Topological Dirac line nodes and superconductivity coexist in SnSe at high pressure

Abstract

Due to fundamental interest and potential applications in quantum computation, tremendous efforts have been invested to study topological superconductivity. However, bulk topological superconductivity seems to be difficult to realize and its mechanism is still elusive. Several possible routes to induce topological superconductivity have been proposed, including proximity efforts, doping or pressurizing a topological insulator or semimetal. Among them, the pressurizing is considered to be a "clean" way to tune the electronic structures. Here we report the discovery of a pressure-induced topological and superconducting phase of SnSe, a material which is highly focused recently due to its superior thermoelectric properties. In situ highpressure electrical transport and synchrotron X-ray diffraction measurements show that the superconductivity emerges along with the formation of a CsCl-type structural symmetry of SnSe above around 27 GPa, with a maximum critical temperature of 3.2 K at 39 GPa. Based on ab initio calculations, this CsCl-type SnSe is predicted to be a Dirac line nodes (DLN) semimetal in the absence of spin-orbit coupling, whose DLN states are protected by the coexistence of timereversal and inversion symmetries. These results make CsCl-type SnSe an interesting model platform with simple crystal symmetry to study the interplay of topological physics and superconductivity.

Effects of temperature and pressure on the optical and vibrational properties of thermoelectric SnSe

“…4 we plot the experimentally measured lattice parameters and the respective pressure-volume (P -V ) data for both the starting P nma and high-pressure Bbmm SnSe phases. The P nma → Bbmm transition pressure of our SnSe sample is close to 10 GPa, in excellent agreement with previous reports [19,21,27,62,69]. In addition, the P nma → Bbmm structural transition can be classified as a second-order transition, i.e.…”

“…4 we show examples of Rietveld refinements for the ambient-and high-pressure phases of SnSe, with the respective XRD patterns included in the Supplement [48]. It becomes clear that the measured XRD pattern of the high-pressure SnSe phase is accurately reproduced with the established Bbmm structural model [19,21,27,62,69], whereas we could not find the additional Bragg peaks supporting the proposed monoclinic modification [22].…”

“…Several high-pressure structural investigations have established the Pnma → Bbmm transition in pristine SnSe between 10-15 GPa at ambient temperature [19,21,27,62,69]. One recent high-pressure XRD study, however, indicated the possibility of a monoclinic modification as the proper choice for indexing the SnSe high-pressure phase [22].…”

“…For this reason, the P nma and Bbmm P -V data can be described by the same equation of state (EoS), with the Davis-Gordon EoS form [70] constituting the best choice [21]. The respective volumes, bulk moduli, and bulk moduli derivatives of the SnSe P nma and Bbmm phases are provided in Table II, alongside literature values [19,21,22,27,69]. In order to verify our experimental structural observations, we have calculated the enthalpies [H(P ) = U + P V ] for the starting P nma and the high-pressure Bbmm SnSe modifications ( Fig.…”

“…1) [19,[23][24][25]. For the sake of completeness, we mention that further compression leads SnSe towards a metallic CsCl-type structure between 30-40 GPa [26,27]. Given that the high-temperature and (first) high-pressure modification of SnSe have the same space group, it was proposed that pressure can effectively lower the P nma → Bbmm transition temperature [10, 11, 15-19, 25, 28].…”