16 research outputs found
Effective Crystalline Electric Field Potential in a j-j Coupling Scheme
We propose an effective model on the basis of a - coupling scheme to
describe local -electron states for realistic values of Coulomb interaction
and spin-orbit coupling , for future development of microscopic
theory of magnetism and superconductivity in -electron systems, where
is the number of local electrons. The effective model is systematically
constructed by including the effect of a crystalline electric field (CEF)
potential in the perturbation expansion in terms of . In this paper,
we collect all the terms up to the first order of . Solving the
effective model, we show the results of the CEF states for each case of
=25 with symmetry in comparison with those of the Stevens
Hamiltonian for the weak CEF. In particular, we carefully discuss the CEF
energy levels in an intermediate coupling region with in the order
of 0.1 corresponding to actual -electron materials between the and
- coupling schemes. Note that the relevant energy scale of is the
Hund's rule interaction. It is found that the CEF energy levels in the
intermediate coupling region can be quantitatively reproduced by our modified
- coupling scheme, when we correctly take into account the corrections in
the order of in addition to the CEF terms and Coulomb interactions
which remain in the limit of =. As an application of the
modified - coupling scheme, we discuss the CEF energy levels of filled
skutterudites with symmetry.Comment: 12 pages, 7 figures. Typeset with jpsj2.cl
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
Electronic Structure and Fermiology of PuCoGa
By using a relativistic linear augmented-plane-wave method, we clarify energy
band structures and Fermi surfaces of recently discovered plutonium-based
superconductor PuCoGa. We find several cylindrical sheets of Fermi surfaces
with large volume, very similar to CeMIn (M=Ir and Co) isostructural with
PuCoGa, in spite of different -electron numbers between Ce and
Pu ions. The similarity is understood by a concept of electron-hole
conversion in a tight binding model constructed based on the - coupling
scheme. Based on the present results, we provide a possible scenario to explain
why a transition temperature is so high as 18.5K in PuCoGa.Comment: 4 pages, Revtex, with 4 figures embedded in the text. Submitted to
Phys. Rev. Let
Strong-coupling theory of superconductivity in a degenerate Hubbard model
In order to discuss superconductivity in orbital degenerate systems, a
microscopic Hamiltonian is introduced. Based on the degenerate model, a
strong-coupling theory of superconductivity is developed within the fluctuation
exchange (FLEX) approximation where spin and orbital fluctuations, spectra of
electron, and superconducting gap function are self-consistently determined.
Applying the FLEX approximation to the orbital degenerate model, it is shown
that the -wave superconducting phase is induced by increasing the
orbital splitting energy which leads to the development and suppression of the
spin and orbital fluctuations, respectively. It is proposed that the orbital
splitting energy is a controlling parameter changing from the paramagnetic to
the antiferromagnetic phase with the -wave superconducting phase
in between.Comment: 4 figures, submitted to PR
Orbital ordering phenomena in - and -electron systems
In recent decades, novel magnetism of - and -electron compounds has
been discussed very intensively both in experimental and theoretical research
fields of condensed matter physics. It has been recognized that those material
groups are in the same category of strongly correlated electron systems, while
the low-energy physics of - and -electron compounds has been separately
investigated rather in different manners. One of common features of both -
and -electron systems is certainly the existence of active orbital degree of
freedom, but in -electron materials, due to the strong spin-orbit
interaction in rare-earth and actinide ions, the physics seems to be quite
different from that of -electron systems. In general, when the number of
internal degrees of freedom and relevant interactions is increased, it is
possible to obtain rich phase diagram including large varieties of magnetic
phases by using several kinds of theoretical techniques. However, we should not
be simply satisfied with the reproduction of rich phase diagram. It is believed
that more essential point is to seek for a simple principle penetrating
complicated phenomena in common with - and -electron materials, which
opens the door to a new stage in orbital physics. In this sense, it is
considered to be an important task of this article to explain common features
of magnetism in - and -electron systems from a microscopic viewpoint,
using a key concept of orbital ordering, in addition to the review of the
complex phase diagram of each material group.Comment: 112 pages, 38 figure
Microscopic Approach to Magnetism and Superconductivity of -Electron Systems with Filled Skutterudite Structure
In order to gain a deep insight into -electron properties of filled
skutterudite compounds from a microscopic viewpoint, we investigate the
multiorbital Anderson model including Coulomb interactions, spin-orbit
coupling, and crystalline electric field effect. For each case of
=113, where is the number of electrons per rare-earth ion, the
model is analyzed by using the numerical renormalization group (NRG) method to
evaluate magnetic susceptibility and entropy of electron. In order to make
further step to construct a simplified model which can be treated even in a
periodic system, we also analyze the Anderson model constructed based on the
- coupling scheme by using the NRG method. Then, we construct an orbital
degenerate Hubbard model based on the - coupling scheme to investigate
the mechanism of superconductivity of filled skutterudites. In the 2-site
model, we carefully evaluate the superconducting pair susceptibility for the
case of =2 and find that the susceptibility for off-site Cooper pair is
clearly enhanced only in a transition region in which the singlet and triplet
ground states are interchanged.Comment: 14 pages, 11 figures, Typeset with jpsj2.cl
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
Construction of microscopic model for f-electron systems on the basis of j-j coupling scheme
We construct a microscopic model for f-electron systems, composed of
f-electron hopping, Coulomb interaction, and crystalline electric field (CEF)
terms. In order to clarify the meaning of one f-electron state, here the j-j
coupling scheme is considered, since the spin-orbit interaction is generally
large in f-electron systems. Thus, the f-electron state at each site is
labelled by , namely, the z-component of total angular momentum j. By
paying due attention to f-orbital symmetry, the hopping amplitudes between
f-electron states are expressed using Slater's integrals. The Coulomb
interaction terms among the -states are written by Slater-Condon or Racah
parameters. Finally, the CEF terms are obtained from the table of Hutchings.
The constructed Hamiltonian is regarded as an orbital degenerate Hubbard model,
since it includes two pseudo-spin and three pseudo-orbital degrees of freedom.
For practical purposes, it is further simplified into a couple of two-orbital
models by discarding one of the three orbitals. One of those simplified models
is here analyzed using the exact diagonalization method to clarify ground-state
properties by evaluating several kinds of correlation functions. Especially,
the superconducting pair correlation function in orbital degenerate systems is
carefully calculated based on the concept of off-diagonal long-range order. We
attempt to discuss a possible relation of the present results with experimental
observations for recently discovered heavy fermion superconductors CeMIn
(M=Ir, Co, and Rh), and a comprehensive scenario to understand superconducting
and antiferromagnetic tendencies in the so-called ``115'' materials such as
CeMIn, UMGa, and PuCoGa from the microscopic viewpoint.Comment: 16 pages, Revtex, with 6 figures embedded in the text. Submitted to
Phys. Rev.