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a
b
1.5
La 2-x Sr x Cu0 4
0.34
0.20
:Cu
:O
:La
1.0
x=0
0.15
0.02
0.10
0.06
0.10
c
0.15
0.5
0.20
0.06
b
a
0.34
0.02
0
0 0
1
2
3
4
h
ω
( eV )
Fig. 5.3 (a) Crystal structure of the 2D cuprate, La 2 CuO 4 .(b) Optical conductivity s ( o ) spectrum
of La 2 x Sr x CuO 4 with various hole carrier concentrations ( x )[ 54 ]
a
b
Fig. 5.4 (a) Schematics of localized 3d electrons ( filled circles ) and antiferromagnetic spin order
( arrows ) in 2D Mott insulator. (b) Conducting (metallic) state realized by the introduction of holes
hole-carrier concentration ( x )inLa 2 x Sr x CuO 4 is increased, the transition from
Mott
0.05. This I-M transition is
schematically illustrated in Fig. 5.4 . Figure 5.4a shows the schematic of the
electronic state of the Mott insulator, in which 3d electrons are localized in each
Cu 2+ ion due to the large on-site Coulomb repulsion energy U between the 3d
electrons. The arrows indicate the antiferromagnetic spin arrangement. Figure 5.4b
shows the holes-doping regime. When the holes (the open circles) are introduced by
the doping, the electrons can move to other sites as indicated by the arrows due
to the presence of the electron vacancies. In this case, a finite density of state is
produced near the Fermi level within the optical gap and then the I-M transition
occurs. As well known, La 2 x Sr x CuO 4 (0.06 < x < 0.25) exhibits superconduc-
tivity at
insulator to metal occurs at around x ¼
low temperatures. In another 2D cuprate, Nd 2 CuO 4 , a similar I-M
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