Quasiparticle interference in antiferromagnetic parent compounds of iron-based superconductors

Abstract

Recently reported quasiparticle interference imaging in underdoped Ca(Fe1−xCox)2As2 shows pronounced C2 asymmetry that is interpreted as an indication of an electronic nematic phase with a unidirectional electron band, dispersive predominantly along the b-axis of this orthorhombic material. On the other hand, even more recent transport measurements on untwinned samples show near isotropy of the resistivity in the ab plane, with slightly larger conductivity along a (and not b). We show that in fact both sets of data are consistent with the calculated ab initio Fermi surfaces, which has a decisevly broken C4, and yet similar Fermi velocity in both directions. This reconciles completely the apparent contradiction between the conclusions of the STM and the transport experiments.PACS numbers: 74.20. Pq,74.25.Jb,74.70.Xa The Fe-based superconductors present a new paradigm for high-T C superconductivity as here Cooper-pairs appear to emerge upon chemical doping from a metallic ground state as opposed from a Mott insulator as found in the celebrated High-T C cuprates 3 . Despite this difference of parent ground state of the Fe-and Cu-based superconductors, similarities lie in that in both cases superconductivity emerges after the suppression of static ordered magnetism 4 . Although band theory has correctly predicted the unusual antiferromagnetic (AFM) order in the parent compounds of the Fe-based superconductors, it consistently overestimates the tendency to magnetism and underestimates the electronic mass, so there is no doubt that electronic interactions can not be ignored in quantitative descriptions, and that they play a different role compared to cuprates. The exact role of correlations, especially once the parent phase of the Fesuperconductors is doped, has been the focus of much debate and controversy.An almost universal feature of the Fe-superconductors is that in the parent phases, there is a tetragonal to orthorhombic structural phase transition that is closely associated with the onset of antiferromagnetic order 5 . Upon chemical doping x, the onset of the structural and magnetic transitions (T S and T N respectively) decrease with x and superconductivity emerges. The physical nature of the cross over from antiferromagnetic order to superconductivity varies between specific materials. In some cases both T S and T N coincide while in others T S is a few degrees higher than T N 5 . Band structure calculations have suggested that the AFM ordering is accompanied by a strong restructuring of the Fermi surface, with the Fermi surface area being reduced by roughly an order of magnitude. This has been confirmed by optical and Hall measurements that register a drastic reduction of the carrier concentration in the AFM state 6 . The calculated AFM Fermi surface consists of several small pockets, which are arranged in the Brillouin zone in a way that strongly breaks the tetragonal symmetry, but each of them is rather isotropic 2 . This led to a prediction of small transport anisotropy. An alternative point of view, th...

Quasiparticle scattering interference in iron pnictides: A probe of the origin of nematicity

“…Similar to the results presented in Fig. 2(e) and (f), the highly anisotropic QPI patterns obtained by previous studies [37][38][39] are directly related to the topology of the SDW distorted constant energy contours. Here, we emphasize another cause leading to the dimer structure in QPI pattern, that is the highly anisotropic SDW gaps in the momentum space which is directly related to the orbital characters of the folded bands as addressed above.…”

“…We further show that due to the inequivalent energies of the two Dirac nodes, the dimer structure in the orbital scenario undergoes a π/2 rotation with the increase of en-ergy, which contrasts clearly with the case of the magnetic scenario. These features of the QPI patterns have not been addressed in previous studies [37][38][39]53,54 . In addition, compared to Ref.…”

“…Theoretically, the features of the QPI patterns in the SDW state have been investigated by previous studies [37][38][39] . Similar to the results presented in Fig.…”

Fluctuating Charge Order: A Universal Phenomenon in Unconventional Superconductivity?

“…Note that extended FS nesting is not a unique prerequisite for CDW order to appear [26,39,40]. A more rigorous approach is the calculation of the susceptibility χ pqq [39][40][41]. This, of course, is anything but trivial in view of the differences mentioned above between the DFT band structure and the ARPES results.…”

Quasi-one-dimensional nanoscale modulation as sign of nematicity in iron pnictides and chalcogenides

“…* dheeraj@postech.ac.kr † alireza@apctp.org This suggests a common origin of the anisotropy in the electronic structure, rather than in specific ordering tendencies.In the SDW state, the orbital occupancy difference that can result from the electronic reconstruction is n xz − n yz ∼ 0.1 [21], which corresponds roughly to an energy splitting of 50meV. According to the experiments, the OS observed above Neel temperature T N can be as large as ∼ 60meV [6], therefore it is natural to explore the consequences of this 'orbital bias' in studying the SDW state as well, ignored in earlier work which may have led to their failure in reproducing the one dimensional (1d) characteristics with correct orientation and lengthscale [22][23][24][25]. Such a term should assume further importance, beyond magnetic anisotropy, in the electrondoped region of SDW state where the magnetic moments are small, and magnetic order induced band reconstruction is less pronounced.Above T N , a non-zero OS has been attributed to the spindriven nematic order with S i · S i+x − S i · S i+y = 0, where average magnetic moment S i = 0, because of the frustration caused by the presence of second nearest-neighbor exchange coupling [26][27][28].…”