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Hall effect of superconducting copper oxide, Cu-1234
PHYSICA ELSEVIER
PhysicaC258 (1996) 384-388
Hall effect of superconducting copper oxide, Cu-1234 M. Ogino a, T. Watanabe a,*, H. Tokiwa b, A. Iyo c, H. Ihara c a Department of Applied Electronics, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan b Department of Applied Physics, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan e Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, lbaraki 305, Japan Received 17 November 1995
Abstract
The resistivity p and Hall coefficient R H have been measured for Cu- 1234 superconductors, (Cu 1- x- ~C x)Ba zCa 3Cu 4Oy (x ~ 0-0.3). The change in Tc due to the different stoichiometry of the constituent elements in (CuC)O2_ 8 block layers is very slight and Tc's for all of these samples were around 117 K, although the magnitude of R H is different. The observed temperature dependence of p ~ T, R H ~ 1 / T are similar to other hole-type cuprate oxide superconductors. We also found an exact T 2 dependence of the Hall angle (cot 0 H = c~T2) up to 300 K for all samples measured.
1. Introduction
Many efforts have been made to discover new high-T~ superconductors which have practical uses. Recently, several authors have succeeded in synthesizing at high pressures and high temperatures, new high-T~ superconducting materials of the B a - C a C u - O system, with high critical temperatures of about 117 K. lhara et al. reported the synthesis of A g l _ x C u x B a 2 C a ._ iCu,O2,+3_z [1] and, thereafter, the synthesis of CUl_xBa2Ca ._ lCu,O2,+4_ z [2] in a series of experiments in which they attempted to replace Hg(T1) by non-toxic A g or Cu in Hg(T1)Ba2Ca3Cu4Oy. Kawashima et al. succeeded in synthesizing a 117 K superconducting phase in the B a - C a - C u - O system, and concluded that the chemical composition of the superconducting phase was
* Corresponding author. Fax: +81 471 22 9195.
(Cu0.sC0.5)Ba2Ca3Cu4011+ z' replacing
Cu atoms by C atoms in the form of CO 3 units [3]. A superstructure of 2 a × 2b × 2c, resulting from the ordering of Cu and C atoms, was also reported, similar to the observation by Ihara et al. in the A g - B a - C a - C u - O system [1]. Jin et al. also prepared a new superconducting compound with T~ = 117 K in the B a - C a C u - O system [4]. They speculated from the X-ray diffraction measurements that the Cu-1234 and Cu1223 phases were responsible for the observed superconductivity, and estimated that lattice constants were a = 3.85 .~, c = 18.30 A. for Cu-1234 and a = 3.88 A, c = 14.94 A for Cu-1223 [4]. Wu et al. proposed that a new homologous series of a cuprate phase is isostructural with Hg and single-layer T1based superconductors [5]. Shimakawa et al. reported the results of a structural refinement using neutron powder-diffraction data for (Cu,C)Ba2Ca3Cu4Oll+z [6]. These compounds have tetragonal symmetry similar to that of Tl(Hg)Ba2Ca3Cu4Oy. They also refined the structure with a model that allowed the
0921-4534/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0 9 2 1 - 4 5 3 4 ( 9 5 ) 0 0 8 1 4 - 4
M. Ogino et a l . / Physica C 258 (1996) 384-388
partial substitution of C as a unit of CO 3 at the Cu site in the (Cu,C)O~+ z layer. Akimoto et al. investigated the crystal structure of Cu0.6Ba2Ca3CuaOt0.8 (not including carbon) using a small single crystal of this compound [7]. They confirmed the existence in this compound of a deficient CuO 2 sheet between two BaO layers, instead of the T102 sheet in the T1-1234 structure. Such a new high-T~ superconductor family of Cu-based B a - C a - C u - O with Tc > 116 K was also thought to be a practical superconductor for present uses. This is because the small CuO 2 block layer spacing suggests a lower anisotropy and a higher Jc, and these materials consist of non-toxic elements, unlike the Hg- and Tl-based materials [8]. In the study of the normal state of cuprate superconductors, the Hall effect provides valiable information on the nature of charge carriers in the CuO 2 planes. Beyond measuring simply the carrier density, the anomalous temperature dependence of the Hall coefficient is assumed to be significant. The T 2 dependence o f the Hall angle 0 (i.e. cot 0 = a T 2 +/3) was found by Chien et al. to hold for Zn-doped YBa2Cu3Oy (YBCO) [9]. Subsequently, this relation has been confirmed for a large number of hole-type cuprate superconductor systems [9-14]. Recently, this behaviour was also shown in a new type of superconductors, HgBa2CaCu2Oy ceramics [15] and HgBa2Ca2Cu30 z single crystals [16], with structures similar to the Cu system. In this paper we report the measurements of the resistivity p and the Hall coefficient R H as a function of temperature in the so-called Cu-1234 compound. The T 2 rule of the Hall angle in this compound was also tested, and the results are compared with those of other cuprate superconducting compounds.
2. Experimental Ceramics of nearly single phase Cu l - x - ~CxBa2Ca3Cu40 z were prepared by the high-pressure synthesis technique described previously [1,2]. Small impurity phases consisting of CaO were detected in the X-ray diffraction pattern. The lattice parameters of these samples change with different carbon contents. In particular, a c-axis constant was deduced by
385
introducing C atoms at Cu sites in the CuO 2_ ~ block layers. Jin et al. [4] reported a c-axis constant of 18.30 ,~ for a Cu-1234 system free of carbon, while a c-axis constant of 17.9512 A for (Cu0.68C0.32)Ba2Ca3CuaOI 1.06 was reported by Shimakawa et al. [6]. A c-axis constant of 17.974 A for Cu0.6Ba 2Ca3Cu4010,8 was reported by Akimoto et aL [7] and is smaller than that of the sample prepared by Jin et al. [4]. This difference may be due to the different stoichiometry of the constituent elements. Thus the lattice parameters of the Cu system change with the stoichiometry of Cu, C and O in the compound. Although the synthesis of single-phase Cu-1234 is greatly stabilized b y the introduction of C in the structure, this time, we tried to prepare samples containing less C. As a result, the c-axis lattice constant of the samp!e which was estimated not to contain C was 18.00 A, and that of the sample which was estimated to contain C of x = 0.2-0.3 was 17.93 ~k. We also prepared samples which showed c-axis lattice constants between these values. However, all of these samples exhibited T~'s around 117 K, in spite o f the inclusion of the different amounts of C as CO 3 units in the structure. As concerns such phenomena, Shimakawa et al. speculated that even though the presence of distorted apical oxygen sites degrades superconductivity for the t w o outer CuO 2 layers, the two inner layers support superconductivity with a high T~ [6]. For the electrical measurements, the samples were cut into bars ~ 4 × 1 × 0.3 mm 3 and then mechnically polished. Good contacts on the samples were made using a special solder and ultrasonic soldering iron (Asahi Glass Co., Cerasolzer #123). The resistivity and the Hall effect were measured using the standard dc four-lead method. The sample current was varied up to a maximum of 26 A / c m 2 and the measured voltage was confirmed to vary linearly with the current. The Hall effect measurements were carried out using a Keithley 1801 nV preamp in a magnetic field of 2 T at 26 A / c m 2. The linearity of the Hall voltage for the magnetic field and the current was checked. We reversed the magnetic field and the sample current at each temperature to cancel the lead misalignment. Temperature control of the sample was performed precisely using a Lakeshore DRC-93CA temperature controller. This is also important for measuring small Hall signals precisely.
386
M. Ogino et aL / Physica C 258 (1996) 384-388 I
3. Results and discussion
C The resistivity is shown as a function of temperature in Fig. 1. The magnitude and the temperature dependence of the resistivity showed slight fluctuations due to the sample preparation procedure. The resistivities of samples A and B exhibited almost the same magnitude and temperature dependence, in spite of their different carbon (C) contents estimated from the variation of the c-axis lattice constants. This indicates the existence of different amounts of defects of Cu and O in the CuO 2_ 8 block layers in the samples. However, for sample C, which may contain a fairly large number of Cu atoms and less C ( ~ 0), the resistivity was slightly higher and showed a steeper temperature dependence. This sample also showed a slightly higher T~ compared with other samples and a narrow transition-temperature width (ATe). The Tc and ATc values were 115.5 and 2.3 K for sample A, 117.2 and 1.6 K for sample B, and 118.0 and 1.6 K for sample C, respectively. Thus the Tc'S of the Cu-1234 system are around 117 K, as reported by several authors. Oxygen or Ar annealing increased the resistivity, maintaining the linear dependence on temperature. We believe that this can be attributed to an insulating phase forming in the grain boundaries during the annealing at 300°C. The temperature dependence o f the Hall coeffi06~-
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Temperature(K) Fig. 2. Temperature dependence of the Hall coefficient Ra.
cients of these samples are shown in Fig. 2. The Hall coefficient(R H) is positive and varies as a function of temperature of y / T . The carrier concentration Nr~ deduced from the R H using the simple relation ( N H = 1 / ( e R H ) ) is shown in Fig. 3. As shown in Figs. I and 3, the sample with higher resistivity exhibits a lower hole concentration. Thus 1/17, H decreases linearly with decreasing temperature. The Hall number per CuO 2 plane was 0.6-0.35 in these samples. These values are rather larger than those of other cuprate superconductors [ 17-19]. For instance, in the compound Hg-1234, which has a structure very similar to Cu-1234, the Hall number is ~ 0.21 [20]. Previously, Shimakawa et al. estimated a nominal Cu valence of 2.37 for the Cu atoms in the CuO 2 plane from the refined composition (Cu0.68Co.32)Ba2Ca 3Cu4Oll.o 6 of their sample [6]. Akimoto et al. also
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387
M. Ogino et a L / Physica C 258 (1996) 384-388
reported a sample of refined composition Cu0. 6Ba2Ca3Cu4010.8 and found an average Cu valence of 2.52 from the charge compensation [7]. Although the compositions of our our samples may be different from those mentioned above, the average charge of Cu in the C u t 2 plane in our samples is in the range of those reported previously. However, it is noteworthy that the T~ in such Cu-1234 compounds is around 117 K in spite of the different contents of Cu atoms, C atoms and O atoms, and consequently the different carder concentrations. This phenomenon seems to be quite strange compared with other cuprate superconductors in which a considerable decreases in Tc were observed by introducing C or other impurities. Concerning this, Shimakawa et al. surmised that carriers in the inner two C u t 2 planes may govern the superconductivitY in Cu-1234 superconductors [6]. If this assumption is correct, the Hall number in the inner C u t 2 plane for the Cu-1234 superconductor may be around 0.35, which is the smallest value shown in Fig. 3. This is the Hall number of sample C, which is estimated to contain no C and shows the best superconducting characteristics in the present study. This implies that the formation of nearly complete C u t 2 block layers and optimization of the carrier concentration through control of the oxygen concentration are important for improving the superconducting characteristics of the Cu-1234 compounds. The Hall angle OH is plotted as cot 0 n vs. T 2 in Fig. 4. The cot OH follows completely the T 2 dependence in the measured temperature range up to 300 K. The relation cot OH = a T 2 + / 3 was proposed theoretically [21,22] and experimentally in many kinds of hole-type cuprate superconductors [9-14]. Thus the scattering mechanisms for hole carders in the Cu-1234 compounds seem to be similar to those of other oxide superconductors. The values of cr for Cu-1234, normalized for a magnetic field of 1 T, were 0.05-0.06 and almost the same as those for YBa2Cu307_;~ (~ 0.04, T < 255 K) [9], H g - 1 2 3 4 ( ~ 0.045, T < 230 K) [20] and TI2Ba2CuO6+ ~ ( ~ 0.06) [11]. In general, impurity scattering causes pronounced /3 values, as shown in the Zn-doped Y-123 superconductor [9]. It is noted that the value of /3 is almost zero in these Cu-1234 compounds. This may indicate that little disorder is introduced for electronic transport in the C u O 2
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T2(104K2) Fig. 4. Cotangentof the Hall angle (cot 0n = p / ( R s B)) and the inverse Hail mobility(l//z n = p / R n ) . The solid lines show fits of the data to cot 0n =otT 2 + ft. planes. In fact, a fairly large positive values of /3 ( ~ 1100) and deviations from the T2-dependence of cot 0 n at low temperature (T < 25 5 K) were observed in an inferior sample of Cu-1234 not shown in Fig. 4. In conclusion, we have presented the first measurements of the Hall effect in the Cu-1234 system of ( C u l _ x _ ~Cx)Ba2CaaCu4Oysynthesized using the high-pressure, high-temperature technique. The results resemble those obtained for other high-Tc oxide superconductors. The temperature dependences p ~ T , R H ~ 1 / T and c o t O H ~ T 2 were observed. The temperature dependence of p and cot OH seems to indicate the low impurity scattering in these samples. The Hall number per C u t 2 plane was 0.35-0.6 and the carrier concentration dependence of Tc is very slight. However, the most excellent superconducting characteristics, related to Tc and ATc, were observed in the sample of Cu l_ 8BaECa3Cu4Oyand the Hall number was ~ 0.35. This implies that the formation of a sample with fully oxidized C u t 2_ block layers is important for drawing out the excellent superconducting properties in the Cu-1234 system.
Acknowledgments
We wish to thank M. Hamao for assistance in the experimental work.
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M. Ogino et aL /Physica C 258 (1996) 384-388
References [1] H. Ihara, K. Tokiwa, H. Ozawa, M. Hirabayashi, H. Matuhata, A. Negishi and Y.S. Song, Jpn. J. Appl. Phys. 33 (1994) 300. [2] H. lhara, K. Tokiwa, H. Ozawa, M. Hirabayashi, A. Negishi, H. Matuhata and Y.S. Song, Jpn. J. Appl. Phys. 33 (1994) 503. [3] T. Kawashima, Y. Matsui and E. Takayama-Muromachi, Physica C 224 (1994) 69. [4] C.-Q. Jin, S. Adachi, X.-J. Wu, H. Yamauchi and S. Tanaka, Physica C 223 (1994) 238. [5] X.-J. Wu, S. Adachi, C.-Q, Jin, H. Yamauchi and S. Tanaka, Physica C 223 (1994) 243. [6] Y. Shimakawa, J.D. Jorgensen, D.G. Hinks, H. Shaked, R.L. Hitterman, F. Izumi, T. Kawashima, E. TakayamaMuromachi and T. Kamiyama, Phys. Rev. B 50 (1994) 16008. [7] J. Akimoto, Y. Oosawa, K. Tokiwa, M. Hirabayashi and H. Ihara, Physica C 242 (1995) 360. [8] A. lyo, K. Tokiwa, T. Kanehira, M. Tokumoto, M. Hirabayashi and H. Ihara, Adv. Superconductivity VII (I994) p. 825.
[9] T.R. Chien, Z.Z. Wang and N.P. Ong, Phys. Rev. Lett. 67 (1991) 2088. [10] J.M. Harris, Y.F. Yan and N.P. Ong, Phys. Rev. B 46 (1992) 14293. [11] Y. Knbo and T. Manako, Physica C 197 (1992) 378. [12] A. Carrington, A.P. Mackenzie, C.T, Lin and J.R. Cooper, Phys. Rev. Lett. 69 (1992) 2855. [13] M.S. Raven and Y.M. Wan, Phys. Rev. B 51 (1995) 561. [14] E.C. Jones, D.P. Norton, D.K. Christen and D.H. Lowndes, Phys. Rev. Lett. 73 (1994) 166. [15] J.M. Harris, H. Wu, N.P. Ong, R.L. Meng and C.W. Chu, Phys. Rev. B 50 (1994) 3246. [16] A. Carrington, D. Colson, Y. Dumont, C. Ayache, A. Bertinotti and J.F. Marucco, Physica C 234 (1994) 1. [17] H. Zhang and H. Sato, Phys. Rev. Lett. 70 (1993) 1697. [18] W.A. Groen, D.M. de Leeuw and L.F. Feiner, Physica C 165 (1990) 55. [19] T. Watanabe, T. Tonozuka, H. Ihara, K. Tokiwa and M. Hirabayashi, Physica C 235-240 (1994) 1495. [20] T. Watanabe, Unpublished. [21] P.W. Anderson, Phys. Rev. Lett. 67 (1991) 2092. [22] A.S. Alexandrov, A.M. Bratkovsky and N.F. Mott, Phys. Rev. Lett. 72 (1994) 1734.
PhysicaC258 (1996) 384-388
Hall effect of superconducting copper oxide, Cu-1234 M. Ogino a, T. Watanabe a,*, H. Tokiwa b, A. Iyo c, H. Ihara c a Department of Applied Electronics, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278, Japan b Department of Applied Physics, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162, Japan e Electrotechnical Laboratory, 1-1-4 Umezono, Tsukuba, lbaraki 305, Japan Received 17 November 1995
Abstract
The resistivity p and Hall coefficient R H have been measured for Cu- 1234 superconductors, (Cu 1- x- ~C x)Ba zCa 3Cu 4Oy (x ~ 0-0.3). The change in Tc due to the different stoichiometry of the constituent elements in (CuC)O2_ 8 block layers is very slight and Tc's for all of these samples were around 117 K, although the magnitude of R H is different. The observed temperature dependence of p ~ T, R H ~ 1 / T are similar to other hole-type cuprate oxide superconductors. We also found an exact T 2 dependence of the Hall angle (cot 0 H = c~T2) up to 300 K for all samples measured.
1. Introduction
Many efforts have been made to discover new high-T~ superconductors which have practical uses. Recently, several authors have succeeded in synthesizing at high pressures and high temperatures, new high-T~ superconducting materials of the B a - C a C u - O system, with high critical temperatures of about 117 K. lhara et al. reported the synthesis of A g l _ x C u x B a 2 C a ._ iCu,O2,+3_z [1] and, thereafter, the synthesis of CUl_xBa2Ca ._ lCu,O2,+4_ z [2] in a series of experiments in which they attempted to replace Hg(T1) by non-toxic A g or Cu in Hg(T1)Ba2Ca3Cu4Oy. Kawashima et al. succeeded in synthesizing a 117 K superconducting phase in the B a - C a - C u - O system, and concluded that the chemical composition of the superconducting phase was
* Corresponding author. Fax: +81 471 22 9195.
(Cu0.sC0.5)Ba2Ca3Cu4011+ z' replacing
Cu atoms by C atoms in the form of CO 3 units [3]. A superstructure of 2 a × 2b × 2c, resulting from the ordering of Cu and C atoms, was also reported, similar to the observation by Ihara et al. in the A g - B a - C a - C u - O system [1]. Jin et al. also prepared a new superconducting compound with T~ = 117 K in the B a - C a C u - O system [4]. They speculated from the X-ray diffraction measurements that the Cu-1234 and Cu1223 phases were responsible for the observed superconductivity, and estimated that lattice constants were a = 3.85 .~, c = 18.30 A. for Cu-1234 and a = 3.88 A, c = 14.94 A for Cu-1223 [4]. Wu et al. proposed that a new homologous series of a cuprate phase is isostructural with Hg and single-layer T1based superconductors [5]. Shimakawa et al. reported the results of a structural refinement using neutron powder-diffraction data for (Cu,C)Ba2Ca3Cu4Oll+z [6]. These compounds have tetragonal symmetry similar to that of Tl(Hg)Ba2Ca3Cu4Oy. They also refined the structure with a model that allowed the
0921-4534/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved SSDI 0 9 2 1 - 4 5 3 4 ( 9 5 ) 0 0 8 1 4 - 4
M. Ogino et a l . / Physica C 258 (1996) 384-388
partial substitution of C as a unit of CO 3 at the Cu site in the (Cu,C)O~+ z layer. Akimoto et al. investigated the crystal structure of Cu0.6Ba2Ca3CuaOt0.8 (not including carbon) using a small single crystal of this compound [7]. They confirmed the existence in this compound of a deficient CuO 2 sheet between two BaO layers, instead of the T102 sheet in the T1-1234 structure. Such a new high-T~ superconductor family of Cu-based B a - C a - C u - O with Tc > 116 K was also thought to be a practical superconductor for present uses. This is because the small CuO 2 block layer spacing suggests a lower anisotropy and a higher Jc, and these materials consist of non-toxic elements, unlike the Hg- and Tl-based materials [8]. In the study of the normal state of cuprate superconductors, the Hall effect provides valiable information on the nature of charge carriers in the CuO 2 planes. Beyond measuring simply the carrier density, the anomalous temperature dependence of the Hall coefficient is assumed to be significant. The T 2 dependence o f the Hall angle 0 (i.e. cot 0 = a T 2 +/3) was found by Chien et al. to hold for Zn-doped YBa2Cu3Oy (YBCO) [9]. Subsequently, this relation has been confirmed for a large number of hole-type cuprate superconductor systems [9-14]. Recently, this behaviour was also shown in a new type of superconductors, HgBa2CaCu2Oy ceramics [15] and HgBa2Ca2Cu30 z single crystals [16], with structures similar to the Cu system. In this paper we report the measurements of the resistivity p and the Hall coefficient R H as a function of temperature in the so-called Cu-1234 compound. The T 2 rule of the Hall angle in this compound was also tested, and the results are compared with those of other cuprate superconducting compounds.
2. Experimental Ceramics of nearly single phase Cu l - x - ~CxBa2Ca3Cu40 z were prepared by the high-pressure synthesis technique described previously [1,2]. Small impurity phases consisting of CaO were detected in the X-ray diffraction pattern. The lattice parameters of these samples change with different carbon contents. In particular, a c-axis constant was deduced by
385
introducing C atoms at Cu sites in the CuO 2_ ~ block layers. Jin et al. [4] reported a c-axis constant of 18.30 ,~ for a Cu-1234 system free of carbon, while a c-axis constant of 17.9512 A for (Cu0.68C0.32)Ba2Ca3CuaOI 1.06 was reported by Shimakawa et al. [6]. A c-axis constant of 17.974 A for Cu0.6Ba 2Ca3Cu4010,8 was reported by Akimoto et aL [7] and is smaller than that of the sample prepared by Jin et al. [4]. This difference may be due to the different stoichiometry of the constituent elements. Thus the lattice parameters of the Cu system change with the stoichiometry of Cu, C and O in the compound. Although the synthesis of single-phase Cu-1234 is greatly stabilized b y the introduction of C in the structure, this time, we tried to prepare samples containing less C. As a result, the c-axis lattice constant of the samp!e which was estimated not to contain C was 18.00 A, and that of the sample which was estimated to contain C of x = 0.2-0.3 was 17.93 ~k. We also prepared samples which showed c-axis lattice constants between these values. However, all of these samples exhibited T~'s around 117 K, in spite o f the inclusion of the different amounts of C as CO 3 units in the structure. As concerns such phenomena, Shimakawa et al. speculated that even though the presence of distorted apical oxygen sites degrades superconductivity for the t w o outer CuO 2 layers, the two inner layers support superconductivity with a high T~ [6]. For the electrical measurements, the samples were cut into bars ~ 4 × 1 × 0.3 mm 3 and then mechnically polished. Good contacts on the samples were made using a special solder and ultrasonic soldering iron (Asahi Glass Co., Cerasolzer #123). The resistivity and the Hall effect were measured using the standard dc four-lead method. The sample current was varied up to a maximum of 26 A / c m 2 and the measured voltage was confirmed to vary linearly with the current. The Hall effect measurements were carried out using a Keithley 1801 nV preamp in a magnetic field of 2 T at 26 A / c m 2. The linearity of the Hall voltage for the magnetic field and the current was checked. We reversed the magnetic field and the sample current at each temperature to cancel the lead misalignment. Temperature control of the sample was performed precisely using a Lakeshore DRC-93CA temperature controller. This is also important for measuring small Hall signals precisely.
386
M. Ogino et aL / Physica C 258 (1996) 384-388 I
3. Results and discussion
C The resistivity is shown as a function of temperature in Fig. 1. The magnitude and the temperature dependence of the resistivity showed slight fluctuations due to the sample preparation procedure. The resistivities of samples A and B exhibited almost the same magnitude and temperature dependence, in spite of their different carbon (C) contents estimated from the variation of the c-axis lattice constants. This indicates the existence of different amounts of defects of Cu and O in the CuO 2_ 8 block layers in the samples. However, for sample C, which may contain a fairly large number of Cu atoms and less C ( ~ 0), the resistivity was slightly higher and showed a steeper temperature dependence. This sample also showed a slightly higher T~ compared with other samples and a narrow transition-temperature width (ATe). The Tc and ATc values were 115.5 and 2.3 K for sample A, 117.2 and 1.6 K for sample B, and 118.0 and 1.6 K for sample C, respectively. Thus the Tc'S of the Cu-1234 system are around 117 K, as reported by several authors. Oxygen or Ar annealing increased the resistivity, maintaining the linear dependence on temperature. We believe that this can be attributed to an insulating phase forming in the grain boundaries during the annealing at 300°C. The temperature dependence o f the Hall coeffi06~-
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Temperature(K) Fig. 2. Temperature dependence of the Hall coefficient Ra.
cients of these samples are shown in Fig. 2. The Hall coefficient(R H) is positive and varies as a function of temperature of y / T . The carrier concentration Nr~ deduced from the R H using the simple relation ( N H = 1 / ( e R H ) ) is shown in Fig. 3. As shown in Figs. I and 3, the sample with higher resistivity exhibits a lower hole concentration. Thus 1/17, H decreases linearly with decreasing temperature. The Hall number per CuO 2 plane was 0.6-0.35 in these samples. These values are rather larger than those of other cuprate superconductors [ 17-19]. For instance, in the compound Hg-1234, which has a structure very similar to Cu-1234, the Hall number is ~ 0.21 [20]. Previously, Shimakawa et al. estimated a nominal Cu valence of 2.37 for the Cu atoms in the CuO 2 plane from the refined composition (Cu0.68Co.32)Ba2Ca 3Cu4Oll.o 6 of their sample [6]. Akimoto et al. also
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Fig. 1. Temperature dependence of the resistivity in (Cuu_xL~sCx)BazCa3Cu4Oy. A, x~0.2-0.3; B, x=0.1; C, x = 0. The inset shows details near the transition.
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Fig. 3. Temperature dependence of the carrier density Nn estimated from the relationship Nn = 1/(eRH).
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reported a sample of refined composition Cu0. 6Ba2Ca3Cu4010.8 and found an average Cu valence of 2.52 from the charge compensation [7]. Although the compositions of our our samples may be different from those mentioned above, the average charge of Cu in the C u t 2 plane in our samples is in the range of those reported previously. However, it is noteworthy that the T~ in such Cu-1234 compounds is around 117 K in spite of the different contents of Cu atoms, C atoms and O atoms, and consequently the different carder concentrations. This phenomenon seems to be quite strange compared with other cuprate superconductors in which a considerable decreases in Tc were observed by introducing C or other impurities. Concerning this, Shimakawa et al. surmised that carriers in the inner two C u t 2 planes may govern the superconductivitY in Cu-1234 superconductors [6]. If this assumption is correct, the Hall number in the inner C u t 2 plane for the Cu-1234 superconductor may be around 0.35, which is the smallest value shown in Fig. 3. This is the Hall number of sample C, which is estimated to contain no C and shows the best superconducting characteristics in the present study. This implies that the formation of nearly complete C u t 2 block layers and optimization of the carrier concentration through control of the oxygen concentration are important for improving the superconducting characteristics of the Cu-1234 compounds. The Hall angle OH is plotted as cot 0 n vs. T 2 in Fig. 4. The cot OH follows completely the T 2 dependence in the measured temperature range up to 300 K. The relation cot OH = a T 2 + / 3 was proposed theoretically [21,22] and experimentally in many kinds of hole-type cuprate superconductors [9-14]. Thus the scattering mechanisms for hole carders in the Cu-1234 compounds seem to be similar to those of other oxide superconductors. The values of cr for Cu-1234, normalized for a magnetic field of 1 T, were 0.05-0.06 and almost the same as those for YBa2Cu307_;~ (~ 0.04, T < 255 K) [9], H g - 1 2 3 4 ( ~ 0.045, T < 230 K) [20] and TI2Ba2CuO6+ ~ ( ~ 0.06) [11]. In general, impurity scattering causes pronounced /3 values, as shown in the Zn-doped Y-123 superconductor [9]. It is noted that the value of /3 is almost zero in these Cu-1234 compounds. This may indicate that little disorder is introduced for electronic transport in the C u O 2
i
0.5
t
i
i
i
i
B=2T
i
'
t
i
~
E 0.3
7
0.2
1000 ~
0.1 0.0 I
o
I
'
I
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8
T2(104K2) Fig. 4. Cotangentof the Hall angle (cot 0n = p / ( R s B)) and the inverse Hail mobility(l//z n = p / R n ) . The solid lines show fits of the data to cot 0n =otT 2 + ft. planes. In fact, a fairly large positive values of /3 ( ~ 1100) and deviations from the T2-dependence of cot 0 n at low temperature (T < 25 5 K) were observed in an inferior sample of Cu-1234 not shown in Fig. 4. In conclusion, we have presented the first measurements of the Hall effect in the Cu-1234 system of ( C u l _ x _ ~Cx)Ba2CaaCu4Oysynthesized using the high-pressure, high-temperature technique. The results resemble those obtained for other high-Tc oxide superconductors. The temperature dependences p ~ T , R H ~ 1 / T and c o t O H ~ T 2 were observed. The temperature dependence of p and cot OH seems to indicate the low impurity scattering in these samples. The Hall number per C u t 2 plane was 0.35-0.6 and the carrier concentration dependence of Tc is very slight. However, the most excellent superconducting characteristics, related to Tc and ATc, were observed in the sample of Cu l_ 8BaECa3Cu4Oyand the Hall number was ~ 0.35. This implies that the formation of a sample with fully oxidized C u t 2_ block layers is important for drawing out the excellent superconducting properties in the Cu-1234 system.
Acknowledgments
We wish to thank M. Hamao for assistance in the experimental work.
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M. Ogino et aL /Physica C 258 (1996) 384-388
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