Citation of the 2019 Nishina Memorial Prize1) Awardee:

Dr. Yoshihiro Iwasa
Professor, Graduate School of Engineering, University of Tokyo

Title of Research Achievements:“Discovery of Electric-field Induced Two-dimensional Superconductivity”
Abstract of Research Achievements:
Thin film superconductors have played a central role in the field of superconductivity, from basic
physical properties to device applications. Main materials in these superconductors are metal vapordeposition films such as Nb, Al, and MoGe, and it has been experimentally shown that thesesuperconducting properties are different from those of bulk three-dimensional systems. Furthermore,in this century, nanotechnology such as thin film fabrication and device fabrication has progressed
dramatically, and superconductors with thickness of 1 nanometer or less and sometimes consisting of
monoatomic layers with high crystallinity have been produced.
Dr. Yoshihiro Iwasa developed a method of high-density carrier-doping by a gate voltage and foundsuperconductivity in two-dimensional systems on oxide insulators and chalcogenide semiconductors[1]. In this method, the carrier doping is achieved by a gate voltage as in the field-effect transistor(FET) consisting of a metal-insulator-semiconductor structure. In the FET structure that constitutes
silicon integrated circuits, high carrier concentration sufficient for superconductivity cannot be
achieved because of breakdown of the insulator.
Dr. Iwasa realized the high-density carrier-doping by replacing the insulator of an FET structurewith an ionic conductor that is an electronic insulator but capable of conducting electricity by ionicmotion. This technique enables the control of carrier concentration for a range much wider than FETwith less influence of impurities on carriers in comparison with other methods like elemental
substitution. His research group discovered, for the first time in the world, the electric-field induced
superconductivity, where the electrical resistivity of induced carriers vanishes above a critical carrier
concentration when it is increased just by changing a gate voltage. Further, his group obtained anelectronic phase diagram ranging from semiconductor to superconductor [2] and a magnetic fieldtemperature phase diagram with large in-plane and out-of-plane anisotropy of the superconducting
transition temperature [3, 4].
This technique of converting an insulator to a superconductor by electric-field induced carrier
doping provides an important experimental method that can eliminate impurity effects in the study ofquantum spin liquids, strongly correlated-electron superconductivity, etc. In addition, it has alreadybeen applied to many other cutting-edge fields in condensed matter research leading to newdevelopments.
References[1] “Electric-field-induced superconductivity in an insulator”, K. Ueno, S. Nakamura, H. Shimotani,A. Ohtomo, N. Kimura, T. Nojima, H. Aoki, Y. Iwasa, and M. Kawasaki, Nature Materials 7, 855(2008).
[2] “Superconducting dome in a gate-tuned band insulator”, J. T. Ye, Y. J. Zhang, R. Akashi, M. S.
Bahramy, R. Arita, and Y. Iwasa, Science 338, 1193 (2012).
[3] “Superconductivity protected by spin-valley locking in ion-gated MoS2”, Y. Saito, Y. Nakamura,
M. S. Bahramy, Y. Kohama, J. T. Ye, Y. Kasahara, Y. Nakagawa, M. Onga, M. Tokunaga, T.
Nojima, Y. Yanase, and Y. Iwasa, Nature Physics 12, 144 (2016).
[4] “Metallic ground state in an ion-gated two-dimensional superconductor”, Y. Saito, Y. Kasahara, J.
T. Ye, Y. Iwasa, and T. Nojima, Science 350, 409 (2015).
[5] “Highly crystalline 2D Superconductors”, Y. Saito, T. Nojima, and Y. Iwasa, Nature Reviews
Materials 12, 144 (2017).

2) Awardees:

Dr. Shigeru Yoshida
Professor, Graduate School of Science, Chiba University

Dr. Aya Ishihara
Professor, Institute for Global Prominent Research, Chiba University

Title of Research Achievements:“Discovery of Ultra-High Energy Cosmic Neutrinos”
Abstract of Research Achievements:
Dr. Shigeru Yoshida and Dr. Aya Ishihara have made essential contributions to the discovery ofultra-high-energy cosmic neutrinos with the IceCube Neutrino Observatory, bringing new insights tothe fields of cosmic ray physics and high-energy neutrino astronomy. While cosmic rays with energies
up to 1020 eV have been observed, their production sources and acceleration mechanisms are largely

unknown. Due to various collisions and magnetic bending while traveling through space, the originsof protons and nuclei cannot be easily reconstructed. Thus, observation of neutrinos is a powerfulmeans to probe information of cosmic rays immediately after their productions.
The IceCube Neutrino Observatory is an array of 5,160 optical sensors embedded in a 1 km3 volumeof deep ice at the South Pole. The optical sensors capture Cherenkov photons radiated by charged
particles in the form of showers and tracks, produced by reactions of cosmic neutrinos inside or nearby
the detector array. Dr. Yoshida and Dr. Ishihara established an analysis method capable of reliably
detecting ultra-high-energy neutrinos. By using this method, two shower events initiated by PeVenergy neutrinos were found in the 20112012 data set [1]. This is the first identification of ultra-highenergy cosmic neutrino candidates, observed with significance of 2.8 sigma (99.71 % confidencelevel). The extensive analysis following this finding has shown that the strongly evolved classes ofastronomical objects such as radio-loud galaxies, vastly more active at the far universe, are unlikely
to be a major origin of ultra-high-energy cosmic rays [2]. This was the first meaningful constraint oncharacteristics of ultra-high-energy cosmic ray origins, realized by neutrino observations.
This very first discovery of PeV-energy neutrinos clarified some characteristics of cosmic neutrinosignals, which prompted a follow-up search to find more neutrino-induced showers at lower energies
using IceCube. The 37 events in total with energies from 30 TeV to 2 PeV were found in the threeyear data set taken during 20102013 [3]. This observation clearly shows the existence of a neutrino
population beyond atmospheric backgrounds of neutrinos and muons. It brought strong evidence ofastrophysical neutrinos to a statistical significance of 5.7 sigma (99.9999994 % confidence level).
After this achievement, the IceCube Collaboration began identification of cosmic neutrino
candidates in real-time and a new online data processing flow where alerts of the observational
information, such as their energy and arrival direction, are sent to the global astronomical community
starting in 2016. Using this system, the high-energy cosmic neutrino event called “IceCube-170922A”
was detected and immediately followed by an alert to the world astronomical facilities on September
23, 2017 (Japan Time). Rapid follow-up observations, based on the neutrino direction, discovered thatthe blazar galaxy housing a massive black hole called “TXS 0506+056” was found to be moreluminous than usual in the visible spectrum. It prompted the rapid analysis by the Fermi-LAT teamfinding that this galaxy was undergoing a giant gamma-ray flare [4]. The MAGIC atmospheric
Cherenkov gamma-ray telescope also detected very high-energy gamma rays extending above 100GeV. The identification of neutrino sources through observations with gamma rays and neutrinos has
opened up a new era of multi-messenger astronomy led by neutrinos. Dr. Yoshida and Dr. Ishihara
both played important roles in the IceCube neutrino alert system and the analyses of IceCube170922A.
References

[1]“First observation of PeV-energy neutrinos with IceCube”, IceCube Collaboration, Phys. Rev.
Lett. 111, 021103 (2013).

[2]“Probing the origin of cosmic rays with extremely high energy neutrinos using the IceCube
Observatory”, IceCube Collaboration, Phys. Rev. D 88, 112008 (2013).

[3]“Observation of high-energy astrophysical neutrinos in three years of IceCube data”, IceCubeCollaboration, Phys. Rev. Lett. 113, 101101 (2014).

[4]“Multimessenger observations of a flaring blazar coincident with high-energy neutrino IceCube170922A”, The IceCube Collaboration et al., Science 361, eaat1378 (2018).