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The 25th Outstanding Paper Award of the Physical Society of Japan

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Each year, for recognizing significant achievements toward the progress in physics, the Physical Society of Japan (JPS) selects outstanding papers among original research articles published in the Journal of the Physical Society of Japan, Progress of Theoretical Physics, Progress of Theoretical and Experimental Physics, and JPS Conference Proceedings.
The selection committee has chosen the following five papers for the 2020 award based on 18 nominations (18 papers) made by the editors of the two journals and representatives of the 19 divisions of the JPS.

Title of Article Spin Chirality Ordering and Anomalous Hall Effect in the Ferromagnetic Kondo Lattice Model on a Triangular Lattice
Journal J. Phys. Soc. Jpn. 79, 083711 (2010)
Authors Yutaka Akagi and Yukitoshi Motome
Citation

The article addresses the question of ground-state properties of the Kondo lattice model on the triangular lattice, which is one of the lattice structures with geometrical frustration. An exciting possibility is that triads of spins on a triangle have an order of noncoplanar spin configuration, which results in scalar chirality. In the presence of the scalar chirality, the time-reversal symmetry and the space inversion are broken. When itinerant electrons are coupled with the spin system with the scalar chirality, the anomalous Hall effect is expected. Hall coefficient of the ordered state is investigated based on the Kubo formula.

The Kondo lattice model on the triangular lattice was first investigated by I. Martin and C. D. Batista (Phys. Rev. Lett. 101, 156402 (2008)), wherein the ground state with the scalar chirality was found at the filling of 3/4th of the conduction electrons along with the anomalous Hall effect. In the present paper, the authors investigated the ground states systematically up to the unit cells with four sites for the whole range of filling of the conduction electrons. As a consequence, a new stable ground state with the scalar chirality was found at the quarter filling. The anomalous Hall effect was also confirmed at this filling. Thus, the systematic exploration for the states with the scalar chirality was completed in the papers of Martin and Batista and the present authors. For these reasons, we conclude that the present paper deserves the Outstanding Paper Award of the Physical Society of Japan.

Title of Article Extremely Large and Anisotropic Upper Critical Field and the Ferromagnetic Instability in UCoGe
Journal J. Phys. Soc. Jpn. 78, 113709 (2009)
Authors Dai Aoki, Tatsuma D. Matsuda, Valentin Taufour, Elena Hassinger, Georg Knebel, and Jacques Flouquet
Citation

Among the types of unconventional superconductivities, U-based superconductivities that coexist with ferromagnetism have attracted much attention as strong candidates for spin-triplet superconductors. The superconductivity of UGe2, UIr, UCoGe, and URhGe, along with UTe2 that was discovered last year, is assisted by the ferromagnetic phase. This link is regarded as strong evidence for non-unitary spin-triplet superconductivity, where a ferromagnetic fluctuation mediates the pairing. Experimental validation for spin-triplet Cooper-pair state is difficult in general, while the linkage between ferromagnetism and superconductivity in U-based materials occupies a significant position in this research area.

The authors of this paper conducted very carefully planned experiments under external magnetic fields on UCoGe single crystals and first pointed out that superconductivity in UCoGe is stabilized especially when the external field is very precisely applied along the b axis. An "S-shaped curve" is observed in the superconducting H-T phase diagram, featuring re-entrant behavior similar to that discovered in URhGe under high pressures. The authors further showed that Tc and quasiparticle state are susceptible to the field orientation. Their findings made a significant impact on the later studies for U-based superconductivity, for example, discoveries of a linkage between anisotropic ferromagnetic fluctuations and superconductivity in UCoGe and re-entrant superconductivity in UTe2. For these reasons, we conclude that this paper deserves the Outstanding Paper Award of the Physical Society of Japan.

Title of Article Anisotropic Magnetoresistance Effects in Fe, Co, Ni, Fe4N, and Half-Metallic Ferromagnet: A Systematic Analysis
Journal J. Phys. Soc. Jpn. 81, 024705 (2012)
Author Satoshi Kokado, Masakiyo Tsunoda, Kikuo Harigaya, and Akimasa Sakuma
Citation

The anisotropic magnetoresistance (AMR) effect has been studied for the longest time among a number of magnetoresistance effects, and it has recently been actively studied as a fundamental phenomenon in the field of spintronics. In this paper, the authors theoretically studied the AMR effect that depends on the s-d scattering process from the conduction band to the localized d state in ferromagnetic transition metal materials. The s-d scattering process, in which only some of the contributions were considered in the previous work, was fully incorporated in this paper, and the most general formula for the AMR ratio was derived. According to this paper, the AMR ratio is proportional to the product of the difference between the localized d-electron up and down spin density of states, and the difference between conduction s-electron up and down spin electrical conductivities at the Fermi level due to spin-orbit and exchange interactions. As a result, the relationship between the sign of the AMR ratio and the primary s-d scattering process that contributes to the AMR effect was clarified. A theoretical formula has been completed with the extreme practicality that applies to a wide range of magnetic materials, such as (strong) ferromagnetism, weak ferromagnetism, and half metal. The most critical example is that screening in an experimental search for half metals has become possible because half metals have a negative AMR ratio. There are also many papers that can be used as starting points for subsequent theoretical studies, such as first-principle calculations. Considering the above points, this paper was recognized to have made an essential contribution to the physical understanding of electrical conductivity in ferromagnetic materials through theoretical research based on original ideas and, hence, deserves the Outstanding Paper Award of the Physical Society of Japan.

Title of Article First-Principles Study of Magnetocrystalline Anisotropy and Magnetization in NdFe12, NdFe11Ti, and NdFe11TiN
Journal J. Phys. Soc. Jpn. 83, 043702 (2014)
Authors Takashi Miyake, Kiyoyuki Terakura, Yosuke Harashima, Hiori Kino, and Shoji Ishibashi
Citation

The lack of heavy rare-earth elements doped in permanent magnets is a serious problem since the demand for permanent magnets utilized in, for instance, motors for electric vehicles increases every year. Based on the background, the authors theoretically studied the magnetic properties of ferromagnetic materials, including NdFe12, to obtain a guideline for designing high-performance permanent magnets. They evaluated the crystal-field parameters at each Nd site by using the first-principles calculation for estimating the magnetic anisotropy based on crystal-field theory. As a result, the authors found that the interstitial N atoms induce strong uniaxial magnetic anisotropy in NdFe12N and NdFe11TiN. They also found that the hybridization between 2p orbitals of the interstitial N atoms and 3d orbitals in Fe atoms plays an important role in the enhancement of magnetization in these materials. Inspired by the theoretical results of this article, NdFe12Nx thin films, which show higher magnetic performances than NdFe14B, have been successfully synthesized.

In conclusion, this article theoretically suggested a promising candidate for high-performance permanent magnets and significantly contributed to the researchers involved in the development of permanent magnets. For these reasons, we conclude that this article deserves the Outstanding Paper Award of the Physical Society of Japan.

Title of Article Can we explain AMS-02 antiproton and positron excesses simultaneously by nearby supernovae without pulsars or dark matter?
Journal Prog. Theor. Exp. Phys. 2016, 021E01 (2016)
Authors Kazunori Kohri, Kunihito Ioka, Yutaka Fujita, and Ryo Yamazaki
Citation

In recent observations of cosmic rays, it has been reported that an anti-particle to particle ratio does not agree with that of the theoretical predictions. That is, clear excesses have been discovered in the fluxes of the cosmic-ray positron and antiproton, compared to the predictions of the standard astrophysical models. In addition to the many theoretical models that have been proposed to explain these excesses by groups in both astrophysics and particle physics communities, the authors of this paper proposed a new theory of nearby old supernova remnants (SNRs), in which high-energy proton was accelerated to be the source for productions of both the observed positron and antiproton. They assumed that the SNRs were born approximately a hundred thousand years ago in molecular clouds located at around a hundred parsec from the solar system. The high-energy protons can scatter off background protons hadronically in the molecular clouds, which then produce both positron and antiproton through hadronic shower processes with their scatterings and decays.

Near the solar system, we observe astronomically diffuse objects, which could be interpreted to be a mark of such old SNRs, and the authors also proposed a method to test their theory by observing anisotropy on directions for their propagation from the sources in the future. It is impressive that their theoretical prediction on a possible antiproton excess reported in their old paper in 2009 agrees with the value observed by the experiment module AMS-02 mounted on the International Space Station in 2015. This paper added more physical interpretations of the observational data and theoretical discussions, making the entire work enormously influential in astrophysics and related fields, including particle physics. For these reasons, we conclude that this paper deserves the Outstanding Paper Award of the Physical Society of Japan.