Fudan University's professor Xiu Faxian's group observed the superconducting state of Cd3As2 on the topological semimetal Cd3As2 and superconducting heterojunction. On May 17, related research results were published online in Nature Communications under the title of "Proximity-induced surface superconductivity in Dirac semimetal Cd3As2" ("Proximity-induced surface superconductivity in Dirac semimetal Cd3As2"). , DOI: 10.1038 / s41467-019-10233-w).
Mayorana Fermion is a novel quasi-particle that satisfies non-Abelian statistics and is the material basis for achieving fault-tolerant topological quantum calculations. In recent years, topological superconducting materials have become one of the important research directions in the field of condensed matter physics due to the existence of Mayorana fermions in their boundary states. Transforming topological materials into superconducting states is one of the effective methods to explore topological superconductivity. As a new type of topological quantum material, the three-dimensional topological semimetal has a Dirac cone structure on its energy band, and has a variety of non-mediocre surface boundary topological states. It is an ideal system for realizing Mayorana fermions. To achieve topological superconductivity in topological semimetals, we first need to transform these boundary states into superconducting states. Although there have been many predictions in theory, there are currently no experiments to fully confirm the superconducting forms of these boundary states.
In order to achieve topological semimetal superconductivity, the Xiu Faxian's group based on the previous Fermi arc surface state transport research of three-dimensional Dirac semimetal Cd3As2 nanosheets, and the research group of the Hong Kong University of Science and Technology (Ku Tuen Law) In cooperation, a layer of superconducting Nb was deposited on Cd3As2 nanosheets to form a heterojunction of Nb / Cd3As2 and Nb / Cd3As2 / Nb, and the superconductivity was transferred to Cd3As2 through the proximity effect. Studies have shown that at the interface of Nb / Cd3As2, a wide conductance platform and a wide peak with zero bias are found through differential conductance spectrum transport measurements, corresponding to the surface Fermi arc and body superconductivity of Cd3As2, respectively. The coupling of the conductance is much stronger than the posture. By changing the thickness of Cd3As2, the specific gravity of the surface Fermi arc and body posture of Cd3As2 can be changed, and the corresponding changes in Fermi arc and body superconducting energy gap can be observed. At the same time, Luo Jintuan's group's theoretical simulation of Nb / Cd3As2 further confirmed the observed Fermi arc and body superconductivity.
Based on the good superconducting neighbor effect of Nb / Cd3As2, the research group studied the physical properties of the Josephson junction (Nb / Cd3As2 / Nb) based on Cd3As2. By superconducting quantum interference (Superconducting Quantum Interference) measurement, it was observed that the superconducting current flows almost completely through the upper and lower surfaces of Cd3As2. The relationship between the superconducting critical current and the magnetic field is a SQUID shape in the experiment. This is very different from the surface state superconducting current characteristics of similar topological insulators. In topological insulators, the surface state is isotropic, and the directions will interfere with each other. It is difficult to form a good boundary current in a single direction, and the Fermi level needs to be adjusted to the body energy gap to avoid the influence of the body state. However, in the three-dimensional Dirac semimetal Cd3As2, because the body state and the surface Fermi arc have a large gap in the coupling capacity for superconductivity, the interference of the body state can be avoided, so that the surface state exists only on both sides of the y direction, and can be formed perfectly. Superconducting current on the surface. These characteristics are strong evidence of Cd3As2 Fermi arc superconductivity.
Schematic of Cd3As2 Fermi Arc Superconductor
The research results for the first time achieved the Fermi arc superconductivity in a three-dimensional topological semimetal (as shown in the figure), which is equivalent to a high-dimensional analogy of the one-dimensional superconducting boundary state of a two-dimensional quantum spin Hall insulator. It is of great significance to understand the boundary state superconductivity. At the same time, based on theoretical predictions, the superconducting Fermi arc of Cd3As2 can realize a new form of Mayorana fermion in the ac Josephson effect: Mayorana flat band. This provides new ideas and experimental methods for exploring topological superconductivity, and also provides a preliminary scientific basis for the final realization of topological quantum computers.
The work was performed by Fu Xianxian and Wang Yihua's group at the Department of Physics of Fudan University, Luo Jin's group at the Hong Kong University of Science and Technology, Han Zheng's group at the Shenyang Institute of Metal Research, Lin Xi's group at Peking University, Zou Jin's group at the University of Queensland, Australia, and Han Xiaodong The research team of Chen Yanhui completed the cooperation. The work has received strong support and funding from the Department of Physics of Fudan University, the State Key Laboratory of Applied Surface Physics, the National Key R & D Program, the Excellent Young Fund of the Fund Committee and general projects. The first unit of the thesis is the Department of Physics of Fudan University. Xiu Faxian, a professor of Physics at Fudan University, is the corresponding author. .
Xiu Faxian's group is mainly engaged in the growth of topological materials, quantum control, and device research of new low-dimensional atomic crystal materials. In Dirac materials, he is committed to the growth of new quantum materials, physical properties measurement, and preparation and characterization of quantum devices. In the field of devices of new low-dimensional atomic crystal materials, their electrical, magnetic and optoelectronic properties are mainly studied.