Slow electromagnetic waves in asymmetric SISIS′ junctions

Splitting of the Swihart velocity in the asymmetric SISIS′ devices (where S is a superconductor and I is an insulator) has been investigated. Analysis based on the existing models of the dynamic state of the stacked junctions shows that if one of the external electrodes, S′, has a considerably lower critical temperature T′_c than the other two electrodes, and the temperature of the system approaches T′_c, but T < T′_c, then the velocities C₊ and c_- approach their limit values: c₊ → c₀, and c_- → 0 (here c₀ is the Swihart velocity in the single SIS junction). Experimentally, both asymmetric Nb/Al-AlO_x-Nb/Al-AlO_x-Ta/Nb and symmetric Nb/Al-AlO_x-Nb/Al-AlO_x-Nb two-terminal devices have been investigated. The values c_- = (1.47 ± 0.04) × 10⁶ m/s and c_- = (1.7 ± 0.1) × 10⁶ m/s have been obtained for the devices of the first and second type, respectively. Although, as expected, the value of c_- is lower in the first case, the difference between the c_- values for the two types of devices is rather small. This is in agreement with the theory, because in the first case the influence of the thickness of the middle electrode on the value of c_- predominates the same effect arising from the reduced superconductivity of the Ta/Nb electrode.