Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Evidence for unidirectional nematic bond ordering in FeSe   The lifting of $d_{xz}$-$d_{yz}$ orbital degeneracy is often considered a
hallmark of the nematic phase of Fe-based superconductors, including FeSe, but
its origin is not yet understood. Here we report a high resolution
Angle-Resolved Photoemission Spectroscopy study of single crystals of FeSe,
accounting for the photon-energy dependence and making a detailed analysis of
the temperature dependence. We find that the hole pocket undergoes a
fourfold-symmetry-breaking distortion in the nematic phase below 90~K, but in
contrast the changes to the electron pockets do not require fourfold
symmetry-breaking. Instead, there is an additional separation of the existing
$d_{xy}$ and $d_{xz/yz}$ bands - which themselves are not split within
resolution. These observations lead us to propose a new scenario of
"unidirectional nematic bond ordering" to describe the low-temperature
electronic structure of FeSe, supported by a good agreement with 10-orbital
tight binding model calculations.

12
The categories of this paper are: nucl-th
11
The categories of this paper are: hep-lat
0
The categories of this paper are: hep-ph
21
The categories of this paper are: adap-org
17
The categories of this paper are: q-fin
2
The categories of this paper are: math
31
The categories of this paper are: ao-sci
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
3
The categories of this paper are: physics
14
The categories of this paper are: math-ph
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
28
The categories of this paper are: alg-geom
8
The categories of this paper are: hep-ex