报告题目:Magnetic force microscopy studies of unconventional superconductors: single vortex mechanics and magnetic penetration depth
 
报告摘要:We have used magnetic force microscopy to image and manipulate individual vortices in YBa2Cu3O6+x (YBCO), and to locally measure the magnetic penetration depth in Ba(Fe1-xCox)2As2. The two experiments were both carried out on single crystalline samples with different doping levels. When the force exerted by the tip is strong enough to overcome the pinning potential, the manner a vortex follows the tip depends on the doping level. In overdoped YBCO­ (x=0.991) vortices stretch like elastic stings. We find that it is easier to drag vortices along the Cu-O chains than across them, consistent with the tilt modulus and the pinning potential being weaker along the chains. In more anisotropic underdoped YBCO (x=0.4), a vortex should be viewed as a stack of two-dimensional pancakes. Rather than tilting, vortices kink, allowing us to determine the interlayer coupling between the pancakes composing a single vortex. In another novel application of MFM, we measure the local penetration depth , as well as obtain its temperature induced change and the spatial homogeneity. We implement this method on the pnictide superconductor Ba(Fe1-xCox)2As2 ( , ), which we also extend  measurements down to 0.4 K by scanning SQUID susceptometry (SSS). We observe that  varies about ten times slower with temperature than previously published, with a dependence that can be well described by a clean two-band fully gapped model over the full temperature range. We find the superfluid density uniform despite the highly disordered vortex configuration.  Our preliminary results on a sample with different doping ( , ) show a similar variation with temperature.
 
报告人:Lan Luan
报告人简介:
Education:
       Stanford University 2004/09-present Ph. D candidate, physics,
       University of Science and Technology of China B.S. 2004
       Major: Applied Physics
       Minor: Electrical Engineering
Honor & Award:
      2004/06 Distinguished graduates, USTC
      2003/09 Guo Moruo Fellowship, USTC (Highest honor in USTC)
      2003/04 “Distinguished Student of Anhui Province”, Anhui Education Bureau
      2000-2004 First-tier scholarship, USTC
Research:
      Magnetic Force Microscopy (MFM)
        • Local penetration depth, pinning force and vortex matter in iron pnictides
        • Single vortex mechanics & dynamics in Cuprates
        • Novel sensor development
     Scanning Hall Probe Microscopy
        • Noise characterization of 80 nm GaAs/AlxGa1-xAs scanning Hall probes
        • Artificial pinning centers in YBCO single crystal
     Colossal Magneto-resistance Material (at USTC)
        • Crystal growth and transport characterization
报告时间:9月16日(星期三)上午10:00
报告地点:中科院物理所D210会议室
联 系 人:周兴江研究员 / 8264-8176