Structures and Optical Properties of Pt and Pd Compounds with Charge-Density-Waves - Material Designs and New Physical Properties in MX-and MMX-Chain Compounds - page 25

600

3

Pt-Br-Pt-II
Pt-Br-Pd

CS pair

2

STE

SS pair

1

400

0

0 5 10 15

l 0 (site)

200

0

0 100 200 300

Temperature (K)

Fig. 2.14 Temperature dependence of the luminescence decay time . The detection energies are

1.19 eV in Pt-Br-Pt-II and 1.52 eV in Pt-Br-Pd. The inset shows the cross-section of the
calculated potential energy surfaces (taken from [ 51 ]). SS and CS show a spin-soliton pair and a
charged-soliton pair, respectively. (Reprinted figure from [ 52 ])

t

includes the STE and the charged-soliton pair and the lower surface includes the
spin-soliton pair and is connected to the CDW ground state. With the parameters
appropriate for MX-chain compounds, a spin-soliton pair is always the lowest
energy excitation, although the energy difference between a spin-soliton pair and
an STE is not so large [ 51 ].
According to more detailed theoretical analyses, a conversion from the STE to
the spin-soliton pair was possible. These expectations were in good agreement
with the experimental results of Pt-Br-Pt-II discussed above. In Pt-Br-Pt-II, it is
reasonable to consider that D is a potential barrier between the STE and the spin-

t 0 1 is a tunneling rate through the barrier. We can imagine in
the inset of Fig. 2.14 that such a barrier exists when the conversion from the STE
to the spin-soliton pair occurs at large values of l 0 , e.g., larger than 8. In
Pt-Br-Pd, the energy of potential surfaces is expected to increase sharply with
increasing l 0 compared with that in Pt-Br-Pt-II (the inset of Fig. 2.14 ), so that the
spin-soliton pair cannot be dissociated. This situation is analogous to bipolarons
in nondegenerate conjugated polymers. In this case, the depth of the potential
well around the STE should be increased. As a result,

soliton pair and

t 0 1
was decreased in Pt-Br-Pd relative to those values in Pt-Br-Pt-II. Thus, it is
expected that the main relaxation path of STEs in Pt-Br-Pd is also the
nonradiative recombination process through the solitonic states, which is essen-
tially the same as that indicated by the arrows in the inset of Fig. 2.14 except for
the slope of potential surfaces.
It is important to comment on the comparison of solitons in MX-chain
compounds with those in trans -(CH) x , in which nature of spin-solitons had also
been extensively studied. In trans -(CH) x , however, a conversion of an electron-hole

D

was enhanced and