1,065 research outputs found
Pseudogap of Color Superconductivity in Heated Quark Matter
We show that the pseudogap of the quark density of states is formed in hot
quark matter as a precursory phenomenon of the color superconductivity on the
basis of a low-energy effective theory. We clarify that the decaying process of
quarks near Fermi surface to a hole and the diquark soft mode (qq)_{soft} is
responsible for the formation of the pseudogap. Our result suggests that the
pseudogap is a universal phenomenon in strong coupling superconductors.Comment: Introduction is largely rewritten and minor changes are made in other
parts of the text. Some referenes with comments are added. Numerical errors
in the figures are corrected. To appear in Phys. Rev.
Analysis of Superconductivity in d-p Model on Basis of Perturbation Theory
We investigate the mass enhancement factor and the superconducting transition
temperature in the d-p model for the high-\Tc cuprates. We solve the
\'Eliashberg equation using the third-order perturbation theory with respect to
the on-site Coulomb repulsion . We find that when the energy difference
between d-level and p-level is large, the mass enhancement factor becomes large
and \Tc tends to be suppressed owing to the difference of the density of
state for d-electron at the Fermi level. From another view point, when the
energy difference is large, the d-hole number approaches to unity and the
electron correlation becomes strong and enhances the effective mass. This
behavior for the electron number is the same as that of the f-electron number
in the heavy fermion systems. The mass enhancement factor plays an essential
role in understanding the difference of \Tc between the LSCO and YBCO
systems.Comment: 4pages, 9figures, to be published in J. Phys. Soc. Jp
Reduction of Tc due to Impurities in Cuprate Superconductors
In order to explain how impurities affect the unconventional
superconductivity, we study non-magnetic impurity effect on the transition
temperature using on-site U Hubbard model within a fluctuation exchange (FLEX)
approximation. We find that in appearance, the reduction of Tc roughly
coincides with the well-known Abrikosov-Gor'kov formula. This coincidence
results from the cancellation between two effects; one is the reduction of
attractive force due to randomness, and another is the reduction of the damping
rate of quasi-particle arising from electron interaction. As another problem,
we also study impurity effect on underdoped cuprate as the system showing
pseudogap phenomena. To the aim, we adopt the pairing scenario for the
pseudogap and discuss how pseudogap phenomena affect the reduction of Tc by
impurities. We find that 'pseudogap breaking' by impurities plays the essential
role in underdoped cuprate and suppresses the Tc reduction due to the
superconducting (SC) fluctuation.Comment: 14 pages, 28 figures To be published in JPS
Controllable Rashba spin-orbit interaction in artificially engineered superlattices involving the heavy-fermion superconductor CeCoIn5
By using a molecular beam epitaxy technique, we fabricate a new type of
superconducting superlattices with controlled atomic layer thicknesses of
alternating blocks between heavy fermion superconductor CeCoIn_5, which
exhibits a strong Pauli pair-breaking effect, and nonmagnetic metal YbCoIn_5.
The introduction of the thickness modulation of YbCoIn_5 block layers breaks
the inversion symmetry centered at the superconducting block of CeCoIn_5. This
configuration leads to dramatic changes in the temperature and angular
dependence of the upper critical field, which can be understood by considering
the effect of the Rashba spin-orbit interaction arising from the inversion
symmetry breaking and the associated weakening of the Pauli pair-breaking
effect. Since the degree of thickness modulation is a design feature of this
type of superlattices, the Rashba interaction and the nature of pair-breaking
are largely tunable in these modulated superlattices with strong spin-orbit
coupling.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let
Perturbation Theory for a Repulsive Hubbard Model in Quasi-One-Dimensional Superconductors
We investigate pairing symmetry and a transition temperature in a
quasi-one-dimensional repulsive Hubbard model. We solve the Eliashberg equation
using the third-order perturbation expansion with respect to the on-site
repulsion . We find that when the electron number density is shifted from
the half-filled, a transition into unconventional superconductivity is
expected. When one dimensionality is weak, a spin-singlet state is favorable.
By contrast, when one dimensionality is strong and electron number density is
far from the half-filled, a spin-triplet state is stabilized. Finally, we
discuss the possibility of unconventional superconductivity caused by the
on-site Coulomb repulsion in -NaVO.Comment: 4 pages, 7 figure
Perturbation Analysis of Superconductivity in the Trellis-Lattice Hubbard Model
We investigate pairing symmetry and transition temperature in the
trellis-lattice Hubbard model. We solve the \'Eliashberg equation using the
third-order perturbation theory with respect to the on-site repulsion . We
find that a spin-singlet state is very stable in a wide range of parameters. On
the other hand, when the electron number density is shifted from the
half-filled state and the band gap between two bands is small, a spin-triplet
superconductivity is expected. Finally, we discuss a possibility of
unconventional superconductivity and pairing symmetry in
SrCaCuO.Comment: 7pages, 10 figures. To be published in J. Phys. Soc. Jp
Random Spin-orbit Coupling in Spin Triplet Superconductors: Stacking Faults in Sr_2RuO_4 and CePt_3Si
The random spin-orbit coupling in multicomponent superconductors is
investigated focusing on the non-centrosymmetric superconductor CePt_3Si and
the spin triplet superconductor Sr_2RuO_4. We find novel manifestations of the
random spin-orbit coupling in the multicomponent superconductors with
directional disorders, such as stacking faults. The presence of stacking faults
is indicated for the disordered phase of CePt_3Si and Sr_2RuO_4. It is shown
that the d-vector of spin triplet superconductivity is locked to be d = k_y x -
k_x y with the anisotropy \Delta T_c/T_c0 \sim \bar{\alpha}^2/T_c0 W_z, where
\bar{\alpha}, T_c0, and W_z are the mean square root of random spin-orbit
coupling, the transition temperature in the clean limit, and the kinetic energy
along the c-axis, respectively. This anisotropy is much larger (smaller) than
that in the clean bulk Sr_2RuO_4 (CePt_3Si). These results indicate that the
helical pairing state d = k_y x - k_x y in the eutectic crystal
Sr_2RuO_4-Sr_3Ru_2O_7 is stabilized in contrast to the chiral state d = (k_x
\pm i k_y) z in the bulk Sr_2RuO_4. The unusual variation of T_c in CePt_3Si is
resolved by taking into account the weak pair-breaking effect arising from the
uniform and random spin-orbit couplings. These superconductors provide a basis
for discussing recent topics on Majorana fermions and non-Abelian statistics.Comment: J. Phys. Soc. Jpn. 79 (2010) 08470
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