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a
b
c
Fig. 2.11 Localized energy levels of (a) spin-solitons ( S 0 ), (b) charged-solitons ( S , S + ), and (c)
polarons ( P , P + ) (Reprinted figure with permission from [ 5 ])
a
b
c
C.B.
P -
S 0
S 0
S -
S +
P +
V.B.
Fig. 2.12 Schematic electronic structures: (a) spin-solitons ( S 0 ), (b) charged-solitons ( S , S þ ),
and (c) polarons ( P , P þ ) (Reprinted figure with permission from [ 5 ])
In Fig. 2.12 , the solid arrows indicate the possible optical transitions. For a polaron,
it was theoretically revealed that oscillator strengths of the transitions indicated by
broken arrows are extremely weak.
Taking into account the theoretical expectations shown in Fig. 2.12 , a doublet
structure a 1 and a 2 related to spins ( S ¼
1/2) in Pt-Br-Pd could be assigned to
polarons. It should be noted that in Pt-Br-Pd, electron polarons exist on the Pt sites
and hole polarons on the Pd sites and therefore, the electron-hole asymmetry exists.
In this case, PA energies for a positively charged polaron and a negatively charged
one will be different from each other due to the difference in the magnitude of U on
the metal sites. Thus the large spectral width of band a 2 was attributed to the
superposition of
the two transitions slightly split due to the electron-hole
asymmetry.
In Pt-Br-Pt-II, bands a , b and band g have different origins as mentioned
above. The important information is that PA bands a and b were not detected in
Pt-Br-Pd. Pt-Br-Pd has the nondegenerate CDW state, so that solitons should not
be stabilized. These results suggested that bands a and b could be attributed to
 
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