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Probing the quantum geometry and topology in condensed matter systems has relied heavily on static electronic transport experiments in magnetic fields. Yet, contact-free optical measurements have rarely been explored. Magnetic dichroism (MCD), the nonreciprocal absorption of circular polarized light, was theoretically linked to the quantized anomalous Hall effect in magnetic insulators and can identify the bands and momenta responsible for the underlying Berry Curvature (BC). Detecting BC through MCD faces two challenges: First, the relevant inter-band transitions usually generate MCD in the infrared (IR) range, requiring large samples with high quality. Second, while most magnetic materials are metallic, the relation between MCD and BC in metals remains unclear. Here, we report the observation of MCD in the IR range along with the anomalous Hall effect in thin film MnBi2Te4. Both phenomena emerge with a field-driven phase transition from an antiferromagnet to a canted ferromagnet. By theoretically relating the MCD to the anomalous Hall effect via BC in a metal, we show that this transition accompanies an abrupt onset of BC, signaling a topological phase transition from a topological insulator to a doped Chern insulator. Our density functional theory calculation suggests the MCD signal mainly originates from an optical transition at the Brillouin zone edge, hinting at a potential new source of BC away from the commonly considered {\Gamma} point. Our findings demonstrate a novel experimental approach for detecting BC and identifying the responsible bands and momenta, generally applicable to magnetic materials.

The synthesis of thin films of magnetic topological materials is necessary to achieve novel quantized Hall effects and electrodynamic responses. EuIn2As2 is a recently predicted topological axion insulator that has an antiferromagnetic ground state and an inverted band structure, but that has only been synthesized and studied as a single crystal. We report on the synthesis of c-axis oriented EuIn2As2 films on sapphire substrates by molecular beam epitaxy. By carefully tuning the substrate temperature during growth, we stabilize the Zintl phase of EuIn2As2 expected to be topologically non-trivial. The magnetic properties of these films reproduce those seen in single crystals, but their resistivity is enhanced when grown at lower temperatures. We additionally find that the magnetoresistance of EuIn2As2 is negative even up to fields as high as 31T. while it is highly anisotropic at low fields, it becomes nearly isotropic at high magnetic fields above 5T. Overall, the transport characteristics of EuIn2As2 appear similar to those of chalcogenide topological insulators, motivating the development of devices to gate tune the Fermi energy and reveal topological features in quantum transport., Comment: accepted Phys. Rev. Materials

InAs/GaSb hosts a broken gap band alignment that has been shown to generate helical topological edge states. Upon the introduction of Mn into the structure, it has been predicted to host a quantized anomalous Hall effect. Here, we show that dilute Mn doping on InAs in InAs/GaSb, allows a tuning of the Fermi level, the introduction of paramagnetism, but also has a non-trivial impact on the band alignment of the system. The measurement of Shubnikov-de-Haas oscillations, cyclotron resonance, and a non-linear Hall effect in Mn-doped samples indicate the coexistence of a high mobility two-dimensional electron gas and a hole gas. Conversely, in undoped InAs/GaSb, pure-n-type transport is observed. We hypothesize that Mn acceptor levels can pin the Fermi energy near the valence band edge of InAs, far from the interface, which introduces a strong band bending to preserve the band offset at the InAs/GaSb interface. The realization of the QAHE in this structure will thus require a careful control of the band alignment to preserve topological insulating character.

We report different growth modes and corresponding magnetic properties of thin EuSe films grown by molecular beam epitaxy on BaF2, Pb1-xEuxSe, GaAs, and Bi2Se3 substrates. We show that EuSe growth predominantly in (001) orientation on GaAs(111) and Bi2Se3, but along (111) crystallographic direction on BaF2 (111) and Pb1-xEuxSe (111). High-resolution transmission electron microscopy measurements reveal an abrupt and highly crystalline interface for both (001) and (111) EuSe films. In agreement with previous studies, ordered magnetic phases include antiferromagnetic, ferrimagnetic, and ferromagnetic phases. In contrast to previous studies, we found strong hysteresis for the antiferromagnetic-ferrimagnetic transition. An ability to grow epitaxial films of EuSe on Bi2Se3 and of Bi2Se3 on EuSe enables further investigation of interfacial exchange interactions between various phases of an insulating metamagnetic material and a topological insulator., Comment: 19pages,13 figures

Most III-nitride semiconductors are grown on non-lattice-matched substrates like sapphire or silicon due to the extreme difficulty of obtaining a native GaN substrate. We show that several layered transition-metal dichalcogenides are closely lattice matched to GaN and report the growth of GaN on a range of such layered materials. We report detailed studies of the growth of GaN on mechanically-exfoliated flakes WS$_2$ and MoS$_2$ by metalorganic vapour phase epitaxy. Structural and optical characterization show that strain-free, single-crystal islands of GaN are obtained on the underlying chalcogenide flakes. We obtain strong near-band-edge emission from these layers, and analyse their temperature-dependent photoluminescence properties. We also report a proof-of-concept demonstration of large-area epitaxial growth of GaN on CVD MoS$_2$. Our results show that the transition-metal dichalcogenides can serve as novel near-lattice-matched substrates for nitride growth., Comment: main manuscript: page 1-15, 5 figures; supplementary: page 16-23