μSR and NMR Study on Quantum Critical Point in Quantum Spin Ladder (CH3)2CHNH3Cu(ClxBr1-x)3

(CH3)2CHNH3Cu(ClxBr1-x)3 abbreviated IPACu(ClxBr1-x)3 is a mixed system of spin-gap compounds IPACuCl3 and IPACuBr3. 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 xc = 0.44, by Muon Spin Relaxation (?SR) and NMR measurements. For x = 0.40, ?SR 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, ?SR and NMR measurements do not show any magnetic order at low temperatures, indicating that the QCP is larger than the reported value of xc = 0.44.