Disappearance of Superconductivity and a Concomitant Lifshitz Transition in Heavily Overdoped Bi_2Sr_2CuO_6 Superconductor Revealed by Angle-Resolved Photoemission Spectroscopy

By partially doping Pb to effectively suppress the superstructure in single-layered cuprate Bi_2Sr_2CuO_(6+δ)(Pb-Bi2201) and annealing them in vacuum or in high pressure oxygen atmosphere, a series of high quality Pb-Bi2201 single crystals are obtained with T_c covering from 17 K to non-superconducting in the overdoped region. High resolution angle resolved photoemission spectroscopy measurements are carried out on these samples to investigate the evolution of the Fermi surface topology with doping in the normal state. Clear and complete Fermi surfaces are observed and quantitatively analyzed in all of these overdoped Pb-Bi2201 samples. A Lifshitz transition from holelike Fermi surface to electron-like Fermi surface with increasing doping is observed at a doping level of ~0.35. This transition coincides with the change that the sample undergoes superconducting-to-non-superconducting states.Our results reveal the emergence of an electron-like Fermi surface and the existence of a Lifshitz transition in heavily overdoped Bi2201 samples. This provides important information in understanding the connection between the disappearance of superconductivity and the Lifshitz transition in the overdoped region.

Distinction between critical current effects and intrinsic anomalies in the point-contact Andreev reflection spectra of unconventional superconductors

In this work,we discuss the origin of several anomalies present in the point-contact Andreev reflection spectra of（Li1-xFex）OHFeSe,LiTi2O4,and La2-xCexCuO4.While these features are similar to those stemming from intrinsic superconducting properties,such as Andreev reflection,electron-boson coupling,multigap superconductivity,d-wave and p-wave pairing symmetry,they cannot be accounted for by the modified Blonder–Tinkham–Klapwijk（BTK） model,but require to consider critical current effects arising from the junction geometry.Our results point to the importance of tracking the evolution of the dips and peaks in the differential conductance as a function of the bias voltage,in order to correctly deduce the properties of the superconducting state.