Physicists create new state of matter made of crystalized bosons

Using a material effect called moiré, physicists coaxed fermions into becoming bosons and force them into solid crystals.
John Loeffler
A boson beneath a moiré honeycomb pattern
A boson beneath a moiré honeycomb pattern

Daniel Dominguez/CERN 

In a remarkable new discovery, physicists have used an innovative technique to coax fermions into becoming crystalized bosons in an exotic new material.

Fermions and bosons are elementary particles in the standard model of particle physics, with fermions being a fundamental building block of matter, with bosons conveying one of the fundamental forces of nature. They are both characterized by their spin, with fermions having a half-integer spin (so 1/2, 3/2, etc.) and bosons having a full integer spin (1, 2, etc).

This produces an interesting phenomenon known as an exciton, which is a state where a negatively-charged electron (a fermion) becomes bound to another positively charged fermion, known as a "hole", and the two half-integer spins combine to form a full-integer spin, forming a bosonic particle.

Normally, bosons are much tougher to study than fermions. Fermions are characterized by their interaction with the electrostatic force, making them relatively easy to interact with and study. Bosons, like photons, tend to not interact with each other, are difficult to create, and so are generally harder to study.

Physicists at the University of California, Santa Barbara (UCSB), wanted to find a reliable way to create and contain bosons in a manner that would allow their characteristics to be better studied and probed, and they have come up with an innovative solution to the problem.

The technique involves taking two lattices of tungsten diselenide and tungsten disulfide and layering them on top of one another, but with a slight twist — literally. Instead of trying to line the patterns of the lattice up, one of them is slightly twisted at an angle, creating a pattern known as moiré.

Physicists create new state of matter made of crystalized bosons
An example of moire using a pair of honeycomb lattices

By pulsing strong light at this pattern, the researchers were able to entice excitons to form between the two lattices and also to study their behavior by creating a solid bosonic crystal they call a bosonic correlated insulator.

“We discovered a new state of matter — a bosonic correlated insulator,” Richen Xiong, a graduate student researcher at UCSB and the lead author of a new paper on the discovery published in the journal Science, said in a UCSB statement.

“Conventionally, people have spent most of their efforts to understand what happens when you put many fermions together,” UCSB condensed matter physicist Chenhao Jin said. “The main thrust of our work is that we basically made a new material out of interacting bosons.”

While bosons are known to condense at super cold temperatures under special conditions, the bosons in the UCSB study would condense at relatively high temperatures and did so until they began to form a solid crystalline structure.

“There are many-body phases with fermions that result in things like superconductivity,” Xiong said. “There are also many-body counterparts with bosons that are also exotic phases. So what we’ve done is create a platform, because we did not really have a great way to study bosons in real materials.”

Jin notes that in addition to studying well-known excitons, this new material might serve as a new way to study the largely unmapped world of condensed matter.

“We know that some materials have very bizarre properties,” he said. “And one goal of condensed matter physics is to understand why they have these rich properties and find ways to make these behaviors come out more reliably.”

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