260 research outputs found
Optical conductivity of filled skutterudites
A simple tight-binding model is constructed for the description of the
electronic structure of some Ce-based filled skutterudite compounds showing an
energy gap or pseudogap behavior. Assuming band-diagonal electron interactions
on this tight-binding model, the optical conductivity spectrum is calculated by
applying the second-order self-consistent perturbation theory to treat the
electron correlation. The correlation effect is found to be of great importance
on the description of the temperature dependence of the optical conductivity.
The rapid disappearance of an optical gap with increasing temperature is
obtained as observed in the optical experiment for Ce-based filled-skutterudite
compounds.Comment: 6 pages, 7 figures, use jpsj2.cls, to appear in J. Phys. Soc. Jpn.
Vol.73, No.10 (2004
Kondo Effect in Fermi Systems with a Gap: A Renormalization Group Study
We present the results of a Wilson Renormalization Group study of the
single-impurity Kondo and Anderson models in a system with a gap in the
conduction electron spectrum. The behavior of the impurity susceptibility and
the zero-frequency response function, are discussed in the
cases with and without particle-hole symmetry. In addition, for the asymmetric
Anderson model the correlation functions, , are computed.Comment: 10 pages, 10 figure
First order valence transition in YbInCu_4 in the (B,T) - plane
The puzzling properties of the first order phase transition in YbInCu and
its alloys in the wide range of magnetic fields and temperatures are perfectly
described in terms of a simple entropy transition for free Yb ions. In
particular, it turns out that the transition line in the -plane is very
close to the elliptic shape, as it has been observed experimentally. Similar
calculations are done, and the experiments are proposed for the
phase transition in Ce in Megagauss fields. We speculate,
that in case of YbInCu the first order transition is a Mott transition
between a higher temperature phase in which localized moments are stabilized by
the entropy terms in the free energy, and a band-like non-magnetic ground state
of the -electrons.Comment: RevTeX, 5 pages, 2 figure
Low temperature specific heat of the heavy fermion superconductor PrOsSb
We report the magnetic field dependence of the low temperature specific heat
of single crystals of the first Pr-based heavy fermion superconductor
PrOsSb. The low temperature specific heat and the magnetic phase
diagram inferred from specific heat, resistivity and magnetisation provide
compelling evidence of a doublet ground state and hence superconductivity
mediated by quadrupolar fluctuations. This establishes PrOsSb as a
very strong contender of superconductive pairing that is neither
electron-phonon nor magnetically mediated.Comment: 4 pages, 4 figure
Formation Mechanism of Hybridization Gap in Kondo Insulators based on a Realistic Band Model and Application to YbB
A new LDA+U band calculation is performed on the Kondo insulator material
YbB and an energy gap of about 0.001Ryd is obtained. Based on this, a
simple tight-binding model with 5d and 4f orbitals on Yb
atoms and the nearest neighbor -bonds between them is constructed with
a good agreement to the above the LDA+U calculation near the gap. The density
of states is also calculated and the shape is found to be very asymmetric with
respect to the gap. A formation mechanism of the gap is clarified for the first
time in a realistic situation with the orbital degeneracies in both conduction
bands and the f states. This model can be a useful starting point for
incorporating the strong correlation effect, and for understanding all the
thermal, thermoelectric, transport and magnetic properties of YbB.Comment: 15 pages, 15 figures, to appear in J. Phys. Soc. Jpn. Vol. 72 No. 5
(2003
Theory of Crystalline Electric Field and Kondo Effect in Pr Skutterudites
Possible Kondo effect in Pr skutterudite is studied with attention to
characteristic features of low-lying crystalline electric field (CEF) levels
and the conduction band. A mechanism for the small CEF splitting between a
singlet and a triplet is proposed as combination of the point-charge
interaction and hybridization of 4f with ligand p states. Provided 4f^3
configurations dominate over 4f^1 as intermediate states, p-f hybridization
favors the triplet, while point-charge interaction favors the singlet. For
realistic parameters for hybridization as well as 4f^1 and 4f^3 levels, these
singlet and triplet can form a nearly degenerate pseudo-quartet. It is found
that one of two spin 1/2 objects composing the pseudo-quartet has a
ferromagnetic exchange, while the other has an antiferromagnetic exchange with
conduction electrons. The magnitude of each effective exchange depends strongly
on a parameter characterizing the triplet wave function under the T_h symmetry.
It is argued that differences of this parameter among Pr skutterdudites are
responsible for the apparent diversity of their physical properties. Numerical
renormalization group is used to derive the renormalization flows going toward
singlet, doublet, triplet or quaret according to the CEF splitting and exchange
interactions.Comment: 19 pages, 6 figures, to be published in Special Invited Section
(Kondo Effect) of Journal of Physical Society of Japa
Spin Fluctuation Induced Superconductivity Controlled by Orbital Fluctuation
A microscopic Hamiltonian reflecting the correct symmetry of -orbitals is
proposed to discuss superconductivity in heavy fermion systems. In the
orbitally degenerate region in which not only spin fluctuations but also
orbital fluctuations develop considerably, cancellation between spin and
orbital fluctuations destabilizes -wave superconductivity.
Entering the non-degenerate region by increasing the crystalline electric
field, -wave superconductivity mediated by antiferromagnetic
spin fluctuations emerges out of the suppression of orbital fluctuations. We
argue that the present scenario can be applied to recently discovered
superconductors CeTIn (T=Ir, Rh, and Co).Comment: 4 pages, 3 figure
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