Quantum Phase Transition of Triangular Spin Tube

研究了我们发现的首个三角形自旋纳米管[CuCl_2tachH]_3Cl]Cl_2的磁激发和量子相变。在超低温比热中，我们发现了由两组分组成的复激发。第一种是缺口激励，可以用伊辛链模型拟合。第二种是具有小态密度的无间隙激发。结果表明，自旋管内存在两种激励。间隙激发的起源是具有伊辛性质的自旋手性激发。这可能与低温下自旋手性的排序有关。第二次激发是由类似自旋波的激发引起的，然而，它的比例只有自旋链的25%或更少。结果表明，该自旋管的基态是无间隙的，具有相当大的能量重整化。为了研究自旋管的相图，我们制作了一个d取代的样品，并成功地在链方向上调制了交换耦合。结果表明，通过d -取代，交换偶联率提高了7%。调制的来源是抑制质子隧道的氢键连接相邻三角形在链方向。在核磁共振中，在极低温度下未发现磁异常，并证实了管间的良好分离。本研究表明，量子自旋管中磁性能的变化是由自旋和自旋手性两个自由度的存在引起的。

The purpose of the research is to investigate the magnetic excitation and the quantum phase transition of a triangular spin nano-tube:[CuCl_2tachH]_3Cl]Cl_2:the first quantum spin tube found by us. In the specific heat at ultra-low temperature, we found complex excitations made of two components. The first one is a gapped excitation, which can be fitted by an Ising chain model. The second one is a gap-less excitation with a small density of state. The result indicates that two types of excitations coexist in a spin tube. The origin of the gapped excitation is the excitation of spin chirality holding the Ising nature. This may be associated with the ordering of spin chirality at low temperature. The second excitation is caused by a spin wave like excitation, however, the fraction of this is only 25 % or less of that of a spin chain. It shows that the ground state of the present spin tube is gapless with considerable renormalization of energy.To investigate the phase diagram of spin tube, we made a D-substituted sample and succeeded in modulating the exchange coupling in the chain direction. It is found that the exchange coupling in increased by 7 % by D-substitution. The origin of the modulation is the suppression of proton tunneling in hydrogen bonds connecting neighboring triangles in chain direction. In NMR, no magnetic anomaly is found down to very low temperature and the good separation between the tubes are confirmed.The present investigation indicates the variety of magnetic properties in a quantum spin tube, which is caused by the existence of two degrees of freedom: spin and spin chirality.