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Stable 'supercrystals' created - Printable Version

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Stable 'supercrystals' created - Kyng - 03-27-2019

http://www.sci-news.com/physics/stable-supercrystals-07028.html

According to a new study, published in the journal Nature Materials, an ultrafast laser pulse plus ‘frustration’ resulted in a new state of matter — a ‘supercrystal.’

They accomplished this by ‘frustrating’ the system — not allowing the material to do what it wants to do, which is to allow it to minimize its energy fully without constraints.

The scientists did this by using single atomic layers of two materials, lead titanate and strontium titanate, stacked in alternating layers on top of each other to build up a 3D structure.

The team grew these layers on top of a crystal substrate whose crystals were intermediate in size between the two layered materials. This provided a second level of frustration, as the strontium titanate layer tried to stretch to conform with the crystal structure of the substrate, and the lead titanate had to compress to conform to it. This put the whole system into a delicate but frustrated state with multiple phases randomly distributed in the volume.

At this point, the researchers zapped the material with a laser pulse, which dumps free charges in the material, adding extra electrical energy to the system, driving it into a supercrystal.


Well, this sounds intriguing - but, sadly, I've only found two articles on it, and neither is really that great. The one I linked above is a bit too technical for most audiences, while the second (from Live Science) attempted to simplify it, but didn't seem to do a particularly good job of it :-/ .

Still, they sound like a very exotic state of matter - and I gather that they managed to get it to persist under normal, room-temperature conditions :O . That makes me wonder: could it be practical to apply these in the real world, and if so, what for?

RE: Stable 'supercrystals' created - Pyrite - 03-27-2019

As the resident ultrafast laser person on here, hopefully I can try to explain what's going on.

They're using a pump - probe technique to mess about with the energy levels inside the atoms. Normally when you do this, you fire two laser beams - a pump pulse and a probe pulse - onto the sample (in this case, onto the atomic layers). The pump pulse has lots of energy, which electrons in the atomic layers absorb, leaving them in 'excited' higher energy states.

At a short time later, the probe pulse comes in and detects changes in absorption. What the article doesn't say is that the probe pulse directly compares a spectrum taken following the pump pulse, to a spectrum taken without the pump pulse. So all the probe does is record the difference that the pump pulse makes to the sample.

If you are able to adjust the time delay of the probe pulse in comparison to the pump pulse, you can see this difference appear and disappear. There is a difference because the population of electrons in different energy levels has changed, and over time this will decay back to zero as the electrons lose their energy and return to their initial state. This gives rise to transient absorption (absorption that arises and decays over time).

What it seems like they've done is engineer a way that the electrons don't drop back to their initial state, and instead remain in their 'excited' state permanently. Alternatively, the electrons may drop into a different state altogether, instead of the initial state. The 'frustation' they mention comes from a mismatch between the structures that they use for the atomic layers. By playing around with magnetism, polarisation and crystal structure, it seems that they've made the initial state as unfavourable as they could, in order to make the 'supercrystal' state relatively as favourable as they can. The system is less likely to want to go back to the original state now, and needs that extra energy push (generated by heating up) to return to it.

I hope this is helpful - if there's stuff people don't understand from what I've said I'd be happy to explain further or simplify it more. :) 

My own personal take on this is that it's a rather shrewd bit of engineering - I wonder if we'll find any use for this, though. It seems as though there's only a select few structures that they can actually do this with at the moment, but perhaps they can find some use for this intermediate state.