Whether staring into space or peering deep into the microscopic realm, there’s always more to see. In the case of solids, there is a world of atoms and particles teeming with activity which ultimately leads to useful properties such as electrical conduction, magnetism and insulation.
One of the most powerful tools for seeing the invisible is a scanning tunneling microscope or STM for short. Rather than an optical lens, its powerful eye comes from an electric current that passes between the tip of the microscope and the sample material. The tip scans the sample and produces a signal that changes depending on the arrangement of atoms in a given material. Taken together, the scans map surfaces with sub-nanometer resolution, revealing electrons and the locations of single atoms.
Recently, a team of IQIST researchers from the University of Illinois Urbana-Champaign added a twist to their STM by replacing the tip with a nanowire made from an exotic material, samarium hexaboride (SmB6). They use the nanowire to image magnetic features in an approach that has potential advantages over other methods. As published in the September 9 issue of Science, their combined measurements and calculations showed the unusual nature of the nanowire itself.
“Lin Jiao, a former postdoc in our group, came up with the idea that this kind of nanowire tip could give us a yes-no answer as to whether a material was magnetic or not,” Vidya Madhavan said. member of IQIST, a professor of physics and corresponding author on the paper. “To our surprise, Anuva Aishwarya, a graduate student in the group, showed that these tips can give much more information than that.”
At the heart of an STM is an effect that allows electrons to “tunnel” through a barrier. Electrons are fundamental particles governed by quantum physics and can act like waves. Unlike water waves, electrons do not necessarily dissipate or rebound completely when they hit a surface. When they encounter an ultra-thin barrier, part of the wave can seep through in a process called quantum tunneling. In an STM, there is a space between the tip of the microscope and the sample material. Electrons can tunnel through this space, creating an electrical signal which, in turn, contains information about the sample.
In addition to charge, electrons have a property called spin, which can be represented by an arrow attached to the electron. Generally, electric currents can contain electrons with their spins pointed in random directions. But scientists can get some materials to carry currents with the direction of rotation locked. For example, spin-fixed (polarized) currents in STMs can be generated with a combination of magnetic tips and external magnets. Unfortunately, the added magnets can be invasive and can inadvertently affect the atoms in the sample. In the new study, the researchers took a different approach to creating spin-polarized currents.
Rather than using a magnetic tip, the team used non-magnetic SmB6. About a decade ago, scientists predicted that this material could be a Kondo topological insulator, which should have unusually stable spin-polarized currents without any added magnets. Thus, on the surface of SmB6 electric currents moving to the right must have spin-up electrons, and vice versa for currents to the left. Currents can even survive unwanted defects in the material. This is a general feature of topological insulators, but scientists have faced challenges in translating this rather exotic physics into real-world technological applications. Additionally, 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 uses its particular currents to simplify magnetic imaging.
In Madhavan’s lab, the team used nanofabrication to modify the STM. Zhuozhen (an undergraduate student in the group) guided by Lin, spent hundreds of hours in a clean room developing this procedure. First, they used an ion beam to cut off the normal tip, which is tungsten. Then they embedded the nanowire in a trench just a few hundred nanometers wide. The threads were about 60 to 100 nanometers in diameter, which is roughly the size of some viruses.
They swept the tip across the surface of iron telluride, which is an antiferromagnetic. Such materials have alternating regions of spin-up and spin-down electrons, and the overall magnetization cancels. This contrasts with the more familiar common bar magnets, which have all electron spins pointed in one direction. Previous STM images with magnetic tips showed light-dark-light stripes, which means the sample is antiferromagnetic. The team collected similar images with the new non-magnetic nanowire configuration, which indicated that tunneling electrons from SmB6 were spin polarized. When the tip was over a region of the antiferromagnet with spins matching the spin orientation of the surface current, the signal increased; otherwise, it has decreased. The STM mapped these variations by scanning the sample and showed clear patterns corresponding to the alternating rotation scratches.
To further confirm that the nanowire signals were related to the unusual currents of SmB6, the team warmed the experiment above 10 Kelvin. At this temperature, SmB6 should no longer be a topological Kondo insulator and will lose its surface spin currents. Above all, the STM no longer observed antiferromagnetic bands, even if the magnetic order of the sample survives at this temperature. They found that spin-polarized currents were simply not present in the nanowire above this temperature. The team performed a third verification of spin-polarized currents by changing the direction of the voltage applied to the tip of the nanowire. This reversed the direction of the tunneling current between the STM and the sample. STM images showed that the contrast in the images is reversed, which can only happen if the tunneling electrons have a spin polarization that reverses when the current changes direction. Together, this evidence showed the exotic nature of SmB6.
“We can change the nanowire on the tip to a different material, which would allow us to probe other potentially unusual aspects of our sample,” said Aishwarya, lead author and graduate student in physics in Madhavan’s group. “I’m very excited about this because it opens the doors to a new nanoscale sensing technique!”
The properties of the tip were surprisingly reproducible, Madhavan said. The team was even able to expose the nanowires to air and they still performed well in STM. Many things are still unknown about SmB6but its robust performance combined with measurement data is consistent with 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 comes from strong many-electron interactions as predicted in SmB6“said Taylor Hughes, IQIST fellow, professor of physics and co-author of the study.
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 new quantum computing devices.
– This press release originally appeared on the University of Illinois Grainger College of Engineering website