We investigate a system of three tunnel-coupled semiconductor quantum dots in
a triangular geometry, one of which is connected to a metallic lead, in the
regime where each dot is essentially singly occupied. Both ferro- and
antiferromagnetic spin-1/2 Kondo regimes, separated by a quantum phase
transition, are shown to arise on tuning the interdot tunnel couplings and
should be accessible experimentally. Even in the ferromagnetically-coupled
local moment phase, the Kondo effect emerges in the vicinity of the transition
at finite temperatures. Physical arguments and numerical renormalization group
techniques are used to obtain a detailed understanding of the problem.Comment: 5 pages, 5 figure
