- Email: [email protected]
Phase diagram of the electron-doped superconductor Pr1−xLaCexCuO4−δ
Physica C 392–396 (2003) 216–220 www.elsevier.com/locate/physc
Phase diagram of the electron-doped superconductor Pr1xLaCexCuO4d S. Kuroshima a
a,* ,
M. Fujita a, T. Uefuji a, M. Matsuda b, K. Yamada
a
Institute for Chemical Research, Solid State Chemistry Lab. 2, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan b Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaragi 319-1195, Japan Received 13 November 2002; accepted 7 February 2003
Abstract Systematic measurements of magnetic susceptibility and neutron diffraction were performed on both as-grown and oxygen-reduced single crystals of Pr1x LaCex CuO4d (x ¼ 0:09, 0.11, 0.15 and 0.18) in order to study the electronic phase diagram as functions of both x and d. Antiferromagnetic ordered phase appears in the x ¼ 0:09 and 0.11 samples for small value of d at low temperature and degrades by either increasing Ce concentration or d. Bulk superconductivity was observed in the wider x and d region compared to that of Nd2x Cex CuO4 . A spin-glass-like phase appears in the limited region of d in between 0.003 and 0.04, which weakly depends on x. Combining both results from susceptibility and neutron scattering measurement, we have obtained x–d phase diagram of Pr1x LaCex CuO4d . Ó 2003 Published by Elsevier B.V. PACS: 74.25.Dw; 74.62.Dh; 74.72.)h; 74.72.Jt; 75.30.Cr Keywords: High-Tc superconductivity; Electron-doped cuprate; Phase diagram
1. Introduction In high-Tc cuprate superconductors, the superconductivity is caused by hole or electron doping into Mott insulator. In La2x Srx CuO4 (LSCO), it is well known that three-dimensional antiferromagnetic (AF) ordered phase quickly disappears and changes into the spin-glass (SG)-like phase upon hole-doping [1,2]. Superconducting (SC) phase appears by further doing. In contrast, in the electron-
* Corresponding author. Tel.: +81-774-38-3114; fax: +81774-38-3118. E-mail address: [email protected] (S. Kuroshima).
0921-4534/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/S0921-4534(03)01233-4
doped Nd2x Cex CuO4 (NCCO), AF phase is robust against electron-doping and narrow SC phase adjoins to the wide AF phase [3]. Therefore, the origin of differences seen in the both phase diagrams has been studied in order to clarify the common mechanism of the high-Tc superconductivity. On the other hand, hole-doped superconductivity is also caused by introducing excess oxygen into La2 CuO4 . Besides, for the appearance of electron-doped superconductivity oxygen reduction procedure is necessary [4], although the role and mechanism is not fully understood. Thus, change of physical properties induced by oxygen is important. We, therefore, investigated magnetic and superconducting properties as a function of oxygen loss (d) for Pr1x LaCex CuO4d (PLCCO)
S. Kuroshima et al. / Physica C 392–396 (2003) 216–220
2. Experimental The single crystals of PLCCO (x ¼ 0:09, 0.11, 0.15 and 0.18) system were grown through a traveling-solvent floating-zone (TSFZ) method under oxygen gas flow. As-grown crystal rod (30 mm in length and 6 mm in diameter) was sliced into thin disk with the thickness of 1 mm. In order to control the oxygen contents in the samples, sliced crystals were carefully annealed under argon gas flow for 12 h at various temperatures between 750 and 950 °C. Relative change in oxygen content (d) per unit formula after the annealing treatment was estimated from the weight loss of the sample. In this paper, we assume d ¼ 0 for as-grown samples. An inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to determine the absolute Ce concentration in each sample. Magnetization measurements were carried out with utilizing a SQUID magnetometer (Quantum Design Co.). Data were collected for both fieldcooled (FC) and zero-field-cooled (ZFC) samples. In order to determine long range magnetic order phase, we performed neutron scattering experiments on the TAS-2 thermal neutron triple-axis spectrometer located at the JAERI JRR-3M reactor. The incident neutron energy was fixed at 14.7 meV. 3. Results In Fig. 1, peak intensity at (1 2 0) magnetic Bragg position in orthogonal notation are shown for x ¼ 0:09 and d ¼ 0:047 sample. Inset shows peak profile through (1 2 0) position for x ¼ 0:15 and d ¼ 0:042 sample. At x ¼ 0:09, peak intensity increases at low temperature, while there is no
Intensity (counts/10sec.)
Pr1-xLaCexCuO4-δ Ei=14.7meV, ω=0meV, (1, 2, 0)ortho 8000
Intensity (counts/4min.)
system, in which the range of oxygen nonstoichiometry is much wider than that in NCCO. And magnetic moments of rear-earth metal atoms are smaller than in NCCO, so PLCCO is more suitable to study the nature of Cu2þ spins that will play important role for superconductivity. In this paper we present phase diagram obtained from the PLCCO samples which oxygen contents were carefully controlled.
217
6000
4000
2000 0
400
x=0.15 δ=0.042
7K 62K
200
0
0.96
1 h (r.l.u.)
1.14
x=0.09 δ=0.047 0
50
100
150
200
Temperature (K) Fig. 1. Temperature dependence of elastic neutron scattering peak intensity at (1 2 0) magnetic Bragg position in orthogonal notation for x ¼ 0:09 and d ¼ 0:047 sample. Inset: peak profile around (1 2 0)ortho for x ¼ 0:15 and d ¼ 0:042 sample.
difference between the profiles at 7 and 62 K in x ¼ 0:15 sample. Thus, this result suggests the existence of long range antiferromagnetic order in x ¼ 0:09 degrades with increasing Ce concentration. The weak peak remaining at higher temperature appears by reduction treatment and is independent of temperature. We next investigated the superconducting properties in the samples with different Ce concentrations as a function of d. According to the previously obtained phase diagram [7], AF ordered phase exists in the sample with x ¼ 0:11 and d < 0:05, while no AF order is expected in the highly Ce-doped sample with x ¼ 0:18. Fig. 2 shows the temperature dependence of magnetic susceptibility for (a) x ¼ 0:11 and (b) 0.18 with various d. All data were taken in ZFC process applying a magnetic field of 10 Oe. At x ¼ 0:11, bulk superconductivity appears in the samples with d ¼ 0:047 and 0.060. Therefore, oxygen reduction is needed to obtain superconducting samples in PLCCO system. However, as seen in the d ¼ 0:060 sample, superconductivity degrades compared to those of d ¼ 0:047 sample, which demonstrates that there exists an optimum condition of oxygen reduction, analogous to the case of NCCO [6]. On the other hand, only fractional
218
S. Kuroshima et al. / Physica C 392–396 (2003) 216–220
Pr1-xLaCexCuO4-δ x=0.11 H=10 Oe
Pr1-xLaCexCuO4-δ
0.01
(a) x=0.11 as-grown
ZFC FC
δ=0.060 -5
2 10
-0.01 ZFC H=10 Oe
δ=0.047 -0.02 0.01
0 (b) x=0.18
δ=0.038 0 δ=0.051 δ=0.070 0
10
20
30
40
Temperature (K) Fig. 2. A series of magnetic susceptibility of as-grown and reduced samples in PLCCO (x ¼ 0:11 and 0.18) system.
superconductivity is obtained for the d up to 0.070. The diamagnetic susceptibility gradually increases with increasing d. Fig. 3 shows temperature dependence of susceptibility for x ¼ 0:11 with as-grown sample, and oxygen-reduced sample with d ¼ 0:025 and 0.050. Result for d ¼ 0:025 shows a SG-like behavior, that is, there exists a clear difference between FC and ZFC susceptibilities. The onset temperature for the appearance of difference in FC and ZFC data (TSG ) is 120 K, which is much higher than TSG (10 K) in the LSCO system [1,2]. This SGlike behavior in the magnetic susceptibility was not observed in the as-grown and highly oxygenreduced samples. Thus SG-like phase lies around d ¼ 0:025. In Fig. 4, we present phase diagrams as a function of d for (a) x ¼ 0:11 and (b) 0.18, respectively. Closed triangles indicate the onset temperature for the magnetic order measured by the neutron scattering measurement. Closed and open circles represent respectively Tc and TSG determined by the magnetic susceptibility measurement. From the comparison between the phase diagrams for two different Ce concentration, it is
Susceptibility (emu/Oe/g)
Susceptibility (emu/Oe/g)
δ=0.038
-0.01
(a) as-grown (δ=0)
-5
4 10
0
(b) δ=0.025
-5
ZFC FC
4 10
-5
2 10
0 (c) δ=0.050
-5
4 10
ZFC FC
-5
2 10
0 0
50
100
150
200
Temperature (K) Fig. 3. Temperature dependence of the magnetic susceptibility for the PLCCO (x ¼ 0:11) samples, which oxygen loss are (a) d ¼ 0, (b) 0.025 and (c) 0.050, respectively. A magnetic field of 10 Oe was applied. Both ZFC and FC data are included.
concluded that AF and bulk SC phases disappear by Ce doping while SG-like phase remains in the x ¼ 0:18 sample with similar d region as that for x ¼ 0:11. Fig. 5 summarizes the phase diagram for x and d. In this phase diagram the positions of marks correspond to the values of x and d for the studied samples. The samples with circle show superconductivity. The SC phase in this diagram is divided into bulk and fractional SC phases. In the fractional SC phase the magnitude of diamagnetic susceptibility is smaller than 0.005 at lowest tem-
S. Kuroshima et al. / Physica C 392–396 (2003) 216–220
Pr1-xLaCexCuO4-δ
Pr1-xLaCexCuO4-δ
300
0.08
(a) x=0.11
219
Bulk SC
200
SG-like 0.06 AF
Temperature (K)
Oxygen loss δ
Fractional SC
100
SC 0 300 (b) x=0.18
Fractional SC 0.04
AF 0.02
SG-like
200 SG-like
0 0.08
100 Fractional SC 0
0
0.02
0.04
0.06
0.1
0.12
0.14
0.16
0.18
0.2
Ce concentration x 0.08
Oxygen loss δ Fig. 4. Phase diagrams to oxygen loss d and temperature for (a) x ¼ 0:11 and (b) 0.18. Closed circles, open circles and closed triangles indicate onset temperatures of superconducting (SC), SG-like and antiferromagnetic (AF) phases, respectively.
perature. Open squares and closed triangles respectively correspond to the samples showing the SG-like magnetic susceptibility and the long ranged AF order at low temperature. The AF phase exists in the low-x and low-d region and competes with the SC phase near the phase boundary. The critical value of d for the appearance of bulk superconductivity rapidly increases with x. On the other hand, the SG-like behavior was observed in the sample with 0:003 < d < 0:04, nearly independent of the Ce concentration. Fractional SC and SG-like phases partially overlap possibly due to inhomogeneous distribution or defect of oxygen atoms in the sample.
4. Discussion Present result clearly demonstrates that the appearance of bulk superconductivity depends on
Fig. 5. Phase diagram to Ce concentration x and oxygen loss d for PLCCO system. Circles, open squares and closed triangles show compositions which have SC, SG-like and AF phases, respectively. A solid line shows a boundary of the AF phase and the boundary between x ¼ 0:11 and 0.15 is not clearly determined.
both values of x and d, namely, more oxygen should be removed from samples with higher Ce concentration. It should be noted that this relation between x and d is not understood if both Cedoping and oxygen reduction dope mobile electrons. In other words, the effective doping rate in this system cannot be simply described by the sum of the doping concentration evaluated from the values of x and d. Carrier localization by Ce-doping and/or excessive oxygen is expected to occur. Comparing to the phase diagram for NCCO, in which the bulk superconductivity appears in the doping range, 0:14 < x < 0:17 and 0:02 < d < 0:06 [5], the SC region is much wider in PLCCO for both x and d. Since AF phase of the PLCCO system is narrower than that of NCCO system, the competing SC phase can extend towards the region with lower x and d. As already discussed [7], such difference between NCCO and PLCCO is ascribed by the difference of lattice constant between the two systems. The SG-like behavior firstly observed in the electron-doped cuprates appears only in the
220
S. Kuroshima et al. / Physica C 392–396 (2003) 216–220
limited region of d. Although the similar SG behavior is observed in the hole-doped LSCO, we speculate the origin is different between LSCO and PLCCO. For the former the SG behavior is related with magnetic frustration induced by doped holes. In addition, TSG is much lower in LSCO than in PLCCO. We speculate the magnetic frustration can be induced by Ce-substituting and the SG-like phase appears only when the doped carrier is localized by the excessive oxygen. However, as seen in the as-grown sample strong localization induces three-dimensional antiferromagnetic order rather than spin-glass.
Japanese Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research on Priority Areas (Novel Quantum Phenomena in Transition Metal Oxides), 12046239, 2000, for Scientific Research (A), 10304026, 2000, for Encouragement of Young Scientists, 13740216, 2001 and for Creative Scientific Research (13NP0201) ‘‘Collaboratory on Electron Correlations––Toward a New Research Network between Physics and Chemistry’’, by the Japan Science and Technology Corporation, the Core Research for Evolutional Science and Technology Project (CREST).
5. Conclusion
References
We performed systematic measurements of the magnetic susceptibility of the carefully reduced PLCCO samples. AF phase is reduced and SC phase spreads in x–d phase diagram. And an SGlike phase is observed in the limited region of d.
[1] F.C. Chou, N.R. Belk, M.A. Kastner, R.J. Birgeneau, A. Aharony, Phys. Rev. Lett. 75 (1995) 2204. [2] S. Wakimoto, S. Ueki, Y. Endoh, K. Yamada, Phys. Rev. B 62 (2000) 3547. [3] H. Takagi, S. Uchida, Y. Tokura, Phys. Rev. Lett. 62 (1989) 1197. [4] A. Serquis, F. Prado, A. Caneiro, Physica C 313 (1999) 271. [5] Y. Koike, A. Kakimoto, M. Yoshida, H. Inozuka, T. Noji, Y. Saito, Physica B 165 & 166 (1990) 1665. [6] K. Kurahashi, H. Matsushita, M. Fujita, K. Yamada, J. Phys. Soc. Jpn. 70 (2002) 910. [7] M. Fujita, T. Kubo, S. Kuroshima, T. Uefuji, K. Kawashima, K. Yamada, I. Watanabe, K. Nagamine, cond-mat/ 0203320, 2002.
Acknowledgements We thank M. Kufu and K. Hirota for technical assistance of ICP measurements at Tohoku University. This work was supported in part by the
Phase diagram of the electron-doped superconductor Pr1xLaCexCuO4d S. Kuroshima a
a,* ,
M. Fujita a, T. Uefuji a, M. Matsuda b, K. Yamada
a
Institute for Chemical Research, Solid State Chemistry Lab. 2, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan b Advanced Science Research Center, Japan Atomic Energy Research Institute, Tokai, Ibaragi 319-1195, Japan Received 13 November 2002; accepted 7 February 2003
Abstract Systematic measurements of magnetic susceptibility and neutron diffraction were performed on both as-grown and oxygen-reduced single crystals of Pr1x LaCex CuO4d (x ¼ 0:09, 0.11, 0.15 and 0.18) in order to study the electronic phase diagram as functions of both x and d. Antiferromagnetic ordered phase appears in the x ¼ 0:09 and 0.11 samples for small value of d at low temperature and degrades by either increasing Ce concentration or d. Bulk superconductivity was observed in the wider x and d region compared to that of Nd2x Cex CuO4 . A spin-glass-like phase appears in the limited region of d in between 0.003 and 0.04, which weakly depends on x. Combining both results from susceptibility and neutron scattering measurement, we have obtained x–d phase diagram of Pr1x LaCex CuO4d . Ó 2003 Published by Elsevier B.V. PACS: 74.25.Dw; 74.62.Dh; 74.72.)h; 74.72.Jt; 75.30.Cr Keywords: High-Tc superconductivity; Electron-doped cuprate; Phase diagram
1. Introduction In high-Tc cuprate superconductors, the superconductivity is caused by hole or electron doping into Mott insulator. In La2x Srx CuO4 (LSCO), it is well known that three-dimensional antiferromagnetic (AF) ordered phase quickly disappears and changes into the spin-glass (SG)-like phase upon hole-doping [1,2]. Superconducting (SC) phase appears by further doing. In contrast, in the electron-
* Corresponding author. Tel.: +81-774-38-3114; fax: +81774-38-3118. E-mail address: [email protected] (S. Kuroshima).
0921-4534/$ - see front matter Ó 2003 Published by Elsevier B.V. doi:10.1016/S0921-4534(03)01233-4
doped Nd2x Cex CuO4 (NCCO), AF phase is robust against electron-doping and narrow SC phase adjoins to the wide AF phase [3]. Therefore, the origin of differences seen in the both phase diagrams has been studied in order to clarify the common mechanism of the high-Tc superconductivity. On the other hand, hole-doped superconductivity is also caused by introducing excess oxygen into La2 CuO4 . Besides, for the appearance of electron-doped superconductivity oxygen reduction procedure is necessary [4], although the role and mechanism is not fully understood. Thus, change of physical properties induced by oxygen is important. We, therefore, investigated magnetic and superconducting properties as a function of oxygen loss (d) for Pr1x LaCex CuO4d (PLCCO)
S. Kuroshima et al. / Physica C 392–396 (2003) 216–220
2. Experimental The single crystals of PLCCO (x ¼ 0:09, 0.11, 0.15 and 0.18) system were grown through a traveling-solvent floating-zone (TSFZ) method under oxygen gas flow. As-grown crystal rod (30 mm in length and 6 mm in diameter) was sliced into thin disk with the thickness of 1 mm. In order to control the oxygen contents in the samples, sliced crystals were carefully annealed under argon gas flow for 12 h at various temperatures between 750 and 950 °C. Relative change in oxygen content (d) per unit formula after the annealing treatment was estimated from the weight loss of the sample. In this paper, we assume d ¼ 0 for as-grown samples. An inductively coupled plasma-atomic emission spectrometry (ICP-AES) was used to determine the absolute Ce concentration in each sample. Magnetization measurements were carried out with utilizing a SQUID magnetometer (Quantum Design Co.). Data were collected for both fieldcooled (FC) and zero-field-cooled (ZFC) samples. In order to determine long range magnetic order phase, we performed neutron scattering experiments on the TAS-2 thermal neutron triple-axis spectrometer located at the JAERI JRR-3M reactor. The incident neutron energy was fixed at 14.7 meV. 3. Results In Fig. 1, peak intensity at (1 2 0) magnetic Bragg position in orthogonal notation are shown for x ¼ 0:09 and d ¼ 0:047 sample. Inset shows peak profile through (1 2 0) position for x ¼ 0:15 and d ¼ 0:042 sample. At x ¼ 0:09, peak intensity increases at low temperature, while there is no
Intensity (counts/10sec.)
Pr1-xLaCexCuO4-δ Ei=14.7meV, ω=0meV, (1, 2, 0)ortho 8000
Intensity (counts/4min.)
system, in which the range of oxygen nonstoichiometry is much wider than that in NCCO. And magnetic moments of rear-earth metal atoms are smaller than in NCCO, so PLCCO is more suitable to study the nature of Cu2þ spins that will play important role for superconductivity. In this paper we present phase diagram obtained from the PLCCO samples which oxygen contents were carefully controlled.
217
6000
4000
2000 0
400
x=0.15 δ=0.042
7K 62K
200
0
0.96
1 h (r.l.u.)
1.14
x=0.09 δ=0.047 0
50
100
150
200
Temperature (K) Fig. 1. Temperature dependence of elastic neutron scattering peak intensity at (1 2 0) magnetic Bragg position in orthogonal notation for x ¼ 0:09 and d ¼ 0:047 sample. Inset: peak profile around (1 2 0)ortho for x ¼ 0:15 and d ¼ 0:042 sample.
difference between the profiles at 7 and 62 K in x ¼ 0:15 sample. Thus, this result suggests the existence of long range antiferromagnetic order in x ¼ 0:09 degrades with increasing Ce concentration. The weak peak remaining at higher temperature appears by reduction treatment and is independent of temperature. We next investigated the superconducting properties in the samples with different Ce concentrations as a function of d. According to the previously obtained phase diagram [7], AF ordered phase exists in the sample with x ¼ 0:11 and d < 0:05, while no AF order is expected in the highly Ce-doped sample with x ¼ 0:18. Fig. 2 shows the temperature dependence of magnetic susceptibility for (a) x ¼ 0:11 and (b) 0.18 with various d. All data were taken in ZFC process applying a magnetic field of 10 Oe. At x ¼ 0:11, bulk superconductivity appears in the samples with d ¼ 0:047 and 0.060. Therefore, oxygen reduction is needed to obtain superconducting samples in PLCCO system. However, as seen in the d ¼ 0:060 sample, superconductivity degrades compared to those of d ¼ 0:047 sample, which demonstrates that there exists an optimum condition of oxygen reduction, analogous to the case of NCCO [6]. On the other hand, only fractional
218
S. Kuroshima et al. / Physica C 392–396 (2003) 216–220
Pr1-xLaCexCuO4-δ x=0.11 H=10 Oe
Pr1-xLaCexCuO4-δ
0.01
(a) x=0.11 as-grown
ZFC FC
δ=0.060 -5
2 10
-0.01 ZFC H=10 Oe
δ=0.047 -0.02 0.01
0 (b) x=0.18
δ=0.038 0 δ=0.051 δ=0.070 0
10
20
30
40
Temperature (K) Fig. 2. A series of magnetic susceptibility of as-grown and reduced samples in PLCCO (x ¼ 0:11 and 0.18) system.
superconductivity is obtained for the d up to 0.070. The diamagnetic susceptibility gradually increases with increasing d. Fig. 3 shows temperature dependence of susceptibility for x ¼ 0:11 with as-grown sample, and oxygen-reduced sample with d ¼ 0:025 and 0.050. Result for d ¼ 0:025 shows a SG-like behavior, that is, there exists a clear difference between FC and ZFC susceptibilities. The onset temperature for the appearance of difference in FC and ZFC data (TSG ) is 120 K, which is much higher than TSG (10 K) in the LSCO system [1,2]. This SGlike behavior in the magnetic susceptibility was not observed in the as-grown and highly oxygenreduced samples. Thus SG-like phase lies around d ¼ 0:025. In Fig. 4, we present phase diagrams as a function of d for (a) x ¼ 0:11 and (b) 0.18, respectively. Closed triangles indicate the onset temperature for the magnetic order measured by the neutron scattering measurement. Closed and open circles represent respectively Tc and TSG determined by the magnetic susceptibility measurement. From the comparison between the phase diagrams for two different Ce concentration, it is
Susceptibility (emu/Oe/g)
Susceptibility (emu/Oe/g)
δ=0.038
-0.01
(a) as-grown (δ=0)
-5
4 10
0
(b) δ=0.025
-5
ZFC FC
4 10
-5
2 10
0 (c) δ=0.050
-5
4 10
ZFC FC
-5
2 10
0 0
50
100
150
200
Temperature (K) Fig. 3. Temperature dependence of the magnetic susceptibility for the PLCCO (x ¼ 0:11) samples, which oxygen loss are (a) d ¼ 0, (b) 0.025 and (c) 0.050, respectively. A magnetic field of 10 Oe was applied. Both ZFC and FC data are included.
concluded that AF and bulk SC phases disappear by Ce doping while SG-like phase remains in the x ¼ 0:18 sample with similar d region as that for x ¼ 0:11. Fig. 5 summarizes the phase diagram for x and d. In this phase diagram the positions of marks correspond to the values of x and d for the studied samples. The samples with circle show superconductivity. The SC phase in this diagram is divided into bulk and fractional SC phases. In the fractional SC phase the magnitude of diamagnetic susceptibility is smaller than 0.005 at lowest tem-
S. Kuroshima et al. / Physica C 392–396 (2003) 216–220
Pr1-xLaCexCuO4-δ
Pr1-xLaCexCuO4-δ
300
0.08
(a) x=0.11
219
Bulk SC
200
SG-like 0.06 AF
Temperature (K)
Oxygen loss δ
Fractional SC
100
SC 0 300 (b) x=0.18
Fractional SC 0.04
AF 0.02
SG-like
200 SG-like
0 0.08
100 Fractional SC 0
0
0.02
0.04
0.06
0.1
0.12
0.14
0.16
0.18
0.2
Ce concentration x 0.08
Oxygen loss δ Fig. 4. Phase diagrams to oxygen loss d and temperature for (a) x ¼ 0:11 and (b) 0.18. Closed circles, open circles and closed triangles indicate onset temperatures of superconducting (SC), SG-like and antiferromagnetic (AF) phases, respectively.
perature. Open squares and closed triangles respectively correspond to the samples showing the SG-like magnetic susceptibility and the long ranged AF order at low temperature. The AF phase exists in the low-x and low-d region and competes with the SC phase near the phase boundary. The critical value of d for the appearance of bulk superconductivity rapidly increases with x. On the other hand, the SG-like behavior was observed in the sample with 0:003 < d < 0:04, nearly independent of the Ce concentration. Fractional SC and SG-like phases partially overlap possibly due to inhomogeneous distribution or defect of oxygen atoms in the sample.
4. Discussion Present result clearly demonstrates that the appearance of bulk superconductivity depends on
Fig. 5. Phase diagram to Ce concentration x and oxygen loss d for PLCCO system. Circles, open squares and closed triangles show compositions which have SC, SG-like and AF phases, respectively. A solid line shows a boundary of the AF phase and the boundary between x ¼ 0:11 and 0.15 is not clearly determined.
both values of x and d, namely, more oxygen should be removed from samples with higher Ce concentration. It should be noted that this relation between x and d is not understood if both Cedoping and oxygen reduction dope mobile electrons. In other words, the effective doping rate in this system cannot be simply described by the sum of the doping concentration evaluated from the values of x and d. Carrier localization by Ce-doping and/or excessive oxygen is expected to occur. Comparing to the phase diagram for NCCO, in which the bulk superconductivity appears in the doping range, 0:14 < x < 0:17 and 0:02 < d < 0:06 [5], the SC region is much wider in PLCCO for both x and d. Since AF phase of the PLCCO system is narrower than that of NCCO system, the competing SC phase can extend towards the region with lower x and d. As already discussed [7], such difference between NCCO and PLCCO is ascribed by the difference of lattice constant between the two systems. The SG-like behavior firstly observed in the electron-doped cuprates appears only in the
220
S. Kuroshima et al. / Physica C 392–396 (2003) 216–220
limited region of d. Although the similar SG behavior is observed in the hole-doped LSCO, we speculate the origin is different between LSCO and PLCCO. For the former the SG behavior is related with magnetic frustration induced by doped holes. In addition, TSG is much lower in LSCO than in PLCCO. We speculate the magnetic frustration can be induced by Ce-substituting and the SG-like phase appears only when the doped carrier is localized by the excessive oxygen. However, as seen in the as-grown sample strong localization induces three-dimensional antiferromagnetic order rather than spin-glass.
Japanese Ministry of Education, Culture, Sports, Science and Technology, Grant-in-Aid for Scientific Research on Priority Areas (Novel Quantum Phenomena in Transition Metal Oxides), 12046239, 2000, for Scientific Research (A), 10304026, 2000, for Encouragement of Young Scientists, 13740216, 2001 and for Creative Scientific Research (13NP0201) ‘‘Collaboratory on Electron Correlations––Toward a New Research Network between Physics and Chemistry’’, by the Japan Science and Technology Corporation, the Core Research for Evolutional Science and Technology Project (CREST).
5. Conclusion
References
We performed systematic measurements of the magnetic susceptibility of the carefully reduced PLCCO samples. AF phase is reduced and SC phase spreads in x–d phase diagram. And an SGlike phase is observed in the limited region of d.
[1] F.C. Chou, N.R. Belk, M.A. Kastner, R.J. Birgeneau, A. Aharony, Phys. Rev. Lett. 75 (1995) 2204. [2] S. Wakimoto, S. Ueki, Y. Endoh, K. Yamada, Phys. Rev. B 62 (2000) 3547. [3] H. Takagi, S. Uchida, Y. Tokura, Phys. Rev. Lett. 62 (1989) 1197. [4] A. Serquis, F. Prado, A. Caneiro, Physica C 313 (1999) 271. [5] Y. Koike, A. Kakimoto, M. Yoshida, H. Inozuka, T. Noji, Y. Saito, Physica B 165 & 166 (1990) 1665. [6] K. Kurahashi, H. Matsushita, M. Fujita, K. Yamada, J. Phys. Soc. Jpn. 70 (2002) 910. [7] M. Fujita, T. Kubo, S. Kuroshima, T. Uefuji, K. Kawashima, K. Yamada, I. Watanabe, K. Nagamine, cond-mat/ 0203320, 2002.
Acknowledgements We thank M. Kufu and K. Hirota for technical assistance of ICP measurements at Tohoku University. This work was supported in part by the