Graduate Thesis Or Dissertation


Angle-Resolved Photoemission Spectroscopy Studies on Cuprate and Iron-Pnictide High-Tc Superconductors Public Deposited
  • The exotic physics in condensed matter systems, such as High-Tc superconductivity in cuprates and the newly discovered iron-pnictide superconductors, is due to the properties of the elementary excitations and their interactions. The "one-electron removal spectral function" measured by angle-resolved photoemission spectroscopy (ARPES) provides a chance to understand these excitations and reveal the mechanism of the high-Tc superconductivity. In most cases, ARPES studies focus on the excitations very close to the Fermi level (usually within tens to hundreds of meVs). In this region, by presuming that the correlation effect is not too strong, we usually can describe the correlated electron system in terms of well-defined "quasiparticles" , i.e. electrons dressed with a manifold of excited states. Then the spectral function measured by ARPES can be separated into two parts: a coherent pole part that contains the information about the dispersion relation E(k) and the lifetime "tau" of the quasiparticles, which is usually the main subject of ARPES studies; and an incoherent smooth part without poles which also contains important information about the many-body interactions in the system but is usually overlooked by physicists due to the lack of analysis techniques and theoretical understanding. In this thesis, we present ARPES measurement on the cuprate High-Tc superconductors PbxBi2-xSr2CaCu2O8 (Pb-Bi2212) and Bi2Sr2CaCu2O8+d (Bi2212) and the iron-pnictide High-Tc superconductor's parent compound CaFe2As2 (Ca122) and BaFe2As2 (Ba122). For Pb-Bi2212 and Bi2212 materials, whose quasiparticle dispersions have already been extensively studied, our work focuses on the incoherent part of the spectral function. By introducing a new ARPES lineshape analysis technique, we separate out the sharp coherent peaks from the higher energy incoherent "background" portions and uncover a new type of scaling behavior of the incoherent portions. In particular, the fraction of weight that is incoherent is found to be intimately linked to the energy of the dispersive coherent feature through a simple quadratic relationship with no special energy scales. This behavior in concert with strong momentum-dependent matrix element effects gives rise to the heavily studied "waterfall" behavior in cuprate superconductors. For the newly discovered Ca122and Ba122 materials, whose intrinsic electronic structure is still missing, our studies aim at understanding its quasiparticle dispersion relation E(k) and the Fermi surface geometry. We observed unequal dispersions and FS geometries along the orthogonal Fe-Fe bond directions. Comparing with the optimized LDA calculations, an orbital-dependent band shifting is introduced to get better agreement, which is consistent with the development of orbital ordering. More interestingly, unidirectional straight and at FS segments are observed near the zone center, which indicates the existence of a unidirectional charge density wave order. Therefore, our studies indicate that beyond the well-known spin density wave (SDW) order and superconducting state (SC), there are other competing orders in the iron-pnictide materials such as the orbital order (OO), the charge density wave (CDW) order and the possible nematic phase. The coexistence of all these competing orders puts strong constraints on theories for describing the iron-pnictide system.
Date Issued
  • 2011
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Last Modified
  • 2019-11-16
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