Topological insulators have attracted much research interest due to their novel band structure and quantum phenomenon. In the past few years, a good deal of attention has been focused on the intrinsic magnetic topological insulators because the interaction between magnetism and topological surface states can produce many exotic topological quantum effects, such as the quantum anomalous Hall effect, Majorana bound states and axion insulator state. In 2019, Prof. Tian Qian, Hongming Weng, Hong Ding, and Wentao Zhang, et al. [Phys. Rev. X 9, 041039 (2019)] reported that EuSn₂As₂ is an intrinsic magnetic topological insulator by combining angle resolved photoemission spectroscopy (ARPES) measurements and first-principles calculation.

Pressure, as a basic thermodynamic parameter, plays an important role in the research of topological materials, and can effectively tune the crystal and electronic structure of material to form a new state of matter. The pressure-induced structural transitions and superconductivity have been successfully observed in the typical topological insulators such as Bi₂Se₃, Bi₂Te₃ and Sb₂Te₃, which crystallize in a rhombohedral structure in R-3m symmetry at ambient pressure. Meanwhile, the layered crystal of EuSn₂As₂ has a Bi₂Te₃-type structure in rhombohedral (R-3m) symmetry under ambient pressure (termed α-EuSn₂As₂ hereafter), however, a systematic high-pressure study on EuSn₂As₂ is lacking.

Recently, Prof. Jian-Tao Wang, Xiaohui Yu, Fang Hong, Jinguang Cheng, etc from Institute of Physics of the Chinese Academy of Sciences collaborated with Dr. Lin Zhao and Prof. Yonghao Han from Jilin University reported a new three-dimensional network structure of β-EuSn₂As₂ [see Figure 1(c)] by combining ab initio calculations and in situ x-ray diffraction measurements. The β-EuSn₂As₂ has a monoclinic (C2/m) symmetry comprising of honeycomb-like Sn sheets and zigzag As chains and can be transformed from the layered α-EuSn₂As₂ via a two-stage phase transition process under pressure [see Fig. 1(a-c)]. At the first stage, the two buckled SnAs layers connect to each other via the nearest neighbor Sn-Sn atoms [see Fig. 1(b)]; at the second stage, the buckled Sn-Sn bonds become planar and form honeycomb-like Sn sheets [see Fig. 1(d)], meanwhile the SnAs layers further connect to each other via the As-As bonds across the Eu layers to form zigzag As chains between the Sn sheets. As a result, a three-dimensional monoclinic network structure comprising honeycomblike Sn sheets and zigzag As chains is achieved via a two-stage reconstruction mechanism. The enthalpy calculations show that β-EuSn₂As₂ is more stable than α-EuSn₂As₂ at above 14.3 GPa [see Fig. 1(e)]. The phonon mode analysis also verifies the dynamic structural stability of β-EuSn₂As₂ structure [see Fig. 2]. Experimentally, the in situ high-pressure XRD experiments identify that the EuSn₂As₂ sample undergoes a structure transition from the layered α-EuSn₂As₂ to the three-dimensional monoclinic β-EuSn₂As₂ at 12.6 GPa [see Fig. 3]. Electrical resistance measurements reveal an insulator-metal-superconductor transition at low temperature around 5 and 15 GPa, respectively [see Fig. 4], according to the two-stage structural conversion process, and the superconductivity with a T_C value of ～4 K is observed up to 30.8 GPa. These discoveries spread over the research fields of superconductivity, topological insulators, and quantum magnetism, and thus will provide a new venue for studying various topics of current condensed matter physics.

The EuSn₂As₂ samples were provided by Dr. Changjiang Yi and Prof. Youguo Shi from the Institute of Physics of Chinese Academy of Sciences. The in situ XRD measurements were performed at 4W2 High Pressure Station, Beijing Synchrotron Radiation Facility (BSRF). The electrical resistance measurements are performed at the Huairou Synergic Extreme Condition User Facility.

This study entitled “Monoclinic EuSn₂As₂: A Novel High-Pressure Network Structure” was published on Phys. Rev. Lett. 126, 155701 (2021).

This work was supported by the National Key R&D Program of China (Grants No. 2016YFA0401503, No. 2016YFA0300604, No. 2018YFA0305700, No. 2018YFA0305900, and No. 2020YFA0711502), the National Natural Science Foundation of China (Grants No. 11974387, No. 12004416, No. 11674328, No. U2032204, No. 11774126, No. 12004014, No. U1832123, and No. U1930401), the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grants No. XDB33000000, No. XDB25000000, and No. QYZDBSSW-SLH013), the K. C. Wong Education Foundation (No. GJTD-2018-01), and the Youth Innovation Promotion Association of Chinese Academy of Sciences (No. 2016006).

Related links:
[1] Monoclinic EuSn₂As₂: A Novel High-Pressure Network Structure, Lin Zhao, Changjiang Yi, Chang-Tian Wang, Zhenhua Chi, Yunyu Yin, Xiaoli Ma, Jianhong Dai, Pengtao Yang, Binbin Yue, Jinguang Cheng, Fang Hong, Jian-Tao Wang,* Yonghao Han,* Youguo Shi,* and Xiaohui Yu,* Phys. Rev. Lett. 126, 155701 (2021)
https://doi.org/10.1103/PhysRevLett.126.155701
[2] Supplemental Materials for “Monoclinic EuSn₂As₂: A Novel High-Pressure Network Structure”, http://link.aps.org/supplemental/10.1103/PhysRevLett.126.155701