Topological insulators constitute a new phase of matter protected by symmetries. Time-reversal symmetry protects strong topological insulators of the Z₂ class, which possess an odd number of metallic surface states with dispersion of a Dirac cone. Topological crystalline insulators are merely protected by individual crystal symmetries and exist for an even number of Dirac cones. Here, we demonstrate that Bi-doping of Pb_1-x Sn _x Se (111) epilayers induces a quantum phase transition from a topological crystalline insulator to a Z₂ topological insulator. This occurs because Bi-doping lifts the fourfold valley degeneracy and induces a gap at [Formula: see text], while the three Dirac cones at the [Formula: see text] points of the surface Brillouin zone remain intact. We interpret this new phase transition as caused by a lattice distortion. Our findings extend the topological phase diagram enormously and make strong topological insulators switchable by distortions or electric fields.Transitions between topological phases of matter protected by different symmetries remain rare. Here, Mandal et al. report a quantum phase transition from a topological crystalline insulator to a Z₂ topological insulator by doping Bi into Pb_1-x Sn _x Se (111) thin films.

中文翻译：

---

从镜像到时间反转对称的拓扑量子相变保护了拓扑绝缘体。

拓扑绝缘体构成了受对称性保护的物质的新阶段。逆时对称性可保护Z ₂类的强拓扑绝缘体，该绝缘体具有奇数个金属表面态，且狄拉克锥的色散。拓扑晶体绝缘体仅受单个晶体对称性的保护，并且存在偶数个狄拉克锥。在这里，我们证明Bi掺杂Pb _1-x Sn _x Se（111）外延层会诱导从拓扑晶体绝缘体到Z ₂的量子相变。拓扑绝缘体。发生这种情况的原因是，双掺杂提升了四倍的峰谷简并在[公式：参见文本]处引起间隙，而在表面布里渊区的三个[狄拉克]圆锥体保持完整。我们将这种新的相变解释为由晶格畸变引起。我们的发现极大地扩展了拓扑相图，并通过变形或电场使强拓扑绝缘体可切换。受不同对称性保护的物质的拓扑相之间的转换仍然很少。在这里，Mandal等人。通过将Bi掺杂到Pb _1-x Sn _x Se（111）薄膜中，报道了从拓扑晶体绝缘体到Z ₂拓扑绝缘体的量子相变。