Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Topology of photonic time-crystals   We introduce topological phases in Photonic Time-Crystals. Photonic
Time-Crystals are materials in which the refractive index varies periodically
and abruptly in time. When the refractive index changes abruptly, the light
propagating in the material experiences time-refraction and time-reflection,
analogous to refraction and reflection in photonic crystals. Interference
between time-refracted and time-reflected waves gives rise to Floquet-Bloch
states and dispersion bands, which are gapped in the momentum. We show that
photonic time-crystals can be in a topologically non-trivial phase, and
calculate the topological invariant associated with the momentum bands of the
Photonic Time-Crystal. The topological invariants are related to the phase
between the forward and backward-propagating waves of the time-crystal, and to
localized edge states in time.

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The categories of this paper are: funct-an
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The categories of this paper are: nlin
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The categories of this paper are: cmp-lg
5
The categories of this paper are: gr-qc
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The categories of this paper are: plasm-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: chao-dyn
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The categories of this paper are: adap-org
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The categories of this paper are: econ
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The categories of this paper are: math-ph
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The categories of this paper are: q-alg
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The categories of this paper are: nucl-ex
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The categories of this paper are: hep-th
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-ex
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The categories of this paper are: mtrl-th