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Pi Josephson junction
Additional recommended knowledge
The supercurrent Is through a conventional Josephson junction (JJ) is given by Is = Icsin(φ), where φ is the phase difference of the superconducting wave functions of the two electrodes, i.e. the Josephson phase. The critical current Ic is the maximum supercurrent that can flow through the JJ. In experiment one usually applies some current through the JJ and the junction reacts by changing the Josephson phase. From the above formula it is clear that the phase φ = arcsin(I / Ic), where I is the applied (super)current.
Since the phase is 2π-periodic, i.e. φ and φ + 2πn are physically equivalent, without loosing generality, we restrict the discussion below to the interval .
When no current (I = 0) is passing through the JJ, e.g. when the junction is disconnected, the JJ is in the ground state and the Josephson phase across it is zero (φ = 0). Attentive reader may notice, that according to the above equations the phase can also be φ = π, also resulting in no current through the JJ. It turns out that the state with φ = π is unstable and corresponds to the Josephson energy maximum, while the state φ = 0 corresponds to the Josephson energy minimum and is a ground state.
In certain cases one may obtain a JJ where the critical current is negative (Ic < 0). In this case the first Josephson relation becomes
Is = − | Ic | sin(φ) = | Ic | sin(φ + π)
Obviously, the ground state of such a JJ is φ = π and corresponds to the Josephson energy minimum, while the conventional state φ = 0 is unstable and corresponds to the Josephson energy maximum. Such a JJ with φ = π in the ground state is called π Josephson junction.
π Josephson junctions have quite unusual properties. For example, if one connects (shorts) the superconducting electrodes with the inductance L (e.g. superconducting wire), as shown in Fig.???, one may expect the spontaneous supercurrent circulating in the loop, passing through JJ and through inductance clockwise or counterclockwise. This supercurrent is spontaneous and belongs to the ground state of the system. The direction of its circulation is chosen at random. This supercurrent will of course induce a magnetic field which can be detected experimentally. The magnetic flux passing through the loop will have the value from 0 to a half of magnetic flux quanta, i.e. from 0 to Φ0 / 2, depending on the value of inductance L.
Technologies and physical principles
Theoretically, for the first time the possibility to have π JJs was discussed by Bulaevskii et al. , who considered a JJ with paramagnetic scattering in the barrier. Almost one decade later the possibility of having a π JJs was discussed in the context of heavy fermion p-wave superconductor .
Experimentally, the first π JJ was a corner JJ made of YBCO (d-wave) and Pb (s-wave) superconductors, see VanHarlingen Review. The π JJs based on ferromagnetic barrier were first fabricated and investigated only a decade later.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Pi_Josephson_junction". A list of authors is available in Wikipedia.|