Quantum disordered ground state in the spin-orbit coupled Jeff = 1/2 distorted honeycomb magnet BiYbGeO5

We delineate quantum magnetism in the strongly spin-orbit coupled, distorted honeycomb-lattice antiferromagnet BiYbGeO${}_5$. Our magnetization and heat capacity measurements reveal that its low-temperature behavior is well described by an effective $J_{\mathrm{e}\mathrm{f}\mathrm{f}}=1\mathrm{/}2$ Kramers doublet of Yb${}^{3+}$. The ground state is nonmagnetic with a tiny spin gap. Temperature-dependent magnetic susceptibility, magnetization isotherm, and heat capacity could be modeled well assuming isolated spin dimers with anisotropic exchange interactions $J_{\mathrm{Z}}\simeq 2.6$~K and $J_{\mathrm{X}\mathrm{Y}}\simeq 1.3$~K. Heat capacity measurements backed by muon spin relaxation suggest the absence of magnetic long-range order down to at least 80,mK both in zero field and in applied fields. This sets BiYbGeO${}_5$ apart from Yb${}_2$Si${}_2$O${}_7$ with its unusual regime of magnon Bose-Einstein condensation and suggests negligible interdimer couplings, despite only a weak structural deformation of the honeycomb lattice.