Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 71

easily without the release of the Br displacements. The relaxation of the Br
displacements (ii) occurs after the formation of the 1D MH domain, accompanied
by the coherent oscillation, which modulates the energy of the 4d z 2 orbital and,
therefore, the Mott-gap transition in the 1D MH domain. As a result, the coherent
oscillation is detected in the D R response. The photogenerated MH domains

subsequently return to the ground state via the 1D random walk of DW pairs,
which was reproduced by the error function.

5.4.2 Ultrafast Photoinduced Transitions in Charge Density
Wave, Mott Insulator, and Metallic Phases in Iodine-
Bridged Platinum-Chain Compound

In the Ni-Br chain compound with the MH-type ground state, the photoinduced
transition to metal occurs as discussed in Sect. 5.3 . On the other hand, in the Pd-Br
chain compound near the CDW and MH phase boundary, the photoinduced CDW
to MH transition is driven. Therefore, it is natural to expect that a CDW to metal
transition can be induced by the photoirradiation on a CDW compound near the
CDW and MH phase boundary. In Pd(chxn

Þ 2 Br Br 2 discussed above, however,
such a CDW to metal transition cannot be induced even by the strong photoexcita-
tion. To realize a photoinduced CDW to metal transition, a compound having more
itinerant electronic states is appropriate. From these considerations, we selected [Pt
(chxn) 2 I]I 2 . In this compound, the 1D electronic state is composed of 5d orbital of
Pt and 5p orbital of I, so that the itinerancy should be enhanced compared to
Pd(chxn

Br 2 with 1D electronic state composed of 4d orbital of Pd and 4p

Þ 2 Br

orbital of Br.
Figure 5.15a shows the reflectivity spectrum of Pt(chxn Þ 2 I I 2 with the light

polarization ( E ) parallel to the chain axis b ( E // b ). The imaginary part of dielectric
constant e 2 [solid line in panel (b)] was obtained by using the KKT of the

reflectivity spectrum. The broad peak at 0.95 eV is due to the CT transition

Pt 2 þ

ð

Pt 4 þ Þ!ð Pt 3 þ ; Pt 3 þ Þ ;

D R )
spectra for ( E // b ) with the polarization of the pump light ( E ex // b ) are presented for
three excitation densities ( x ph ). x ph is the averaged photon (ph) density of the pump
light absorbed within the absorption depth (ca. 300 ˚ ). The delay time t d of the
probe light relative to the pump light is 0.16 and 1.7 ps. Errors of

. In Fig. 5.15c, e , photoinduced reflectivity changes (

D R are smaller

than 10 3 (10 4 ) in the mid-IR (near-IR) region. At t d ¼ 0.16 ps, the reflectivity

increases below 1 eV. For x ph ¼ 0.005 and 0.02 ph/Pt,

D R has a peak at ca. 0.4 eV,

while for x ph ¼ 0.05 ph/Pt,

D R has no peak but monotonically increases with

decrease in energy. At t d ¼

D R has a peak at ca. 0.4 eV in common. The
results demonstrate that there is a metastable-photoinduced phase characterized by
the reflection peak at ca. 0.4 eV.
To get information of the photoinduced phase, the photoinduced change in
e 2 (

1.7 ps,

D e 2 ) (Fig. 5.15d, f ) was obtained by KKT of R + D R .At t d ¼

0.16 ps,

e 2 decreases at ca. 0.95 eV in common. For x ph ¼ 0.005 and 0.02 ph/Pt, e 2 rather