3
CHAPTER II LITERATURE STUDY
Superconductivity is a phenomenon observed in many kinds of metal and ceramic material that have zero electrical resistivity when they are cooled down to sufficiently
low temperatures. The specimen showing this phenomenon is called a superconductor and the temperature at which the electrical resistivity starts to be zero is called the critical
temperature T
c
. Superconductivity was discovered for the first time by H. K. Onnes in 1911.
The era of high-T
c
superconductivity began when Bednorz and Muller reported possible superconductivity in single-layer cuprates called 214 cuprates Bednorz
et al.
, 1986. The parent compound of these cuprates is La
2
CuO
4
. Superconductivity appears by randomly substituting some Sr atoms for La and forming La
2-x
Sr
x
CuO
4
. When a La
3+
ion is replaced by Sr
2+
, the CuO
2
plane loses one electron so that one mobile hole remains in the CuO
2
planes, leading to the formation of a hole-doped cuprate. Another family of single-layer cuprates is the electron-doped Nd, Pr, Sm-Ce-Cu-O. In these materials,
when a Nd
3+
or Pr
3+
or Sm
3+
is replaced by Ce
4+
, the CuO
2
plane gets an excess electron, leading to the formation of an electron-doped cuprates Tokura
et al.
, 1989. All of the crystals have common structures consist of both the conduction layer CuO
2
plane and the charge reservoir layer. CuO
2
planes are responsible for their electronic properties.
0.2
0.1 0.1
0.2 T
x x
AF SC
4 Figure 1. Schematic phase diagram of the hole- and electron-doped 214 cuprate. AF and
SC indicate antiferromagnetic order and superconductivity, respectively.
The so-called hole-electron doping symmetry in the high-T
c
cuprates has been one of central interests in relation to the mechanism of the high-T
c
superconductivity. Phase diagrams of the hole- and electron-doped systems are very similar to each other. That is,
the parent compounds are both Mott insulators exhibiting long-range antiferromagnetic order with similar values of the Neel temperature. The superconducting phases appear
through doping holes or electrons into the Mott insulators as shown in Fig 1. These properties lead to the view of hole-electron doping symmetry. On the other hand, some
properties in the electron-doped superconductors have been found to be different from those in the hole-doped superconductors, leading to the hole-electron doping asymmetry.
First, the effectiveness of carriers for destroying the long-range antiferromagnetic order is different between the hole- and electron-doped systems. In the electron-doped system of
Nd
2-x
Ce
x
CuO
4
, the long-range antiferromagnetic order survives up to x ~ 0.13 Takagi
et al.
, 1989, while it survives only up to x ~ 0.02 in the hole-doped system of La
2-x
Sr
x
CuO
4
. Secondly, in the inelastic neutron-scattering measurements, an incommensurate spin-
correlation, which may be due to the so-called dynamically fluctuating stripes of spins and holes Tranquada
et al.
, 1995, has been found in the hole-doped system Yamada
et al.
, 1998. In the electron-doped system, on the other hand, a commensurate spin- correlation, which is related to the simple antiferromagnetic order, has been observed
Yamada
et al.
, 2003. As for impurity effects, different behaviors between the hole- and electron-doped systems
are also observed. For examples, the superconductivity in the electron-doped system is suppressed through the substitution of magnetic Ni for Cu more markedly than through
the substitution of nonmagnetic Zn for Cu Tarascon
et al.
, 1990, which is contrary to the result in the hole-doped system Xiao
et al.
, 1990. From the view of the Cu-spin dynamics properties, one of important phenomena in the
214 cuprates is that in hole-doped system, charge in the CuO
2
plane are not distributed homogeneously but form quasi-one-dimensional charge stripes. The charge stripes are
5 manifestation of a self-organized state. In 1995, Tranquada
et al
. suggested a stripe model of spins and holes from neutron scattering measurements to understand the mechanism of
the 18 anomaly around
p
= 18 Tranquada
et al.
, 1995. That is, dynamically fluctuating stripes are pinned and stabilized by the tetragonal low-temperature structure space
group: P4
2
ncm, leading to the static stabilization and suppression of superconductivity at
p
= 18. These results can be well described using the stripe model. This model consists of hole charge domains and spin domains. Charge stripes refer to a charge
density wave CDW and spin stripes refer to a spin density wave SDW. From ZF- SR
measurements, it has been found that a slight amount of Zn tends to induce slowing down of the Cu-spin fluctuations in the whole superconducting regime of hole-doped system,
which is able to be interpreted as being due to pinning and stabilization of the dynamically fluctuating stripes of spins and holes Risdiana
et al.
, 2008, Adachi
et al.
, 2008.
Figure 2. ZF-
SR time spectra of hole-doped La
2-x
Sr
x
Cu
0.97
Zn
0.03
O
4
with
x
= 0.15 - 0.30 at 0.3 K Risdiana
et al.
, 2008. Figure 2 shows the ZF-
SR time spectra at 0.3 K for La
2-x
Sr
x
Cu
0.97
Zn
0.03
O
4
with
x
= 0.15 - 0.30 Risdiana
et al.
, 2008. For the optimally doped
x
= 0.15, a fast depolarization and a following flat spectrum around 13 are observed, suggesting the occurrence of a
short-range magnetic order Adachi
et al.
, 2008. Compared with the spectrum of
x
= 0.15, the muon-spin depolarization becomes weak with increasing
x
but still shows an exponential-like depolarization at
x
= 0.27. The exponential-like depolarization disappears at
x
= 0.30 where the superconductivity also disappears in the Zn-free
y
= 0.
6 Therefore, these results suggest that the pinning of the dynamically fluctuating stripes
occurs in the overdoped regime as well, that is, the stripe-pinning model holds good even for overdoped La
2-x
Sr
x
CuO
4
. This result might point to the importance of the dynamical stripe correlations in the appearance of high-T
c
superconductivity in the hole-doped system Emery
et al.
, 1999. In other hand, the mechanism of superconductivity in the electron-doped high-T
c
cuprates has not yet been clarified also. It is of great interest whether the stripe-pinning model holds good even for the electron-doped system or not
and whether the mechanism in the electron-doped high- T
c
cuprates is different from that in the hole-doped ones or not.
Figure 3. ZF-
SR time spectra of electron-doped Pr
0.86
LaCe
0.14
Cu
1-y
Zn
y
O
4+ -
with
y
= 0, 0.01, 0.02, 0.05 and large
values 0.04 ≤ ≤ 0.09 Risdiana
et al.
, 2010.
7 Figure 3 shows the ZF-
SR time spectra of Pr
0.86
LaCe
0.14
Cu
1-y
Zn
y
O
4+ -
with
y
= 0, 0.01, 0.02, 0.05 and large
values Risdiana
et a l.
, 2010. It is found that the spectra are independent of the Zn concentration, meaning that no impurity-induced slowing down of
the Cu-spin fluctuations is detected in electron-doped Pr
0.86
LaCe
0.14
CuO
4+ -
. That is, Zn impurities do not appear to affect the Cu-spin dynamics, which is very different from the
results of hole-doped La
2-x
Sr
x
Cu
1-y
Zn
y
O
4
as shown in Fig. 2. This may be understood in two ways. First, there may be no dynamically fluctuating stripes of spins and electrons in
the electron-doped cuprates, because the dynamical stripes lead to the impurity-induced magnetic order in the hole-doped cuprates Adachi
et al.
, 2004. Second, the effect of Pr
3+
moments may be too strong for the effect of a small amount of Zn impurities to be observed. To be conclusive, another electron-doped cuprate without Pr
3+
moments such as Eu
2-x
Ce
x
Cu
1-y
Zn
y
O
4
should be used for this study. Therefore, the study of spin dynamics in the electron-doped system and the following comparison with the results of
the hole-doped system will lead to an understanding of the general mechanism of the superconductivity.
8
CHAPTER III OBJECTIVE AND BENEFIT
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