Adiabatic Landau-Zener-Stückelberg transition with or without dissipation in the low-spin molecular system V 15

The spin-$\frac{1}{2}$ molecular system ${\mathrm{V}}_{15}$ shows no barrier against spin reversal. This makes possible direct phonon activation between the two levels. By tuning the field sweeping rate and the thermal coupling between the sample and the thermal reservoir we have control over the phonon-bottleneck phenomena previously reported in this system. We demonstrate adiabatic motion of molecule spins in time dependent magnetic fields and with different thermal coupling to the cryostat bath. We also discuss the origin of the zero-field tunneling splitting for a half-integer spin.