Abstract
We investigate the Hall conductivity in a Sierpinski carpet, a fractal of Hausdorff dimension $d_f=\ln(8)/\ln(3) \approx 1.893$, subject to a perpendicular magnetic field. We compute the Hall conductivity using linear response and the recursive Green function method. Our main finding is that edge modes, corresponding to a maximum Hall conductivity of at least $\sigma_{xy}=\pm \frac{e^2}{h}$, seems to be generically present for arbitrary finite field strength, no mater how one approaches the thermodynamic limit of the fractal. We discuss a simple counting rule to determine the maximal number of edge modes in terms of paths through the system with a fixed width. This quantized edge conductance, as in the case of the conventional Hofstadter problem, is stable with respect to disorder and thus a robust feature of the system.
Highlights
Seem to be generically present for arbitrary finite field strength, no matter how one approaches the thermodynamic limit of the fractal
The prospect of topological order in fractals was investigated in Refs. [26,27] and recently revived in Ref. [28] and we compare our results to this latter work
While we found that making the fractal cuts deeper generically decreases the number of edge modes, the remaining single edge mode regions become increasingly stable with increasing system size
Summary
Mikael Fremling , Michal van Hooft, Cristiane Morais Smith, and Lars Fritz Institute for Theoretical Physics, Center for Extreme Matter and Emergent Phenomena, Utrecht University, Princetonplein 5, 3584 CC Utrecht, the Netherlands (Received 5 July 2019; revised manuscript received 30 October 2019; published 13 January 2020). We investigate the Hall conductivity in a Sierpinski carpet, a fractal of Hausdorff dimension d f = ln(8)/ ln(3) ≈ 1.893, subject to a perpendicular magnetic field. We compute the Hall conductivity using linear response and the recursive Green function method. Our main finding is that edge modes, corresponding to a maximum
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