Determining ground states of correlated electron systems is fundamental to understanding novel phenomena in condensed matter physics. A difficulty, however, arises in a geometrically frustrated system in which the incompatibility between the global t
opology of an underlying lattice and local spin interactions gives rise to macroscopically degenerate ground states, potentially prompting the emergence of quantum spin states, such as resonating valence bond (RVB) and valence bond solid (VBS). Although theoretically proposed to exist in a kagome lattice -- one of the most highly frustrated lattices in two dimensions (2D) being comprised of corner-sharing triangles -- such quantum-fluctuation-induced states have not been observed experimentally. Here we report the first realization of the pinwheel VBS ground state in the S=1/2 deformed kagome lattice antiferromagnet Rb2Cu3SnF12. In this system, a lattice distortion breaks the translational symmetry of the ideal kagome lattice and stabilizes the VBS state.
Magnetic properties of the spin doughnut Mo_{75}V_{20}, which is a frustrated magnetic cluster with twenty spins, are studied by using the finite-temperature Lanczos method. Our model Hamiltonian consists of one ten-spin ring and two five-spin rings
with nearest-neighbor antiferromagnetic couplings J and J. The five-spin rings are coupled to the ten-spin ring by an antiferromagnetic exchange interaction J so as to form isosceles triangles of one J-bond and two J-bonds. Fitting the theoretical magnetic susceptibility with the experimental one measured by Muller et al., the exchange parameters are estimated as J=388 K, J=163 K and J=81 K. The most interesting feature of this system is that the two five-spin rings behave like two free-spins at low temperatures in despite of J>J. We also calculate temperature dependence of the specific heat, and find three peaks at T sim J, J and J/100.
