In recognition of important achievements toward progress in physics, the Physical Society of Japan (JPS) annually selects outstanding papers from 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 five papers for the 2021 award based on 19 nominations (for 16 papers) made by the editors of the JPS journals and representatives of the 19 divisions of the JPS.

On the morning of March 14, 2021, the 2021 award ceremony was held as a Zoom webinar.

The titles of the five selected papers, together with their citations, follow below.

1. |
Observation of a Be double-Lambda hypernucleus in the J-PARC E07 experiment **Prog. Theor. Exp. Phys. 2019, 021D02 (2019) H Ekawa, K Agari, J K Ahn, T Akaishi, Y Akazawa, S Ashikaga, B Bassalleck, S Bleser, Y Endo, Y Fujikawa, N Fujioka, M Fujita, R Goto, Y Han, S Hasegawa, T Hashimoto, S H Hayakawa, T Hayakawa, E Hayata, K Hicks, E Hirose, M Hirose, R Honda, K Hoshino, S Hoshino, K Hosomi, S H Hwang, Y Ichikawa, M Ichikawa, M Ieiri, K Imai, K Inaba, Y Ishikawa, A Iskendir, H Ito, K Ito, W S Jung, S Kanatsuki, H Kanauchi, A Kasagi, T Kawai, M H Kim, S H Kim, S Kinbara, R Kiuchi, H Kobayashi, K Kobayashi, T Koike, A Koshikawa, J Y Lee, J W Lee, T L Ma, S Y Matsumoto, M Minakawa, K Miwa, A T Moe, T J Moon, M Moritsu, Y Nagase, Y Nakada, M Nakagawa, D Nakashima, K Nakazawa, T Nanamura, M Naruki, A N L Nyaw, Y Ogura, M Ohashi, K Oue, S Ozawa, J Pochodzalla, S Y Ryu, H Sako, Y Sasaki, S Sato, Y Sato, F Schupp, K Shirotori, M M Soe, M K Soe, J Y Sohn, H Sugimura, K N Suzuki, H Takahashi, T Takahashi, Y Takahashi, T Takeda, H Tamura, K Tanida, A M M Theint, K T Tint, Y Toyama, M Ukai, E Umezaki, T Watabe, K Watanabe, T O Yamamoto, S B Yang, C S Yoon, J Yoshida, M Yoshimoto, D H Zhang, Z Zhang** |

The lambda particle is the lightest hyperon that contains one strange quark. In order to determine the structure of neutron stars, one of the most important pieces of information is the interaction between lambda particles. This interaction plays a major role in gravitational waves generated from the merger of neutron stars and in r-process nucleosynthesis. Because the lambda particle is short-lived, studying the structure of double lambda hypernuclei is critical to determining the interaction between two lambda particles. This paper reported on the successful production of a double hypernucleus of ^{11}_{ΛΛ}Be. The experiment was performed at J-PARC by combining an emulsion technique and high-resolution spectrometry of charged particles. Even though seven events had previously been reported for the production of double hypernuclei, the nuclide of the produced hypernuclei had been uniquely identified in only one case. The present study constitutes the second case in which nuclides were successfully identified. The experimental data had much higher statistics than in previous measurements, and the _{ΛΛ} binding energy ΔB_{ΛΛ} could be determined with higher precision. The data confirmed that the attraction between L particles was significantly weaker than that between nucleons. Moreover, it was determined that the value of ΔB_{ΛΛ} of ^{11}_{ΛΛ}Be was significantly different from that of ^{6}_{ΛΛ}He, indicating that the value of ΔB_{ΛΛ} depended on the core nucleus in a hypernucleus. These results significantly improve our understanding of hyperon-hyperon interactions. It was therefore determined that this article deserves the Outstanding Paper Award of the Physical Society of Japan.

2. |
Superconductivity in Ca1-xLaxFeAs2: A Novel 112-Type Iron Pnictide with Arsenic Zigzag Bonds J. Phys. Soc. Jpn. 82, 123702 (2013) Naoyuki Katayama, Kazutaka Kudo, Seiichiro Onari, Tasuku Mizukami, Kento Sugawara, Yuki Sugiyama, Yutaka Kitahama, Keita Iba, Kazunori Fujimura, Naoki Nishimoto, Minoru Nohara, and Hiroshi Sawa |

Following the discovery of iron-pnictide superconductors (*T*_{c}~26 K) by Hosono's group, tremendous efforts have been devoted to synthesizing new superconductors with similar crystal structures and higher superconducting transition temperatures (*T*_{c}). The highest *T*_{c} in this class of materials has increased to 55 K. The following research aimed at increasing *T*_{c} and developing new structures to investigate the relationship between the superconductivity mechanism and electronic dimensionality. For this latter purpose, various spacer layers between FeAs layers were developed.

This paper reported on the discovery of a new iron-based superconductor with a zigzag As chain as a spacer layer. Its distinct properties are described as follows: (i) the crystal structure is monoclinic, in contrast to most of the iron-based superconductors, which have an orthorhombic structure; (ii) the spacer layer is a one-dimensional zigzag chain of the As dimer; and (iii) although a sharp transition was observed at 34 K, the presented data indicated the possibility of a higher *T*_{c} of 45 K, which may be realized by adjusting the compositions.

Although signs of superconductivity in the Ca-La-Fe-As system were already known since the early stages of research on iron-based superconductors, its composition and structure have remained unknown. The present study provided a clear solution to this problem by synthesizing a theoretically proposed compound. This paper qualifies as a high-level experimental study and distinguishes itself from many other works reporting on new superconductors in the following points: the growth of high-quality single crystals, the determination of the electron density distribution map by maximum entropy method (MEM) analysis using X-ray diffraction data taken at SPring-8, and the determination of the Fermi surface by first-principles calculation.

Motivated by this study, many researchers have investigated this system and have actually reported a higher *T*_{c} (> 40 K) than that suggested in this study. This paper has contributed significantly to the study of iron-based superconductors, which is evidenced by the more than 100 citations. This work is a good example of high level Japanese research in the field of iron-based high-temperature superconductors, and therefore, this paper deserves the Outstanding Paper Award from the Physical Society of Japan.

3. |
Superconductivity of Au-Ge-Yb Approximants with Tsai-type Clusters **J. Phys. Soc. Jpn. 84, 023705 (2015) Kazuhiko Deguchi, Mika Nakayama, Shuya Matsukawa, Keiichiro Imura, Katsumasa Tanaka, Tsutomu Ishimasa, and Noriaki K. Sato** |

The development of new superconductors and the elucidation of their non-trivial superconducting mechanisms have been vigorously studied globally. In particular, the following questions related to this area of research have been posed: "Does superconductivity appear in quasi crystals without crystal translational symmetry?" and "What is the mechanism of superconductivity?" Studies have been conducted to answer these perplexing questions.

In this paper, the authors reported on the discovery of superconductivity for two novel Tsai-type cluster compounds: AGY(I) [Au_{64.0}Ge_{22.0}Yb_{14.0}, *T*_{c} = 0.68 K] and AGY(II) [Au_{63.5}Ge_{20.5}Yb_{16.0}, *T*_{c} = 0.36 K]. These are called "approximant crystals" and are located between ordinary crystals and "quasicrystals". In particular, in AGY(II), structural analysis has revealed that magnetic Yb ions occupy the center of the Tsai cluster, and the superconducting mechanism has been investigated in terms of its correlation with magnetism. Recently, the authors also discovered superconductivity (*T*_{c} = 0.05 K) in the "quasicrystal" Al-Zn-Mg [K. Kamiya, et al., Nat. Commun. 9, 154 (2018)], which is a development of this "approximant crystal," and non-trivial superconductivity has been investigated. In this paper, the first superconductivity in "approximant crystals," which is the cornerstone of "quasicrystals" showing superconductivity, is reported, and the mechanism is described as a curious superconductivity involving magnetic ions, which is a discovery that has tremendous impact. For these reasons, we determined that the present paper deserves the Outstanding Paper Award of the Physical Society of Japan.

4. |
Anomaly Polynomial of General 6D SCFTs **Prog. Theor. Exp. Phys. 2014, 103B07 (2014) Kantaro Ohmori, Hiroyuki Shimizu, Yuji Tachikawa, Kazuya Yonekura** |

In this study, the authors developed a method to obtain anomaly polynomials systematically in general six-dimensional *N* = (2,0) and *N* = (1,0) superconformal field theories (SCFT), and they demonstrated this in many concrete examples. The study of this type of six-dimensional SCFT is important in both string theory and quantum field theory. It often appears in string theory, as it is realized on a six-dimensional object called an M5-brane in M-theory, which is considered to be a unified theory of string theory. Furthermore, it has remarkable properties with profound applications. For example, dualities in some four-dimensional gauge theories can be derived by compactifying this six-dimensional theory to four dimensions. However, in general, analysis of this theory is extremely difficult, as no reliable weakly coupled description exists. Even the Lagrangian has not been written down in a covariant form because it contains a rank-2 antisymmetric tensor field with a self-dual field strength. The major achievement of this study is that it succeeds in determining the structure of an anomaly by circumventing all these difficulties.

The authors noticed that even if the scalar field related to the antisymmetric tensor field by supersymmetry acquired a non-zero vacuum expectation value, the symmetry of interest was not broken and the anomaly remained unchanged. Based on this fact, an analysis was performed in an appropriate vacuum in which the low-energy effective theory became simpler. In this case, the contribution of the anomaly from the one loop diagram was already known, and what they needed to evaluate was the contribution of the so-called GS term found by Green and Schwarz, which may exist in a theory with antisymmetric tensor fields. They showed that the GS term could be determined in quite general cases by imposing gauge anomaly cancellation and by comparing the results with compactified theories. Applying this idea, they successfully obtained explicit expressions for various systems, reproduced known results, and provided new formulas.

They also made an interesting observation, namely, that the number of physical degrees of freedom suggested by the anomaly analysis was proportional to *Q*^{3}, where *Q* is the number of M5-branes in the large *Q* limit. This was consistent with the prediction of the AdS_{7}/CFT_{6} correspondence in M-theory. They also confirmed that their results were consistent with the anomaly inflow in M-theory.

This paper provided reliable and fulfilling insights and has had tremendous influence as a basic study in this field. Therefore, we determined that this paper deserves the Outstanding Paper Award of the Physical Society of Japan.

5. |
Quantum Thermal Hall Effect in a Time-Reversal-Symmetry-Broken Topological Superconductor in Two Dimensions: approach from Bulk Calculations **J. Phys. Soc. Jpn. 82, 023602 (2013) Hiroaki Sumiyoshi and Satoshi Fujimoto** |

This study examined the thermal Hall conductivity (THC) of two-dimensional topological superconductors. The authors derived the formula of THC from the Bogoliubov-de Gennes (BdG) equation and showed that its coefficient is given by half of the Chern number, that is, it is quantized to one-half of that in the quantum Hall system. This half quantization itself was predicted by other authors based on the picture of the Majorana edge state. The present work is original in the derivation from the BdG equation that describes bulk states and in expressing the result using the bulk topological (Chern) number. The formula was derived without limiting the generality and is quite versatile, e.g., it is not restricted to two dimensions. In fact, the formula has been applied to three-dimensional chiral superconductors with a nodal gap [N. Yoshioka et al., J. Phys. Soc. Jpn. **87**, 124602 (2018)]. As observed experimentally in a Kitaev magnet α-RuCl^{3}, [Y. Kasahara et al., Nature **559**, 227 (2018)] half quantization of THC is currently a hot topic in the field of topological materials, to which this study made a significant contribution.

In their calculation, the authors used Luttinger's gravitational potential and followed the recent leading-edge transport theory that considers energy magnetization along with the Kubo formula [T. Qin et al., Phys. Rev. Lett. **107**, 236601 (2011)]. Examining carefully the crucial role in the half quantization played by the particle-hole symmetry of the BdG equation, the present work extended the aforementioned transport theory for normal metals and insulators to superconductors.

These discoveries show that this paper deserves the Outstanding Paper Award of the Physical Society of Japan.