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固体薄膜被广泛认为是近乎理想的二维量子自旋系统。另一方面，它拥有异常强烈的挫折感。本研究对氦-3固体薄膜在零磁场和弱磁场下的热容和热导率进行了测量，发现：1)弱磁场对氦-3固体薄膜的热容有很强的影响，弱磁场的影响比交换作用弱十多年。在反铁磁性薄膜中，自旋隙似乎是由磁场引入的。在铁磁性薄膜中，磁场抑制了短程自旋有序化。这些反常行为可能是由于强烈的挫折感造成的。2)在氦-3薄膜与石墨衬底之间，热量只通过局部斑点传递，这应该是石墨衬底中的杂质。在氦-3薄膜中，热通过声子和自旋激发进行传递。这些信息不仅在低温应用上有价值。3)最近提出的第二层4/7相的吸附结构提供了在Kagome晶格上实现S=1/2量子自旋的可能性，方法是用4/7相中四分之一的第二层氦-3取代1/4。这种氦3-氦4混合薄膜的热容测量有力地表明了Kagome晶格的实现。它们也与Kagome晶格上短程自旋有序的增长慢于三角晶格的理论预言一致。

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.