October 9, 2020
The University of Tokyo
Tokyo Institute of Technology
Researchers at Nippon Telegraph and Telephone Corporation (NTT; Head office: Chiyoda-ku, Tokyo; President & CEO: Jun Sawada) have succeeded, for the first time in the world, in preparing ultrahigh-quality thin films of SrRuO3*1 and observing quantum transport phenomena in them, which is peculiar to an exotic state called “magnetic Weyl semimetal*2”. This work was accomplished by magnetotransport measurements performed in collaboration with the Tanaka Research Group at The University of Tokyo (UTokyo, President: Makoto Gonokami, Bunkyo-ku, Tokyo). They have also revealed the existence of the exotic state in SrRuO3 by theoretical calculation as well, which was carried out in collaboration with the Das Research Group at the Tokyo Institute of Technology (Titech, Headquarter: Meguro-ku, Tokyo; President: Kazuya Masu). Working together, these teams have provided the first set of experimental and theoretical evidence for the existence of the “magnetic Weyl semimetal” state in oxide materials.
SrRuO3 is a metal that exhibits ferromagnetism*3 when cooled to less than -120 ℃. While the ability to prepare large bulk single crystals*4 remains elusive, the large-area single-crystalline thin films necessary for fabricating devices and other purposes have been widely used in research on oxide electronics.*5 The ultrahigh-quality SrRuO3 thin films exploited in this study were achieved through a combination of a sui generis thin-film growth technique developed by NTT for multication oxides and machine-learning-based optimization (process informatics) of the growth conditions. The specimens have an extremely high residual resistivity ratio (RRR),*6 a measure of quality for metallic materials, whose value surpasses the RRR record in SrRuO3 thin films for the first time in 20 years.
The results provide robust evidence for the existence of the magnetic Weyl semimetal state in materials as well as insight into the quantum transport properties in such an exotic state and their emerging mechanisms. As only three years have passed since the first experimental report*7 on the magnetic Weyl semimetal state, applying the present results for device fabrication or other purposes may require considerable time. Nevertheless, the insight gained by the present research will lead to innovative oxide materials and novel quantum devices in the future.
This research was reported in Nature Communications on October 9, 2020.
It has recently been revealed that the concept of topology plays a fundamental role in comprehending some quantum states emerging in materials, and such topological materials and exotic states in them are being actively investigated. Among those exotic states, however, the magnetic Weyl semimetal state is still not well understood from the experimental standpoint, though a lot has been predicted theoretically. Observations of the magnetic Weyl semimetal state have been reported in Mn3Sn (The University of Tokyo et al., 2017*7), in Co2MnGa (Princeton University et al., 2019*8), and in Co3SnS2 (Max Planck Institute of Microstructure Physics, Shanghai Tech University et al., 2019*9, and Weizmann Institute of Science et al., 2019*10). However, there is a dearth of knowledge about the quantum transport properties peculiar to that state, especially quantum oscillations*11. Besides, for the design and fabrication of devices in the future, magnetic Weyl semimetals that are versatile and can be readily implemented in single-crystalline thin films are highly desirable. Therefore, researchers have sought to develop such magnetic Weyl semimetals and establish guiding principles to uncover such materials.
Researchers at NTT Basic Research Laboratories (NTT-BRL), working with those at NTT Communication Science Laboratories (NTT-CSL), have prepared ultrahigh-quality thin films of SrRuO3 using a combination of a unique oxide thin-film growth technique and a machine learning technique (process informatics), both of which they have been refining over many years (Fig. 1). The SrRuO3 films exhibit an extremely high residual resistivity ratio (RRR) *6 value of more than 84, surpassing the RRR record for SrRuO3 thin films for the first time in 20 years.
Measurements of temperature- and magnetic-field-dependence of magnetoresistance in such ultrahigh-quality specimens, carried out with UTokyo, conduced to the first set of quantum transport properties peculiar to the magnetic Weyl semimetal state. Such characteristic transport properties were observed in the low-temperature and high-RRR regime (Fig. 2), indicating the essential importance of sample quality.
When the magnetic Weyl semimetal state exists in materials, it is generally anticipated that five quantum transport properties characteristic to the exotic state should be observed, which stem from linear dispersion *12, violation of time-reversal symmetry *13, and the topological nature*14, inherent to the state (Fig. 3). In the present research, all five properties were observed, providing robust evidence for the existence of the magnetic Weyl semimetal state in SrRuO3. Amongst them, the observation of quantum oscillations (Fig. 4) deserves special mention as it is only allowed for ultrahigh-quality specimens in which electron scattering is sufficiently suppressed. The quantum oscillations have elucidated that quasi-particles*15 specific to the magnetic Weyl semimetal state have light cyclotron masses*16 and high quantum mobility*17 and show Berry phase shift*18, as expected (see Fig. 3).
Furthermore, the existence of the magnetic Weyl semimetal state in SrRuO3 was confirmed by density functional theory *19 calculations carried out by the Titech team, providing the first set of experimental and theoretical evidence for the existence of the magnetic Weyl semimetal state in oxide materials. The present work provides guiding principles for discovering similarly fascinating materials and will certainly contribute to further developments in this research field.
In addition, this is the first successful observation of the magnetic Weyl semimetal state in single-crystalline thin film specimens, which have high compatibility with device fabrication processes. As we are still at the dawn of this research field, applying the present results to something practical such as device fabrication may require considerable time. Nevertheless, topologically protected quantum states like magnetic Weyl semimetal have high immunity against impurities and noises. Therefore, the insight gained in the present work will contribute to the design of quantum devices operating on novel principles in the future.
We prepared the SrRuO3 thins films with a perovskite structure*20 (Fig. 5a) by means of molecular beam epitaxy*21 augmented by machine-learning techniques (process informatics). To grow high-quality SrRuO3 thin films, precise control of the flux rates of each constituent cation (Sr, Ru) is mandatory. Although controlling the flux of Ru is a challenge because of its high melting point (higher than 2000℃), we have succeeded in precisely controlling its flux rate as well as that of Sr. We accomplished this by monitoring the flux rates with an atomic emission spectrometer and feeding them back to the evaporation source power supplies in real time (Fig. 7), which enabled preparation of ultrahigh-quality SrRuO3 thin films with the Sr and Ru atoms arranged in a highly ordered structure (Fig. 5b).
In our quest for better comprehending the fundamentals of the magnetic Weyl semimetal state in SrRuO3, we will further investigate the electronic structures of SrRuO3 using advanced spectroscopy techniques offered by synchrotron radiation facilities.*22 At the same time, as a part of a strategy toward quantum devices, we will examine whether or not we can electrically control the transport properties of Weyl particles in a manner similar to that in conventional semiconductor devices.
“Quantum transport evidence of Weyl fermions in an epitaxial ferromagnetic oxide ”
Kosuke Takiguchi, Yuki K. Wakabayashi, Hiroshi Irie, Yoshiharu Krockenberger, Takuma Otsuka, Hiroshi Sawada, Sergey A. Nikolaev, Hena Das, Masaaki Tanaka, Yoshitaka Taniyasu, and Hideki Yamamoto
Nature Communications (2020): https://doi.org/10.1038/s41467-020-18646-8.
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