When two superconducting materials are separated by an insulator of very few thickness and are connected by a wire, a dc current flows in the external circuit (dc.Josephson effect). However, if the two superconducting materials are connected to a dc power, an ac microwaves are produced at the junction (ac josephson effect).
In this case, the current persists only for a short time unlike the first case where this current was persisted longer. Is it the alone difference between the two kinds of Josephson effects ?
The Josephson effect is a macroscopic quantum phenomenon of super current. To observe the Josephson
effect, the superconductor is cooled until the electrons are bound to one another in pairs, called cooper pairs,
due to interactions with phonons. These cooper pairs are very weakly bound fermions. Two bound fermions
are a system with integer spin, so a cooper pair is a boson, and bosons can flow into the same quantum
mechanical state and become a superconductor. Josephson predicted that these cooper pairs could flow
across an insulating layer of Josephson junction, effectively causing a dc super current, up to a maximum
value called the current Ic, without developing a voltage drop across the weak link. This is called the dc
Josephson effect. The supercurrent flowing through the junction is related to the difference in the phase of
two wave functions as I=IcSinϕ. Josephson also predicted that when a current greater than Ic is forced
through the weak link, a dc voltage appears across the weak links. In the presence of a dc voltage V acrossthe junction, in addition to normal conducting current, an ac super current also flows across the weak links
at a frequency fJ given by 2eV=hfJ, where V is the dc voltage drop. This ac super current can be frequency
modulated by an applied ac voltage of frequency, f, and the current then has Fourier components at
frequencies (2eV/h= nf), where n is an integer. If for a particular value of n, the super current has a dc
component depending on the magnitude and phase of the ac voltage. In the current–voltage characteristics, a
voltage step with a differential resistance equal to zero, therefore, occurs. The current width of the steps
depends on the magnitude of the ac voltage. The current width of the nth step varies with the magnitude of
ac voltage like the nth order Bessel function. This is called the ac Josephson effect. If an unbiased junction
is irradiated with rf radiation, a dc voltage is generated across the junction. In case of hysteretic tunnel
junction this dc voltage is quantized. This effect is called the inverse ac Josephson effect.
When the applied voltage bias (V) across a S-I-S (superconductor-insulator-superconductor) tunnel junction is zero, there is a dissipationless dc current, which is called dc Josephson effect. Whereas, with a non-zero voltage bias the current (dissipative) oscillates with a frequency f=2eV/h and ac Josephson effect is observed.
These are the differences between dc and ac Josephson effects.
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