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Topological surface-states can acquire an energy gap when time-reversal symmetry is broken by interfacing with a magnetic insulator. This gap has yet to be measured. Such topological-magnetic insulator heterostructures can host a quantized anomalous Hall effect and can allow the control of the magnetic state of the insulator in a spintronic device. In this work, we observe the energy gap of topological surface-states in proximity to a magnetic insulator using magnetooptical Landau level spectroscopy. We measure Pb_1-xSn_xSe–EuSe heterostructures grown by molecular beam epitaxy exhibiting a record mobility and low Fermi energy. Through temperature dependent measurements and theoretical calculations, we show this gap is likely due to quantum confinement and conclude that the magnetic proximity effect is weak in this system. This weakness is disadvantageous for the realization of the quantum anomalous Hall effect, but favorable for spintronic devices which require the preservation of spin-momentum locking at the Fermi level.

 

Abstract Ferromagnetic semiconductor Ga 1– x Mn x As 1– y P y thin films go through a metal–insulator transition at low temperature where electrical conduction becomes driven by hopping of charge carriers. In this regime, we report a colossal negative magnetoresistance (CNMR) coexisting with a saturated magnetic moment, unlike in the traditional magnetic semiconductor Ga 1– x Mn x As. By analyzing the temperature dependence of the resistivity at fixed magnetic field, we demonstrate that the CNMR can be consistently described by the field dependence of the localization length, which relates to a field dependent mobility edge. This dependence is likely due to the random environment of Mn atoms in Ga 1– x Mn x As 1– y P y which causes a random spatial distribution of the mobility that is suppressed by an increasing magnetic field.