137 research outputs found
Superconductivity from a long-range interaction: a crossover between the electron gas and the lattice model
We explore how the superconductivity arising from the on-site
electron-electron repulsion will change when the repulsion is changed to a
long-ranged, 1/r-like one by introducing an extended Hubbard model with the
repulsion extending to distant (12th) neighbors. With a simplified
fluctuation-exchange approximation, we have found for the square lattice that
(i) as the band filling becomes dilute enough, the charge susceptibility
becomes comparable with the spin susceptibility, where p and then s pairings
become dominant, in agreement with the result for the electron gas by Takada,
while (ii) the d-wave, which reflects the lattice structure, dominates well
away from the half filling. All these can be understood in terms of the spin
and charge structures along with the shape and size of the Fermi surface.Comment: 5 pages, 6 figure
Comment on "Isoelectronic Ru substitution at Fe-site in Sm(FeRu)AsOF compound and its effects on structural, superconducting and normal state properties" (arXiv:1004.1978)
Based on the five-orbital model, we derive the reduced impurity scattering
rate in Sm(FeRu)AsOF
from the residual resistivity. At , the transition temperature is
K. For ) the obtained value
of ranges from 1.5 to 2.9, which suggests that the -wave state
cannot survive. We point out that the magnetoresistance frequently gives an
underestimated value of in correlated electron systems.Comment: 2 page
A unified origin for the 3D magnetism and superconductivity in NaCoO
We analyze the origin of the three dimensional (3D) magnetism observed in
nonhydrated Na-rich NaCoO within an itinerant spin picture using a 3D
Hubbard model. The origin is identified as the 3D nesting between the inner and
outer portions of the Fermi surface, which arise due to the local minimum
structure of the band at the -A line. The calculated spin wave
dispersion strikingly resembles the neutron scattering result. We argue that
this 3D magnetism and the spin fluctuations responsible for superconductivity
in the hydrated systems share essentially the same origin.Comment: 5pages, 6figure
Orbital Order, Structural Transition and Superconductivity in Iron Pnictides
We investigate the 16-band d-p model for iron pnictide superconductors in the
presence of the electron-phonon coupling g with the orthorhombic mode which is
crucial for reproducing the recently observed ultrasonic softening. Within the
RPA, we obtain the ferro-orbital order below TQ which induces the
tetragonal-orthorhombic structural transition at Ts = TQ, together with the
stripe-type antiferromagnetic order below TN. Near the phase transitions, the
system shows the s++ wave superconductivity due to the orbital fluctuation for
a large g case with TQ > TN, while the s+- wave due to the magnetic fluctuation
for a small g case with TQ < TN. The former case is consistent with the phase
diagram of doped iron pnictides with Ts > TN.Comment: 5 pages, 5 figures, minor changes, published in J. Phys. Soc. Jp
Correlation-driven electronic nematicity in the Dirac semimetal BaNiS2
In BaNiS2 a Dirac nodal-line band structure exists within a two-dimensional
Ni square lattice system, in which significant electronic correlation effects
are anticipated. Using scanning tunneling microscopy, we discover signs of
correlated-electron behavior, namely electronic nematicity appearing as a pair
of C2-symmetry striped patterns in the local density-of-states at ~60 meV above
the Fermi energy. In observations of quasiparticle interference, as well as
identifying scattering between Dirac cones, we find that the striped patterns
in real space stem from a lifting of degeneracy among electron pockets at the
Brillouin zone boundary. We infer a momentum-dependent energy shift with d-form
factor, which we model numerically within a density wave equation framework
that considers spin-fluctuation-driven nematicity. This suggests an unusual
mechanism driving the nematic instability, stemming from only a small
perturbation to the Fermi surface, in a system with very low density of states
at the Fermi energy. The Dirac points lie at nodes of the d-form factor, and
are almost unaffected by it. These results highlight BaNiS2 as a unique
material in which Dirac electrons and symmetry-breaking electronic correlations
coexist.Comment: 11 pages, 5 figures (plus 6 pages, 4 figures
Simple Real-Space Picture of Nodeless and Nodal s-wave Gap Functions in Iron Pnictide Superconductors
We propose a simple way to parameterize the gap function in iron pnictides.
The key idea is to use orbital representation, not band representation, and to
assume real-space short-range pairing. Our parameterization reproduces fairly
well the structure of gap function obtained in microscopic calculation. At the
same time the present parameterization is simple enough to obtain an intuitive
picture and to develop a phenomenological theory. We also discuss
simplification of the treatment of the superconducting state.Comment: 4 page
Superconductivity in the Three-Fold Charge-Ordered Metal of the Triangular-Lattice Extended Hubbard Model
The quarter-filling extended Hubbard model on the triangular lattice is
studied to explore pairing instability in the three-fold charge-ordered (CO)
metal. We derive a second-order strong-coupling effective Hamiltonian of doped
carriers into the three-fold CO insulator at electron density of , and
then study the - and -wave superconductivities down to by
using the BCS mean-field approximation. It is found that the triplet -wave
pairing is more stable than the -wave one. We also point out that this
coexisting state of the charge ordering and superconductivity is possible to
have critical temperature .Comment: 4 pages, 7 figure
Coherence effect in a two-band superconductor: Application to iron pnictides
From a theoretical point of view, we propose an experimental method to
determine the pairing symmetry of iron pnictides. We focus on two kinds of
pairing symmetries, and , which are strong candidates for the
pairing symmetry of iron pnictides. For each of these two symmetries, we
calculate both the density and spin response functions by using the two-band
BCS model within the one-loop approximation. As a result, a clear difference is
found between the - and -wave states in the temperature
dependence of the response functions at nesting vector , which connects
the hole and electron Fermi surfaces. We point out that this difference comes
from the coherence effect in the two-band superconductor. We suggest that the
pairing symmetry could be clarified by observing the temperature dependence of
both the density and spin structure factors at the nesting vector in
neutron scattering measurements.Comment: 15 pages, 7 figures, 1 tabl
Study of Ni-doping Effect of Specific Heat and Transport Properties for LaFe1-yNiyAsO0.89F0.11
Specific heats and transport quantities of the LaFe1-yNiyAsO0.89F0.11 system
have been measured, and the results are discussed together with those reported
previously by our group mainly for LaFe1-yCoyAsO0.89F0.11 and
LaFeAsO0.89-xF0.11+x systems. The y dependence of the electronic specific heat
coefficient gamma can basically be understood by using the rigid-band picture,
where Ni ions provide 2 electrons to the host conduction bands and behave as
nonmagnetic impurities. The superconducting transition temperature Tc of
LaFe1-yNiyAsO0.89F0.11 becomes zero, as the carrier density p (=2y+0.11) doped
to LaFeAsO reaches its critical value p_c_ ~0.2. This p_c_ value of ~0.2 is
commonly observed for LaFe1-yCoyAsO0.89F0.11 and LaFeAsO0.89-xF0.11+x systems,
in which the relations p = x+0.11 and p = y+0.11 hold, respectively. As we
pointed out previously, the critical value corresponds to the disappearance of
the hole-Fermi surface. These results indicate that the carrier number solely
determines the Tc value. We have not observed appreciable effects of pair
breaking, which originates from the nonmagnetic impurity scattering of
conduction electrons and strongly suppresses T_c_ values of systems with
sign-reversing of the order parameter over the Fermi surface(s). On the basis
of the results, the so-called s_+-_ symmetry of the order parameter with the
sign-reversing is excluded.Comment: 4 pages, 7 figures, submitted to J. Phys. Soc. Jpn, (modified
version
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