The dimensionality of the electronic and magnetic structure of a given material is generally predetermined by its crystal structure. Here, using elastic and inelastic neutron scattering combined with magnetization measurements, we find unusual magnetic behavior in three-dimensional (3D) ${\mathrm{Ba}}_2{\mathrm{CoO}}_4$. In spite of isolated ${\mathrm{CoO}}_4$ tetrahedra, the system exhibits a 3D noncollinear antiferromagnetic order in the ground state with an anomalously large Curie-Weiss temperature of 110 K compared to $T_{\mathrm{N}}=26$ K. More unexpectedly, spin dynamics displays quasi-two-dimensional spin-wave dispersion with an unusually large spin gap, and one-dimensional magnetoelastic coupling. Our results indicate that ${\mathrm{Ba}}_2{\mathrm{CoO}}_4$ is a unique system for exploring the interplay between isolated polyhedra, low-dimensional magnetism, and spin states in oxides.

We report an experimental observation of superconductivity in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ thin films without application of external pressure. The films under study were synthesized by magnetron sputtering. Surface studies suggest that the observed transport characteristics are related to the polycrystalline continuous part of the investigated films with a homogeneous distribution of elements and the Cd-to-As ratio close to stoichiometric ${\mathrm{Cd}}_3{\mathrm{As}}_2$. The latter is also supported by Raman spectra of the studied films where two pronounced peaks inherent to ${\mathrm{Cd}}_3{\mathrm{As}}_2$ were observed. The obtained x-ray diffraction patterns for studied films also correspond to the ${\mathrm{Cd}}_3{\mathrm{As}}_2$ lattice. The formation of a superconducting phase in the films under study is confirmed by the characteristic behavior of the temperature and the magnetic field dependence of the sample resistivity, as well as by the presence of pronounced zero-resistance plateaus in the $dV/dI$ characteristics. The corresponding $H_c\text{−}{T}_c$ plots reveal a clearly pronounced linear behavior within the intermediate temperature range, similar to that observed for bulk ${\mathrm{Cd}}_3{\mathrm{As}}_2$ and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ films under pressure, suggesting the possibility of a nontrivial pairing in the films under investigation. We discuss a possible role of the sample inhomogeneities and crystal strains in the observed phenomena.

Using numerical simulations, we examine the nonlinear dynamics of skyrmions driven over random pinning. For weak pinning, the skyrmions depin elastically, retaining sixfold ordering; however, at the onset of motion there is a dip in the magnitude of the structure factor peaks due to a decrease in positional ordering, indicating that the depinning transition can be detected using the structure factor even within the elastic depinning regime. At higher drives, the moving skyrmion lattice regains full ordering. For increasing pinning strength, we find a transition from elastic to plastic depinning that is accompanied by a sharp increase in the depinning threshold due to the proliferation of topological defects, similar to the peak effect found at the elastic to plastic depinning transition in superconducting vortex systems. For strong pinning and strong Magnus force, the skyrmions in the moving phase can form a strongly clustered or phase-separated state with highly modulated skyrmion density, similar to that recently observed in continuum-based simulations for strong disorder. As the Magnus force is decreased, the density phase-separated state crosses over to a dynamically phase-separated state with uniform density but with flow localized in bands of motion, while in the strongly damped limit, both types of phase-separated states are lost. In the strong pinning limit, we find highly nonlinear velocity-force curves in the transverse and longitudinal directions, along with distinct regions of negative differential conductivity in the plastic flow regime. The negative differential conductivity is absent in the overdamped limit. The Magnus force is responsible for both the negative differential conductivity and the clustering effects since it causes faster moving skyrmions to partially rotate around slower moving or pinned skyrmions.

Magnetism of the $S=1$ Heisenberg antiferromagnets on the spatially anisotropic square lattice has been scarcely explored. Here, we report a study of the magnetism, specific heat, and thermal conductivity on $\mathrm{Ni}{\left[\mathrm{SC}{\left({\mathrm{NH}}_2\right)}_2\right]}_6{\mathrm{Br}}_2$ (DHN) single crystals. ${\mathrm{Ni}}^{2+}$ ions feature an $S=1$ rectangular lattice in the $bc$ plane, which can be viewed as an unfrustrated spatially anisotropic square lattice. A long-range antiferromagnetic order is developed at $T_{\mathrm{N}}=2.23$ K. Below $T_{\mathrm{N}}$, an upturn is observed in the $b$-axis magnetic susceptibility and the resultant minimum might be an indication for the $XY$ anisotropy in the ordered state. A gapped spin-wave dispersion is confirmed from the temperature dependence of the magnetic specific heat. Anisotropic temperature-field phase diagrams are mapped out and possible magnetic structures are proposed.

The realization of topological quantum phases of matter remains a key challenge to condensed matter physics and quantum information science. In this work, we demonstrate that progress in this direction can be made by combining concepts of tensor network theory with Majorana device technology. Considering the topological double-semion string-net phase as an example, we exploit the fact that the representation of topological phases by tensor networks can be significantly simpler than their description by lattice Hamiltonians. The building blocks defining the tensor network are tailored to realization via simple units of capacitively coupled Majorana bound states. In the case under consideration, this yields a remarkably simple blueprint of a synthetic double-semion string net, and one may be optimistic that the required device technology will be available soon. Our results indicate that the implementation of tensor network structures via mesoscopic quantum devices opens up a powerful avenue toward the realization and quantum simulation of synthetic topological quantum matter.

It is currently understood that high temperature superconductivity (SC) in the transition metal $\left(M\right)$ substituted iron arsenides $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{M}_x\right)}_2{\mathrm{As}}_2$ is promoted by magnetic excitations with wave vectors $(\pi ,0)$ or $(0,\pi )$. It is known that while a small amount of Co substitution lead to SC, the same does not occur for Mn for any value of $x$. In this work, magnetic excitations in the iron arsenides $\mathrm{Ba}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Mn}}_x\right)}_2{\mathrm{As}}_2\left(x=0.0,0.007,0.009,0.08\right)$ are investigated by means of resonant inelastic x-ray scattering (RIXS) at the Fe $L_3$ edge, for momentum transfer $\mathit{q}$ along the high symmetry Brillouin zone $(\pi ,0)$ and $(\pi ,\pi )$ directions. It is shown that with increasing Mn content $\left(x\right)$, the excitations become anisotropic both in dispersion and lineshape. Both effects are detected even for small values of $x$, evidencing a cooperative phenomenon between the Mn impurities, that we ascribe to emerging Néel order of the Mn spins. Moreover, for $x=0.08$, the excitations along $\mathit{q}\parallel (\pi ,0)$ are strongly damped and nearly nondispersive. This result suggests that phases of arsenides containing local moments at the FeAs layers, as in Mn or Cr substituted phases, do not support high temperature SC due to the absence of the appropriate magnetic excitations.

Using ab initio wave-function-based calculations, we provide valuable insights with regard to the magnetic exchange in $5d$ and $4d$ oxides with face-sharing ligand octahedra, ${\mathrm{BaIrO}}_3$ and ${\mathrm{BaRhO}}_3$. Surprisingly strong antiferromagnetic Heisenberg interactions as large as 400 meV are computed for idealized iridate structures with ${90}^{\circ }$ Ir-O-Ir bond angles and in the range of 125 meV for angles of ${80}^{\circ }$ as measured experimentally in ${\mathrm{BaIrO}}_3$. These estimates exceed the values derived so far for corner-sharing and edge-sharing systems and motivate more detailed experimental investigations of quantum magnets with extended $5d/4d$ orbitals and networks of face-sharing ligand cages. The strong electron-lattice couplings evidenced by our calculations suggest rich phase diagrams as a function of strain and pressure, a research direction with much potential for materials of this type.

Results from illumination-sensitive infrared absorption and photocurrent spectroscopy measurements on the hydrogen defect in ZnO characterized by a local vibrational mode with a frequency of 3326 ${\mathrm{cm}}^{-1}$ are reported. It is shown that photoexcitation with $\hslash \omega \ge 2.7$ eV results in two new axial oriented $\mathrm{O}\text{−}\mathrm{H}$ stretch vibrational modes at 3170 and 3589 ${\mathrm{cm}}^{-1}$, which appear at the expense of the previously reported azimuthal oriented 3326- and 3358-${\mathrm{cm}}^{-1}$ modes. We propose a tentative configurational-coordinate energy diagram of the defect and discuss its microscopic origin.

The optical properties of the new iron-based superconductor ${\mathrm{CsCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ with $T_c\simeq 29$ K have been determined. In the normal state a good description of the low-frequency response is obtained with a superposition of two Drude components of which one has a very low scattering rate (narrow Drude peak) and the other a rather large one (broad Drude peak). Well below $T_c\simeq 29$ K, a pronounced gap feature is observed which involves a complete suppression of the optical conductivity below $\sim 110{\mathrm{cm}}^{-1}$ and thus is characteristic of a nodeless superconducting state. The optical response of the broad Drude component can be described with a dirty-limit Mattis-Bardeen-type response with a single isotropic gap of $2\mathrm{\Delta }\simeq 14$ meV. To the contrary, the response of the narrow Drude component is in the ultraclean limit and its entire spectral weight is transferred to the zero-frequency $\delta \left(\omega \right)$ function that accounts for the loss-free response of the condensate. These observations provide clear evidence for a band-selective coexistence of clean- and dirty-limit superconductivity with nodeless gaps in ${\mathrm{CsCa}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$.

In order to study the origin of metallization of ${\mathrm{VO}}_2$ induced by electron injection, we deposit K atoms onto the surface of ${\mathrm{VO}}_2$ films grown on ${\mathrm{TiO}}_2$ (001) substrates, and we investigate the change in the electronic and crystal structures using in situ photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The deposition of K atoms onto a surface of insulating monoclinic ${\mathrm{VO}}_2$ leads to a phase transition from insulator to metal. In this metallization state, the V-V dimerization characteristic to the monoclinic phase of ${\mathrm{VO}}_2$ still exists, as revealed by the polarization dependence of the XAS spectra. Furthermore, the monoclinic metal undergoes a transition to a monoclinic insulator with decrease in temperature, and to a rutile metal with increase in temperature. These results indicate the existence of a metallic monoclinic phase around the boundary between the insulating monoclinic and metallic rutile phases in the case of electron-doped ${\mathrm{VO}}_2$.

We analytically investigate the effect of a noncentrosymmetric geometry in the optical second harmonic (SH) generation from a nanowire made of a centrosymmetric material, in the interior of which quadratic optical processes are suppressed. We consider an infinite cylinder with a cross section that is slightly deformed away from a circle and with a radius much smaller than the wavelength. We calculate the induced linear and nonlinear fields perturbatively in terms of the deformation parameter, and we obtain the nonlinear dipolar and quadrupolar hyperpolarizabilities, whose spectra we evaluate for metallic and dielectric materials. We show that for very small deformations, the dipolar contribution to the response competes with the quadrupolar term, and may even be dominant. We explore the spectra of the hyperpolarizability and identify the contributions to its structure for metallic and dielectric nanowires. We also discuss the nature of the SH radiation at various frequencies, and we find that it may be dominated by the dipolar or the quadrupolar term, or that both may compete yielding nonsymmetric radiation patterns. Our calculation may be employed to assess, calibrate, and test numerical SH calculations.

Exploration of optical nonlinear response of graphene predominantly relies on ultrashort time domain measurements. Here we propose an alternate technique that uses frequency shifted continuous wavefront optical fields, thereby probing graphene's steady state nonlinear response. We predict frequency sideband generation in the reflected field that originates from coherent electron dynamics of the photoexcited carriers. The corresponding threshold in input intensity for optimal sideband generation provides a direct measure of the third-order optical nonlinearity in graphene. Our formulation yields analytic forms for the generated sideband intensity, is applicable to generic two-band systems and suggests a range of applications that include switching of frequency sidebands using nonlinear phase shifts and generation of frequency combs.

Coupling localized surface plasmons (LSPs) in Au nanostructure arrays to Fabry-Pérot cavity modes, both peak splitting and peak locking behaviors can be observed when light is incident normally into the cavity from different directions. These phenomena can be quantitatively described by a model based on modified Fresnel equations, and thus be interpreted as an extended Fano resonance effect. Both the peak splitting and peak locking behaviors arise from the interference between localized surface plasmon resonance and cavity modes with different initial phase difference. When the phase difference of the cavity state and the LSP state is $\pi$ or 0, the superposition between a cavity resonance mode and a localized surface plasmon mode can result in the peak splitting or peak locking. Experimental results demonstrate that the separation energy between the split peaks changes with the volumes and densities of the Au nanoparticles and agree with the numerical calculation results quite well.

In this paper, we use a many-body approach to study the absorption spectra of electron-doped two-dimensional semiconductors. Optical absorption is modeled by a many-body scattering Hamiltonian which describes an exciton immersed in a Fermi sea. The interaction between electron and exciton is approximated by an effective scattering potential, and optical spectra are calculated by solving for the exciton Green's function. From this approach, the trion can be assigned as a bound state of an electron-exciton scattering process, and the doping-dependent phenomena observed in the spectra can be attributed to several many-body effects induced by the interaction with the Fermi sea. While the many-body scattering Hamiltonian cannot be solved exactly, we reduce the problem to two limiting solvable situations. The first approach approximates the full many-body problem by a simple scattering process between the electron and the exciton, with a self-energy obtained by solving a Bethe-Salpeter equation (BSE). An alternate approach assumes an infinite mass for the exciton, such that the many-body scattering Hamiltonian reduces to a Mahan-Noziéres-De Dominicis (MND) model. The exciton Green's function can then be solved numerically exactly by a determinantal formulation, with an optical spectra that show signatures of the Fermi-edge singularity at high doping densities. The full doping dependence and temperature dependence of the exciton and trion line shapes are simulated via these two approximate approaches, with the results compared to each other and to experimental expectations.

We study the confined bound states in circular bilayer graphene quantum dots. Using the semiclassical Einstein-Brillouin-Keller quantization rule, we predict that valley levels inside the quantum dot undergo three stages: splitting, crossing, and recombining when varying the applied magnetic field. This exotic phenomenon originates from the Berry phase, which increases from zero to $2\pi$ with the magnetic field and has opposite signs for the two inequivalent valleys. To further confirm this phenomenon, we perform a fully quantum mechanical calculation based on a low-energy continuum model and a real space tight-binding model, and show that the valley effect can be explicitly observed by scanning-tunneling spectroscopy measurements. Moreover, we propose an efficient valley filter device as an application of this valley effect, which can be used to manipulate the valley degree of freedom in valleytronics.

Recent experiments and theoretical studies have proposed that spin-transport in organic semiconductors, in particular in ${\mathrm{Alq}}_3$, may occur in an impurity band. Here we model the electronic and magnetic properties of such an impurity band by treating the effect of disorder in a numerically accurate way. The calculations are carried out by solving the Anderson-Hubbard model within the mean-field approximation and by accounting for magnetic excitations via the Bethe-Salpeter equation. We find that some impurities form clusters where electrons are delocalized, while others develop localized magnetic moments, which are antiferromagnetically correlated. The excitations of these correlated magnetic moments are spin waves, which can enable spin transport.

The spin Hall effect is considered a phenomenon in which a charge current is converted to a spin current due to the spin-orbit coupling (SOC). Recently, large values of the spin Hall angle (SHA), as the conversion efficiency between spin and charge currents, were observed in metal oxide structures in ferromagnetic/metal (FM/M) oxide bilayers, although their underlying mechanisms are ambiguous. The present study aims to analytically indicate that a mixed region of metal and oxide formed at the metal/oxide interface includes the surface oxide charge, which allows for introducing an SOC term in the electron Hamiltonian. Based on the results, the SHA increases by two orders of magnitude in a Cu/oxide thin film through the side jump and skew scattering mechanisms, reaching that of heavy metals. In addition, a comprehensive model is provided for the influence of oxidation on the SHA and spin-orbit torques (SOTs) in FM/M structures. We find that the magnitude of SOTs is greatly dependent on (i) the surface oxidation condition, (ii) the current flow path, and (iii) the electronic interface condition. The model represented in the present study is regarded as a promising model and prediction mechanism which explains recent observations. The findings can be implemented in generating spin current without any use of external magnetic fields and heavy metals in spintronic devices.