Atomic path from insulator to metal messier than thought

Atomic Path from Insulator to Metal Messier than Thought
A simulation shows how atoms in vanadium dioxide shift during the brief time of an ultrafast laser pulse.

Researchers have peeked behind the curtain of the ultrafast phase transition of vanadium dioxide and found its atomic theatrics are much more complicated than they thought. It’s a material that has fascinated scientists for decades for its ability to shift from being an electrical insulator to a conductor.

The study, which appears Nov. 2 in the journal Science, is a collaboration between researchers at Duke University, the SLAC National Accelerator Laboratory at Stanford, the Barcelona Institute of Science and Technology, Oak Ridge National Laboratory, and the Japan Synchrotron Radiation Research Institute.

Vanadium dioxide has been intensely studied by researchers for more than five decades because of its unusual ability to switch from insulator to conductor at the conveniently attainable temperature of 152 degrees Fahrenheit. While other materials are also capable of this transition, most occur well below room temperature, making vanadium dioxide a better option for practical applications.

More recently, materials scientists have explored how this same phase transition takes place when the material’s atomic structure is excited by an extremely short, ultrafast laser pulse. What makes the phenomenon so challenging to study is the remarkable speed at which it happens—about 100 femtoseconds. That’s one tenth of a millionth of a millionth of a second.

The ultra-bright X-ray pulses at SLAC’s Linac Coherent Light Source (LCLS), however, are even faster.

By triggering vanadium dioxide’s electrical phase transition with a femtosecond laser and then pinging its atoms with X-ray pulses just tens of femtoseconds long, researchers were able to watch the transition unfold in full detail for the first time. They found that, rather than transitioning from one atomic structure to another in a direct, collaborative manner, the vanadium atoms arrived at their destinations through more unpredictable routes and independently of each other.

“It was proposed that the material would go from one crystalline structure to the other by following a deterministic, well-defined shuffling,” said Olivier Delaire, associate professor of mechanical engineering and materials science at Duke and one of the leaders of the study. “Instead we discovered that, even within a single transition, each atom is doing its own thing independently of the others.”

“The disorder we found is very strong, which means we have to rethink how we study all of these materials that we thought were behaving in a uniform way,” said Simon Wall, an associate professor at the Institute of Photonic Sciences in Barcelona and one of the leaders of the study.

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