Microscopic Phase Separation in Triangular-Lattice Quantum Spin Magnet κ-(BEDT-TTF)2Cu2(CN)3 Probed by Muon Spin Relaxation

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