The addition of energy to any material, for example by heating it, almost always makes its structure less ordered. Ice, for example, with its crystalline structure, melts to become liquid water in no order.
But in the new experiments of physicists at MIT and elsewhere, the opposite is true: When a model called a charge density wave in a given material is struck by a fast laser pulse, a whole new charge density wave is created ̵
The discovery was reported today in the journal in Natural Physics in an article by MIT professors Nuh Gedik and Pablo Jarillo-Herrero, postdoc Anshul Kogar, graduate student Alfred Zong and 17 others at MIT, Harvard University, National University SLAC accelerators, Stanford University and the Argon National Laboratory.
The experiments use a material called lanthanum tritelluride, which naturally forms in a layered structure. In this material, a wavy model of electrons in high and low density regions forms spontaneously, but is limited to one direction in the material. But when struck by ultra-fast burst of laser light – less than a picosecond long or less than a trillionth of a second, this model, called charge density wave or CDW, also erases a new CDW, at right angles to the original  This new, perpendicular CDW is something that has never been seen before in this material. It only exists for lightning, disappearing within a few more picoseconds. As it disappears, the original returns to view again, suggesting that its presence has been suppressed in some way by the new one.
Gedic explains that in ordinary materials, the electron density in the material is constant throughout their volume, but in some materials, when cooled at a certain temperature, the electrons are organized in CDWs with alternating regions of high and low electron density. In lanthanum tritelluride or LaTe 3 CDW is one fixed direction in the material. In the other two dimensions, the electron density remains constant, as in ordinary materials.
The perpendicular version of the CDW, which appears after the laser light has burst, has never been observed in this material, Gedick says. "It just blinks for a while, and then it's gone," Kogar says, to be replaced by the original CDW model, which immediately pops back.
Gedic points out that "it's pretty unusual. material, you reduce order. "
" It is as if these two [kinds of CDW] are racing – when one comes, the other goes, "Kogar says. "I think the really important concept here is phase competition."
The idea that two possible states of matter may be in competition and that the dominant regime suppresses one or more alternative regimes is quite common in quantum materials, researchers say. This suggests that there may be latent states, lurking unseen in many types of matter, which could be revealed if a way to suppress the dominant state is found. This is what seems to happen in the case of those competing states of CDW, which are considered analogous to crystal structures because of the predictable, ordered patterns of their subatomic constituents.
Typically, all stable materials are in a minimum energy state – that is, of all possible configurations of their atoms and molecules, the material is restored to a state that requires the least energy to be maintained. But for a chemical structure, there may be other possible configurations that the material could potentially have, except that they are suppressed by the lowest-energy dominant state.
"By striking this dominant state with light, perhaps these other states can be realized," says Gedic. And because new states emerge and disappear so quickly, "you can turn them on and off," which can
The possibility that suppression of other phases may reveal entirely new material properties opens many new areas of study, Kogar says. "The goal is to find phases of material, which can only exist out of balance, " he says – in other words, states that would never be achievable without a method, such as this system of fast laser pulses, to suppress the dominant phase.
Gedik adds that "usually to change the phase of the material, try chemical changes, or pressure, or magnetic fields. In this work, we use light to make these changes. "
New discoveries can help to better understand the role of phase competition in other systems. This, in turn, can help answer questions such as why superconductivity occurs in some materials at relatively high temperatures and can help in the pursuit of even higher temperature superconductors. Gedic says, "Well, if all you have to do is shine a light on the material, and this new state arises ?
This work was supported by the US Department of Energy, the SLAC National Accelerator Laboratory, the Skoltech-MIT NGP Program, the Center for Excitonics and the Gordon and Betty Moore Foundation.
The study shows what happens when ultra-fast laser pulses rather than heat cause the material to change phase
Light-induced charge density wave in LaTe 3 Physics of Nature (2019). DOI: 10.1038 / s41567-019-0705-3, https://nature.com/articles/s41567-019-0705-3
Massachusetts Institute of Technology
Physical Experiment with Ultrafast Laser Pulses Creates an Unprecedented Phase of Matter (2019, November 12)
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