The ground state of the quantum spin system κ-(BEDT-TTF) 2 Cu 2 (CN) 3 in which antiferromagnetically-interacting S=½ 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 μSR, we have investigated its spin dynamics below 0.1 K, unveiling its microscopically phase separated ground state at zero field.KEYWORDS: triangular lattice, frustration, μSR, quantum spin antiferromagnet, organic superconductorsThe ground state of the antiferromagnetic spin systems on the equilateral triangular lattice attracts considerable interest, because the geometrical frustration inhibits a simple magnetic order, so that appearance of an exotic ground state is expected [1][2][3][4][5][6][7] . The organic molecular charge-transfer salt κ-(BEDT-TTF) 2 Cu 2 (CN) 3 , where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene, is a Mott insulator and considered to be a quantum Heisenberg antiferromagnet on the triangular lattice. The salt contains two dimerized BEDT-TTF in a unit cell, and one hole carrier localizes on each dimer due to strong Coulomb repulsive onsite correlations. Then the system can be considered as a dimer Mott insulator.An S=½ localized spin on each localized hole forms a nearly equilateral triangular lattice reflecting a characteristic κ-type arrangement of BEDT-TTF molecules.So far, 1 H/ 13 C-NMR 8,9) and specific heat measurements 10) have shown that there is no magnetic order at low temperatures down to 20 mK, which is much lower than the exchange interaction J ≈ 250 K, though it maintains gapless. This result has been believed to be quite legitimate in that the frustration effect may suppress a possible magnetic order. However, quite recently M. Yamashita et al. 11) has found that the thermal conductivity shows the thermal-activation-type temperature dependence, suggesting a gapped ground state with Δ=0.46 K. This observation is calling back many arguments on its ground state until now.Furthermore, the gap survives the field of 10 T, though the thermal conductivity depends significantly on magnetic field 11). Furthermore, NMR spectra and transverse-field (TF) μSR 13) at such high fields report the existence of field-induced static moments 9,12) . Recently, an anomalous charge degrees of freedom is probed in this system as a dipole glass-like dielectric response at T < 50 K. In addition, the freezing of charge distribution fluctuation is clearly observed at T = 6 K 17). The anomaly is theoretically argued to be a segregation of the charge in the half-filled HOMO onto either side of the BEDT-TTF molecules in the dimer 14,15) . At the same temperature, Mann...

CH 3 ) 2 CHNH 3 Cu(Cl x Br 1-x ) 3 abbreviated IPACu(Cl x Br 1-x ) 3 is a mixed system of spin-gap compounds IPACuCl 3 and IPACuBr 3 . This mixed system is reported by macroscopic measurements to show a magnetic order when the value of x is between the quantum critical points (QCPs) 0.44 and 0.87. We have investigated microscopically the ground state of the samples x = 0.40, 0.45 and 0.50 which are located near the QCP of x c = 0.44, by Muon Spin Relaxation (mSR) and NMR measurements. For x = 0.40, mSR measurements in zero and longitudinal fields have shown that the spin fluctuation shows a significant slowing down with decreasing temperature, although the magnetic order does not appear down to T = 15 mK, indicating that the ground state is an exotic one rather than a gapped one. For x = 0.45 and 0.50, mSR and NMR measurements do not show any magnetic order at low temperatures, indicating that the QCP is larger than the reported value of x c = 0.44.

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Solid solution of the two spin gap systems (CH 3) 2 CHNH 3 CuX 3 (X=Cl, Br) was reported from results of macroscopic measurements to be gapless when fraction of Cl content x is in between 0.44 and 0.87, and otherwise remains gapped. We have investigated by µSR the ground state of Cl-rich gapped region x=0.95 and 0.88 to find that they are microscopically phase separated into two components, magnetic tiny islands and singlet sea surrounding them. With decreasing temperature, the characteristic frequency of spin fluctuation in islands showed a significant slow down. However, the slow down ceased around 1 K, below which the spin fluctuation spectrum did not change down to 15 mK. These results indicate that unlike the Brrich system x <0.44 where appearance of an exotic ground state has been argued, the Cl-rich system x >0.87 persists to be paramagnetic without showing a long-range order. This difference is considered to be originated in the robustness of Haldane gap against disorder.