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

氦-3固体膜被广泛认为是一种理想的二维量子自旋体系。另一方面，它又具有极强的挫败感。本研究测量了氦-3固体薄膜在零磁场和弱磁场下的热容和热导率，揭示了一些新的信息。1)弱磁场对氦-3薄膜热容量的影响较大，弱于十多年的交换作用。在反铁磁薄膜中，自旋间隙似乎是由磁场引入的。在铁磁薄膜中，磁场抑制了短程自旋有序。这些异常行为可能是由强烈的挫败感引起的。2)氦-3薄膜与石墨衬底之间仅通过局部斑点传递热量，这些斑点应该是石墨衬底中的杂质。在氦-3薄膜中，热量通过声子和自旋激发传递。这一信息不仅在低温应用方面有价值。测量的热导率给出了自旋相关长度的温度变化信息。3)最近提出的第二层4/7相的吸附结构，给出了用4/7相中的氦-4取代第二层氦-3的四分之一，在kagome晶格上实现S=1/2量子自旋的可能性。这种氦3 -氦4混合膜的热容测量有力地证明了kagome晶格的实现。他们也同意理论预测，在kagome晶格上短程自旋有序的增长比在三角形晶格上慢。

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.