ENDOR and Optical Studies of Fractional S = 1/2 Spins and Haldane Gap in S = 1 Antiferromagnetic Linear Chains

本研究在多种不同材料中对S=1线性反铁磁链进行了一系列扩展的EPR和光学实验。这些闭合时具有单线态基态和激发谱间隙（霍尔丹间隙）的链代表了当前对量子无序基态的强烈兴趣。开链具有在链端显示分数自旋S=1/2（自旋子）的非常新颖的性质。电子-核双共振（ENDOR）将用于绘制自旋1/2在链端延伸的位置上的空间分布。在各种含镍链的有机和无机化合物中，用少量的非磁性离子代替镍离子（S=1）可以产生链断裂和带有自旋子的开放端。人们还将努力通过光学泵出一些Ni离子的基态来制造这些断链。通过这种方式，可以通过改变光强度来改变链的长度，并且可以观察到自旋子之间的相互作用。此外，Francis Bitter实验室的合作者正在进行拉曼散射，以测量k=0时的霍尔丹间隙，以帮助解决一些重要的问题，包括它的性质和相对于k=..磁性中心排列成链的化合物的磁性行为，链之间的相互作用可以忽略不计，代表了理论磁学与创造性材料合成的前沿课题的交叉点。直到最近，这种理想的链式化合物还不存在，因为很难保持足够远的磁链来抑制链之间的相互作用，最终导致三维磁性行为。然而，近年来合成的新材料显示了这种理想的一维磁性。首席研究员与at&t贝尔实验室的材料科学家密切合作，后者正在为上述实验创造这些材料。希望对这些一维系统的进一步了解将导致对二维系统的进一步了解，并通过扩展到高温超导。

This research involves an extended series of EPR and optical experiments on S=1 linear antiferromagnetic chains in a variety of different materials. These chains which when closed have a singlet ground state and a gap in their excitation spectrum (the Haldane gap) are representative of the strong current interest in quantum disordered ground states. Open chains have the very novel property of displaying fractional spin, S=1/2, (spinons) at the chain ends. Electron- nuclear double resonance (ENDOR) will be used to map out the spatial distribution of the spin 1/2 on the sites extending in from the chain ends. Breaks in the chains and resulting open ends with spinons will be created by substituting small amounts of non- magnetic ions for the Ni ions(S=1) in a variety of organic and inorganic compounds with Ni chains. Efforts will also be made to create these chain breaks by optically pumping some of the Ni ions out of their ground state. In this way chain lengths can be varied by changing the light intensity and interaction between the spinons may be observed. In addition Raman scattering is underway with collaborators at The Francis Bitter lab to measure the Haldane gap at k=0 to help resolve some of the important questions involving its nature and magnitude relative to the gap at k=.. %%% The magnetic behavior of compounds in which the magnetic centers are arrayed in chains with negligible interaction between the chains represents an intersection of a forefront topic in theoretical magnetism with creative materials synthesis. Until recently such ideal chain compounds did not exist as it was difficult to keep the magnetic chains far enough apart to inhibit interaction among the chains, ultimately resulting in three dimensional magnetic behavior. However, new materials have been synthesized in recent years which display this ideal one- dimensional magnetic behavior. The principal investigator works closely with materials scientists at AT&T Bell Laboratories who are creating these materials for the experiments mentioned above. It is hoped that an enhanced understanding of these one dimensional systems will lead to further insight into two-dimensional systems and by extension to high temperature superconductivity.