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Papers by Akira Kofuji

Research paper thumbnail of Unconventional gap dependence of high-order harmonic generation in the extremely strong light-matter-coupling regime

Physical Review A

High-order harmonic generation (HHG) is one of the most commonly studied nonlinear optical phenom... more High-order harmonic generation (HHG) is one of the most commonly studied nonlinear optical phenomena, originating in the ultrafast dynamics of electrons in atomic gases and semiconductors. It has attracted much attention because of its nonperturbative nature and potential for future attosecond laser pulse sources. On the theory side, a semiclassical picture based on tunneling ionization of electrons is successfully used in explaining key characteristics of the HHG. This model assumes that electric fields nonperturbatively excite electrons beyond the ionization potential or the band gap. Thus, intuitively, a larger gap should lead to an exponentially smaller HHG emission. Despite this intuition, the HHG in the Mott insulator Ca 2 RuO 4 has shown an unconventional exponential increase with respect to the gap width. This experiment implies effects beyond the semiclassical theory. However, most theoretical works have focused on the dependence of the HHG on external control parameters, and the gap dependence of the HHG is poorly understood even in noninteracting systems. Thus, it is essential to clarify the gap dependence of the HHG in a fully quantum mechanical approach. Here, we analyze numerically exactly the gap dependence of the HHG in two-level systems. We find an increase in the strength of the HHG when the Rabi frequency is large compared to the gap width. Furthermore, the relaxation and scattering of electrons increase the visibility of this gap dependence. Finally, we find that the enhancement rate follows a universal scaling law regardless of the driving frequency. The existence of this gap dependence in two-level systems suggests that this unconventional gap dependence is a universal behavior that can be found not only in Mott insulators but also in atomic gases and semiconductors.

Research paper thumbnail of Quantum theory of the Intrinsic Orbital Magnetoelectric Effect in itinerant electron systems at finite temperatures

arXiv (Cornell University), Feb 26, 2023

Research paper thumbnail of Quantum theory of the intrinsic orbital magnetoelectric effect in itinerant electron systems at finite temperatures

Physical Review B

Magnetization can be induced by an electric field in systems without inversion symmetry P and tim... more Magnetization can be induced by an electric field in systems without inversion symmetry P and time-reversal symmetry T. This phenomenon is called the magnetoelectric (ME) effect. The spin ME effect has been actively studied in multiferroics. The orbital ME effect also exists and has been mainly discussed in topological insulators at zero temperature. In this paper, we study the intrinsic orbital ME response in metals at finite temperature using the Kubo formula. The intrinsic response originates from the Fermi sea and does not depend on the dissipation. Especially in systems with PT-symmetry, the extrinsic orbital ME effect becomes zero, and the intrinsic ME effect is dominant. We apply the response tensor obtained in this work to a PT-symmetric model Hamiltonian with antiferromagnetic loop current order demonstrating that the intrinsic ME effect is enhanced around the Dirac points.

Research paper thumbnail of Effects of strong correlations on the nonlinear response in Weyl-Kondo semimetals

Physical Review B, 2021

Nonlinear responses give rise to various exciting phenomena, which are forbidden in linear respon... more Nonlinear responses give rise to various exciting phenomena, which are forbidden in linear responses. Among them, one of the most fascinating phenomena is the recently observed giant spontaneous Hall effect in Ce3Bi4Pd3. This material is a promising candidate for a Weyl-Kondo semimetal, and this experiment implies that strong correlation effects can enhance the nonlinear Hall effect. However, most theoretical studies on nonlinear responses have been limited to free systems, and the connection between nonlinear responses and strong correlation effects is poorly understood. Motivated by these experiments and recent theoretical advances to analyze strong correlation effects on the nonlinear response, we study a periodic Anderson model describing Ce3Bi4Pd3 using the dynamical mean-field theory. We calculate the nonlinear longitudinal conductivity and the nonlinear Hall conductivity using the Kubo formula extended to the nonlinear response regime and clarify their temperature dependences. We numerically show that strong correlations can enhance nonlinear conductivities, and we conclude that the magnitude of the experimentally observed giant nonlinear Hall effect can be explained by strong correlation effects.

Research paper thumbnail of Unconventional gap dependence of high-order harmonic generation in the extremely strong light-matter-coupling regime

Physical Review A

High-order harmonic generation (HHG) is one of the most commonly studied nonlinear optical phenom... more High-order harmonic generation (HHG) is one of the most commonly studied nonlinear optical phenomena, originating in the ultrafast dynamics of electrons in atomic gases and semiconductors. It has attracted much attention because of its nonperturbative nature and potential for future attosecond laser pulse sources. On the theory side, a semiclassical picture based on tunneling ionization of electrons is successfully used in explaining key characteristics of the HHG. This model assumes that electric fields nonperturbatively excite electrons beyond the ionization potential or the band gap. Thus, intuitively, a larger gap should lead to an exponentially smaller HHG emission. Despite this intuition, the HHG in the Mott insulator Ca 2 RuO 4 has shown an unconventional exponential increase with respect to the gap width. This experiment implies effects beyond the semiclassical theory. However, most theoretical works have focused on the dependence of the HHG on external control parameters, and the gap dependence of the HHG is poorly understood even in noninteracting systems. Thus, it is essential to clarify the gap dependence of the HHG in a fully quantum mechanical approach. Here, we analyze numerically exactly the gap dependence of the HHG in two-level systems. We find an increase in the strength of the HHG when the Rabi frequency is large compared to the gap width. Furthermore, the relaxation and scattering of electrons increase the visibility of this gap dependence. Finally, we find that the enhancement rate follows a universal scaling law regardless of the driving frequency. The existence of this gap dependence in two-level systems suggests that this unconventional gap dependence is a universal behavior that can be found not only in Mott insulators but also in atomic gases and semiconductors.

Research paper thumbnail of Quantum theory of the Intrinsic Orbital Magnetoelectric Effect in itinerant electron systems at finite temperatures

arXiv (Cornell University), Feb 26, 2023

Research paper thumbnail of Quantum theory of the intrinsic orbital magnetoelectric effect in itinerant electron systems at finite temperatures

Physical Review B

Magnetization can be induced by an electric field in systems without inversion symmetry P and tim... more Magnetization can be induced by an electric field in systems without inversion symmetry P and time-reversal symmetry T. This phenomenon is called the magnetoelectric (ME) effect. The spin ME effect has been actively studied in multiferroics. The orbital ME effect also exists and has been mainly discussed in topological insulators at zero temperature. In this paper, we study the intrinsic orbital ME response in metals at finite temperature using the Kubo formula. The intrinsic response originates from the Fermi sea and does not depend on the dissipation. Especially in systems with PT-symmetry, the extrinsic orbital ME effect becomes zero, and the intrinsic ME effect is dominant. We apply the response tensor obtained in this work to a PT-symmetric model Hamiltonian with antiferromagnetic loop current order demonstrating that the intrinsic ME effect is enhanced around the Dirac points.

Research paper thumbnail of Effects of strong correlations on the nonlinear response in Weyl-Kondo semimetals

Physical Review B, 2021

Nonlinear responses give rise to various exciting phenomena, which are forbidden in linear respon... more Nonlinear responses give rise to various exciting phenomena, which are forbidden in linear responses. Among them, one of the most fascinating phenomena is the recently observed giant spontaneous Hall effect in Ce3Bi4Pd3. This material is a promising candidate for a Weyl-Kondo semimetal, and this experiment implies that strong correlation effects can enhance the nonlinear Hall effect. However, most theoretical studies on nonlinear responses have been limited to free systems, and the connection between nonlinear responses and strong correlation effects is poorly understood. Motivated by these experiments and recent theoretical advances to analyze strong correlation effects on the nonlinear response, we study a periodic Anderson model describing Ce3Bi4Pd3 using the dynamical mean-field theory. We calculate the nonlinear longitudinal conductivity and the nonlinear Hall conductivity using the Kubo formula extended to the nonlinear response regime and clarify their temperature dependences. We numerically show that strong correlations can enhance nonlinear conductivities, and we conclude that the magnitude of the experimentally observed giant nonlinear Hall effect can be explained by strong correlation effects.

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