|The Davis group does it again|
|Contact: Paul Preuss, firstname.lastname@example.org|
When in a letter to Nature published January 24, 2002, a group led by J.C. Séamus Davis reported obtaining the first-ever direct evidence of "granular" superconductivity in BSCCO, a high-temperature (high-Tc) superconductor, it was only the latest in a series of landmark explorations of the mysteries of these strange materials.
Just a week earlier, on January 18, Davis's group had published a report
in Science describing another novel use of scanning tunneling microscopy
(STM) to shed light on the relationship of magnetic and electronic properties
in the same material -- results commentators called "a breakthrough."
Davis is a member of Berkeley Lab's Materials Sciences Division and a
professor of physics at the University of California at Berkeley. In the
last two years his group has produced a spate of scientific papers presenting
"eye-witness" experimental evidence in support of some theoretical
ideas and against others. A February, 2000, Nature article reported using
STM to image zinc-atom impurities in BSCCO, produced the first real-space
image of d-wave symmetry in a high-Tc superconductor. And in June of 2001
another Nature article presented STM imaging of nickel-atom impurities
in BSCCO, showing visible evidence of magnetism's role in high-Tc superconductivity.
Using materials specially prepared by Shin-ichi Ushida of the University
of Tokyo, Davis and his group obtained images of regular variations in
these electronic states, seen as a visible checkerboard pattern surrounding
each vortex core. The patterns are consistent with several explanations
of how electronic states may coexist in high-Tc superconductors, including
the granularity reported last month in Nature, with nanometer-scale superconducting
islands scattered in an insulating sea, as well as the "stripe phase,"
in which one-dimensional lines of charged particles alternate with insulating
In a Science Perspective, Subir Sachdev and Shou-Cheng Zhang described the Davis group's results as a breakthrough for helping researchers choose among competing explanations of why electronic states vary with the "doping" of the material. The results support the notion of competition between the spatial arrangement of spin and charge densities in high-Tc superconductors: at low doping, spin density dominates and the material behaves as an insulator; at optimum doping, charge density dominates, allowing superconductivity.
But Sachdev and Zhang remark that "many mysteries remain" -- a state of affairs that will keep the Davis group busy producing papers for some time to come.
"A Four Unit Cell Periodic Pattern of Quasi-Particle States Surround
Vortex Cores in Bi2Sr2CaCu2O8+delta," by Jenny Hoffman, Eric Hudson,
Kristine Lang, Vidya Madhavan, Hiroshi Eisaki, Shin-ichi Ushida, and J.
C. Davis, appeared in the 18 January 2002 issue of Science. It
and the other papers mentioned here can be found on the Davis