Experimental observation of magnetic dimers in dilutedYb:YAlO3

We present a comprehensive experimental investigation of Yb magnetic dimers in ${\mathrm{Yb}}_0.04{\mathrm{Y}}_0.96{\mathrm{AlO}}₃$, an Yb-doped yttrium aluminum perovskite ${\mathrm{YAlO}}₃$, by means of specific heat, magnetization, and high-resolution inelastic neutron scattering (INS) measurements. In our sample, the Yb ions are randomly distributed over the lattice and $\ensuremath{\sim}7%$ of Yb ions form quantum dimers due to nearest-neighbor antiferromagnetic coupling along the $c$ axis. At zero field, the dimer formation manifests itself in an appearance of an inelastic peak at $\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}0.2\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ in the INS spectrum and a Schottky-like anomaly in the specific heat. The structure factor of the INS peak exhibits a cosine modulation along the $L$ direction, in agreement with the $c$-axis nearest-neighbor intradimer coupling. A careful fitting of the low-temperature specific heat shows that the excited state is a degenerate triplet, which indicates a surprisingly small anisotropy of the effective Yb-Yb exchange interaction despite the low crystal symmetry and anisotropic magnetic dipole contribution, in agreement with previous reports for the Yb parent compound, ${\mathrm{YbAlO}}₃$ [L. Wu et al., Nat. Commun. 10, 698 (2019), L. S. Wu et al., Phys. Rev. B 99, 195117 (2019)], and in contrast to ${\mathrm{Yb}}₂{\mathrm{Pt}}₂\mathrm{Pb}$ [L. S. Wu et al., Science 352, 1206 (2016), W. J. Gannon et al., Nat. Commun. 10, 1123 (2019)]. The obtained results are precisely reproduced by analytical calculations for the Yb dimers.