Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 63

a

b

0.4

Δ N eff (1.0 eV)

0.2

0.3

N eff =

x

0.2

Δ N eff (0.2 eV)

0.1

0.1

Δ N

eff = x

N D

0 0

0 0.05 0.1 0 0.1 0.2 0.3 0.4

x ph x

Fig. 5.8 (a) Photoinduced changes of the effective number of electrons

D N eff (1.0 eV) and D N eff

(0.2 eV) at t d 0.1 ps as functions of excitation photon density x ph in [Ni(chxn) 2 Br]Br 2 .(b) The

effective number of electrons as a function of the chemical-doping concentration x for
La 2 x Sr x CuO 4 [ 54 ]. N eff and N D are the total spectral weight in the IR region and the Drude
component, respectively

¼

materials. In the Ni-Br chain compound, it has been clarified from the measurement
of the DC conductivity [ 60 ] and the Seebeck coefficient (H Okamoto, unpublished
result) that a small number of electron carriers exists in an as-grown sample. A
weak midgap absorption due to such carriers or small polarons is indeed observed in
the

spectrum ( E // b ) as shown by the broken line in Fig. 5.6 . The photoinduced
absorption observed for small x ph (

s

10 4 ) resembles this midgap absorption
in spectral shape and energy position. This resemblance suggests that electron-type
carriers are responsible for the photoinduced midgap absorption. The hole-type
carriers with the Br p-character may be strongly localized by the strong
electron-lattice interaction and hence make the least contribution to the spectral
weight of the midgap absorption. When the metallic state is formed for x ph >

¼ 6.2

0.1,
however, there should be no distinction between electrons and holes for the strongly
d-p hybridized state near the Fermi level.

5.3.2 Relaxation Dynamics of the Photoinduced Metallic State

The ultrashort lifetime of the photogenerated metallic state is another important
aspect of the present photoinduced Mott transition. Figure 5.9a, b show the tempo-
ral characteristics of D R at 1.39 eV, respectively. Both two

characteristics are almost the same with each other. In Fig. 5.9a , we also show the
temporal evolutions of

D R at 0.12 eV and

D N eff (0.5 eV) for x ph ¼ 0.012 and x ph ¼

0.5 by the open

circles.

D N eff (0.5 eV) are also in good agreement with each other.
Therefore, the time characteristics of D R can be considered to reflect those of D N eff

D R (0.12 eV) and