Possible permanent Dirac- to Weyl-semimetal phase transition by ion implantation

Three-dimensional (3D) topological semimetals (TSMs) are a new class of Dirac materials that can be viewed as 3D graphene and are referred to as Dirac semimetals (DSMs) or Weyl semimetals (WSMs) depending on whether time reversal symmetry and/or inversion symmetry are protected, respectively. Despite some interesting results on Dirac- to Weyl-semimetal phase transitions under conditions of low temperature or strong magnetic field (B), all of them are reversible phenomena. Here, we report for the first time a possible permanent transition in a single TSM by ion implantation. A Dirac- to Weyl-semimetal phase transition in a Bi_0.96Sb_0.04 DSM results from inversion-symmetry breaking induced by implantation with nonmagnetic Au ions for implant fluences (ϕ_G) ≥ 3.2 × 10¹⁶ Au cm⁻². This phenomenon is evidenced by the ϕ_G-dependent behavior of the Raman spectra and quantum-oscillation parameters extracted from magnetoresistance (MR) measurements, which show abrupt changes at ϕ_G ≥ 3.2 × 10¹⁶ Au cm⁻². The verification of the transition is further supported by observations of negative MR in the longitudinal B // electric field orientation, indicating the existence of a chiral anomaly in Weyl fermions induced by implantation with nonmagnetic Au ions. In contrast, implantation with magnetic Mn ions exhibits no such particular behavior. Our findings demonstrate the first realization of a possible permanent DSM-to-WSM phase transition in a single material by the simple approach of implantation using nonmagnetic elements.