Steve Winter
Junior Project Leader


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Spin-Orbit Physics in Organic and Inorganic Materials

In solid state materials, the coupling of electronic spin and orbital degrees of freedom play an important role in many recently proposed and discovered exotic states such as topological insulators, superconductors, and magnetic phases. The realization of such model states in real materials requires an understanding of the relationship between crystal structure and the nature of the spin-orbit effects, as well as synthetic strategies for controlling the resulting interactions. A regime of particular interest occurs when there is a balance of all electronic energy scales, such as spin-orbit coupling, kinetic energy, and Coulomb repulsion between electrons. However, while spin-orbit effects and kinetic energy are typically enhanced in materials composed of heavier atoms, Coulomb repulsion is typically suppressed, so that this balance may be challenging to achieve in real systems.

For inorganic systems, this balance seems to be met in materials of atoms clustered around Iridium in the periodic table. We are working on refining the magnetic and electronic models for such systems through a combination of ab-initio and analytical methods. An alternate design strategy has also recently been suggested by our work on organic materials, for which Coulomb repulsion may be more directly tuned. Through analysis of magnetic resonance spectra of organic radical magnets, we demonstrated that spin-orbit effects play a prominent role in the magnetic properties of sulfur and selenium based organic materials. This result challenges the typical assumption that anisotropic magnetic terms are weak in organic materials. Most recently, we developed an ab-initio scheme that provides accurate estimates of such spin-orbit terms for organic systems, and are working on extending this approach to the dimer Mott insulator systems.


Magnetism and Mott Transition in Multiband Organics

When the strength of Coulomb repulsion exceeds that of the electronic kinetic energy in the solid state, electrons may become localized to their parent atoms or molecules, forming a magnetic Mott insulator. For organic materials, where the crystals are held together by weak van der Waals forces, pressure may be used to greatly compress such samples, significantly enhancing the kinetic energy, and possibly driving an insulator to metal transition. We recently reported a class of organic radicals that undergo such a transition, but it is driven not by simple pressure-induced band widening, but rather by rehybridization of the highest occupied orbitals. Combining results from neutron diffraction, magnetic resonance, thermal expansion and high pressure transport, optical and magnetic measurements a complete picture of this transition is beginning to emerge.

 

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