2017/10/26(Thu) 14:00 一樓演講廳 1F, Auditorium
Title
Tuning the electronic properties of topological materials by strain
Speaker
Prof. Vidya Madhavan (Department of Physics and Frederick Seitz Materials Research Laboratory, University of Illinois Urbana-Champaign, Urbana, Illinois 61801, USA)Abstract
Topological insulators host zero mass electrons which can be modeled by the same massless Dirac equation that is used to describe relativistic particles travelling close to the speed of light. In normal topological insulators, the symmetry that protects the Dirac point is time-reversal symmetry. In a distinct class of topological materials called Topological Crystalline Insulators (TCIs) [1,2,3], the Dirac point is protected by a crystalline symmetry (mirror symmetry). Due to the importance of crystalline symmetry in generating and protecting the Dirac surface states, the topological states of TCIs are expected to be highly sensitive to changes in structure. In particular, strain is predicted to shift the momentum space position of the Dirac bands, create pseudo magnetic fields and in extreme cases create a topological to trivial phase transition. In this talk I will describe our experimental and theoretical investigations of TCIs using scanning tunneling microscopy on TCI thin films [3,4,5,6]. We grew heteroepitaxial thin films of SnTe on PbSe. Due to the large lattice mismatch, a 2D buckled periodic structure arises which encodes position dependent strain. I will show how using STM we can map out the strain tensor at the nanoscale and explicitly identify regions of tensile compressive strain as well as uniaxial strain. Furthermore we can correlate these different types of strains with local changes in the Dirac electronic structure. We find that the momentum space position of the Dirac nodes in the kx and ky directions move in phase in response to tensile/compressive strain and out of phase due to uniaxial strain. Interestingly we find that the effects of uniaxial strain are counterintuitive and strongly influenced by the orbital nature of the bands.
[1] L. Fu, Topological Crystalline Insulators. Phys. Rev. Lett. 106, 106802 (2011).
[2] T. H. Hsieh et al., Topological crystalline insulators in the SnTe material class. Nat.Commun. 3, 982 (2012).
[3] Y. Okada, et al., Observation of Dirac node formation and mass acquisition in a topological crystalline insulator, Science 341, 1496-1499 (2013)
[4] Ilija Zeljkovic, et al., Mapping the unconventional orbital texture in topological crystalline insulators, Nature Physics 10, 572–577 (2014)
[5] Ilija Zeljkovic, et al., Dirac mass generation from crystal symmetry breaking on the surfaces of topological crystalline insulators, Nature Materials 14, 318–324 (2015)
[6] Ilija Zeljkovic, Daniel Walkup, Badih Assaf, Kane L Scipioni, R. Sankar, Fangcheng Chou, Vidya Madhavan, Strain engineering Dirac surface states in heteroepitaxial topological crystalline insulator thin films, Nature Nanotechnology 10, 849–853 (2015)
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Language
演講語言 (Language): in English