24 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
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
Giant orbital diamagnetism of three-dimensional Dirac electrons in Sr3PbO antiperovskite
This work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI (Grants No. 24224010, No. 15K13523, No. JP15H05852, No. JP15K21717,, No. 17H01140, No. 18H01162, and No. 17J05243), JSPS Core-to-Core Program (A) Advanced Research Networks, and the Alexander von Humboldt Foundation. S.S. acknowledges financial support by JSPS and the Materials Education program for the future leaders in Research, Industry, and Technology (MERIT).In Dirac semimetals, interband mixing has been known theoretically to give rise to a giant orbital diamagnetism when the Fermi level is close to the Dirac point. In Bi1−xSbx and other Dirac semimetals, an enhanced diamagnetism in the magnetic susceptibility χ has been observed and interpreted as a manifestation of such giant orbital diamagnetism. Experimentally proving their orbital origin, however, has remained challenging. The cubic antiperovskite Sr3PbO is a three-dimensional Dirac electron system and shows the giant diamagnetism in χ as in the other Dirac semimetals. 207Pb NMR measurements are conducted in this study to explore the microscopic origin of diamagnetism. From the analysis of the Knight shift K as a function of χ and the relaxation rate T1–1 for samples with different hole densities, the spin and the orbital components in K are successfully separated. The results establish that the enhanced diamagnetism in Sr3PbO originates from the orbital contribution of Dirac electrons, which is fully consistent with the theory of giant orbital diamagnetism.Publisher PDFPeer reviewe