Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Lattice dynamics and electronic excitations in a large family of lacunar
  spinels with a breathing pyrochlore lattice structure   Reproducing the electronic structure of AM$_4$X$_8$ lacunar spinels with a
breathing pyrochlore lattice is a great theoretical challenge due to the
interplay of various factors. The character of the M$_4$X$_4$ cluster orbitals
is critically influenced by the Jahn-Teller instability, the spin-orbit
interaction, and also by the magnetic state of the clusters. Consequently, to
reproduce the narrow-gap semiconducting nature of these moderately correlated
materials requires advanced approaches, since the strength of the inter-cluster
hopping is strongly affected by the character of the cluster orbitals. In order
to provide a solid experimental basis for theoretical studies, we performed
broadband optical spectroscopy on a large set of lacunar spinels, with
systematically changing ions at the A and M sites as well as the ligand (A=Ga,
Ge, Al; M=V, Mo, Nb, Ta; X=S, Se). Our study covers the range of phonon
excitations and also electronic transitions near the gap edge. In the phonon
excitation spectrum a limited subset of the symmetry allowed modes is observed
in the cubic state, with a few additional modes emerging upon the
symmetry-lowering structural transition. All the infrared active modes are
assigned to vibrations of the ligands and ions at the A sites, with no obvious
contribution from the M-site ions. Concerning the electronic states, we found
that all compounds are narrow-gap semiconductors ($E_\mathrm{g} = 130 -
350\,$meV) already in their room-temperature cubic state and their structural
transitions induce weak, if any, changes in the band gap. The gap value is
decreased when substituting S with Se and also when replacing $3d$ ions by $4d$
or $5d$ ions at the M sites.

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The categories of this paper are: hep-ph
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The categories of this paper are: chao-dyn
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
21
The categories of this paper are: adap-org
26
The categories of this paper are: bayes-an
17
The categories of this paper are: q-fin
9
The categories of this paper are: nlin
33
The categories of this paper are: plasm-ph
15
The categories of this paper are: nucl-ex
14
The categories of this paper are: math-ph
30
The categories of this paper are: q-alg
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The categories of this paper are: alg-geom
23
The categories of this paper are: solv-int
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The categories of this paper are: hep-lat