Coexistence of a Spin-Singlet Ground State with the 3D Magnetic Order in theCsC60Polymerized Fulleride

The low temperature behavior of the ${\mathrm{CsC}}_{60}$ polymer was investigated by NMR measurements on ${}^{13}\mathrm{C}$ and ${}^{133}\mathrm{Cs}$ nuclei at ambient and under hydrostatic pressure up to 9 kbar. The existence of a second-order structural phase transition to a spin-singlet (nonmagnetic) ground state at $T_S=13.8\mathrm{}\mathrm{K}$ is clearly established at ambient pressure. This state coexists with the magnetic order that develops at $T_N=30\mathrm{}\mathrm{K}$. The application of pressure first suppresses the magnetic order and a homogeneous nonmagnetic ground state is stabilized at 5 kbar.