Physicists find new ‘control knob’ for the quantum topological world

A “topological” alloy of iron and tin has been found to respond surprisingly strongly to magnetic fields, according to research carried out by Zahid Hasan at Princeton University and colleagues in the US, China and Taiwan. The material, which is layered, can be easily cleaved to create surfaces with honeycomb crystal structures with six-fold rotational symmetry. When studying the surface, Hasan and colleagues found that the symmetry of the electronic structure does not match that of the atomic lattice. Materials with properties that respond strongly to electric or magnetic fields are interesting because they are so useful for technological applications. Those boasting giant magnetoresistance (GMR), for example, are used in a wide range of magnetic-field sensors, notably in hard-disk drives. These materials have a resistance that changes dramatically when subject to a magnetic field. We found a new control knob for the quantum topological world Zahid Hasan Topological properties can arise in materials with a strong quantum-mechanical coupling between the spin and orbital angular momentum of electrons. On the surface of some materials, this spin-orbit coupling can lock the motion of an electron to its spin, preventing the electron from scattering. This results in a topological insulator that conducts electrons on its surface, but not in its bulk. Given that electron spin is related to the magnetic properties of a material, any magnetic materials with strong spin-orbit coupling could have exotic and potentially useful topological properties. Cleaved crystal In an ordinary crystalline solid, the electronic structure normally has the same symmetry as the surrounding atomic lattice. But instead of having the expected six-fold symmetry, the researchers found the electronic structure on the surface of the iron-tin alloy has two-fold rotational symmetry. “We had expected to find something six-fold, as in other topological materials, but we found something completely unexpected,” says Princeton’s Songtian Sonia Zhang. “We kept investigating, and we found more unexpected things. It’s interesting because the theorists didn’t predict it at all. We just found something new.” Big surprise The next big surprise was that that the axis defining the two-fold symmetry can be rotated by applying a magnetic field. Furthermore, the electronic structure could be aligned in any direction that the researcher chose. Hasan’s team is unable to explain why a magnetic field has such a dramatic effect on the electronic properties of the material. Indeed, the response to the magnetic field is about 100 times stronger than predicted by current theory. The implication is that the g-factor of the material’s electrons – which relates the magnetic moment of a particle to its spin – is about 100-times greater than for electrons in free space. “Nobody predicted that in topological materials,” says Hasan, “This gigantic and tunable quantum effect opens up the possibilities for new types of quantum technologies and nanotechnologies.” He believes his team has found “a new control knob” for the quantum topological world. “We expect this is tip of the iceberg,” he adds. “There will be a new subfield of materials or physics grown out of this.” David Hsieh from Caltech, who was not involved in the research, adds that the research could be evidence of a new quantum phase of matter. “That’s, for me, exciting,” he says. “They’ve given a few clues that something interesting may be going on, but a lot of follow-up work needs to be done, not to mention some theoretical backing to see what really is causing what they’re seeing”. The research is described in Nature.