Imaging device tracks and controls photocurrent streamlines in electronic devices

SOPHIA CHEN: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Sophia Chen. Lots of materials conduct electricity when you shine a light on them. Think ordinary metals or solar panels, where electrons begin to flow through the material when you illuminate it. But here’s a basic question. When the light hits the material, what path do the electrons take? That’s what drives physicist Nathan Gabor’s recent work at the University of California, Riverside.

NATHAN GABOR: There are these kinds of paths of least resistance that the electrons would take. So when I put an electron in, it actually follows that path. And we wanted to be able to image those.

SOPHIA CHEN: But Gabor’s team didn’t just want to image the electron flow – they also wanted to steer the electrons’ trajectory. To do that, they designed a unique micron-scale device. They took a crystal of yttrium iron garnet, which does not conduct electricity, and put a nanometers-thick layer of platinum, which does conduct electricity, on top of it. When they illuminate the device with a laser, this device produces an electric current, as desired. It also turns out, when you combine the crystal with the platinum, the interface between the two materials exhibits some surprising magnetic properties.

NATHAN GABOR: You take a seemingly boring insulator, and you take a regular metal platinum, but when you glue them together, and you have this very, very, you know, little narrow interface between the two, they actually become sensitive to a magnetic field.

SOPHIA CHEN: Gabor’s team used this sensitivity to a magnetic field to steer the electron flow in the device. Specifically, the direction of the electrons’ motion responds to the magnetic field according to a phenomenon known as the Nernst effect.

NATHAN GABOR: So we heat with the laser, and that generates the current, and that creates a temperature gradient. And then I apply a magnetic field, and then the Nernst effect says that if the magnetic field is in one direction, than the direction of the current flow is perpendicular to that. 

SOPHIA CHEN: In addition to creating the electric current in the device, the laser light also allowed them to image the electron flow in the device. They mapped the electrons’ paths while varying the direction of an external magnetic field. Gabor likens these paths to fluid “streamlines,” such as air around an airplane wing. These streamlines map out a sort of average path that the electrons take in the material.

NATHAN GABOR: It's just like, you would think with wind - we would say that, Oh, the wind is traveling at five miles per hour to the west. But really, inside the wind are all these little microscopic particles that are kind of colliding with each other in random directions, not all the time, but they have a net flow that is five miles per hour to the west. So it's the same exact thing, it's that those flow lines show the kind of net flow of these electrons as they would diffuse through the material.

SOPHIA CHEN: They found that they could make the electron streamlines squeeze or contort around the device by changing the device’s shape, much like you can change the air flow around an airplane wing by changing its shape. This technique enables them to analyze electron flow, which could lead to more efficient electronic devices.

NATHAN GABOR: Even in conventional materials, you know, when electrons get bunched together, they heat up. And that's actually what causes devices to die. So you could actually use a technique like this to optimize the shape of devices to kind of make them cause less heat.

SOPHIA CHEN: Gabor is interested in applying this current imaging technique in exotic materials, such as those where electrons form vortices and eddy currents. This work was published in a recent issue of the Proceedings of the National Academy of Sciences. My name is Sophia Chen from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.