Effects of Impurities and Magnetic Field on Competition of Multiple Spin Exchange Interaction in Two-Dimensional Quantum Solid

吸附在石墨衬底上的氦-3固体膜被广泛认为是一个几乎理想的二维量子自旋系统。另一方面，它又具有非常强烈的挫败感。本研究在零磁场和弱磁场条件下测量了氦-3固体薄膜的热容和热导率，得到了以下新的结果：1）氦-3薄膜的热容在弱磁场条件下也受到强烈的影响，弱磁场的影响比交换作用弱十多倍.在反铁磁薄膜中，自旋能隙似乎是由磁场引入的。在铁磁薄膜中，短程自旋有序被磁场抑制。2）氦-3薄膜与石墨衬底之间的热传递是通过局部的斑点进行的，这些斑点应该是石墨衬底中的杂质。在氦-3膜中，热通过声子和自旋激发传递。这些信息不仅在低温应用中是有价值的。测量的热导给出了自旋相关长度随温度变化的信息。3）最近提出的第二层4/7相的吸附结构给出了通过在4/7相中用氦-4取代第二层氦-3的四分之一来实现S=1/2量子自旋的可能性。对氦3 -氦4混合膜的热容测量结果表明，这种混合膜是kagome晶格。它们也同意理论预言的短程自旋有序在kagome晶格上的生长比在三角晶格上慢。

Helium-3 solid film adsorbed on a graphite substrate is widely accepted to be almost ideal two-dimensional quantum spin system. On the other hand, it possesses extraordinary strong frustration. In this research, heat capacities and thermal conductivities of helium-3 solid films have been measured in zero or weak magnetic fields, and some following new information has been revealed.1) Heat capacity of helium-3 films is strongly affected even by weak magnetic field, which is weaker than the exchange interaction more than one decade. In the antiferromagnetic film, spin gap seems to be introduced by the magnetic field. In the ferromagnetic films, short range spin ordering is suppressed by the magnetic filed. These anomalous behaviors might arise from strong frustration.2) Between helium-3 films and graphite substrates, heat is transferred only by local spots, which should be impurities in graphite substrates. Within the helium-3 films, heat is transferred by phonons and spin excitations. This information is valuable not only in cryogenics application. The measured thermal conductance gives information about the temperature variation of the spin correlation length.3) The adsorption structure of the second layer 4/7 phase, which has recently been proposed, gives a possibility of a realization of S=1/2 quantum spins on kagome lattice by replacing a quarter of second layer helium-3 with helium-4 in the 4/7 phase. Heat capacity measurements of such helium 3 - helium 4 mixture film strongly suggest the realization of kagome lattice. They also agree with theoretical predictions that growth of short range spin ordering on kagome lattice is slower than on triangular lattice.