The exciton condensate in bilayer systems
Final Report Abstract
In this project, we made the observation of the fractional quantum Hall effect in the lowest Landau level of a two-dimensional electron system (2DES), residing in the diluted magnetic semiconductor Cd 1−x MnxTe. The presence of magnetic impurities results in a giant Zeeman splitting leading to an unusual ordering of composite fermion Landau levels. In experiment, this results in an unconventional opening and closing of fractional gaps around the filling factor ν =3/2 as a function of an in-plane magnetic field, i.e., of the Zeeman energy. By including the s-d exchange energy into the composite Landau level spectrum the opening and closing of the gap at filling factor 5/3 can be modeled quantitatively. The widely tunable spin-splitting in a diluted magnetic 2DES provides a means to manipulate fractional states. In the part of the project where we aimed at realizing an exciton condensate in complex graphene heterostructures, we established a fabrication scheme capable of assembling complex layer sequences of up to five alternating layers of hBN and graphene. We demonstrated independent carrier density control of one of those layers by gate-dependent Raman spectroscopy. The carrier mobility in a single encapsulated graphene sheet was enhanced dramatically, allowing us to observe ballistic electron motion in antidot superlattices. Although the ultimate goal of two coupled, high-quality graphene layers could not be achieved within the funding period, our work has cleared the way for future experiments in this direction.
Publications
- Phys. Rev. B 90, 115302 (2014). Fractional quantum Hall effect in a dilute magnetic semiconductor
C. Betthausen, P. Giudici, A. Iankilevitch, C. Preis, V. Kolkovsky, M. Wiater, G. Karcezwski, B.A. Piot, J. Kunc, M. Potemski, T. Wojotowicz, D. Weiss
(See online at https://doi.org/10.1103/PhysRevB.90.115302) - Ballistic transport in graphene antidot lattices. Nano Lett. 15, 8402 (2015)
A. Sandner, T. Preis, C. Schell, P. Giudici, K. Watanabe, T. Taniguchi, D. Weiss, and J. Eroms
