Andreev spectroscopy of iron-based superconductors: temperature dependence of the order parameters and scaling of $ \Delta_{\rm L, S}$ with $ T_{\rm C}$

Abstract

IntroductionUnexpected discovery in 2006 [1] of the first layered Fe-based high-temperature superconductor (HTSC) -LnOFePn (where Ln -lanthanide, Pn -pnictide; hereafter referred to as "1111") becomes a key issue of modern solid state physics. Since 2008, the class of iron-based superconductors has much expanded: several families of iron pnictides and chalcogenides have been synthesized [2][3][4]. The crystal structure of oxypnictides recalls that of cuprates and is in fact a stack of the superconducting Fe-As layers alternating along the c-direction with spacers, the nonsuperconducting oxide blocks, Ln-O. In spite of the pronounced layered structure and anisotropic physical properties, the electron subsystem in Fe-based superconductors is less quasi-two-dimensional in comparison with that in cuprate HTSC, because the height of the Fe-As blocks exceeds the thickness of the Cu-O planes, whereas the distance between superconducting blocks in iron-based superconductors is significantly less than that in cuprates. The latter seems to be a reason [5] why the critical temperature of Fe-based superconductors, though being as high as T C ≈ 57.5 K [6] still does not reach the cuprate one. Superconductivity in novel materials emerges with the suppression of spin density wave ground state under doping of the superconducting Fe-As layers or under external pressure [7]. The key distinction from cuprates, however, is the multiband nature of newly-discovered superconductivity in iron-based materials. Band structure calculations showed (for a review, see [8]) the coexistence of the electron and the hole quasi-two-dimensional bands in these compounds, whereas the Fermi surfaces consist of slightly warped along the c-direction cylinders (near Γ and M points), where several superconducting condensates could form. Contrary to the observation of the strong isotope effect [9], an early theoretical study showed [10] that the high temperature superconductivity in Fe-based compounds could not to be based on the electron-phonon interaction solely. Although the latter plays an important role, it is incapable [10] to explain the observable values of T C in the framework of the Eliashberg theory [11]. Taking into account the nesting of the Fermi surface sheets along the Γ-M-direction, the vicinity of the antiferromagnetic state [12,13], and the appearance of the experimentally observed [14] peak of dynamic spin susceptibility ("magnetic resonance"), Mazin et al. suggested the theoretical description of the mechanism of superconductivity in iron pnictides and chalcogenides based on sign-changing (in different bands) order parameter -the so-called s ± -model. The simplest initial model considers two isotropic order parameters (in the electron and the hole bands, correspondingly), which have equal amplitudes but are in antiphase (i.e. having opposite signs). The strong interband interaction mediated by spin fluctuations along the Γ-M-direction plays the key role in this model, whereas the intraband electron-phonon coupling is an order of magnit...

On the structure of the superconducting order parameter in high-temperature Fe-based superconductors

“…Nonetheless, the experimentally observed magnetic resonance peak seems [13,14] not so pronounced as predicted for s ± , and the s ± system would be unstable in a presence of impurities [13,39]. The absence of nesting evident from ARPES measurements [7] and a strong intraband coupling evaluated from direct temperature dependences of the gaps [34,40] also support s ++ . On the other hand, a proximity to a Lifshitz transition, the larger gap developed in the smallest Fermi surface cylinder [7,17], and the observed nanoscale phase separation and Feshbach-type resonance [41,42] indirectly facilitate the superstripe model.…”

“…In the bands with the small gap, although their quasi-twodimensionality, a weak superconductivity is developed, with an "eigen" BCS-ratio close to 3.5. Remarkably, in iron-based oxypnictides the intraband coupling within the small gap bands seems a bit stronger: according to our estimations, in average, 2S/kBTc S ≈ 4 [34,77,82]. Nonetheless, the weak superconductivity of S-bands in Ba-122 is not exceptional: in magnesium diborides, "eigen" superconductivity in the 3D -condensate similarly tends to the BCS-limit [76,77].…”

“…No model has got an unambiguous experimental confirmation yet, thus making the main issues still unanswered. For example, the experimentally determined BCS ratio in Ba-122 family varies by a factor of six [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], for a review, see 12,34]. A possible reason is the strong out-of-plane gap anisotropy in k-space which seems to "smear" the gap values obtained in bulk probes, and a sensitivity-to-surface of the superconducting properties.…”

Signature of multigap nodeless superconductivity in fluorine-doped NdFeAsO

“…As can be seen, the gap values differ for different compounds and the small gap increases linearly with T c . The family of 1111 superconductors with Gd, La, Ce, and Nd follow this tendency [13,53]. It is clearly seen in Fig.…”

High pressure crystal growth of the antiperovskite centrosymmetric superconductor SrPt3P

“…As it is well-known from textbooks, the two-band MgB2 superconductor and Fe-based pnictides, multiple sheet Fermi-surface topology may lead to a complicated gap structures with different energy gaps on each sheet [25][26][27][28][29]. The Tc value and the gap ratio in the multiple-band systems are currently being investigated due to a rather complicated interplay of intra-and interband couplings.…”