Superconducting gaps via Raman scattering in iron superconductors

Abstract

We investigate non-resonant Raman response for iron-based superconductors using the framework of an effective S4 model that was recently proposed to capture the essential electronic and magnetic properties of Iron based superconductors. We compute the momentum matrix elements and the resulting Raman vertices exactly for different polarization geometries and amazingly find that a simple coskxcosky superconducting gap function is in good agreement with experimental data measured in both iron-pnictides and iron-chalcogenides. The Raman peaks are also matched quantitatively with the measured superconducting gaps by other experimental techniques. The result strengthens the validity of the S4 model and the dominance of the s-wave pairing for iron-based superconductors.The discovery of Iron based superconductors [1,2] has inspired continued searches for new ways of understanding the mechanism of superconductivity. The major difficulty in understanding their electronic structure is the occurrence of multiple bands at the Fermi level. Although many effective model Hamiltoniansconstructed by including different number of orbitals like two [3], three [4] and five [5] orbitals -have been proposed, their successes are limited; that is, while simple two and three orbital models leave out several essential features observed in experiments, five orbital models lack analytical and numerical tractability which makes it hard for theoretical calculations to predict meaningful results and therefore, fail to isolate the vital ingredients which control the diverse and interesting phenomena observed in experiments.Recently it was suggested that the underlying, low energy, electronic structure in the iron based superconductors was governed by an S 4 symmetric, two weakly coupled, single orbital models [6]. The motivation for such a model, henceforth referred to as the S 4 model, was derived from the fact that if the hopping parameters in the original five band model [5] are tranformed to a different gauge setting, then, one can use the effective d xz and d yz orbitals to construct a model that accurately describes the essential band structure near Fermi levels. In the real space picture, the two orbitals can be naturally segmented into two groups : one, coupled to the As atoms on the top of the F e layer and the other, to those at the bottom, with each group containing contributions from both the sub-lattices. It was also demonstrated that the gauge transformation maps the s−wave pairing symmetry in the original one F e Brillouin Zone (BZ) to the d− wave pairing symmetry in the folded BZ defined on the two sublattices, thus establishing that the relevant symmetry which unifies the different families of high T c superconductors is a combination of both the hopping and the pairing symmetries. It remains, now, to compute the consequences and predictions of such a model, examine its accuracy, and determine its suitability in describing the observed experimental phenomena. It is important to note that the critical difference between the S 4 ...

Charge nematicity and electronic Raman scattering in iron-based superconductors

“…The SC peak is only observed in B 1g symmetry. It was initially interpreted as a Cooper pair-breaking peak associated to the creation of pairs of Bogoliubov quasiparticles in a BCS state [165][166][167][168][169]. Its energy is indeed close to twice the energy gap 2∆ detected by ARPES experiments at the electron pockets in similarly doped crystals [170,171].…”

Low-energy inelastic response in the superconducting phases of PrOs4Sb12

“…Second, in the presence of long range Coulomb screening and vertex corrections, there are additional contributions that need to be incorporated into the inelastic response [27] which might, in general, make it difficult to isolate pure representations. Third, the effective mass approximation used in our analysis is valid only when the frequency of the incoming and scattered light is either small or large in comparison to the inter-band spacings [27,43]. This may not always be accurate [43] and one must be careful in not overestimating the vertex in certain regions of the Fermi surface.…”

“…Third, the effective mass approximation used in our analysis is valid only when the frequency of the incoming and scattered light is either small or large in comparison to the inter-band spacings [27,43]. This may not always be accurate [43] and one must be careful in not overestimating the vertex in certain regions of the Fermi surface. Due to the above outlined reasons, our calculations should not be considered any more than qualitative.…”

“…( 10) as a band curvature [25,27]. Note that this approximation breaks down when the bands touch each other at or near the fermi level and could yield qualitatively different results [43]. Keeping in mind that there are three distinct irreducible representations, −A 1g , B 1g , B 2g −, that have been accessed in the square lattice point group, each IRRP (selected through its appropriate polarization geometry) can selectively average different regions of the Fermi surface, yielding distinct responses in the superconducting state.…”

“…One could still make reasonably accurate comparisons with the corresponding theoretical predictions of the responses in mixed geometries Second, in the presence of vertex corrections, there are additional contributions that need to be incorporated into the inelastic response 31,47 which might, in general, make it difficult to isolate pure representations. Third, the effective mass approximation used in our analysis is valid only when the frequency of the incoming and scattered light is either small or large in comparison to the inter-band spacings 31,48 . This may not always be…”