Fig. 7
a Multilevel conductance states during reset process by applying a consecutive increase in reset voltage sweep from 0.7 to 1.0 V. Arrow indicates the sweep direction. The increment of sweep voltage was 0.01 V. b Retention tests were performed at fifteen different HRSs for 500 s. The resistance values were extracted at read voltage of 0.2 V. c The potentiation and depression characteristics under 50 negative pulses (− 1.0 V, 500 μs) and 50 positive pulses (+ 1.5 V, 500 μs), respectively. d The PPF index vs. pulse interval ∆t, where a series of double spikes with different inter-spike timings were employed, showing an exponential decreasing trend