Weyl semimetals are characterized by unconventional electromagnetic response. We present analytical expressions for all components of the frequency-and wave-vector-dependent charge-spin linear-response tensor of Weyl fermions. The spin-momentum locking of the Weyl Hamiltonian leads to a coupling between charge and longitudinal spin fluctuations, while transverse spin fluctuations remain decoupled from the charge. A real Weyl semimetal with multiple Weyl nodes can show this charge-spin coupling in equilibrium if its crystal symmetry is sufficiently low. All Weyl semimetals are expected to show this coupling if they are driven into a non-equilibrium stationary state with different occupations of Weyl nodes, for example by exploiting the chiral anomaly. Based on the response tensor, we investigate the low-energy collective excitations of interacting Weyl fermions. For a local Hubbard interaction, the charge-spin coupling leads to a dramatic change of the zero-sound dispersion: its velocity becomes independent of the interaction strength and the chemical potential and is given solely by the Fermi velocity. In the presence of long-range Coulomb interactions, the coupling transforms the plasmon modes into spin plasmons. For real Weyl semimetals with multiple Weyl nodes, the collective modes are strongly affected by the presence of parallel static electric and magnetic fields, due to the chiral anomaly. In particular, the zero-sound frequency at fixed momentum and the spin content of the spin plasmons go through cusp singularities as the chemical potential of one of the Weyl cones is tuned through the Weyl node. We discuss possible experiments that could provide smoking-gun evidence for Weyl physics. arXiv:1810.12560v2 [cond-mat.mes-hall]

A three-orbital itinerant-electron model involving d xz , d yz and d xy Fe 3d orbitals is proposed for iron pnictides towards understanding the ( , 0 π ) ordered magnetism and magnetic excitations in these materials. It is shown that this model at half filling yields a gapped ( , 0 π ) magnetic state, and simultaneously reproduces several experimentally observed features such as the electronic structure, spin excitations, as well as the ferro orbital order between the d xz and d yz orbitals.interaction strength U c for ( , 0 π ) ordering. However, spin wave dispersion in this model does not agree in detail with INS measurements [27,28]. Moreover, it does not include d xy Fe orbital which contributes some portions of the electron pockets. Similarly, three-orbital models with one-third filling (i.e. two electrons in three orbitals) [29], two-third filling (i.e. four electrons in three orbitals) [30,31], and four-orbital model at half-filling [32] can reproduce the desired FS structure, but spin wave excitations in these models have not been investigated yet. More realistic five-orbital models [33][34][35], which yield the FS topology similar to experimental findings, were proposed aimed at investigation of pairing instabilities, but spin excitations were not studied. Although anisotropic spin wave excitation was obtained in a recent study [36] for a five-orbital model [37], investigation of spin excitations over the entire BZ was not carried out.In this context, we present and investigate a three-orbital itinerant-electron model in this paper. We find that this minimal model simultaneously yields the correct FS structure as well as spin wave dispersion consistent with INS experiments. Moreover, ferro orbital order of appropriate sign is also obtained in this model.The organization of this paper is as follows. The importance of ferromagnetic (F) spin couplings on the experimentally measured spin wave dispersion in the ( , 0 π ) state is briefly discussed in section 2. Then in section 3, it is shown that no F spin coupling is generated in the two-band model with FS nesting [24] ,although this model yields correct FS topology. A third d xy Fe orbital is therefore necessary to overcome this shortcoming, and a three-orbital model having d xz , d yz and d xy Fe 3d orbitals is presented in section 4 highlighting the FS and DOS. The investigation of magnetic excitations and orbital ordering in the ( , 0 π ) magnetic state is carried out in section 5. Finally conclusions are discussed in section 6. Spin wave energy at the ferromagnetic zone boundary and ferromagnetic spin couplingIn order to highlight the effect of induced F spin couplings on the spin wave dispersion and spin wave energy at the FZB, we consider the case of single band Hubbard model with nearest-neighbor (NN) hopping t and nextnearest-neighbor (NNN) hopping t′. In this case, the spin wave dispersion in the strong coupling limit [38] is given by ( ) J J J b q J J q q H U n n U J n n J J a a a a S S

Weyl semimetals exhibit exotic Fermi-arc surface states, which strongly affect their electromagnetic properties. We derive analytical expressions for all components of the composite density-spin response tensor for the surfaces states of a Weyl-semimetal model obtained by closing the band gap in a topological insulating state and introducing a time-reversal-symmetry-breaking term. Based on the results, we discuss the electromagnetic susceptibilities, the current response, and other physical effects arising from the density-spin response. We find a magnetoelectric effect caused solely by the Fermi arcs. We also discuss the effect of electron-electron interactions within the random phase approximation and investigate the dispersion of surface plasmons formed by Fermi-arc states. Our work is useful for understanding the electromagnetic and optical properties of the Fermi arcs. arXiv:2001.00438v2 [cond-mat.mes-hall] 4 Apr 2020

Spin wave dispersion in the frustrated fcc type-III antiferromagnet MnS 2 has been determined by inelastic neutron scattering using a triple-axis spectrometer. Existence of multiple spin wave branches, with significant separation between high-energy and low-energy modes highlighting the intrinsic magnetic frustration effect on the fcc lattice, is explained in terms of a spin wave analysis carried out for the antiferromagnetic Heisenberg model for this S = 5/2 system with nearest and next-nearest-neighbor exchange interactions. Comparison of the calculated dispersion with spin wave measurement also reveals small suppression of magnetic frustration resulting from reduced exchange interaction between frustrated spins, possibly arising from anisotropic deformation of the cubic structure.