To Joseph Falson, who recently joined Caltech as assistant professor of materials science, electrons are like exotic supercars because they possess amazing capabilities.
Consider the phenomenon of superconductivity, in which paired electrons fly unimpeded, resulting in materials that display zero electrical resistance. When electrons move through impure materials, however, or when superconductivity is disrupted, those talents are lost. In Falson’s analogy, he likens that to driving the supercar down a cobblestone street that limits its speed. “Our job is not to make the supercar, it’s just to make the highway,” he says.
At Caltech, he approaches the materials challenge in two ways. First, he and his students will attempt to synthesize new materials expected to display novel electronic features. Theoreticians have proposed many such materials, and a wealth of these are now ripe to be created in a stable experimental environment. The second approach is to test and improve known materials, especially ultrathin films, some of which are exotic superconductors.
Using the latter approach, Falson and colleagues at Tsinghua University in China and the Max Planck Institute for Solid State Research in Germany found a superconducting material that retains its superconductivity even after exposure to magnetic fields that, according to the predictions of conventional theories, normally should disrupt that property. The work is described in a study published in the March 27 issue of the journal Science.
Breaking the Rules
The problem for those who seek to study superconductivity and eventually make practical use of it is that, so far, it has been realized only at ultracold temperatures no warmer than -70 degrees Celsius. “There is a very strong push to realize room-temperature superconductivity—it is one of the holy grails of science,” Falson says, “because then you are going to employ these materials in motors or transmission lines, and the loss would be significantly less. It would revolutionize society.”
However, there are many challenges for scientists trying to reach that goal. Three major factors disrupt superconductivity: increased temperature, exposure to a magnetic field, or carrying a high density of electric current. Moreover, these factors are interrelated: the warmer the setting, the smaller the critical magnetic field needed to disrupt its superconductivity. In their new study, however, Falson and colleagues tested a material that was suspected to have exotic electronic properties and demonstrated that its superconductivity was more resistant to an external magnetic field than theory would suggest.
The researchers began with a common silicon substrate upon which they laid down many thin crystalline layers of materials such as bismuth telluride and lead telluride. The top layer of gray tin was just a few atomic layers thick, at which point the tin becomes effectively two-dimensional; the electrons within that layer can move only within the plane of the tin, not up and down. Earlier theoretical work had suggested this multilayered material would have exotic electronic properties, and other researchers unexpectedly found that the films were superconducting. Falson cooled this material to extremely low temperatures to further investigate the superconductivity and found the unexpected anomalous behavior.
The key to Falson’s study is a quantum property of electrons known as spin. The spin of an electron refers to its angular momentum; the property can be measured in terms of both magnitude and direction. In a normal superconducting material, he says, electrons with opposing spins become a Cooper pair, the particle responsible for superconductivity. In bulk materials, the spin can point in any direction. In certain thin films, however, the direction of the spin becomes coupled to the underlying electronic structure of the material, resulting in spins that prefer to point “out of the plane.” In other words, if you picture the tin layer as a piece of paper, the spins tend to align perpendicular to the flat plane of the paper.
When electrons are exposed to a magnetic field, however, the spins of the electrons like to line up with the direction of the magnetic field. In the study of the tin layer, Falson’s group applied a large magnetic field “in-plane,” meaning parallel to the sheet of paper. They found that changing the spin direction, and thus killing superconductivity, required 40 percent more magnetic field strength than established theory would have predicted, and this effect was only evident as the experimental temperature approached absolute zero. A new theoretical approach, developed to explain these results, established that an underlying structural property of Falson’s layered material makes the electrons reluctant to flip.
Some of Falson’s collaborators published an associated study that explained the implications of this finding and predicted 200 more theoretical materials with the same resilience to magnetic fields. Falson’s new Caltech lab, which is under construction in the Harry G. Steele Laboratory, will attempt to find out which of those predicted materials could show similar physics in the real world. He and his colleagues will synthesize the materials via a technique called molecular beam epitaxy: within a vacuum chamber, a variety of elements will be evaporated to form a molecular beam. The materials will then accumulate upon a carefully chosen and prepared substrate in layers just a couple of nanometers thick. “We have to grow crystals on top of other crystals. This is what separates us from bulk crystal growers,” Falson explains. “We can look at sandwiches, or heterostructures, of dissimilar materials and study their emergent properties.”
Around the World
Falson first became interested in the physical properties of materials while growing up and attending college in his native Australia, then he followed a globetrotter’s path to Caltech. After undergraduate studies at the University of New South Wales, he obtained a scholarship from a program the Japanese government uses to encourage foreign scholars to study and research in the country. At the University of Tokyo, during his doctoral studies, he learned about crystal growth and condensed matter physics, which explores the macroscopic and microscopic properties of matter, while also picking up a little Japanese. Postdoctoral work took him to the Max Planck Institute in Germany before he arrived in Pasadena at the start of 2020.
The Caltech campus has provided some sense of home, Falson says. “The eucalyptus trees here, I love,” he says. “It just reminded me of home. I remember walking around campus during the interview and just smelling these eucalyptus trees.”
Caltech’s approach to basic science attracted him as well. Rather than working at a university or national lab with many people researching the same subjects, he wanted to be somewhere that allowed him to go in new directions, such as his fundamental work on synthesizing new materials. “I was really happy they were willing to take that risk and support that direction,” he says.