Title: Pressure-tuning of bond-directional exchange interactions and magnetic frustration in hyperhoneycomb iridate $β$-$\mathrm{Li_2IrO_3}$,
Abstract: We explore the response of Ir $5d$ orbitals to pressure in
$\beta$-$\mathrm{Li_2IrO_3}$, 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
$\beta$-$\mathrm{Li_2IrO_3}$ further away from the pure $J_{\rm eff}=1/2$
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.