New Theory Explains Quick Switch in Switchable Mirrors

March 19, 1997

Cincinnati -- University of Cincinnati physicist Fu-Chun Zhang and collaborators who include UC graduate student Kwai-Kong Ng have developed a theory to explain how a shiny mirror made from rare earth elements can suddenly change into a transparent window. The unusual materials were discovered in 1996 and have many potential applications, including the development of "smart" windows for energy savings.

Zhang will explain his theory Wednesday, March 19 during the annual meeting of the American Physical Society in Kansas City, Missouri. It covers a class of new materials consisting of thin films of rare earth elements such as yttrium and lanthanum combined with hydrogen.

Ronald Griessen of the Netherlands led a group which first produced these materials and demonstrated that they can change from a metallic mirror to an insulating window in a matter of seconds.

Zhang says standard physics theory does not explain the switching phenomenon from mirror to window, because it is a single-particle theory and does not account for the strong electron correlation in the hydrides. Instead, Zhang considers a many-particle theory where strong correlation is built in.

"The answer turns out to be quite simple. Hydrogen is the smallest atom in the world, so the hydrogen is like a ping pong ball. It's very small, and the rows of the metallic atoms are very big like basketballs. The ping pong ball can move in and out of the basketball lattice very easily."

The rare earth hydrides are also unusual, because unlike semiconductors, they can take in an enormous number of electrons at the switching point. Those electrons change their behavior when the hydrogen atoms enter the lattice. One positively charged proton plus one negatively charged electron equals one hydrogen atom. Once inside the lattice, however, the hydrogen atoms attract a second electron. That sets the stage for the big switch.

"The electric charge does not change much, but the electron state changes dramatically," explained Zhang.

The electrons become localized around hydrogen atoms, no longer free to move around. When enough become locked into place, light which would have bounced off the mobile electrons is now able to move directly through the material. Instead of reflecting light (the mirror state), the material now transmits light (the window state).

The switch was depicted in dramatic fashion last year on the cover of Nature (March 21, 1996). In the mirror state, a thin film of ytrrium hydride reflected the image of a knight from a chess set. After the switch to the window state, a chessboard could be clearly seen behind the knight.

Zhang says his theory can explain the switch in both the lanthanum and yttrium-based materials, and should be useful in studying other rare earth hydrides as well. An early version of his theory was published earlier this year in Physical Review Letters. The co-authors were Ng, Professor T.M. Rice at the Swiss Federal Institute of Technology in Zurich and Professor V.I. Anisimov of the Russian Academy of Sciences.

University of Cincinnati

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