Topological insulators (TI) are a new state of quantum matter, which support two-dimensional Dirac-type metallic surface states while their bulk states are simultaneously insulating.1–3 These strongly spin–orbit coupled (SOC) materials have been shown to carry topologically protected phase order which offers tantalizing possibilities for both fundamental and applied research areas. A TI with a ferromagnetic perturbation can further enhance the functionality of these materials since magnetism can lead to the quantum anomalous Hall (QAH) effect,4–6 topological magnetoelectric effects, and image monopoles. Given the novel nature of these phenomena, many experiments, from angle-resolved photoemission spectroscopy (ARPES),7 scanning tunneling spectroscopy8 to electrical transport measurements, have been devoted to the study of pristine and magnetically doped TIs which initially belonged to the Bi2Se3 family and later extended to other systems.4 

In order to lead us towards a better understanding of TIs and applications, it is, however, necessary to develop techniques that will enable high quality TI materials to be obtained in a routine and reliably way, but so far this has been an enormous challenge. Since highly volatile chalcogenide components are involved in most TIs, whether in bulk single crystal or epitaxial thin films, maintaining perfect stoichiometry has proven to be universally difficult. Observing the predicted transport properties of TIs, particularly surface carriers of super high mobility whilst maintaining bulk insulating states, is seriously impeded by the unintentional doping (arising from nonstoichiometry) of bulk carriers. Moreover, in thin films and heterostructures, at the all-important thickness range of a few quintuple layers, the additional limitation of the film-substrate lattice mismatch and resulting strain in films is a major concern. Doping of TIs with impurities can lead to secondary phase formation in the bulk or locally on the surface which can lead to unreliable properties, unless the above factors are carefully addressed. Device structure fabrication needed for the investigation of transport properties using liquid based lithography seriously affects surface properties. There has been progress in addressing some of these issues, but much work is still required in order to move forward.

The quantum Hall (QH) effect exhibits a topological electronic state with dissipationless edge currents circulating in one direction of a two dimensional electron layer under a strong magnetic field.9 The QAH effect shares similar characteristic features as the QH effect, whereas its physical origin stems from the intrinsic spin-orbit coupling and ferromagnetism. QAH displays the dissipationless quantized Hall transport in ferromagnetic materials in the absence of external magnetic fields. Realizing the QAH effect in realistic materials requires that the system be a ferromagnetic insulator with topologically non-trivial electronic band structures. It is possible to achieve this in a TI by doping it with transition metal atoms, such as Cr,5 Mn,10,11 and V,6 thereby introducing ferromagnetism in it. Careful optimization of thin film growth and concurrent control of the dopant and carrier concentration in the ferromagnetic TI has been found to be very important to observe ideal QAH state and dissipationless edge chiral current.

Alternately, proximity-induced ferromagnetism in a TI is attainable in a heterostructure consisting of a TI and a ferromagnetic material—and preferably an insulating ferromagnet such as EuS12 or YIG (Y 3Fe5012).13 The short-range nature of magnetic proximity coupling between the TI and the ferromagnet necessarily requires extreme control of the interface. In this approach, the advantage, while also not introducing defects, is that the ferromagnetism gets introduced mostly at the interface layers of the TI, thus modifying the behavior of the all-important surface, rather than affecting the majority bulk states. Hence, the properties of TI can be tuned independently.

Considering the great difficulties in creating highly stoichiometric, highly perfect materials, as outlined above, and considering the strong dependence of device performance on materials properties, it is a very opportune time to focus on the materials aspects of TIs. Hence, this special issue of APL Materials highlights the materials science aspects of the field. These include growth, fabrication, and characterization of TI films and devices in addition to showcasing some of the important developments and exciting results so far discovered.

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