Abstract
The interplay between time-reversal symmetry (TRS) and band topology plays a crucial role in topological states of quantum matter. In time-reversal-invariant (TRI) systems, the inversion of spin-degenerate bands with opposite parity leads to nontrivial topological states, such as topological insulators and Dirac semimetals. When the TRS is broken, the exchange field induces spin splitting of the bands. The inversion of a pair of spin-splitting subbands can generate more exotic topological states, such as quantum anomalous Hall insulators and magnetic Weyl semimetals. So far, such topological phase transitions driven by the TRS breaking have not been visualized. In this work, using angle-resolved photoemission spectroscopy, we have demonstrated that the TRS breaking induces a band inversion of a pair of spin-splitting subbands at the TRI points of Brillouin zone in , when a long-range ferromagnetic order is developed. The dramatic changes in the electronic structure result in a topological phase transition from a TRI ordinary insulator state to a TRS-broken topological semimetal (TSM) state. Remarkably, the magnetic TSM state has an ideal electronic structure, in which the band crossings are located at the Fermi level without any interference from other bands. Our findings not only reveal the topological phase transition driven by the TRS breaking, but also provide an excellent platform to explore novel physical behavior in the magnetic topological states of quantum matter.
- Received 18 November 2020
- Revised 25 January 2021
- Accepted 24 February 2021
DOI:https://doi.org/10.1103/PhysRevX.11.021016
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Topological states of matter are dictated by global properties of the electron wave function and thus are insensitive to local disorder and defects in materials. The past decade has seen tremendous progress in researching magnetic topological states, where magnetism and topology intertwine. This interplay may lead to the emergence of novel quantum phenomena such as the quantum anomalous Hall effect, in which quantized conductance in 2D materials free of magnetic fields could enable next-generation electronic devices with ultralow power requirements. However, further exploration and application has been hampered by the rarity of experimentally confirmed magnetic topological materials with all conducting electrons being topologically nontrivial. Here, we report such an ideal magnetic topological state in a ferromagnet .
Specifically, we observe a topological phase transition in crystals as the material develops long-range ferromagnetic order with decreasing temperature. In the high-temperature paramagnetic state, is an ordinary semiconductor with a narrow gap between the occupied and unoccupied electron bands—the broadening of the energy level of the atomic orbital in crystal. In the low-temperature ferromagnetic state, the highest occupied band and lowest unoccupied bands are inverted, leading to a semimetal state with nontrivial topology. Based on this discovery in crystals, we expect that the quantum anomalous Hall effect can be realized in 2D films.
The outstanding properties in the electronic structure of make it a promising system to study the physical behavior associated with magnetic topological states in both crystals and thin films.
