Understanding the Mysteries of High-Temperature
By Heather Rock Woods
electronic states in a high-temperature superconducting material
are strongest along the diagonal momentum direction.
(Image by Donghui Lu)
High-temperature superconductors (HTSCs) operate in
mysterious ways, but scientists are starting to understand their
peculiarities by using a state-of-the-art spectroscopy system at SSRL.
One of the biggest mysteries is how a material that starts as an
insulator—which does not conduct electricity—can become a
high-temperature superconductor after being doped with electric
Researchers Kyle Shen and Donghui Lu (both ESRD), working in Zhi-xun
Shen’s group at SSRL and Stanford, looked at the evolution from
insulator to superconductor by studying an HTSC material at different
doping concentrations, including ones that are insulating. The team used
angle-resolved photoemission spectroscopy (ARPES), a method of probing
the electronic states in solids.
published in Science magazine on February 11, contribute to creating a
fundamental understanding of the perplexing physics of HTSCs.
“The materials were discovered almost 20 years ago, but they are very
complicated and not well understood,” Lu said. “We’d like to have a
microscopic theory that tells us why they can be superconducting at a
temperature much higher than conventional superconductors, and thereby
how to improve the materials.”
HTSCs have huge
potential for industry because they conduct electrical current without
heat loss, yet need to be cooled only to liquid nitrogen temperatures
(77 Kelvin) rather than the liquid helium temperatures (4 Kelvin) needed
for conventional superconductors. While still chilly, that ‘high’
temperature is much less expensive to reach. HTSCs are used in niche
applications, as the materials are currently too brittle for widespread
Below the superconducting transition
temperature, electrons pair up and travel free of resistance. The pairs
in conventional superconductors join up through well-understood
interactions. In HTSCs, however, it is unclear what mechanism causes the
electrons to pair up.
One clue came from another
group that used scanning tunneling microscopy (STM) to look at where the
electrons were distributed across the two-dimensional sheets that make
up the HTSC material. They discovered an interesting checkerboard
pattern, indicating an unusual charge ordering (the repeating pattern or
arrangement of electrons).
The ARPES data added to
the unexpected picture. It revealed electronic states that were much
stronger along the nodal momentum direction (diagonal to the
checkerboard squares) than along the anti-nodal (straight) direction.
The anti-nodal direction is the one in which superconductivity is the
strongest and where charge ordering manifests.
fact that there are fewer electronic states along the anti-nodal
direction is surprising,” Lu said. “We thought they would be stronger to
give rise to the checkerboard pattern observed by STM.”
These results show that the difference in momentum direction is
important to electronic structure, and therefore put strong constraints
on proposed models trying to explain how HTSCs work.
For more information, see: http://www.sciencemag.org/content/vol307/issue5711/index.shtml