Intrinsic defect engineering - a new approach for realizing topological crystalline insulators

The discovery of topological insulators (TIs) not only enhanced peoples’ understanding of condensed matter, but also revealed TIs to be promising prospects for applications in dissipationless electronic and spintronic devices and quantum computing due to their exotic properties. TIs are insulating in the bulk, just like trivial insulators, but boast conducting surface/edge states which are protected by time reversal symmetry. These surface states are not only stable, but also show spin-momentum-locking, that is to say electrons of opposite spins propagate in opposite directions, which makes it possible to communicate information via spins rather than charges. Topological crystalline insulators (TCIs) are to some extent analogous to TIs, the difference being that TCIs are protected by the crystal symmetry instead of time reversal symmetry, and they do not require spin-orbit coupling. Therefore, TCIs are easier to be realized in real materials and may be more promising for applications.

After the concept of a TCI was proposed by L. Fu (Phys. Rev. Lett., 2011), SnTe which is a IV-VI narrow-gap compound, was first predicted to be a TCI, and this was soon proved by independent experiments. As the counterpart of SnTe, PbTe resembles SnTe in its crystal structure, but PbTe was shown both theoretically and experimentally to be a trivial insulator. An interesting question arises: can PbTe be tuned into a TCI? The key point for achieving a topologically nontrivial PbTe is to induce a band inversion, which could be realized through alloying, pressure or strain. For example, alloying PbTe with SnTe can lead to topologically nontrivial Pb_1-xSn_xTe if the Sn content exceeds a critical value. However, the thermodynamically favorable Sn vacancy defects in a SnTe TCI lead to a high bulk carrier density, which will unavoidably affect the surface transport. To the best of our knowledge, there are no experimental studies on pressurizing PbTe into a TCI. Therefore, it is important to seek new approaches for inducing band inversions and realizing novel TCIs.

Recently, the collaboration of two research groups in Department of Physics (Prof. Huizhen Wu and Prof Huiqiu Yuan) and Dr. Yunhao Lu in School of Materials Science and Engineering of Zhejiang University led to the realization of a new PbTe TCI. Mr. Songsong Ma and Mr. Chunyu Guo are co-first authors. They theoretically predicted that Te antisite defects change the band order of PbTe and there exist topological surface states on high symmetry crystal surfaces. Experimentally, they synthesized Te-antisite-doped [111]-oriented PbTe films using molecular beam epitaxy technology and conducted quantum transport experiments on the samples at low temperatures and high magnetic fields. The observed quantum oscillations reveal a nontrivial Berry phase in PbTe: Te_Pb, which demonstrates the Dirac fermion nature of the topological surface states. Further experiments on PbTe: Te_Pb upon applying external pressure up to 2.2 GPa show that pressure can enlarge the inverted gap and reduce the bulk carrier contribution to the quantum transport. The intrinsic defect engineering approach developed in this work could be applied to other materials, especially to chalcogenides, which provides the opportunity to discover more topological materials.  This work is published in Advanced Functional Materials (DOI: 10.1002/adfm.201803188).