Quantum transport phenomena in topological materials
The world has shifted,
Yet still, I am who I am
Oath of symmetry
Topology – the mathematical concept that characterizes the robustness of forms under continuous deformation, has opened up a new chapter in the story of quantum materials. The traditional framework for classifying phases and phase transitions follows the Landau theory of spontaneous symmetry breaking, a must-cover topic in statistical mechanics textbooks. This framework works beautifully in describing a zoo of phenomena, ranging from the boiling and freezing of water to superconductivity and superfluidity; however, a broad class of “misfits” – the topological phases, exposes its limitation. A well-known example is the quantum Hall effect, where the transition between electron states of distinct integer Hall conductance does not break any symmetry. Further research efforts reveal that the topological phases are rooted in the “shape” of the electron band and are protected by the underlying symmetries. Taking topology as the guiding principle, a new paradigm for classifying quantum phases emerges, which fuels the exploration of novel states of matter that are not only of fundamental interest but may hold potential technological applications.
In our lab, the experimental journey in the topological realm leads us to discoveries of quantum transport phenomena that defy the common sense. One such prime finding is the surprisingly large anomalous Hall effect (AHE) in non-ferromagnetic materials, namely, spin liquids and antiferromagnets. The AHE, where voltage flows transversely to the current direction without external magnetic field, is one of the most fundamental and widely-studied transport phenomena in physics. Traditionally, the AHE is considered to be proportional to net magnetization, and therefore, is expected to occur only in ferromagnets. Later theoretical advances have established an intimate connection between the AHE and the Berry curvature – a virtual magnetic field in the momentum space that serves as an indicator of the electron band topology. We design materials that feature unique topological state with strongly enhanced Berry curvature, leading to significant AHE despite the absence of net magnetization. The observed AHE enables us to electrically manipulate and detect antiferromagnetically coupled spins, giving rise to a new direction for realizing memory devices that are much faster and energy-efficient than today’s version. For more details on this research topic, please refer to the following list of publications.
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 S. Nakatsuji, N. Kiyohara, and T. Higo, Nature 527, 212 (2015).
 N. Kiyohara, T. Tomita, and S. Nakatsuji, Phys. Rev. Applied 5, 064009 (2016).
 K. Kuroda, T. Tomita et al., Nat. Mater. 16, 1090 (2017).
 M.-T. Suzuki et al., Phys. Rev. B 95, 094406 (2017).
 M. Ikhlas, T. Tomita et al., Nat. Phys. 13, 1085 (2017).
 T. Higo et al., Nat. Photon. 12, 73 (2018).
 H. Tsai, T. Higo et al., Nature 580, 608 (2020).