Band Engineering to Achieve a Wide Band Gap Topological Insulator

Graduate #61
Discipline: Nanoscience
Subcategory: Materials Science
Session: 1
Room: McKinley

Ido Levy - City College of New York
Co-Author(s): Cody Youmans, Thor A. Garcia, Haiming Deng, Steven Alsheimer, Pouyan Ghaemi, Lia Krusin-Elbaum, Maria C. Tamargo, City College of New York, and City University of New York, NY

Three dimensional topological insulators (3D TIs) are being widely researched today for their attractive and unique transport properties [1]. These materials present a bandgap in the bulk and highly conducting metallic surface states. Heterostructures of these TIs are predicted to show promising properties [2]. We have recently shown that a short period superlattice of two TI materials, such as Bi2Se3 and Sb2Te3, can present promising electrical properties, for example a decrease in carrier concentration in the bulk and an increase of resistance as a function of the superlattice period. This can be explained by the formation of a bulk bandgap in the superlattice due to confinement effects in each of the layers, as a result of their “broken gap” band alignment. A question remains as to the presence of the topological surface states in such a short period superlattice structure. To investigate this, magnetoconductance measurements were performed for the superlattice structure with the smallest periodicity; one that showed the reduced bulk conductivity previously observed. Fitting these data to the typically used Hikami-Larkin-Nagaka theory [3] suggests the presence of two two-dimensional conduction channels in the small period superlattice as expected for a 3D TI layer. Angle dependent magnetoresistance measurements and a fit of the dephasing length (lφ) dependence on temperature both give further supporting evidence of the preservation of the topological surface states. Thus, we conclude that this short period Bi2Se3-Sb2Te3TI-TI superlattice behaves as a designer 3D TI with different properties to the two individual TI constituents, which are conducting in the bulk. Tight binding calculation for such short period TI-TI superlattices were performed and the results suggest that for the appropriate combination of materials, it may be possible to achieve a “designer” 3D TI with a bulk bandgap that is larger than the gaps of either of the component TI materials. Due to high bulk doping, the superlattice will be characterized in the near future using advanced methods (ARPES, FTIR, STM) to validate the gap enhancement. [1] Y. Xia et al., Nat. Phys. 5, 398 (2009) [2] K.M. Masum-Habib et al., Phys. Rev. Lett. 114, 176801 (2015) [3] S. Hikami et al., Prog. Theor. Phys. 63, 707 (1980)

Funder Acknowledgement(s): This work was supported by NSF Grant Nos. HRD-1547830 (IDEALS CREST) and DMR-1420634 (MRSEC PAS3).

Faculty Advisor: Maria Tamargo, mtamargo@ccny.cuny.edu

Role: I grew all the samples for the research using Molecular Beam Epitaxy. In addition, I characterized the samples using Atomic Force Microscopy, X-Ray Diffraction and parts of the transport measurements.