Tailoring the band structure of thin films by strain and by the growth of artificial heterostructures
Final Report Abstract
The electronic band structure of a material determines its electronic, optical, magnetic and catalytic properties and thus a precise adjustment of the band structure is of fundamental importance for a wide range of applications. This requires a comprehensive understanding of the correlation of the band structure with the atomic constitution. Within this project we have explored two possible ways to tailor the band structure of thin films, namely by the growth of artificial heterostructures and by imposing of mechanical strain. To this end, we have developed a novel flip-chip preparation technique enabling the preparation of ultrathin epitaxial films with predetermined thicknesses on polycrystalline and/or flexible substrates fresh from in-situ cleavage. Using our flip-chip preparation technique, we have studied the superconducting proximity effect in the arguably simplest system of prototypical TI Bi2Se3 on top of the elementary, isotropic s-wave superconductor Nb. Our angle-resolved photoemission spectroscopy measurements of the film surface disclosed superconducting gaps and coherence peaks of similar magnitude for both the topological surface states and bulk states. The ARPES spectral map as a function of temperature and TI film thickness up to 10 QLs revealed key characteristics relevant to the mechanism of coupling between the topological surface states and the superconducting Nb substrate; the effective coupling length was found to be much larger than the decay length of the topological surface states. Furthermore, we explored the effect of elastic strain on the band structure of topological materials and showed that the Dirac surface states of the topological insulator Bi2Se3 can be reversibly tuned by an externally applied elastic strain. Performing in-situ x-ray diffraction and in-situ angle-resolved photoemission spectroscopy measurements during tensile testing of epitaxial Bi2Se3 films bonded onto a flexible substrate, we demonstrated elastic strains of up to 2.1% and quantify the resulting reversible changes in the topological surface state. Our study thus established the functional relationship between the lattice and electronic structures of Bi2Se3 and, more generally, demonstrated a new route toward momentum-resolved mapping of strain-induced band structure changes. Using ARPES in combination with first principle calculations we also showed that an epitaxial compressive strain can turn α-Sn films into a topological Dirac semimetal. With decreasing film thickness the nature of the Dirac cone crosses over from 3D to 2D and in the ultrathin limit of a single-layer of stanene on InSb(111) ARPES measurements revealed a sizable gap of 0.44 eV, interesting for roomtemperature electronic quantum spin Hall applications.
Publications
- In-situ strain tuning of the Dirac surface states in Bi2Se3 films
D. Flötotto, Y. Bai, Y.-H. Chan, P. Chen, X. Wang, P. Rossi, C.-Z. Xu, C. Zhang, J. A. Hlevyack, J. D. Denlinger, H. Hong, M.-Y. Chou, E. J. Mittemeijer, J. N. Eckstein and T.-C. Chiang
(See online at https://doi.org/10.1021/acs.nanolett.8b02105) - Experimental and Theoretical Electronic Structure and Symmetry Effects in Ultrathin NbSe2 Films, Phys. Rev. Materials 2, 064002 (2018)
C.-Z. Xu, X. Wang, P. Chen, D. Flötotto, J. A. Hlevyack, M.-K. Lin, G. Bian, S.-K Mo and T.-C. Chiang
(See online at https://doi.org/10.1103/PhysRevMaterials.2.064002) - Gapped electronic structure of epitaxial stanene on InSb(111), Phys. Rev. B 97, 035122 (2018)
C.-Z.i Xu, Y.-H. Chan, P. Chen, X. Wang, D. Flötotto, J. A. Hlevyack, G. Bian, S.-K. Mo, M.-Y. Chou, and T.-C. Chiang
(See online at https://doi.org/10.1103/PhysRevB.97.035122) - Superconducting Pairing of Topological Surface States in Bismuth Selenide Films on Niobium, Science Advances 4, eaar7214 (2018)
D. Flötotto, Y. Ota, Y. Bai, C. Zhang, K. Okazaki, A. Tsuzuki, T. Hashimoto, J. N. Eckstein, S. Shin and T.-C. Chiang
(See online at https://doi.org/10.1126/sciadv.aar7214)
