Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Magnetic Order in 3D Topological Insulators -- Wishful Thinking or
  Gateway to Emergent Quantum Effects?   Three-dimensional topological insulators (TIs) are a perfectly tuned
quantum-mechanical machinery in which counter-propagating and oppositely
spin-polarized conduction channels balance each other on the surface of the
material. This topological surface state crosses the bandgap of the TI, and
lives at the interface between the topological and a trivial material, such as
vacuum. Despite its balanced perfection, it is rather useless for any practical
applications. Instead, it takes the breaking of time-reversal symmetry (TRS),
and the appearance of an exchange gap to unlock hidden quantum states. The
quantum anomalous Hall effect, which has first been observed in Cr-doped
(Sb,Bi)$_2$Te$_3$, is an example of such a state in which two edge channels are
formed at zero field, crossing the magnetic exchange gap. The breaking of TRS
can be achieved by magnetic doping of the TI with transition metal or rare
earth ions, modulation doping to keep the electronically active channel
impurity free, or by proximity coupling to a magnetically ordered layer or
substrate, in heterostructures or superlattices. We review the challenges these
approaches are facing in the famous 3D TI (Sb,Bi)$_2$(Se,Te)$_3$ family, and
try to answer the question whether these materials can live up to the hype
surrounding them.

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The categories of this paper are: solv-int
2
The categories of this paper are: math
14
The categories of this paper are: math-ph
29
The categories of this paper are: funct-an
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The categories of this paper are: supr-con
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The categories of this paper are: hep-lat
30
The categories of this paper are: q-alg
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The categories of this paper are: adap-org
8
The categories of this paper are: hep-ex
33
The categories of this paper are: plasm-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: quant-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: ao-sci
7
The categories of this paper are: hep-th
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The categories of this paper are: nlin