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.     

一种新的分析和检查需求模型

由于ＳＣＲ（ＳｏｆｔｗａｒｅＣｏｓｔＲｅｄｕｃｔｉｏｎ）是较实用的定义软件系统需求的方法，以此方法为基础，提出一个形式化的分析和检查需求的模型．此分析和检查模型主要根据综合知识表示模型的一些基本原理，使用状态转换规则、语义计算规则和属性来形式化地描述ＳＣＲ中表格符号的语义，并通过执行规则来自动分析由ＳＣＲ定义的需求．为便于理解，给出了一个简单的实例来介绍如何分析和检查基于ＳＣＲ的需求的一致性和完整性．     

基于M_(AS)的形式化数据流图的方法

由于ＤｅＭａｒｃｏ的数据流图（ＤＤＦＤ）缺乏形式化的描述方法，本文提出了一种使用综合知识表示模型ＭＡＳ来形式化描述ＤＤＦＤ的新方法．使用此方法，能方便地建立一些有关ＤＤＦＤ的知识规则，并能对ＤＤＦＤ自动进行一致性和完全性检查     

Electronic structure and nematic phase transition in superconducting multiple-layer FeSe films grown by pulsed laser deposition method

We report comprehensive angle-resolved photoemission investigations on the electronic structure of single crystal multiple-layer FeSe films grown on CaF_2 substrate by pulsed laser deposition(PLD) method. Measurements on FeSe/CaF_2 samples with different superconducting transition temperatures T_c of 4 K, 9 K, and 14 K reveal electronic difference in their Fermi surface and band structure. Indication of the nematic phase transition is observed from temperature-dependent measurements of these samples; the nematic transition temperature is 140-160 K, much higher than ~90 K for the bulk FeSe. Potassium deposition is applied onto the surface of these samples; the nematic phase is suppressed by potassium deposition which introduces electrons to these FeSe films and causes a pronounced electronic structure change. We compared and discussed the electronic structure and superconductivity of the FeSe/CaF_2 films by PLD method with the FeSe/SrTiO_3 films by molecular beam epitaxy(MBE) method and bulk FeSe. The PLD-grown multilayer FeSe/CaF_2 is more hole-doped than that in MBE-grown multiple-layer FeSe films. Our results on FeSe/CaF_2 films by PLD method establish a link between bulk FeSe single crystal and FeSe/SrTiO_3 films by MBE method, and provide important information to understand superconductivity in FeSe-related systems.