PHYSICS/BK21 Colloquium(09/3/13)

"                  * PHYSICS/BK21 Colloquium *

                       “Investigations of the Effects of Defects
                  on the First Order Vortex Melting Transition
                         in YBa2Cu3O7-δ Single Crystals”

  
ØSpeaker : Prof. Lisa Paulius [Western Michigan University]  
ØPlace : Physics Seminar Room (Science Bldg, 3-201)
ØDate & Time : Mar. 13(Fri)  4:00 ~ 5:00 pm

Abstract
  
The first order vortex melting transition is very sensitive to the presence of disorder and
can be supplanted by a second order phase transition in the  presence  of a sufficiently
high density of defects. We present the results  of systematic  studies on the effects of
defects on the first order vortex melting transition in YBa2Cu3O7-δ single crystals, with a
special focus on the upper and lower critical points. The experiments were  performed
with electrical transport and ac specific heat  measurements. The geometry and strength
of the defects varied,  including  naturally occurring twin boundaries as well as detwinned
single crystals  irradiated with successive doses of electrons,  protons, or heavy  ions.
Twin boundaries yield planar defects,  electron irradiation yielded weak point  defects,
proton irradiation yielded strong point-like defects and  heavy  ion  irradiation yielded
strong columnar defects. Our results  indicate that the  origins  of the upper and lower
critical points of the first order melting transition arise from different mechanisms. The
position of the lower critical  point is determined by the pinning strength of the defect,
whereas the upper critical point shifts in response to the geometrical correlation of the
defects.

                        Contact Person : Prof. Ki Bong Lee (054-279-2068, kibong@postech.ac.kr)    "