Kondo Lattice thin films: Fundamental Insights and Novel Device Applications

Interaction between the localized f moments and band-like conduction electrons in certain inter-metallic systems leads to the formation of the so-called ‘Kondo Lattice’ that can host a variety of exotic ground states, including heavy Fermi liquids, hidden order, unconventional superconductivity, and quantum critical phases. Epitaxial synthesis of these compounds with atomic precision opens up the possibility of rationally manipulating these interactions and exploiting them in devices for potential applications. It also allows access to atomically smooth sample surfaces for advanced spectroscopic measurements such as angle-resolved photoemission spectroscopy (ARPES). This is particularly beneficial for compounds that cannot be cleaved in a bulk single crystal form.

In this talk, I will describe anapplication of such an approach using thin films of a prototypical mixed-valence compound, YbAl3where the Ytterbium (Yb) valence fluctuates between two different valence configurations, Yb2+(4f13)and Yb3+(4f12). I will establish a precise one-to-one correspondence between the change in Ytterbium (Yb) valence and a topological transition of the Fermi surface, which is accompanied by an enhancement of the Yb 4fdensity of states (DOS) at the Fermi level. I will further show how interactions in this system can be modified by fabricating ultra-thin films and superlattices, where YbAl3 atomic layers are separated by LuAl3 layers. Finally, by fabricating bi-layer heterostructures consisting of atomic layers of YbAl3anda ferromagnet (iron), I will show that the spin hall conductivity in YbAl3 undergoes a dramatic enhancement from ~ 104ħ/2e-1m-1 at 300 K to greater than106ħ/2e-1m-1at 37 K. This enhancement follows the same temperature scaling as that of the Yb 4f DOS,thereby revealing the connection between heavy fermion formation and the observation of giant spin hall conductivity at low temperatures.