Local spin moment configuration in the frustrated s=1∕2 Heisenberg triangular antiferromagnet V15 determined by NMR

We report very low-temperature measurements of ⁵¹V NMR spectra as a function of external magnetic field in the s=1∕2 Heisenberg triangular antiferromagnet cluster K₆[V₁₅As₆O₄₂(H₂O)]∙8H₂O (V15), a simple prototype of a geometrically magnetic frustrated system. It is known that the frustration is relieved by a splitting of the degenerate ground state into two S_T=1∕2 states. Our measurements allow the microscopic determination of the local spin configuration both in the nonfrustrated total spin S_T=3∕2 state at high fields and in the two quasidegenerate S_T=1∕2 ground states at low fields, where it is shown that the simple frustrated chiral configuration in the triangle is broken down into two different spin configurations corresponding to the two quasidegenerate S_T=1∕2 states.