  Quantum phase transitions have captured the interest of a large community in
condensed-matter and atom physics research. The common feature of these very
different material classes lies in the fact that the competition between
low-energy scales can be tuned by a nonthermal parameter, such as pressure,
magnetic or electric field, and chemical composition for the condensed-matter
systems. In heavy-fermion materials, the strong exchange J between f-electrons
and conduction electrons can lead to quenching of the f-electron-derived
(nearly) localized magnetic moments via the Kondo effect or, if J becomes
weaker, to long-range magnetic order via the Ruderman-Kittel-Kasuya-Yosida
interaction mediated by the conduction electrons. In addition it has been
suggested that magnetic order can be suppressed by quantum fluctuations which
may be enhanced by geometric frustration. Here we report on the observation of
a quantum phase transition in a partially frustrated antiferromagnetic metallic
system. In antiferromagnetic CePdAl the magnetic Ce ions form a network of
equilateral triangles in the (001) plane, similar to the kagom\'e lattice, with
one third of the Ce moments not participating in long-range order. The N\'eel
temperature T_N = 2.7 K can be driven to zero upon replacing 14.4% of Pd by Ni.
Here the specific heat C exhibits a C/T ~ - log T dependence. Within the
Hertz-Millis-Moriya model of quantum criticality, this behavior can be
attributed to two-dimensional critical antiferromagnetic fluctuations arising
from the decoupling of three-dimensional magnetic order by frustration. The
intermediate planes of frustrated moments are a possible candidate for a
two-dimensional spin-liquid. The simultaneous presence of magnetic order,
geometric frustration, and Kondo effect in this system might thus entail a new
route to quantum criticality.
