[ad_1]
The researchers peeked behind the curtain of the ultra-fast phase transition of vanadium dioxide and discovered that her atomic theatricality was far more complicated than she thought. It's a material that has fascinated scientists for decades for its ability to switch from an electrical isolator to a driver.
The study, which appears on November 2 in the newspaper Science, is a collaboration between researchers from Duke University, Stanford's SLAC National Accelerator Laboratory, the Barcelona Institute of Science and Technology, the Oak Ridge National Laboratory, and the Institute for Research on Synchrotron Radiation. from Japan.
Researchers have been closely studying vanadium dioxide for more than five decades because of its unusual ability to pass from an insulator to a driver at easily 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 choice for practical applications.
More recently, material scientists have explored how this same phase transition occurs when the atomic structure of the material is excited by an extremely short and ultra-fast laser pulse. What makes the phenomenon so difficult to study is the remarkable speed with which it occurs – about 100 femtoseconds. It's a tenth of a millionth of a millionth of a second.
However, the ultra-bright X-ray pulses at the SLAC coherent light source (LCLS) are even faster.
By triggering the electrical phase transition of vanadium dioxide with a femtosecond laser, and then sending X-ray pulses on its atoms barely tens of femtoseconds, the researchers were able to observe the transition taking place in every detail. They discovered that rather than moving from one atomic structure to another in a direct and collaborative way, vanadium atoms arrived at their destination in more unpredictable ways and independently of each other.
"It has been proposed that the material passes from one crystalline structure to the next following a deterministic and well defined brewing," said Olivier Delaire, associate professor of mechanical engineering and materials science at Duke and one of the makers of the study. "Instead, we discovered that even within a single transition, each atom was doing its own thing independently of the others."
"The mess we've seen is very strong, which means we have to rethink the way we study all these materials that we thought would behave in a consistent way," said Simon Wall, associate professor at Institute of Photonic Sciences of Barcelona. leaders of the study.
"They do not easily take up their new duties, like the members of a group parading in a field, they are staggering like party goers leaving a bar at the closing," Wall said. "If our ultimate goal is to control the behavior of these materials in order to be able to pass them from one phase to the other, it is much harder to control the drunk chorus than the fanfare."
To understand the meaning of experimental observations, Delaire's group at Duke also directed supercomputer simulations of atomic dynamics in the material. The simulations were performed on supercomputers at the National Energy Research Computing Center and the Oak Ridge Leadership Computing facility.
"It was amazing to see that my student Shan Yang presented the results of his quantum atomic motion simulations," Delaire said. "This is almost exactly the same as the experimental" movies "of X-ray intensity recorded, even without the need for adjustable parameters."
Previous studies did not have access to the spatial and temporal resolution offered by the LCLS and could only measure the averages of the atomic behavior of the material. Because of these limitations, they could not see the importance of random deviations from the average vanadium atom motions.
However, with the sensitivity of the LCLS, researchers could get a much clearer picture of what was happening.
"It's a bit like astronomers studying the night sky," Delaire said. "Previous studies could only see the brightest stars visible to the naked eye, but with ultra-clear and ultra-fast X-ray pulses we could see the weak and diffuse signals of the galaxy of the Milky Way between them. "
This study, and others like it, are essential to understanding the behavior of photo-excited materials. For example, if properly exploited, the atomic reaction of vanadium dioxide revealed in this study could form the basis of high-speed transistors for computers combining photons and electrons. Researchers also use this general concept in the pursuit of the dream of superconductors at room temperature.
"The new insights gained in the photoinduced transition process of insulator on the metal in vanadium dioxide should be directly relevant to reassess our understanding of other materials," Delaire said. "We are just beginning to explore this new area of the ability to control the behavior of materials simply by illuminating them and combining state-of-the-art X-ray facilities with supercomputers to keep up with what's happening. that the atomic dynamics at play is even more complicated than previously thought. "
Source link
