Scientists used supercomputers to find a new class of materials that possess an exotic state of matter known as the quantum spin Hall effect. The researchers published their results in the journal Science in December 2014, where they propose a new type of transistor made from these materials.

The computational allocation was made through XSEDE, the Extreme Science and Engineering Discovery Environment, a single virtual system funded by the National Science Foundation (NSF) that scientists use to interactively share computing resources, data and expertise. The study was funded by the U.S. Department of Energy and the NSF.

"To me, national computing resources like XSEDE, or specifically the Stampede and Lonestar supercomputers, are extremely helpful to computational scientists," Xiaofeng Qian said. In January 2015, Qian left MIT to join Texas A&M University as the first tenure-track assistant professor at its newly formed Department of Materials Science and Engineering.

"To me, national computing resources like XSEDE, or specifically the Stampede and Lonestar supercomputers, are extremely helpful to computational scientists." Xiaofeng Qian, Assistant Professor in the Department of Materials Science and Engineering at Texas A&M University.

What Qian and colleagues did was purely theoretical work, using Stampede for part of the calculations that modeled the interactions of atoms in the novel materials, two-dimensional transition metal dichalcogenides (TMDC). Qian used the molecular dynamics simulation software Vienna Ab initio Simulation Package to model a unit cell of atoms, the basic building block of the crystal lattice of TMDC.

Scientists diagram the electronic band structure of materials to show the energy ranges an electron is allowed, with the band gap showing forbidden zones that basically block the flow of current. Spin-orbit coupling accounts for the electromagnetic interactions between electron's spin and magnetic field generated from the electron's motion around the nucleus.

"We found a very convenient method to control the topological phase transition in these quantum spin Hall interlayers." Xiaofeng Qian, Assistant Professor in the Department of Materials Science and Engineering at Texas A&M University.

The complexity lies in the details of these interactions, for which Qian applied many-body perturbation theory with the GW approximation, a state-of-the-art first principles method, to calculate the quasiparticle electronic structures for electrons and holes. The 'G' is short for Green's Function and 'W' for screened Coulomb interaction, Qian explained.

The big picture for Qian and his colleagues is the hunt for new kinds of materials with extraordinarily useful properties. Their target is room-temperature quantum spin Hall insulators, which are basically near-two-dimensional materials that block current flow everywhere except along their edges. "Along the edges you have the so-called spin up electron flow in one direction, and at the same time you have spin down electrons and flows away in the opposite direction," Qian explained. "Basically, you can imagine, by controlling the injection of charge carriers, one can come up with spintronics, or electronics."

The scientists in this work proposed a topological field-effect transistor, made of sheets of hexagonal boron interlaced with sheets of TMDC. "We found a very convenient method to control the topological phase transition in these quantum spin Hall interlayers," Qian said. "This is very important because once we have this capability to control the phase transition, we can design some electronic devices that can be controlled easily through electrical fields."

Qian stressed that this work lays the theoretical ground for future real experiments in the lab. He hopes it might develop into an actual transistor suitable for a quantum computer, basically an as-yet-unrealized machine that manipulates data beyond just the binary of ones and zeros.

"So far, we haven't looked into the detailed applications for quantum computing yet," Qian said. "However, it is possible to combine these materials with superconductors and come up with the so-called Majorana fermion zero mode for quantum computing."