Pressure-driven collapse of the relativistic electronic ground state in a honeycomb iridate

Honeycomb-lattice quantum magnets with strong spin-orbit coupling are promising candidates for realizing a Kitaev quantum spin liquid. Although iridate materials such as Li2IrO3 and Na2IrO3 have been extensively investigated in this context, there is still considerable debate as to whether a localized relativistic wavefunction (Jeff = 1/2) provides a suitable description for the electronic ground state of these materials. To address this question, we have studied the evolution of the structural and electronic properties of α-Li2IrO3 as a function of applied hydrostatic pressure using a combination of x-ray diffraction and x-ray spectroscopy techniques. We observe striking changes even under the application of only small hydrostatic pressure (P ≤ 0.1 GPa): a distortion of the Ir honeycomb lattice (via X-ray diffraction), a dramatic decrease in the strength of spin-orbit coupling effects (via X-ray absorption spectroscopy), and a significant increase in non-cubic crystal electric field splitting (via resonant inelastic X-ray scattering). Our data indicate that α-Li2IrO3 is best described by a Jeff = 1/2 state at ambient pressure, but demonstrate that this state is extremely fragile and collapses under the influence of applied pressure. A Kitaev quantum spin liquid is an exotic state of matter in which spins do not order even at very low temperature — it can be realized in materials with a honeycomb lattice and strong spin-orbit coupling, such as Li2IrO3. Two different descriptions have been put forward to describe the electronic ground state of this material, one involving localized electrons, the other itinerant electrons. Only the localized picture is compatible with the realization of a Kitaev spin liquid. To discriminate between these scenarios, Young-June Kim at the University of Toronto, Canada and colleagues studied the effect of applying hydrostatic pressure combining different X-ray techniques. They found that a localized electronic state is observed at room pressure, but it is very fragile and extremely small pressures are sufficient to disrupt it.