Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Strong correlation effects of the Re 5$d$ electrons on the
  metal-insulator transition in Ca$_2$FeReO$_6$   We have investigated the electronic structure of polycrystalline
Ca$_2$FeReO$_6$ using photoemission spectroscopy and band-structure
calculations within the local-density approximation+$U$ (LDA+$U$) scheme. In
valence-band photoemission spectra, a double-peak structure which is
characteristic of the metallic double perovskite series has been observed near
the Fermi level ($E_{\rm F}$), although it is less distinct compared to the
Sr$_2$FeMoO$_6$ case. The leading near-$E_{\rm F}$ structure has a very weak
spectral weight at $E_{\rm F}$ above the metal-insulator transition (MIT)
temperature $T_{\rm MI}$ of $\sim$140 K, and it loses the $E_{\rm F}$ weight
below $T_{\rm MI}$, forming a small energy gap.
  To reproduce this small energy gap in the calculation, we require a very
large effective $U$ ($U_{\rm eff}$) for Re (4 eV) in addition to a relatively
large $U_{\rm eff}$ for Fe (4 eV). Although the most of the experimental
features can be interpreted with the help of the band theory, the overall
agreement between the theory and the experiment was not satisfactory. We
demonstrate that the effective transfer integral between Fe and Re is actually
smaller than that between Fe and Mo in Ca$_2$FeMoO$_6$, which can explain both
MIT and very high ferrimagnetic transition temperature.

27
The categories of this paper are: comp-gas
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
13
The categories of this paper are: quant-ph
6
The categories of this paper are: astro-ph
19
The categories of this paper are: eess
28
The categories of this paper are: alg-geom
34
The categories of this paper are: chem-ph
8
The categories of this paper are: hep-ex
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The categories of this paper are: dg-ga
3
The categories of this paper are: physics
2
The categories of this paper are: math
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The categories of this paper are: q-fin
37
The categories of this paper are: supr-con
14
The categories of this paper are: math-ph
5
The categories of this paper are: gr-qc