Research Presents New Possibilities for Making Artificial Topological Materials

A group of international researchers have developed a new, unique structure that helps tune topological properties in such a manner that these unique behaviors can be turned on or off. The structure presents new possibilities for studying the characteristics of topological states of matter.

“This is an exciting new direction in topological matter research,” said M. Zahid Hasan, professor of physics at Princeton University and an investigator at Lawrence Berkeley National Laboratory in California who headed the study, which was reported March 24th in the Science Advances journal. “We are engineering new topological states that do not occur naturally, opening up numerous exotic possibilities for controlling the behaviors of these materials.”

The novel structure contains alternating layers of normal, or trivial, and topological insulators - a unique layout that enables the team to switch on or off the current flowing through the structure. The potential to regulate the current indicates new possibilities for circuits that are based on topological behaviors, but more importantly, may present an innovative artificial crystal lattice structure for investigating quantum behaviors.

The 2016 Nobel Prize in physics was conferred to Princeton University’s F. Duncan Haldane and two other researchers. Theories behind the matter’s topological properties were the focus of this Nobel Prize. One group of matter is topological insulators, which behave like insulators on the inside but still enable the current to pass through without any resistance on the surfaces.

In the novel structure, interfaces between the alternating layers produce a one-dimensional (1D) lattice, where topological states can occur.  The lattice’s one-dimensional nature can be compared to cutting into the material and removing an extremely thin slice, and then looking at the slice’s thin edge. This 1D lattice looks like a chain of synthetic atoms. This is an emergent behavior because it emerges only when multiple layers are stacked together.

By altering the layers’ composition, the research team will be able to regulate the jumping of electron-like particles, known as Dirac fermions, via the material. For instance, if the trivial-insulator layer is made relatively thick – just about four nanometers – the Dirac fermions will not be able to pass through it. This effectively makes the whole structure a trivial insulator. On the other hand, if the trivial-insulator layer is made thin – measuring just one nanometer thick – the Dirac fermions will travel from one topological layer to the next.

In order to design the two materials, the Princeton researchers collaborated with another research team at Rutgers University headed by Seongshik Oh, associate professor of physics, who in partnership with Hasan and others demonstrated earlier in 2012 that when indium is added to bismuth selenide – a topological insulator – it becomes a trivial insulator. Before that, Hasan’s team hypothetically and experimentally identified bismuth selenide (Bi₂Se₃) as a topological insulator, a finding which was reported in 2009 in the Nature journal.

“We had shown that, depending on how much indium you add, the resulting material had this nice tunable property from trivial to topological insulator,” Oh said, referring to the work published in Physical Review Letters in 2012.

Graduate students Nikesh Koirala of Rutgers and Ilya Belopolski of Princeton University integrated two advanced methods with the latest instrumentation development and worked on layering the two materials – indium bismuth selenide and bismuth selenide – to create the optimal structure. However, it was very difficult to match up the lattice structures of both materials, so that the Dirac fermions can jump from layer to layer. Suyang Xu and Belopolski collaborated with colleagues at Lawrence Berkeley National Laboratory, Princeton University¸ and several other institutions to employ high resolution angle-resolved photoemission spectroscopy to improve the behavior of the Dirac fermions depending on a growth to measurement feedback loop.

While no topologically analogous states can occur naturally, the team observed that similar behavior can be detected in a chain of polyacetylene, which is a popular model of 1D topological behavior as elucidated by the 1979 Su-Schrieffer-Heeger’s hypothetical model of an organic polymer.

According to Hasan, the study opens up new possibilities for making synthetic topological materials.

In nature, whatever a material is, topological insulator or not, you are stuck with that. Here we are tuning the system in a way that we can decide in which phase it should exist; we can design the topological behavior.

Hasan

The potential to control the movement of light-like Dirac fermions may help future scientists to exploit the resistance-less flow of current observed in topological materials.

These types of topologically tunable heterostructures are a step toward applications, making devices where topological effects can be utilized.

Hasan

The Hasan team is looking for new ways to adjust the thickness and ultimately study the topological states in relation to superconductivity, magnetism, the quantum Hall effect, as well as Majorana and Weyl fermion states of matter.

Apart from the work performed at Rutgers and Princeton, the study featured contributions from South University of Science and Technology of China; National University of Singapore; Swiss Light Source, Paul Scherrer Institute; University of Central Florida; Diamond Light Source, Didcot, U.K.; Universität Würzburg; and Synchrotron SOLEIL, Saint-Aubin, France.

Synchrotron-based ARPES measurements headed by Princeton research team and Work at Princeton University were supported by the U.S. Department of Energy under Basic Energy AQ29 Sciences grant no. DE-FG-02-05ER46200 (to M.Z.H.). I.B. was supported by an NSF Graduate Research Fellowship. S.O., N.K., and M.B. were supported by the Emergent Phenomena in Quantum Systems Initiative of the Gordon and Betty Moore Foundation under grant no. GBMF4418 and by the NSF under grant no. NSF-EFMA-1542798. M.N. was supported by start-up funds from the University of Central Florida. H.L. acknowledges support from the Singapore National Research Foundation under award no. NRF-NRFF2013-03. The work acknowledges support from Diamond Light Source, Didcot, U.K., for time on beamline I05 under proposal SI11742-1. Certain measurements were performed at the ADRESS beamline (24) of the Swiss Light Source, Paul Scherrer Institute, Switzerland. This research was partly supported by grant AQ30 no. 11504159 of the National Natural Science Foundation of China (NSFC), grant no. 2016A030313650 of NSFC Guangdong, and project no. JCY20150630145302240 of the Shenzhen Science and Technology Innovations Committee.

The paper titled, “A novel artificial condensed matter lattice and a new platform for one-dimensional topological phases,” by Ilya Belopolski, Su-Yang Xu, Nikesh Koirala, Chang Liu, Guang Bian, Vladimir Strocov, Guoqing Chang, Madhab Neupane, Nasser Alidoust, Daniel Sanchez, Hao Zheng, Matthew Brahlek, Victor Rogalev, Timur Kim, Nicholas C. Plumb, Chaoyu Chen, François Bertran, Patrick Le Fèvre, Amina Taleb-Ibrahimi, Maria-Carmen Asensio, Ming Shi, Hsin Lin, Moritz Hoesch, Seongshik Oh and M. Zahid Hasan, was featured in the journal Science Advances on March 24, 2017. (Belopolski et al., Sci. Adv. 2017;3: e1501692 24 March 2017)