27 research outputs found
Coulomb blockade without potential barriers
We study transport through a strongly correlated quantum dot and show that
Coulomb blockade can appear even in the presence of perfect contacts. This
conclusion arises from numerical calculations of the conductance for a
microscopic model of spinless fermions in an interacting chain connected to
each lead via a completely open channel. The dependence of the conductance on
the gate voltage shows well defined Coulomb blockade peaks which are sharpened
as the interaction strength is increased. Our numerics is based on the
embedding method and the DMRG algorithm. We explain the emergence of Coulomb
blockade with perfect contacts by a reduction of the effective coupling matrix
elements between many-body states corresponding to successive particle numbers
in the interacting region. A perturbative approach, valid in the strong
interaction limit, yields an analytic expression for the interaction-induced
suppression of the conductance in the Coulomb blockade regime.Comment: Fixed problems with eps figure
Possible Pairing Symmetry of Three-dimensional Superconductor UPt -- Analysis Based on a Microscopic Calculation --
Stimulated by the anomalous superconducting properties of UPt, 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 . As results of the 2D calculation, we obtain
distinct two types of stable spin-triplet pairing states. One is the
-wave(B) pairing around and in a small region, which is
caused by the ferromagnetic fluctuation. Then, the other is the (or
)-wave(E) pairing in large region far from the half-filling () which is caused by the vertex corrections only. However, we find that the
former -wave pairing is destroyed by introduced 3D dispersion. This is
because the 3D dispersion breaks the favorable structures for the -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. We also study the case
of the pairing symmetry with a polar gap. This -wave(A) is stabilized
by the large hopping integral along c-axis . It is nearly degenerate with
the suppressed (or )-wave(E) in the best fitting parameter region
to UPt (). 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 SrRuO.Comment: 15 pages, 12 figure
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.
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
The embedding method beyond the single-channel case: Two-mode and Hubbard chains
We investigate the relationship between persistent currents in multi-channel
rings containing an embedded scatterer and the conductance through the same
scatterer attached to leads. The case of two uncoupled channels corresponds to
a Hubbard chain, for which the one-dimensional embedding method is readily
generalized. Various tests are carried out to validate this new procedure, and
the conductance of short one-dimensional Hubbard chains attached to perfect
leads is computed for different system sizes and interaction strengths. In the
case of two coupled channels the conductance can be obtained from a statistical
analysis of the persistent current or by reducing the multi-channel scattering
problem to several single-channel setups.Comment: 14 pages, 13 figures, submitted for publicatio
Phylogeographic insights into the invasion history and secondary spread of the signal crayfish in Japan
Successful invasion by nonindigenous species is often attributed to high propagule pressure, yet some foreign species become widespread despite showing reduced genetic variation due to founder effects. The signal crayfish (Pacifastacus leniusculus) is one such example, where rapid spread across Japan in recent decades is believed to be the result of only three founding populations. To infer the history and explore the success of this remarkable crayfish invasion, we combined detailed phylogeographical and morphological analyses conducted in both the introduced and native ranges. We sequenced 16S mitochondrial DNA of signal crayfish from across the introduced range in Japan (537 samples, 20 sites) and the native range in western North America (700 samples, 50 sites). Because chela size is often related to aggressive behavior in crayfish, and hence, their invasion success, we also measured chela size of a subset of specimens in both introduced and native ranges. Genetic diversity of introduced signal crayfish populations was as high as that of the dominant phylogeographic group in the native range, suggesting high propagule pressure during invasion. More recently established crayfish populations in Japan that originated through secondary spread from one of the founding populations exhibit reduced genetic diversity relative to older populations, probably as a result of founder effects. However, these newer populations also show larger chela size, consistent with expectations of rapid adaptations or phenotypic responses during the invasion process. Introduced signal crayfish populations in Japan originate from multiple source populations from a wide geographic range in the native range of western North America. A combination of high genetic diversity, especially for older populations in the invasive range, and rapid adaptation to colonization, manifested as larger chela in recent invasions, likely contribute to invasion success of signal crayfish in Japan. © 2016 The Authors. Ecology and Evolution published by John Wiley & Sons Ltd
Role of spin-orbit coupling on the spin triplet pairing in Na_{x}CoO_{2}yH_{2}O I: d-vector under zero magnetic field
The d-vector in possibile spin triplet superconductor Na_{x}CoO_{2}yH_{2}O is
microscopically investigated on the basis of the multi-orbital Hubbard model
including the atomic spin-orbit coupling. As a result of the perturbation
theory, we obtain the stable spin triplet superconductivity where the p-wave
and f-wave states can be stabilized. If we neglect the spin-orbit coupling,
superconducting state has 6-fold (3-fold) degeneracy in the p-wave (f-wave)
state. This degeneracy is lifted by the spin-orbit coupling. We determine the
d-vector within the linearlized Dyson-Gorkov equation. It is shown that the
d-vector is always along the plane when the pairing symmetry is p-wave, while
it depends on the parameters in case of the f-wave state. The lifting of
degeneracy is significant in the p-wave state while it is very small in the
f-wave state. This is because the first order term with respect to the
spin-orbit coupling is effective in the former case, while it is ineffective in
the latter case. The consistency of these results with NMR and \muSR
measurements are discussed.Comment: To appear in J. Phys. Soc. Jpn. 74 (2005) No.
Nonconventional flash annealing on shallow indium implants in silicon
The diffusion behavior and the electrical characteristics of indium doped layers in silicon were studied. Indium was implanted in silicon at energies of 70 and 25 keV to doses of 5.8 and 3E14, respectively. The implants were performed both in amorphous and crystalline silicon. The implants were submitted to a combination of thermal annealing, RTA, and flash annealing to regrow the implanted layers and activate the dopant. Four point probe sheet resistance measurements and Hall effect measurements were carried out to test the electrical properties of the implanted layers. The atomic concentration profiles were assessed using secondary ion mass spectrometry. A drastic increase in the dopant activation was observed following co-implanting with carbon. Moreover, the carbon presence inhibits the indium diffusion and segregation in damaged areas. The preamorphizing treatment affects the indium diffusion in two ways. For low thermal budget anneals the diffusion is suppressed, conversely the diffusion is enhanced under severe annealing conditions