Magnetism and superfluidity in low dimensional ^3He adsorbed on graphite

吸附在石墨表面的低密度固体~3He膜是最理想的二维三角形晶格S=1/2反铁磁体之一。由于~3He原子之间的核心势，高阶多重自旋交换(MSE)过程以及两粒子交换起着重要的作用。偶数对的交换是反铁磁(AFM)，而奇数对的交换是铁磁(FM)。它们之间的竞争，加上几何上的挫折，使这个系统非常沮丧。最近对有限系统精确对角化的一项计算预测了一种可能的具有激发带隙的“量子自旋液体”基态。用核磁共振方法研究了吸附在两层和三层HD预镀石墨上的亚单层~3He的核磁共振谱，结果表明吸附在两层和三层HD预镀石墨上的亚单层~3He的核磁共振谱最低可达100μK，比交换能J低一个数量级以上。在高度受挫的反铁磁性固体区，磁化强度在J附近遵循居里-魏斯定律，然后逐渐增加到100mK。然而，它并没有表现出与自旋能隙的存在相对应的任何反常行为。归一化磁化强度与还原温度(T/J)的关系与凝固后的密度无关。这一事实与在高温区观察到的主要多自旋交换具有相似的密度依赖性的结果是一致的。

A low density solid ^3He film adsorbed on graphite is proved to be one of the most ideal two dimensional S=1/2 antiferromagnets on a triangular lattice. Due to the hard-core potential between ^3He atoms, the higher order multiple spin exchange (MSE) processes as well as two-particle exchange play important. The exchange of even number of pairticles is antiferromagnetic (AFM), whereas that of odd ones is ferromagnetic (FM). The competition between them, in addition to the geometrical frustration, makes this system strongly frustrated. A recent calculation of the exact diagonalization for a finite system predicts a possible "quantum spin liquid" ground state with an excitation gap. NMR studies of sub-monolayer ^3He adsorbed on two and three layers of HD preplated graphite have been made down to 100μK which is more than one order of magnitude smaller than the exchange energy (J). In the highly frustrated antiferromagnetic solid region, the magnetization obeys a Curie-Weiss law even at the temperatures around J and then increases gradually down to 100 mK. Nevertheless it does not show any anomalous behavior corresponding to the existence of spin gap. The normalized magnetization vs. the reduced temperature (T/J) is independent of the density just after solidification. This fact is consistent with the result observed in the high temperature region that the main multiple spin exchanges have similar density dependence.