Chemistry Reference
In-Depth Information
addition to the electron hopping in the 3D random network of metallic
nanographene sheets, the Π-electron carriers interact strongly with the
edge-state localized spins of a given sheet. At high temperatures, the fast
hopping process evidenced by the high conductivity (Fig. 5(a)) makes
the edge-state spins subjected to the motional narrowing, yielding a
homogeneous spin system in the entire of the network. This is justified
by the Lorentzian shape of the ESR signal and its less saturated behavior
in the high temperature range. Therefore, the spin system is modeled
merely as a metallic system with the interaction between the localized
edge-state spins and conduction Π-carriers. Here, the strong coupling
between the edge-state spins and the conduction Π-carriers, and a weak
spin-orbit interaction, the latter of which is evident from a considerably
small deviation of the g-value from that of the free electron spin (∆
G
~
10
-3
),
30
lead to a bottleneck in energy dissipation from the conduction
carriers to the lattice. In the bottleneck regime, the effective spin-lattice
relaxation time, which is inversely proportional to the line width, is given
as;
31,32
(
)(
)
∆
H
~ 1 /
T
=
1 /
T
T
/
T
,
(3)
1,eff
n
Π
Π
n
Π
L
where
T
and
Π
T
are the relaxation times of from the edge-state
spin to the Π-carrier, from the Π-carrier to the edge-state spin, and from
the Π-carrier to the lattice, respectively. The relaxation from the edge-
state spins to the Π-carriers,
T
,
nΠ
Πn
T
is given by the Korringa relation with
Πn
.
33
Then, as shown in Eq. (3), the
temperature dependence of
T
∝
1
/
T
n
Π
line width is narrowed by
(
)
T
/
T
due to the bottleneck effect with a
Π
n
Π
L
slow relaxation rate
T
is
independent of temperature
31
and
Π
T
, which is governed by the boundary
scattering in the nanographene, is also less temperature dependent.
34
Eventually, the linear temperature dependence of the line width observed
above
T
c
~
20 K (Fig. 6(b)) is in good agreement with that expected by
Eq. (3). This suggests the importance of the interplay between the edge-
state spins and conduction Π-carriers strongly interacting with each other
in the metallic medium.
The situation becomes different in the magnetism in the low
temperature regime below
T
c
. The appearance of the hole-burning in the
vicinity of the transition temperature demonstrates the serious effect of
1
T
Here, it should be noted that
ΠL
Πn
