  The paradigm shift of the Hermitian systems into the non-Hermitian regime
profoundly modifies the inherent topological property, leading to various
unprecedented effects such as the non-Hermitian skin effect (NHSE). In the past
decade, the NHSE effect has been demonstrated in quantum, optical and acoustic
systems. Besides in those non-Hermitian wave systems, the NHSE in diffusive
systems has not yet been explicitly demonstrated, despite recent abundant
advances in the study of topological thermal diffusion. Here we first design a
thermal diffusion lattice based on a modified Su-Schrieffer-Heeger model which
enables the observation of diffusive NHSE. In the proposed model, the periodic
heat exchange rate among adjacent unit cells and the asymmetric temperature
field coupling inside unit cells can be judiciously realized by appropriate
configurations of structural parameters of unit cells. The transient
concentration feature of temperature field on the boundary regardless of
initial excitation conditions can be clearly observed, indicating the
occurrence of transient thermal skin effect. Nonetheless, we experimentally
demonstrated the NHSE and verified the remarkable robustness against various
defects. Our work provides a platform for exploration of non-Hermitian physics
in the diffusive systems, which has important applications in efficient heat
collection, highly sensitive thermal sensing and others.
