Angle-resolved Photoemission Studies on Hole Doped Iron Pnictides Ba1-xKxFe2As2

Abstract

The discovery of the high-Tc superconductivity in iron-arsenic materials in 2008 immediately became one of the hottest topics in the condensed matter physics. This dissertation presents a systematic study on the pairing symmetry and electronic structure on the hole doped materials of BaFe2As2 (so called “122”-system), by angle-resolved photoemission spectroscopy (ARPES). In the early ARPES studies on “122”-pnictides, we observed two hole-like Fermi surfaces (FSs) centered at the Brillouin zone (BZ) center, (Γ), and two electron-like FSs centered at the zone corner (M), which is (π, π) in the BZ or (π, 0) in the unfolded BZ. The size of these FS sheets can be changed by carrier doping, which causes change of the chemical potential. In the superconducting state, temperature (T) and momentum (k) dependence of ARPES measurements reveals the Fermi-surface-dependent nodeless superconducting gaps in this system and shows that an s-wave symmetry is the most natural interpretation for our findings in terms of the pairing order parameter. The ratio 2Δ/kBTc switches from weak to strong coupling on different FS sheets. Large superconducting gaps are observed with a strong coupling coefficient (2Δ/kBTc) on the near-nested FSs connected by the antiferromagnetic (AF) wave vector ((π, π) in the BZ or (π, 0) in the unfolded BZ). When Tc is suppressed in the heavily overdoped materials, the near-nesting condition vanishes, or more precisely, the (π, π) inter-FS scattering disappears due to the absence of either the hole-like or the electron-like FS at the Fermi energy (EF). We have also performed ARPES measurements on kz-dependence of the superconducting gap and band structure of the optimally hole doped sample Ba0.6K0.4Fe2As2. By varying the photon energy, we can tune kz continuously. While significant kz dispersion of the superconducting gaps is observed on the hole-like bands, much weaker kz dispersion of the superconducting gaps is observed on the electron-like bands. Remarkably, we find that a 3D gap function based on short-range pairing can fit the superconducting gaps on all the FS sheets. Moreover, an additional hole-like FS (referred as the α‘ FS) predicted by local density approximation (LDA) calculations is observed around the Z point. The disappearance of intensity of the α‘ band near EF at kz = π/2 suggests that the α‘ band could either sink below EF or be degenerate with the inner hole (α) band. The studies on the α‘ band in the superconducting state reveal a nearly isotropic superconducting gap on this FS sheet. Underdoped samples Ba0.75K0.25Fe2As2 are used to study how the AF fluctuations and superconductivity interplay in the underdoped regime that is closer to the AF phase. we observe that the superconducting gap of the underdoped pnictides scales linearly with Tc. A distinct pseudogap develops upon underdoping and coexists with the superconducting gap. Remarkably, this pseudogap occurs mainly on the FS sheets that are connected by the AF wave vector, where the superconducting pairing is stronger as well. This suggests that both the pseudogap and the superconducting gap are driven by the AF fluctuations, and the long-range AF ordering competes with the superconductivity. The observed dichotomic behaviour of the pseudogap and the SC gap on different FS sheets in the underdoped pnictides shares similarities with those observed in the underdoped copper oxide superconductors, providing a possible unifying picture for both families of high-temperature superconductors.