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JAPAN NANONET BULLETIN - 89th Issue - February 8, 2007

YOUNG RESEARCHERS' INTRODUCTION

Shinichiro IWAI
Associate Professor, Department of Physics, Tohoku University

Photoinduced insulator to metal transition in strongly correlated electron system

(Issued in Japanese: November 30, 2005)

Photoinduced phase transition is a new mechanism for Tb/s -class ultrafast switching devices. In strongly correlated electron systems, such as a Mott insulator and a charge ordered insulator, ultrafast phase switching with small or no large lattice displacement is possible. We explore the ultrafast dynamics of a photoinduced insulator to metal transition in 3d transition metal compounds and organic charge transfer complexes.

We have prepared a one-dimensional (1D) halogen bridged Ni complex ([Ni(chxn)2Br]Br2) that undergoes a photoinduced insulator to metal switching (or equivalently the photoinduced Mott transition). Fig.1 (a) and (b) show a structure of Ni compound and transient optical responses, respectively. Immediately after photoexcitation, a Drude-like large reflectivity increase that is indicative of a metallic state was observed. A large reflectivity change of 300% in the mid IR region and an ultrafast response on the order of picoseconds was observed. The photoinduced Mott transition has also been observed with the oxide LaCoO3.

Some of low-dimensional organic conductors, as well as the 3d transition metal compounds, also show strong electronic correlation. The two-dimensional organic charge transfer (CT) compound (BEDT-TTF)2X (X denotes a counter anion) is well known because it has a wide variety of electronic properties from insulator to superconductor. Because of strong intersite coulomb repulsive interactions, a charge ordered insulator is formed at low temperature. Fig. 2 shows a schematic illustration of the insulator to metal transition in (BEDT-TTF)2X. A photoinduced insulator to metal switching in this compound occurs with an excitation that is 10 times smaller than that for a 3d transition metal compound. Furthermore, the ultrafast recovery dynamics for returning to the original insulating state strongly depend on the materials, reflecting a critical energy balance needed for electron transfer, electron correlation and electron-phonon interaction.

Shinichiro IWAI
Shinichiro IWAI
Associate Professor, Department of Physics, Tohoku University
 
1994Ph.D., Department of Applied Physics, Nagoya University
1994Joined National Institute of Materials and Chemical Research, Agency of Industrial Science and Technology (AIST)
2001Senior Researcher, Correlated Electron Research Center, AIST
2002Researcher, Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology (JST)
2003Associate Professor, Tohoku University
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Fig. 1
Fig. 1
(a) Structure of [Ni(chxn)2Br]Br2 .
(b) Transient reflectivity (upper) and optical conductivity(lower) spectra for various time delays.

Fig. 2
Fig. 2
Schematic illustration of insulator (charge ordering state) to metal transition in (BEDT-TTF)2X. Red circles in the insulator represents a localized charges (holes).

Relevant papers
  1. Iwai, S., Tanaka, S., Fujinuma, K., Kishida, H., Okamoto, H. & Tokura, Y.
    Ultrafast Optical Switching from an Ionic to a Neutral State in Tetrathiafulvalene-p-Chloranil (TTF-CA) Observed in Femtosecond Reflection Spectroscopy
    Phys. Rev. Lett. 88, 57402,1-4 (2002)
  2. Iwai, S., Ono, M., Maeda, A., Matsuzaki, H., Kishida, H., Okamoto, H. & Tokura, Y.
    Ultrafast Optical Switching to a Metallic State by Photoinduced Mott Transition in a Halogen-Bridged Nickel-Chain Compound
    Phys. Rev. Lett. 91,057401,1-4(2003)
  3. Iwai, S., Ishige, Y., Okimoto, Y., Tokura, Y. & Okamoto, H.
    Coherent Control of Charge and Lattice Dynamics in a Photoinduced Neutral -to-Ionic Transition of a Charge-Transfer Compound
    Phys. Rev. Lett. 96, 057403, 1-4(2006).