  The Kitaev spin liquid provides a rare example of well-established quantum
spin liquids in more than one dimension. It is obtained as the exact ground
state of the Kitaev spin model with bond-dependent anisotropic interactions.
The peculiar interactions can be yielded by the synergy of spin-orbit coupling
and electron correlations for specific electron configuration and lattice
geometry, which is known as the Jackeli-Khaliullin mechanism. Based on this
mechanism, there has been a fierce race for the materialization of the Kitaev
spin liquid over the last decade, but the candidates have been still limited
mostly to $4d$- and $5d$-electron compounds including cations with the low-spin
$d^5$ electron configuration, such as Ir$^{4+}$ and Ru$^{3+}$. Here we discuss
recent efforts to extend the material perspective beyond the Jackeli-Khaliullin
mechanism, by carefully reexamining the two requisites, formation of the
$j_{\rm eff}=1/2$ doublet and quantum interference between the exchange
processes, for not only $d$- but also $f$-electron systems. We present three
examples: the systems including Co$^{2+}$ and Ni$^{3+}$ with the high-spin
$d^7$ electron configuration, Pr$^{4+}$ with the $f^1$-electron configuration,
and polar asymmetry in the lattice structure. In particular, the latter two are
intriguing since they may realize the antiferromagnetic Kitaev interactions, in
contrast to the ferromagnetic ones in the existing candidates. This partial
overview would stimulate further material exploration of the Kitaev spin
liquids and its topological properties due to fractional excitations.

Strongly Correlated Electrons