## 1 Introduction  
Negative differential resistance (NDR) is an electrical property exhibited by certain electronic circuits and components. NDR has a variety of applications, including storage memory, threshold logic devices, oscillators, and even the emulation of biological neuronal dynamics [1; 2; 3]. NDR devices have been realized in a large variety of materials, and several corresponding physical mechanisms have been proposed to explain NDR behavior [4; 5; 6; 7; 8]. In the context of this study, the relevant mechanism is self-heating effects [1]. NDR caused by self-heating effects is characterized by an S-shaped I-V characteristic and is often also named current-controlled negative differential resistance. This is because when experimentally recorded, the current in the NDR region is usually controlled by an external resistor. S-shape NDRs have been reported for carbon-based nanostructures [2; 3], carbon silica nanocomposites [4], in NbO\({}_{2}\)[5] transition-metal dichalcogenides [6] and in organic-based transistors [7]. The theoretical framework for S-NDR was recently presented in a study by [PERSON] [1] where it is also claimed that any material where the charge transport is strongly temperature-dependent and the thermal conduction is low enough to generate local Joule heating can exhibit S-NDR. The scaling behavior of devices exhibiting NDR due to electrothermal effects was addressed by [PERSON] et al. [8] and a physics-based compact model of the NDR was proposed in a recent review [9]. A more general model, known as the field-triggered thermal runway (FTTR) model, has been proposed to explain the NDR observed in NbO\({}_{2}\)[10]. This model assumes that the NDR is caused by a combination of an electric-field-based increase in conductivity and Joule heating. A study by [PERSON] et al. presents a detailed model of current-controlled negative differential resistance (NDR) in NbOx-based devices and explains a wide range of discrete and compound NDR characteristics observed in these devices [11]. One important characteristic of devices with NDR in their I-V curve is the ability to control the magnitude of the NDR region. For instance, being able to adjust the peak current and the peak/valley voltage ratio allows for the control of oscillation conditions and the power consumption of oscillator circuits. However, tuning the NDR remains a challenge. So far, only transistor devices have shown this potential. For example, ambipolar transistors made of black phosphorus can modulate the NDR by altering the charge type through the inversion of the gate voltage polarity [12]. Double-gate transistors made of graphene can also modulate the magnitude of the NDR by changing the tunneling current through the application of a gate voltage [13]. Modulation of the NDR was also reported in black phosphorus and molybdenum disulfide heterostructures [14]. The main contribution of this study is the demonstration of a simple two-terminal device with the ability to electrically adjust the magnitude of the NDR. The device consists of a pyrogallol-formaldehyde/ZrO\({}_{2}\) composite carbon composite material sandwiched between two silver electrodes, and the magnitude of the NDR can be changed by an order of magnitude when subjected to an electrical current that produces mild Joule heating. The NDR adjustment is caused by an electrothermal effect that generates a metastable change in the electrical conductance of the sample. The metastable state relaxes to equilibrium over a period of approximately 20 h and is fully reversible, with a relaxation process following power-law kinetics. The slow decay of the metastable state allows for the electrical adjustment of the NDR magnitude on a time scale of seconds or minutes, depending on the desired NDR magnitude. To the best of our knowledge, this is the first report of an electrical adjustment of NDR properties using a two-terminal device. This paper starts by presenting the basic I-V characteristics, including the S-shaped NDR behavior and its agreement with a Joule heating effect.