Electrons in Fractal Geometries

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The development of two-dimensional materials has enabled the discovery of new physical phenomena and led to the development of devices that are of crucial importance to our society. For example, we now base the definition of electrical resistance on the quantum Hall effect, a phenomenon exclusive to two dimensions. Similarly, the transistor that underpins every electronic device in use today relies on the manipulation of a two-dimensional electron gas. The realization that a material can have very different properties in two-dimensions compared to three-dimensions was a huge conceptual leap. Here, I propose a similar leap: from integer (0,1,2,3) to non-integer, or fractional, dimensions. Specifically, I will study the properties of electrons in geometric fractals with dimension between 2 and 1. Fractals are objects that are self-similar on different length scales with two unique properties (i) a non-integer dimension and (ii) expansion symmetry but no periodicity. Studying fractals will not only allow me to verify the existence of predicted exotic properties but also to address urgent questions related to the interplay of dimensionality and symmetry on the one hand and the existence of exotic states of matter (e.g. long-range entangled states, topological phases) on the other. Building on a recently published proof-of-concept, I will assemble geometric fractals atom-by-atom in a scanning tunnelling microscope. A unique advantage of this approach is that it provides control over all relevant parameters: type of fractal, its size and symmetry, the coupling strength, whether or not spin-orbit coupling and electron correlation are included, defects. My extensive experience with creating and characterising such systems makes me uniquely suited to develop the nascent field of electronic fractals. My results will provide engineering rules for truly novel electronic materials that are based on fractal geometries (e.g. fabricated by lithographic patterning of semiconductors)