Mechanical Control of Spin States and Conductance Achieved in Cobalt Complex
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Mechanical Control of Spin States and Conductance Achieved in Cobalt Complex Progress in the ability to fabricate controllable single-molecule electrical devices has enabled these devices to be used as scientific tools to perform detailed measurements of electron correlations on nanometer-length scales. Recently, D.C. Ralph of Cornell University; T.A. Costi of Forschungszentrum Jülich, Germany; P.S. Cornaglia of Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Argentina; and their colleagues have studied the spin states in single molecules attached to gold electrodes and the Kondo effect by which the molecular spin can be screened by electrons, as a function of stretching the molecule. Electronic states depend on symmetry; for example, a complex composed of a transition metal and ligands breaks spherical symmetry and splits the metal’s originally degenerate d orbitals. For spin S > 1,
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distortion of the ligand geometry causes additional splitting of the (2S + 1)-degenerate spin states, resulting in magnetic anisotropy. The researchers modified the symmetry of a spin S = 1 Co complex by controllably stretching it, and simultaneously measuring the current flowing through it. They were able to use mechanical control of the Co complex to demonstrate the underscreened Kondo effect, where conduction electrons only partially screen the molecular spin, from S = 1 to 1/2. As reported in the June 11th issue of Science (DOI: 10.1126/science.1186874; p. 1370), the researchers used mechanically controllable break-junction devices to stretch individual Co(tpy-SH)2 complexes (where tpy-SH is 4′-mercapto2,2′:6′,2′′-terpyridine). The researchers measured differential conductance (dI/dV) as a function of increasing electrode sep
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