163 research outputs found
Superconducting states in frustrating t-J model: A model connecting high- cuprates, organic conductors and NaCoO
The two-dimensional t-J model on a frustrating lattice is studied using
mean-field variational theories with Gutzwiller approximation. We find that a
superconducting state with broken time-reversal symmetry (d+id state) is
realized in the parameter region close to the triangular lattice. The
frustration enlarges the region of superconductivity when for the
hole-doped case, which is equivalent to for electron doping. We also
discuss the SU(2) degeneracy at half-filling. The d+id state probably
corresponds to the spin gap state at half-filling.Comment: 4 pages, 4 figure
Spontaneous deformation of the Fermi surface due to strong correlation in the two-dimensional t-J model
Fermi surface of the two-dimensional t-J model is studied using the
variational Monte Carlo method. We study the Gutzwiller projected d-wave
superconducting state with an additional variational parameter t'_v
corresponding to the next-nearest neighbor hopping term. It is found that the
finite t'_v<0 gives the lowest variational energy in the wide range of
hole-doping rates. The obtained momentum distribution function shows that the
Fermi surface deforms spontaneously. It is also shown that the van Hove
singularity is always located very close to the Fermi energy. Using the
Gutzwiller approximation, we show that this spontaneous deformation is due to
the Gutzwiller projection operator or the strong correlation.Comment: 4 pages, 3 eps figures, revte
Electronic states around a vortex core in high-Tc superconductors based on the t-J model
Electronic states around vortex cores in high-Tc superconductors are studied
using the two-dimensional t-J model in order to treat the d-wave
superconductivity with short coherence length and the antiferromagnetic (AF)
instability within the same framework. We focus on the disappearance of the
large zero-energy peak in the local density of states observed in high-Tc
superconductors. When the system is near the optimum doping, we find that the
local AF correlation develops inside the vortex cores. However, the detailed
doping dependence calculations confirm that the experimentally observed
reduction of the zero-energy peak is more reasonably attributed to the
smallness of the core size rather than to the AF correlation developed inside
the core. The correlation between the spatial dependence of the core states and
the core radius is discussed.Comment: 4 pages, 4 figure
Eomesodermin Is a Localized Maternal Determinant Required for Endoderm Induction in Zebrafish
SummaryIn zebrafish, endoderm induction occurs in marginal blastomeres and requires Casanova (Cas), the first endoderm-specific factor expressed in the embryo. Whereas the transcription factors Gata5 and Bon are necessary and sufficient for cas expression in marginal blastomeres, Bon and Gata5 are unable to induce cas in animal pole cells, suggesting that cas expression requires an additional, unidentified factor(s). Here, we show that cas expression depends upon the T box transcription factor Eomesodermin (Eomes), a maternal determinant that is localized to marginal blastomeres. Eomes synergizes potently with Bon and Gata5 to induce cas, even in animal pole blastomeres. We show that Eomes is required for endogenous endoderm induction, acting via an essential binding site in the cas promoter. Direct physical interactions between Eomes, Bon, and Gata5 suggest that Eomes promotes endoderm induction in marginal blastomeres by facilitating the assembly of a transcriptional activating complex on the cas promoter
Mott Transitions and d-wave Superconductivity in Half-Filled-Band Hubbard Model on Square Lattice with Geometric Frustration
Mechanisms of Mott transitions and dx2-y2-wave superconductivity (SC) are
studied in the half-filled-band Hubbard model on square lattices with a
diagonal hopping term (t'), using an optimization (or correlated) variational
Monte Carlo method. In the trial wave functions, a doublon-holon binding effect
is introduced in addition to the onsite Gutzwiller projection. We mainly treat
a d-wave singlet state and a projected Fermi sea. In both wave functions,
first-order Mott transitions without direct relevance to magnetic orders take
place at U=Uc approximately of the bandwidth for arbitrary t'/t. These
transitions originate in the binding or unbinding of a doublon to a holon.
d-wave SC appears in a narrow range immediately below Uc. The robust d-wave
superconducting correlation are necessarily accompanied by enhanced
antiferromagnetic correlation; the strength of SC becomes weak, as t'/t
increases.Comment: 18 pages, 30 figure
Disorder-driven quantum phase transition from antiferromagnetic metal to insulating state in multilayered high-Tc cuprate (Cu,C)Ba2Ca4Cu5Oy
We report on superconducting(SC) characteristics for oxygen-reduced Cu-based
five-layered high-temperature superconductor (Cu,C)Ba2Ca4Cu5Oy(Cu-1245(OPT)),
which includes five-fold outer planes (OP) and four-fold inner planes (IP).As a
result of the reduction of the carrier density, the bulk SC for Cu-1245 (OPT)
takes place at the nearly optimally-doped OP with Tc= 98 K that is different
from previously-reported Cu-1245(OVD) where IP plays a primary role for the
onset of SC. It gives an evidence that the carrier density of the
optimally-doped layer determines its bulk Tc.Static antiferromagnetic(AFM)
order is evidenced at IP's by zero-field Cu-NMR at low temperature,
irrespective of the SC transition at OP's below 98K. This AFM state at IP's is
characterized by a carrier localization at low temperatures due to disorder
effect, whereas the carrier densities in each layer are similar to Hg-1245(OPT)
where the AFM metallic state are realized in IP's. This finding reinforces the
phase diagram in which the AFM metallic phase exists between AFM insulator and
SC states for the case of ideally-flat CuO2 plane without disorder.Comment: 4 pages, 5 figure
Dynamical Induction of s-wave Component in d-wave Superconductor Driven by Thermal Fluctuations
We investigated the mutual induction effects between the d-wave and the
s-wave components of order parameters due to superconducting fluctuation above
the critical temperatures and calculated its contributions to paraconductivity
and excess Hall conductivity based on the two-component stochastic TDGL
equation. It is shown that the coupling of two components increases
paraconductivity while it decreases excess Hall conductivity compared to the
cases when each component fluctuates independently. We also found the singular
behavior in the paraconductivity and the excess Hall conductivity dependence on
the coupling parameter which is consistent with the natural restriction among
the coefficients of gradient terms.Comment: 10 pages, 4 figures included, submitted to J.Phys.Soc.Jp
Superconducting Gap Modulation in Weak Stripe States
The superconducting gap modulation is investigated in the presence of a weak
stripe structure, using the Bogoliubov-de Gennes theory on the two-dimensional
Hubbard model with nearest-neighbor site pairing interaction. We calculate the
local density of states and discuss the recently observed scanning tunneling
spectroscopy spectra with four lattice periodicity on Bi_2 Sr_2 Ca Cu_2
O_{8+delta} We also consider the spectral weight in the reciprocal space, where
the Fermi surface and the superconducting gap are modulated by the band folding
effect of the stripe structure.Comment: 4 pages, 4 figures, to be published in J. Phys. Soc. Jp
Quasiparticle States at a d-Wave Vortex Core in High-Tc Superconductors: Induction of Local Spin Density Wave Order
The local density of states (LDOS) at one of the vortex lattice cores in a
high Tc superconductor is studied by using a self-consistent mean field theory
including interactions for both antiferromagnetism (AF) and d-wave
superconductivity (DSC). The parameters are chosen in such a way that in an
optimally doped sample the AF order is completely suppressed while DSC
prevails. In the mixed state, we show that the local AF-like SDW order appears
near the vortex core and acts as an effective local magnetic field on the
quasiparticles. As a result, the LDOS at the core exhibits a double-peak
structure near the Fermi level that is in good agreement with the STM
observations on YBCO and BSCCO. The presence of local AF order near the votex
core is also consistent with the recent neutron scattering experiment on LSCO.Comment: 4 pages, 2 ps figure
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
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