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October 01, 2007

Using Terahertz Radiation to Control Material Properties

Ultrafast pulses of terahertz radiation has been used to change a manganite crystal from an electrical insulator into a conductor

This work is using terahertz radiation to change the properties of a solid crystal material by 100,000 times at super high speeds. It is better and more useful than ancient dreams of alchemy.

The ability to induce dramatic phase-changes in solid materials through select vibrations holds great promise for future exploitation of prized technological phenomena such as superconductivity and magnetoresistance. The methods present a new way of studying electron correlation effects and the coupling between crystal structure and the conduction properties of strongly correlated electrons.

Rini, working under Schoenlein and with a group of collaborators that included Ra’anan Tobey, Nicky Dean, Jiro Itatani, Yasuhide Tomioka, Yoshinori Tokura and Andrea Cavalleri, flashed single crystals of the strongly correlated manganite with femtosecond pulses of terahertz (trillion-cycles-per-second) radiation. Terahertz (abbreviated THz) radiation is the frequency of molecular vibrations; the femtosecond (millionths of a billionth of a second) timescale is the measure of atoms in motion.

Rini, Schoenlein and their colleagues found that a frequency of about 17 THz set off vibrations in the manganite crystal which resulted in a stretching of the electronic bonds that connect its principal constituent atoms - manganese and oxygen. This mild distortion of the crystal’s geometry caused a profound change in its electronic properties.

“By selectively exciting an individual vibrational mode of the insulating manganite, we increased the crystal’s electrical conductivity by five orders of magnitude,” said Rini. “What we observed was that the excitation of the manganese-oxide molecule’s vibrational mode promptly induced an ultrafast transition of the molecule to a metallic phase.”

This marks the first experimental demonstration that the selective excitation of a single vibrational mode can be used to induce phase changes in a crystal. It also demonstrates that the dynamics of a phase change in a solid can be observed when the solid resides in the electronic ground state - the electronic state in which most chemical reactions and phase transitions take place.

In the future, Rini said the Schoenlein group would like to use longer wavelength radiation to selectively excite other vibrational modes, and femtosecond x-ray beams to explore other aspects of vibrationally induced phase transitions. For now, their experimental technique is already shedding new light on the physics behind CMR, which should prove valuable for the future use of this phenomenon in magnetic data storage devices. The technique might also be used to address the unresolved physics behind the phenomenon of high-temperature superconductivity – copper-oxide (cuprate) materials that lose all electrical resistance at temperatures much higher than conventional superconductors.

“The complex and remarkable behavior of strongly correlated electron systems poses among the most intriguing questions in condensed matter physics,” said Rini. “Our vibrational excitation approach enables time-resolved measurements under the unique conditions created by the localization of energy in specific vibrational modes, and helps elucidate the coupling between particular vibrations and related electronic and magnetic properties. We believe our technique will find extensive application in other complex solids.”

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