Staff directory Christian Koch

Christian Koch

Fellowship Master Student
Advanced Electron Nanoscopy - Inorganic Nanoparticles



  • Coherent Epitaxial Semiconductor-Ferromagnetic Insulator InAs/EuS Interfaces: Band Alignment and Magnetic Structure

    Liu Y., Luchini A., Martí-Sánchez S., Koch C., Schuwalow S., Khan S.A., Stankevič T., Francoual S., Mardegan J.R.L., Krieger J.A., Strocov V.N., Stahn J., Vaz C.A.F., Ramakrishnan M., Staub U., Lefmann K., Aeppli G., Arbiol J., Krogstrup P. ACS Applied Materials and Interfaces; 12 (7): 8780 - 8787. 2020. 10.1021/acsami.9b15034. IF: 8.456

    Hybrid semiconductor-ferromagnetic insulator heterostructures are interesting due to their tunable electronic transport, self-sustained stray field, and local proximitized magnetic exchange. In this work, we present lattice-matched hybrid epitaxy of semiconductor-ferromagnetic insulator InAs/EuS heterostructures and analyze the atomic-scale structure and their electronic and magnetic characteristics. The Fermi level at the InAs/EuS interface is found to be close to the InAs conduction band and in the band gap of EuS, thus preserving the semiconducting properties. Both neutron and X-ray reflectivity measurements show that the overall ferromagnetic component is mainly localized in the EuS thin film with a suppression of the Eu moment in the EuS layer nearest the InAs and magnetic moments outside the detection limits on the pure InAs side. This work presents a step toward realizing defect-free semiconductor-ferromagnetic insulator epitaxial hybrids for spin-lifted quantum and spintronic applications without external magnetic fields. Copyright © 2019 American Chemical Society.

  • Semiconductor-Ferromagnetic Insulator-Superconductor Nanowires: Stray Field and Exchange Field

    Liu Y., Vaitiekėnas S., Martí-Sánchez S., Koch C., Hart S., Cui Z., Kanne T., Khan S.A., Tanta R., Upadhyay S., Cachaza M.E., Marcus C.M., Arbiol J., Moler K.A., Krogstrup P. Nano Letters; 20 (1): 456 - 462. 2020. 10.1021/acs.nanolett.9b04187. IF: 12.279

    Nanowires can serve as flexible substrates for hybrid epitaxial growth on selected facets, allowing for the design of heterostructures with complex material combinations and geometries. In this work we report on hybrid epitaxy of freestanding vapor-liquid-solid grown and in-plane selective area grown semiconductor-ferromagnetic insulator-superconductor (InAs/EuS/Al) nanowire heterostructures. We study the crystal growth and complex epitaxial matching of wurtzite and zinc-blende InAs/rock-salt EuS interfaces as well as rock-salt EuS/face-centered cubic Al interfaces. Because of the magnetic anisotropy originating from the nanowire shape, the magnetic structure of the EuS phase is easily tuned into single magnetic domains. This effect efficiently ejects the stray field lines along the nanowires. With tunnel spectroscopy measurements of the density of states, we show that the material has a hard induced superconducting gap, and magnetic hysteretic evolution which indicates that the magnetic exchange fields are not negligible. These hybrid nanowires fulfill key material requirements for serving as a platform for spin-based quantum applications, such as scalable topological quantum computing. Copyright © 2019 American Chemical Society.