University of Twente Student Theses


Superconductor-Bi1:5Sb0:5Te1:7Se1:3 Topological Insulator Hybrid Devices

Scholten, Thijs (2013) Superconductor-Bi1:5Sb0:5Te1:7Se1:3 Topological Insulator Hybrid Devices.

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Abstract:Topological insulators in proximity to a superconductor and topological superconductors are interesting states of matter, not least because of their potential to host Majorana fermions. Thin flakes of the topological insulator Bi1:5Sb0:5Te1:7Se1:3 (BSTS) are studied in Nb-BSTSNb junctions of different lengths. These nanojunctions are produced by photo and e-beam lithography and standard sputtering techniques. Four-point transport measurements were performed at temperatures of about 30 mK and in magnetic fields up to 3 T. The smallest two junction were shorted due to an e-beam overexposure, but showed Josephson effects attributed to weak links of Nb ears or microbridges. The observed hysteresis in the I; V - curves of these junctions were attributed to electron heating (hot electrons). Larger junctions (realized junction lengths between 52 and 250 nm) revealed no superconductivity but did reveal interesting resistance peaks at zero bias (ZBRP). These ZBRP are attributed to 2D electron-electron interactions (EEI). The observed side resistance peaks are speculated to be indicative of p-wave induced superconductivity in BSTS. The magnetoresistance was found to be suppressed at lower magnetic fields. This is attributed to weak antilocalization (WAL) (perhaps in combination with EEI). Although the fitting parameters of the 2D HLN-equation deviate from the expected values, the shape does correspond. Several explanations for the deviation are presented. Additionally, flakes of the topological superconductor Cu0:3Bi2:1Se3 (CBS) are investigated. Samples with gold leads deposited on CBS flakes were manufactured using photolithography and sputter deposition techniques. Four-point transport measurements down to 1.5 K were performed to test if the flakes poses superconductivity. Whereas the bulk crystal was superconducting before and after the measurements on flakes of the same crystal, the flakes itself were not superconducting (resistance in the order of 1Ω). This is attributed to non-uniform Cu doping by intercalation, giving rise to a small superconducting volume fraction or, possibly, to an easier cleaving of non-superconducting flakes.
Item Type:Essay (Master)
Faculty:TNW: Science and Technology
Subject:51 materials science
Programme:Nanotechnology MSc (60028)
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