Theory of Andreev reflection in junctions with iron-based high-$T_c$ superconductors

We construct a theory for low-energy quantum transport in $\text{normal}\mid \text{superconductor}$ junctions involving the recently discovered iron-based high-$T_c$ superconductors. We properly take into account both Andreev bound surface states and the complex Fermi-surface topology in our approach and investigate the signatures of the possible order-parameter symmetries for the FeAs lattice. Our results could be helpful in determining the symmetry of the superconducting state in the iron-pnicitide superconductors.

Figure 1

(Color online) (a) Illustration of the two-dimensional FeAs plane with the $d_{xz}$ and $d_{yz}$ orbitals and hopping between them, as proposed in Ref. 8. (b) Sketch of the Fermi-surface topology for the long-lived quasiparticle excitations in a minimal two-band model (see main text for parameter values).

Figure 2

(Color online) Plot of the conductance spectra for tunneling along the (100) axis in an iron-pnicitide $\text{N}\mid \text{S}$ junction for several possible order-parameter symmetries. Only in the $d_{xy}$-wave case $\Delta ={\Delta }_0{s}_x{s}_y$ is there a considerable ZBCP. Note the different scale on the voltage axis for this case due to the narrowness of the ZBCP. High (low) values of the parameter $Z$ denote low (high) transmissivity interfaces.

Figure 3

(Color online) [(a) and (b)] Plot of the conductance spectra for tunneling along the (100) axis in an iron-pnicitide $\text{N}\mid \text{S}$ junction for the case of one fully gapped OP and one nodal OP. (c) Evolution of the Fermi-surface topology for $\mu =\left\{1.24,1.54,1.84\right\}$ in the direction of the arrow. (d) Conductance spectra for the $c_x+c_y$ symmetry with $Z=3$ for different doping levels.