Photo-control of material properties on femto- (10−15) and pico- (10−12) second timescales at room temperature has been a long-sought goal of materials science. Here we demonstrate a unique ultrafast conversion between the metallic and insulating state and the emergence of a hidden insulating state by tuning the carrier coherence in a wide temperature range in the two-leg ladder superconductor Sr14-xCaxCu24O41 through femtosecond time-resolved reflection spectroscopy. We also propose a theoretical scenario that can explain the experimental results. The calculations indicate that the holes injected by the ultrashort light reduce the coherence among the inherent hole pairs and result in suppression of conductivity, which is opposite to the conventional photocarrier-doping mechanism. By using trains of ultrashort laser pulses, we successively tune the carrier coherence to within 1 picosecond. Control of hole-pair coherence is shown to be a realistic strategy for tuning the electronic state on ultrafast timescales at room temperature.
The photoinduced transition from the charge-density-wave (CDW) phase to the metallic phase in low-dimensional ladder-type cuprate Sr 14 Cu 24 O 41 was investigated in terms of femtosecond time-resolved reflection spectroscopy. Following the melting of the CDW order just after photoexcitation, a Drude-like metallic state was formed with a large optical response and maintained for more than 50 ps. The Drude weight increased with increasing fluence with threshold behavior as a result of the cooperative interactions in the CDW domains and reached the value of the Drude weight in the hole-doped metallic compound of Sr 4 Ca 10 Cu 24 O 41 . These results indicate the photoinduced formation of a similar metallic state driven by hole doping.
Ultrafast carrier dynamics caused by photoexcitation in quasi-one-dimensional two-leg ladder cuprate Sr4Ca10Cu24O41 was investigated by the femtosecond reflection spectroscopy. After the photoexcitation along the leg direction, the transient reflectivity changes (∆R/R) in the mid-infrared region showed instant decrease within 150 fs. The suppressed ∆R/R increased so rapidly on the picosecond time scale that the reflectivity value finally became larger than that in the initial state. Such a successive response in ∆R/R, which was also observed in other photon energy regions, is discussed in terms of ultrafast variation of the Drude weight in the ladder system by the photoirradiation.
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