The study on a new quantum-spin phase in a quantum ladder system

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New quantum antiferromagnetic chains, spin-ladder systems, have attracted a lot of theoretical and experimental interest. An even-leg ladder is believed to be a spin-gap system and to have a short-range resonating valence bond (RVB) ground state. The compound Cu_2 (C_5H_<12>N_2)_2Cl_4 (hereafter, CHpC) is a typical two-leg ladder system with exchange interaction for the rungs (J_⊥ /k=13.2K) and for the legs (J _<//>/k=2.4K) of the ladder and a spin-gap energy of Δ/k=l0.5K.Recently, we reported experimental results of a new compound Cu_2(C_5H_<12>N_2)_2Br_4 (hereafter, CHpB) of which crystal structure is isostrucural with CHpC.The results suggest that the compound CHpB is a spin-ladder system with a spin-gap like CHpC.We estimated the exchange interactions J _⊥/k=24.8K, J _<//>/k=3.5K and a spin-gap energy of Δ/k=21.3K.Therefore, the random mixture compound Cu_2(C_5H_<12>N_2)_2 (Cl_<1-x>Br_x)_4 seems to be a ladder system with random antiferromagnetic bonds. The magnetic properties of a … More random mixture compound Cu_2(C_5H_<12>N_2)_2 (Cl_<1-x>Br_x)_4 are rather different from those of the pure systems. Paramagnetic divergence is significantly observed for the susceptibility at low temperatures and the magnetization appears even at low fields. For random systems, the susceptibilities do not follow the Curie law and diverge stronger than the Curie law. The systems with strong randomness are in a random singlet phase or are in a Griffice phase of which susceptibility diverges at low temperatures. The heat capacity of both pure and random systems was measured at low temperatures. In the cases of pure systems, the behavior of heat capacities is of the exponential type T^2exp (-Δ/kT), where Δ is the gap energy for the collective excitation. For random systems, the heat capacity at low temperatures shows similar behavior observed for pure systems in applied field, of which gap energy decreases with increasing H.The results suggest that the ground state of the random-bond ladder is a Griffiths phase. Less