5
CHAPTER II LITERATURE STUDY
Superconductor is a material showing a phenomenon that has zero electrical resistivity when they are cooled down to sufficiently low temperatures. The
temperature at which the electrical resistivity starts to be zero is called the critical temperature T
c
. Because of this phenomenon, some applications are developed for future technology especially for energy saving such as for superconductor cable
without any energy loss of electric power transmission and high performance power storage devices.
Most of superconductors are cuprate compounds. These superconducting cuprates are often called high-T
c
superconducting cuprates HTSC. The era of high- T
c
superconducting cuprates 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.
Figure 1. Schematic phase diagram of the hole- and electron-doped 214 cuprate. AF and SC indicate antiferromagnetic order and superconductivity, respectively.
0.2 0.1
0.1 0.2
T
x x
AF S
S Electron-doped
Hole-doped
Electron doping Hole doping
0.3
100 200
300
6 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. Impurity-substitution effects to the some physical properties have been also
intensively studied to understand the role and mechanism of superconductivity. 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 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
7 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. 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. 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. For Fe substitution samples of La
2-x
Sr
x
Cu
1-y
Fe
y
O
4
, it has been found that the magnetic transition temperature and magnetic correlation are enhanced through the 1 Fe
substitution in a wide range of hole concentration where superconductivity appears in Fe-free La
2 −x
Sr
x
CuO
4
Suzuki et al., 2012.
8 Figure 3.
Temperature dependence of the depolarization rate of the slowly depolarizing component
λ for typical values of x in La
2 −x
Sr
x
Cu
1 −y
Fe
y
O
4
with y = 0.01 Suzuki et al., 2012.
Figure 3 show the temperature dependence of the depolarization rate of the slowly depolarizing component
λ for typical values of x in La
2 −x
Sr
x
Cu
1 −y
Fe
y
O
4
with y = 0.01 Suzuki et al., 2012. It appears that two peaks of
λ observed in x = 0.16
originate from the stripe order of Cu spins at low temperatures below ~ 4 K and from the spin-glass state of Fe spins at temperatures below ~ 15 K. Therefore, it is guessed
that the two magnetic state change with changing hole doping. 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. We measured 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 al., 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
9 be conclusive, another electron-doped cuprate without Pr
3+
moments such as Eu
2- x
Ce
x
CuO
4
should be used for this study. An advantage of this sample rather than other types of electron-doped systems is that the absence of rare earth moments such as Pr
3+
moments in Pr
1-x
LaCe
x
CuO
4
. Using Eu
2-x
Ce
x
CuO
4
samples, we can directly observe the dynamic behavior of Cu-spins and effects of impurity to them without any
magnetic disturbance. Thus, this system would be a good candidate to achieve the answer to the objective of this proposed study.
Due to the previous study of effect of Fe on the Cu-spin dynamics in hole- doped system, partially Fe substitution in electron-doped cuprates is potential
impurity to study the Cu-spin dynamics in the electron-doped system. 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.
10
CHAPTER III RESEARCH OBJECTIVE AND ADVANTAGES
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