  Recent reports of a possible room-temperature superconductor called LK-99
have generated a lot of attention worldwide. In just a few days, a large amount
of experimental works attempted to reproduce this sample and verify its
properties. At the same time a large amount of theoretical works have also been
reported. However, many experiments have drawn different conclusions, and many
theoretical results are not consistent with experimental results. For one of
the structures of LK-99 with the chemical formula as Pb9Cu(PO4)6O, many
first-principles calculations did not consider spin-orbit coupling and
concluded that it is a flat band metal. However, spin-orbit coupling is often
not negligible in systems with heavy elements, and LK-99 contains a large
amount of heavy element Pb. We performed calculations of electronic structure
of Pb9Cu(PO4)6O with spin-orbit coupling, and the results show that it's not a
metal but a semiconductor. This is consistent with many experimental results.
In the ferromagnetic state Pb9Cu(PO4)6O is an indirect-bandgap semiconductor
with a bandgap of 292 meV. Moreover, its conduction band is a flat band. At an
electron doping level of 0.5 e/unit cell, Pb9Cu(PO4)6O becomes metallic and has
a flat band with a width of only 25 meV at the Fermi level in the ferromagnetic
state. While in the antiferromagnetic-A state, Pb9Cu(PO4)6O is a direct-bandgap
semiconductor with a bandgap of 300 meV. As a magnetic narrowband
semiconductor, Pb9Cu(PO4)6O may have potential application value in the field
of optoelectronic device, photocatalytic, photodetector and spintronics device.
