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 such problems.
Strong electron-correlation effects, originating from Coulomb interaction between localized electrons, are intractable in a standard theory of solids but are considered to be responsible for various interesting phenomena, such as ferromagnetism, the Mott metal-insulator transition, and high-temperature superconductivity.
The understanding as well as quantitative description of the electron correlations is not only a challenge in fundamental physics but holds a potential for future technological applications.
During the last 20 years, 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.
In particular, we have 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. We clarified the evolution of the poles and zeros of the single-electron Green's function, 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.
As in conventional superconductors, the frequency dependence of these functions may hold a key to the superconducting mechanism. In the superconducting state of 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].
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 experimental observations of superconductivity in several quasicrystals and approximant crystals. The question is whether the supercondctivity occurs in the absence of translational symmetry, and if yes, how the Cooper pairs are formed . This would also be relevant to ultracold fermion atom experiments, where optical quasiperiodic lattices have been available.
May 2014 - present
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.
H. Braganca, S. Sakai, M. C. O. Aguiar, and M. Civelli, Correlation-driven Lifshitz transition at the emergence of the pseudogap phase in the two-dimensional
Hubbard model, arXiv:1708.02084.
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).