When adding energy to a material, such as heating, its structure almost always becomes more messy. Ice, for example, melts with its crystalline structure to liquid water without any order.
However, in physicists' new experiments at MIT and elsewhere, the opposite is true: when a pattern called the charge density wave in a given material is hit with a fast laser pulse, a completely new charge density wave is created ̵
The discovery is reported today in the journal Nature Physics in a work by MIT professors Nuh Gedik and Pablo Jarillo-Herrero. Postdoc Anshul Kogar, PhD student Alfred Zong and 17 others at MIT, Harvard University, the SLAC National Accelerator Laboratory, Stanford University and the Argonne National Laboratory.
The experiments used a material called lanthanum tritelluride, which naturally forms a layered structure itself. This material spontaneously forms a wavelike electron pattern in high and low density regions, but is confined to a single direction within the material. With an ultrafast laser beam – less than one picosecond or less than a trillionth of a second – this pattern, called the charge density wave or CDW, is canceled out, and a new CDW appears at right angles to the original.
This new vertical CDW is something that has never been seen in this material before. It only exists for a flash and disappears within a few picoseconds. When it disappears, the original appears again, suggesting that its presence was somehow suppressed by the new.
Gedik explains that in ordinary materials, the electron density within the material is constant throughout the volume, but in certain materials, when cooled below a certain temperature, the electrons organize themselves into a CDW with alternating high and low regions electron density. In Lanthanum Tritelluride or LaTe 3 the CDW travels along a fixed direction within the material. In the other two dimensions, the electron density remains constant as with ordinary materials.
The vertical version of the CDW, which appears after the laser beam, has never been observed in this material before, says Gedik. It "just flashes briefly and then goes away," says Kogar, replacing it with the original CDW pattern, which immediately becomes visible again.
Gedik points out that "this is pretty unusual." In most cases, when you add energy to a material, you reduce the order. "
" It's as if these two [kinds of CDW] are competing with each other – when one appears, the other disappears, "says Kogar. "I think the really important concept here is the phase competition."
The idea that two possible states of matter compete with each other and that the dominant mode suppresses one or more alternative modes is quite common in quantum materials, the researchers say. This suggests that there could be latent states that are hidden in many kinds of matter that could be revealed if a way could be found to suppress the dominant state. This seems to be the case with these competing CDW states, which are considered analogous to crystal structures because of the predictable, ordered patterns of their subatomic constituents.
Normally, all stable materials are at their minimum energy states – that is, of all possible configurations of their atoms and molecules, the material settles in the state that requires the least energy to sustain itself. But for a particular chemical structure, there may be other possible configurations that the material could possibly have, other than being suppressed by the dominant state with the lowest energy.
"By shutting off this predominant state with light, possibly through these other states, can be realized," says Gedik. And because the new states appear and disappear so quickly, "you can turn them on and off," which can be useful for some information-processing applications.
The ability to suppress other phases can open up completely new material properties, says Kogar. "The goal is to find material phases that can only exist out of balance," he says – in other words, states that would not be achievable without a method such as this system of fast laser pulses to suppress the dominant phase.
Gedik adds that "normally, to change the phase of a material, chemical changes or pressure or magnetic fields are made.In this work, we use light to make these changes."
The new findings could be helpful to better understand the role of phase competition in other systems. This, in turn, may help in answering questions as to why superconductivity occurs in some materials at relatively high temperatures, and may be helpful in finding even higher superconductor material, and this new condition arises?
The work was supported by the US Department of Energy, the SLAC National Accelerator Laboratory, the Skoltech-MIT NGP program, the Center for Excitons, and the Gordon and Betty Moore Foundation.
Study shows what happens when ultrafast laser pulses, not heat, cause a phase change of a material
Light-induced charge density wave in LaTe 3 Natural physics (2019). DOI: 10.1038 / s41567-019-0705-3, https://nature.com/articles/s41567-019-0705-3
Physical experiment with ultrafast laser pulses creates a hitherto invisible phase of matter (2019, 12 November)
retrieved on November 12, 2019
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