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Photo-Induced Phase Transition due to Cooperative Coupling of Phonon and Intra- and Inter-Molecule Electronic Transitions

The photo-induced phase transitions, in which material properties such as color, conductivity, magnetism, etc. suddenly change under light illumination, occur as a result of interactions among light, electrons, and lattice vibrations. Because a sudden phase transition can be controlled by light illumination, the photo-induced phase transitions are attracting attention also from the point of view of applications such as ultrafast switching devices. Photoinduced melting of a charge order is a typical example of such phenomena and has been observed in various materials. In a charge ordered state, the number of electrons localized in a molecule becomes different between neighboring molecules, causing the formation of a symmetry-broken superlattice as shown in Fig. 1.

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Fig. 1: Molecular crystal with charge order.

Recently the research group in the Physics Department of Chuo University performed an extensive theoretical study for the purpose of understanding the initial processes of photo-induced melting in quasitwo-dimensional organic conductor Et2Me2Sb[Pd(dmit)2]2. In this study, a model including the internal structure of molecules was introduced and a new mechanism due to cooperative couplings of lattice vibrations and electron transitions within a molecule and between neighboring molecules was proposed for its photo-induced melting of the charge order.

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Fig. 2: Schematic illustration of the process of photoinduced melting of charge order.

Reference

K. Nishioka and K. yonemitsu, J. Phys. Soc. Jpn. 82 (2013) 094716.

 
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