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Speed Limit for Electrical Switching Revealed

Photonics.com
Jul 2013
MENLO PARK, Calif., July 31, 2013 — An optical laser pulse has shattered the ordered electronic structure in an insulating sample of magnetite, switching the material to electrically conducting in a picosecond (one-trillionth of a second). The discovery could lead to faster, more powerful computers.

Scientists have known the basic properties of the naturally magnetic mineral magnetite for thousands of years, but its more exotic electronic properties are now just being learned.

To tap into these properties, researchers at SLAC National Accelerator Laboratory used the Linac Coherent Light Source (LCLS) x-ray laser to send pulses of visible laser light at the mineral, clocking the fastest-possible electrical switching. They discovered that it takes only a picosecond to flip the on/off electrical switch in magnetite, which is thousands of times faster than in transistors now in use.


An optical laser pulse (red streak from upper right) shatters the ordered electronic structure (blue) in an insulating sample of magnetite, switching the material to electrically conducting (red) in one-trillionth of a second. Courtesy of Greg Stewart/SLAC.

“This breakthrough research reveals for the first time the ‘speed limit’ for electrical switching in this material,” said Roopali Kukreja, a materials science researcher at SLAC and Stanford University and a lead author of the study.

The LCLS experiment also showed researchers how the electronic structure of the sample rearranged into nonconducting “islands” surrounded by electrically conducting regions, which began to form just hundreds of quadrillionths of a second after a laser pulse struck the sample. This showed how such conducting and nonconducting states can coexist and create electrical pathways in next-generation transistors.

When the samples were hit with visible light, its electronic structure was fragmented at an atomic scale, rearranging it to form the islands. The laser blast was followed closely by an ultrabright, ultrashort x-ray pulse that enabled the investigators to study the timing and details of changes in the sample excited by the initial laser strike.


In its insulating state, the magnetite sample has electrical charges locked into structures known as “trimerons,” which are composed of three iron atoms (a). An optical laser pulse was used to fracture trimerons (b), creating strands of electrical conductivity (red) surrounding islands of nonconducting trimeron structures (c). Courtesy of S. de Jong et al /Nature Materials.

Slight adjustments to the intervals of the x-ray pulse enabled the precise measurement of the length of time it took the material to shift from nonconducting to an electrically conducting state, and the observation of the structural change during the switch.

Scientists worked for decades to resolve this electrical structure at the atomic level, and just last year, another research team identified its building blocks as “trimerons” — formed by three iron atoms that lock in the charges. That finding provided key insights in interpreting the results from the LCLS experiment.

The magnetite had to be cooled to –190 ºC to lock its electrical charges in place, so the next step is to study more complex materials and room-temperature applications, Kukreja said. Follow-up studies have been conducted focusing on a hybrid material that exhibits similar ultrafast switching properties at near room temperature, which would make it a better candidate for commercial use than magnetite.


The magnetite experiment was conducted at the Soft X-ray Materials Science (SXR) experimental station at SLAC National Accelerator Laboratory’s Linac Coherent Light Source X-ray laser. Courtesy of Brad Plummer/SLAC.

Future experiments will aim to identify exotic compounds and test new techniques to induce the switching and tap into other properties that are superior to modern-day silicon transistors.

The research, which appeared in Nature Materials (doi: 10.1038/nmat3718), was performed in collaboration with scientists from Helmholtz-Zentrum Berlin for Materials and Energy; Hamburg University/Center for Free Electron Laser Science (CFEL); the University of Amsterdam; the T-REX laboratory at ELETTRA-Sincrotrone Trieste and the University of Trieste; Cologne, Potsdam Regensburg and Purdue universities; the Advanced Light Source at Lawrence Berkeley National Laboratory; and SwissFEL. 

For more information, visit: www.slac.stanford.edu


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