Experimental investigation of quantum phase transition on one-dimensional spin gapped system

研究了典型的S=1/2海森堡交替自旋链体系；五氟苯基硝基氮氧化物（F_5PNN）和具有两个临界磁场区的新型磁系统；（CH_3）_2NH_2CuCl_3 （DMACuCl_3）研究了自旋间隙体系的场诱导相变和Tomonaga-Luttinger液相。在F_5PNN中，我们发现在T=250mK下，Hc1=3T和Hc2=6.5T之间存在场感应磁有序。在磁场感应相的H-T相边界外，观察到的特征行为取决于高达Hc2以上的磁场。在S=1/2自旋间隙体系中，比热对温度和场的依赖关系与理论计算结果在定性上是一致的。在DMACuCl_3中，在Hc1=2T以下存在自发磁有序，而在Hc1=2T至Hc2 =3.5T的1/2磁化平台区不存在磁有序。在Hc2=3.5T以上出现磁场感应磁有序现象，直至H=8T，磁场感应磁有序随着磁场的增大而增大。

We have studied a typical S=1/2 Heisenberg alternating spin chain system ; pentafulorophenyl nitronyl nitroxide (F_5PNN) and a novel magnetic system with two critical field regios ; (CH_3)_2NH_2CuCl_3 (DMACuCl_3) to investigate field induced phase transitions and Tomonaga-Luttinger liquid phase in spin-gapped system.In F_5PNN, we found a field induced magnetic ordering between Hc1=3T and Hc2=6.5T below T=250mK. Characteristic behaviors are observed depending on the magnetic field up to above Hc2 outside of the H-T phase boundary for the field induced phase. The temperature and field dependence of the specific heat are qualitatively in good agreement with the theoretical calculation on an S=1/2 spin gapped system.In DMACuCl_3,a spontaneous magnetic ordering is observed below Hc1=2T, while no magnetic ordering is in the 1/2 magnetization plateau field region from Hc1=2T to Hc2 =3.5T. A field induced magnetic ordering emerges above Hc2=3.5T, which grows with increasing magnetic field up to H=8T.