  Nonreciprocal nonhermitian systems provide an unconventional localization
mechanism of topological zero modes via the nonhermitian skin effect. While
fundamental theoretical characterizations of this effect involve the
biorthogonal system of right and left eigenmodes, the recent demonstration of
this effect for a zero mode in a robotic metamaterial (Ghatak et al.,
arXiv:1907.11619) is based on the direct experimental observation of the
conventional right eigenvectors. Here I show that such nonreciprocal mechanical
metamaterials reveal their underlying biorthogonality in the directly
observable response of the system to external excitation. Applied to the
ground-breaking experiment, this nonreciprocal response theory predicts that
the zero-mode skin effect goes along an extended phase where the system is
highly sensitive to physical perturbations, leading to a diverging response in
the limit of a large system.
