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Superconducting and magnetic properties of La1−xPrxBa2Cu3Oy
PHYSICAi
Physica B 194-196 (1994) 1937-1938 North-Holland
Superconducting
and
magnetic
properties of
Lal_,Pr,
Ba2CuzOy
K. Sekizawa% Y. Taka~o% K. Kanno% H. Ikuta b, H. Ozaki b and H. Enomoto ~ ~Department of Physics, College of Science eald Technology, Nihon University, Kanda-Surugadai 1-8, Chiyoda-ku, Tokyo 101, Japan bDepartment of Electrical Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 160, Japan ~Deparment of Solid State Electronics, Osaka Electro-Communication University, Osaka 572, Japaal The electronic state of Pr ions in L a a _ , P r , B a 2 C u 3 O y was examined by their depression effect on superconductivity, magnetic properties and the XPS spectra. The superconducting transition temperature Tc of La]-,Pr~Ba2CuaO~ keeps 90 K for x < 0.03 and decreases drastically with x and it becomes semiconducting above x = 0.37. This decrease rate of Te is larger than that of Y l _ , P r , BazCuaO.v. The temperature dependence of the inverse magnetic susceptibility 1/X ,, is a little convex and is well explained by ]?r 3+ ion in the crystal field mad the exchange field. The XPS spectra of PrBa2CuzO~ show that the 4f levels of Pr ions fall on the valence band aald are smeared out.
In RBa2 Cu30~ (R=rare earth ions) type compounds, the superconducting transition temperature Tc is about 90 K for almost all rare earth ions. The exception is PrBa2Cu3Oy, which is semiconducting. It is generally accepted that in RBa~CuzO~ with heavy rare earth ions, 4f electrons are almost completely isolated from superconducting electrons except they suffer a crystalline electric field (CEF). An interaction between 4felectrons in rare earth ions and the CuO2 planes increases with decreasing atomic number of rare earth ions [1]. A Pr ion, the lightest rare earth ion having 4f electrons in RBa2CuzOy depresses superconductivity. Although the depression effect of Pr ions on superconductivity was investigated in Y l - z P r ~ B a 2 C u 3 0 ~ by many researchers [2], the origin of this depression has not been elucidated yet. Previously', we reported the preliminary data of traalsport, magnetic aald structural properties of L a l _ , P r , Ba2Cu3Oy [3, 4, 5]. We performed a numerical calculation of the magnetic susceptibility of Pr 3+ ions taking account of a CEF and an exchange interaction between Pr 3+ ions, in the framework of Racah's tensor operator method, where all the J states belonging to the lowest LS multiplet of Pr 3+ ion were included [6]. In this study, we investigated the condition of sample preparation more precisely and also made XPS measurements near the Fermi level in order to ob-
tain information about the hybridization of Pr-4J orbitals with O-2p orbitals in the valence band. The results of a numerical calculation were compared with the experimental results. We succeeded to obtain almost single phase of La]_zPr~Ba2Cu30~ in the whole raaxge of x by a particular process of heating and cooling in flowing N2 gas and the final annealing in flowing 02 gas [7]. The oxygen content y is nearly 7.0 and almost independent of x. Lattice parameters a, b and c/3 decreases linearly with x. This indicates that the complete solid solutions are formed over the whole range of x. The Pr concentration dependence of Tc is shown in Fig. 1. The superconducting transition temperature decreases slowly in a range of 0 _< x _< 0.03 and then decreases drastically. The critical concentration at which superconductivity disappears is about Xcr = 0.37. 100
i
i
"%
"-~ ,5o
eo
I.°°
% 0
i
o
i
|
i
L
O.5
I
I
I
I
1.0
x
Fig. 1 The dependence of Tc on x.
0921-4526/94//$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSD! 0921-4526(93)1600-Q
1938
The temperatttre dependence of the inverse magnetic susceptibility 1 / Z m of L a l _ , P r , Ba2 Cua Oy is shown in Fig. 2. It deviates from a Curie-Weiss law and is a little convex. Our numericM calculation shows that ~ ,n of Pr a+ ions under the CEF [8, 9] and the inolecular field has a similar temperature dependence to the experimentM one [6]. 500
,
,
,
,
400
'
4] and O-2p levels. The hybridization may bring about the hole localization and the magnetic pah" breaking.
5
x=o,e
; ¢D
i
300
x=o.8
200
x--1.o'
5
10
0
-2
Binding Energy (oV) 100 0 0
50
100
150
200
250
300
Fig. 3 XPS spectra near Fermi level; . . . . : P r 2 C u O 4 , - - PrBa2Cu3Oy aad---:LaBa2Cu3Oy.
T(K)
Fig. 2 The 1/Z , , - T curves of molar Lal _~Pr~Ba,~CuaOy. ExperimentM curves:-- and calculated curves: .....
In the series of rare earth elements, the 4] orbitals are stabilized and contract in the radial extent with incre&sing the atomic nmnber. To obtain the energy level relation of 4] orbitals to the valence bmM, XPS measurements were made on LaBa2CuaOy, PrBa2CuaOy and Pr2CuO4 (Fig.3). The peak due to Pr-4feleetrons ( ~ 1 eV) distinglfished in the spectrum of Pr2CuO4 disappeared in the spectrum of PrBaeCuaOy. This indicates that the 4] levels of Pr ions overlap the valence band and are smeared out in PrBa2CuaO~, which shows the occurrence of the hybridization of Pr-4f with O-2p electrons. The magnetic susceptibility of PrBa2CuaOy is strongly influenced by the CEF effect. The experimentM results is well reproduced by the calculation of the susceptibility of Pr ions of trivalent with antiferromagnetic interaction between thein, accompanied by a small contribution of Cu ions, without introducing a Pr 4+ state. The depression of T~ is considered to be due to the hybridization of 4/orbitals with O-2p orbitals in the CuO2 planes, which depends not orrly on the distance between Pr and O ions in the CuO2 planes but also on the energy level relation of the Pr-
References [1] T. Inaba, Y. T a t ~ l o and K. Sekizawa, Solid State Commtm. 70 (1989) 725. [2] A. Kebede, C. S. Jee, J. Schwelger, J. E. Crow, T. Mihalisin, G. H. Myer, R. E. Sa lomon, P. Schlottmarm, M. V. Kurie, S. H. Bloom and R. P. Guertin, Phys. Rev. B 40 (1989) 4453. [3] K. Sekizawa, Y. Takano, K. Kanno and K. Ohtani, Physica C 185-189 (1991) 1273. [4] Y. Takano aald K. Sekizawa, Phase Traalsitions 41 (1993) 217.
JJAP .~erie~ 7 Mechanisms o] Superconductivity, ed. Y.
[5] K. Sekizawa and Y.Takano,
Muto, (.lpn. J. Appl. Phys., 1992) p.106. [6] Y. Takano mid K. Sekizawa, to appear. [7] Y. Takano, T. Inaba, K. Yamamoto and K. Sekizawa, Physica B 169 (1991) 683. [8] G. L. Goodman, C-K. Loong and L. Soderholm, J. Phys. 3 (1991) 49. [9] L. Soderholm, C-K. Loong, G. L. Goodman and B. D. Dabrowski, Phys. Rev. B 43 (1991) 7923.
Physica B 194-196 (1994) 1937-1938 North-Holland
Superconducting
and
magnetic
properties of
Lal_,Pr,
Ba2CuzOy
K. Sekizawa% Y. Taka~o% K. Kanno% H. Ikuta b, H. Ozaki b and H. Enomoto ~ ~Department of Physics, College of Science eald Technology, Nihon University, Kanda-Surugadai 1-8, Chiyoda-ku, Tokyo 101, Japan bDepartment of Electrical Engineering, Waseda University, Ohkubo 3-4-1, Shinjuku-ku, Tokyo 160, Japan ~Deparment of Solid State Electronics, Osaka Electro-Communication University, Osaka 572, Japaal The electronic state of Pr ions in L a a _ , P r , B a 2 C u 3 O y was examined by their depression effect on superconductivity, magnetic properties and the XPS spectra. The superconducting transition temperature Tc of La]-,Pr~Ba2CuaO~ keeps 90 K for x < 0.03 and decreases drastically with x and it becomes semiconducting above x = 0.37. This decrease rate of Te is larger than that of Y l _ , P r , BazCuaO.v. The temperature dependence of the inverse magnetic susceptibility 1/X ,, is a little convex and is well explained by ]?r 3+ ion in the crystal field mad the exchange field. The XPS spectra of PrBa2CuzO~ show that the 4f levels of Pr ions fall on the valence band aald are smeared out.
In RBa2 Cu30~ (R=rare earth ions) type compounds, the superconducting transition temperature Tc is about 90 K for almost all rare earth ions. The exception is PrBa2Cu3Oy, which is semiconducting. It is generally accepted that in RBa~CuzO~ with heavy rare earth ions, 4f electrons are almost completely isolated from superconducting electrons except they suffer a crystalline electric field (CEF). An interaction between 4felectrons in rare earth ions and the CuO2 planes increases with decreasing atomic number of rare earth ions [1]. A Pr ion, the lightest rare earth ion having 4f electrons in RBa2CuzOy depresses superconductivity. Although the depression effect of Pr ions on superconductivity was investigated in Y l - z P r ~ B a 2 C u 3 0 ~ by many researchers [2], the origin of this depression has not been elucidated yet. Previously', we reported the preliminary data of traalsport, magnetic aald structural properties of L a l _ , P r , Ba2Cu3Oy [3, 4, 5]. We performed a numerical calculation of the magnetic susceptibility of Pr 3+ ions taking account of a CEF and an exchange interaction between Pr 3+ ions, in the framework of Racah's tensor operator method, where all the J states belonging to the lowest LS multiplet of Pr 3+ ion were included [6]. In this study, we investigated the condition of sample preparation more precisely and also made XPS measurements near the Fermi level in order to ob-
tain information about the hybridization of Pr-4J orbitals with O-2p orbitals in the valence band. The results of a numerical calculation were compared with the experimental results. We succeeded to obtain almost single phase of La]_zPr~Ba2Cu30~ in the whole raaxge of x by a particular process of heating and cooling in flowing N2 gas and the final annealing in flowing 02 gas [7]. The oxygen content y is nearly 7.0 and almost independent of x. Lattice parameters a, b and c/3 decreases linearly with x. This indicates that the complete solid solutions are formed over the whole range of x. The Pr concentration dependence of Tc is shown in Fig. 1. The superconducting transition temperature decreases slowly in a range of 0 _< x _< 0.03 and then decreases drastically. The critical concentration at which superconductivity disappears is about Xcr = 0.37. 100
i
i
"%
"-~ ,5o
eo
I.°°
% 0
i
o
i
|
i
L
O.5
I
I
I
I
1.0
x
Fig. 1 The dependence of Tc on x.
0921-4526/94//$07.00 © 1994 - Elsevier Science B.V. All rights reserved SSD! 0921-4526(93)1600-Q
1938
The temperatttre dependence of the inverse magnetic susceptibility 1 / Z m of L a l _ , P r , Ba2 Cua Oy is shown in Fig. 2. It deviates from a Curie-Weiss law and is a little convex. Our numericM calculation shows that ~ ,n of Pr a+ ions under the CEF [8, 9] and the inolecular field has a similar temperature dependence to the experimentM one [6]. 500
,
,
,
,
400
'
4] and O-2p levels. The hybridization may bring about the hole localization and the magnetic pah" breaking.
5
x=o,e
; ¢D
i
300
x=o.8
200
x--1.o'
5
10
0
-2
Binding Energy (oV) 100 0 0
50
100
150
200
250
300
Fig. 3 XPS spectra near Fermi level; . . . . : P r 2 C u O 4 , - - PrBa2Cu3Oy aad---:LaBa2Cu3Oy.
T(K)
Fig. 2 The 1/Z , , - T curves of molar Lal _~Pr~Ba,~CuaOy. ExperimentM curves:-- and calculated curves: .....
In the series of rare earth elements, the 4] orbitals are stabilized and contract in the radial extent with incre&sing the atomic nmnber. To obtain the energy level relation of 4] orbitals to the valence bmM, XPS measurements were made on LaBa2CuaOy, PrBa2CuaOy and Pr2CuO4 (Fig.3). The peak due to Pr-4feleetrons ( ~ 1 eV) distinglfished in the spectrum of Pr2CuO4 disappeared in the spectrum of PrBaeCuaOy. This indicates that the 4] levels of Pr ions overlap the valence band and are smeared out in PrBa2CuaO~, which shows the occurrence of the hybridization of Pr-4f with O-2p electrons. The magnetic susceptibility of PrBa2CuaOy is strongly influenced by the CEF effect. The experimentM results is well reproduced by the calculation of the susceptibility of Pr ions of trivalent with antiferromagnetic interaction between thein, accompanied by a small contribution of Cu ions, without introducing a Pr 4+ state. The depression of T~ is considered to be due to the hybridization of 4/orbitals with O-2p orbitals in the CuO2 planes, which depends not orrly on the distance between Pr and O ions in the CuO2 planes but also on the energy level relation of the Pr-
References [1] T. Inaba, Y. T a t ~ l o and K. Sekizawa, Solid State Commtm. 70 (1989) 725. [2] A. Kebede, C. S. Jee, J. Schwelger, J. E. Crow, T. Mihalisin, G. H. Myer, R. E. Sa lomon, P. Schlottmarm, M. V. Kurie, S. H. Bloom and R. P. Guertin, Phys. Rev. B 40 (1989) 4453. [3] K. Sekizawa, Y. Takano, K. Kanno and K. Ohtani, Physica C 185-189 (1991) 1273. [4] Y. Takano aald K. Sekizawa, Phase Traalsitions 41 (1993) 217.
JJAP .~erie~ 7 Mechanisms o] Superconductivity, ed. Y.
[5] K. Sekizawa and Y.Takano,
Muto, (.lpn. J. Appl. Phys., 1992) p.106. [6] Y. Takano mid K. Sekizawa, to appear. [7] Y. Takano, T. Inaba, K. Yamamoto and K. Sekizawa, Physica B 169 (1991) 683. [8] G. L. Goodman, C-K. Loong and L. Soderholm, J. Phys. 3 (1991) 49. [9] L. Soderholm, C-K. Loong, G. L. Goodman and B. D. Dabrowski, Phys. Rev. B 43 (1991) 7923.