Artificial Electro-Optical Neuron Integrating Hot Electrons in a Mott Insulator

Mott insulators are a class of strongly correlated materials with emergent properties important for modern electronics applications, such as artificial neural networks. Under an electric field, these compounds undergo a resistive switching that may be used to build up artificial neurons. However, the mechanism of this resistive switching is still under debate and may depend on the Mott material involved. Some works suggest an electronic avalanche phenomenon, while others propose an electrothermal scenario. As electric pulses produce both Joule heating and hot carriers, disentangling their respective roles requires the use of another external stimulus. Here, an ultrashort light pulse is used to tune the number of photogenerated carriers and the energy provided to the system. In these pump-pump-probe experiments, a crystal of the Mott insulator ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$ is simultaneously excited by electric and laser pulses while an electric probe monitors its conductivity. The study shows that the resistive switching is affected by the number of generated photocarriers rather than by the accumulation of energy deposited by the femtosecond laser. It supports therefore a mechanism driven by generation of hot carriers. Finally, our work opens the possibility to build up an artificial electro-optical “Mott” neuron tuned by a femtosecond laser pulse.

Figure 1

(a) Resistivity versus temperature measured on a single crystal of the Mott insulator ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$. The insets show the crystallographic structure of ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$ and a picture of the ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$ single crystal connected with four gold wires. (b)–(d) A typical example of an electric Mott transition induced at 85 K by the application of a 120-µs (90-V) electric pulse on a circuit made of the ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$ crystal in series with a 1-kΩ load resistance. The temporal evolutions of the current (b), of the sample voltage (c), and of the sample resistance (d) clearly indicate a resistive switching occurring after a time delay of approximately 80 µs.

Figure 2

(a) Transport measurements performed at different temperatures and low electric field for a ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$ crystal. All measurements fall on a master curve following the predicted evolution of the carrier multiplication rate $\tilde{n}$ (blue dotted line). (b) Conductivity ratio dynamics $\tilde{\sigma }=\sigma /{\sigma }_0$ measured under electric field for a ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$ crystal for $E<E_{\mathrm{th}}$ and $E>E_{\mathrm{th}}$.

Figure 3

Temporal evolution of the normalized conductivity $\tilde{\sigma }=\sigma /{\sigma }_0$ during a voltage pulse applied to different circuits containing a ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$ crystal and that is synchronized or not with a single-shot 100-fs laser pulse applied at the beginning of the pulse. (a) Schematic representation of the setup and impact of laser fluence or wavelength on the number of electrons excited from the lower (LHB) to the upper Hubbard band (UHB). (b) Evolution of $\tilde{\sigma }(t)$ measured under a 60-V voltage pulse and when femtosecond laser pulses of constant energy (800 nm, hν = 1.55 eV) and of increasing fluences are applied (from 1 to 10 µJ/pulse). (c) Evolution of $\tilde{\sigma }(t)$ measured on a different ${\mathrm{Ga}\mathrm{Ta}}_4{\mathrm{Se}}_8$ crystal under an 85-V voltage pulse and when femtosecond laser pulses leading to the same number of induced photocarriers are applied with two different energies (hν = 0.5 and 2.3 eV).

Figure 4

(a) Schematic of a biological neuron receiving input spikes from other neurons and triggering an output action potential when the membrane potential reaches a threshold value. This behavior of the neuron membrane was first modeled by a RC circuit and led to the definition of the simplest model of artificial neuron called LIF. (b) Sketch of an electro-optic Mott neuron setup and graphical representation of the electronic dynamics with or without laser pulse. The artificial Mott neuron follows dynamics similar to that of the LIF model but the integrated quantity is not the membrane potential (or the charge in the capacitor) but the carrier multiplication rate $(\tilde{n})$ in the Mott device.

Figure 5

Experimental resistive switching obtained by applying trains of 25-µs electric pulses of 63 or 52 V synchronized or not with a 100-fs laser pulse. (a) Profile of applied voltage pulses. Sample conductivity time profiles measured (b) without and (c) with laser pulse. The measurements are performed under conditions similar to the ones of Fig. 3.