Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Dirac lines and loop at the Fermi level in the Time-Reversal Symmetry
  Breaking Superconductor LaNiGa$_2$   Unconventional superconductors have Cooper pairs with lower symmetries than
in conventional superconductors. In most unconventional superconductors, the
additional symmetry breaking occurs in relation to typical ingredients such as
strongly correlated Fermi liquid phases, magnetic fluctuations, or strong
spin-orbit coupling in noncentrosymmetric structures. In this article, we show
that the time-reversal symmetry breaking in the superconductor LaNiGa$_2$ is
enabled by its previously unknown topological electronic band structure. Our
single crystal diffraction experiments indicate a nonsymmorphic crystal
structure, in contrast to the previously reported symmorphic structure. The
nonsymmorphic symmetries transform the $k_z=\pi/c$ plane of the Brillouin zone
boundary into a node-surface. Band-structure calculations reveal that distinct
Fermi surfaces become degenerate on the node-surface and form Dirac lines and a
Dirac loop at the Fermi level. Two symmetry related Dirac points remain
degenerate under spin-orbit coupling. ARPES measurements confirm the
calculations and provide evidence for the Fermi surface degeneracies on the
node-surface. These unique topological features enable an unconventional
superconducting gap in which time-reversal symmetry can be broken in the
absence of other typical ingredients. LaNiGa$_2$ is therefore a topological
crystalline superconductor that breaks time-reversal symmetry without any
overlapping magnetic ordering or fluctuations. Our findings will enable future
discoveries of additional topological superconductors.

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The categories of this paper are: math-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: mtrl-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: gr-qc
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: math
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The categories of this paper are: plasm-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: eess
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The categories of this paper are: physics
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The categories of this paper are: q-alg
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-th
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The categories of this paper are: bayes-an