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VISUALIZATION OF TOPOLGICAL STATES IN PRISTINE NANOWIRES

Periodic Reporting for period 4 - TOPO-NW (VISUALIZATION OF TOPOLGICAL STATES IN PRISTINE NANOWIRES)

Reporting period: 2020-07-01 to 2021-06-30

Over the past decade and a half our comprehension of materials has been revolutionized by their topological classification. We now know that insulators, semimetals and metals can be subclassified into various topological classes. The topological bulk assures the formation of exotic modes on the boundary of the sample. The unique properties of these boundary modes can be harnessed as a platform for future electronics. A prime example is topological superconductivity induced in semiconducting nanowires. At the ends of the topological segments Majorana zero-modes are thought to bind, which are prime resource for topologically protected quantum computation. However, probing the topological class of a material, beyond the existence of boundary modes, is highly non-trivial. We find that confining the topological surface states at the circumference of a nanowire made of topologically classified material provides novel tools for investigating, controlling and manipulating it.
To achieve this goal we combine scanning tunneling microscopy and spectroscopy with molecular beam epitaxy growth. Measuring the nanowires with such a local prove poses a technological challenge due to their brittleness and high reactivity. We thus maintain the nanowires under ultra high vacuum at all stages between growth and measurement. This has indeed allowed us to probe the electronic states in semiconducting InAs nanowires as well as in nanowires epitaxially deposited with superconducting Al.
In the project TopoNW we investigated fundamental aspects of topological states of matter including topological superconductivity through the development of the mesoscopic methodology of nanowires. We have succeeded to substantially advance our comprehension of topological states of matter. We have progressed in three main research trajectories:
Spectroscopic investigation of semiconducting nanowires – We have developed unique technology to investigate spectroscopically nanowires grown in MBE with STM without breaking ultra high vacuum. Together with development of new nanowire growth protocols we have advanced our fundamental understanding of electronic properties in those nanowires towards realization and manipulation of topological Majorana modes. We have found revival of coherence of 1D hot electrons in InAs nanowires. We have found robust pinning of chemical potential at InAs/Al interface. We have demonstrated tuning of the quantized band structure in tapered nanowires towards manipulation of topological superconductivity. To achieve these we have developed new growth protocols of reclined InAs Nanowires and demonstrated their advantage for Al coating.
Spectroscopic investigation of novel topological states of matter - We have conducted pioneering research on timely topological materials. We have identified and characterized the inversion symmetry broken Weyl semimetal TaAs and the time reversal symmetry broken Weyl semimetal Co3SnS2. We have used topological defects to resolve the topological classification in bismuth. We have identified dual topological classification in Bi2TeI. We have reported topological superconductivity in 4Hb-TaS2
Spectroscopic investigation of topological nanowires – We have developed both the growth protocols of SnTe nano-crystals and nanowires in molecular beam epitaxy, which are nowadays investigated spectroscopically in STM. We have also developed the theoretical backbone for flux threading responses in such complex nanowires
We have established the unique technology and methodology needed to map spectrscopically nanowires in situ, that will allow us to continue to explore toplogical superconductivity and putative Majorana end modes in them.
We have introduced functionalized substrates as superconducting and magnetic, that supports greater device complexity.
We have put forward and demonstrated new methodolgy for their tuning and manipulation based on tapered nanowires.
Topographic image of aluminum deposited InAs nanowire taken with a scanning tunnelinc microscope