Theoretical study of spherical kagome clusters in Mo72 V30 and W72 V30

Abstract

Spin-1/2 spherical kagomé clusters, or quantum-spin icosidodecahedrons, occurring in Mo|$_{72}$|V|$_{30}$| and W|$_{72}$|V|$_{30}$| are analyzed using the Lanczos method. Magnetic susceptibility analysis reveals that, whereas Mo|$_{72}$|V|$_{30}$| contains some degree of structural distortion that decreases its spin gap, the structure of W|$_{72}$|V|$_{30}$| can be described as a regular (non-distorted) icosidodecahedron. We determine the four types of exchange parameter for Mo|$_{72}$|V|$_{30}$| and clarify the mechanism of distortion that causes spin-gap suppression. The non-distorted W|$_{72}$|V|$_{30}$| system has about eighty low-lying singlet states below the first triplet excitation. The specific heats of the two materials are calculated and compared in order to illustrate the effects of high numbers of low-energy states on the low-temperature specific heat of W|$_{72}$|V|$_{30}$|⁠. Numerical calculations of the Raman spectrum of W|$_{72}$|V|$_{30}$| are carried out in order to determine how the low-lying singlet states can be observed through Raman scattering experiments.