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SPST Researchers Discover New Topological Quantum Materials and Reveal the Electrochemical Double Layer Structure
Author:            Date:2016-10-08            Browse:723

The brilliant X-ray sources from large synchrotron radiation facilities have enabled many state-of-art material characterization tools and techniques. Recently, with the help of the synchrotron based advanced photoelectron spectroscopy, research groups from the division of condensed matter physics and photon science (DCPS), School of Physical Science and Technology (SPST), ShanghaiTech University made several new discoveries in both topological quantum materials and electrochemistry.

Topological quantum materials represent a new class of matter with both exotic physical properties and novel application potentials. Since 2010, many Heusler compounds that exhibit rich emergent properties such as unusual magnetism, superconductivity and heavy fermion behavior, have been predicted to host non-trivial topological electronic structures and attracted wide attention. The coexistence of topological order and other unusual properties makes Heusler materials ideal platform to search for new topological quantum phases (such as quantum anomalous Hall insulator and topological superconductor). However, despite the great interests and intensive research efforts, the topological electronic structure on the Heusler family remains elusive.

It the latest work published online in Nature Communications on Sept. 27th, researchers from ShanghaiTech and collaborators led by Prof. Zhongkai Liu, Prof. Binghai Yan and Prof. Yulin Chen investigated the half-Heusler compound family LnPtBi (Ln=Lu, Y) using a technique known as Angle Resolved Photoemission Spectroscopy (ARPES) [1]. Their research teams discovered unique topological surface states (TSSs) in the compound family and explained the experimental findings by ab-initio calculations, thus proving the non-trivial topological nature in LnPtBi. Remarkably, in contrast to many topological insulators that have TSSs inside their bulk gap, the TSSs in LnPtBi show their unusual robustness by lying well below the Fermi energy and strongly overlapping with the bulk valence bands. Moreover, these non-centrosymmetric superconducting LnPtBi compounds are promising candidates for the investigation of topological superconductivity, Majorana fermions, and spintronic applications.

Figure. 1 (a) ARPES Fermi surface maps of Half-Heusler alloy LuPtBi (111) surface shows complex surface states. (b,c) Zoom-in plot of Fermi surface map around the Brillouin zone center (b) and zone boundary (c). (d,e) Plot of 3D electronic structure around the Brillouin zone center (b) and zone boundary (e). The TSS and the Dirac point is clearly observed in the center of (d).

Meanwhile, the electrochemical double layer (EDL) originally conceived by Hermann von Helmholtz in the nineteenth century, constitutes a key concept in modern electrochemistry of electrified interface. While there have been many theoretical models predicting structural and electrical organization of the EDL, the experimental verification of these models has been challenging due to the limitations of available experimental techniques. The induced potential drop in the electrolyte has never been directly observed and verified experimentally.

In a first-of-its-kind experiment at Berkeley Lab's Advanced Light Source, researchers led by Prof. Zhi Liu (DCPS, ShanghaiTech) and Dr. Ethan Crumlin (Berkeley Lab) studied the active chemistry of a gold electrode and a water-containing electrolyte that also contained a neutrally charged molecule called pyrazine. They used a technique called ambient pressure X-ray photoelectron spectroscopy (APXPS) to measure the potential distribution for water and pyrazine molecules across the solid/liquid interface in response to changes in the electrode potential and the electrolyte concentration.

The experiment demonstrated a new, direct way to precisely measure a potential drop in the stored electrical energy within the double layer's electrolyte solution. These measurements also allowed researchers to determine associated charge properties across the interface. This study, “Unravelling the electrochemical double layer by direct probing of the solid/liquid interface”, was published Aug. 31 in Nature Communications. [2]

Figure 2. Schematization of the electrochemical double layer probing by APXPS under polarization conditions.

Hubert Gasteiger, a chemistry professor at the Technical University of Munich and the university's chair of technical electrochemistry who is familiar with the latest experiment, said, "This work is already quite applicable to real problems," as it provides new insight about the potential distribution within the double layer.

"No one has been able to look into this roughly 10-nanometer-thin region of the electrochemical double layer in this way before," he said. "This is one of the first papers where you have a probe of the potential distribution here. Using this tool to validate double-layer models I think would give us insight into many electrochemical systems that are of industrial relevance."[3]

ShanghaiTech research groups in this work were supported by the National Natural Science Foundation of China under Contract No. 11227902, the Chinese Academy of Sciences-Shanghai Science Research Center under Grant No: CAS-SSRC-YH-2015-01.

[1] Paper link: http://www.nature.com/articles/ncomms12924

[2] Paper link:http://www.nature.com/articles/ncomms12695

[3] Berkeley lab news release and media link:

http://newscenter.lbl.gov/2016/08/31/chemistry-with-tender-x-rays;

https://nature.altmetric.com/details/10948367/news.