  The electronic ground state in many iridate materials is described by a
complex wave-function in which spin and orbital angular momenta are entangled
due to relativistic spin-orbit coupling (SOC). Such a localized electronic
state carries an effective total angular momentum of $J_{eff}=1/2$. In
materials with an edge-sharing octahedral crystal structure, such as the
honeycomb iridates Li2IrO3 and Na2IrO3, these $J_{eff}=1/2$ moments are
expected to be coupled through a special bond-dependent magnetic interaction,
which is a necessary condition for the realization of a Kitaev quantum spin
liquid. However, this relativistic electron picture is challenged by an
alternate description, in which itinerant electrons are confined to a
benzene-like hexagon, keeping the system insulating despite the delocalized
nature of the electrons. In this quasi-molecular orbital (QMO) picture, the
honeycomb iridates are an unlikely choice for a Kitaev spin liquid. Here we
show that the honeycomb iridate Li2IrO3 is best described by a $J_{eff}=1/2$
state at ambient pressure, but crosses over into a QMO state under the
application of small (~ 0.1 GPa) hydrostatic pressure. This result illustrates
that the physics of iridates is extremely rich due to a delicate balance
between electronic bandwidth, spin-orbit coupling, crystal field, and electron
correlation.
