Metallic Spin-Liquid Behavior of the Geometrically Frustrated Kondo Lattice Pr
2
Ir
2
O
7
S. Nakatsuji,
1
Y. Machida,
1
Y. Maeno,
1,2
T. Tayama,
3
T. Sakakibara,
3
J. van Duijn,
4
L. Balicas,
5
J. N. Millican,
6
R. T. Macaluso,
6
and Julia Y. Chan
6
1
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
2
International Innovation Center, Kyoto University, Kyoto 606-8501, Japan
3
Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa 277-8581, Japan
4
ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom
5
National High Magnetic Field Laboratory (NHMFL), Tallahassee, Florida 32310, USA
6
Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803, USA
(Received 23 July 2005; published 3 March 2006)
Strongly frustrated magnetism of the metallic pyrochlore oxide Pr
2
Ir
2
O
7
has been revealed by single
crystal study. While Pr 4f moments have an antiferromagnetic RKKY interaction energy scale of jT
j
20 K mediated by Ir 5d-conduction electrons, no magnetic long-range order is found except for partial
spin freezing at 120 mK. Instead, the Kondo effect, including a lnT dependence in the resistivity, emerges
and leads to a partial screening of the moments below jT
j. Our results indicate that the underscreened
moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.
DOI: 10.1103/PhysRevLett.96.087204 PACS numbers: 75.20.Hr, 75.40.Cx, 75.50.Ee
Geometrically frustrated magnets have attracted great
interest because of the possible emergence of novel mag-
netic phases at low temperatures resulting from the sup-
pression of conventional order. Among them, the three-
dimensional pyrochlore lattice of corner sharing tetrahedra
has been studied extensively [1]. It is predicted theoreti-
cally that Heisenberg spins on a pyrochlore lattice with
nearest-neighbor antiferromagnetic (AF) coupling form a
spin-liquid state at T 0K [2]. However, only a few
compounds are believed to display a spin-liquid phase,
such as the insulator Tb
2
Ti
2
O
7
[3].
In metallic systems, the frustration inherent to the pyro-
chlore lattice might also lead to new types of electronic
behavior. One remarkable possibility is the predominance
of the Kondo effect, and concomitant heavy-fermion be-
havior, in nearly localized d- and f-electron systems where
the Kondo temperature is generally too small to overcome
magnetic order without the frustration. Prominent ex-
amples are the heavy-fermion behavior in LiV
2
O
4
and
YScMn
2
with itinerant d-electron spins on a pyrochlore
lattice [4,5].
Connecting the two exotic states of frustrated magnets,
insulating spin-liquid and itinerant heavy fermions, there is
another exciting yet unprecedented possibility of metallic
spin liquid [6,7]. Ground states in f-electron based Kondo
lattices are generally classified into Fermi liquid and mag-
netic regimes as the result of the competition between the
Kondo effect and RKKY interactions. If the lattice has
geometrical frustration and the transition temperature is
depressed, the underscreened moments may stay disor-
dered even in the magnetic regime, and form a metallic
spin liquid on the geometrically frustrated Kondo lattice.
(See the inset of Fig. 1.)
There has been a number of reports on metallic systems
among the A
2
B
2
O
7
pyrochlore oxides possessing localized
moments [1]. Yet, none is known to remain magnetically
disordered down to the lowest temperatures except for the
newly developed pyrochlore iridates [8]. In particular, the
AF correlated Pr 4f moments of Pr
2
Ir
2
O
7
remain para-
magnetic down to at least 0.3 K in the metallic state due to
the Ir 5d-conduction bands [8]. This places Pr
2
Ir
2
O
7
as a
candidate for a geometrically frustrated Kondo lattice.
Here we report on strongly frustrated magnetism in
single crystals of Pr
2
Ir
2
O
7
. We find that the h111i Ising-
like Pr
3
moments have an AF RKKY interaction energy
scale jT
j 20 K. However, the dc magnetization down to
70 mK does not exhibit any trace of long-range order
(LRO), except for an indication of partial freezing at
120 mK. Instead, the Kondo effect emerges below jT
j
and leads to a partial screening of the 4f moments, re-
Temperature
Fermi
Liquid
AF
Order
Spin
Liquid
Control Parameter
0
1
2
0
200
400
600
0 100 200 300
[100]
[110]
[111]
1/(χ χ -
0
) (mole-Pr/emu)
T (K)
ρ (mΩcm)
Pr
2
Ir
2
O
7
FIG. 1 (color online). Zero-field resistivity T (left axis), and
the inverse of susceptibility
0
1
T (right axis) measured
under a field of 100 mT along 100, 110, and 111. The solid
line represents a fit to the Curie-Weiss law, while the broken line
indicates
CEF
vv
1
based on the crystal electric field
analysis. Inset: the schematic phase diagram for geometrically
frustrated Kondo lattices.
PRL 96, 087204 (2006)
PHYSICAL REVIEW LETTERS
week ending
3 MARCH 2006
0031-9007= 06=96(8)=087204(4)$23.00 087204-1 © 2006 The American Physical Society