Disordered ground state in a spin-orbit coupled pseudospin-$\frac{1}{2}$ cobalt-based metal-organic framework magnet with orthogonal spin dimers

We present the magnetic properties of a quantum dimer magnet based on ${\mathrm{Co}}^{2+}$. The metal-organic framework compound ${\mathrm{Co}}_2{\left(\mathrm{BDC}\right)}_2{\left(\mathrm{DPTTZ}\right)}_2·\mathrm{DMF}$ features ${\mathrm{Co}}^{2+}$ dimers arranged nearly orthogonal to each other, similar to the Shastry-Sutherland lattice. Our assessment based on the magnetization and heat capacity experiments reveals that the magnetic properties at low temperatures can be described by pseudospin-1/2. The magnetic ground state of the material is found to be a singlet with a tiny spin gap. The thermodynamic properties are described by a model of isolated spin dimers with the anisotropic exchange couplings $J_{xy}\simeq 3.5$ K and $J_z\simeq 11$ K. Interestingly, no field-induced quantum phase transition is detected down to 100 mK around the critical field of gap closing, suggesting the absence of Bose-Einstein condensation of triplons and establishing isolated dimers with a negligible interdimer coupling.