Josephson effects at iron pnictide superconductors: Approaching phase‐sensitive experiments

Abstract

The electrical transport in the pnictides especially the quasiparticle tunnelling and Josephson effects in inhomogeneous superconducting systems can provide information on the symmetry of the order parameter. Different kinds of Josephson junctions with one or two pnictide electrodes were realised and their properties were experimentally studied in detail (e.g., current–voltage characteristics, microwave response, current‐phase relations, dependencies on temperature and external magnetic field). These results were compared to well‐known Josephson junctions and recent theories taking into account the special properties (multiband superconductivity, exotic pairing symmetry) of the pnictides. By combination of two junctions in different crystal directions with different barrier transparencies one forms corner junctions or SQUIDs as phase‐sensitive devices. If the technological problems can be solved the investigations on such devices will finally clear up the symmetry of the pnictides, directly. These investigations will lead to a better understanding of fundamental properties of the pnictides as well as to a realistic judgement on the potential for applications of pnictides in superconducting electronics and sensor applications, both established ones and novel ones utilising the special pnictide properties.

Probing the current-phase relation in Josephson point-contact junctions between $$\hbox {Pb}_{0.6}$$In$$_{0.4}$$ and $$\hbox {Ba}_{0.6}$$K$$_{0.4}$$(FeAs)$$_2$$ superconductors

“…It is now worth noticing, after what has just been shown above, that there is a number of studies of Josephson contacts between “ordinary” and high-temperature superconductors based on Cu or Fe, where it was also not possible to approximate the oscillations of the current steps as a function of applied microwave power by using the renormalized frequency obtained from the current-voltage characteristics. In those cases, in order to reproduce the experimental behavior , the authors handpicked the value and the contact capacity 23 , 48 . As it has already been mentioned before, in our case the problems arising in the correct determination of , necessary for approximating the measured dependencies by means of the RSJ model, are associated to a number of deviations of the investigated Josephson point contacts from the ideal behavior and with the determination of the value from the low-voltage part of the curve.…”

Shot noise as a probe for the pairing symmetry of iron pnictide superconductors

“…From Eqs. (25)(26)(27)(28), all the scattering amplitudes can be determined when spin up/down electron is incident in band 1 or 2.…”

“…In certain samples, a small non zero resistance has been observed below T c due to the presence of inter-growth defect [25] that may affect the experimental results in Josephson junctions. Real measurements are often influenced by thermal noise, which smears the shape of the current near the critical current [26]. We can avoid these difficulties by calculating the shot noise in the tunnel limit, i.e., at z →Large, where differential shot noise vanishes for s ± pairing but is finite for s ++ pairing when the interband coupling strength(α) is large and shows opposite behavior when α is small.…”

Phase-Sensitive Evidence for the Sign-Reversal s± Symmetry of the Order Parameter in an Iron-Pnictide Superconductor Using Nb/Ba1−x

“…Alternatively, the strong reduction of the gap can be related with the change of the orbital character from d xz/yz to d z 2 −1 [25].Josephson effect facilitates phase-sensitive probe of the order parameters [1,[4][5][6][7]. So far few reliable phasesensitive experiments were reported for pnictides [26][27][28][29]. Both integer and half-integer flux-quantum transitions were observed [26] and large variations of the I c R n product, where I c is the critical current and R n is the junction resistance, were reported [27].…”

“…So far few reliable phasesensitive experiments were reported for pnictides [26][27][28][29]. Both integer and half-integer flux-quantum transitions were observed [26] and large variations of the I c R n product, where I c is the critical current and R n is the junction resistance, were reported [27]. Interpretation of such results is ambiguous because half-flux quantum transitions may occur even in conventional s/s junctions due to influence of Abrikosov vortices [30], and for s/s ± junctions phase shifts depend on the tunneling direction and the I c R n depends on tunneling probabilities from the two bands [5][6][7]28].…”