2021 Nishina Memorial Prize

Dr. Takahisa Arima
Professor, Graduate School of Engineering, University of Tokyo,
concurrently appointed as Team Leader, Strong Correlation Quantum Structure ResearchTeam, RIKEN Center for Emergent Matter ScienceandDr. Tsuyoshi Kimura
Professor, Graduate School of Engineering, University of Tokyo

“Discovery and Exploration of Spin-Induced Multiferroics”

Magnetism and dielectricity are among the most fundamental properties of materials.
Application of magnetic fields induces magnetization in many materials as a result of unevenelectronic spin distribution. Application of electric fields, on the other hand, produces a flow ofelectrons (electric current) in metals and, in insulators, asymmetric distribution of electronic
and ionic charge, resulting in electric polarizations. The electromagnetic effect, whereapplication of magnetic fields (electric field) induces electric polarization (magnetization), hasbeen known in certain materials since 1960s. However, this effect was very weak and had nevergrown into a major research field.
This situation has changed significantly in this century. Dr. Takahisa Arima and Dr.
Tsuyoshi Kimura discovered in 2003 together with coworkers that a change of magneticstructure induces a ferroelectric state in the magnetic insulator TbMnO3 [1]. This material had
been known to show an incommensurate magnetic order of Mn3+ spins below the Neeltemperature TN=41 K. The ferroelectric order with a spontaneous polarization along c-axis of
the crystal was observed at zero magnetic field below TC=27 K, at which a change of themagnetic structure had been suggested. Furthermore, this polarization along c-axis wassuppressed by magnetic fields along b-axis, which in turn induced a polarization along a-axis.
“Ferroics” is the term to collectively describe the states, in which microscopic magnetic orelectric moments of atoms and ions are ordered in a macroscopic scale, such as ferro(antiferro-) magnetic and ferro- (antiferro-) electric states. This discovery indicated the presenceof an unknown coupling between two different ferroics.
Because of the relativistic spin-orbit interaction, the shape of electronic orbital relevant tothe superexchange interaction of Mn-O-Mn bonding in TbMnO3 depends on the relativeorientation of two Mn spins. For non-collinear spin arrangements, it was theoretically shownby Katsura, Nagaosa, and Balatsky that the distorted orbital shape should lead to thespontaneous electric polarization unless the vector connecting the two spins is orthogonal to thespin plane. Subsequently, Dr. Arima and coworkers investigated the spin structure of TbMnO3
by neutron diffraction experiments and confirmed a collinear spin structure above TC with asinusoidal modulation in the amplitude of Mn moments and a non-collinear cycloidal spin
structure below TC, consistent with the theory [2].
Since non-collinear spin structures are rather ubiquitous for magnets with competingexchange interactions, coexistence of ferroelectricity and magnetic order has been confirmed
in a large number of frustrated magnets after the discovery in TbMnO3. Many of these materials
enable us to control electric polarization by magnetic fields or magnetization by electric fields,which are also important for application. Thus multiferroics has rapidly grown into a major
field of condensed matter physics.
Dr. Takahisa Arima has continued to contribute to the development of multiferroics
through precise structural analysis by X-ray and neutron diffraction and through proposal of a
new mechanism for the electric polarization in CuFeO2, which has a proper screw spin structure[3, 4]. Dr. Tsuyoshi Kimura has succeeded in synthesizing many new multiferroic materials,
including a hexaferrite system exhibiting magnetoelectric effects at room temperature and low
magnetic fields, demonstrating universality of multiferroics and possibility for applications[5−7].
References:[1] T. Kimura, T. Goto, H. Shintani, K. Ishizaka, T. Arima, and Y. Tokura, “Magnetic controlof ferroelectric polarization”, Nature 426, 55 (2003).
[2] T. Arima, A. Tokunaga, T. Goto, H. Kimura, Y. Noda, and Y. Tokura, “Collinear to Spiral
Spin Transformation without Changing the Modulation Wavelength upon FerroelectricTransition in Tb1-xDyxMnO3”, Phys. Rev. Lett. 96, 097202 (2006).
[3] K. Taniguchi, N. Abe, T. Takenobu, Y. Iwasa, and T. Arima, “Ferroelectric Polarization
Flop in a Frustrated Magnet MnWO4 Induced by a Magnetic Field”, Phys. Rev. Lett. 97,097203 (2006).
[4] T. Arima, “Ferroelectricity Induced by Proper-Screw Type Magnetic Order”, J. Phys. Soc.
Jpn. 76, 073702 (2007).
[5] T. Goto, T. Kimura, G. Lawes, A. P. Ramirez, and Y. Tokura, “Ferroelectricity and Giant
Magnetocapacitance in Perovskite Rare-Earth Manganites”, Phys. Rev. Lett. 92, 257201(2004).
[6] T. Kimura, J. C. Lashley, and A. P. Ramirez; “Inversion-symmetry breaking in thenoncollinear magnetic phase of the triangular-lattice antiferromagnet CuFeO2”, Phys. Rev.
B 73, 220401(R) (2006).
[7] Y. Kitagawa, Y. Hiraoka, T. Honda, T. Ishikura, H. Nakamura, and T. Kimura, “Low-fieldmagnetoelectric effect at room temperature”, Nat. Mater. 9, 797 (2010).

Dr. Masato Takita
Professor, Institute for Cosmic Ray Research, the University of Tokyo“Establishment of the sub-PeV gamma ray astronomy and elucidation of the origin of galactic
cosmic rays”

Modifying an observational instrument for cosmic-ray induced air showers located at
4,300 m in Tibet, China and observing high-energy cosmic gamma rays, Dr. Masato Takita andhis collaborators succeeded in detection of sub-PeV (1014 eV − 1015 eV = 100 TeV − 1 PeV)gamma rays for the first time in the world and elucidated that PeV cosmic rays originate in ourGalaxy. These achievements have led to the establishment of the sub-PeV gamma-rayastronomy.
As cosmic rays have an electric charge and they are bent by the galactic magnetic field in
our Galaxy, high-energy gamma rays from π0 produced by accelerated cosmic rays colliding
with the interstellar medium have been attracting attention as a means to elucidate their origin.
Dr. Takita proposed and applied a method to separate a cosmic-ray induced muon-rich airshower from a gamma-ray induced muon-poor air shower by installing a large (3,400 m2 inarea) water Cherenkov-type muon detector (large water tanks with 20-inch-in-diameterphotomultiplier tubes), at 2.4 m underground of the Tibet ASγ air shower surface detector array(65,700 m2 in area).
In 2019, the Tibet ASγ experiment succeeded in the first detection of gamma rays beyond
100 TeV from the Crab Nebula at a 5.6 σ significance (Ref. [1]). Following the first detection,the Tibet ASγ experiment observed gamma rays above 100 TeV not only from the two sources(TASG J2019 +368 and TASG J2032 +414) in the Cygnus constellation direction (Ref. [2]),but also from a supernova remnant G106.3+2.7 for the first time (Ref. [3]). Especially, thegamma rays from G106.3+2.7 are identified as those of the π0 origin produced by cosmic-ray,
based on discussions on their origin (not from the pulsar, but from the overlapping molecularclouds), other-wavelength observations, and the energy budget. Thus, the observation
demonstrates for the first time that G106.3+2.7 is a potential candidate object that accelerates
cosmic rays up to PeV energies.
Furthermore, in 2021, the Tibet ASγ experiment discovered many diffuse gamma rays
with energies between 400 TeV and 1 PeV, coming ubiquitously along the Galactic disk at a
5.9 σ significance, and what is more surprising, their directions turned out to be away from theknown TeV gamma-ray sources in our Galaxy (Ref. [4]). The detected gamma rays areconcluded to be produced by collisions of PeV cosmic rays emitted from cosmic-ray sources inour Galaxy with interstellar medium in the Galactic disk and their features are consistent withtheoretical predictions. This is the first experimental evidence for the belief since 1958 that PeV
cosmic rays originate in our Galaxy.
Although these research achievements belong to a Japan-China international joint researchgroup, the Tibet ASγ collaboration, Dr. Takita, who directs the collaboration, played anessential role in these discoveries, and his contribution is judged to be sufficiently significant.
References:[1] M. Amenomori et al. (Tibet ASγ collaboration), “First Detection of Photons with Energy
beyond 100 TeV from an Astrophysical Source”, Phys. Rev. Lett. 123, 051101 (2019).
[2] M. Amenomori et al. (Tibet ASγ collaboration), “Gamma-Ray Observation of the Cygnus
Region in the 100-TeV Energy Region”, Phys. Rev. Lett. 127, 031102 (2021).
[3] Tibet ASγcollaboration, “Potential PeVatron supernova remnant G106.3+2.7 seen in thehighest-energy gamma rays”, Nature Astronomy 5, 460 (2021).
[4] M. Amenomori et al. (Tibet ASγ collaboration), “First Detection of sub-PeV Diffuse
Gamma Rays from the Galactic Disk: Evidence for Ubiquitous Galactic Cosmic Rays
beyond PeV Energies”, Phys. Rev. Lett. 126, 141101 (2021).

Dr. Satoshi Miyazaki
Professor, Advanced Technology Center, National Astronomical Observatory"Observational cosmology through the development of wide field cameras for Subaru
Telescope"

Dark matter and dark energy are major mysteries confronting physics and cosmology inthe 21st century. Investigating their distribution and temporal changes in the universe byanalyzing the distribution of galaxies over the cosmological scale could be a clue forunderstanding their nature.
In the 1990s, Dr. Satoshi Miyazaki began developing large format CCD devices incollaboration with the members of the Lincoln Laboratory of Massachusetts Institute ofTechnology. For observations of galaxies in distant universe, the high sensitivity in the nearinfrared is an important advantage due to cosmological redshift. He succeeded in developing alarge, back-illuminated CCD device with four times the sensitivity of conventional CCDs at
950 nm together with his collaborators. The CCD device has a rectangular form and keeps highsensitivity up to the edge in three directions. Ten such devices are arranged in a mosaic
configuration such that wide field imaging is possible with almost no gaps. His group completed
this unique camera, Suprime-Cam, in 2000 in time for the first light of the Subaru Telescope.
This camera has become an unrivaled instrument in the world [1] and has made significant
achievements in the studies of galaxies in the early universe and the identification of the cosmic
reionization epoch.
Based on this successful operation, he proposed developing an ambitious Hyper SuprimeCam (HSC) with a field of view seven times larger than that of Suprime-Cam. The successfulfabrication of the HSC in 2013 [2] was led by incorporating, (1) the development of a CCD that
doubles the sensitivity at a wavelength of 1 micron by dramatically increasing the thickness ofthe depletion layer, (2) the development of a corrective lens system and atmospheric dispersion
correction mechanism to achieve high resolution images over a wide field of view, and (3) thedevelopment of a lightweight ceramic housing tube of the camera that enables mounting on theSubaru Telescope.
The cosmic background radiation observation satellite WMAP, launched in 2001, hasverified the overall validity of the standard cosmic evolution model called the ΛCDM model.
However, the number of galaxy clusters predicted by this model is known to be much larger
than the numbers identified by X-rays. He proposed a project to search for faint clusters ofgalaxies by measuring systematically the "weak gravitational lensing effect" caused by thegravitational field of dark matter in the clusters. His group succeeded to identify the distributionof dark matter using high quality images from Suprime-Cam [3]. In the latest study using theHSC, 95% of the detected dark matter halos are confirmed to have corresponding faint clustersof galaxies. However, the number count of galaxy clusters is still lower than the theoreticalprediction, which might indicate the need to revise the standard cosmic evolution model.
In 2014, he proposed a strategic plan to survey distant universe using the HSC over a totalof 330 nights, as a representative of an international joint observation project involving theNational Astronomical Observatory of Japan, the Kavli Institute for the Physics andMathematics of the Universe, and Princeton University to study dark matter and dark energy.
The survey group has already published 40 papers from the first 90 nights of data in a special
issue of the Publications of the Astronomical Society of Japan, in January 2018.
In addition to the aforementioned study on the identification of dark matter haloes using
weak gravitational lensing, his group revealed a larger-scale three-dimensional spatial
distribution of dark matter haloes from a study of weak gravitational lensing at different times
over billions of years. As a result, they revealed how dark matter haloes coalesced into clusters
over the billion years of cosmic time [4].
His group also succeeded for the first time in determining the parameters of the cosmic
evolution model from the analysis of the power spectrum of the weak gravitational lensing
effect seen in the HSC data at redshifts up to about 1. The results suggest that there may be a
deviation from the parameters of the standard cosmological model obtained from the analysisof the cosmic background radiation at redshift 1000 [5].
Although most of these studies are the results of collaborative research, they were notpossible without the contribution of Dr. Satoshi Miyazaki, who made a large amount of datawith the world's highest image quality and maximum sensitivity available to the science
community with his meticulous efforts to minimize systematic errors in HSC images. He notonly led the production of the mosaic CCD camera, but also performed various data calibration
analyses and released high-quality image data to provide astronomers around the world with
opportunities to use the Subaru Telescope for science. He has led a strategic observation
program to elucidate dark matter and dark energy and thereby has made a significant
contribution to the advancements of astrophysics and astronomy.
References:[1] “Subaru Prime Focus Camera – Suprime-Cam”, Miyazaki, S. et al., 2002a, PASJ, 54, 833[2] “Hyper Suprime-Cam: System design and verification of image quality”, Miyazaki, S. etal., 2018a, PASJ, 70, 1[3] “A large sample of shear-selected clusters from the Hyper Suprime-Cam Subaru Strategic
Program S16A Wide field mass maps”, Miyazaki, S. et al., 2018b, PASJ, 70, 27[4] “Two-and three-dimensional wide-field weak lensing mass maps from the Hyper SuprimeCam Subaru Strategic Program S16A data”, Oguri, M., Miyazaki, S., Hikage, C. et al.,2018, PASJ, 70, 26.
[5] “Cosmology from cosmic shear power spectra with Subaru Hyper Suprime-Cam first-yeardata”, Hikage, C., Oguri, M., Hamana, T. et al., 2019, PASJ, 71, 43.