Material Designs and New Physical Properties in MX- and MMX-Chain Compounds by Masahiro Yamashita & Hiroshi Okamoto

Author:Masahiro Yamashita & Hiroshi Okamoto
Language: eng
Format: epub
Publisher: Springer Vienna, Vienna

Fig. 8.22Calculated spectra for the dp model. The curve of corresponds to the NiBr case. Reprinted from Iwano [39]. Copyright (2006) by the American Physical Society

8.3.3 Photoinduced Metallic States

As was first demonstrated by Iwai et al., strong photoirradiation on these materials can convert them into a type of metallic states [40]. As has been discussed already, one-photon absorbed states are well described as a bound or unbound doublon–holon pair. Then, the next question is about the nature of the strongly excited states. In more detail, weak or intermediate excitation also causes some photoinduced effect, which is characterized as a photoinduced absorption within the gap region. However, this is not a so-called Drude peak, which is usually centered at ω = 0, being a clear marker of the metallic state. We therefore know that there is some threshold for the formation of this metallic state in these compounds. This makes a contrast to the carrier-number dependency known for the one-dimensional Hubbard model, in which the singularity exists only at the half filling.

To understand this discrepancy, one key is the existence of the e–l interactions. There is a possibility that the e–l interactions exist to some extent and work as an inhibitor to the metallic state. However, as we have already discussed, the e–l interaction that works between the halogen stretching and the metal orbital is suppressed effectively in the limit of low carrier density. Much more detailed analyses about the time scales of polaron formations, actual associated modes, and the excitation-density dependency will be necessary for better understanding.

Another key will be the result by Takahashi et al., which traced numerically the real-time evolution of photoexcited states within the Hubbard model [41]. Their results show some critical intensity of photoexcitation realizing the metallization and seems to be consistent with the observation. This result is therefore attractive, since it suggests a different situation in the photoexcited states from the doped cases, more specifically, a trend that the nature of the excited state is changed drastically beyond a certain amount of excitations. Unfortunately, the numerical calculation is limited to a small system size and does not allow a definite conclusion. A proof based on analytical treatments will be further required to finish this argument.

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