Sequential resonance for giant quantum oscillations above the energy barrier and its classical counterpart

It has recently been shown that giant quantum oscillations above the energy barrier (GQOAB) of a spin S can be coherently induced by the simultaneous application of 2S alternating fields (shaped field) associated with successive level separations Phys. Rev. Lett. 131, 066701 (2023). We begin the present paper with a more detailed study of the properties of GQOAB, such as the dependence of their frequency on the value of the anisotropy constant or on that of the amplitude of the alternating fields. We then extend the concept of GQOAB to the case where these ac-fields are applied, not simultaneously, but sequentially, showing that such a protocol also leads to coherent GQOAB but with a number of differences that are discussed in detail. We conclude with a classical approach of this problem showing that ``giant classical oscillations above the barrier (GCOAB) '' can also be designed at zero Kelvin.