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Quantum phase transitions have captured the interest of a large community incondensed-matter and atom physics research. The common feature of these verydifferent material classes lies in the fact that the competition betweenlow-energy scales can be tuned by a nonthermal parameter, such as pressure,magnetic or electric field, and chemical composition for the condensed-mattersystems. In heavy-fermion materials, the strong exchange J between f-electronsand conduction electrons can lead to quenching of the f-electron-derived(nearly) localized magnetic moments via the Kondo effect or, if J becomesweaker, to long-range magnetic order via the Ruderman-Kittel-Kasuya-Yosidainteraction mediated by the conduction electrons. In addition it has beensuggested that magnetic order can be suppressed by quantum fluctuations whichmay be enhanced by geometric frustration. Here we report on the observation ofa quantum phase transition in a partially frustrated antiferromagnetic metallicsystem. In antiferromagnetic CePdAl the magnetic Ce ions form a network ofequilateral triangles in the (001) plane, similar to the kagom\'e lattice, withone third of the Ce moments not participating in long-range order. The N\'eeltemperature 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 theHertz-Millis-Moriya model of quantum criticality, this behavior can beattributed to two-dimensional critical antiferromagnetic fluctuations arisingfrom the decoupling of three-dimensional magnetic order by frustration. Theintermediate planes of frustrated moments are a possible candidate for atwo-dimensional spin-liquid. The simultaneous presence of magnetic order,geometric frustration, and Kondo effect in this system might thus entail a newroute to quantum criticality.

Approaching quantum criticality in a partially geometrically frustrated heavy-fermion metal