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A discussion of topological insulator applications and strain optimization of optical properties as well as two novel and exciting phenomena in acoustic structures are presented. In the first part of the presentation, three-dimensional topological insulators (TI) like Bi2Se3, Bi2Te3 and Sb2Te3 are important materials [1] for spintronics and provide a solid-state system displaying, on a low-energy scale, much of the high-energy physics of fermions including the presence of a Dirac cone in the dispersion. We present a group-theoretical discussion of the electronic bandstructure of the Bi2Se3 class of topological insulators and discuss energy harvesting properties [2]. The second part presents the coupling of piezoelectric equations and semiconductor drift-diffusion equations and demonstrates, in the plasma frequency range where the permittivity approaches zero, that large acoustic gains can be achieved under the action of a constant electric field by virtue of the Cherenkov effect. The gain can be several orders of magnitude. Device structures for acoustic amplification are also demonstrated theoretically. The third part of the presentation explores realization of parity-time symmetry phononic devices by designing a non-hermitean system Hamiltonian and the use of cleverly devised combinations of loss and gain. The gain blocks are piezoelectric materials activated in the Cherenkov regime [3]. In the final part of my presentation, I will discuss contact electrification and the influence of quantum mechanical effects.
References
1.H. Zhang, C. Liu, X. Qi, X. Dai, Z. Fang, and S. Zhang, Nat. Phys. 5, 438 (2009).
2.M. R. Brems, J. Paaske, A. M. Lunde, and M. Willatzen, Phys. Rev. B 97, 081402(R) (2018).
3.J, Christensen, M. Willatzen, V. R. Velasco, and M.-H. Lu, Phys. Rev. Lett. 116, 207601 (2016).
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Prof.