Prof. WANG Zhengfei from University of Science and Technology of China and his coworkers reported a novel one dimensional topological edge state in the high-Tc superconductor FeSe/SrTiO3. The work was published with the title “Topological edge states in a high-temperature superconductor FeSe/SrTiO3(001) film” in Nature Materials on July 4, 2016. Reviewers have commented that "Iron-based superconductor and topological insulator are hot topics since 2008. It is very interesting to see the combination of them after years." This advance was also highlighted in the "News and Views" column of the same issue under the title, "Topological insulators and superconductivity: the integrity of two sides".
Superconducting and topological states are two most intriguing quantum phenomena in the condensed-matter physics. The entanglement of these two states, the topological superconducting state, will give rise to even more exotic quantum phenomena. The topological superconductors are characterized by the superconducting bulk states and topologically protected gapless metallic surface or edge states, which can generate the Majorana fermion at the center of its magnetic vortex. The Majorana fermion is its own antiparticle, so it’s very stable and cannot be easily destroyed by conventional electromagnetic or physical interaction, making it a suitable qubit in quantum computing. However, natural topological superconductors have not been found yet. Previously, people have tried proximity effect to realize the topological superconductor by growing superconductor and topological insulator two materials together. However, due to the interface quality, crystallization temperature and other factors, the growth conditions for such composites are very harsh, greatly limiting its research progress.
To overcome the above research bottleneck, the research term between theoretical and experimental physicists has systematically studied the antiferromagnetic electronic structures of the high-Tc superconductor FeSe/SrTiO3. Combining theoretical calculations, experimental STM and APRES measurements, they identified the checkerboard antiferromagnetic order of FeSe/SrTiO3 to be the best match to the ARPES band structures. Most importantly, a novel real-space localized one dimensional topological edge state is observed within the energy window of the nontrivial spin-orbital coupling gap of FeSe/SrTiO3. While many materials are found to be either a superconductor or a topological insulator, it is very rare that both states exist in one material. Therefore, for the first time in both reciprocal space and real space, the researchers demonstrated the coexistence of superconducting and topological states in FeSe/SrTiO3. Furthermore, using the PN junction to realize the electron and hole doping, the energy level of the superconducting and topological gap can be tuned together, making it possible to achieve the high-Tc topological superconductor and Majorana fermion in this single-component material. Meantime, this work will also inspire people to further think about the physical origin of the high-Tc in FeSe/SrTiO3, and promote the mechanism study in iron-based high temperature superconducting materials.
This research was supported by Chinese Youth 1000 Talents Program, NSFC, Strategic Priority Research Program (B) of CAS, MOST of China and DOE-BES.
Supported by the Chinese Youth 1000 Talents Program, NSFC, Strategic Priority Research Program (B) of CAS, MOST of China and DOE-BES.
Left panel: FeSe/SrTiO3 PN junction, showing the electron doped superconducting state and hole doped topological edge state(Provided by http://d4sci.com).Right panel: FeSe/SrTiO3 topological gap and topological edge state comparison between theory and experiment.
(HFNL)
