Magnetic excitations in the spin-12tetramer substanceCu2Cd11411B2O6obtained by inelastic neutron scattering experiments

We performed inelastic neutron scattering experiments on ${\mathrm{Cu}}_{2}^{114}\mathrm{Cd}{}^{11}{\text{B}}_{2}{\mathrm{O}}_{6}$ powder. The magnetic excitations at low temperatures are similar to those of the interacting spin-$\frac{1}{2}$ tetramers in the ordered state. The weak excitations existing above 3 meV suggest that the Higgs mode appears in ${\mathrm{Cu}}_{2}{\mathrm{CdB}}_{2}{\mathrm{O}}_{6}$ at ambient pressure and zero magnetic field. We evaluated ${J}_{1}=27.3\ifmmode\pm\else\textpm\fi{}1.0$ and ${J}_{2}=\ensuremath{-}14.0\ifmmode\pm\else\textpm\fi{}1.4$ meV for the intratetramer interactions and ${J}_{3}=\ensuremath{-}0.4\ifmmode\pm\else\textpm\fi{}0.2$ and ${J}_{4}=1.4\ifmmode\pm\else\textpm\fi{}0.2$ meV for the intertetramer interactions. The spin gap in the isolated spin tetramer was calculated to be 1.6 meV, which is less than the effective intertetramer interaction value $(3.6\ifmmode\pm\else\textpm\fi{}0.8$ meV). Therefore, antiferromagnetic long-range order is possible, although the ground state of the isolated spin tetramer is the spin-singlet state. We discuss the temperature dependence of the magnetic excitations.