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Study of carrier doping across the parent Mott insulator La2CuO4
Physica C 470 (2010) S49–S50
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Study of carrier doping across the parent Mott insulator La2CuO4 Seiki Komiya *, Ichiro Tsukada Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa 240-0196, Japan
a r t i c l e
i n f o
Article history: Accepted 26 February 2010 Available online 2 March 2010
a b s t r a c t Ce substituted La2CuO4 single crystals are investigated to try doping electrons into the parent Mott insulator. Transport properties of slightly Ce substituted La2CuO4 show that carriers are still holes activated from an impurity level of which activation energy is the same as the parent La2CuO4. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Mott insulator Hall coefficient Thermal activation
1. Introduction
3. Results and discussion
High-T c superconductivity arises when sufficient carriers are doped into a parent Mott insulator, and how the electronic states develop with doping is one of the most important issues in highT c research. In the La-214 system, we have clarified that a finite density of states is created already with 0.5% hole doping by heat capacity measurements, where the system still keeps long range antiferromagnetic order [1]. In this paper, we report physical properties of Ce substituted La-214 single crystals, in which we try doping electrons into the parent La2CuO4 (LCO) with keeping T-structure [2].
Fig. 1 shows the powder X-ray diffraction data of LCO and LCCO crystals using CuKa X-rays. All the peaks of the LCCO sample are indexed as the T-structure, and no traces of the T 0 -structure or impurity phases are found. The Ce concentration is analyzed by the ICP-AES method, and found to be 0.003, which is much smaller than the nominal composition 0.01. It looks rather hard to substitute Ce at the La site. To see the difference of structures between LCO and LCCO in more detail, lattice parameters are determined by the Cohen’s method. The calculated lattice parameters are: a ¼ 5:364 Å; b ¼ 5:410 Å; c ¼ 13:165 Å for LCO, and a ¼ 5:355 Å; b ¼ 5:402 Å; c ¼ 13:146 Å for LCCO. This is qualitatively in agreement with the thin film results [2], and this result would show that Ce is properly substituted with La even though the Ce concentration in the crystal is smaller than that of the nominal one. Figs. 2 and 3 show the temperature dependence of the in plane resistivity ðqab Þ and the Arrhenius plot of the Hall coefficient ðRH Þ data, respectively, for Ce substituted and undoped samples. With Ce substitution, qab becomes larger than that of the parent material, and RH also increases. These transport results suggest that the hole density decreases with Ce substitution. The low temperature RH behavior of LCCO is thermal activation type, and the activation energy looks unchanged from that of the parent LCO. The magnetization and heat capacity measurements of LCCO show that the Neél temperature is 320 K and the electronic specific heat coefficient ðcÞ is zero, which is the same as the parent LCO (data not shown here). These results are consistent with each other, indicating that there is a finite energy gap near the Fermi level in the density of states of LCCO. It has been observed that there is a small concentration (0.6%) of the impurity states which provide holes in the parent LCO [3]. So, these data indicate
2. Experimental Single crystals of Ce substituted La2CuO4 (LCCO) with the nominal composition of La1.99Ce0.01CuO4 are grown by the traveling solvent floating zone method. Samples are cut from the grown rod and carefully annealed in pure Ar flow to remove excess oxygen. The crystal structure is analyzed by the X-ray diffraction measurements using powder samples to which grown crystals are ground, and the chemical composition is determined by the ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy) method. Transport properties are measured by conventional four terminal method and heat capacity is measured by relaxation method using Quantum Design’s Physical Property Measurement System. Magnetization is measured with Magnetic Property Measurement System. * Corresponding author. E-mail address: [email protected] (S. Komiya). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.02.094
S50
S. Komiya, I. Tsukada / Physica C 470 (2010) S49–S50
1500
10 La
Ce
1.997
4
CuO
0.003
4
1000
103
R (cm /C)
0
10
2
10
1
10
0
3
I (cps)
500
La CuO 2
4
H
1000
500
0
10
20
30
40
50
60
10
-1
2 (deg.) 10-2
Fig. 1. Powder X-ray diffraction data of LCO and LCCO crystals.
0
0.005
0.01
0.015
0.02
1/T (1/K) Fig. 3. Arrhenius plot of the Hall coefficient.
electronic conduction, and another contributes only by thermal activation. Sr substitution immediately modifies the electronic states with only 0.5% substitution [1], but the holes from the impurity states do not. Therefore, if Ce substitution works in the same way, the electronic specific heat coefficient c would show some finite value even if the amount of Ce substitution is very small, because at very low temperatures holes from the impurity states are almost bound at the impurity sites. The results of this study may suggest that Ce substitution does not provide mobile electrons in the T-structure 214 system.
1
ab
( cm)
10
10
0
4. Summary
10
-1
0
50
100
150
200
250
300
Temperature (K) Fig. 2. Temperature dependences of the in plane resistivity.
that the hole concentration existing in the parent material is compensated by Ce substitution. Indeed, the concentration of the impurity state in LCCO calculated using the RH data is 0.3%, which is consistent with the concentration of impurity states in LCO and the amount of Ce substitution. In the case of Sr substitution, there are two kinds of hole carriers: one creates finite density of states and contributes much to
Single crystals with the nominal composition of La1.99Ce0.01 CuO4 are grown to study the behavior of carriers across the parent Mott insulator without changing crystal structure. The Ce concentration of the grown crystal is 0.003, and the transport properties indicate that this Ce substitution compensates hole carriers which exist already in the parent LCO. In this study, we have not found any evidence of mobile electrons in the T-structure 214 system. Samples with more Ce are necessary for further study of the electron dynamics in this system. References [1] S. Komiya, I. Tsukarda, J. Phys.: Conf. Ser. 150 (2009) 052118. [2] A. Tsukada, H. Yamamoto, M. Naito, Phys. Rev. B 74 (2006) 174515. [3] S. Ono, S. Komiya, Y. Ando, Phys. Rev. B 75 (2007) 024515.
Contents lists available at ScienceDirect
Physica C journal homepage: www.elsevier.com/locate/physc
Study of carrier doping across the parent Mott insulator La2CuO4 Seiki Komiya *, Ichiro Tsukada Central Research Institute of Electric Power Industry, Yokosuka, Kanagawa 240-0196, Japan
a r t i c l e
i n f o
Article history: Accepted 26 February 2010 Available online 2 March 2010
a b s t r a c t Ce substituted La2CuO4 single crystals are investigated to try doping electrons into the parent Mott insulator. Transport properties of slightly Ce substituted La2CuO4 show that carriers are still holes activated from an impurity level of which activation energy is the same as the parent La2CuO4. Ó 2010 Elsevier B.V. All rights reserved.
Keywords: Mott insulator Hall coefficient Thermal activation
1. Introduction
3. Results and discussion
High-T c superconductivity arises when sufficient carriers are doped into a parent Mott insulator, and how the electronic states develop with doping is one of the most important issues in highT c research. In the La-214 system, we have clarified that a finite density of states is created already with 0.5% hole doping by heat capacity measurements, where the system still keeps long range antiferromagnetic order [1]. In this paper, we report physical properties of Ce substituted La-214 single crystals, in which we try doping electrons into the parent La2CuO4 (LCO) with keeping T-structure [2].
Fig. 1 shows the powder X-ray diffraction data of LCO and LCCO crystals using CuKa X-rays. All the peaks of the LCCO sample are indexed as the T-structure, and no traces of the T 0 -structure or impurity phases are found. The Ce concentration is analyzed by the ICP-AES method, and found to be 0.003, which is much smaller than the nominal composition 0.01. It looks rather hard to substitute Ce at the La site. To see the difference of structures between LCO and LCCO in more detail, lattice parameters are determined by the Cohen’s method. The calculated lattice parameters are: a ¼ 5:364 Å; b ¼ 5:410 Å; c ¼ 13:165 Å for LCO, and a ¼ 5:355 Å; b ¼ 5:402 Å; c ¼ 13:146 Å for LCCO. This is qualitatively in agreement with the thin film results [2], and this result would show that Ce is properly substituted with La even though the Ce concentration in the crystal is smaller than that of the nominal one. Figs. 2 and 3 show the temperature dependence of the in plane resistivity ðqab Þ and the Arrhenius plot of the Hall coefficient ðRH Þ data, respectively, for Ce substituted and undoped samples. With Ce substitution, qab becomes larger than that of the parent material, and RH also increases. These transport results suggest that the hole density decreases with Ce substitution. The low temperature RH behavior of LCCO is thermal activation type, and the activation energy looks unchanged from that of the parent LCO. The magnetization and heat capacity measurements of LCCO show that the Neél temperature is 320 K and the electronic specific heat coefficient ðcÞ is zero, which is the same as the parent LCO (data not shown here). These results are consistent with each other, indicating that there is a finite energy gap near the Fermi level in the density of states of LCCO. It has been observed that there is a small concentration (0.6%) of the impurity states which provide holes in the parent LCO [3]. So, these data indicate
2. Experimental Single crystals of Ce substituted La2CuO4 (LCCO) with the nominal composition of La1.99Ce0.01CuO4 are grown by the traveling solvent floating zone method. Samples are cut from the grown rod and carefully annealed in pure Ar flow to remove excess oxygen. The crystal structure is analyzed by the X-ray diffraction measurements using powder samples to which grown crystals are ground, and the chemical composition is determined by the ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy) method. Transport properties are measured by conventional four terminal method and heat capacity is measured by relaxation method using Quantum Design’s Physical Property Measurement System. Magnetization is measured with Magnetic Property Measurement System. * Corresponding author. E-mail address: [email protected] (S. Komiya). 0921-4534/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2010.02.094
S50
S. Komiya, I. Tsukada / Physica C 470 (2010) S49–S50
1500
10 La
Ce
1.997
4
CuO
0.003
4
1000
103
R (cm /C)
0
10
2
10
1
10
0
3
I (cps)
500
La CuO 2
4
H
1000
500
0
10
20
30
40
50
60
10
-1
2 (deg.) 10-2
Fig. 1. Powder X-ray diffraction data of LCO and LCCO crystals.
0
0.005
0.01
0.015
0.02
1/T (1/K) Fig. 3. Arrhenius plot of the Hall coefficient.
electronic conduction, and another contributes only by thermal activation. Sr substitution immediately modifies the electronic states with only 0.5% substitution [1], but the holes from the impurity states do not. Therefore, if Ce substitution works in the same way, the electronic specific heat coefficient c would show some finite value even if the amount of Ce substitution is very small, because at very low temperatures holes from the impurity states are almost bound at the impurity sites. The results of this study may suggest that Ce substitution does not provide mobile electrons in the T-structure 214 system.
1
ab
( cm)
10
10
0
4. Summary
10
-1
0
50
100
150
200
250
300
Temperature (K) Fig. 2. Temperature dependences of the in plane resistivity.
that the hole concentration existing in the parent material is compensated by Ce substitution. Indeed, the concentration of the impurity state in LCCO calculated using the RH data is 0.3%, which is consistent with the concentration of impurity states in LCO and the amount of Ce substitution. In the case of Sr substitution, there are two kinds of hole carriers: one creates finite density of states and contributes much to
Single crystals with the nominal composition of La1.99Ce0.01 CuO4 are grown to study the behavior of carriers across the parent Mott insulator without changing crystal structure. The Ce concentration of the grown crystal is 0.003, and the transport properties indicate that this Ce substitution compensates hole carriers which exist already in the parent LCO. In this study, we have not found any evidence of mobile electrons in the T-structure 214 system. Samples with more Ce are necessary for further study of the electron dynamics in this system. References [1] S. Komiya, I. Tsukarda, J. Phys.: Conf. Ser. 150 (2009) 052118. [2] A. Tsukada, H. Yamamoto, M. Naito, Phys. Rev. B 74 (2006) 174515. [3] S. Ono, S. Komiya, Y. Ando, Phys. Rev. B 75 (2007) 024515.