Affiliation Faculty of Science and Engineering, Waseda Research Institute for Science and Engineering Job title Senior Researcher(Professor) Homepage URL

### Education 【 display / non-display 】

• 1976.04
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1981.03

University of Tokyo   Faculty of Science   Department of Physics

• 東京大学   理学博士

### Research Experience 【 display / non-display 】

• 2019.04
-
Now

Waseda University   Waseda Research Institute for Science and Engineering   Senior Researcher/Research Professor

• 2019.04
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Now

Toyota Physical and Chemical Research Institute   Fellow

•
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Now

University of Tokyo   professor Emeritus

• 2006.04
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2019.03

University of Tokyo   Department of Applied Physics   Professor

• 1997.09
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2006.03

University of Tokyo   Institute for Solid State Physics   Professor

### Professional Memberships 【 display / non-display 】

•

THE PHYSICAL SOCIETY OF JAPAN

### Research Areas 【 display / non-display 】

• Magnetism, superconductivity and strongly correlated systems

• Semiconductors, optical properties of condensed matter and atomic physics

• Computational science

• Mathematical physics and fundamental theory of condensed matter physics

### Research Interests 【 display / non-display 】

• fluid mechanics

• topological materials

• computational physics

• quantum many-body physics

• superconductivity

### Papers 【 display / non-display 】

• Resonant Inelastic X-Ray Scattering Spectra of Cuprate Superconductors Predicted by Model of Fractionalized Fermions

Journal of the Physical Society of Japan   90 ( 7 ) 074702 - 074702  2021.07  [Refereed]

• High-temperature superconductivity

Xingjiang Zhou, Wei-Sheng Lee, Masatoshi Imada, Nandini Trivedi, Philip Phillips, Hae-Young Kee, Paivi Torma, Mikhail Eremets

NATURE REVIEWS PHYSICS   3 ( 7 ) 462 - 465  2021.07

View Summary

Despite decades of intense theoretical, experimental and computational effort, a microscopic theory of high-temperature superconductivity is not yet established. Eight researchers share their contributions to the search for a better understanding of unconventional superconductivity and their hopes for the future of the field.

• Local moments versus itinerant antiferromagnetism: Magnetic phase diagram and spectral properties of the anisotropic square lattice Hubbard model

Physical Review B   103 ( 12 )  2021.03  [Refereed]  [International journal]

View Summary

Using a cluster extension of the dynamical mean-field theory (CDMFT) we map out the magnetic phase diagram of the anisotropic square lattice Hubbard model with nearest-neighbor intrachain t and interchain t(perpendicular to) hopping amplitudes at half filling. A fixed value of the next-nearest-neighbor hopping t' = -t(perpendicular to)/2 removes the nesting property of the Fermi surface and stabilizes a paramagnetic metal phase in the weak-coupling regime. In the isotropic and moderately anisotropic regions, a growing spin entropy in the metal phase is quenched out at a critical interaction strength by the onset of long-range antiferromagnetic (AF) order of preformed local moments. It gives rise to a first-order metal-insulator transition consistent with the Mott-Heisenberg picture. In contrast, a strongly anisotropic regime t(perpendicular to)/t less than or similar to 0.3 displays a quantum critical behavior related to the continuous transition between an AF metal phase and the AF insulator. Hence, within the present framework of CDMFT, the opening of the charge gap is magnetically driven as advocated in the Slater picture. We also discuss how the lattice-anisotropy-induced evolution of the electronic structure on a metallic side of the phase diagram is tied to the emergence of quantum criticality.

• Single-Particle Spectral Function Formulated and Calculated by Variational Monte Carlo Method with Application to d -Wave Superconducting State

Physical Review X   10 ( 4 )  2020.11  [Refereed]

View Summary

A method to calculate the one-body Green's function for ground states of correlated electron materials is formulated by extending the variational Monte Carlo method. We benchmark against the exact diagonalization (ED) for the one- and two-dimensional Hubbard models of 16-site lattices, which proves high accuracy of the method. The application of the method to a larger-sized Hubbard model on the square lattice correctly reproduces the Mott insulating behavior at half-filling and gap structures of the d-wave superconducting state of the hole-doped Hubbard model in the ground state optimized by enforcing the charge uniformity, evidencing a wide applicability to strongly correlated electron systems. From the obtained d-wave superconducting gap of the charge-uniform state, we find that the gap amplitude at the antinodal point is several times larger than the experimental value when we employ a realistic parameter as a model of the cuprate superconductors. The effective attractive interaction of carriers in the d-wave superconducting state inferred for an optimized state of the Hubbard model is as large as the order of the nearest-neighbor transfer, which is far beyond the former expectation in the cuprates. We discuss the nature of the superconducting state of the Hubbard model in terms of the overestimate of the gap and the attractive interaction in comparison to the cuprates.

• Ido, K., Imada, M., Misawa, T.

Physical Review B   101 ( 7 )  2020  [Refereed]

### Books and Other Publications 【 display / non-display 】

• 統計力学 = Statistical mechanics 1

東京大学工学教程編纂委員会, 宮下 精二, 今田 正俊

東京大学  2018

• 計算と物質

押山, 淳, 天能, 精一郎, 杉野, 修, 大野, かおる, 今田, 正俊, 高田, 康民

岩波書店  2012.07 ISBN: 9784000113038

• p電子系およびナノ構造物質における強相関第一原理手法による電子状態の解明

今田 正俊

今田正俊  2008

• Physics of strongly correlated electron systems : proceedings of the Yukawa International Seminar 2004 : (YKIS 2004) : Yukawa Institute for Theoretical Physics, Kyoto University, November 1-19, 2004

Yukawa International Symposium, 今田, 正俊, 川上, 則雄, 斯波, 弘行, 常次, 宏一

Progress of Theoretical Physics  2006

• 統計物理学

今田 正俊

丸善  2004 ISBN: 4621074830

### Misc 【 display / non-display 】

• Hidden self-energies as origin of cuprate superconductivity revealed by machine learning

Youhei Yamaji, Teppei Yoshida, Atsushi Fujimori, Masatoshi Imada

2019.03

View Summary

Experimental data are the source of understanding matter. However, measurable
quantities are limited and theoretically important quantities are often hidden.
Nonetheless, recent progress of machine-learning techniques opens possibilities
of exposing them only from available experimental data. In this article, the
Boltzmann-machine method is applied to the angle-resolved photoemission
spectroscopy spectra of cuprate superconductors. We find prominent peak
structures both in normal and anomalous self-energies, but they cancel in the
total self-energy making the structure apparently invisible, while the peaks
make dominant contributions to superconducting gap, hence providing a decisive
testimony for the origin of superconductivity. The relation between superfluid
density and critical temperature supports involvement of universal carrier
relaxation time associated with dissipative strange metals. The present
achievement opens avenues for innovative machine-learning spectroscopy method.

• Representing Quantum Many-Body States by Machine Learning

Nomura Yusuke, Yamaji Youhei, Imada Masatoshi

Butsuri   74 ( 2 ) 72 - 81  2019

View Summary

&lt;p&gt;Machine learning is used to extract essential pattern from big data. This technique can be used to extract the essential feature of quantum many-body wave function (=a vector with exponentially large dimensions), and to obtain compact representation of many-body states. In this article, we review representations of many-body states using Boltzmann machine, a type of artificial neural network. We introduce an efficient representation using restricted Boltzmann machines (RBM) and also discuss the efforts to improve the RBM wave functions.&lt;/p&gt;

• M. Horio, S. Sakai, K. Koshiishi, Y. Nonaka, H. Suzuki, J. Xu, M. Hashimoto, D. Lu, Z. -X. Shen, T. Ohgi, T. Konno, T. Adachi, Y. Koike, M. Imada, A. Fujimori

2018.01

Internal/External technical report, pre-print, etc.

View Summary

The pseudogap phenomena have been a long-standing mystery of the cuprate<br />
high-temperature superconductors. Unlike the pseudogap in hole-doped cuprates,<br />
however, the pseudogap in the electron-doped counterpart has been attributed to<br />
band folding due to short-range antiferromagnetic (AFM) order. We performed<br />
angle-resolved photoemission spectroscopy measurements on electron-doped<br />
cuprates showing spin-glass and disordered AFM behaviors at low temperatures,<br />
and found that the gap magnitude \textit{decreases} in the antinodal region<br />
contrary to the hole-doped case. Moreover, the gap opening position was not<br />
always on the AFM Brillouin zone boundary in contradiction with the requirement<br />
of the AFM band-folding picture. These features are consistent with cluster<br />
dynamical-mean-field-theory calculations which predict an $s$-symmetry<br />
pseudogap that shrinks in the andinodal region. The present results support the<br />
scenario that the proximity to the Mott insulator, without relying on the<br />
well-developed AFM correlation, gives rise to a momentum-dependent pseudogap of<br />
$s$-symmetry with indirect gap commonly in the electron-doped and hole-doped<br />
cuprates, implying a universal origin of the pseudogap with a similarity to the<br />
Mott gap formation.

• Superconductivity Emerging from Excitonic Mott insulator - Theory of Alkaline Doped Fullerene

2017.11

View Summary

A three-orbital model derived from the two-dimensional projection of the $ab$
$initio$ Hamiltonian for alkaline doped fullerene A$_3$C$_{60}$ with A=Cs,Rb,K
is studied by a variational Monte Carlo method. We correctly reproduce the
experimental isotropic s-wave superconductivity around the $ab$ $initio$
parameters. With narrowing the bandwidth, the transition to an insulator is
also reproduced, where orbital symmetry is found to be spontaneously broken
with emergence of an excitonic Mott insulator for two orbitals and an
antiferromagnetic insulator nearly degenerate with a spin liquid for the third
orbital. The superconductivity is a consequence of exciton melting.

• Understanding of high-Tc superconductivity emerging from multi-scale ab initio methods and emergent concepts

今田 正俊

固体物理   52 ( 11 ) 649 - 660  2017.11