Symmetry breaking orbital anisotropy on detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition

Abstract: Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high temperature superconductors. Similarly, the new iron-based high temperature superconductors exhibit a tetragonal to orthorhombic structural transition (i.e. a broken C4 symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here we present an angle resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe1-xCox)2As2 in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant dxz and dyz character, which is consistent with anisotropy observed by other probes. For compositions x>0, for which the structural transition (TS) precedes the magnetic transition (TSDW), an anisotropic splitting is observed to develop above TSDW, indicating that it is specifically associated with TS. For unstressed crystals, the band splitting is observed close to TS, whereas for stressed crystals the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.

• The paper demonstrates a major energy splitting between dₓz and dᵧz orbitals, evidencing strong in-plane electronic anisotropy.
• It employs ARPES on detwinned, uniaxially stressed crystals to observe anisotropic band splitting above the spin density wave transition.
• Findings underscore the crucial role of orbital order alongside spin and lattice effects in shaping high-temperature superconductivity.

Symmetry Breaking Orbital Anisotropy in Electron-Doped Iron Arsenide Superconductors

The paper presents a detailed angle-resolved photoemission spectroscopy (ARPES) study on detwinned single crystals of the electron-doped iron-arsenide superconductors, particularly focusing on Ba(Fe_{1-x}Co_x)_2As_2. This investigation reveals a symmetry-breaking orbital anisotropy in the electronic structure observed above the spin density wave (SDW) transition temperature. The research targets a deeper understanding of the nematic phase associated with this class of superconductors.

Key Findings

The study reports on a tetragonal-to-orthorhombic structural transition accompanied by a spin density wave order in the iron pnictide superconductors, specifically investigating the in-plane electronic anisotropy that becomes evident upon reducing the temperature. In this research, detwinning of the crystals was achieved through the application of in-plane uniaxial stress, thereby allowing the study of single-domain electronic structures.

Notably, the findings demonstrate that:

• In-plane Electronic Anisotropy: A large energy splitting between two orthogonal bands related to the d_{xz} and d_{yz} orbital character is observed, indicative of strong in-plane electronic anisotropy.
• Anisotropic Splitting above T_SD: For compositions where the structural transition precedes the magnetic one, an anisotropic band splitting emerges above the SDW transition (T_SD), correlating with the structural transition (T_S). This highlights a large in-plane nematic susceptibility in the electronic structure.
• Impact of Uniaxial Stress: In unstressed crystals, band splitting is noticeable near T_S, while in stressed samples, the splitting is observed at much higher temperatures. This provides evidence of significant nematic fluctuations spanning well above the long-range magnetic order.

Implications and Future Prospects

This research supports the significance of the orbital degree of freedom in the nematic phase of iron arsenides, presenting a compelling study of nematic susceptibility. The observed large anisotropic band splitting substantially informs theoretical models related to the electronic structure of iron-based superconductors, guiding towards a more nuanced picture of orbital order contributions.

The large amplitude of the observed effects suggests that the spin and lattice degrees of freedom alone cannot fully explain the nematic state, leading to implications that orbital order also plays an integral role. These insights contribute to the broader understanding of high-temperature superconductivity mechanisms.

For future investigations, extending this study across different doping levels and superconductor families could further elucidate the role of orbital fluctuations and their dynamic interaction with other electronic phases. Understanding the full landscape of nematic susceptibility could be transformative for developing theoretical models addressing superconductivity in iron-based compounds and potentially other high-temperature superconductors.

Conclusion

The findings from this paper underscore the complexity of the interplay between structural, magnetic, and electronic phenomena in iron-based superconductors. They present a refined perspective on how symmetry-breaking phenomena are intrinsic to these materials and are pivotal in shaping their superconducting properties. This work lays a vital foundation for future explorations into quantum phases and may inspire advanced theoretical investigations into the superconducting behavior of strongly correlated electron systems.