Nuclear Magnetic Relaxation Rate in Iron-Pnictide Superconductors

Nuclear magnetic relaxation rate 1/T_1 in iron-pnictide superconductors is calculated using the gap function obtained in a microscopic calculation. Based on the obtained results, we discuss the issues such as the rapid decrease of 1/T_1 just below the transition temperature and the difference between nodeless and nodal s-wave gap functions. We also investigate the effect of Coulomb interaction on 1/T_1 in the random phase approximation and show its importance in interpreting the experimental results.Comment: Proceedings of 9th International Conference on Materials and Mechanisms of Superconductivity. To be published in Physica

Single Impurity Problem in Iron-Pnictide Superconductors

Single impurity problem in iron-pnictide superconductors is investigated by solving Bogoliubov-de Gennes (BdG) equation in the five-orbital model, which enables us to distinguish s$_{+-}$ and s$_{++}$ superconducting states. We construct a five-orbital model suitable to BdG analysis. This model reproduces the results of random phase approximation in the uniform case. Using this model, we study the local density of states around a non-magnetic impurity and discuss the bound-state peak structure, which can be used for distinguishing s$_{+-}$ and s$_{++}$ states. A bound state with nearly zero-energy is found for the impurity potential $I\sim 1.0$ eV, while the bound state peaks stick to the gap edge in the unitary limit. Novel multiple peak structure originated from the multi-orbital nature of the iron pnictides is also found.Comment: 5 page

Fermi-Suface Evolution by Transition-metal Substitution in the Iron-based Superconductor LaFeAsO

We study how Co- and Ni-substitution affect the electronic structure of the iron-based superconductor, LaFeAsO. We perform {\it ab initio} supercell calculations and unfold the first Brillouin zone (BZ) to calculate the spectral function in the BZ for the normal cell. The charge density distribution in real space shows that doped extra electrons are trapped around Co (Ni) atom. This seems to mean that Co(Ni)-substitution does not work as carrier doping. However, the present momentum-space analysis indicates that the Fermi-surface volume indeed expands by substitutions, which can be well described by the rigid-band shift. By taking into account this effective doping, we discuss whether the sign-reversing s-wave ($s_{\pm}$-wave) scenario is compatible with experiments.Comment: 4 pages, 3 figure

Effect of transition-metal substitution in iron-based superconductors

We study theoretically the current debatable issue about the effect of transition-metal (TM) substitution in iron-based superconductors through treating all of the TM ions as randomly distributed impurities. The extra electrons from TM elements are localized at the impurity sites. In the mean time the chemical potential shifts upon substitution. The phase diagram is mapped out and it seems that the TM elements can act as effective dopants. The local density of states (LDOS) is calculated and the bottom becomes V-shaped as the impurity concentration increases. The LDOS at the Fermi energy $\rho(\omega=0)$ is finite and reaches the minimum at the optimal doping level. Our results are in good agreement with the scanning tunneling microscopy experiments.Comment: 5 pages, 4 figure

Low-Energy Effective Hamiltonian and the Surface States of Ca_3PbO

The band structure of Ca_3PbO, which possesses a three-dimensional massive Dirac electron at the Fermi energy, is investigated in detail. Analysis of the orbital weight distributions on the bands obtained in the first-principles calculation reveals that the bands crossing the Fermi energy originate from the three Pb-p orbitals and three Ca-dx2y2 orbitals. Taking these Pb-p and Ca-dx2y2 orbitals as basis wave functions, a tight-binding model is constructed. With the appropriate choice of the hopping integrals and the strength of the spin-orbit coupling, the constructed model sucessfully captures important features of the band structure around the Fermi energy obtained in the first-principles calculation. By applying the suitable basis transformation and expanding the matrix elements in the series of the momentum measured from a Dirac point, the low-energy effective Hamiltonian of this model is explicitely derived and proved to be a Dirac Hamiltonain. The origin of the mass term is also discussed. It is shown that the spin-orbit coupling and the orbitals other than Pb-p and Ca-dx2y2 orbitals play important roles in making the mass term finite. Finally, the surface band structures of Ca_3PbO for several types of surfaces are investigated using the constructed tight-binding model. We find that there appear nontrivial surface states that cannot be explained as the bulk bands projected on the surface Brillouin zone. The relation to the topological insulator is also discussed.Comment: 11 page

Spin-Polarization in Magneto-Optical Conductivity of Dirac Electrons

A mechanism is proposed based on the Kubo formula to generate a spin-polarized magneto-optical current of Dirac electrons in solids which have strong spin-orbit interactions such as bismuth. The ac current response functions are calculated in the isotropic Wolff model under an external magnetic field, and the selection rules for Dirac electrons are obtained. By using the circularly polarized light and tuning its frequency, one can excite electrons concentrated in the spin-polarized lowest Landau level when the chemical potential locates in the band gap, so that spin-polarization in the magneto-optical current can be achieved.Comment: 4 pages, 3 figure

Orbital-Selective Superconductivity and the Effect of Lattice Distortion in Iron-Based Superconductors

The superconducting (SC) state of iron-based compounds in both tetragonal and orthorhombic phases is studied on the basis of an effective Hamiltonian composed of the kinetic energy including the five Fe 3d-orbitals, the orthorhombic crystalline electric field (CEF) energy, and the two-orbital Kugel'-Khomski\u{i}-type superexchange interaction. Our basic assumption is that the antiferromagnetic (AF) state in the parent compounds can be described by the $d_{xz}$ and $d_{yz}$ orbitals, and that the electrons in these orbitals have relatively strong electron correlation in the vicinity of the AF state. In order to study the physical origin of the structure-sensitive SC transition temperature, the effect of orthorhombic distortion is taken into account as the energy-splitting, $\Delta_{\textrm{ortho.}}$, between the $d_{xz}$ and $d_{yz}$ orbitals. We find that the eigenvalue of the linearized gap equation decreases accompanied with the reduction of the partial density of states for the $d_{xz}$ and $d_{yz}$ orbitals as $\Delta_{\textrm{ortho.}}$ increases, and that the dominant pairing symmetry is an unconventional fully gapped $s_{+-}$-wave pairing. We also find large anisotropy of the SC gap function in the orthorhombic phase. We propose that the CEF energy plays an important role in controlling $T_{\textrm{c}}$ and the SC gap function, and that orbital-selective superconductivity is a key feature in iron-based superconductors, which causes the structure-sensitive $T_{\textrm{c}}$.Comment: 11 pages, To appear in J. Phys. Soc. Jp

Topological sound in active-liquid metamaterials

Liquids composed of self-propelled particles have been experimentally realized using molecular, colloidal, or macroscopic constituents. These active liquids can flow spontaneously even in the absence of an external drive. Unlike spontaneous active flow, the propagation of density waves in confined active liquids is not well explored. Here, we exploit a mapping between density waves on top of a chiral flow and electrons in a synthetic gauge field to lay out design principles for artificial structures termed topological active metamaterials. We design metamaterials that break time-reversal symmetry using lattices composed of annular channels filled with a spontaneously flowing active liquid. Such active metamaterials support topologically protected sound modes that propagate unidirectionally, without backscattering, along either sample edges or domain walls and despite overdamped particle dynamics. Our work illustrates how parity-symmetry breaking in metamaterial structure combined with microscopic irreversibility of active matter leads to novel functionalities that cannot be achieved using only passive materials

Spin-Density-Wave Gap with Dirac Nodes and Two-Magnon Raman Scattering in BaFe2As2

Raman selection rules for electronic and magnetic excitations in BaFe2As2 were theoretically investigated and applied them to the separate detection of the nodal and anti-nodal gap excitations at the spin density wave (SDW) transition and the separate detection of the nearest and the next nearest neighbor exchange interaction energies. The SDW gap has Dirac nodes, because many orbitals participate in the electronic states near the Fermi energy. Using a two-orbital band model the electronic excitations near the Dirac node and the anti-node are found to have different symmetries. Applying the symmetry difference to Raman scattering the nodal and anti-nodal electronic excitations are separately obtained. The low-energy spectra from the anti-nodal region have critical fluctuation just above TSDW and change into the gap structure by the first order transition at TSDW, while those from the nodal region gradually change into the SDW state. The selection rule for two-magnon scattering from the stripe spin structure was obtained. Applying it to the two-magnon Raman spectra it is found that the magnetic exchange interaction energies are not presented by the short-range superexchange model, but the second derivative of the total energy of the stripe spin structure with respect to the moment directions. The selection rule and the peak energy are expressed by the two-magnon scattering process in an insulator, but the large spectral weight above twice the maximum spin wave energy is difficult to explain by the decayed spin wave. It may be explained by the electronic scattering of itinerant carriers with the magnetic self-energy in the localized spin picture or the particle-hole excitation model in the itinerant spin picture. The magnetic scattering spectra are compared to the insulating and metallic cuprate superconductors whose spins are believed to be localized.Comment: 38 pages, 11 figure