Low-energy excitations in the S=1/2 molecular nanomagnet K-6[(V15As6O42)-As-IV(H2O)]center dot 8H(2)O from proton NMR and mu SR

Zero- and longitudinal-field muon-spin-rotation (mu SR) and H-1 NMR measurements on the S=1/2 molecular nanomagnet K-6[(V15As6O42)-As-IV(H2O)](.)8H(2)O are presented. In LF experiments, the muon asymmetry P(t) was fitted by the sum of three different exponential components with fixed weights. The different muon relaxation rates lambda(i) (i=1,2,3) and the low-field H=0.23 T H-1 NMR spin-lattice relaxation rate 1/T-1 show a similar behavior for T > 50 K: starting from room temperature they increase as the temperature is decreased. The increase of lambda(i) and 1/T-1 can be attributed to the 'condensation' of the system toward the lowest-lying energy levels. The gap Delta similar to 550 K between the first and second S=3/2 excited states was determined experimentally. For T < 2 K, the muon relaxation rates lambda(i) stay constant, independently of the field value H <= 0.15 T. The behavior for T < 2 K strongly suggests that, at low T, the spin fluctuations are not thermally driven but rather originate from quasielastic intramolecular or intermolecular magnetic interactions. We suggest that the very-low-temperature relaxation rates could be driven by energy exchanges between two almost degenerate S=1/2 ground states and/or by quantum effects.