Speed Limit Set for Ultrafast Electrical Switch

Switching process in magnetite takes only a trillionth of a second.

The experiments were conducted at the SXR beamline at LCLS (Photo: Brad Plummer/SLAC).

An international team of researchers have clocked the fastest-possible electrical switching in magnetite, a naturally occurring magnetic mineral, at the X-ray laser LCLS in California. Their results could drive innovations in ultrafast tiny transistors that control the flow of electricity across silicon chips, enabling faster, more powerful computing devices.

With the Linac Coherent Light Source (LCLS), the world’s most powerful X-ray laser, scientists including researchers from the Center for Free-Electron Laser Science CFEL in Hamburg and from European XFEL, found that it takes only 1 trillionth of a second to flip the on-off electrical switch in samples of magnetite, which is thousands of times faster than in transistors now in use. The results were published July 28 in Nature Materials.

Scientists first hit each sample with a visible-light laser pulse and then investigated it with an ultra-bright and ultra-short X-ray blast from the LCLS facility. The visible-light laser pulse fragmented the material's electronic structure, rearranging it to form islands. The laser blast was followed closely by an ultrabright, ultrashort X-ray pulse from the LCLS facility that allowed researchers to study, for the first time, the timing and details of changes in the sample excited by the initial laser strike.

"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 who is a lead author of the study.

  “The ultra-short X-ray flashes of these new free-electron lasers allow finding and carefully observing these fast natural processes,” said Prof. Wilfried Wurth from CFEL and the University of Hamburg. “With the understanding of these processes, we will create the basis for completely new applications – in this case possibly a new generation of electronic components.” The CFEL scientists did not only participate in this experiment but – jointly with researchers from Stanford and Berkeley – substantially contributed to the design and construction of the soft X-ray (SXR) beamline at the LCLS which was used in this experiment.

By slightly adjusting the interval of the X-ray pulses, the scientists precisely measured how long it took the material to shift from a non-conducting to an electrically conducting state, and observed the structural changes during this switch. The experimenters had to cool down the magnetite to minus 190 degrees Celsius to lock/”freeze” its electrical charges in place.

Scientists had worked for decades to resolve the electrical structure of magnetite at the atomic level, and just last year another research team had identified its building blocks as "trimerons" – a characteristic periodic formation of three charged iron atoms. That finding provided key insights in interpreting results from the current LCLS experiments.

The next step for the scientists is to study more complex materials that are also switchable at room-temperature. 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 researchers have already conducted follow-up studies focusing on a hybrid material that exhibits similar ultrafast switching properties at near room temperature, which makes it a better candidate for commercial use than magnetite.

Currently, there is a major global effort underway to go beyond modern semiconductor transistors using new materials to satisfy demands for smaller and faster computers. X-ray lasers as the LCLS and the European XFEL, which is now being built in Northern Germany, have the unique ability to home in on processes that occur at the scale of atoms in trillionths and quadrillionths of a second.

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

CFEL Publications