Student Colloquium: Junjing Zhao and Dingfei Ai
Scaling relations in underdoped hjigh Tc superconductors
presented by Dingfei Ai
In the underdoped high temperature superconductors, instead of a complete Fermi surface above Tc, only disconnected Fermi arcs appear, separated by regions that still exhibit an energy gap. The nature of pseudogap phase becomes a very important problem in the study of high Tc superconductor. Senthil proposed an ansatz (Phys. Rev. B 78, 035103 (2008)) that there is a quantum critical line just above the pseudogap phase (multiple quantum critical points, one for each doping value).
The scaling function is a multi-dimension function. Thus we fix doping and one variable of the scaling function at a time and vary the other variable to check the scaling relation. Then we repeat for different doping values and variables. We found the scaling function works in 5 different samples which are of different doping. In the future, we will analyze overdoped samples to see whether the scaling function describes all doping values or is specific to a description of the behavior of underdoped superconductors.
Doping versus disorder in cation-doped Bi2212 high Tc superconductor
presented by Junjing Zhao
Parent compounds for High temperature Superconductors (HTSCs) are antiferromagnetic Mott insulators. As they are doped with holes, superconductivity emerges through a dome shaped region in the temperature versus doping phase plane. In Bi2Sr2CaCu2O8+δ HTSCs doping levels can be altered either via changing oxygen concentration or via substituting Sr atoms with some trivalent elements, e.g. Dy, La etc. Quite naively, one could think that both ways of changing carrier concentration would lead to similar results. However, by comparing our systematic and detailed ARPES measurements on Bi2Sr2CaCu2O8+δ samples with those on Bi2(Sr,Dy)2CaCu2O8+δ samples, we found that the two ways of changing carrier concentration give rise to qualitatively different behaviors.
We previously found that irrespective of doping level, the momentum dependence of superconducting gap follows a simple d-wave form in Bi2Sr2CaCu2O8+δ samples1. Quite remarkably the same gap anisotropy persists even through the superconductor-to-insulator transition. Moreover, we observed that quasiparticel (QP) peak exists all around the underlying Fermi Surface (FS) for all doping levels in the superconducting state. But in Bi2(Sr,Dy)2CaCu2O8+δ samples, QPs vanish even in the superconducting state for lower carrier concentrations. In addition, the anisotropy of superconducting gap shows a strong deviation from the simple d-wave form, especially in underdoped side. Most likely, the addition of Dy not only changes carrier concentration, but also leads to disorders in the samples, which could be responsible for non d-wave gap and the destruction of QPs at lower dopings.