assistant: Here is a summarization of the paper on the topic found here:
https://www.nature.com/articles/s43246-022-00242-6

When a topological insulator is made into a nanowire, the interplay between topology and size quantization gives rise to peculiar one-dimensional states whose energy dispersion can be manipulated by external fields. In the presence of proximity-induced superconductivity, these 1D states offer a tunable platform for Majorana zero modes. While the existence of such peculiar 1D states has been experimentally confirmed, the realization of robust proximity-induced superconductivity in topological-insulator nanowires remains a challenge. Here, we report the realization of superconducting topological-insulator nanowires based on (Bi1−xSbx)2Te3 (BST) thin films. 

When two rectangular pads of palladium are deposited on a BST thin film with a separation of 100–200 nm, the BST beneath the pads is converted into a superconductor, leaving a nanowire of BST in-between. We found that the interface is epitaxial and has a high electronic transparency, leading to a robust superconductivity induced in the BST nanowire. Due to its suitable geometry for gate-tuning, this platform is promising for future studies of Majorana zero modes. https://doi.org/10.1038/s43246-022-00242-6 OPEN 1 Physics Institute II, University of Cologne, Zülpicher Straße 77, 50937 Köln, Germany. 2 Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich GmbH, 52425 Jülich, Germany. 3 KU Leuven, Quantum Solid State Physics, Celestijnenlaan 200 D, 3001 Leuven, Belgium. 4 These authors contributed equally: Mengmeng Bai, Xian-Kui Wei, Junya Feng. ✉email: ando@ph2.uni-koeln.de COMMUNICATIONS MATERIALS | (2022) 3:20 | https://doi.org/10.1038/s43246-022-00242-6 | www.nature.com/commsmat 1 1234567890():,; 

The decisive characteristic of a topological insulator (TI) is the existence of gapless surface states whose gapless nature is protected by time-reversal symmetry. However, in a TI nanowire, the size quantization effect turns the topological surface states into peculiar one-dimensional (1D) states that are gapped due to the formation of subbands1,2. Interestingly, their energy dispersion can be manipulated by both magnetic and electric fields, and these tunable 1D states have been proposed to be a promising platform to host Majorana zero modes (MZMs) in the presence of proximity-induced superconductivity3,4. Therefore, realization of robust superconductivity in TI nanowires using the superconducting proximity effect is important for the future Majorana research. So far, the existence of the 1D subbands in TI nanowires has been probed by various means5–11, but it has been difficult to induce robust superconductivity in TI nanowires12, because a suitable method to realize an epitaxial interface between a TI nanowire and a superconductor has been lacking. In terms of the realization of an epitaxial interface, it was previously reported13 that when Pd is sputter-deposited on a (Bi1−xSbx)2Te3 (BST) thin film, the diffusion of Pd into BST at room temperature results in a self-formation of PdTe2 superconductor (SC) in the top several quintuple-layers (QLs) of BST, leaving an epitaxial horizontal interface. 

Within the self-formed PdTe2 layer, Bi and Sb atoms remain as substitutional or intercalated impurities. This self-epitaxy was demonstrated to be useful for the fabrication of TI-based superconducting nanodevices involving planar Josephson junctions13. In such devices, the BST remains a continuous thin film and the superconducting PdTe2 proximitizes the surface states from above. In the present work, we found that depending on the Pd deposition method, Pd can diffuse deeper into the BST film and the conversion of BST into a PdTe2-based superconductor can take place in the whole film. Specifically, this full conversion occurs when Pd is thermally-deposited onto BST films with the thickness up to ~30 nm. This finding opened the possibility to create a TI nanowire sandwiched by SCs, by converting most of the BST film into the SC and leaving only a ~100-nm-wide BST channel to remain as the pristine TI. This way, one may induce robust superconductivity in the BST naowire proximitized by the PdTe2 superconductor on the side through an epitaxial interface. Furthermore, this structure allows for gate-tuning of the chemical potential in the BST nanowire from both top and bottom sides (called dual-gating), which offers a great advantage to be able to independently control the overall carrier density and the internal electric field14. This is useful, because the realization of Majorana zero modes (MZMs) in a TI nanowire requires to lift the degeneracy of the quantum-confined subbands3,4, which can be achieved either by breaking time-reversal symmetry (by threading a magnetic flux along the nanowire)3 or by breaking inversion symmetry (by creating an internal electric field)4. In this paper, we show that the devices consisting of BST nanowire sandwiched by PdTe2 superconductor indeed present robust proximity-induced superconductivity, evidenced by signatures of multiple Andreev reflections. Furthermore, in our experiment of the ac Josephson effect, we observed that the first Shapiro step is systematically missing at low rf frequency, low rf power, and at low temperature, which points to 4π-periodic current-phase relation and gives possible evidence for Majorana bound states. This result demonstrates that the superconducting TI nanowire realized here is a promising platform for future studies of MZMs. 
Results Nanowire formation. 

To confirm whether TI nanowires can indeed be created by Pd diffusion, we prepared BST/Pd bilayer samples specially designed for scanning transmission electron microscope (STEM) studies: a ~30-nm-thick BST film was epitaxially grown on an InP (111)A substrate and ~20-nm-thick rectangular Pd pads were thermally-deposited on BST with distances of ~200 nm between them. Figure 1a shows a high-angle annular-dark-field (HAADF) STEM image of the cross-section of such a sample; here, the two Pd pads are separated by 231.7 nm. The corresponding elemental maps of Pd, Bi, Sb and Te detected by energy-dispersive X-ray (EDX) spectroscopy (Fig. 1b–e) show that the Pd not only penetrates fully into the BST film beneath the pad, but also propagates outward by ~90 nm from each edge (Fig. 1b); note that in this sample, the outward diffusion was along the 〈110〉 direction (see Fig. 1e inset). 
As a result, a BST nanowire with a width of ~40 nm is left in-between the two Pdcontaining regions. One can also see the swelling of Pd-absorbed BST (hereafter called Pd-BST) along the out-of-plane direction compared to the pristine BST region. We note that this STEM observation was made about 3 months after the Pd deposition, so that the in-plane Pd-diffusion length of ~90 nm is probably longer than that in devices that were measured soon after the Pd deposition. For additional STEM data for various Pd-pad separations, see Supplementary Note 1 and Supplementary Fig. 1. After confirming the creation of a TI nanowire, the next important question is the morphology of the system and identification of the converted phase. First, atomic-resolution examination of the TI nanowire region (Fig. 1f) confirms highquality epitaxial growth of BST on the InP (111) surface, with QLs remain intact. Near the interface between BST and Pd-BST (Fig. 1g and Supplementary Fig. 2), one can see that the existence of Pd completely transforms QLs into a triple-layer (TL) structure characteristic of the PdTe2-like phase (Fig. 1i)13. Interestingly, the forefront of Pd-BST forms a V-shaped "epitaxial" interface to make a compromise between the different thicknesses of the QL and the TL. 
In the Pd-BST region closer to the Pd pad (Fig. 1h), we found that the TL structure changes to a zigzag pattern (Fig. 1k). Our image-simulation-based atomic structure analysis suggests that the TL- and zigzag-structured Pd-BST correspond to PdTe2-like and PdTe-like structural phases (Fig. 1j), respectively15. Depending on the concentration of Pd at different locations, the chemical formula may vary between Pd(Bi,Sb,Te)2 and Pd(Bi,Sb,Te), possibly with partial occupation of the Pd sites by Sb as well. Note that both PdTe2 and PdTe are SCs, with reported Tc of 2.0 and 4.5 K, respectively16,17. Our transport measurements show that Pd-BST is also a SC with Tc ≃ 1 K, which indicates that PdTe2 and PdTe still superconduct after heavy intercalation/substitution with Bi and Sb, albeit with a lower Tc. dc transport properties of the sandwich Josephson junction. To characterize how these TI nanowires are proximitized by the superconductivity in Pd-BST, we fabricated devices as shown in Fig. 2a in a false-color scanning electron microscope (SEM) image (for the transport characterization of the BST thin films used for device fabrication, see Supplementary Note 2 and Supplementary Fig. 3). These devices are SNS-type Josephson junctions (JJs) in which the TI nanowire is sandwiched by SCs. We can measure the supercurrent across the nanowire, but the 1D transport properties of the nanowire cannot be measured in these devices.