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

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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.