spatiotemporal full-wave time-domain simulation of Maxwell's equations on the basis of a finite-difference time-domain (FDTD) method beyond the slowly varying amplitude or paraxial approximations. This offers a platform to realistically model, in particular, the typical ultrafast optical excitation experiments in micro- and nanocavities. Our simulations confirm that the weak screening of the Coulomb interaction in TMDC monolayers yields pronounced exciton lines in the linear spectrum and we uncover the second-order nonlinearity represented in the semiconductor Maxwell-Bloch equations by an intraband dipole moment. This allows us to calculate the spectral shape of the exceptionally strong second-harmonic generation around the exciton lines of TMDC monolayers. We demonstrate that the second-harmonic signal can remarkably be further enhanced by several orders of magnitude through a suitably designed (one-dimensional) photonic microcavity. Due to its self-consistency, flexibility, explicit spatio-temporal resolution on the nanoscale and the ready access to light field and electron dynamics, our theory and computational approach is an ideal platform to design and explore spatiotemporal nonlinear and quantum dynamics in complex photonic or plasmonic micro- and nanostructures for optoelectronic, nanophotonic and quantum applications of TMDC monolayers.

Understanding the response of low-dimensional electronic systems to external perturbations is of great interest for fundamental science as well as future applications. Here we employ plasmon spectroscopy to study the modification of metallicity in Au-induced quantum wires on the Si(553) surface by hydrogenation. The present study shows that no direct bond formation to the Au chains is necessary for controllable modifications of metallicity. Instead, changes in band structure of the whole system by H adsorbed at the Si step edges suffice via indirect interaction, as proven by the quantitative agreement between first-principles calculations and the unoccupied band structure derived from plasmon spectroscopy. Interestingly, simple electron donation has a similar effect as hydrogen adsorption at the step edges with respect to the Au bands and the dimerization of the Au chains.

We present an effective model for surface states at the Si-terminated (001) surface of ternary rare earth (R) intermetallic compounds ${\mathit{RT}}_2{\mathrm{Si}}_2$ with the transition metal element $T$ = Ir or Rh. The model is based on a fully ab initio derivation of the relativistic $\mathbf{k}·\mathbf{p}$ Hamiltonian and is thereby capable of accurately reproducing the spin polarization of the spin-orbit split surface states, which is very different from the classical Rashba effect. The reliable treatment of spin in our model enables a predictive analysis of the effect of magnetic exchange interaction of the surface-state electrons with ferromagnetically ordered $4f$ moments of the subsurface rare-earth layer in the magnetic phases of the ${\mathit{RT}}_2{\mathrm{Si}}_2$ compounds.

Using high-resolution inelastic neutron scattering, we examine the spin dynamics of ${\mathrm{Mn}}_{1-x}{\mathrm{Co}}_x{\mathrm{WO}}_4$ in the collinear AF1, the $ac\text{−}b$ spiral AF2, and the $ac$ cycloidal AF5 phases. The spin wave excitations are well described by a Heisenberg model with competing long-range exchange interactions $(J_i$ up to 12th nearest neighbors) and the single-ion anisotropy $K$ induced by the spin-orbit interaction. While the exchange constants are relatively unchanged, the dominant effect of doping is to change the single-ion anisotropy from easy axis $\left(K>0\right)$ in the collinear AF1 phase to easy plane $\left(K<0\right)$ in the two multiferroic phases.

By the Monte Carlo method, magnetic properties have been investigated for Eu-doped GaN involving internal nanostructures induced by nanoscale spinodal decomposition, where these nanostructures are spontaneously or artificially formed. In the present simulations, hysteretic and nonhysteretic magnetization curves are observed in the systems with the large-sized and small-sized nanostructures, respectively. These nanostructures affect the blocking temperatures as well. Furthermore, they influence temperature-dependent energy barriers of spin flipping; therefore, the simulations suggest that the magnetization is thermally stable. However, we observe that the blocking temperatures are smaller than the experimental values, which may be due to atomic vacancies.

Here we report that femtosecond laser pulses are able to trigger oscillations of the magneto-optical Faraday rotation in the ferromagnetic semiconductor $\mathrm{CdC}{\mathrm{r}}_2\mathrm{S}{\mathrm{e}}_4$ in the presence of an applied magnetic field. The frequency of these oscillations is a linear function of the magnetic field and corresponds to the ferromagnetic resonance (FMR). Tuning the photon energy of the pump pulses we reveal two different mechanisms, which induce FMR precession in this material. In the case of pumping from the valence band deep into the conduction band (photon energy 3.1 eV), the phase of the spin oscillations is not sensitive to the polarization of the pump, but can be reversed over 180 deg by changing the polarity of the applied magnetic field. We assign these oscillations to the coherent spin precession triggered by ultrafast laser-induced heating. This mechanism requires a strong optical absorption in the material and becomes inactive if the pump photon energy is below the band gap. Tuning the photon energy in a wide range from 0.88 to 2.1 eV reveals the second mechanism of optical excitation of coherent spin oscillations with a maximum around 1.2 eV, i.e., very close to the energy of the band gap in the semiconductor. Contrary to the laser-induced heating, this excitation mechanism is pump polarization dependent, being the most efficient if the pump is circularly polarized. The phase of the spin oscillations is independent of the polarity of the applied magnetic field, but changes by 180 deg under reversing the helicity of light. We suggest that the effect can be interpreted in terms of spin transfer torque experienced by the network of the ordered $\mathrm{C}{\mathrm{r}}^{3+}$ spins as a result of excitation of electrons from the top of the $p$-type valence band to the bottom of the $s$-type conduction band. In particular, a strong spin-orbit interaction experienced by the carriers in the valence band is responsible for the coupling of the spins of the photogenerated carriers and the polarization of light. Due to strong $pd$- and $sd$-exchange interactions the spins of the photocarriers appear to be coupled to the network of ordered spins of the $\mathrm{C}{\mathrm{r}}^{3+}$ ions.

CoFeB/MgO heterostructures are a promising candidate for an integral component of spintronic devices due to their high magnetic anisotropy, low Gilbert damping, and excellent magnetoresistive properties. Here, we present experimental measurements and atomistic simulations of the temperature and CoFeB thickness dependence of spontaneous magnetization and magnetic anisotropy in CoFeB/MgO ultrathin films. We find that the thermal fluctuations are different between the bulk and interface magnetizations, and that the interfacial anisotropy originates from a two-site anisotropic exchange interaction. These effects lead to a complex temperature and thickness dependence of the magnetic properties critical to device operation and stability at elevated temperatures.

It is demonstrated that the Landau diamagnetism of the free electron gas and a monovalent metal can be considered as a Fermi surface effect. Only a relatively small number of electron states close to the Fermi surface are diamagnetically active whereas the majority of the electron states inside the Fermi surface are diamagnetically inert. This partitioning of the occupied electron states is driven by the structure of Landau levels, around which one can introduce magnetic tubes in the reciprocal space. Completely filled magnetic tubes do not change their energy in an applied magnetic field, and only partially occupied magnetic tubes in the neighborhood of the Fermi surface exhibit a diamagnetic response. Using this partitioning of the occupied electron states we derive a general expression for the steady diamagnetic susceptibility, for calculation of which one needs to know the shape of the Fermi surface and the energy gradient on it. The method is applied to alkali metals, whose Fermi surfaces and energy gradients have been obtained from ab initio band structure calculations. It has been found that the Landau diamagnetic susceptibility is anisotropic depending on the direction of the applied magnetic field with respect to the Fermi surface. This effect is more pronounced for Li and Cs, whose Fermi surfaces show a noticeable deformation from the spherical shape. The method opens a route for ab initio calculations of the Landau diamagnetism of metals or intermetallic compounds. In the case of free electron gas it is shown that this approach also fully describes the oscillatory de Haas–van Alphen part of the diamagnetic susceptibility. Small oscillations of the Fermi energy found in the model are caused by redistribution (inflow or outflow) of electrons from the equatorial region of the Fermi surface.