How This New State of Matter Is Made With Lasers, Crystals, and “Frustration”


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. Make sure to subscribe and thanks for watching.

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