Center for Emergent Matter Science, RIKEN,
2-1 Hirosawa, Wako, Saitama 351-0198, Japan
shiro.sakai_at_riken.jp (_at_ → @)
My reseach interest is in electron-correlation problems in solids, and development of new numerical tools to address these problems.
Strong electron-correlation effects, originating from Coulomb interaction between localized electrons, are intractable with a standard theory of solids but are considered to be responsible for various interesting phenomena, such as magnetism, the Mott metal-insulator transition, and high-temperature superconductivity.
The understanding as well as quantitative description of the electron-correlation effects is not only a challenge in fundamental physics but holds a potential for future technological applications.
During the last few decades, the dynamical mean-field theory (DMFT) has turned out to be a useful tool to study the electron-correlation problems.
Among its achievements, the DMFT succeeded in describing the Mott metal-insulator transition in infinite dimensions, reproduced reasonable values of the Curie temperature in iron and nickel, and, through its cluster extension applied to the two-dimensional Hubbard model, reproduced a phase diagram similar to that of copper-oxide high-temperature superconductors.
Development of a more realistic calculation scheme in conjunction with density functional theory is also underway in the field.
In this circumstance I have focused my efforts on the following issues (numbers in [ ] refer to Publication list):
Effect of Hund's coupling in multiorbital systems.
We studied a possible Hund-induced superconductivity of an s-wave spin-triplet orbital-antisymmetric pairing [1,2], and the mechanism of itinerant ferromagnetism in transition metals such as nickel .
Mott metal-insulator transition and pseudogap state in the two-dimensional Hubbard model.
This model is relevant to the high-Tc superconducting cuprates. In its electronic structure, two different singularities are present; a pole of Green's function and a pole of self-energy. The former defines the Fermi surface while the latter generates a correlation-induced gap like the Mott gap. We clarified the evolution of these singularities with carrier doping from undoped Mott insulator to overdoped Fermi-liquid metal [6,7,9,16]. Between them, we found an anomalous metallic state with an unusual "s-wave" pseudogap, in which the gap shifts to the unoccupied side in the nodal region, and its evidence in the electronic Raman response. This result suggests the importance of measuring momentum-resolved unoccupied spectra in cuprates [9,10,13,14].
Real-frequency structure of the self-energy and superconducting gap function in unconventional superconductors.
It was a study on the frequency dependence of the superconducting gap function that established the phonon-mediated mechanism of conventional superconductors. Then, the frequency dependence may hold a key to the high-Tc superconducting mechanism in cuprates as well. By scrutinizing this property in the two-dimensional Hubbard model, we found a hidden fermionic excitation at the origin of both the pseudogap and high-Tc superconductivity [22,26,30 (review in Japanese)].
Schemes bridging first-principles and many-body-model calculations and its application to real materials [12,19,20,23,27,28].
Extension of the DMFT to more realistic situations, such as multioribital [1,3,4,17], cluster [11,24], and dynamical interaction.
Possible superconductivity in quasicrystals.
This study is motivated by a recent experimental search for superconductivity in quasicrystals. The question is whether the supercondctivity occurs in the absence of translational symmetry (i.e., absence of well-defined momentum space as well as Fermi surface), and if yes, how the Cooper pairs are formed and what interesting properties can be expected. This issue would also be relevant to ultracold atomic systems, where optical quasiperiodic lattices have been available. Interplay of the fractal geometry and superconductivity may lead to a novel fascinating field! 
You can find the coordinates of Penrose tiling here.
April 2018 -
Senior Research Scientist in the group of Prof. Ryotaro Arita at CEMS, RIKEN.
May 2014 - March 2018
Research Scientist in the group of Prof. Ryotaro Arita at CEMS, RIKEN.
January 2013 - April 2014
Research Associate in the group of Prof. Ryotaro Arita at Department of Applied Physics, University of Tokyo.
October 2011 - January 2013
Postdoc in the group of Prof. Antoine Georges at Center for Theoretical Physics, Ecole Polytechnique, France.
October 2009 - September 2011
Postdoc in the group of Prof. Karsten Held at Institute for Solid State Physics, Vienna University of Technology, Austria.
April 2007 - September 2009
Postdoc in the group of Prof. Masatoshi Imada at Department of Applied Physics, University of Tokyo.
April 2002 - March 2007
Master- and doctor-course student in the group of Prof. Hideo Aoki at Department of Physics, University of Tokyo.
April 1998 - March 2002
Physics student at Kyoto University.
1980 - 1998
Kushiro, Hokkaido, Japan.
S. Sakai. M. Civelli, and M. Imada, Direct Connection between Mott Insulator and d-Wave High-Temperature Superconductor Revealed by Continuous Evolution of Self-Energy Poles arXiv:1804.09271
S. Sakai, Frequency-dependent structure of superconducting gap function in two-dimensional Hubbard model
J. Phys.: Conf. Ser., accepted
M. Horio, S. Sakai, K. Koshiishi, Y. Nonaka, M. Hashimoto, D. Lu, Z.-X. Shen, T. Ohgi, T. Konno, T. Adachi, Y. Koike, M. Imada, A. Fujimori, Common origin of the pseudogap in electron-doped and hole-doped cuprates governed by Mott physics, arXiv:1801.04247.
S. Kunisada, S. Adachi, S. Sakai, N. Sasaki, M. Nakayama, S. Akebi, K. Kuroda, T. Sasagawa, T. Watanabe, S. Shin, and T. Kondo, Observation of Bogoliubov Band Hybridization in the Optimally Doped Trilayer Bi2Sr2Ca2Cu3O10+d, Phys. Rev. Lett. 119, 217001 (2017) (Editor's suggestion)
B. Loret, S. Sakai, S. Benhabib, Y. Gallais, M. Cazayous, M. A. Measson, R. D. Zhong, J. Schneeloch, G. D. Gu, A. Forget, D. Colson, I. Paul, M. Civelli, and A. Sacuto, Vertical temperature boundary of the pseudogap under the superconducting dome in the phase diagram of Bi2Sr2CaCu2O8+d, Phys. Rev. B 96, 094525 (2017); arXiv:1703.00794.
R. Arita, T. Koretsune, S. Sakai, R. Akashi, Y. Nomura, and W. Sano, Non-empirical calculation of superconducting transition temperatures in light-element superconductors, Advanced Materials, 1602421 (2017).