H. Uemura

Quantum interference between charge excitation paths in a solid-state Mott insulator

Ultrafast spectroscopy reveals the many-body effects behind the metallization of a one-dimensional Mott insulator. Unlike in ultracold gases, these femtosecond excitation studies of quantum dynamics occur at room temperature.

Role of Coulomb Interactions and Spin-Peierls Dimerizations in Neutral-to-Ionic Phase Transition Investigated by Femtosecond Reflection Spectroscopy

To clarify hidden mechanisms of neutral-to-ionic (NI) transition in an organic molecular compound, tetrathiafulvalene- p -chloranil, we investigated temperature- and excitation-density dependences of photoinduced NI transition dynamics. With increasing temperature from 90 K just above the NI transition temperature (81 K), transient dimerizations due to the spin-Peierls (SP) mechanism monotonically decrease. The result suggests that a virtual SP transition temperature locates near 300 K. When photogenerated I states increase with increase of the excitation density, each I domain is further stabilized by interchain Coulomb interactions, but rather destabilized via the suppression of dimerizations. Roles of Coulomb interactions, SP dimerizations, and interchain interactions in the NI transition are discussed on the basis of the results.

Large second-order optical nonlinearity in a ferroelectric molecular crystal of croconic acid with strong intermolecular hydrogen bonds

Linear and nonlinear optical responses in a molecular crystal, croconic acid, showing electronic-type ferroelectricity were studied by reflection and second harmonic generation spectroscopy. The second-order nonlinear susceptibility χ(2) was very large, exceeding 10−6 esu in the near-infrared region. The enhancement of χ(2) was attributed to the large dipole moment of the lowest π–π* transition and the large difference between the molecular dipole moments for the ground state and the photoexcited state. We deduced the molecular orbitals (MOs) and dipole moments responsible for the large χ(2) by comparing the experimental optical parameters and MO calculation results based upon density functional theory.

Enhancement of Photoinduced Charge-Order Melting via Anisotropy Control by Double-Pulse Excitation in Perovskite Manganites: Pr_{0.6}Ca_{0.4}MnO_{3}.

To control the efficiency of photoinduced charge-order melting in perovskite manganites, we performed femtosecond pump-probe spectroscopy using double-pulse excitation on Pr_{0.6}Ca_{0.4}MnO_{3}. The results revealed that the transfer of the spectral weight from the near-infrared to infrared region by the second pump pulse is considerably enhanced by the first pump pulse and that the suppression of crystal anisotropy, that is, the decrease of long-range lattice deformations due to the charge order by the first pump pulse is a key factor to enhance the charge-order melting. This double-pulse excitation method can be applied to various photoinduced transitions in complex materials with electronic and structural instabilities.

Clocking the Collapse of a Mott Gap

We probe the dynamics of band gap collapse in the Mott (electronic) insulator ETF2TCNQ and Peierls (structural) insulator K-TCNQ. The collapse of the Mott gap is a purely electronic process occurring within 20 fs. In the Peierls insulator, the gap collapse takes 300 fs, only after a lattice distortion has relaxed.

Ultrafast photoinduced melting of spin-Peierls phase in potassium-tetracyanoquinodimethane

Ultrafast dynamics of photoinduced melting of spin-Peierls (SP) phase was investigated in K-tetracyanoquinodimethane by using fs pump-probe reflection spectroscopy. It was revealed that the photocarriers are localized with ~70 fs and behave as nonmagnetic impurities, which lead to the melting of the SP phase within ~ 400 fs. In the melting process of the SP phase, several kinds of coherent oscillations were observed, suggesting that the corresponding several phonon modes contribute to the stabilization of the SP phase.