Enhancement of Magnetic Stripe Order in Iron-Pnictide Superconductors from the Interaction between Conduction Electrons and Magnetic Impurities

Abstract

Recent experimental studies have revealed several unexpected properties of Mn-doped BaFe2As2. These include extension of the stripe-like magnetic (π, 0) phase to high temperatures above a critical Mn concentration only, the presence of diffusive and weakly temperature dependent magnetic (π, π) checkerboard scattering, and an apparent absent structural distortion from tetragonal to orthorhombic. Here, we study the effects of magnetic impurities both below and above the Néel transition temperature within a real-space five-band model appropriate to the iron pnictides. We show how these experimental findings can be explained by a cooperative behavior of the magnetic impurities and the conduction electrons mediating the Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions between them.PACS numbers: 74.62. En, 74.70.Xa, 75.30.Hx Whether the electronic fluctuations in Fe-based superconductors are predominantly of magnetic or orbital nature remains controversial.[1] The structural transition from tetragonal to orthorhombic symmetry has been argued to arise from an electronically driven nematic instability caused either by orbital order [2][3][4][5] or so-called spin-Ising order.[6-9] These order parameters are, however, intimately linked by symmetry and cannot exist on their own,[1] making it hard to determine experimentally which order exhibits the dominant susceptibility in the high-T normal phase and hence drive e.g. the structural transition. A resolution to this question is of great interest since the dominant fluctuations are likely to also mediate the pairing required for superconductivity. Therefore, the presence of a magnetic tetragonal phase has attracted a lot of attention [10,11]; this phase exhibits magnetic order at the same ordering vectors (π, 0), (0, π) ≡ Q stripe as the standard stripe magnetic order, but without a concomitant breaking the tetragonal symmetry. The existence of magnetic non-orthorhombic phases argues against orbital order as the driving instability of these materials, and has been interpreted in terms of C 4 symmetric magnetic structures with ordering at Q stripe . [12,13] For the pnictides, the above issues have largely focussed on hole-doped BaFe 2 As 2 where particularly Ba(Fe 1−x Mn x ) 2 As 2 constitutes an interesting case in point. For this compound, Kim et al. [10] found that the structural phase transition disappears at a critical Mn doping (x c ∼ 0.1) whereas the Q stripe magnetic order remains. Remarkably, at higher doping than x c the magnetic order exhibits a new high-temperature component as seen by the persistence of a broad Q stripe magnetic Bragg peak well in excess of the Néel temperature T N of the lower doped x < x c samples. In addition, inelastic neutron scattering revealed that Mn dopants induce short-range quasi-elastic spin scattering at (π, π) ≡ Q Néel , which persists at all measured T , and coexists with the long-range ordered Q stripe phase at low T .[14] The presence of local antiferromagnetic (AF) correlations induced by Mn ions have also been detected by ...

Competing superconducting and magnetic order parameters and field-induced magnetism in electron-dopedBa(Fe1−xCox)2As2Larsen1, Uranga2, Stieber3 et al. 2015Phys. Rev. B101100View full textAdd to dashboardCite

“…Other recent examples of this behavior include several Mn-substituted samples revealing unusual competitive effects between magnetism and superconductivity as well as the emergence of new high-temperature impurityinduced magnetic phases [24][25][26][27].…”

Towards a quantitative description of tunneling conductance of superconductors: Application to LiFeAs

“…From the experimental point of view, there are no indications that Co and Ni carry a significant magnetic moment. As for Mn impurities, on the other hand, they are expected to be magnetic [33][34][35] but only weakly coupled to the itinerant electron spins and hence one expects their magnetic potential to not strongly modify the LDOS 36 . Finally, we discuss consequences of our basis transformation for the spatial modulations of the tunneling current, spectra and topographs close to impurities, e.g., using Eq.…”

Scanning tunnelling spectroscopy as a probe of multi-Q magnetic states of itinerant magnets

“…Indeed, Mössbauer experiments have been recently employed to distinguish single- Q versus double- Q phases in iron-based superconductors18. Several theoretical calculations16192021222324 and experimental studies1825262728293031323334 have revealed that these materials support not only a single- Q magnetic stripe (MS) phase, with ordering vectors Q 1 =( π , 0) or Q 2 =(0, π ), but also two possible types of double- Q phases: a collinear double- Q phase with non-uniform Fe-magnetization, called charge-spin density wave (CSDW), and a coplanar double- Q phase called spin-vortex crystal (SVC)5. In the former, the staggered magnetization vectors corresponding to the two Q vectors are parallel to each other, whereas in the latter they are perpendicular.…”