Searches for experimental materialization of Kitaev’s quantum spin liquid ground state [1] have taken central stage in condensed matter physics. This non-trivial ground state contains topologically protected excitations and holds promise for applications in quantum information and quantum computation areas [1]. Honeycomb-based systems with strong spin-orbit (SO) coupled Ir4+ions and related bond-directional exchange anisotropies are the prominent candidates but the spin-liquid regime is precluded at ambient pressure by competing exchange interactions that preserve long-range magnetic ordering [2-4].
Within this context, a myriad of experimental efforts has been put forward to tune the competing exchange interactions of these materials under external stimuli [5-10]. For example, evidence in favor of spin-liquid state under applied pressure has been found in ß-Li2IrO3 after independent magnetic probes revealed that its magnetic ordering is suppressed at ~1.5-2 GPa [2, 5-6]. Based on structural data obtained at room temperature, this order-disorder magnetic transition is believed to originate in small lattice perturbations that preserve crystal symmetry, and related changes in bond-directional anisotropic exchange interactions [5, 8]. We have performed thorough investigation of the evolution of the crystal structure of ß-Li2IrO3 under pressure at low temperatures (T ≤ 50 K) to reveal that the suppression of magnetism coincides with a change in lattice symmetry involving Ir-Ir dimerization [11]. We find that the critical pressure for dimerization significantly shifts from 4.4(2) GPa at room temperature to ~1.5-2 GPa below 50 K. The low temperature transitions involve new as well as coexisting dimerized phases, departing from the direct Fddd→C2/c transformation observed at room temperature. Our new investigation of the Ir (L3/L2) isotropic branching ratio in x-ray absorption spectra reveals that the previously reported reconstruction of the electronic ground state and departure from the Jeff=1/2 picture are closely related to the onset of dimerized phases. Formation of Ir2 dimers as the leading mechanism driving the magnetically disordered ground state in ß-Li2IrO3 constitutes an important result against the putative spin-liquid state in this material.
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[2]
T. Takayama et. al., Phys. Rev. Lett., 114, 077202 (2015)
[3] S. C. Williams et.al., Phys. Rev. B 93, 195158 (2016)
[4] K. A. Modic et. al., Nat. Comm. 5, 4203 (2014)
[5] L. S. I. Veiga et. al., Phys. Rev. B 96, 140402(R) (2017)
[6] N. P. Breznay et. al., Phys. Rev. B 96, 020402(R) (2017)
[7] V. Hermann et. al., Phys. Rev. B 97, 020104(R) (2018)
[8] M. Majumder et. al., Phys. Rev. Lett. 120, 237202 (2018)
[9] V. Hermann et. al., Phys. Rev. B 96, 195137 (2017)
[10] T. Takayama et. al., Phys. Rev. B 99, 125127 (2019)
[11] L. S. I. Veiga et. al., to be submitted.