Resolving the Metal-Insulator Transition Mechanism of NbO₂ for Memristor Applications

Electroformed NbO_x films show potential for memristor applications, however, their memristive mechanism is not understood. NbO₂ filaments with a temperature-dependent MIT have been theorized to be the culprit,[1,2] but this has not been proven due in part to a lack of understanding regarding the MIT nature. Crystalline NbO₂ undergoes an MIT with accompanying structural transition at ~810°C, but there are competing claims as to whether it is Mott, Peierls, or cooperative Mott-Peierls like VO₂.[3,4] We have investigated crystalline NbO₂ using synchrotron-based temperature-dependent and variable energy XPS, µ-LEED, and LEEM/PEEM, and have conclusively found it be a gradual 2^nd-order Peierls transition. The gradual nature is then attributed to weakening Nb-Nb dimers causing the formation of a pseudogap as temperature increases. Moreover, calculations fully reproduce experimental spectra without explicit consideration of correlations, indicating minimal Mott character.
[1] S. Kumar et al., Nature 548, 318–321 (2017).
[2] S. Kumar et al., Nat. Commun. 8, 658 (2017).
[3] V. Eyert, EPL 58(6), 851–856 (2002).
[4] A. O’Hara at al., Phys. Rev. B 91(9), 094305 (2015).