Condensed-Matter Physics & Materials Science Seminar

The current that can cross grain boundaries of arbitrary misorientation remains the single largest determinant of possible conductor architecture. In cuprates, experience with YBCO and Bi-2223 shows that misorientations of only a few degrees decrease the critical current density significantly, the critical angle at which depression of the intragrain current density starts in [001] tilt grain boundaries being as small as 2-3°. In a broad sense the better properties of YBCO compared to Bi-2223, which corresponds to the 1st to 2nd generation HTS conductor technology change, can be plausibly explained just in terms of the much worse uniaxial texture (FWHM ~12°) of Bi-2223, as compared to the biaxial texture of ~5° possible in YBCO. But in fact the situation is more complex and more promising than this simple explanation would suggest. Neither Bi-2223 nor pure YBCO can be significantly overdoped, making the case of round wire Bi-2212 which can be overdoped of great interest. Moreover there are marked benefits of allowing ex situ growth in which grain boundaries meander so as to avoid separate populations of Abrikosov-Josephson vortices in GBs. Segregation effects can also very beneficially enhance the Jc of GBs. Moreover, GBs in pnictides appear to show similar sensitivity to misorientation as in the cuprates, making the detailed understanding of how far GBs can be pushed of great practical and scientific interest. I will review recent work of my collaborators and I on Bi-2212, YBCO and its related compounds and pnictides with a particular view towards expanding the domain in which GB properties can be enhanced.