Negative differential resistance: another banana?
  Just like the artefact found in ferroelectric hysteresis loops, the nearly identical NDR effect shown in Sr3Co2Fe24O41, TiO2, Al2O3, glass and even banana skins is confirmed to be a kind of water behavior. The combination of water induced tunneling effect, water decomposition and absorption plays a crucial role in the NDR effect. The results and mechanism demonstrated here illustrate that much attention should be paid to the chemical environment when studying electrical properties of materials / devices. 
J. Li, H.-F. Zhang, G.-Q. Shao, B.-L. Wu, S.-X. Ouyang
cond-mat.mtrl-sci