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Reporting period: 2019-01-01 to 2020-06-30

Over the past decade our comprehension of materials has been revolutionized by their topological classification. We now know that insulators, semimetals and metals can be subclassified into trivial and 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 aluminum.
We have developed and constructed an ultra high vacuum suitcase that allows us to transport nanowires from their molecular beam epitaxy chamber to the scanning tunneling microscope. This has uniquely enabled us to investigate the spectroscopic properties of electrons confined to one dimension. We have imaged the quantized sub-bands in the nanowires through the interference patterns the electrons embed in the local density of states along the nanowire as they scatter off impurities, crystallographic defects and the nanowire end. We identified a new high-energy regime of extended phase coherence that grows with increasing energies. More recently we have studied superconducting aluminum islands grown epitexially on the InAs nanowires in their Coulomb blockade regime. From it we could extract fundamental system parameters as the surprising absence of charge transfer from the aluminum islands to the semiconducting nanowires and the existence of a substantial energy barrier at the interface between the two.
We have continued with the exploration and characterization of bulk topological materials including the inversion symmetry broken Weyl semimetal TaAs, a time reversal symmetry broken Weyl semimetal, a dual weak and crystalline topological insulator Bi2TeI and the potential higher order topological insulating class in bismuth.
During this period we have begun the growth of a nanocrystals and nanowires of a topological compound in a recently purchased molecular beam epitaxy machine and have started their spectroscopic characterization in a home built scanning tunneling microscope. We are also constructing a new dilution fridge scanning tunneling microscope that will allow us to investigate topological states in general and Majorana modes in topological superconductors with higher energy resolution and at higher magnetic fields.
Following an intense technological efforts into the construction of the new ultra low temperature scanning tunneling microscope and the installment, integration and initialization of the purchased molecular beam epitaxy machine we have now begun to grow nanocrystals and nanowires made of a topological material that has many intriguing properties. We are just about to start its spectroscopic characterization which will reveal the quantized behavior and response of its topological surface states. We have also developed scanning tunneling microscopy compatible gating architecture that will next allow us to modify and tune the electron occupation of the nanowires. The combination of these unique capabilities will surely provide unprecedented insight into the electronic nature of topological surface states and a variety of Majorana end modes with ultimate spatial and energy resolution.
Topographic image of aluminum deposited InAs nanowire taken with a scanning tunnelinc microscope