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Physics of Topological Matter and its Device Applications

The Ando Laboratory, nicknamed Topological Matter Laboratory Cologne, focuses on experimental studies of materials called topological matter. This is a peculiar class of materials in which a nontrivial topology of the quantum-mechanical wavefunctions leads to unusual, and often useful, physical properties. The prime example of topological matter is topological insulators, and there are also topological superconductors, topological semimetals, etc.

Our core strategy is to perform three key ingredients, (i) state-of-the-art materials syntheses, (ii) nanodevice fabrications, and (iii) difficult low-temperature experiments, in the same laboratory in a synergistic manner. For example, we grow some of the best single crystals and MBE thin films of topological insulators and superconductors, which are used for advanced magnetotransport studies and for nanodevice fabrications to study various functionalities desirable for spintronics or topological quantum computation. We also grow single crystals of new materials or MBE thin films of novel heterostructures, with the aim of expanding the horizon of topological matter and discovering new quantum phenomena. Our goal is not only to deepen the fundamental knowledge on topological matter, but also to develop the basic principles for its actual device applications.

Our research is performed in a dedicated lab building, which was erected in 2016 to house the clean room facilities, MBE machines, ultra-low-temperature experimental facilities, single crystal growth facilities, and various characterization facilities. With these facilities, we can complete the synthesis-fabrication-measurement cycle to perform the world-leading research on topological matter.

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What is Topological Insulator?

In the topologically-protected surface states of topological insulators, the orientation of electron spin is perpendicularly locked to the momentum.

Topological insulators are a new class of materials where an insulating bulk state supports an intrinsically metallic surface state that is “topologically protected” by time reversal symmetry. Intriguingly, the resulting metallic surface state is helically spin-polarized (i.e., right- and left-moving electrons carry up and down spins, respectively) and consist of massless Dirac fermions (i.e., the energy of quasiparticles is linearly dependent on the momentum). Those peculiar properties of the surface state open exciting new opportunities for novel spintronics devices with ultra-low energy consumptions. Professor Ando is one of the pioneers of the research field of topological insulators, and his pedagogical review article (http://journals.jps.jp/doi/pdf/10.7566/JPSJ.82.102001) provides a good introduction to this new field.

What is Topological Superconductor?

The first data which probed the existence of Majorana fermions on the surface of a topological superconductor. This is a plot of differential conductivity vs bias voltage, measured in Ando Laboratory by point-contact spectroscopy on CuxBi2Se3, which is a superconductor derived from the topological insulator Bi2Se3. [Sasaki et al., Phys. Rev. Lett. 107, 217001 (2011).

Even more exotic state of matter is realized in topological superconductors, which are predicted to host exotic quasiparticles called “Majorana fermions” on the surface. Some superconductors derived from topological insulators are intrinsic bulk topological superconductors. Also, by putting a conventional superconductor on top of a topological insulator, one can induce 2D topological superconductivity on the surface.

Majorana fermions are peculiar in that particles are their own antiparticles, and they were originally conceived as a model for mysterious neutrinos. Currently their realization in condensed matter is of significant interest, because when they are localized, they become topologically-protected zero modes, which are predicted to obey non-Abelian statistics. This unusual statistics, which is distinct from neither Fermi nor Bose statistics, is fundamentally new in physics and allows for performing topologically-protected quantum computation. Also, Majorana fermions can store quantum information in a non-local manner and endow Majorana-based qubits with special robustness against decoherence. These remarkable characteristics of Majorana fermions make them a particularly promising building block of future quantum information technologies.

Current Research Themes

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  • Topological insulator spin-voltage device
  • Topological Josephson junctions
  • Spintronic devices based on topological insulators

  • Chiral anomaly in Weyl semimetals

  • New types of topological matter

  • Quantum anomalous Hall effect

  • Helical Majorana fermions on the surface of topological superconductors

  • Topological superconductor nanowires

  • Topological Josephson junctions

  • Majorana zero-modes in topological superconductors

  • Majorana qubits

  • Detection of non-Abelian statistics

Who should join?

We offer excellent research environment and interesting projects to talented young physicists (or physicists to be) at all levels: Bachelor students, Master students, PhD students, Postdocs, and even summer-research students. If you are interested in doing research in the Topological Matter Laboratory Cologne, please contact Professor Ando.