Bai, Mengmeng
(2022).
Josephson junctions fabricated via Pd diffusion
into the topological insulator (Bi1−xSbx)2Te3.
PhD thesis, Universität zu Köln.
Abstract
The work in this thesis was motivated by the theoretical prediction that a pair of Majorana bound states (MBSs) will be formed in Josephson junctions (JJs), which consist of s-wave superconductors (SCs) and three-dimensional topological insulators (3D TIs). A fractional Josephson effect is expected in such JJs with a 4π-periodic current phase relation. Our JJs are based on the MBE-grwon (Bi1−xSbx)2Te3 (BST) film. Both planar JJs and sandwich JJs were
fabricated and characterized by transport measurements.
First, we discovered that SCs are formed by magnetron sputtering Pd onto BST film. The transition temperature (TC) and the upper critical field (HC2) of the SCs were characterized in Hall-bar devices based on Pd/BST bilayer films. To identify the SC phase in our samples, scanning
transmission electron microscopy (STEM) Energy-dispersive x-ray spectroscopy (EDX) were performed. STEM and EDX analyses show the result that the self-formed PdTe1,2 SCs are
epitaxially formed on the BST film. A planar SC-TI-SC JJ is built when two Pd electrodes are deposited on BST film with a small gap. A supercurrent and a Fraunhofer-like pattern were observed on this JJ, suggesting that BST film is proximitized by self-formed SCs successfully.
To characterize the junction transparency, a superconducting quantum interference device (SQUID) was fabricated. A high transparency ∼ 0.96 is obtained based on the Octavio-Tinkham-Blonder-Klapwijk (OTBK) theory. For most of the planar JJs, the interface between SCs and TIs is on the surface of TI. The surface can be polluted by water and oxide layers during nanofabrication. While the interface is inside the TI film and the SCs are epitaxially grown in
our device. These are possible reasons for the high junction transparency. However, there is a lack of features for the existence of multiple Andreev reflections, which is usually observed in high-transparency JJs. Epitaxial growth of a superconductor on the TI surface is still challenging.
Our discovery of unexpected self-epitaxy to form SCs on the surface of BST makes the nanofabrication of devices with an epitaxial SC/TI interface much more straightforward.
These devices provide a good platform to realize the Fu-Kane proposal in TIs to engineer a topological superconducting (TSC) state. Second, we found that SCs can also form when Pd is thermally-deposited onto BST films.
The SCs of PdTe and PdTe2 with an “epitaxial” interface is confirmed by STEM and EDX analyses. It is different from the case of the sputter-deposited Pd. The Pd penetrates fully
through the BST film when Pd was deposited by thermal evaporation. This finding opened the possibility to create a TI nanowire sandwiched by SCs, by converting the BST film into the SC and leaving only a narrow BST channel to remain as the pristine TI. TI nanowires are created in this way that is confirmed by STEM. The finding of the complete conversion of BST into SCs also opened the possibility to build SC-TI-SC sandwiched JJs. The TI nanowire was
proximitized by self-formed SCs successfully. Standard Fraunhofer patterns were observed on such JJs, suggesting that the supercurrent is not due to a superconducting short-circuit and the distribution of the supercurrent is homogeneous. The junction transparency is about 0.8, and the resonance conductance peaks resulting from multiple Andreev reflection (MAR) were observed. The MAR feature observed with the index up to 9 and at temperatures up to 1.0 K which is close to the transition temperature of self-formed SCs, suggesting that the superconducting proximity effect in JJs is strong. Finally, we did the Shapiro measurements.
The m = 1 Shapiro step is clearly missing at low frequencies and low power. Two possible topologically trivial origins of the first step missing are Landau-Zener transitions (LZTs) and hysteretic I − V characteristics. Temperature dependence of the Shapiro steps shows that the m = 1 step is still missing at a high enough temperature where the hysteretic I − V behavior is completely gone. Landau-Zener transition is considered when the junction transparency is close to 1, but our junction transparency is not that high.We proved that the origin of the missing first Shapiro step is neither hysteretic I−V characteristic nor Landau-Zener transitions.
These phenomena are reproduced in another two devices. Hence, our Shapiro-step suggest that a 4π-periodic contribution in the sandwich JJs. This gives possible evidence for topological Majorana bound states.
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