Proximity and Josephson effects in microstructures based on multiband superconductors (Review Article)

Abstract

Emerging in the 1950s, the multiband superconductivity has been considered for a long time as an approximate model in the form of a generalization of the BCS theory to the case of two bands for a more accurate quantitative description of the properties and characteristics of such superconductors as cuprates, heavy fermions compounds, metal boron carbides, fullerides, strontium ruthenate etc. due to their complex piecewise-continuous Fermi surfaces. However the discovery of the multiband structure of the superconducting state in magnesium diboride in 2001 and iron oxypnictides and chalcogenides in 2008 led to the appearance of many papers in which effects and different dependences well known for conventional single-band s-wave superconductors were reexamined. The main purpose of these studies was to reveal the symmetry type of the order parameter, which provides an important information about the mechanism of Cooper pairing in these superconductors. One of the most effective methods of obtaining information on the symmetry properties of the order parameter in the multiband superconductors is phase-sensitive techniques. This review summarizes the results of theoretical and experimental studies of the proximity and Josephson effects in systems based on multiband superconductors in contact with normal metals, insulators and other superconductors.

Effects of Pair-Hopping Coupling on Properties of Multi-Band Iron-Based Superconductors

“…Similar deflection of the gap from the BCS-like shape has been observed experimentally for, e.g., 111 [51], 122 [52], and 1111 [53] families of the iron-based superconductors (in a more realistic three band model, there is the pair of leading bands with similar but unresolved gaps coupled by spin fluctuations and the weakly coupled third band [54,55]) as well as in other multi-band superconductors such as PuCoGa 5 [56], (Li 1−x Fe x )OHFeSe [57], LnOFeAs [58], Sn(Pb)Mo 6 S 8 [59], or classical two-band MgB 2 [60][61][62][63]. In a case of MgB 2 one has detected also a similar character of the interplay between intra-and inter-band quantities, which has been observed in a temperature dependence of the critical Josephson currents from one band to the other of the MgB 2 -insulator-MgB 2 tunnel junctions [61,64], or in a more general case of the two-band Josephson junction [65].…”

Coexistence of giant Cooper pairs with a bosonic condensate and anomalous behavior of energy gaps in the BCS-BEC crossover of a two-band superfluid Fermi gas

“…(3) and performing dimensionless procedure for Eqs. (3) and (6) in units of the total Fermi momentum and the total Fermi energy, we get the system of equations for the energy gaps and the particle densities that will be solved numerically (see Appendix A). Besides the energy gaps ∆ i and the particle densities n i another important characteristic of the pairing regimes through out the BCS-BEC crossover in a two-band superfluid Fermi gas is the intrapair correlation lengths of the Cooper pairs, which is determined by the expression…”

Josephson effect in two-band superconductors

“…The CPR remains sinusoidal even at such a nontrivial solution because the π shift in θ does not break time-reversal symmetry as shown in Eq. (15). Thus, we conclude that the CPR is sinusoidal as…”

“…(35) and (36), however, the phases of the pair potentials have already been settled self-consistently under the condition in Eq. (15). Thus, our remaining task is to determine | 1 | and | 2 |, which is not difficult because of the simple mathematical structure in Eqs.…”

“…When such a two-band superconductor couples to a single-band s-wave superconductor through an insulator, the junction energy depends on the phase of the single-band superconductor ϕ s , ϕ 1 , and ϕ 2 . Actually, theoretical studies on such Josephson junctions have shown the complicated energy diagrams as a function of these phases [12][13][14][15][16]. Especially, an s +− state frustrates the junction energy and stabilizes a spontaneously time-reversal symmetry breaking state near the junction interface [12,13].…”