We performed optical studies on CaFeAsF single crystals, a parent compound of the 1111-type iron-based superconductors that undergoes a structural phase transition from tetragonal to orthorhombic at $T_s=121$ K and a magnetic one to a spin density wave (SDW) state at $T_N=110$ K. In the low-temperature optical conductivity spectrum, after the subtraction of a narrow Drude peak, we observe a pronounced singularity around $300{\mathrm{cm}}^{-1}$ that separates two regions of quasilinear conductivity. We outline that these characteristic absorption features are signatures of Dirac fermions, similar to what was previously reported for the ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ system [Z.-G. Chen et al., Phys. Rev. Lett. 119, 096401 (2017)]. In support of this interpretation, we show that for the latter system this singular feature disappears rapidly upon electron and hole doping, as expected if it arises from a van Hove singularity in between two Dirac cones. Finally, we show that one of the infrared-active phonon modes (the Fe-As mode at $250{\mathrm{cm}}^{-1})$ develops a strongly asymmetric line shape in the SDW state and note that this behavior can be explained in terms of a strong coupling with the Dirac fermions.

A new $A$-site-ordered perovskite $\mathrm{BiC}{\mathrm{u}}_3\mathrm{C}{\mathrm{r}}_4{\mathrm{O}}_{12}$ is synthesized under a high pressure of 7.7 GPa. A phase transition from a paramagnetic metal to a ferrimagnetic metal is observed at $T_{\mathrm{c}}=190\mathrm{K}$ accompanied with a structural change from cubic to monoclinic. Structural analysis of the low-temperature monoclinic phase reveals that this transition represents a charge disproportionation of $\mathrm{C}{\mathrm{r}}^{3.75+}$ into $\mathrm{C}{\mathrm{r}}^{4+}$ and $\mathrm{C}{\mathrm{r}}^{3.5+}$. We argue that the asymmetric displacement of Bi caused by a lone-pair effect triggers the formation of a dimeric ${{\mathrm{Cr}}^{4+}}_2{\mathrm{O}}_5$ unit and leads to an ordering of $\mathrm{C}{\mathrm{r}}^{4+}$ and $\mathrm{C}{\mathrm{r}}^{3.5+}$ below the transition.

We consider a family of quantum loop models in 2+1 spacetime dimensions with marginally long-ranged and statistical interactions mediated by a $U\left(1\right)$ gauge field, both purely in 2+1 dimensions and on a surface in a (3+1)-dimensional bulk system. In the absence of fractional spin, these theories have been shown to be self-dual under particle-vortex duality and shifts of the statistical angle of the loops by $2\pi$, which form a subgroup of the modular group, $\mathrm{PSL}(2,\mathbb{Z})$. We show that careful consideration of fractional spin in these theories completely breaks their statistical periodicity and describe how this occurs, resolving a disagreement with the conformal field theories they appear to approach at criticality. We show explicitly that incorporation of fractional spin leads to loop model dualities which parallel the recent web of (2+1)-dimensional field theory dualities, providing a nontrivial check on its validity.

We report an approach for examining electron properties using information about the shape and size of a nanostructure as a measurement reference. This approach quantifies the spin precession angles per unit length directly by considering the time-reversal interferences on chaotic return trajectories within mesoscopic ring arrays (MRAs). Experimentally, we fabricated MRAs using nanolithography in InGaAs quantum wells which had a gate-controllable spin-orbit interaction (SOI). As a result, we observed an Onsager symmetry related to relativistic magnetic fields, which provided us with indispensable information for the semiclassical billiard ball simulation. Our simulations, developed based on the real-space formalism of the weak localization/antilocalization effect including the degree of freedom for electronic spin, reproduced the experimental magnetoconductivity (MC) curves with high fidelity. The values of five distinct electron parameters (Fermi wavelength, spin precession angles per unit length for two different SOIs, impurity scattering length, and phase coherence length) were thereby extracted from a single MC curve. The methodology developed here is applicable to wide ranges of nanomaterials and devices, providing a diagnostic tool for exotic properties of two-dimensional electron systems.

We have studied the influence of one-dimensional periodic nanostructured substrates on graphene band structure. One-monolayer-thick graphene is extremely sensitive to periodic terrace arrays, as demonstrated on two different nanostructured substrates, namely Ir(332) and multivicinal curved Pt(111). Photoemission shows the presence of minigaps related to the spatial periodicity. The potential barrier strength of the one-dimensional periodic nanostructuration can be tailored with the step-edge type and the nature of the substrate. The minigap opening further demonstrates the presence of backward scattered electronic waves on the surface and the absence of Klein tunneling on the substrate, probably due to the fast variation of the potential, of a spatial extent of the order of the lattice parameter of graphene.

We study Andreev processes and nonlocal transport in a three-terminal graphene-superconductor hybrid system under a quantizing perpendicular magnetic field [G.-H. Lee et al., Nat. Phys. 13, 693 (2017)]. We find that the amplitude of the crossed Andreev reflection (CAR) processes crucially depends on the orientation of the lattice. By employing Landauer-Büttiker scattering theory, we find that CAR is generally very small for a zigzag edge, while for an armchair edge it can be larger than the normal transmission, thereby resulting in a negative nonlocal resistance. In the case of an armchair edge and with a wide superconducting region (as compared to the superconducting coherence length), CAR exhibits large oscillations as a function of the magnetic field due to interference effects. This results in sign changes of the nonlocal resistance.

We present the application of the $\mathrm{SU}(N)$ spin-wave theory to spin-orbital Mott insulators whose ground states exhibit magnetic orders. When taking both spin and orbital degrees of freedom into account rather than projecting Hilbert space onto the Kramers doublet, which is the lowest spin-orbital locked energy levels, the $\mathrm{SU}(N)$ spin-wave theory should take the place of the $\mathrm{SU}(2)$ one due to the inevitable spin-orbital multipole exchange interactions. To implement the application, we introduce an efficient general local mean-field method, which involves all local fluctuations, and develop the $\mathrm{SU}(N)$ linear spin-wave theory. Our approach is tested firstly by calculating the multipolar spin-wave spectra of the $\mathrm{SU}(4)$ antiferromagnetic model. Then, we apply it to spin-orbital Mott insulators. It is revealed that the Hund's coupling would influence the effectiveness of the isospin-$1/2$ picture when the spin-orbital coupling is not large enough. We further carry out the $\mathrm{SU}(N)$ spin-wave calculations of two materials, $\alpha \text{−}{\mathrm{RuCl}}_3$ and ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$, and find that the magnonic and spin-orbital excitations are consistent with experiments.

Nodal loop semimetals are close descendants of Weyl semimetals and possess a topologically dressed band structure. We argue by combining the conventional theory of magnetic oscillation with topological arguments that nodal loop semimetals host coexisting topological and trivial magnetic oscillations. These originate from mapping the topological properties of the extremal Fermi surface cross sections onto the physics of two dimensional semi-Dirac systems, stemming from merging two massless Dirac cones. By tuning the chemical potential and the direction of magnetic field, a sharp transition is identified from purely trivial oscillations, arising from the Landau levels of a normal two dimensional (2D) electron gas, to a phase where oscillations of topological and trivial origin coexist, originating from 2D massless Dirac and semi-Dirac points, respectively. These could in principle be directly identified in current experiments.

Extreme ultraviolet (XUV) transient reflectivity around the germanium $M_{4,5}$ edge ($3d$ core-level to valence transition) at 30 eV is advanced to obtain the transient dielectric function of crystalline germanium [100] on femtosecond to picosecond time scales following photoexcitation by broadband visible-to-infrared (VIS/NIR) pulses. By fitting the transient dielectric function, carrier-phonon induced relaxations are extracted for the excited carrier distribution. The measurements reveal a hot electron relaxation rate of $3.2\pm 0.2\mathrm{ps}$ attributed to the $X\text{−}L$ intervalley scattering and a hot hole relaxation rate of $600\pm 300\mathrm{fs}$ ascribed to intravalley scattering within the heavy hole (HH) band, both in good agreement with previous work. An overall energy shift of the XUV dielectric function is assigned to a thermally induced band gap shrinkage by formation of acoustic phonons, which is observed to be on a timescale of 4–5 ps, in agreement with previously measured optical phonon lifetimes. The results reveal that the transient reflectivity signal at an angle of ${66}^{\circ }$ with respect to the surface normal is dominated by changes to the real part of the dielectric function, due to the near critical angle of incidence of the experiment $\left({66}^{\circ }\text{–}{70}^{\circ }\right)$ for the range of XUV energies used. This work provides a methodology for interpreting XUV transient reflectivity near core-level transitions, and it demonstrates the power of the XUV spectral region for measuring ultrafast excitation dynamics in solids.

We investigate the excitonic spectrum of ${\mathrm{MoS}}_2$ monolayers and calculate its optical absorption properties over a wide range of energies. Our approach takes into account the anomalous screening in two dimensions and the presence of a substrate, both cast by a suitable effective Keldysh potential. We solve the Bethe-Salpeter equation using as a basis a Slater-Koster tight-binding model parameterized to fit the ab initio ${\mathrm{MoS}}_2$ band structure calculations. The resulting optical conductivity is in good quantitative agreement with existing measurements up to ultraviolet energies. We establish that the electronic contributions to the $C$ excitons arise not from states at the $\mathrm{\Gamma }$ point, but from a set of $\mathit{k}$ points over extended portions of the Brillouin zone. Our results reinforce the advantages of approaches based on effective models to expeditiously explore the properties and tunability of excitons in TMD systems.

Friedel oscillations (FO) of electron density caused by a deltalike neutral impurity in two-dimensional (2D) systems in a magnetic field are calculated. Three 2D cases are considered: free electron gas, monolayer graphene, and group-VI dichalcogenides. An exact form of the renormalized Green's function is used in the calculations, as obtained by a summation of the infinite Dyson series and regularization procedure. Final results are valid for large ranges of potential strengths $V_0$, electron densities $n_e$, magnetic fields $B$, and distances from the impurity $r$. Realistic models for the impurities are used. The first FO of induced density in ${\mathrm{WS}}_2$ are described by the relation $\mathrm{\Delta }n\left(\mathit{r}\right)\propto sin(2\pi r/T_{\mathrm{FO}})/r^2$, where $T_{\mathrm{FO}}\propto 1/\sqrt{E_F}$. For weak impurity potentials, the amplitudes of FO are proportional to $V_0$. For attractive potentials and high fields, the total electron density remains positive for all $r$. On the other hand, for low fields, repulsive potentials and small $r$, the total electron density may become negative, so that many-body effects should be taken into account.

The peculiar electron dispersion in Dirac materials makes lowest-order Auger processes prohibited or marginally prohibited by energy and momentum conservation laws. Thus, Auger recombination (AR) in these materials is very sensitive to many-body effects. We incorporate them at the level of the $GW$ approximation into the nonequilibrium Green's functions approach to AR and study the role of dynamic screening, spectrum broadening, and renormalization in the case of weakly pumped undoped graphene. We find that incorrect treatment of many-body effects can lead to an order-of-magnitude error in the recombination rate. We show that the AR time depends weakly (sublinearly) on the background dielectric constant, which limits the possibility to control recombination by the choice of substrate. However, the AR time can be considerably prolonged by placing graphene under a metal gate or by introducing a band gap. With carrier cooling taken into account, our results comply with experiments on photoexcited graphene.

We reexamine the relaxation process of a single spin embedded in an elastic medium, a problem studied recently by Garanin and Chudnovsky (GC) [Phys. Rev. B 92, 024421 (2015)] from the viewpoint of angular-momentum transfer. Using Noether's theorem, we identify two distinct angular momenta of the medium, one Newtonian discussed by GC and the other field-theoretical, both of which consist of an orbital part and a spin part. For both angular momenta, we found that the orbital part is as essential as the spin part in the relaxation process. In particular, the angular-momentum transfer from the (real) spin to the Newtonian orbital part may be considered as an incipient rotation that leads to the Einstein–de Haas effect.

Symmetry and localization properties of defect modes of a one-dimensional bicomponent magnonic superlattice are theoretically studied. The magnonic superlattice can be seen as a periodic array of nanostripes, where stripes with different widths, termed defect stripes, are periodically introduced. By controlling the geometry of the defect stripes, a transition from dispersive to practically flat spin-wave defect modes can be observed inside the magnonic band gaps. It is shown that the spin-wave profile of the defect modes can be either symmetric or antisymmetric, depending on the geometry of the defect. Due to the localized character of the defect modes, a particular magnonic superlattice is proposed wherein the excitation of both symmetric and antisymmetric flat magnonic modes is enabled at the same time. Also, it is demonstrated that the relative frequency position of the antisymmetric mode inside the band gap does not significantly change with the application of an external field, while the symmetric modes move to the edges of the frequency band gaps. The results are complemented by numerical simulations, where excellent agreement is observed between the methods. The proposed theory allows exploring different ways to control the dynamic properties of the defect modes in metamaterial magnonic superlattices, which can be useful for applications on multifunctional microwave devices operating over a broad frequency range.

The new intermetallic uranium beryllium germanide UBeGe and its thorium analogon ThBeGe crystallize with the hexagonal ZrBeSi type of structure. Studies of magnetic, thermal, and transport properties were performed on polycrystalline samples between 1.8 and 750K. UBeGe is a uniaxial ferromagnet and there are indications for two magnetic transitions at $T_{\mathrm{C}}{}^{\left(1\right)}\approx 160\mathrm{K}$ and $T_{\mathrm{C}}^{\left(2\right)}\approx 150\mathrm{K}$. The high paramagnetic effective moment ${\mu }_{\mathrm{eff}}\approx 3.1{\mu }_{\mathrm{B}}$, x-ray absorption near-edge spectroscopy (XANES, 17–300 K), as well as theoretical DFT calculations indicate localized U $5f^2$ states in UBeGe. ThBeGe is a diamagnetic metallic material with low density of states at the Fermi level.

Three-dimensional spin current density injected onto the surface of a topological insulator (TI) produces a two-dimensional charge current density on the surface of the TI, which is the so-called inverse Edelstein effect (IEE). The ratio of the surface charge current density on the TI to the spin current density injected across the interface defined as the IEE length was shown to be exactly equal to the mean free path in the TI determined to be independent of the electron transmission rate across the interface [Phys. Rev. B 94, 184423 (2016)]. However, we find that the transmission rate across the interface gives a nonzero contribution to the transport relaxation rate in the TI as well as to the effective IEE relaxation rate (over and above any surface hybridization effects), and the IEE length is always less than the original mean free path in the TI without the interface. We show that both the IEE relaxation time and the transport relaxation time in the TI are modified by the interface transmission time. The correction becomes significant when the transmission time across the interface becomes comparable to or less than the original momentum scattering time in the TI. This correction is similar to experimental results in Rashba electron systems in which the IEE relaxation time was found shorter in the case of direct interface with metal in which the interface transmission rate will be much higher, compared to interfaces incorporating insulating oxides. Our results indicate the continued importance of the interface to obtain a better spin-to-charge current conversion and a limitation to the conversion efficiency due to the quality of the interface.

In this paper, we present a simple model of a three-dimensional insulating magnetic structure which represents a magnonic analog of the layered electronic system described by A. A. Burkov and L. Balents [Phys. Rev. Lett. 107, 127205 (2011)]. In particular, our model realizes Weyl magnons as well as surface states with a Dirac spectrum. In this model, the Dzyaloshinskii-Moriya interaction is responsible for the separation of opposite Weyl points in momentum space. We calculate the intrinsic (due to the Berry curvature) transport properties of Weyl and so-called anomalous Hall effect magnons. The results are compared with fermionic analogs.

We have performed ${}^{75}\mathrm{As}$ NMR measurements on single crystals to investigate the nematic behavior via the in-plane anisotropy of the electronic state at the As site far from Co impurities in the representative iron arsenides $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$. From the analysis of the angular dependence of the NMR satellites in the $c$ plane using the binominal distribution, we find that there is the in-plane fourfold symmetry breaking, namely, the orthorhombic-type anisotropy in the electric field gradient (EFG) at the As site with no Co atom at the nearest neighboring Fe sites even in the tetragonal phase of both ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ and $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2(x\ne 0)$. The NMR spectrum in the antiferromagnetically ordered state of ${\mathrm{BaFe}}_2{\mathrm{As}}_2$ is shown not to support a nanotwin model on the basis of the nematic order proposed from the pair-distribution analysis of neutron scattering data. Based on results of the $x$ and temperature $T$ dependences of the in-plane anisotropy in the wide $x$ and $T$ ranges, the symmetry breaking is concluded to come from the local orthorhombic domains induced by disorder such as Co impurities or lattice imperfections. Furthermore, we find that the asymmetry parameter of EFG $\eta$ obeys the Curie-Weiss law which may be governed by nematic susceptibility, and the Weiss temperature becomes zero at $x_{\mathrm{c}}\sim 0.05$ in $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2{\mathrm{As}}_2$.

We revise the theory of the indirect exchange interaction between magnetic impurities beyond the linear response theory to establish the effect of impurity resonances in the surface states of a three-dimensional topological insulator. The interaction is composed of isotropic Heisenberg, anisotropic Ising, and Dzyaloshinskii-Moriya types of couplings. We find that all three contributions are finite at the Dirac point, which is in stark contrast to the linear response theory which predicts a vanishing Dzyaloshinskii-Moriya-type contribution. We show that the spin-independent component of the impurity scattering can generate large values of the Dzyaloshinskii-Moriya-type coupling in comparison with the Heisenberg and Ising types of couplings, while these latter contributions drastically reduce in magnitude and undergo sign changes. As a result, both collinear and noncollinear configurations are allowed magnetic configurations of the impurities.