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

新的量子反铁磁链--自旋梯形系统吸引了许多理论和实验的兴趣。偶数阶梯被认为是一个自旋能隙系统，并且具有短程共振价键(RVB)基态。化合物Cu2(C5H&lt；12&gt；N_2)_2Cl4(以下简称CHPc)是一个典型的两腿阶梯体系，阶梯的阶梯(J_⊥/k=13.2K)和支腿(J_1t；//&gt；/k=2.4K)具有交换作用，其自旋能隙能为Δ/k=1.5K。结果表明，化合物CHpB是一个类似CHPC的自旋梯体系，其交换作用J_⊥/k=24.8K，J_&lt；//&gt；/k=3.5K，自旋隙能Δ/k=21.3K。Brx)4似乎是一个具有随机反铁磁键的梯形体系。…的磁性更多的无规混合化合物Cu2(C5H&lt；12&gt；N_2)_2(Cl_&lt；1-x&gt；Brx)_4与纯体系有很大的不同。低温下的磁化率存在明显的顺磁发散，即使在低场下也会出现磁化现象。对于随机系统，极化率不遵循居里定律，并且比居里定律发散得更强。具有较强随机性的系统处于随机单重态相或在低温下磁化率发散的Griffice相。在低温下测量了纯体系和无规体系的热容。在纯系统的情况下，热容的行为是指数型T^2exp(-Δ/kT)，其中Δ是集体激发的能隙。对于随机体系，在外场作用下，其低温热容表现出与纯体系相似的行为，其能隙随H的增加而减小。结果表明，随机键梯的基态为Griffiths相。较少

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