Correlated electron phenomena in Ce- and Pr- based filled skutterudite
arsenides and antimonides
M. B. Maple, R. E. Baumbach, T. A. Sayles, N. P. Butch, P.-C. Ho,
T. Yanagisawa and W. M. Yuhasz
Department of Physics and Institute for Pure and Applied Physical Sciences
University of California, San Diego
La Jolla, CA 92093 USA
Z. Henkie, R. Wawryk, T. Cichorek and A. Pietraszko
Institute of Low Temperature and Structure Research
Polish Academy of Sciences, 50-950 Wroclaw, Poland
Abstract
The filled skutterudite compounds MT4X12 (M = alkali metal, alkaline earth, lanthanide, actinide; T
= Fe, Ru, Os; X = P, As, Sb) display a wide range of correlated electron phenomena including
conventional BCS superconductivity, unconventional superconductivity, magnetic order,
quadrupolar order, valence fluctuation behavior, heavy fermion behavior, non-Fermi liquid
behavior, and hybridization gap insulator behavior. In this talk, we describe recent experiments
on single crystals of ternary Ce- and Pr-based filled skutterudite arsenides and pseudoternary Pr-
based skutterudite antimonides Pr(Os1-xRux)4Sb12and Pr1-xNdxOs4Sb12. The ground states of these
compounds are determined by a delicate interplay between hybridization of localized 4f- and
ligand states of the pnictogen atoms that form the atomic cages within which the lanthanide ion
resides, crystalline electric field splitting of the lanthanide 4f energy levels, magnetic and
quadrupolar interactions, and electron band structure. For example, CeRu4As12 exhibits non-
Fermi liquid (NFL) behavior at low temperatures, suggesting the proximity to a quantum critical
point (QCP), similar to that previously observed in CeRu4Sb12. The Ce ions appear to have an
intermediate valence between 3+ and 4+ in CeRu4As12 and a nearly integral valence of 3+ in
CeRu4Sb12. This suggests that the appropriate description of the NFL behavior may be a valence
fluctuation picture for CeRu4As12 and a Kondo lattice scenario for CeRu4Sb12. Since many of the
other Ce-based skutterudites (e.g., CeFe4P12, CeOs4As12, CeOs4Sb12) are apparently
hybridization gap semiconductors (or Kondo insulators), the NFL behavior of CeRu4X12 (X = As,
Sb) could arise from proximity to a metal-insulator transition. The heavy fermion compound
PrOs4Sb12 exhibits unconventional strong coupling superconductivity below Tc = 1.85 K that
breaks time reversal symmetry, apparently consists of several distinct superconducting phases,
some of which may have point nodes in the energy gap, and may involve triplet spin pairing of
electrons. A high field ordered phase, identified with antiferroquadrupolar order, occurs in
PrOs4Sb12 between 4.5 T and 16 T and below ~1 K. Magnetic and quadrupolar excitations
between the Pr3+ singlet ground state and a low-lying triplet first excited state, with a splitting D12 ~
7 K, may play a role in the heavy fermion behavior and unconventional superconductivity in
PrOs4Sb12. In the Pr(Os1-xRux)4Sb12 system, D12 increases with x to ~70 K at x = 1, the Tc vs x
curve exhibits a minimum at x ≈ 0.6, and the superconductivity apparently changes from
unconventional to conventional BCS at x ≈ 0.3. In the Pr1-xNdxOs4Sb12 system, Tc decreases
linearly with x in the superconducting region below xc ≈ 0.5, while the Curie temperature qc
increases linearly with x in the ferromagnetic region above xc ≈ 0.5. The compound PrOs4As12
undergoes transitions at 2.3 K and 2.2 K in zero-field into two ordered phases, of which the one
below 2.2 K is antiferromagnetic, that can be suppressed to 0 K with magnetic fields of 2 T and
3.2 T. The temperature and field dependences of the specific heat and electrical resistivity
indicate that PrOs4As12 is a Kondo lattice system with a small Kondo temperature TK of ~1 K and
an electronic specific heat coefficient of ~1 J/mol K2. The compound PrFe4As12 is ferromagnetic
with qc ≈ 18 K, while PrRu4As12 is a BCS superconductor with Tc ≈ 2.4 K. Research at UCSD was
supported by the US Department of Energy under Grant No. DE FG02-04ER46105 and the
National Science Foundation under Grant No. 0335173.

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