Abstract Body

By combining superconducting single flux quantum (SFQ) digital circuits and complementary metal oxide semiconductor (CMOS) circuits, it is expected to realize super high-performance hybrid devices. A three-terminal superconducting nanowire cryotron, nTron [1], has been developed as an interface to connect SFQ and CMOS circuits. The nTron utilizes the transition from the superconducting state to the normal state of a superconducting nanowire. To understand the basic direct-current and pulse-current operations, we have simulated the three-terminal response of the nTron [2,3]. In the present work, we conduct the numerical simulation for the case where a pulse current from the SFQ circuit is injected to the gate of the nTron.

  We have developed the numerical technique to simulate the operation of the nTron by using the finite element method to solve the time-dependent Ginzburg-Landau (TDGL) equation and heat-diffusion equation [3]. In our simulation, we first apply the finite channel bias current (I_bias) and inject the finite pulse input to the gate, and we investigate the channel voltage (V_ch). Initially, we used a current source model to simulate the pulse input to the gate, but it resulted in unrealistically large voltage at the gate. The gate voltage was greater than , which corresponded to tens of times the signal from SFQ circuits. We, therefore, changed the gate input to a voltage source model, where a pulse wave of the same magnitude as the signal from SFQ circuits was injected to the gate.

  Figure 1(A) shows snapshots of the transition from the superconducting state (blue) to the normal state (red) when the pulse signal (height 0.5 mV, width 5 ps) is applied to the gate. The pulse input causes a vortex to enter from the gate, resulting in the normal transition at the center of the channel. As time goes on, the normal-state area expands and eventually recovers to the superconducting state. Figure 1(B) shows the numerical results of the time evolution of the channel voltage V_ch. When a pulse signal is input at t=0 ps, V_ch is generated and reaches a maximum of V_ch =5.3 mV at t=18 ps. V_ch then decreases and finally returns to a zero-voltage state at t ~ 50 ps.

Figure 1. (A) Distribution of normalized super-electron density . (B) Output characteristic for the pulse input operation of the nTron.

This work was supported by JSPS KAKENHI, Grant Numbers JP20K05314 and JST SPRING, Grant Number JPMJSP2151.

[1] A. N. McCaughan and K. K. Berggren, Nano Lett. 14, 5748 (2014).

[2] N. Yasukawa et al., The 36th International Symposium on Superconductivity (ISS 2023) ED-3-4.

[3] N. Yasukawa et al., Supercond. Sci. Technol. 37, 065013 (2024).

Keywords: superconducting nanowire cryotron, three-terminal device, TDGL simulation