  We demonstrate the appearance of a purely entropic stochastic resonance (ESR)
occurring in a geometrically confined system, where the irregular boundaries
cause entropic barriers. The interplay between a periodic input signal, a
constant bias and intrinsic thermal noise leads to a resonant ESR-phenomenon in
which feeble signals become amplified. This new phenomenon is characterized by
the presence of two peaks in the spectral amplification at corresponding
optimal values of the noise strength. The main peak is associated with the
manifest stochastic resonance synchronization mechanism involving the
inter-well noise-activated dynamics while a second peak relates to a regime of
optimal sensitivity for intra-well dynamics. The nature of ESR, occurring when
the origin of the barrier is entropic rather than energetic, offers new
perspectives for novel investigations and potential applications. ESR by itself
presents yet another case where one constructively can harvest noise in driven
nonequilibrium systems.
