A team of researchers led by Foundry users has observed chirality for the first time in polar skyrmions – quasiparticles akin to tiny magnetic swirls – in a material with reversible electrical properties. The combination of polar skyrmions and these electrical properties could one day lead to applications such as more powerful data storage devices that continue to hold information – even after a device has been powered off. Their findings were reported this week in the journal Nature.
When the team of researchers began this study in 2016, they had set out to find ways to control how heat moves through materials. So they fabricated a special crystal structure called a superlattice from alternating layers of lead titanate (an electrically polar material, whereby one end is positively charged and the opposite end is negatively charged) and strontium titanate (an insulator, or a material that doesn’t conduct electric current).
But once they took STEM (scanning transmission electron microscopy) measurements of the lead titanate/strontium titanate superlattice at the Foundry’s National Center for Electron Microscopy (NCEM) facility, they saw something strange that had nothing to do with heat: Bubble-like formations had cropped up all across the device.
Those bubbles, it turns out, were polar skyrmions – or textures made up of opposite electric charges known as dipoles. Researchers had always assumed that skyrmions would only appear in magnetic materials, where special interactions between magnetic spins of charged electrons stabilize the twisting chiral patterns of skyrmions. So when the researchers discovered skyrmions in an electric material, they were astounded.
Using STEM, the researchers took atomic snapshots of skyrmions’ chirality at room temperature in real time, the researchers discovered that the forces placed on the polar lead titanate layer by the nonpolar strontium titanate layer generated the polar skyrmion “bubbles” in the lead titanate.
To confirm their observations, the team went to the Advanced Light Source (ALS) and probed the chirality by using a spectroscopic technique known as RSXD-CD (resonant soft X-ray diffraction circular dichroism).
Light waves can be “circularly polarized” to also have handedness, so the researchers theorized that if polar skyrmions have handedness, a left-handed skyrmion, for example, should interact more strongly with left-handed, circularly polarized light – an effect known as circular dichroism. They found that incoming circularly polarized X-rays, like a screw whose threads rotate either clockwise or counterclockwise, interact with skyrmions whose dipoles rotate in the same direction, even at room temperature. In other words, they found evidence of circular dichroism – where there is only a strong interaction between X-rays and polar skyrmions with the same handedness.
The researchers next plan to study the effects of applying an electric field on the polar skyrmions.