Yu Kanamori

Photoinduced Change in the Spin State of Itinerant Correlated Electron Systems

A photoinduced spin-state change in the itinerant correlated electron system is studied. A photon introduced in the low-spin band insulator induces a bound state of the high-spin state and a photoexcited hole. This bound state brings a characteristic peak in the pump-probe optical absorption spectra which are completely different from the spectra in thermal-excited states. The present results well explain the recent experiments of the ultrafast optical spectroscopy in perovskite cobaltites.

Dynamical coupling and separation of multiple degrees of freedom in a photoexcited double-exchange system.

We present a theory of ultrafast photoinduced dynamics in a spin-charge coupled system, motivated by pump-probe experiments in perovskite manganites. A microscopic picture for multiple dynamics in spin and charge degrees is examined. Real-time simulations are carried out by two complimentary methods. Our calculation demonstrates that electron motion governs a short-time scale where charge and spin dynamics are combined strongly, while, in a long-time scale controlled by spin relaxation, the charge sector does not follow the remarkable change in the spin sector. The present results are in contrast to a conventional double-exchange picture in equilibrium states.

Photoinduced magnetic bound state in an itinerant correlated electron system with a spin-state degree of freedom

Photo-excited state in correlated electron system with spin-state degree of freedom is studied. We start from the two-orbital extended Hubbard model where energy difference between the two orbitals is introduced. Photo-excited metastable state is examined based on the effective model Hamiltonian derived by the two-orbital Hubbard model. Spin-state change is induced by photo-irradiation in the low-spin band insulator near the phase boundary. High-spin state is stabilized by creating a ferromagnetic bound state with photo-doped hole carriers. An optical absorption occurs between the bonding and antibonding orbitals inside of the bound state. Time-evolution for photo-excited states is simulated in the time-dependent mean-field scheme. Pair-annihilations of the photo-doped electron and hole generate the high-spin state in a low-spin band insulator. We propose that this process is directly observed by the time-resolved photoemission experiments.

Numerical study of photoinduced dynamics in a double-exchange model

Photo-induced spin and charge dynamics in double-exchange model are numerically studied. The Lanczos method and the density-matrix renormalization-group method are applied to one-dimensional finite-size clusters. By photon irradiation in a charge ordered (CO) insulator associated with antiferromagnetic (AFM) correlation, both the CO and AFM correlations collapse rapidly, and appearances of new peaks inside of an insulating gap are observed in the optical spectra and the one-particle excitation spectra. Time evolutions of the spin correlation and the in-gap state are correlated with each other, and are governed by the transfer integral of conduction electrons. Results are interpreted by the charge kink/anti-kink picture and their effective motions which depend on the localized spin correlation. Pump-photon density dependence of spin and charge dynamics are also studied. Roles of spin degree of freedom are remarkable in a case of weak photon density. Implications of the numerical results for the pump-probe experiments in perovskite manganites are discussed.

Optical manipulation of magnetism in spin-charge-coupled correlated electron systems

Photoirradiation effects in correlated electrons coupled with localized spins are studied based on the extended double-exchange model. In particular, we examine melting of an antiferromagnetic (AFM) charge order insulating state by varying the light intensity. When intense light is irradiated, the AFM insulating characteristics are strengthened, rather than change into the ferromagnetic metallic characteristic, which are expected from the conventional double exchange interaction when carriers are introduced by weak light irradiation or chemical doping. This provides a new principle for optically manipulating magnetism.

Photo-induced phenomena in correlated electron system with multi-degree of freedom

We present a theoretical study of photo-induced dynamics in a correlated electron system where electronic charge couples with spin and lattice. This is motivated by the recent experiments in perovskite manganites and related compounds. The generalized double exchange model is analyzed by utilizing the two complementary methods, the exact diagonalization and inhomogeneous Hartree-Fock methods. Time evolutions of the optical absorption spectra, spin correlation, and charge correlation are calculated. It is shown that photo-induced dynamics in this system is governed by strong coupling between itinerant electrons and localized spins.

Transient spectra of photo-excited states in double exchange model

We investigate photo-induced effect in strongly correlated electron systems where conduction electrons couple with localized spins. In particular, the photo-induced phenomena in a charge-ordered insulator associated with antiferromagnetic order are examined. Transient excitation spectra are calculated by the exact diagonalization method in one-dimensional extended double exchange model. We find that in the photo-excited states, finite spectral weights in the one-particle excitation spectra appear inside the insulating gap and its band width increases with increasing time. This change of the electronic structure is correlated with the time development of the localized spin correlation. These results indicate that a strong coupling between spin and charge governs the photo-induced phenomena in this system.