A scanning electron microscope image of a samarium hexaboride nanowire bonded to a STM (left), together with images of an antiferromagnet produced by that STM (middle and right). The middle image is a zoomed in view, showing the light-dark-light striping that occurs in an antiferromagnet. Image courtesy of the authors.
A scanning electron microscope image of a samarium hexaboride nanowire bonded to a STM (left), together with images of an antiferromagnet produced by that STM (middle and right). The middle image is a zoomed in view, showing the light-dark-light striping that occurs in an antiferromagnet. Image courtesy of the authors.

One of the most powerful tools for peering deep into the nanoscale realm is a scanning tunneling microscope (STM). Rather than an optical lens, its powerful eye comes from an electrical current that passes between the tip of the microscope and the surface of a material. The tip scans across the surface and produces a signal that changes based on how atoms are arranged within the material. Taken together, the scans can map out surfaces with sub-nanometer resolution, revealing electrons and single atoms.

A team of researchers in the University of Illinois at Urbana-Champaign’s Illinois Quantum Information Science and Technology Center (IQUIST) has now added a twist to their STM by replacing the tip with a nanowire made from an exotic material known as samarium hexaboride (SmB6). They then used this nanowire to image magnetic features in an approach that has several potential advantages over other methods. As they report in a paper in Science, their combined measurements and calculations showed evidence of the unusual nature of the nanowire itself.

“Lin Jiao, a former post-doc in our group, proposed the idea that this kind of nanowire tip may be able to give us a yes-no answer as to whether a material was magnetic or not,” said IQUIST member Vidya Madhavan, a physics professor and corresponding author of the paper. “Much to our surprise, Anuva Aishwarya, a graduate student in the group, showed that these tips could give much more information than that.”

At the heart of an STM is an effect that lets electrons ‘tunnel’ through a barrier. Electrons are fundamental particles governed by quantum physics, and can act like waves. Unlike water waves, electrons don’t necessarily dissipate or bounce back completely when they hit a surface. As they encounter a super-thin barrier, a bit of the wave can leak through in a process called quantum tunneling.

In an STM, there is a gap between the tip of the microscope and the sample material. The electrons can tunnel through this gap, creating an electrical signal that, in turn, contains information about the sample.

In addition to charge, electrons have a property called spin, which can be pictured as an arrow attached to the electron. Typically, electrical currents contain electrons with their spins pointed in random directions. But scientists can coax some materials into carrying currents with the spin direction locked. For example, fixed-spin (polarized) currents in STMs can be generated with a combination of magnetic tips and external magnets. Unfortunately, the added magnets can be invasive and may inadvertently affect the sample atoms. In the new study, the researchers took a different approach to creating spin-polarized currents.

Rather than employing a magnetic tip, the team used non-magnetic SmB6. Around a decade ago, scientists predicted that this material could be a Kondo topological insulator, able to produce unusually stable spin-polarized currents without any added magnets. This means that on the surface of SmB6, electric currents moving to the right should have electrons with spin-up, while currents moving to the left should have electrons with spin-down. These currents can even survive in the face of unwanted defects in the material.

This is a general feature of topological insulators, yet scientists have faced challenges translating this rather exotic physics into real-world technology applications. Moreover, scientists are still trying to understand the different varieties of topological materials. This new study provides strong evidence that SmB6 is indeed a Kondo topological insulator, and puts its peculiar currents to work simplifying magnetic imaging.

In Madhavan’s laboratory, the team used nanofabrication to modify a STM. Guided by Jiao, Zhuozhen Cai (an undergraduate in the group) spent hundreds of hours in a cleanroom developing this procedure. First, they used a beam of ions to chop off the normal STM tip, which is made of tungsten. Then they embedded the nanowire, which is 60–100nm wide, into a trench that is a few hundred nanometers wide.

They scanned this nanowire tip across the surface of iron telluride, which is an antiferromagnet. Such materials have alternating regions of spin-up and spin-down electrons, and so the overall magnetization cancels out. This is in contrast to the more familiar bar magnets, where all the electron spins point in the same direction.

Previous images produced by STMs with magnetic tips showed light-dark-light stripes, which is the hallmark of an antiferromagnet. The team produced similar images with the new non-magnetic nanowire setup, indicating that the tunneling electrons from SmB6 were spin-polarized. When the tip was over a region of the antiferromagnet with spins that matched the orientation of the spins of the surface current, the signal increased; otherwise, it decreased. The STM mapped out these variations as it scanned over the sample and showed clear patterns corresponding to the alternating spin stripes.

To further confirm that the nanowire signals were related to the unusual currents of SmB6, the team warmed up the experiment above 10K. At this temperature, SmB6 should no longer be a Kondo topological insulator and will lose its surface spin currents. Crucially, the STM no longer observed any antiferromagnetic stripes, even though the sample’s magnetic ordering survives at this temperature. The researchers found that spin-polarized currents were simply not present in the nanowire above 10K.

They performed a third check of the spin-polarized currents by switching the direction of the voltage applied to the nanowire tip. This reversed the direction of the tunneling current between the STM and the sample. The STM images showed that the contrast in the images is reversed, which can only happen if the tunneling electrons have a spin-polarization that flips when the current changes direction. Together, this evidence confirmed the exotic nature of SmB6.

“We can switch the nanowire on the tip to a different material, which would let us probe other, potentially unusual, aspects of our sample,” said Anuva Aishwarya, lead author and a physics graduate student in Madhavan’s group. “I am very excited about this because it opens doors to a new nanoscale sensing technique!"

The tip properties were surprisingly repeatable, said Madhavan. The team could even expose the nanowires to air and they still did well in the STM. Much is still unknown about SmB6, but its robust performance combined with the measurement data is consistent with the predictions about its topological nature.

"This technique is perhaps the first real application of a topological insulator, and remarkably, for it to work, it is crucial that the origin of the topology is from strong many-electron interactions as expected in SmB6," said IQUIST member Taylor Hughes, who is a professor of physics and a co-author of the paper.

In future studies, the team plans to modify the nanowire to see if it can reveal even more material features. For example, they are interested in creating and detecting exotic particle-like entities such as Majorana fermions, which have long been proposed as the basis for novel quantum computing devices.

This story is adapted from material from the University of Illinois at Urbana-Champaign, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.