Origin of electronic dimers in the spin-density wave phase of Fe-based superconductors

Abstract

We investigate the emergent impurity-induced states arising from point-like scatterers in the spin-density wave phase of iron-based superconductors within a microscopic five-band model. Independent of the details of the band-structure and disorder potential, it is shown how stable magnetic (π, π) unidirectional nematogens are formed locally by the impurities. Interestingly, these nematogens exhibit a dimer structure in the electronic density, are directed along the antiferromagnetic a-axis, and have typical lengths of ∼ 10 lattice constants in excellent agreement with recent scanning tunnelling experiments. These electronic dimers provide a natural explanation of the dopant-induced transport anisotropy found e.g. in the 122 iron pnictides. [19][20][21][22][23][24]. Since impurities are known to nucleate magnetic order locally [25][26][27], it is reasonable to assume that incipient nematic order in a fluctuating state can be similarly condensed around a defect, to create a local nematic electronic state. Once present, such anisotropic defect structures can influence macroscopic anisotropy as well, and it has been suggested [3,23,28,29] that they are responsible for the resistivity anisotropy observed in detwinned Ba-122 crystals [5,6,29].Because impurities represent a well-defined perturbation which can be examined locally, studying the electronic states they create can yield important information on the background correlations present in the pure system [25]. In YBCO, for example, magnetic droplets formed around Zn impurities are known to have a size consistent with the pure antiferromagnetic (AF) correlation length.[25] At present, the microscopic mechanism responsible for the creation of local defect states in Febased materials and for breaking of rotational symmetry is unclear. Some clues are offered by STM experiments on defects in the underdoped, magnetically ordered phase, where the symmetry is already broken by the (π, 0) spindensity wave (SDW) order (the wave vector is given in the effective 1-Fe Brillouin zone). Fourier transform scanning tunneling spectroscopy (STS) deduced the existence of electronic defects with C 2 symmetry[3] nucleated by the Co dopants in Ca(Fe 1−x Co x ) 2 As 2 , while a more detailed analysis reported a dimer structure.[23] These dimers are approximately eight lattice constants (a) long and oriented along the AF a-axis, consistent with the larger resistivity found along the ferromagnetic b-axis.As a first step in understanding the origin of local C 4 symmetry breaking observed in several experiments on various materials, it seems useful to study a situation where a known chemical impurity substitutes at a known position, and ask why such a dimer-like structure (with charge or local density of states (LDOS) peaks located such a great distance from the impurity site) should be induced. As in the cuprates, this problem should be accessible to weak-coupling theories of these systems, provided they account for the electronic states of the system to which the impurity couples and tre...

Magnetic interactions in iron superconductors: A review

“…Furthermore, the nematic susceptibility shows clean critical exponents [47], suggesting irrelevance of disorder. Theoretically, nematogens around impurities have been found in models of iron pnictides driven by orbital [227] or spin correlations [228]. On the other hand, in calculations in the magnetic phase of multi-orbital models the role of the topology and morphology of the Fermi surface in the resistivity anisotropy has been emphasised without [145] and with impurities [229].…”

In-plane anisotropy of transport coefficients in electronic nematic states: Universal origin of nematicity in Fe-based superconductors

“…In Ba122, in contrast, the anisotropic elastic scattering (nematogen) also gives sizable contribution as discussed in Refs. [20][21][22][23][24].…”

“…To answer this question, the strong in-plane anisotropy of transport coefficients has been studied intensively as a key electronic property in the nematic state [2,[15][16][17][18][19][20][21][22][23][24]. In Ba(Fe 1−x Co x ) 2 As 2 , Ba(As 1−x P x ) 2 and EuFe 2 (As 1−x P x ) 2 , large C 2 anisotropy in the resistivity ρ ≡ ρ x − ρ y < 0 appears in detwinned samples below T S , where ρ μ is the resistivity along the μ axis [15][16][17].…”

“…The anisotropic elastic scattering due to the impurityinduced C 2 local orbital order (orbital nematogen) [20,21] and the magnetic nematogen [22,23], and the anisotropic quasiparticle velocity [24] have been discussed. On the other hand, the anisotropic inelastic scattering due to the C 2 spin fluctuations was discussed based on the spin-nematic scenario [2].…”

Pair breaking due to orbital magnetism in iron-based superconductors

“…Since iron-based superconductors are only close to this SO(6)-symmetric fixed point, the SDW or SC instabilities occur first, and iCDW order has not been observed in any iron-based superconductor so far, although being close in energy. It is, however, a conceivable scenario that such order could nucleate around impurities in these materials, similar to SDW order [15][16][17] . Such iCDW-type impurities break time-reversal symmetry and thereby are responsible for orbital magnetism.…”