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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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