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MX-chains, more specifically, the Pd compound, and prompt further studies, for

example, on entangled dynamics in which the two collective degrees of freedom in

the electronic system and the lattice system evolve in time, affecting each other.

The second progress is the further photoconversion of the PdBr compound, that

is, the second transition from the Mott-Hubbard state to a metallic state. As we

already know that the Ni compound as another type of Mott insulator becomes

metallic after the strong photoexcitation, this sounds very likely. In fact, Matsuzaki

et al. have tried such detection and obtained a result supporting this (H. Matsuzaki,

unpublished). Although a theoretical investigation has not been tried yet, the

theorists face a novel field of dynamics in which a domain in its transient growth

process is converted again into the third phase.

The last progress we want to mention is challenges toward more detailed and

accurate understanding of this photoconversion process. So far, we have looked at

the domain growth only focusing on its coherent aspect. In this case, we mean by

the word of “coherent” a description based solely on the collective coordinate of the

domain formation. Very recently, Uemura et al. have found the generations of

various types of coherent phonons in TTF-CA during its photoconversion process

[
51
]. This observation suggests substantial couplings between the collective coor-

dinate and the other degrees of freedom, and tell us the necessity of a new theory

that includes such effects. In a naive image, we expect that the collective motion of

the domain growth will receive a kind of friction from the other degrees of freedom

such as phonons, magnons, electron-hole excitations, and so on. Regarding this, the

idea of quantum friction [
52
] is attractive, although we must keep the following two

points in mind. First, at least at the initial stage of dynamics, the relevant modes are

rather limited since the coherent phonons are observed. In this sense, this is a

different problem from that considering all the other modes as a reservoir. Second,

the domain has a translational invariance in ideal or sufficiently large systems. Not

only its spatial extension but also the motion of its center of gravity will receive

scattering effects from the other degrees of freedom. Such complexity will also be

of central interest in the very recent paper [
53
] and is expected to make our

understanding much deeper.

References

1. Nasu K (1984) J Phys Soc Jpn 53:302

2. Mishima A, Nasu K (1989) Phys Rev B 39:5758

3. Mishima A, Nasu K (1989) Phys Rev B 39:5763

4. Mishima A, Nasu K (1989) Phys Rev B 40:5593

5. Hubbard J (1963) Proc R So0063 A 276:238

6. Tanino H, Kobayashi K (1983) J Phys Soc Jpn 52:1446

7. Su WP, Schrieffer JR, Heeger AJ (1979) Phys Rev Lett 42:1698

8. Gammel JT, Saxena A, Batistic I, Bishop AR, Phillpot SR (1992) Phys Rev B 45:6408

9. Matsuzaki M, Iwano K, Aizawa T, Ono M, Kishida H, Yamashita M, Okamoto H (2004) Phys

Rev B 70:035204

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