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 (muSR) and 1H NMR measurements on the S= 1/2 molecular nanomagnet K6[V15As6O42(H2O)]·8H2O 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 l_i (i=1,2,3) and the low-field H=0.23 T 1H NMR spin-lattice relaxation rate 1/T1 show a similar behavior for T>50 K: starting from room temperature they increase as the temperature is decreased. The increase of l_i and 1/T1 can be attributed to the “condensation” of the system toward the lowest-lying energy levels. The gap Delta, around 550 K, between the first and second S= 3/2 excited states was determined experimentally. For T<2 K, the muon relaxation rates l_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-lowtemperature relaxation rates could be driven by energy exchanges between two almost degenerate S= 1/2 ground states and/or by quantum effects.