February 15, 2002
Berkeley Lab Science Beat Berkeley Lab Science Beat
The Davis group does it again
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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."
Superconductivity is suppressed in the center of magnetic vortices, but superconducting currents flow around each core. (Credit: Sachdev and Zhang, Science)

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.

The Davis group's latest article in Science takes advantage of the fact that materials in the superconducting state normally exclude magnetic fields. However, if a high-Tc superconductor is subjected to a strong enough magnetic field at right angles to the superconducting copper-oxygen layers, a pattern of magnetic "vortexes" penetrates the material. Superconductivity is suppressed in the vortex cores, but superconducting currents circulate around them, effects which depend on the density and spin orientation of electrons in the core regions.

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 bands.

An STM image of a single magnetic vortex in BSCCO. Density waves of electron charge and spin centered in the vortex interfere in a checkerboard pattern.  

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 group website.

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