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 four papers for the 2023 award based on 18 nominations (for 17 papers) made by the editors of the JPS journals and representatives of the 10 divisions of JPS.

The award ceremony for the 28th Outstanding Paper Award will be held at noon on September 18, 2023.

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

Magnetic Properties of Layered Itinerant Electron Ferromagnet Fe₃GeTe₂

J. Phys. Soc. Jpn. 82, 124,711 (2013)

Bin Chen, JinHu Yang, HangDong Wang, Masaki Imai, Hiroto Ohta, Chishiro Michioka, Kazuyoshi Yoshimura, and MingHu Fang

This paper is a pioneering report on the experimental growth of a single crystal of Fe₃GeTe₂, which has recently been the subject of intense research as a two-dimensional itinerant ferromagnet, and on its various physical properties. The authors determined the basic properties of Fe₃GeTe_2, such as its magnetic phase transition and magnetic fluctuation. The critical behavior of the magnetic properties of Fe₃GeTe₂ are due to strong spin fluctuations near the ferromagnetic critical point, reflecting the fact that it is a layered compound weakly coupled by van der Waals interactions, and is well explained by Moriya’s self-consistent renormalization (SCR) theory and Takahashi’s theory developed for quasi-two-dimensional ferromagnets. Furthermore, from the analysis of the Rhodes-Wohlfarth ratio, which is a measure of itinerant and localized properties, it is concluded that Fe₃GeTe₂ is a quasi-two-dimensional itinerant magnet.

Recently, Fe₃GeTe₂ has attracted much attention as one of the few materials that exhibit itinerant ferromagnetism in monolayer systems. Monolayer materials have been intensively studied by the international research community in recent years since the discovery of graphene, and van der Waals layered compounds are one of the main material groups in this field. This paper led to the discovery of Fe₃GeTe₂ as a monolayer itinerant ferromagnet, and its contribution to the research of various physical properties is also appreciated. The pioneering work, focusing on two-dimensional itinerant ferromagnets and its significant influence on recent studies, is now recognized for its high scientific importance. Therefore, this paper deserves the Outstanding Paper Award from the Physical Society of Japan.

Gapless Spin-Liquid Phase in an Extended Spin 1/2 Triangular Heisenberg Model

J. Phys. Soc. Jpn. 80, 093,707 (2014)

Ryui Kaneko, Satoshi Morita, and Masatoshi Imada

Quantum spin liquids, i.e., the phase of a quantum spin system in which the spin is not frozen at absolute zero temperature, have attracted much attention since Anderson proposed the idea in the 1970s. In the early stages of research, the Heisenberg model on the frustrated triangular lattice was considered to be a candidate, but later studies revealed that the ground state of the triangular lattice Heisenberg model with only the nearest-neighbor interaction is antiferromagnetic.

Although the search for quantum spin liquids has continued since then, their existence has not been established due to difficulties in numerical approaches, except for examples with exact solutions such as the Kitaev spin model. In this paper, the authors report the quantum spin liquid phase in the J1-J2 model on a triangular lattice with next-nearest-neighbor interaction in addition to the nearest-neighbor interaction when the ratio of J2 to J1 is in the region between 0.1 and 0.135, using a many-variable variational Monte Carlo method for up to relatively large sizes.

The results presented in this paper have had a strong impact on the study of quantum spin liquids and triggered many related studies. The theoretical model discussed in this paper is still recognized as one of the most promising candidates for quantum spin liquids. It is of great academic significance that this paper suggests a nontrivial quantum spin liquid phase not in a special model like the Kitaev spin model but in a relatively simple spin model. Therefore, this paper deserves the Outstanding Paper Award from the Physical Society of Japan.

Multiple Superconducting Phases and Unusual Enhancement of the Upper Critical Field in UTe₂

J. Phys. Soc. Jpn. 89, 053705 (2020)

Dai Aoki, Fuminori Honda, Georg Knebel, Daniel Braithwaite, Ai Nakamura, DeXin Li, Yoshiya Homma, Yusei Shimizu, Yoshiki J. Sato, Jean-Pascal Brison, and Jacques Flouquet

This paper reports the multiple superconducting phases of the recently discovered superconductor, UTe₂. The authors performed precise measurements of magnetoresistance, specific-heat, and magnetocaloric effect under high pressures. Their high-quality data made it possible to demonstrate that UTe₂ possesses multiple superconducting phases. Such phases are akin to the superfluid phases of ³He. In addition, they found that some of the phases exhibit upper critical fields that exceed the Pauli limit. These results strongly suggest that UTe₂ has spin triplet superconductivity with complex superconducting order parameters. This is an outstanding achievement that deserves the Outstanding Paper Award of the Physical Society of Japan.

Can an off-axis gamma-ray burst jet in GW170817 explain all the electromagnetic counterparts?

Prog. Theor. Exp. Phys. 2018, 043E02

Kunihito Ioka and Takashi Nakamura

The gravitational wave event GW170817, associated with the merger of two neutron stars observed on August 17, 2017, was a historic event that heralded the beginning of multi-messenger astrophysics by observing not only gravitational waves but also electromagnetic counterparts across all wavelength ranges from gamma rays to radio waves. In particular, the observed gamma rays supported a 30-year-old hypothesis that binary neutron star mergers produce short-duration gamma-ray bursts (GRBs). However, the luminosity was three or four orders of magnitude lower than standard one. In this paper, which was released on the same day as the LIGO-Virgo press conference, the authors proposed an off-axis model that naturally explains why the luminosity was lower; this model is now widely accepted as the standard model. This model gives a simple but consistent explanation of the observational results, in which the collimated emission from the relativistic jet appears many orders of magnitude fainter when observed from a direction different from the jet axis. Although different models were proposed shortly after its discovery, the authors’ off-axis model was eventually proven correct when apparent superluminal motion was discovered in long-duration observations of radio waves. The authors have been proposing the possibility of GRBs being observed from off-axis since the early 2000s, and their many years of original research came to fruition in this event. In the future era of multi-messenger astrophysics, the off-axis model of GRBs will be used as the standard model to interpret observational results and will continue to play an important role. This paper is therefore worthy of the JPS Outstanding Paper Award.

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The authors declare that they have no competing interests.