Two-Dimensional Tunable Reactance Element Free from Electromagnetic Coupling

A capacitor modeled as a parallel combination of a resistance (R) and a capacitance (C) exhibits three distinct operating regimes when both parameters depend on the applied voltage (V): a positive-capacitance regime (dR/R>dV/V), an Ohmic regime (dR/R=dV/V), and a negative-capacitance regime (dR/Rinfinity), the device behaves as a conventional permittivity-based capacitor, whereas in the limit (R -> 0), negative capacitance emerges due to nonlinear current-voltage characteristics. To verify this mechanism, we fabricated nanometer-spaced two-electrode structures using multi-walled carbon nanotubes (MWCNTs) and Si crystals. The measurements confirmed negative capacitance consistent with theoretical predictions. Unlike ferroelectric negative capacitance, the effect demonstrated here arises solely from the nonlinear I-V characteristics at the electrode interfaces, without involving any ferroelectric polarization dynamics. This negative capacitance can be interpreted as an equivalent inductance, enabling a two-dimensional tunable reactance element (TDTRE) that operates without electromagnetic coupling and is compatible with conventional IC technologies.