Quantum spin Hall effect in two-dimensional transition metal dichalcogenides

Exotic states materialize with supercomputers. Materials with novel electrical properties discovered using XSEDE computational resources Stampede and Lonestar supercomputers of TACC

UNIVERSITY OF TEXAS AT AUSTIN, TEXAS ADVANCED COMPUTING CENTER

The science team included Ju Li, Liang Fu, Xiaofeng Qian, and Junwei Liu, experts in topological phases of matter and two-dimensional materials research at the Massachusetts Institute of Technology (MIT). They calculated the electronic structures of the materials using the Stampede and Lonestar supercomputers of the Texas Advanced Computing Center.

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.

This picture tells quite a story to scientists. It's a portrait of what they call a topological insulator, materials that conduct only at their edges. Technically it shows the edge density of states calculated for a monolayer transition metal dichalcogenide in the 1T'-MoS2 structural phase.

There's a black gap between the purple blobs at the bottom and top. What's more, there's crisscrossing reddish lines that bridge the gap. The lines indicate the edge state of the material, allowing electrons to cross the gap and conduct electricity. CREDIT: (Credit: Qian et. al.) USAGE RESTRICTIONS: None

"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.

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.

"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-orbit coupling as accurately as possible," Qian said.

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.

"In order to carry out these calculations to obtain reasonable convergence in the results, we have to use 96 cores, sometimes even more," Qian said. "And then we need them for 24 hours. The Stampede computer is very efficient and powerful. The work that we have been showing is not just one material; we have several other materials as well as different conditions. In this sense, access to the resources, especially Stampede, is very helpful to our project."

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."

Media Contact: Faith Singer-Villalobos faith@tacc.utexas.edu 512-232-5771

Quantum spin Hall effect in two-dimensional transition metal dichalcogenides. More about this image and story at Nanotechnology Today - http://nanotechnologytoday.blogspot.com/2015/03/quantum-spin-hall-effect-in-two.html

Exotic states materialize with supercomputers. Materials with novel electrical properties discovered using XSEDE computational resources Stampede and Lonestar supercomputers of TACC.

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.

Photon transport enhanced by transverse Anderson localization in disordered superlattices

A breakthrough by a team of researchers from UCLA, Columbia University and other institutions could lead to the more precise transfer of information in computer chips, as well as new types of optical materials for light emission and lasers.

The researchers were able to control light at tiny lengths around 500 nanometers — smaller than the light’s own wavelength — by using random crystal lattice structures to counteract light diffraction. The discovery could begin a new phase in laser collimation — the science of keeping lasers precise and narrow instead of spreading out.

The study’s principal investigator was Chee Wei Wong, associate professor of electrical engineering at the UCLA Henry Samueli School of Engineering and Applied Science.

Artist’s depiction of light traveling through a photonic crystal superlattice, where holes have been randomly patterned. The result is a more narrow beam of light. Image by Nicoletta Barolini

Think of shining a flashlight against a wall. As the light moves from the flashlight and approaches the wall, it spreads out, a phenomenon called diffraction. The farther away the light source is held from the wall, the more the beam diffracts before it reaches the wall.

The same phenomenon also happens on a scale so small that distances are measured in nanometers — a unit equal to one-billionth of a meter. For example, light could be used to carry information in computer chips and optical fibers. But when diffraction occurs, the transfer of data isn’t as clean or precise as it could be.

Technology that prevents diffraction and more precisely controls the light used to transfer data could therefore lead to advances in optical communications, which would enable optical signal processing to overcome physical limitations in current electronics and could enable engineers to create improved optical fibers for use in biomedicine.

To control light on the nanoscale, the researchers used a photonic crystal superlattice, a lattice structure made of crystals that allows light through. The lattice was a disorderly pattern, with thousands of nanoscale heptagonal, square and triangular holes. These holes, each smaller than the wavelength of the light traveling through the structure, serve as guideposts for a beam of light.

Engineers had understood previously that uniformly patterned holes can control the spatial diffraction somewhat. But the researchers found in the new study that the structures with the most disorderly patterns were best able to trap and collimate the beam into a narrow path, and that the structure worked over a broad part of the infrared spectrum.

The study’s lead author was Pin-Chun Hsieh, who was advised by Wong during his doctoral studies at Columbia University’s Fu Foundation School of Engineering and Applied Science.

The effect of disorder, known as Anderson localization, was first proposed in 1958 by Nobel laureate Philip Anderson. It is the physical phenomenon that explains the conductance of electrons and waves in condensed matter physics.

The new study was the first to examine transverse Anderson localization in a chip-scale photonic crystal media. It was published online today by Nature Physics.

“This study allows us to validate the theory of Anderson localization in chip-scale photonics, through engineered randomness in an otherwise periodic structure,” Wong said. “What Pin-Chun has observed provides a new path in controlling light propagation at the wavelength scale, that is, delivering structure arising out of randomness.”

Hsieh, who also is chairman and majority owner of Taiwan-based Quantumstone Research, said the findings are completely counterintuitive because one might think that disorder in the structures would lead the light to spread out more. “This effect, based on intuition gained from electronic systems, where introduced impurities can turn an insulator into a semiconductor, shows unequivocally that controlling disorder can arrest transverse transport, and really reduce the spreading of light.”

The numerical simulation was performed at University College London, and the sample fabrication was carried out at the Brookhaven National Laboratory in New York and at National Cheng Kung University in Taiwan.

The research was supported primarily by a grant from the U.S. Office of Naval Research. Additional support was provided by the National Science Foundation, the Department of Energy and the government of the United Kingdom. Hsieh is supported by a scholarship from Taiwan’s Department of Education.

Media Contact Matthew Chin mchin@support.ucla.edu 310-206-0680

Photon transport enhanced by transverse Anderson localization in disordered superlattices. More about this image and story at Nanotechnology Today - http://nanotechnologytoday.blogspot.com/2015/02/photon-transport-enhanced-by-transverse.html

A breakthrough by a team of researchers from UCLA, Columbia University and other institutions could lead to the more precise transfer of information in computer chips, as well as new types of optical materials for light emission and lasers.

The researchers were able to control light at tiny lengths around 500 nanometers — smaller than the light’s own wavelength — by using random crystal lattice structures to counteract light diffraction.