33 research outputs found
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 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 (-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
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 and 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, , between the and
orbitals. We find that the eigenvalue of the linearized gap equation decreases
accompanied with the reduction of the partial density of states for the
and orbitals as increases, and
that the dominant pairing symmetry is an unconventional fully gapped
-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 and the SC gap function, and that
orbital-selective superconductivity is a key feature in iron-based
superconductors, which causes the structure-sensitive .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