Evidence of low-energy singlet excited states in the spin- 12 polyhedral clusters { M o72 V30} and { W72 V30} with strongly frustrated kagome networks

Magnetization, specific heat, and electron spin resonance (ESR) measurements are carried out to clarify the low-energy excitations for the S=1/2 polyhedral clusters {Mo72V30} and {W72V30}. The clusters provide unique model systems of a kagomé network on a quasisphere. The linear field variation of magnetization at low temperatures indicates that the ground state is singlet for both clusters. The temperature and the magnetic field dependence of specific heat shows a distinct difference between the two clusters with differing structural symmetries. In {W72V30} with Ih symmetry, the existence of several tens of low-energy singlet excited states below the lowest triplet excited state is revealed. The specific heat of the slightly distorted {Mo72V30} with D5h symmetry leads to a drastic decrease of the singlet contribution, which is consistent with the partial lifting of the frustration and the decrease of degenerated low-energy states. The singlet excitation existence is confirmed further by the temperature dependence of the ESR spectra. Comprehensive experimental studies have demonstrated unique low-energy excitations of the spherical kagomé networks and their sensitivity to the cluster symmetry.