Symmetry-breaking orbital anisotropy observed for detwinned Ba(Fe _{1- x} Co _x ) ₂ As ₂ 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 C 4 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, BaðFe 1-x Co x Þ 2 As 2 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 d xz and d yz character, which is consistent with anisotropy observed by other probes. For compositions x > 0, for which the structural transition (T S ) precedes the magnetic transition (T SDW ), an anisotropic splitting is observed to develop above T SDW , indicating that it is specifically associated with T S . For unstressed crystals, the band splitting is observed close to T S , 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.C orrelated electron systems owe their emergent phenomena to a complex array of competing electronic phases. Among these, a nematic phase is one where rotational symmetry is spontaneously broken without breaking translational symmetry (1, 2). Two well-established examples are found in certain quantum Hall states (3) and in the bilayer ruthenate Sr 3 Ru 2 O 7 (4), both of which exhibit a large transport anisotropy under the application of large magnetic fields, even though they seem to originate from apparently different physics. Recently, evidence of nematicity has also been reported in the pseudogap phase of cuprate high-temperature (high-T C ) superconductors, in both YBa 2 Cu 3 O y (5) and Bi 2 Sr 2 CaCu 2 O 8þδ (6). The proximity of the pseudogap phase to superconductivity raises the question of what role nematicity plays in relation to the mechanism of high-T C superconductivity. Intriguingly, the newly discovered iron pnictide high-T C superconductors also exhibit a nematic phase in the form of a tetragonal-to-orthorhombic structural transition that either precedes or accompanies the onset of long-range antiferromagnetic order (7,8), both of which are suppressed with doping leading to...

Distinguishing spin-orbit coupling and nematic order in the electronic spectrum of iron-based superconductors

“…We can now discuss the implications of our results to the interpretation of ARPES experiments in the nematicparamagnetic state, which takes place in the temperature range T SDW < T < T nem for unstrained samples and in the temperature range T SDW < T for detwinned samples [4][5][6][7][8][9][10][11][12] . The observed splitting of the E M1 doublet has been mostly attributed to an anisotropy in the onsite energies of the d xz and d yz orbitals (ferro-orbital order), which in our model is given by ϕ Γ = ϕ 1 .…”

“…Angle-resolved photo-emission spectroscopy (ARPES), being sensitive to the electronic energy-momentum dispersion 3 , is an attractive tool to probe how these distinct ordered states manifest themselves in the electronic spectrum [4][5][6][7][8][9][10][11][12] . However, the close proximity of the different electronic energy scales, together with the multi-orbital character of the band structure, render this task non-trivial 13 .…”

Competition between superconductivity and nematic order: Anisotropy of superconducting coherence length

“…[2][3][4][5][6][7][8] In the cuprates, nematic order is signaled by the anisotropy of resistivity and bond ordered density [2][3][4][5][6] . In the pnictides, orthorhombic structure distortion is ubiquitous, and is confirmed by various tools such as neutron scattering 9 , STM 10 , transport [11][12][13] , and ARPES 14 . A natural question associated with nematic ordering is its interaction with other order parameters.…”

Nematicity driven by hybridization in iron-based superconductors

“…The normal state properties of these materials, in contrast, at first seemed rather unremarkablea bad metal behavior at high temperatures, which can be followed by structural transition and antiferromagnetic phase at low temperatures. This simple picture was considerably complicated by the growing evidence of nematic fluctuations [6][7][8] or even long-range order 9 present in some compounds at temperatures well above the antiferromagnetic and structural transitions. Furthermore, this nematicity seems to be of purely electronic (rather than structural) origin 10 .…”