Pressure tuning of bond-directional exchange interactions and magnetic frustration in the hyperhoneycomb iridate β−Li2IrO3

Abstract

We explore the response of Ir 5d orbitals to pressure in β-Li2IrO3, a hyperhoneycomb iridate in proximity to a Kitaev quantum spin liquid (QSL) ground state. X-ray absorption spectroscopy reveals a reconstruction of the electronic ground state below 2 GPa, the same pressure range where x-ray magnetic circular dichroism shows an apparent collapse of magnetic order. The electronic reconstruction, which manifests a reduction in the effective spin-orbit (SO) interaction in 5d orbitals, pushes β-Li2IrO3 further away from the pure J eff = 1/2 limit. Although lattice symmetry is preserved across the electronic transition, x-ray diffraction shows a highly anisotropic compression of the hyperhoneycomb lattice which affects the balance of bond-directional Ir-Ir exchange interactions driven by spin-orbit coupling at Ir sites. An enhancement of symmetric anisotropic exchange over Kitaev and Heisenberg exchange interactions seen in theoretical calculations that use precisely this anisotropic Ir-Ir bond compression provides one possible route to realization of a QSL state in this hyperhoneycomb iridate at high pressures.The novel electronic ground states of 5d-based compounds driven by spin-orbit interactions continue to provide an excellent playground for the realization of unconventional quantum phases of matter including topological insulators [1-4] and quantum spin-liquids (QSLs) [5][6][7]. One example of the latter is the non-trivial QSL ground state of the Kitaev model [8], a rare example of a solvable interacting quantum model with Majorana fermions as its elementary excitations. Material candidates for possible realization of the Kitaev model include honeycomb-based-lattice systems with strong spin-orbit coupling [6,9], such as the two and three-dimensional honeycomb iridates, α-Li(Na) [7,[20][21][22] as well as α-RuCl 3 [23,24]. However, it is experimentally established that these materials order magnetically at low temperatures [17,18,20,[25][26][27], spoiling numerous attempts to realize the Kitaev QSL. Hence, tuning structure and related intricate interactions present in these materials through chemical or physical pressure provides a potential route to introduce magnetic frustration and realize novel phases of matter.In this Letter we have investigated the electronic and structural response of β-Li 2 IrO 3 to high pressure. Xray absorption near edge structure (XANES) measurements at Ir L-edges reveal a dramatic suppression of the isotropic Ir (L 3 /L 2 ) branching ratio at P ∼ 1.5 GPa, signaling a reduction in the effective strength of spinorbit interactions in the 5d band. This is the same pressure at which net magnetization in applied field collapses [17]. The reconstructed electronic state preserves the L z / S z orbital-to-spin moment ratio of Ir magnetic moments and the insulating ground state indicating that spin-orbit interactions and Mott physics con-

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

“…In γ-Li 2 IrO 3 , single crystal x-ray diffraction (XRD) measurements of the a-axis lattice parameter under pressure show a smooth evolution across the low temperature magnetic transition, implying that structural distortions were not at play 33 . However, studies of the first-order, pressure-induced structural phase transition in β-Li 2 IrO 3 at P S ∼ 4.4(2) GPa at room temperature show a strong non-linear response of the b-axis compressibility across the structural boundary while the compressibility of the a-axis remains rather linear 32 . The high-pressure monoclinic structure features significantly shortened Ir-Ir bond length within the zigzag chain, and the anticipated formation of Ir 2 dimers at ∼ 4 GPa was further confirmed by recent neutron diffraction experiments at room temperature 35 .…”

“…Indeed, the apparent collapse of magnetic ordering in β-Li 2 IrO 3 20,32 (T N = 38 K) and γ-Li 2 IrO 3 33 (T N = 38 K) at unexpectedly low pressures (P m ∼ 1.5−2 GPa) has generated significant interest in understanding what kind of local moment interactions develop in the high-pressure phase, what mechanism drives them, and whether such interactions would ultimately lead to a QSL state. In β-Li 2 IrO 3 , X-ray magnetic circular dichroism (XMCD) reveals that the ferromagnetic response in an applied magnetic field disappears at ∼ 2 GPa, the same pressure range where X-ray absorption spectroscopy (XAS) shows a reconstruction of the electronic structure and departure from the strong SO coupling limit 32 . Muon spin relaxation (µSR) measurements also reveal a breakdown of magnetic ordering at similar pressures (∼ 1.4 GPa) and emergence of a new ground state marked by the coexistence of disordered states, namely, spin liquid and spin glass 34 .…”

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 .…”

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…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 .…”