130th CEMS Colloquium

Title

Exploring Topological Band Structure and Relevant Properties by ARPES

Abstract

Angle-resolved photoelectron spectroscopy (ARPES) is one of the most powerful experimental techniques for visualizing the band structure of materials. It is expected to contribute to the development of low-power consuming devices and highly efficient thermoelectric materials, as will be demonstrated in this talk. We first show that time-resolved ARPES, pumped with terahertz (THz) light, can aid in the creation of low-loss electronic devices. We have uncovered the momentum distribution of Dirac fermions accelerated by the carrier wave of a THz pulse, and found that spin-momentum locking on the surface of topological insulators (TIs) enables fully ballistic lightwave currents over distances of several hundred nanometers. This could pave the way for spintronic devices that operate at optical clock rates [1]. Transverse thermoelectric conversion, known as the anomalous Nernst effect (ANE), holds significant promise for the development of next-generation thermoelectric devices. Recently, Co-based Heusler alloys and related binary alloys have emerged as promising ANE materials, exhibiting thermopower more than two orders of magnitude higher than that of pure Fe [2,3]. Understanding the origin of this remarkable Nernst thermopower requires careful examination of the band structure, as the Berry curvature is highly sensitive to the position of the Fermi level. However, experimental studies of the band structure have been challenging due to the difficulty in cleaving bulk crystals. In this talk, we demonstrate that the use of a vacuum suitcase allows for direct observation of the spin-resolved band structure of in-situ grown, ordered Co_₂MnGa (L2₁) and Fe_₃Ga (D0₃) thin films via spin- and angle-resolved photoelectron spectroscopy (spin-ARPES) with synchrotron radiation [4,5]. By replacing Ge atoms with Ga atoms, we precisely position the topological band crossing at the Fermi level, resulting in a high Berry curvature. Through a detailed comparison of the iso-energy surfaces of Co_₂MnGa and Co_₂MnGe, we confirm the validity of the rigid-band model, which suggests that replacing Ga with Ge can fine-tune the carrier concentration to maximize the anomalous Nernst conductivity [5]. These findings provide valuable insights for optimizing Nernst thermopower.

1. J. Reimann, AK, U. Höfer, R. Huber et al., Nature 562, 396 (2018).
2. A. Sakai et al., Nat. Phys. 14, 1119 (2018). / A. Sakai et al., Nature 581, 53 (2020).
3. H. Nakayama et al., Phys. Rev. Mater. 3, 114412 (2019).
4. T. Kono and AK et al., Phys. Rev. Lett. 125, 216403 (2020).
5. K. Sumida, Y. Sakuraba, K. Masuda and AK et al., Commun. Mater. 1, 89 (2020).

Contact

email: k-akiyama@riken.jp
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