Observation of Zero-Point Quantum Fluctuations of a Single-Molecule Magnet through the Relaxation of its Nuclear Spin Bath.

A single-molecule magnet placed in a magnetic field perpendicular to its anisotropy axis can be truncated to an effective two-level system, with easily tunable energy splitting. The quantum coherence of the molecular spin is largely determined by the dynamics of the surrounding nuclear spin bath. Here we report the measurement of the nuclear spin-lattice relaxation rate 1= T-1n in a single crystal of the singlemolecule magnet Mn-12-ac, at T approximate to 30 mK in perpendicular fields B-vertical bar up to 9 T. The relaxation channel at B approximate to 0 is dominated by incoherent quantum tunneling of the Mn 12 -ac spin S, aided by the nuclear bath itself. However for B-vertical bar > 5 T we observe an increase of 1= T-1n by several orders of magnitude up to the highest field, despite the fact that the molecular spin is in its quantum mechanical ground state. This striking observation is a consequence of the zero-point quantum fluctuations of S, which allow it to mediate the transfer of energy from the excited nuclear spin bath to the crystal lattice at much higher rates. Our experiment highlights the importance of quantum fluctuations in the interaction between an “effective two-level system” and its surrounding spin bath.