Tsunetsugu Group

Professor

TSUNETSUGU,Hirokazu

Research Associate

IKEDA,Tatsuhiko

Research Associate

FUJII,Tatsuya

Strongly correlated electron systems, particularly compounds including d- or f-orbital electrons, are the main subjects of our research. In these systems where electron-electron interactions are very strong, a variety of interesting phenomena emerge at low temperatures, and various magnetic orders, unconventional superconductivity and density waves are typical cases. Targets of our recent study include novel types of quantum order and quantum fluctuations in frustrated spin and strongly correlated electronic systems with multiple degrees of freedom. In these systems, many soft modes of fluctuations are coupled, and this affects the nature of quantum phase transitions, as well as electronic states and dynamical properties including transport phenomena. One of our recent achievements is about optical conductivity near an antiferromagnetic phase transition. We have shown that vertex corrections due to magnetic order and fluctuations provide a substantial contribution to optical conductivity, which is in sharp contrast to the behavior near the Mott transition in frustrated electron systems.

Numerical results of the temperature dependence of the dc conductivity in the half-filled Hubbard model on a square lattice calculated by the cluster dynamical mean field theory. The antiferromagnetic transition temperature is *T*_{N}/*t* = 0.34. In approaching zero temperature, compared with the data without vertex corrections (●), the full data (◆) including vertex corrections start to show a decrease of conductivity at a higher temperature. Energy unit is the electron transfer integral *t*.

Vertex corrections in the optical conductivity in the same model; paramagnetic (para) part and magnetic (mag) part. The data without vertex corrections are shown by black dotted lines, while red lines are the full data.

- Electronic states of strongly correlated systems with d- or
*f*-electrons - Statistical physics of frustrated systems
- Theory of novel phases in quantum magnets
- Unconventional superconductivity in strongly correlated electron systems

1.

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Entanglement prethermalization in an interaction quench between two harmonic oscillators: T. N. Ikeda, T. Mori, E. Kaminishi and M. Ueda, Phys. Rev. E **95** (2017) 022129/1-8.

2.

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Theory of antiferromagnetic Heisenberg spins on a breathing pyrochlore lattice: H. Tsunetsugu, Prog. Theor. Exp. Phys. **2017** (2017) 033101/1-29.

3.

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Classical Monte Carlo Study for Antiferro Quadrupole Orders in a Diamond Lattice: K. Hattori and H. Tsunetsugu, J. Phys. Soc. Jpn. **85** (2016) 094001/1-14.

4.

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Cluster dynamical mean field theory study of antiferromagnetic transition in the square-lattice Hubbard model: Optical conductivity and electronic structure: T. Sato and H. Tsunetsugu, Phys. Rev. B **94** (2016) 085110/1-12.

5.

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Fano resonance through Higgs bound states in tunneling of Nambu-Goldstone modes: T. Nakayama, I. Danshita, T. Nikuni and S. Tsuchiya, Phys. Rev. A **92** (2015) 043610/1-19.