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Science 20 April 2007:
Vol. 316. no. 5823, pp. 425 - 429
DOI: 10.1126/science.1138834
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Reports

Nonequilibrium Phase Transitions in Cuprates Observed by Ultrafast Electron Crystallography

Nuh Gedik,1 Ding-Shyue Yang,1 Gennady Logvenov,2 Ivan Bozovic,2 Ahmed H. Zewail1*

Nonequilibrium phase transitions, which are defined by the formation of macroscopic transient domains, are optically dark and cannot be observed through conventional temperature- or pressure-change studies. We have directly determined the structural dynamics of such a nonequilibrium phase transition in a cuprate superconductor. Ultrafast electron crystallography with the use of a tilted optical geometry technique afforded the necessary atomic-scale spatial and temporal resolutions. The observed transient behavior displays a notable "structural isosbestic" point and a threshold effect for the dependence of c-axis expansion ({Delta}c) on fluence (F), with {Delta}c/F = 0.02 angstrom/(millijoule per square centimeter). This threshold for photon doping occurs at ~0.12 photons per copper site, which is unexpectedly close to the density (per site) of chemically doped carriers needed to induce superconductivity.

1 Physical Biology Center for Ultrafast Science and Technology, California Institute of Technology (Caltech), Pasadena, CA 91125, USA.
2 Brookhaven National Laboratory (BNL), Upton, NY 11973–5000, USA.

* To whom correspondence should be addressed. E-mail: zewail{at}caltech.edu

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Science. ISSN 0036-8075 (print), 1095-9203 (online)