From Single Ion to Correlation: Probing the Physics of Kondo Topological Insulator with Lattice Constant Engineering

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Non-technical abstract: Wonder what happens to the electronic properties of a material when you stretch it? In an electronic material, the valence of an atom tells us the strength of its combining power with other atoms, and does not usually change with time. In some very special materials, called 'mixed valence compounds', the valence rapidly fluctuates between more than one value. It is recently discovered by the principle investigator that stretching some of these mixed valence compounds could slow down or speed up the valence fluctuation. As a result, the electronic properties are effectively tuned to meet the requirements of device applications. In this project, the research team investigates the effect of stretching on mixed valence in several so-called 'strongly correlated materials'. Cousins of these materials have already found technological importance: high temperature superconductors for MRI diagnosis, giant magneto-resistance materials inside iPads, just to name two. This project pursues crystal growth, measurements and device fabrication on several mixed valence materials to unveil the nature of mixed valence physics and by extension explore their unusual electronic and thermal properties for technological applications such as ultra-high frequency oscillator and quantum metrology. Several educational components are integrated into this project, consisting of course development, K-12 outreach and scientific training for students. Internships for high school volunteers from disadvantaged regions in Santa Ana, as well as undergraduate researcher from UC Irvine's California Alliance for Minority Participation program are enabled by this project to generate interest and prepare for careers in science and technology.Technical Abstract The normally well localized and magnetic 4f-electrons in rare earth compounds can in some materials become itinerant electrons with effective electron mass orders of magnitude larger than that of free electrons, leading variously to unusual 'strongly correlated' superconducting, magnetic and semiconducting states, these last known as Kondo insulators. Theory suggests that the Kondo insulator SmB6 belongs to the exotic class of so-called topological insulators possessing a surface conducting state with interesting technological potential. SmB6 is at present the only such strongly correlated f-band semiconductor for which experiment finds good evidence for this. The semiconducting energy gap scale in SmB6 is approximately 40K with the surface conducting state developing only below 4K. The principal investigators find that uniaxial strain strongly increases the gap energy and moves the development of the surface conducting state to 200K, approaching a range useful for application. The goals of this project are to discover what sets the energy gap scale and accompanying development of the surface state, how this scale can be controlled and where other strongly correlated topological insulators can be found. This will be pursued in single crystal alloys of SmB6 and other Kondo insulators via transport, magnetic and thermodynamic measurements at ambient pressure and under strain. There is no reason to suppose that SmB6 is a unique material and understanding what sets the energy gap scale can present the rationale for discovery of other strongly correlated semiconductors in its exotic class with potential for application.