Pressure-induced collapse of the spin-orbital Mott state in the hyperhoneycomb iridate β−Li2IrO3

Abstract

Hyperhoneycomb iridate β-Li2IrO3 is a three-dimensional analogue of two-dimensional honeycomb iridates, such as α-Li2IrO3, which recently appeared as another playground for the physics of Kitaevtype spin liquid. β-Li2IrO3 shows a non-collinear spiral ordering of spin-orbital-entangled J eff = 1/2 moments at low temperature, which is known to be suppressed under a pressure of ∼2 GPa. With further increase of pressure, a structural transition is observed at PS ∼ 4 GPa at room temperature. Using the neutron powder diffraction technique, the crystal structure in the high-pressure phase of β-Li2IrO3 above PS was refined, which indicates the formation of Ir2 dimers on the zig-zag chains, with the Ir-Ir distance even shorter than that of metallic Ir. We argue that the strong dimerization stabilizes the bonding molecular orbital state comprising the two local dzx-orbitals on the Ir-O2-Ir bond plane, which conflicts with the equal superposition of dxy-, dyz-and dzx-orbitals in the J eff = 1/2 wave function produced by strong spin-orbit coupling. The results of resonant inelastic x-ray scattering (RIXS) measurements and the electronic structure calculations are fully consistent with the collapse of the J eff = 1/2 state. A subtle competition of various electronic phases is universal in honeycomb-based Kitaev materials.

Pressure-induced structural dimerization in the hyperhoneycomb iridateβ−Li2IrO3at low temperatures

“…Note that a rather significant contraction of the c lattice parameter is also observed in the P 2 1 /n phase, a fact that could explain the unexpected transition to the F ddd structure in a mixed phase, with presence of shortened Z-bonds. While the latter was not detected in any of the experiments conducted in the past 32,35 , the presence of dimerized Z-bonds was predicted theoretically for structure optimizations in absence of SO coupling or Coulomb interaction 53 . Although our results point towards a dimerization of all Ir-Ir bond types, a preferable dimerization of the Ir-Ir X, Y -bonds forming the zig-zag chains is observed as a consequence of the significant contraction of the a and b lattice parameters for almost all the structures.…”

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

“…It is furthermore interesting to note that the pressure-induced changes in α-Li 2 IrO 3 seem to be quite similar to those in the threedimensional analogue β-Li 2 IrO 3 , due to the similarities in the local environment of the Ir atoms: It was predicted that β-Li 2 IrO 3 undergoes an electronic phase transition from Mott insulator to band insulators, concomitant to the structural phase transition to a dimerized phase [23,55,56]. The three-peak profile of the optical conductivity of α-Li 2 IrO 3 at high pressures, reported here, appears to be indicative for the dimerized state in iridates [55].…”

“…In Na 2 IrO 3 in its dimerized phase, which is predicted at high pressure [21,23], the dimerization introduces a Mott insulator to band insulator transition, whereby the low-energy excitations could possibly preserve their j 1/2 -character [23]. Such a Mott to band insulator transition is common to other j 1/2 Mott insulators in their dimerized phase [22,55,56]. It is furthermore interesting to note that the pressure-induced changes in α-Li 2 IrO 3 seem to be quite similar to those in the threedimensional analogue β-Li 2 IrO 3 , due to the similarities in the local environment of the Ir atoms: It was predicted that β-Li 2 IrO 3 undergoes an electronic phase transition from Mott insulator to band insulators, concomitant to the structural phase transition to a dimerized phase [23,55,56].…”

Anisotropic temperature-field phase diagram of single crystalline β−Li2IrO3 : Magnetization, specific heat, and Li7

“…In the following, we show that the field H ⊥ b has minor influence on β-Li 2 IrO 3 indeed and does not break the Q = 0 incommensurate order. Moreover, we probe the field-induced state above H c for H b and juxtapose it with the pressure-induced state of β-Li 2 IrO 3 [23], where thermodynamic measurements and local probes detect the breakdown of the incommensurate order above 1.4 GPa and the formation of a partially frozen spin liquid [24], although these effects may also result from a structural dimerization [25] that occurs in the same pressure range at low temperatures [26]. We also use nuclear magnetic resonance (NMR) as a local probe of the field-induced state above H c .…”