Magnetization and transport properties in the superconducting Pr2 Ba4 Cu7 O15−δ withmetallic double-chain
Shibuki Toshima,1 Michiaki Matsukawa,1, ∗ Taiji Chiba,1 Satoru Kobayashi,1 Sigeki Nimori,2 and Makoto Hagiwara3

arXiv:1202.6363v1 [cond-mat.supr-con] 28 Feb 2012

1

Department of Materials Science and Engineering, Iwate University , Morioka 020-8551 , Japan
2
National Institute for Materials Science, Tsukuba 305-0047 ,Japan
3
Kyoto Institute of Technology, Kyoto 606-8585,Japan(Dated: March 1, 2012)

We have reported the effect of pressure on the magnetization, and transport properties in thenominal composition Pr2 Ba4 Cu7 O15−δ synthesized by a sol-gel technique. A reduction treatmentof the as-sintered sample in vacuum causes higher superconductivity achieving Tc,on =∼ 30 K
for δ = 0.94. Application of hydrostatic pressure on the oxygen depleted sample enhances its onset
temperature up to 36 K at 1.2 GPa, indicating the nearly optimum doping level of the charge carrierin comparison to the pressure dependence of lower Tc samples with δ = 0.45. Seebeck coefficient of
the superconducting sample shows a metallic conduction, followed by a clear drop below Tc,on andis in its temperature dependence below 100 K quite different from that of the non-superconducting
one. This finding strongly suggests a dramatic change of the electronic state along the CuO double
chain due to the reduction treatment for the appearance of superconductivity .
PACS numbers: 74.25.Ha,74.25.F-,74.90.+nI.

INTRODUCTION

Since the discovery of high-Tc copper-oxide superconductors, extensive studies on strongly electron correlated
system have been in progress on the basis of physicalproperties of two-dimensional (2D) CuO2 planes. Moreover, from the viewpoint of low-dimensional physics, particular attention is paid to the physical role of onedimensional (1D) CuO chains included in some families of high-Tc copper oxides such as Y-based superconductors with the transition temperature Tc =∼ 92K. It is well known that the Pr-substitution for Y-sites
in YBa2 Cu3 O7−δ (Y123) and YBa2 Cu4 O8 (Y124) compounds dramatically suppresses Tc and superconductivity in CuO2 planes disappears beyond the critical valueof Pr , xc =0.5 and 0.8, respectively.1,2 Such a suppression effect due to Pr-substitution on superconductivity has been explained in terms of the hybridization
model with respect to Pr-4f and O-2p orbitals.3 Y124
compound with double chains is thermally stable up to800 ◦ C ,while in Y123/7-δ oxygen deficiencies are easily introduced at lower annealing temperatures .4 Intermediate between PrBa2 Cu3 O7−δ (Pr123) with single chains and PrBa2 Cu4 O8 (Pr124) with double chainsis the Pr2 Ba4 Cu7 O15−δ (Pr247) compound with an alternative repetition of the CuO single chain and double chain blocks along the c-axis, isostructural with superconducting Y2 Ba4 Cu7 O15−δ (Y247).5,6 In contrast toY123, Y247 retains its orthorhombic structure and its
superconducting properties, even after oxygen along thesingle chains is completely depleted.7 This finding is considered to be due to the presence of the double CuO
chain block in Y247. For Pr247, it is possible to examine
physical properties of metallic double chains in terms ofvarying oxygen contents along single chains.
Recently, Matsukawa et al., discovered that oxygen removed polycrystalline Pr2 Ba4 Cu7 O15−δ with CuO

metallic double chain shows superconductivity aroundTc,on =∼ 15 K.8 Matsushita et al., revealed that Hall
coefficient of oxygen defect Pr247 is varied from positive
sign to negative one upon decreasing temperatures, indicating an electron doped superconductor.9 The nuclear
quadrapole resonance experiment indicated that the superconductivity of reduced Pr247 is realized at the CuO
double chains.10 For the appearance of Pr247 superconductor, it is essential to remove oxygen from the as sintered sample of Pr247.8,11 Through several works on synthesis of higher Tc samples in nominal Pr247 composition, it has been made clear that the heterogeneous structure containing both Pr123 and Pr124 phases in Pr247
system plays a crucial role on attaining higher Tc .12,13
High-resolution transmission electron microscopy (TEM)
analyses of the heterogeneous Pr247 sample exhibits that
there exists an irregular long-period stacking structure
along the c axis such as {-S-S-S-S-D-} sequence, where
S and D denote single CuO chain and double CuO chain
blocks, respectively.12
In this paper, we have reported the magnetic, electronic and thermal transport properties of the oxygen removed Pr2 Ba4 Cu7 O15−δ . In particular, the δ =0.94 sample exhibits higher superconductivity attaining
Tc,on =∼ 30 K.

II.

EXPERIMENT

Polycrystalline samples of the nominal composition
Pr2 Ba4 Cu7 O15−δ (Pr247) were synthesized using sol-gel
technique. High purity powders of praseodymium oxide(Pr6 O11 ), barium carbonate (BaCO3 ), and copper carbonate (CuCO3 ) were mixed to the stoichiometric composition, they were then added to a suitable amount of
citric acid solution and finally their mixture was magnetically stirred in a container. The solution was con-

2

(a)

Pr124

Pr247

Intensity (arb. units)

Intensity (arb. units)

Pr123

5

5.5

6

6.5

7

7.5

8

2θ (deg)

10

20

30

40

50

60

70

80

2θ (degree)

oxygen pressure (as-sintered sample). For the sinteredprocess, the electric furnace equipped with three temperature controllers was used to reduce thermal gradient along the axis of furnace tube. The resultant sample was post annealed in vacuum at 500 ◦ C for 48∼72
h (reduced sample). X-ray powder diffraction measurement was carried out with CuKα radiation at diffraction
angle range of 2θ = 5◦ − 80◦ . The electron probe micro analyzer (EPMA) analysis on the superconductive
sample annealed in vacuum revealed that the atomic ratio of Pr:Ba:Cu = 1.99:4.18:6.72, through an average ofcomposition analyses at several points within its sample.
The oxygen deficiency was estimated to be δ = 0.70 and
0.94 from gravimetric analysis for 48 h and 72 h reduction treatments,respectively. The electric resistivity wasmeasured with dc four terminal method. Seebeck coefficient S(T ) was determined from both measurements of athermoelectric voltage and temperature difference along
the longitudinal direction of the measured sample. The
Seebeck coefficient of copper lead was subtracted fromthe measured values using the published data. The thermal conductivity data were collected with a conventional
heat flow method. The dc magnetization was performed
using the commercial superconducting quantum interference device (SQUID) magnetometers both at Iwate University and National Institute for Materials Science. In
particular, to remove the remanent magnetic field below 1 mOe, the fluxgate device was utilized in the zerofield cooled (ZFC) and field cooled (FC) magnetization
measurements under H = 1 Oe. Hydrostatic pressure inmagnetization was applied by using a clamp-type CuBe
cell up to 1.2 GPa. Fluorinert was used as a pressuretransmitting medium. The magnitude of pressure wascalibrated by the pressure dependence of the critical temperature of lead.

III.

Fig. 1: (color online) (a)X-ray powder diffraction pattern and(b) SEM image of the superconductive sample with the nominal Pr2 Ba4 Cu7 O15−δ . The inset of (a) shows the low angle
diffraction data enlarged near 2θ = 5◦ − 8◦ . Here, three arrows point to the peaks corresponding to the Miller indexes(002), (004), and (001) of Pr124, Pr247, and Pr123, respectively upon increasing 2θ.

densed on a hot plate at 80 ◦ C for 30 min and the resultant citrate gel was pyrolyzed at 450 ◦ C for 2 h, toremove the organic compounds. The obtained precursor
was ground into a powder by a glass rod. The precursor
was calcined in air at 800 ◦ C for 24 h. The resultant
powder was pressed into a pellet and then sintered at875∼876 ◦ C for a long time over 180 h under ambient

RESULTS AND DISCUSSIONFirst of all, let us show in Fig.1 X-ray powder diffraction pattern of the superconductive sample with the nominal Pr2 Ba4 Cu7 O15−δ . The low angle diffraction data
enlarged near 2θ = 5◦ − 8◦ are presented in the inset
of Fig.1(a), where three arrows point to the peaks corresponding to the Miller indexes of Pr124, Pr247, andPr123 upon increasing 2θ. These findings indicate thatthe nominal phase of Pr247 obtained coexists with both
Pr124 and Pr123 phases. Under ambient pressure condition, it seems to be difficult to synthesize high quality
polycrystalline sample with nominal composition. However, to our knowledge, the heterogeneous samples haveshown good superconductive properties in comparison topure Pr247 with lower Tc,on =∼ 15 K.12 The scanningelectron microscope (SEM) micrographs of the superconducting sample with δ = 0.94 displayed in Fig.1(b) revealthat its typical grain size is estimated to be several µm.
Figure 2 (a) shows the temperature variation of zero
field cooled (ZFC) and field cooled (FC) magnetization

3
2

0

(a) H= 1 Oe(a)ZFC 50 Oe

0
T

C ON

0GPa

= 30.5 K

0.8GPa

-4

FC

-5

3x10

T

C ON

= 30.5 K

ZFC 50 Oe

-4

T

-6

2.8x10-5 ZFC

1.04

C ON

T

1

=33.5K

C ON

T

-5

2.6x10

ZFC

-5

28

30

32

-3

-2x10

0

5

10

34
T (K)

36

0.96
25

38

30

-10

15 20 25 30
Temperature (K)

35

0

40

=36K

C ON

0.98

-8

2.4x10

5

10

35

T (K)

15 20 25 30
Temperature (K)

40

35

40

40

0.005
(b) ZFC 20 Oe

TC ON = 21 K(b)

TC ON = 30K

35

0-0.005-0.015-0.02

5.4x10-4

5.2x10

-4

5x10

-4

4.8x10-4

4.6x10

0

5

10

T

C ON

δ=0.62

25

= 30K

20
15

δ=0.45-4

28-0.025

ZFC

c on

FC

2.5GPaT-0.01

δ=0.9430(K)

as-sintered
48 h
72 h

M (emu/g)

M (emu/g)

= 30K

1.02

-3

-1.5x100GPa
0.8GPa1.2GPa1.06

M/M(40K)

-3

-1x10

-5

3.2x101.2GPa-2

M /M(40K)FC
M (emu/g)

M (emu/g)

-5x10

30

32

34
T (K)

15 20 25 30
Temperature (K)

36

38

35

10
40

Fig. 2: (color online) (a)Temperature variation of zero fieldcooled (ZFC) and field cooled (FC) magnetization of oxygen removed Pr247 under an applied field of 1 Oe using thefluxgate device. (b) The magnetization curves of the oxygen
removed Pr247 under 20 Oe for several reduced conditions.
In the inset, the magnetization data are magnified to clarify
the definition of Tc,on .

of the oxygen removed Pr247 sample under an appliedfield of 1 Oe. In the inset of Fig. 2, the magnified data
are plotted, to clarify the definition of onset Tc . The
as-sintered sample exhibits no superconducting anomaly
as previously reported. However, the reduction treatment on its as-sintered one in vacuum results in diamagnetic character from Tc,on =∼ 21 K for 48 h up to
Tc,on =∼ 30K K for 72 h, which reaches highest value
over the previous works.8,11,12 The magnetic susceptibility data taken under 1 Oe using the fluxgate device arepresented for the 72 h reduction sample. From the ZFC
magnetization at 2 K, the superconducting volume fraction is estimated to be ∼ 10%, indicating bulk supercon-

0

0.2

0.4

0.6 0.8
P (GPa)1

1.2

Fig. 3: (color online) (a) Temperature variation of normalized
magnetization of oxygen removed Pr247 under hydrostaticpressures of 0, 0.8 and 1.2GPa. The enlarged plots are shownin the inset, to clarify the effect of external pressure on Tc,on .
(b) The pressure dependence of Tc,on of the Pr247 sample
with the oxygen defect δ = 0.94 as a function of the appliedpressure up to 1.2 GPa. For comparison, the data of pure
Pr247 with δ = 0.45 and 0.62 prepared by a high oxygenpressure technique are cited.14,15

ductivity. Next, let us examine the effect of external pressure on the low-T magnetization in the oxygen removed
Pr247. First of all, application of hydrostatic pressure
on the superconducting sample enhances Tc,on =∼ 30 K
at 0 GPa up to 36 K at 1.2 GPa as shown in Fig. 3,
where the data included the magnetization of the pressure cell are normalized by the value of magnetization at
40 K. In the inset of Fig. 3(a), the enlarged plots are
displayed to emphasize the variation of Tc,on with pressure. In Fig.3 (b), the effect of pressure on Tc,on of pure
Pr247 with δ = 0.45 prepared by a high pressure oxygen
technique cited from ref.14 shows a rapid rise of Tc,on

4from 15 K at ambient pressure up to 24 K at 1 GPa, indicating the stronger dependence of Tc,on on pressure incomparison to the present result. Recently, Ishikawa hasreported that the application of pressure on the onset of
Tc in Pr2 Ba4 Cu7 O15−δ (δ=0.62) increases from 20.6 K at
0 GPa up to 33.1K at 2.5 GPa. Furthermore, increasingpressure above 3 GPa, the superconductivity disappeared
and under high pressure above 5 GPa the temperature
variation of the resistivity showed a semiconducting likebehavior.15 These findings suggest that if the pure Pr247
sample is located in the underdoped region, the present
Pr247 sample is near the optimum one. Second, the magnitude of negative magnetization (the volume of the superconducting state) is suppressed with the applied pressure of 1.2 GPa, which is contrast to the pressure effect ofTc,on . Sano et al discussed the suppression of the superconductivity in the CuO double chain under pressure on
the basis of Tomonaga-Luttinger liquid theory.17 If thevolume shrinkage due to the application of pressure gives
rise to a decrease of the distance between the two chainsof a CuO double chain, we then expect that an increaseof the hopping term between the nearest neighbor oxygensites occupying 2p orbitals, resulting in a suppression of
the superconducting corerelation. Their calculation does
not take into account for the pressure dependence of carrier concentration, giving no explanation for the overallchange of the superconducting state with pressure. Theδ dependence of Tc,on shows a monotonous increase from15 K at δ = 0.45 up to 30 K at δ = 0.94, which is qualitatively in good agreement with a theoretical prediction by
Nakano.18 Comparing the effect of pressure on Tc,on withthe δ dependence of it, we expect that the application ofpressure gives rise to an increase in the superconducting
carrier concentration.
For several reduction conditions, the temperature variation of electrical resistivity of the present sample Pr247
is given in Fig. 4 (a). For 72h reduced case, ρ(T) curve
exhibits its dramatic drop near 30 K followed by zero
resistance at low temperatures. We notice that the resistivity in the normal state for except lower temperature
region rises gradually with increasing the reduction time.
This tendency seems to be not anomalous but common
behavior in the present system because an increase of theresistivity in CuO2 plane, ρpl , due to the reduction treatment is predominant over a decrease of that along CuO
double chain, ρch at higher temperatures.
Finally, we check thermal transport properties of thesuperconducting and non-superconducting Pr247 (Fig.
4(b)).(Thermal conductivity data are not shown here)
Seebeck coefficient S is a sensitive probe to examine theelectronic state of highly inhomogeneous materials since
the thermal current is hard to be disturbed by the grainboundaries in contrast to the electric current. Upon decreasing T , Seebeck coefficient S(T ) of the superconducting sample reaches a maximum peak, then shows a rapid
decrease below 200 K, and finally followed by a substantial drop below Tcon . It is true that the behavior of S(T )of the as-sintered non-superconducting sample is, bothin its temperature dependence and magnitude, similar
to that of Pr124 with metallic double chain.19 The magnitude of S of Pr124 reaches a small value less than 1µV/K below 100 K, which is explained on the basis of theone- to two-dimensional (1D-2D) crossover. For conventional metal, Seebeck coefficient is expressed in terms of
S = (πk)2 T /3e{∂ln(N (E))/∂E}, where N (E) is the density of electronic state.20 In our rough estimation, if we
assume that the magnitude of S is proportinal to the energy derivative of N (E), the density of state for 2D metal
is independent of the energy, giving S(T ) ∼ 0. However,the S value of the superconducting Pr247 shows a finite
value of at most a few µV/K down to Tcon . This finding
is probably ascribed to a variation of the electronic stateof the CuO double chain associated with carrier doping
into the double-chain block due to the reduction treatment. Upon decreasing T , the Hall coefficient RH of the
24 h reduced sample changes its sign across 100 K andthen exhibits a rapid increases with negative sign. We
expect that the disagreement in sign between RH and
S in the oxygen removed sample is ascribed to the dramatic variation of the density of states with reduction
treatment since ∂(N (E))/∂E near the Fermi energy isrelated to the sign of Seebeck coefficient in addition to
the sign of charge carrier.
A previous work21 on role of oxygen in Pr123 compound revealed that the oxygen deficient Pr123 phaseremains highly insulating state, causing neither metallic
or superconducting sign. Double chain conduction in single crystalline Pr124 exhibits a strongly metallic character but no superconducting transition down to 2 K.22 We
concentrate on single-chain rich microdomains present in
the nominal Pr247 as observed by TEM measurements12 ,which is probably close to the appearance of higher
Tc,on = 30 K accompanied by the larger oxygen defectδ = 0.94. It is, thus, expected that a reduction treatment
removes oxygen from CuO single chain rich sequence suchas {S-D-S-S-S-D} and results in higher carrier doping into
its surrounding CuO double chain in comparison to thatin the regular sequence {S-D-S-D-S-D}.
In summary, we have demonstrated the pressure effectof Tc,on , and the transport properties of the nominal composition Pr2 Ba4 Cu7 O15−δ . Magnetization and resistivity
measurements of the δ=0.94 sample reduced in vacuum
exhibit Tc,on =∼ 30 K. The pressure dependence of Tc,onof the δ = 0.94 sample strongly suggests the nearly optimum doping of the superconducting carries. Comparing
the effect of pressure on Tc,on with the δ dependence of it,
we notice that the application of pressure gives rise to anincrease in the carrier concentration. We believe that theheterogeneous structure of the nominal Pr247 is closely
related to the appearance of higher TC . Seebeck coefficient of the δ=0.94 sample shows a metallic conduction
below 100 K and is in its temperature dependence quite
different from the data of the as-sintered one, leading toa dramatic change of the density of state along the CuO
metallic double chain.

5
Acknowledgments

supported by a Grant-in-Aid for Scientific Research fromJapan Society of the Promotion of Science.

The authors are grateful for Dr. K. Nonaka for his assistance in EPMA experiments. This work was partiallyElectronic address: matsukawa@iwate-u.ac.jp
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11

6

0.03
(a)

as sintered
48h
72h

0.025

ρ (Ω cm)

0.02
0.015
0.01
0.005
0
0

50

100

150

200

250

300

T (K)

35

(b)

30

as-sintered
72h reduction

20

5

15
4

Tc on

10

S (µV/K)

S (µV/K)

25

3
2

5
1

0

0
0

10

-5
0

50

100

150
T (K)

200

20
30
T (K)

250

40

50

300

Fig. 4: (color online) (a)Temperature dependence of the resistivity of oxygen removed Pr247 for several annealing conditions.(b)Temperature dependence of Seebeck coefficient for
the as-sintered and oxygen removed Pr247. In the inset, themagnified data of the reduced sample are presented.