In March of 2019, a team of scientists reported they managed to use a laser pulse to create
a crystal with giant repeating structures that are much larger than those in ordinary
crystals, what’s known as a “supercrystal.”
While that’s admittedly cool, lasers have actually been used to transform materials
into more ordered states for decades, but what made this special was their supercrystal
could stay in that state for at least a year.
This is one of the first examples of a material that achieved long-term stability after it
had been rearranged using such a short laser pulse, and the key was a lot of “frustration”.
The scientists were looking for hidden states of matter by taking the matter out of its
comfortable state, what’s known as its ground state.
When blasted by photons from a laser, the electrons in matter get excited, before minimizing
their energy again and quickly returning to their normal state.
In that heightened phase, or on the way back down, the material may have properties the
scientists are looking for, but they have to act fast to spot them because they may
not stick around for long.
To accomplish this scientists blast the material with a laser for just less than a picosecond
before hitting it with a gentler probe light that reveals what’s happening.
Of course, if they do find a state of a material that’s doing something useful, it doesn’t
do them much practical good if it’s gone in the blink of an eye.
So scientists figured out a way to get a material to an excited state and keep it there using
what’s known as frustration.
Now I’m not talking about the frustration you feel when a parent tells you to pause
your multiplayer game in the middle of a match.
Frustration in this sense is when the material is not allowed to do what it wants to do.
So...okay I guess they are kind of similar, actually.
But while you may want to play Fortnite, materials want to minimize their energy without constraints.
So the scientists grew a material that would have constraints, one layer at a time.
They started with a crystal substrate they would use to grow single atomic layers of
their material.
Their material was made of lead titanate and strontium titanate.
But the substrate they used to grow those two compounds was a size in between them,
so, the strontium titanate had to stretch out, while the lead titanate had to compress
to conform.
As these contorted layers were grown, they were stacked in an alternating pattern.
This added another level of frustration.
Lead titanate is ferroelectric, meaning the material has positive and negative electric
poles.
Strontium titanate is not.
Alternating layers with these two properties caused the electric polarization vectors to
curve in on themselves unnaturally, like a vortex.
When the scientists put it all together they got one frustrated material with multiple
phases that are spread randomly throughout.
All it needed to fall in line was just a little nudge.
With a laser.
A sub-picosecond pulse of light excited the material.
With the added energy the material arranged itself into repeating unit cells with a volume
a million times greater than the lead titanate or strontium titanate it was based on.
It wasn’t just a crystal anymore.
It was a supercrystal.
it might even stay that way forever at room temperature, but the study only lasted one year.
At last, an intermediate phase was captured and frozen, instead of vanishing as quickly
as it came.
The results will help scientists learn about and model these types of phase transitions,
and may one day lead to nanoscale materials that aren’t possible to create with traditional
fabrication.
These materials could have properties, like new forms of polar, magnetic, and electronic
states, that don’t exist in nature.
Looks like a little bit of frustration now, could have a big reward some day later.
While the crystal was stable at room temperature, it returned to its previous state when heated
to 176 degrees celsius.
Another pulse from a laser transformed it right back again.
If you liked this video, check out this one I did on a new state of matter that’s both
liquid and solid at the same time.
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