262 research outputs found
Antiferro-quadrupole state of orbital-degenerate Kondo lattice model with f^2 configuration
To clarify a key role of orbitals in the emergence of
antiferro-quadrupole structure in PrPb, we investigate the ground-state
property of an orbital-degenerate Kondo lattice model by numerical
diagonalization techniques. In PrPb, Pr has a
configuration and the crystalline-electric-field ground state is a non-Kramers
doublet . In a - coupling scheme, the state is
described by two local singlets, each of which consists of two electrons
with one in and another in orbitals. Since in a cubic
structure, has localized nature, while orbitals are
rather itinerant, we propose the orbital-degenerate Kondo lattice model for an
effective Hamiltonian of PrPb. We show that an antiferro-orbital state is
favored by the so-called double-exchange mechanism which is characteristic of
multi-orbital systems.Comment: 3 pages, 3 figures, Proceedings of Skutterudite2007 (September 26-30,
2007, Kobe
Field-angle-dependent specific heat measurements and gap determination of a heavy fermion superconductor URu2Si2
To identify the superconducting gap structure in URu2Si2 we perform
field-angle-dependent specific heat measurements for the two principal
orientations in addition to field rotations, and theoretical analysis based on
microscopic calculations. The Sommerfeld coefficient \gamma(H)'s in the mixed
state exhibit distinctively different field-dependence. This comes from point
nodes and substantial Pauli paramagnetic effect of URu2Si2. These two features
combined give rise to a consistent picture of superconducting properties,
including a possible first order transition of Hc2 at low temperatures.Comment: 4 pages, 5 figures, to appear in Phys. Rev. Let
Anomalous Anisotropic Magnetoresistance in Heavy-Fermion PrFe4P12
We have investigated the anisotropy of the magnetoresistance in the Pr-based
HF compound PrFe4P12. The large anisotropy of effective mass and its strong
field dependence have been confirmed by resistivity measurements. Particularly
for H||[111], where the effective mass is most strongly enhanced, the non-Fermi
liquid behavior has been observed. Also, we have found the angular dependence
of the magnetoresistance sharply enhanced at H||[111], which is evidently
correlated with both the non-Fermi liquid behavior and the high-field ordered
state (B-phase).Comment: 3 pages, 3 figures. J. Phys. Soc. Jpn. Vol.77, No.8, in pres
High magnetic field phase diagram of PrOs4Sb12
The magnetic phase diagram of PrOsSb has been investigated by
specific heat measurements between 8 and 32 T. A new Schottky anomaly due to
excitations between two lowest crystalline-electric-field (CEF) singlets, has
been found for both and above the field
where the field-induced ordered phase (FIOP) is suppressed. The constructed
phase diagram shows weak magnetic anisotropy and implies a crossing of
the two CEF levels at about 8 - 9 T for both field directions. These results
provide an unambiguous evidence for the singlet being the CEF ground
state and suggest the level crossing (involving lowest CEF levels) as the
driving mechanism of FIOP.Comment: Submitted to Phys. Rev. Let
Microscopic Mechanism for Staggered Scalar Order in PrFe4P12
A microscopic model is proposed for the scalar order in PrFe4P12 where f2
crystalline electric field (CEF) singlet and triplet states interact with two
conduction bands. By combining the dynamical mean-field theory and the
continuous-time quantum Monte Carlo, we obtain an electronic order with
staggered Kondo and CEF singlets with the total conduction number being unity
per site. The ground state becomes semimetallic provided that the two
conduction bands have different occupation numbers. This model naturally
explains experimentally observed properties in the ordered phase of PrFe4P12
such as the scalar order parameter, temperature dependence of the resistivity,
field-induced staggered moment, and inelastic features in neutron scattering.
The Kondo effect plays an essential role for ordering, in strong contrast with
ordinary magnetic orders by the RKKY interaction.Comment: 4 pages, 4figure
Realization of Heavy Local Fermi Liquid and Non-Fermi Liquid in f Crystalline-Electric-Field Singlet-Triplet Configuration
Using the numerical renormalization group method, we investigate an extended
Anderson model, in which correlated electrons with the
(singlet)-(triplet) f crystalline-electric-field (CEF)
configuration hybridize with conduction electrons of (doublet) and
(quartet) under cubic symmetry, from a strong spin-orbit
interaction limit. For the case of the parameters relevant to PrFe4P12, the
system is under competition between the CEF singlet fixed point and the
multichannel
Kondo non-Fermi liquid fixed point arising from the quadrupolar coupling
between the impurity with pseudospin 1 and the conduction electron with
pseudospin 3/2 . We consider that this result reveals the origin of the
heaviness of the effective mass and non-Fermi liquid behavior of the Pr- based
filled skutterudite compounds.Comment: 9 pages, 5 figure
Theoretical Description of Nearly Discontinuous Transition in Superconductors with Paramagnetic Depairing
Based on a theoretical argument and Monte Carlo simulations of a
Ginzburg-Landau model derived microscopically, it is argued that, in type-II
superconductors where {\it both} the paramagnetic {\it and} orbital depairings
are important, a strong first-order transition (FOT) at expected in
the mean field (MF) approximation never occurs in real systems and changes due
to the fluctuation into a crossover. The present result explains why a {\it
nearly} discontinuous crossover at with {\it no} intrinsic hysteresis
is observed only in a clean superconducting material with a singlet pairing and
a high condensation energy such as CeCoIn.Comment: Publication version. See cond-mat/0306060 regarding a corresponding
long pape
On the origin of multiple ordered phases in PrFe4P12
The nature of multiple electronic orders in skutterudite PrFe_4P_{12} is
discussed on the basis of a model with antiferro-quadrupole (AFQ) interaction
of \Gamma_3 symmetry. The high-field phase can be reproduced qualitatively
provided (i) ferro-type interactions are introduced between the dipoles as well
as between the octupoles of localized f-electrons, and (ii) separation is
vanishingly small between the \Gamma_1-\Gamma_4^{(1)} crystalline electric
field (CEF) levels. The high-field phase can have either the same ordering
vector q=(1,0,0) as in the low-field phase, or a different one q=0 depending on
the parameters. In the latter case, distortion of the crystal perpendicular to
the (111) axis is predicted. The corresponding anomaly in elastic constants
should also appear. The electrical resistivity is calculated with account of
scattering within the CEF quasi-quartet. It is found that the resistivity as a
function of the direction of magnetic field shows a sharp maximum around the
(111) axis at low temperatures because of the level crossing.Comment: 16 pages, 5 figure
Novel features in the flux-flow resistivity of the heavy fermion superconductor PrOsSb
We have investigated the electrical resistivity of the heavy fermion
superconductor PrOsSb in the mixed state. We found unusual double
minima in the flux-flow resistivity as a function of magnetic field below the
upper critical field for the first time, indicating double peaks in the pinning
force density (). Estimated at the peak exhibits
apparent dependence on applied field direction; composed of two-fold and
four-fold symmetries mimicking the reported angular dependence of thermal
conductivity (). The result is discussed in correlation with the double
step superconducting (SC) transition in the specific heat and the multiple
SC-phases inferred from the angular dependence of .Comment: 5 pages, 7 figures, to appear in J. Phys. Soc. Jpn. Vol. 74, No. 6 or
Spin-Dependent Mass Enhancement under Magnetic Field in the Periodic Anderson Model
In order to study the mechanism of the mass enhancement in heavy fermion
compounds in the presence of magnetic field, we study the periodic Anderson
model using the fluctuation exchange approximation. The resulting value of the
mass enhancement factor z^{-1} can become up to 10, which is significantly
larger than that in the single-band Hubbard model. We show that the difference
between the magnitude of the mass enhancement factor of up spin (minority spin)
electrons z^{-1}_up and that of down spin (majority spin) electrons z^{-1}_down
increases by the applied magnetic field B//z, which is consistent with de
Haas-van Alphen measurements for CeCoIn_5, CeRu_2Si_2 and CePd_2Si_2. We
predict that z^{-1}_up >z^{-1}_down in many Ce compounds, whereas z^{-1}_up <
z^{-1}_down in Yb compounds.Comment: 5 pages, 4 figure
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