515 research outputs found
Co-NMR Knight Shift of NaxCoO2 \dot yH2O Studied in Both Superconducting Regions of the Tc-nuQ3 Phase Diagram Divided by the Nonsuperconducting Phase
In the temperature (T)-nuQ3 phase diagram of NaxCoO2 \dot yH2O, there exist
two superconducting regions of nuQ3 separated by the nonsuperconducting region,
where nuQ3 is usually estimated from the peak position of the 59Co-NQR spectra
of the 5/2-7/2 transition and well-approximated here as nuQ3~3nuQ,nuQ being the
interaction energy between the nuclear quadrupole moment and the electric field
gradient. We have carried out measurements of the 59Co-NMR Knight shift (K) for
a single crystal in the higher-nuQ3 superconducting phase and found that K
begins to decrease with decreasing T at Tc for both magnetic field directions
parallel and perpendicular to CoO2-planes. The result indicates together with
the previous ones that the superconducting pairs are in the spin-singlet state
in both phases, excluding the possibility of the spin-triplet superconductivity
in this phase diagram. The superconductivity of this system spreads over the
wide nuQ3 regions, but is suppressed in the narrow region located at the middle
point of the region possibly due to charge instability.Comment: 8 pages, 5 figures, submitted to J. Phys. Soc. Jp
Partial Disorder in the Periodic Anderson Model on a Triangular Lattice
We report our theoretical results on the emergence of a partially-disordered
state at zero temperature and its detailed nature in the periodic Anderson
model on a triangular lattice at half filling. The partially-disordered state
is characterized by coexistence of a collinear antiferromagnetic order on an
unfrustrated honeycomb subnetwork and nonmagnetic state at the remaining sites.
This state appears with opening a charge gap between a noncollinear
antiferromagnetic metal and Kondo insulator while changing the hybridization
and Coulomb repulsion. We also find a characteristic crossover in the
low-energy excitation spectrum as a result of coexistence of magnetic order and
nonmagnetic sites. The result demonstrates that the partially-disordered state
is observed distinctly even in the absence of spin anisotropy, in marked
contrast to the partial Kondo screening state found in the previous study for
the Kondo lattice model.Comment: 4 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
Indirect RKKY interaction in any dimensionality
We present an analytical method which enables one to find the exact spatial
dependence of the indirect RKKY interaction between the localized moments via
the conduction electrons for the arbitrary dimensionality . The
corresponding momentum dependence of the Lindhard function is exactly found for
any as well. Demonstrating the capability of the method we find the RKKY
interaction in a system of metallic layers weakly hybridized to each other.
Along with usual in-plane oscillations the RKKY interaction has the
sign-reversal character in a direction perpendicular to layers, thus favoring
the antiferromagnetic type of layers' stacking.Comment: 3 pages, REVTEX, accepted to Phys.Rev.
Conductance of nano-systems with interactions coupled via conduction electrons: Effect of indirect exchange interactions
A nano-system in which electrons interact and in contact with Fermi leads
gives rise to an effective one-body scattering which depends on the presence of
other scatterers in the attached leads. This non local effect is a pure
many-body effect that one neglects when one takes non interacting models for
describing quantum transport. This enhances the non-local character of the
quantum conductance by exchange interactions of a type similar to the
RKKY-interaction between local magnetic moments. A theoretical study of this
effect is given assuming the Hartree-Fock approximation for spinless fermions
in an infinite chain embedding two scatterers separated by a segment of length
L\_c. The fermions interact only inside the two scatterers. The dependence of
one scatterer onto the other exhibits oscillations which decay as 1/L\_c and
which are suppressed when L\_c exceeds the thermal length L\_T. The
Hartree-Fock results are compared with exact numerical results obtained with
the embedding method and the DMRG algorithm
Giant Extrinsic Spin Hall Effect due to Rare-Earth Impurities
We investigate the extrinsic spin Hall effect in the electron gas model due
to magnetic impurities, by focusing on Ce- and Yb-impurities. In the dilute
limit, the skew scattering term dominates the side jump term. For
Ce-impurities, the spin Hall angle due to skew scattering is
given by , where is the phase shift
for partial wave. Since reaches if
\delta_2 \simge 0.03, the spin Hall effect is anticipated to be considerable
in metals with rare-earth impurities. The giant extrinsic SHE originates from
the large orbital angular momentum, which is also significant for the intrinsic
SHE.Comment: 5 pages, 3 figures, to be published in New Journal of Physic
Partial Disorder and Metal-Insulator Transition in the Periodic Anderson Model on a Triangular Lattice
Ground state of the periodic Anderson model on a triangular lattice is
systematically investigated by the mean-field approximation. We found that the
model exhibits two different types of partially disordered states: one is at
half filling and the other is at other commensurate fillings. In the latter
case, the kinetic energy is lowered by forming an extensive network involving
both magnetic and nonmagnetic sites, in sharp contrast to the former case in
which the nonmagnetic sites are rather isolated. This spatially extended nature
of nonmagnetic sites yields a metallic partially-disordered state by hole
doping. We discuss the mechanism of the metal-insulator transition by the
change of electronic structure.Comment: 4 pages, 4 figures, accepted for publication in J. Phys. Soc. Jp
From Kondo Effect to Fermi Liquid
The Kondo effect has been playing an important role in strongly correlated
electon systems. The important point is that the magnetic impurity in metals is
a typical example of the Fermi liquid. In the system the local spin is
conserved in the ground state and continuity with respect to Coulomb repulsion
is satisfied. This nature is satisfied also in the periodic systems as far
as the systems remain as the Fermi liquid. This property of the Fermi liquid is
essential to understand the cuprate high-Tc superconductors (HTSC). On the
basis of the Fermi liquid theory we develop the transport theory such as the
resistivity and the Hall coefficient in strongly correlated electron systems,
such as HTSC, organic metals and heavy Fermion systems. The significant role of
the vertex corrections for total charge- and heat-currents on the transport
phenomena is explained. By taking the effect of the current vertex corrections
into account, various typical non-Fermi-liquid-like transport phenomena in
systems with strong magnetic and/or superconducting flucutations are explained
within the Fermi liquid theory.Comment: 14 pages, an article for the special edition of JPSJ "Kondo Effect --
40 Years after the Discovery
Endoscopy in the Diagnosis of Small Intestinal Tumors
The importance of endoscopy in the diagnosis of small intestinal tumors was evaluated
in 15 patients with small intestinal tumors treated in our hospital. Two tumors were
benign, and 13 were malignant (carcinoma in 5 patients, malignant lymphoma in 5 and
leiomyosarcoma in 3). The presence of lesions could be determined by X-rays before
surgery, but definitive diagnoses were difficult. When preoperative endoscopy of the
small intestine was possible accurate preoperative diagnoses could be made based on the
endoscopic findings and biopsies taken under direct vision. Endoscopy is therefore very
important for the diagnosis of small intestinal tumors. It is necessary to develop small
intestinal endoscopes that are easier to insert
Numerical Renormalization Group Calculations for the Self-energy of the impurity Anderson model
We present a new method to calculate directly the one-particle self-energy of
an impurity Anderson model with Wilson's numerical Renormalization Group method
by writing this quantity as the ratio of two correlation functions. This way of
calculating Sigma(z) turns out to be considerably more reliable and accurate
than via the impurity Green's function alone. We show results for the
self-energy for the case of a constant coupling between impurity and conduction
band (ImDelta = const) and the effective Delta(z) arising in the Dynamical Mean
Field Theory of the Hubbard model. Implications to the problem of the
metal-insulator transition in the Hubbard model are also discussed.Comment: 18 pages, 9 figures, submitted to J. Phys.: Condens. Matte
Theoretical Analysis of the "Double-q" Magnetic Structure of CeAl2
A model involving competing short-range isotropic Heisenberg interactions is
developed to explain the "double-q" magnetic structure of CeAl. For
suitably chosen interactions, terms in the Landau expansion quadratic in the
order parameters explain the condensation of incommensurate order at
wavevectors in the star of (1/2 , 1/2 , 1/2), where
is the cubic lattice constant. We show that the fourth order terms in the
Landau expansion lead to the formation of the so-called "double-q" magnetic
structure in which long-range order develops simultaneously at two
symmetry-related wavevectors, in striking agreement with the magnetic structure
determinations. Based on the value of the ordering temperature and of the
Curie-Weiss of the susceptibility, we estimate that the nearest
neighbor interaction is ferromagnetic, with K and the
next-nearest neighbor interaction is antiferromagnetic with K.
We also briefly comment on the analogous phenomenon seen in the similar system
TmS.Comment: 22 pages, 6 figure
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