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

Abstract

A pressure-induced collapse of magnetic ordering in β-Li2IrO3 at Pm ∼ 1.5−2 GPa has previously been interpreted as evidence for possible emergence of spin liquid states in this hyperhoneycomb iridate, raising prospects for experimental realizations of the Kitaev model. Based on structural data obtained at room temperature, this magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions. Here we report on the evolution of the crystal structure of β-Li2IrO3 under pressure at low temperatures (T ≤ 50 K) and show that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization. The critical pressure for dimerization shifts from 4.4(2) GPa at room temperature to ∼ 1.5 − 2 GPa below 50 K. While a direct F ddd → C2/c transition is observed at room temperature, the low temperature transitions involve new as well as coexisting dimerized phases. Further investigation of the Ir (L3/L2) isotropic branching ratio in x-ray absorption spectra indicates that the previously reported departure of the electronic ground state from a J eff = 1/2 state is closely related to the onset of dimerized phases. In essence, our results suggest that the predominant mechanism driving the collapse of magnetism in β-Li2IrO3 is the pressure-induced formation of Ir2 dimers in the hyperhoneycomb network. The results further confirm the instability of the J eff = 1/2 moments and related non-collinear spiral magnetic ordering against formation of dimers in the low-temperature phase of compressed β-Li2IrO3. arXiv:1905.08211v2 [cond-mat.str-el]

High-temperature magnetic anomaly in the Kitaev hyperhoneycomb compound β−Li2IrO3

“…However, the µSR data unambiguously rules this out, as the presence of such a phase would lead to oscillations in the muon relaxation. Another possibility is a valencebond transition, similar to that seen under pressure in α-RuCl 3 51 or β-Li 2 IrO 3 [52][53][54][55] . However the spin dimerization has an associated structural distortion that leads to strong hysteresis on warming and cooling, and this is absent in the current data.…”

Field evolution of low-energy excitations in the hyperhoneycomb magnet β−Li2IrO3

“…This compound is magnetically ordered at ambient pressure and in zero magnetic field, but its incommensurately ordered state appears to be rather fragile. Pressure breaks down the magnetic order and triggers the formation of a partially frozen spin liquid above 1.4 GPa [22], which may be concomitant with a structural transformation [23]. External field applied along the b direction alters the magnetically ordered state too, although in this case the incommensurate order is gradually replaced by the commensurate one above H c = 2.8 T [24][25][26], and no spin-liquid state is observed.…”

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 .…”