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MIT Team Studies 2D Materials With Supercomputers

Research suggests a topological field-effect transistor could be made of sheets of boron interlaced with transition metal dichalcogenide

The edge density of states calculated for a monolayer transition metal dichalcogenide in the 1T'-MoS2 structural phase.

Scientists at the Massachusetts Institute of Technology (MIT) have used supercomputers to study 2D transition metal dichalcogenides, a new class of materials that possess the quantum spin Hall effect. 

They published their results in the journal Science in December 2014, where they proposed a new type of transistor: a topological field-effect transistor, made from sheets of hexagonal boron interlaced with sheets of  transition metal dichalcogenide.

The team included Ju Li, Liang Fu, Xiaofeng Qian, and Junwei Liu, experts in topological phases of matter and 2D materials research at MIT. They calculated the electronic structures of the materials using the Stampede and Lonestar supercomputers of the Texas Advanced Computing Centre.

What Qian and colleagues did was purely theoretical work, using Stampede for part of the calculations that modeled the interactions of atoms in the 2D transition metal dichalcogenides. 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 transition metal dichalcogenide (TMDC).

"If you look at the unit cell, it's not large. They are just a few atoms. However, the problem is that we need to predict the band structure of charge carriers in their excited states in the presence of spin coupling as accurately as possible," Qian said.

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

The complexity lies in the details of these interactions, for which Qian applied many-body perturbation theory with the GW approximation, a 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 MIT team's vision of a new kind of electronic device has the 2D material at the middle of a layered 'sandwich' with layers of boron nitride, at top and bottom. When an electric field is applied to the material, it switches the quantum state of the middle layer. The boundaries of these 'switched' regions act as perfect quantum wires, potentially leading to new electronic devices with low losses.

 "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."

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-2Dl 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."

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-unrealised 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."

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 US Department of Energy and the NSF.

'Quantum spin Hall effect in two-dimensional transition metal dichalcogenides' by Qian et al, Science 12, Vol. 346 no. 6215 (2014)



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