32 research outputs found

### A Mechanism of Spin-Triplet Superconductivity in Hubbard Model on Triangular La ttice: Application to UNi_2Al_3

We discuss the possibility of spin-triplet superconductivity in a
two-dimensional Hubbard model on a triangular lattice within the third-order
perturbation theory. When we vary the symmetry in the dispersion of the bare
energy band from D_2 to D_6, spin-singlet superconductivity in the
D_2-symmetric system is suppressed and we obtain spin-triplet superconductivity
in near the D_6-symmetric system. In this case, it is found that the vertex
terms, which are not included in the interaction mediated by the spin
fluctuation, are essential for realizing the spin-triplet pairing. We point out
the possibility that obtained results correspond to the difference between the
superconductivity of UNi_2Al_3 and that of UPd_2Al_3.Comment: 11pages, 5figure

### Gate-voltage dependence of Kondo effect in a triangular quantum dot

We study the conductance through a triangular triple quantum dot, which are
connected to two noninteracting leads, using the numerical renormalization
group (NRG). It is found that the system shows a variety of Kondo effects
depending on the filling of the triangle. The SU(4) Kondo effect occurs at
half-filling, and a sharp conductance dip due to a phase lapse appears in the
gate-voltage dependence. Furthermore, when four electrons occupy the three
sites on average, a local S=1 moment, which is caused by the Nagaoka mechanism,
is induced along the triangle. The temperature dependence of the entropy and
spin susceptibility of the triangle shows that this moment is screened by the
conduction electrons via two separate stages at different temperatures. The
two-terminal and four-terminal conductances show a clear difference at the gate
voltages, where the SU(4) or the S=1 Kondo effects occurring.Comment: 4 pages, 4 figs: typos just below (4) are corrected, results are not
affecte

### Possible spin triplet superconductivity in Na$_x$CoO$_{2}\cdot y$H$_{2}$0

Combining symmetry based considerations with inputs from available
experimental results, we make the case that a novel spin-triplet
superconductivity triggered by antiferromagnetic fluctuations may be realized
in the newly discovered layered cobaltide Na$_x$CoO$_{2}\cdot y$H$_2$O. In the
proposed picture, unaccessable via resonating-valence-bond physics extrapolated
from half-filling, the pairing process is similar to that advanced for
Sr$_{2}$RuO$_4$, but enjoys a further advantage coming from the hexagonal
structure of the Fermi-surface which gives a stronger pairing tendency.Comment: 4 page

### Anisotropy of Resonant Inelastic X-Ray Scattering at the K Edge of Si:Theoretical Analysis

We investigate theoretically the resonant inelastic x-ray scattering (RIXS)
at the $K$ edge of Si on the basis of an ab initio calculation. We calculate
the RIXS spectra with systematically varying transfered-momenta,
incident-photon energy and incident-photon polarization. We confirm the
anisotropy of the experimental spectra by Y. Ma {\it et al}. (Phys. Rev. Lett.
74, 478 (1995)), providing a quantitative explanation of the spectra.Comment: 18 pages, 11 figure

### Fourth Order Perturbation Theory for Normal Selfenergy in Repulsive Hubbard Model

We investigate the normal selfenergy and the mass enhancement factor in the
Hubbard model on the two-dimensional square lattice. Our purpose in this paper
is to evaluate the mass enhancement factor more quantitatively than the
conventional third order perturbation theory. We calculate it by expanding
perturbatively up to the fourth order with respect to the on-site repulsion
$U$. We consider the cases that the system is near the half-filling, which are
similar situations to high-$T_c$ cuprates. As results of the calculations, we
obtain the large mass enhancement on the Fermi surface by introducing the
fourth order terms. This is mainly originated from the fourth order
particle-hole and particle-particle diagrams. Although the other fourth order
terms have effect of reducing the effective mass, this effect does not cancel
out the former mass enhancement completely and there remains still a large mass
enhancement effect. In addition, we find that the mass enhancement factor
becomes large with increasing the on-site repulsion $U$ and the density of
state (DOS) at the Fermi energy $\rho(0)$. According to many current reseaches,
such large $U$ and $\rho(0)$ enhance the effective interaction between
quasiparticles, therefore the superconducting transition temperature $T_c$
increases. On the other hand, the large mass enhancement leads the reduction of
the energy scale of quasiparticles, as a result, $T_c$ is reduced. When we
discuss $T_c$, we have to estimate these two competitive effects.Comment: 6pages,8figure

### Origin of the Weak Pseudo-gap Behaviors in Na_{0.35}CoO_2: Absence of Small Hole Pockets

We analyze the ``normal electronic states'' of Na_{0.35}CoO_2 based on the
effective d-p model with full d-orbital freedom using the fluctuation-exchange
(FLEX) approximation. They sensitively depend on the topology of the Fermi
surfaces, which changes as the crystalline electric splitting (CES) due to the
trigonal deformation. We succeed in reproducing the weak pseudo-gap behaviors
in the density of states (DOS) and in the uniform magnetic susceptibility below
300K, assuming that six small hole-pockets predicted by LDA band calculations
are absent. When they exist, on the contrary, then ``anti-pseudo-gap
behaviors'' should inevitably appear. Thus, the present study strongly supports
the absence of the small hole-pockets in Na_{0.35}CoO_2, as reported by recent
ARPES measurements. A large Fermi surface around the \Gamma-point would account
for the superconductivity in water-intercalated samples.Comment: 5pages, to appear in J. Phys. Soc. Jpn. Vol.74 (2005) No.

### Multi-orbital analysis on the Superconductivity in Na_{x}CoO_{2} \cdot y H_{2}O

We preform a multi-orbital analysis on the novel superconductivity in
Na_{x}CoO_{2} \cdot yH_{2}O. We construct a three-orbital model which
reproduces the band structure expected from the LDA calculation. The effective
interaction leading to the pairing is estimated by means of the perturbation
theory. It is shown that the spin triplet superconductivity is stabilized in
the wide parameter region. This is basically owing to the ferromagnetic
character of spin fluctuation. The p-wave and f-wave superconductivity are
nearly degenerate. The former is realized when the Hund's rule coupling is
large, and vice versa. In a part of the parameter space, the d-wave
superconductivity is also stabilized. We point out that the orbital degeneracy
plays an essential role for these results through the wave function of
quasi-particles. The nearly degeneracy of p-wave and f-wave superconductivity
is explained by analysing the orbital character of each Fermi surface. We
discuss the validity of some reduced models. While the single band Hubbard
model reproducing the Fermi surface is qualitatively inappropriate, we find an
effective two-orbital model appropriate for studying the superconductivity. We
investigate the vertex corrections higher than the third order on the basis of
the two-orbital model. It is shown that the vertex correction induces the
screening effect but does not affect on the qualitative results.Comment: To appear in J. Phys. Soc. Jpn. 74 (2005) No.

### Magnetic-Field-Induced Antiferromagnetism in Two-Dimensional Hubbard Model: Analysis of CeRhIn$_5$

We propose the mechanism for the magnetic-field-induced antiferromagnetic
(AFM) state in a two-dimensional Hubbard model in the vicinity of the AFM
quantum critical point (QCP), using the fluctuation-exchange (FLEX)
approximation by taking the Zeeman energy due to the magnetic field $B$ into
account. In the vicinity of the QCP, we find that the AFM correlation
perpendicular to $B$ is enhanced, whereas that parallel to $B$ is reduced. This
fact means that the finite magnetic field increases $T_N$, with the AFM order
perpendicular to $B$. The increment in $T_N$ can be understood in terms of the
reduction of both quantum and thermal fluctuations due to the magnetic field,
which is caused by the self-energy effect within the FLEX approximation. The
present study naturally explains the increment in $T_N$ in CeRhIn_5 under the
magnetic field found recently.Comment: 5 page

### Possible Pairing Symmetry of Three-dimensional Superconductor UPt$_3$ -- Analysis Based on a Microscopic Calculation --

Stimulated by the anomalous superconducting properties of UPt$_3$, we
investigate the pairing symmetry and the transition temperature in the
two-dimensional(2D) and three-dimensional(3D) hexagonal Hubbard model. We solve
the Eliashberg equation using the third order perturbation theory with respect
to the on-site repulsion $U$. As results of the 2D calculation, we obtain
distinct two types of stable spin-triplet pairing states. One is the
$f$-wave(B$_1$) pairing around $n = 1.2$ and in a small $U$ region, which is
caused by the ferromagnetic fluctuation. Then, the other is the $p_x$(or
$p_y$)-wave(E$_1$) pairing in large $U$ region far from the half-filling ($n =
1$) which is caused by the vertex corrections only. However, we find that the
former $f$-wave pairing is destroyed by introduced 3D dispersion. This is
because the 3D dispersion breaks the favorable structures for the $f$-wave
pairing such as the van Hove singularities and the small pocket structures.
Thus, we conclude that the ferromagnetic fluctuation mediated spin-triplet
state can not explain the superconductivity of UPt$_3$. We also study the case
of the pairing symmetry with a polar gap. This $p_z$-wave(A$_1$) is stabilized
by the large hopping integral along c-axis $t_z$. It is nearly degenerate with
the suppressed $p_x$(or $p_y$)-wave(E$_1$) in the best fitting parameter region
to UPt$_3$ ($1.3 \le t_z \le 1.5$). These two p-wave pairing states exist in
the region far from the half-filling, in which the vertex correction terms play
crucial roles like the case in Sr$_2$RuO$_4$.Comment: 15 pages, 12 figure

### Orbital-Controlled Superconductivity in f-Electron Systems

We propose a concept of superconductivity controlled by orbital degree of
freedom taking CeMIn5 (M= Co, Rh, and Ir) as typical examples. A microscopic
multiorbital model for CeMIn5 is analyzed by fluctuation exchange
approximation. Even though the Fermi-surface structure is unchanged, the ground
state is found to change significantly among paramagnetic, antiferromagnetic,
and d-wave superconducting phases, depending on the dominant orbital component
in the band near the Fermi energy. We show that our picture naturally explains
the different low-temperature properties of CeMIn5 by carefully analyzing the
crystalline electric field states.Comment: 5 pages, 4 figure