  The ground state of the quantum spin system kappa-(BEDT-TTF)2Cu2(CN)3 in
which antiferromagnetically-interacting S=1/2 spins are located on a nearly
equilateral triangular lattice attracts considerable interest both from
experimental and theoretical aspects, because a simple antiferromagnetic order
may be inhibited because of the geometrical frustration and hence an exotic
ground state is expected. Furthermore, recent two reports on the ground state
of this system have made it further intriguing by showing completely
controversial results; one indicates the gapless state and the other gapped. By
utilizing microscopic probe of muSR, we have investigated its spin dynamics
below 0.1 K, unveiling its microscopically phase separated ground state at zero
field.
