Photoinduced Phase Transitions in MX-Chain Compounds - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 64

Fig. 5.9 Temporal profiles of
D R measured (a) at 0.12 eV
and (b) at 1.39 eV for various
excitation photon density x ph
in [Ni(chxn) 2 Br]Br 2 .
Temporal profiles of D N eff
(0.5 eV) for x ph ¼ 0.5 and
0.012 are also shown by the
open circles

a

0.12 eV

b

1.39 eV

x ph =6.2x10 -4

x ph =6.2x10 -4 1.0

1.0

0.5 0.5

0

0

1.0

1.0

0.012

0.012

0.5

0.5

0 0

1.0 1.0

0.25

0.25

0.5

0.5

0

0

1.0 1.0

0.5

0.5

0.5

0.5

0 0

0 10 20 0 10 20

Delay time (ps)

Delay time (ps)

(0.5 eV). The time characteristics of

D R (0.12 eV) can be well reproduced by the
sum of the three exponential functions whose time constants t d (weights) are 3 ps

500 ps (10 %) for x ph ¼

(40 %), 8 ps (50 %), and 0.012 and 0.5 ps (60 %), 3 ps

(15 %), and 8 ps (25 %) for x ph ¼

t d

0.5. The component with 500 ps for

x ph ¼

0.012 is attributable to some long-lived-trapped carriers. The ultrafast decay
component with

0.5, while such an ultrafast
decay was not detected for x ph ¼ 0.012. Therefore, it is likely that the ultrafast
decay is characteristic of the photogenerated metallic states.
The decay dynamics of the photoexcited state in Mott insulators has also been
studied on the 2D cuprates (La 2 CuO 4 ,Nd 2 CuO 4, and Sr 2 CuO 2 Cl 2 )[ 20 , 21 , 61 , 62 ]and
1D cuprate (Sr 2 CuO 3 )[ 63 ], and 1D organic CT complex [(BEDT-TTF)-(F 2 TCNQ),
BEDT-TTF

t d ¼ 0.5 ps is dominant for x ph ¼

difluorotetracyano-
quinodimethane] [ 18 ]. In La 2 CuO 4 ,Nd 2 CuO 4 and (BEDT-TTF)-(F 2 TCNQ), in which
photoinduced I-M transitions were demonstrated, decay time

¼ bis(ethylenedithio)tetrathiafulvalene, F 2 TCNQ ¼

of metallic states was
revealed to be shorter than 40 fs in the 2D cuprates and shorter than 200 fs in (BEDT-
TTF)-(F 2 TCNQ). These values of

t

are significantly shorter than that in conventional
semiconductors. The electron-electron scattering with emission of spin-excitations or
spinons is considered as a possible mechanism for the ultrafast relaxation in the Mott
insulators [ 20 , 21 , 61 - 63 ]. In the present bromine-bridged Ni-chain compound,
enhancement of similar electron-electron scattering in the quasi-1D metallic state
may play a major role in the increase of the recombination rate of photocarriers.

t