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

证明了一种吸附在石墨上的低密度固体^3He薄膜是三角晶格上最理想的二维S=1/2反铁磁体之一。由于^3He原子之间的硬核电位，高阶多自旋交换（MSE）过程以及双粒子交换发挥了重要作用。偶数粒子的交换是反铁磁性的（AFM），奇数粒子的交换是铁磁性的（FM）。它们之间的竞争，加上几何上的挫败感，使得这个系统产生了强烈的挫败感。最近对有限系统精确对角化的计算预测了一种可能的具有激发间隙的“量子自旋液体”基态。对吸附在两层和三层HD预镀石墨上的亚单层^3He的核磁共振研究已降至100μK，比交换能(J)小一个数量级以上。在高度受挫的反铁磁固体区，即使在J附近的温度下，磁化强度也遵循居里-魏斯定律，然后逐渐增加到100 mK，但没有表现出与自旋间隙存在相对应的异常行为。归一化磁化强度与还原温度的比值（T/J）与凝固后的密度无关。这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.