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

Abstract

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) [1,2]. Such a localized electronic state carries an effective total angular momentum of J ef f = 1/2 [3,4]. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and Na2IrO3, these J ef f = 1/2 moments are expected to be coupled through a special bonddependent magnetic interaction [5][6][7], which is a necessary condition for the realization of a Kitaev quantum spin liquid [8]. 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 [9,10]. 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 ef f = 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. arXiv:1803.04056v1 [cond-mat.str-el]

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…We first consider α‐Li 2 IrO 3 . The pressure effects in this system has been studied experimentally by X‐ray diffraction measurements with synchrotron radiation and more recently by other X‐ray techniques . These studies reveal that α‐Li 2 IrO 3 undergoes a transition from the monoclinic phase ( space group) to a lower symmetry triclinic phase ( space group) at the critical pressure .…”

“…Details of the interplay between electronic correlations ( U ), spin‐orbit coupling (SOC) effects, magnetism and covalent bonding in α‐Li 2 IrO 3 have been explored by DFT simulations and quantum chemistry calculations . Two competing interactions controlling the structural transition at finite pressure are (a) the energy gain due to the local moment formation and (b) the dimerization and the consequent quenching of local moments.…”

“…it is possible to steer the systems through various phase transitions in a controlled way. As a matter of fact, this technique has proven to be utterly useful to uncover the nature of quantum phases, such as a Mott insulator, spin liquid, correlated metal, multiferroic, or unconventional superconductor . Materials where such a tuning parameter is very effective are, for instance, organic charge transfer salts, where, due to the softness of the organic components, small pressures induce large changes in the electronic properties of the materials giving rise to complex phase diagrams .…”

Optical signature of the pressure-induced dimerization in the honeycomb iridate α−Li2IrO3

“…Experiments finally showed that the electronic ground state of Na 2 IrO 3 can be well described in terms of a relativistic Mott insulator with a half-filled j eff =1/2-band split by the Hubbard U [17,27,32,33], although it exhibits a significant intrahexagon itinerant character which could lead to a mixing of j eff =1/2 (j 1/2 ) and j eff =3/2 (j 3/2 ) states in the Ir t 2g level [17,34,35]. Compared to Na 2 IrO 3 , for α-Li 2 IrO 3 signs for an increased QMO character was observed at ambient conditions [17] and at low pressures [36].…”

Spin–lattice and electron–phonon coupling in 3d/5d hybrid Sr3NiIrO6

“…5d materials are unique for several reasons. First, strong spin-orbit coupling competes with magnetic, crystalfield, many-body Coulomb, and other interactions to drive new physical behaviors [7], such as the J eff = 1/2 state in certain iridates [8,9]. Second, the bonding interactions associated with the larger 5d orbitals promote inter-cation dimerization in pairwise, chain-like, and other complex orderings [10,11].…”