The iron-based superconductors (FeSC), discovered in 2008 and with transition temperatures as high as 55 K, became the second family of materials capable of achieving high-Tc after the cupper oxides. In many ways, the FeSC became a playground for theorists and experimentalists to compare and contrast the two families of high-Tc materials in the hope of identifying the important bare minimal ingredients for the mechanisms of high-temperature superconductivity.
Juxtaposing the two families, two major questions stand out:
- What is the appropriate ground state theoretical starting point for FeSC? Is it similar to or different from the cuprates?
- What is the nature of the phases in proximity to high-Tc superconductivity?
Below are our projects guided by these questions.
Electron Correlation Effects in FeSC
One important issue in the field is whether the magnetism in the pnictides is driven by local moment physics like the cuprates or itinerant physics. We have been studying this issue with ARPES by measuring in detail the electronic structures such as band dispersions and Fermi surfaces of various pnictide compounds, and comparing them with local-density approximation (LDA) calculations. In the simplest system of LaFePO, we found quantitative agreement between LDA and measured band dispersions after a uniform shift and renormalization of the LDA bands . For the intermediate case of BaFe2As2 in the paramagnetic state, we have also shown that nonmagnetic LDA calculations capture the details in the band dispersion after a momentum dependent shift and renormalization.
In the iron-chalcogenide materials, however, we found the electron correlation to be much stronger. In particular, we observed the low-temperature state of the AxFe2-ySe2 (A=K,Rb) superconductors to exhibit an orbital-dependent renormalization of the band near the Fermi level - the dxy bands heavily renormalized compared to the dxz/dyz bands. Upon raising the temperature to above 150 K, the system evolves into a state in which the dxy bands have depleted spectral weight while the dxz/dyz bands remain metallic. Combined with theoretical calculations, our observations can be consistently understood as a temperature-induced crossover from a metallic state at low temperatures to an orbital-selective Mott phase at high temperatures. Moreover, the fact that the superconducting state of AxFe2-ySe2 is near the boundary of such an orbital-selective Mott phase constrains the system to have sufficiently strong on-site Coulomb interactions and Hund's coupling, highlighting the non-trivial role of electron correlation in this family of iron-based superconductors.
Competing Phases in FeSC
Nematicity, defined as broken rotational symmetry, has recently been observed in the competing phases in proximity to the superconducting phase in the high-Tc cuprates. Similarly, the new iron-based high temperature superconductors exhibit symmetry breaking competing phases in the form of a tetragonal to orthorhombic structural transition preceding or coincident with the development of collinear spin density wave (SDW) in the undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping.
Evidence for strong in-plane anisotropy in the SDW state has been reported by various probes, pointing to the possibility that the pnictides may also harbor in its competing phases a nematic order, whose nature is still elusive. We studied detwinned pnictide family Ba(Fe1-xCox>)2As2 in the underdoped region using ARPES, resolving single domain electronic structure in the orthorhombic SDW state which exhibits strong in-plane anisotropy consistent with other probes.
Remarkably, we observe that this anisotropy manifests a broken symmetry in the orbital degree of freedom via a large splitting of the dxz and dyz bands. Moreover, such orbital anisotropy is seen to develop almost fully above the onset of the long range magnetic order, suggesting a nematic order involving the orbital degree of freedom. As such, our study shows that the orbital degree of freedom plays a nontrivial role in pnictide physics, and should be taken into account when formulating the theoretical models to describe superconductivity in iron pnictides .
Furthermore, we observe this nematicity, together with SDW order, directly compete with superconductivity in a K-doped Ba122 sample. This is done by first observing a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials .
 D. H. Lu et al. Electronic structure of the iron-based superconductor LaOFeP. Nature 455, 81 (2008)
 M. Yi et al. Observation of temperature-induced crossover to an orbital-selective Mott phase in AxFe2-ySe2 (A = K,Rb) superconductors. Phys. Rev. Lett. 110, 067003 (2013)
 M. Yi et al. Symmetry breaking orbital anisotropy observed for detwinned Ba(Fe1-xCox)2As2 above the spin density wave transition. PNAS 108, 6878 (2011)
 M. Yi et al. Dynamic competition between spin-density wave order and superconductivity in underdoped Ba1-xKxFe2As2. Nature Comm. 5, 3711 (2014)