Honeycomb-based materials have been attracting a lot of attention owing to their fascinating physics as exemplified by the emergence of a massless Dirac fermion and a spin liquid state. We have investigated magnetoelectric (ME) phenomena in a honeycomb magnet ${\mathrm{Co}}_4{\mathrm{Nb}}_2{\mathrm{O}}_9$ and observed a unique ME response associated with a simple antiferromagnetic ordering on the honeycomb network. The induced electric polarization changes its direction by an angle $-2\theta$ around the trigonal axis upon rotating the magnetic field by an angle $\theta$. We attribute the variation of electric polarization direction in a rotating magnetic field to the continuous rotation of the magnetic moment on the honeycomb. Our findings may open a variety of novel ME responses based on a honeycomb magnet.

We analyze the possible types of ordering in a boson-fermion model. The Hamiltonian is inherently related to the Bose-Hubbard model for vector two-species bosons in optical lattices. We show that such a model can be reduced to the Kugel-Khomskii type spin-pseudospin model, but in contrast to the usual version of the latter model, we are dealing here with the case of spin $S=1$ and pseudospin $1/2$. We show that the interplay of spin and pseudospin degrees of freedom leads to a rather nontrivial magnetic phase diagram including the spin-nematic configurations. Tuning the spin-channel interaction parameter $U_s$ gives rise to quantum phase transitions. We find that the ground state of the system always has the pseudospin domain structure. On the other hand, the sign change of $U_s$ switches the spin arrangement of the ground state within domains from a ferro- to antiferromagnetic one. Finally, we revisit the spin (pseudospin)-1/2 Kugel-Khomskii model and see the inverse picture of phase transitions.

We report high-pressure ${}^{77}\mathrm{Se}$ NMR studies on FeSe single crystals that reveal a prominent inhomogeneous NMR linewidth broadening upon cooling, with the magnetic field applied along the tetragonal [110] direction. The data indicate the existence of short-range-ordered, inhomogeneous electronic nematicity, which has surprisingly long time scales over milliseconds. The short-range order survives temperatures up to eight times the structural transition temperature, and remains robust against pressure, in contrast to the strong pressure-dependence of the orbital ordering, structural transition, and the ground state magnetism. Such an extended region of static nematicity in the ($P,T$) space of FeSe indicates an enormously large fluctuating regime, and provides fresh insights and constraints to the understanding of electronic nematicity in iron-based superconductors.

We study the spin dynamics of the resonant excitonic state (RES) proposed, within the theory of an emergent SU(2) symmetry, to explain some properties of the pseudogap phase of cuprate superconductors. The RES can be described as a proliferation of particle-hole patches with an internal modulated structure. We model the RES modes as a charge order with multiple $2{\mathbf{p}}_{\text{F}}$ ordering vectors, where $2{\mathbf{p}}_{\text{F}}$ connects two opposite sides of the Fermi surface. This simple modelization enables us to propose a comprehensive study of the collective mode observed at the antiferromagnetic wave vector $\mathbf{Q}=(\pi ,\pi )$ by inelastic neutron scattering in both the superconducting state and also in the pseudogap regime. In this regime, we show that the dynamic spin susceptibility exhibits a loss of coherence terms except at special wave vectors commensurate with the lattice. We argue that this phenomenon could explain the change of the spin response shape around $\mathbf{Q}$. We demonstrate that the hole dependence of the RES spin dynamics is in agreement with the experimental data in ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$.

The magnetic field driven superconductor-to-insulator transition in thin films is theoretically understood in terms of the notion of vortex-charge duality symmetry. The manifestation of such symmetry is the exchange of roles of current and voltage between the superconductor and the insulator. While experimental evidence obtained from amorphous indium oxide films supported such duality symmetry, it is shown to be broken, counterintuitively, at low temperatures where the insulating phase exhibits discontinuous current-voltage characteristics. Here, we demonstrate that it is possible to effectively restore duality symmetry by driving the system beyond the discontinuity into its high current, far from equilibrium, state.

We have systematically studied the antiferromagnetic and nematic transitions in ${\mathrm{Sr}}_{1-x}{\mathrm{Ba}}_x{\mathrm{Fe}}_{1.97}{\mathrm{Ni}}_{0.03}{\mathrm{As}}_2$ by magnetic susceptibility and uniaxial-pressure resistivity measurements, respectively. The derivatives of the temperature dependence of both magnetic and nematic susceptibilities show clearly sharp peaks when the transitions are first order. Accordingly, we show that while both of the magnetic and nematic transitions change from first-order to second-order with increasing Barium doping level, there is a narrow doping range where the former becomes second order but the latter remains first order, which has never been realized before in other systems. Moreover, the antiferromagnetic and nematic transition temperatures become different and the jump of nematic susceptibility becomes small in this intermediate doping range. Our results provide key information on the interplay between magnetic and nematic transitions. Concerning the current debate on the microscopic models for nematicity in iron-based superconductors, these observations agree with the magnetic scenario for an itinerant fermionic model.

The electronic structure of ${\mathrm{BaFe}}_2{X}_3$ ($X$ = S and Se) and ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$ in which two-leg ladders are formed by the Fe sites are studied by means of x-ray absorption and resonant inelastic x-ray scattering spectroscopy. The x-ray absorption spectra at the Fe $L$ edges for ${\mathrm{BaFe}}_2{X}_3$ exhibit two components, indicating that itinerant and localized Fe $3d$ sites coexist. Substantial x-ray linear dichroism is observed in polarization dependent spectra, indicating the existence of orbital order or fluctuation in the Fe ladder even above the Néel temperature $T_{\mathrm{N}}$. Direct exchange interaction along the legs of the Fe ladder stabilizes the orbital and antiferromagnetic orders in ${\mathrm{BaFe}}_2{\mathrm{S}}_3$, while the ferromagnetic molecular orbitals are realized between the rungs in ${\mathrm{CsFe}}_2{\mathrm{Se}}_3$.

We analyze the magnetically ordered phases in the Kondo lattice model on a square lattice around quarter filling using the dynamical mean field theory. We find that close to quarter filling besides the paramagnetic phase, at least three magnetic phases compete. These phases are a ferromagnetic state, a partial Kondo screened (PKS) state, which is a combination of charge order and magnetic order, and a striped magnetic phase, which is ferromagnetically ordered in one direction and antiferromagnetically in the other. We analyze the spectral properties of these states and show that the PKS state is insulating with a flat band above the Fermi energy, while the other states are metallic. Furthermore, we demonstrate that the energy gain by the Kondo effect is larger in the PKS state, while the striped magnetic state can gain more energy from the Ruderman-Kittel-Kasuya-Yosida interaction. Thus, while the striped magnetic state is stable at weak coupling, the PKS state becomes stable at intermediate coupling.

The basic theoretical foundation for the modeling of phonon-assisted absorption spectra in direct band-gap semiconductors, introduced by Elliott 60 years ago [R. J. Elliott, Phys. Rev. 108, 1384 (1957)] using second order perturbation theory, results in a square root shaped dependency close to the absorption edge. A careful analysis of the experiments [N. Naka et al., Jpn. J. Appl. Phys. 44, 5096 (2005)] reveals that for the yellow S excitons in ${\mathrm{Cu}}_2\mathrm{O}$ the lineshape does not follow that square root dependence. The reexamination of the theory shows that the basic assumptions of constant matrix elements and constant energy denominators is invalid for semiconductors with dominant exciton effects like ${\mathrm{Cu}}_2\mathrm{O}$, where the phonon-assisted absorption proceeds via intermediate exciton states. The overlap between these and the final exciton states strongly determines the dependence of the absorption on the photon energy. To describe the experimental observed line shape of the indirect absorption of the yellow S exciton states we find it necessary to assume a momentum dependent deformation potential for the optical phonons.

Atomic-scale helices exist as motifs for several material lattices. We examine a tight-binding model for a single one-dimensional monatomic chain with a $p$-orbital basis coiled into a helix. A topologically nontrivial phase emerging from this model supports a chiral symmetry-protected zero-energy mode localized to a boundary, always embedded within a continuum band, regardless of termination site. We identify a topological invariant for this phase that is related to the number of zero energy end modes by means of the bulk-boundary correspondence, and give strict conditions for the existence of the bound state. An additional class of gapped edge modes in the model spectrum has practical consequences for surface states in, e.g., trigonal tellurium and selenium and other van der Waals-bonded one-dimensional semiconductors.

Topological phases of matter in (2+1) dimensions are commonly equipped with global symmetries, such as electric-magnetic duality in gauge theories and bilayer symmetry in fractional quantum Hall states. Gauging these symmetries into local dynamical ones is one way of obtaining exotic phases from conventional systems. We study this using the bulk-boundary correspondence and applying the orbifold construction to the (1+1)-dimensional edge described by a conformal field theory (CFT). Our procedure puts twisted boundary conditions into the partition function and predicts the fusion, spin, and braiding behavior of anyonic excitations after gauging. We demonstrate this for the electric-magnetic self-dual ${\mathbb{Z}}_N$ gauge theory, the twofold symmetric $\text{SU}{\left(3\right)}_1$, and the $S_3$-symmetric $\text{SO}{\left(8\right)}_1$ Wess-Zumino-Witten theories.

The Imbert-Fedorov (IF) shift in optics describes the transverse shift of light beams at the reflection interface. Recently, the IF shift of Weyl fermions at the interface between Weyl semimetals (WSMs) with single monopole charge has been studied. Here, we study the IF shift at the interface between two WSMs, each of which is carrying an arbitrary integer monopole charge. We find a general relation between the monopole charges of the two WSMs and the IF shift. In particular, the IF shift is proportional to the monopole charge if both WSMs have the same one. Our results can be used to infer the topology of the materials by experimentally measuring their IF shift. Furthermore, we consider the possibility that the Weyl fermions are scattered to other Weyl cones during the reflection, which results in qualitatively different behavior of the IF shift. While we use a quantum mechanical approach to solve the problem, semiclassical equations of motion and the conservation of total angular momentum can help us intuitively interpret our results in special cases.

We report on observation of pronounced terahertz radiation-induced magnetoresistivity oscillations in AlGaAs/GaAs two-dimensional electron systems, the terahertz analog of the microwave induced resistivity oscillations (MIRO). Applying high-power radiation of a pulsed molecular laser we demonstrate that MIRO, so far observed at low power only, are not destroyed even at very high intensities. Experiments with radiation intensity ranging over five orders of magnitude from 0.1 to ${10}^4{\mathrm{W}/\mathrm{cm}}^2$ reveal high-power saturation of the MIRO amplitude, which is well described by an empirical fit function $I/{(1+I/I_s)}^{\beta }$ with $\beta \sim 1$. The saturation intensity $I_s$ is of the order of tens of watts per square centimeter and increases by a factor of 6 by increasing the radiation frequency from 0.6 to 1.1 THz. The results are discussed in terms of microscopic mechanisms of MIRO and compared to nonlinear effects observed earlier at significantly lower excitation frequencies.

We report a high-field-induced magnetic phase in a single crystal of $\mathrm{U}{\left({\mathrm{Ru}}_{0.92}{\mathrm{Rh}}_{0.08}\right)}_2{\mathrm{Si}}_2$. Our neutron study, combined with high-field magnetization, shows that the magnetic phase above the first metamagnetic transition at ${\mu }_0{H}_{c1}=21.6$ T has an uncompensated commensurate antiferromagnetic structure with a propagation vector ${\mathit{Q}}_2=\left(\frac{2}{3}00\right)$ possessing two single-$\mathit{Q}$ domains. U moments of $1.45\left(9\right){\mu }_B$ directed along the $c$ axis are arranged in an up-up-down sequence propagating along the $a$ axis, in agreement with bulk measurements. The U magnetic form factor at high fields is consistent with both the ${\mathrm{U}}^{3+}$ and ${\mathrm{U}}^{4+}$ types. The low-field short-range order that emerges from pure ${\mathrm{URu}}_2{\mathrm{Si}}_2$ due to Rh doping is initially strengthened by the field but disappears in the field-induced phase. The tetragonal symmetry is preserved across the transition, but the $a$-axis lattice parameter increases already at low fields. Our results are in agreement with an itinerant electron model with $5f$ states forming bands pinned in the vicinity of the Fermi surface that is significantly reconstructed by the applied magnetic field.

A simple spin-$\frac{1}{2}$ frustrated antiferromagnetic Heisenberg model (AFHM) on the square lattice—including chiral plaquette cyclic terms—was argued [A. E. B. Nielsen, G. Sierra, and J. I. Cirac, Nat. Commun. 4, 2864 (2013)] to host a bosonic Kalmeyer-Laughlin (KL) fractional quantum Hall ground state [V. Kalmeyer and R. B. Laughlin, Phys. Rev. Lett. 59, 2095 (1987)]. Here, we construct generic families of chiral projected entangled pair states (chiral PEPS) with low bond dimension ($D=3,4,5$) which, upon optimization, provide better variational energies than the KL Ansatz. The optimal $D=3$ PEPS exhibits chiral edge modes described by the Wess-Zumino-Witten $\text{SU}{\left(2\right)}_1$ model, as expected for the KL spin liquid. However, we find evidence that, in contrast to the KL state, the PEPS spin liquids have power-law dimer-dimer correlations and exhibit a gossamer long-range tail in the spin-spin correlations. We conjecture that these features are genuine to local chiral AFHM on bipartite lattices.

Physics arising from two-dimensional (2D) Dirac cones has been a topic of great theoretical and experimental interest to studies of gapless topological phases and to simulations of relativistic systems. Such 2D Dirac cones are often characterized by a $\pi$ Berry phase and are destroyed by a perturbative mass term. By considering mean-field nonlinearity in a minimal two-band Chern insulator model, we obtain a different type of Dirac cone that is robust to local perturbations without symmetry restrictions. Due to a different pseudospin texture, the Berry phase of the Dirac cone is no longer quantized in $\pi$, and can be continuously tuned as an order parameter. Furthermore, in an Aharonov-Bohm (AB) interference setup to detect such Dirac cones, the adiabatic AB phase is found to be $\pi$ both theoretically and computationally, offering an observable topological invariant and a fascinating example where the Berry phase and AB phase are fundamentally different. We hence discover a nonlinearity-induced quantum phase transition from a known topological insulating phase to an unusual gapless topological phase.

Disjointed Fermi arcs in Weyl semimetals can intertwine with chiral bulk modes and participate in unusual closed magnetic orbits in the presence of a vertical magnetic field. Here, we carry out a quantum oscillation study of such unusual Weyl magnetic orbits in the Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$, a close cousin of Weyl semimetals. We find that extra two-dimensional (2D) quantum oscillations emerge at high fields, which superimpose on a three-dimensional bulk background and can be attributed to the Weyl magnetic orbits, when the thickness of the nanoplates is smaller than the mean free path of the electrons. Further evidence of 2D quantum oscillations from the Weyl magnetic orbits is provided by nonlocal detection, which demonstrates an alternative way to study the quantum transport properties of Fermi arcs under magnetic fields.

We study the transport properties at the surface state of a topological insulator (TI) with a potential barrier. Due to the hexagonal warping effect, the number of reflected propagating states for an incident electron beam can change from one to two, corresponding to a change from normal reflection to double reflection, by controlling the Fermi energy. Associated with the change, the corresponding reflection probability shows a significant jump in its derivative. In addition, for a junction with potential barrier, the reflection probability should oscillate as the potential energy varies due to the tunneling resonance. The oscillation period in the TI junction is closely related to the warping strength. Thus, these two proposals both can be used to identify the hexagonal warping strength of the TI surface state and especially, are robust against the influence of lateral surfaces, as they depend on the relative variation of transport. Remarkably, the latter proposal is compatible with the recent experiment in graphene [Chen et al., Science 353, 1522 (2016)], which achieves a direct measurement of the angle-resolved scattering probability.