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Role of interlayer coupling in alkaline-substituted (Bi, Pb)-2223 superconductors
Journal of Alloys and Compounds 804 (2019) 348e352
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Role of interlayer coupling in alkaline-substituted (Bi, Pb)-2223 superconductors J.Y. Oh a, Tien M. Le b, d, A.T. Pham b, D.H. Tran b, D.S. Yang c, B. Kang a, * a
Department of Physics, Chungbuk National University, Cheongju, South Korea Faculty of Physics, VNU University of Science, Hanoi, Viet Nam c Department of Physics Education, Chungbuk National University, Cheongju, South Korea d Department of Physics, Sungkyunkwan University, Suwon, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2019 Received in revised form 1 July 2019 Accepted 3 July 2019 Available online 4 July 2019
In this work, we investigated a correlation between superconductivity and interlayer coupling of two different alkaline (Na and K)-substituted Bi1$6Pb0$4Sr2Ca2Cu3O10þd (BSCCO) polycrystalline samples. The excess conductivity analysis by the Aslamazov-Larkin (AL) and Lawrence-Doniach (LD) theories showed that Na substitution at the Ca site induced a gradual broadening of 3D fluctuation region with increasing interlayer coupling strength, which explains a systematic increase of Tc and a decrease of normal state resistivity. On the other hand, exactly the opposite results were observed in the K-substituted samples in place of Sr. Extended x-ray absorption fine structure (EXAFS) studies revealed that substitution of Na and K generated completely different effects on the local structure around Cu atoms. It is noticeable that the CueO bond distance was found to decrease monotonically with the varying amounts of Na, which indicates that the CuO2 layer is stabilized. On the while, the opposite was observed to occur with the varying amounts of K. Unlike the CueCa bond which was the least affected by the substitution, the CueSr bond distance increased drastically with K substitution. All these findings indicate that Na substitution at the Ca site enhances superconductivity with no loss of interlayer interaction, while K substitution at the Sr site weakens superconductivity due to the diminished interlayer interaction. © 2019 Published by Elsevier B.V.
Keywords: BSCCO Critical temperature Excess conductivity Interlayer coupling EXAFS
1. Introduction Ever since the discovery of high temperature superconductor (HTS) in layered cuprates, intensive studies have been carried out to identify the mechanism of high-Tc superconductivity in these systems [1]. In recent years, there have been a lot of studies on the investigation of BSCCO with a general formula of Bi2Sr2Can1CunO2nþ4þy, which is identified as three different phases depending on the numbers of CuO2 layers (n ¼ 1, 2, or 3). Each phase yields a different value of the superconducting transition temperature (Tc), i.e., Bi2Sr2CuO6 (Bi-2201, Tc ¼ 20 K), Bi2Sr2CaCu2O8 (Bi-2212, Tc ¼ 80 K), and Bi2Sr2Ca2Cu3O10 (Bi-2223, Tc ¼ 110 K) [2,3]. Among them, the Bi-2223 phase has attracted a large amount of attention as a potential candidate for wire/cable applications not only due to remarkably high Tc but also due to larger values of the critical current density Jc and the critical magnetic field Hc2, as
* Corresponding author. E-mail address: [email protected] (B. Kang). https://doi.org/10.1016/j.jallcom.2019.07.029 0925-8388/© 2019 Published by Elsevier B.V.
compared to other phases [3,4]. It is well known that the Bi-2223 system consists of a conducting CuO2 layer and a space layer (SrO, BiO, and Ca). The conducting CuO2 layer is in charge of the electrical conduction, while the space layer is to transfer charge to the conducting CuO2 layer in order to generate superconductivity [4,5]. Therefore, carrier density is the most important parameter of this system, which determine both the normal state and the superconducting state properties. Due to a strong dependence of superconductivity on carrier density, many scientists have tried to modify the space layer in order to improve superconductivity. Among the various ways of studies, cationic substitution is known to be an effective method to change the number of carriers in the CuO2 layer and to understand the underlying mechanism of superconductivity. The effect of cationic substitution on the Bi-2223 system has been studied by several groups [6e12]. Substitution of rare-earth (RE) materials into the Bi2þ, the Sr2þ, and the Ca2þ sites has been studied, and some groups displayed that there is a close relationship between rare-earth substitution and hole filling effect, thereby resulting in a decrement in the Tc values [4,8]. As an alternative,
J.Y. Oh et al. / Journal of Alloys and Compounds 804 (2019) 348e352
substitution of alkaline materials, which have a lower valence (þ1) state and similar ionic radii with Bi, Sr, Ca, and Cu [9e12], was carried out to change carrier concentration. Some of these substitutions (Ca replacement by Na and Cu replacement by Li) promoted an optimum carrier density, hence the Tc values were enhanced [9e11], while the case of Sr replacement by K showed an opposite trend [11,12]. For the BSCCO system, intrinsic characteristics such as a short coherence length and a high anisotropy induce superconducting fluctuation which arises from the formation of Cooper pairs above Tc [13]. In order to understand the underlying mechanism between optimization of charge carriers and Tc, excess conductivity studies based on the Aslamazov and Larkin (AL), and Lawrence and Doniach (LD) theories [14,15] were performed by several groups. Previous studies reported that excess conductivity was greatly affected by cationic substitution [16e19], and that the Tc suppression was probably due to weakened interlayer coupling between the CuO2 layers. Despite intense experimental studies carried out, the origin of weakened interlayer coupling and the mechanism of Tc enhancement or suppression by alkaline substitution have not been clearly resolved yet [9e12]. In this work, we report a comparative study of the excess conductivities of Na- and K- substituted Bi-2223 system, i.e., Bi1$6Pb0$4Sr2Ca2-xNaxCu3O10þd (Na-substituted) and Bi1$6Pb0$4Sr2yKyCa2Cu3O10þd (K-substituted). Na and K were chosen as dopants due to the following reasons: (i) their ionic radii are similar to those of Ca and Sr, and (ii) they have lower oxidation state (þ1) than Ca and Sr (þ2). Therefore, carrier concentration is expected to change [9e12], which may produce alteration in local structure. To identify the distribution of atomic displacements by two differnet alkaline substitutions, the local atomic structure of the CuO2 plane was characterized by using extended X-ray absorption fine structure (EXAFS) spectroscopy. We found a close correlation between Tc and the local structure and proposed an effective doping mechanism which may improve the superconductivity of the Bi-2223 system.
Ds ¼
e2 4J 0:5 1 þ ε 16Zdε
349
(2)
Where J ¼ ½2xc ð0Þ=d2 . For the superconducting system with weak coupling, this expression can be reduced to Ds ¼ Aε1:0 ð2DÞ. This equation well suits for the BSCCO system and predicts a crossover of conduction dimensionality from 2D to 3D at TLD, where TLD ¼ Tc ð1 þ JÞ [15]. 3. Experimental details Polycrystalline Bi1$6Pb0$4Sr2Ca2-xNaxCu3O10þd (x ¼ 0.00, 0.02, 0.04 and 0.06) and Bi1$6Pb0$4Sr2-yKyCa2Cu3O10þd (y ¼ 0.02, 0.04 and 0.06) samples were prepared by a solid-state reaction technique. The Pb content was chosen to be 0.4 based on the optimized value for the highest Tc [7]. Appropriate weights of high-purity (99.99%) Bi2O3, PbO, SrCO3, CuO and Na2CO3 (for Na-doped), and Bi2O3, PbO, SrCO3, CuO and K2CO3 (for K-doped) were thoroughly mixed and ground. The mixed powders were calcined by four steps in air at 670 C/48 h þ750 C/48 h þ800 C/48 h þ820 C/48 h with several intermittent grindings in order to ensure homogeneity. The powders were then pressed into pellets and sintered in air at 850 C for 160 h. According to the alkaline material contents x and y, the samples were labeled as Pure, Na002, Na004, Na006, K002, K004, and K006. The temperature dependence of electrical resistivity was measured in the temperature range from 50 K to 300 K by using a four probe method in a closed-loop Helium system. The local atomic displacements were investigated by analyzing extended Xray absorption fine structure (EXAFS) data with 8C Beam line of the Pohang Light Source (PLS) at room temperature. The collected data were analyzed by the ATHENA and ARTEMIS codes of the IFEFFIT (Newville, 2001) software program [24,25]. 4. Results and discussion
2. Theoretical approaches Aslamazov and Larkin provided a theoretical expression of excess conductivity Ds by using microscopic approach in the mean field region (MFR), where the fluctuations are small. Ds is expressed to diverge as a power-law given by Ds ¼ Aεl , where ε is the reduced temperature given by the relation ε ¼ (T - Tc)/Tc with Tc corresponding to the maximum peak in the dr/dT vs. T plot, and l is the critical exponent related to the conduction dimensionality expressed as; l ¼ 0.3 for critical region (CR), l ¼ 0.5 for 3D, l ¼ 1.0 for 2D, and l ¼ 3.0 for short wave fluctuations (SWF). A is a temperature independent constant and is expressed as A ¼ e2 = 32Zxc ð0Þ and A ¼ e2 =16Zd for 3D and 2D, respectively, where xc ð0Þ is the c-axis coherence length and d is the effective inter-layering spacing between the two CuO2 layers [14,15,20e22]. From an experimental point of view, excess conductivity can be calculated:
Ds ¼
1
rðTÞ
1
rn ðTÞ
The temperature dependences of the ab-plane resistivity rab ðTÞ of all samples are shown in Fig. 1. The resistivity vs. temperature plot is separated into two regimes by different behaviors; the first is attributed to the characterization of the normal state, and the other is characterized by the superconducting fluctuation of Cooper pairs above Tc. In the first regime, resistivity exhibited a metallic behavior
(1)
where rn ðTÞ is extrapolated normal state resistivity obtained from the Anderson and Zou relation [23]. Lawrence and Doniach (LD) modified the AL theory to be suitable for strong anisotropic superconductor by introducing the interlayer coupling strength J, and excess conductivity is expressed as:
Fig. 1. The temperature dependence of the resistivity of pure, Na-substituted and Ksubstituted samples. The inset shows a plot of dr =dT as a function of temperature and the solid line is the best fit to the Anderson-Zou relation to obtain Tc and rn .
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J.Y. Oh et al. / Journal of Alloys and Compounds 804 (2019) 348e352
with a nearly constant slope, while in the second regime where rðTÞ is deviating from a linearity, the fluctuation of Cooper pairs contributed to the conductivity near Tc [12,26e28]. The values of Tc were determined from the temperatures corresponding to the maximum dr =dT as shown in the inset of Fig. 1. The resistivity curves for Na substitution and the K substitution displayed opposite behaviors as dopant contents increased. For the samples with Na substitution, the values of Tc were found to increase systematically from 105.4 K (pure) to 108.5 K (Na006), whereas the normal state resistivity at 250 K decreased from 0.037 Ucm (pure) to 0.025 Ucm (Na006). On the other hand, the resistivity curves for K substitution showed an exactly opposite trend. Unlike the expectation that the substitution of alkaline ions with þ1 valence state would increase the carrier density in the CuO2 layer, the carrier density seemed to be systematically reduced with K substitution. These behaviors may be explained as a result of different modifications in the space layers by introducing Na and K ions. In the HTS cuprates, strong interlayer coupling facilitates transferring charge carriers from the space layer to the CuO2 layer, and may increase carrier density in the CuO2 layer, manifested by a reduction of the normal state resistivity [15,20]. Therefore, the rðTÞ behaviors may indicate an increase or a decrease of the CuO2 interlayer coupling induced by introducing Na and K ions into the space layer. To investigate a change in the CuO2 interlayer coupling by alkaline substitution, the excess conductivity data of all samples were analyzed according to the LD theory [15]. The inset of Fig. 1 presented a fit of the normal state resistivity from 150 K to 250 K to the Anderson and Zou's linear relation to obtain an extrapolated resistivity, which was used to calculate the excess conductivity. The double logarithmic plots of calculated excess conductivity ðDsÞ as a function of reduced temperature ðεÞ of five representative samples (Pure, Na002, Na006, K002 and K006) were displayed in Fig. 2. By fitting the data with the critical exponent values of the AL model, the reduced temperature can be separated into three regions i.e., short wave fluctuation (SWF), mean field region (MFR) and critical region (CR). In the SWF, the excess conductivity rapidly decreased above Tc with an exponent value of ly3, indicating that short wave fluctuations play a dominant role [29]. In the MFR, the critical exponent decreased to ly1:0, which is characterized by 2D fluctuations, indicating that conductivity in this region mainly occurs from Cooper pairs limited in the CuO2 layer [15,30]. As temperature is further decreased close to Tc, the critical exponent decreases to ly 0:5 (3D fluctuation), where Cooper pairs cross a barrier layer to reach the conducting CuO2 layer [15]. The final region with ly 0:3 is the dynamic CR, where the GL theory breaks down and interaction between Cooper pairs is considered [22,31]. As shown in Fig. 2, the critical exponents of the Ds data in the CR for all samples were found to be ~0.3 in good agreement with the theoretical prediction of dynamic scaling effect [22], which justify the adaption of the LD theory. In addition, the Ds data in the MFR for all samples represented two distinct linear parts with different exponents, indicating that a crossover from 2D to 3D was occurred at TLD. It is noted that the values of 2D-3D crossover temperature TLD and the interlayer coupling J calculated from TLD turned out to depend upon the doping contents and increased systematically for the Na-substituted case, while they decreased for the K-substituted case, as summarized in Table 1. The increase of TLD prolonged the 3D fluctuation region and resulted in an increase of interlayer coupling strength. The Fermi velocity of charge carriers could be deduced from the excess conductivity data in MFR. According to the LD theory, the excess conductivity in 2D is reduced to Ds ¼ Aε1:0 ð2DÞ and the slope b of Ds1 as a function of temperature is given as b ¼ 16Zd= e2 Text , where Text is extrapolated temperature obtained from the
Fig. 2. Double logarithmic plot of excess conductivity Ds as a function of reduced temperature ε for a) Pure, b) Na002, c) Na006, d) K002, and e) K006 samples. The black dot-dashed lines with a slope of ~0.3 correspond to the CR in the LD model. The red and the blue lines are the fits to the 3D and the 2D models in the MFR, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Parameters calculated from the excess conductivity analysis of pure, Na-substituted and K-substituted samples. Sample
T mf c (K)
TLD (K)
J
b(K1)
d (Å)
xc ð0Þ(Å)
Vf (106 m/s)
Pure Na002 Na004 Na006 K002 K004 K006
105.4 107.1 107.6 108.5 104.6 103.6 102.9
106.6 108.6 109.3 111.1 105.6 104.5 103.7
0.0028 0.0036 0.0040 0.0061 0.0024 0.0021 0.0019
0.041 0.036 0.032 0.028 0.037 0.040 0.046
65.4 62.8 57.2 49.1 68.7 72.9 88.9
1.59 1.67 1.74 1.76 1.58 1.51 1.46
1.44 1.53 1.61 1.63 1.43 1.33 1.29
slope b. The temperature dependence of Ds1 and an enlarged plot near Text for all samples are shown in Fig. 3 and the inset of Fig. 3, respectively. The values of d and xc ð0Þ determined from the extrapolation of the Ds1 plots revealed an inverse relationship with each other. By varying amounts of Na, the former decreased and the latter increased, while exactly the opposite trend was observed in the K-substituted case, as listed in Table 1. The increase of xc ð0Þ by Na substitution may reduce the anisotropy of the Bi2223 system and leads to an improved superconducting property, consistent with the result of increasing Tc. The Fermi velocity VF of carriers was calculated by using the following expression:
J.Y. Oh et al. / Journal of Alloys and Compounds 804 (2019) 348e352
Fig. 3. The temperature dependence of Ds1 plots for all samples. Inset: The linear fit line determines the value of b.
VF ¼
5pkB Tc xc ð0Þ 2KZ
(3)
where kB is the Boltzman constant, K is a coefficient of proportionality (K y0:12Þ, and Z is the reduced Planck constant. The values of VF were found to increase with increasing Na content, while they were found to decrease with varying amounts of K, as listed in Table 1. The increase or decrease of VF indicates that the efficiency of charge carrier transfer to the CuO2 layer is enhanced or reduced by different substitutions, which may lead to an increase or a decrease of carrier numbers in the CuO2 layer. This result provides a plausible explanation on enhanced/weakened CuO2 interlayer coupling by different dopants, which is known to be sensitive to the local structure in the normal state [32]. To have a direct access to the local structural change, we carried out interactive analysis of EXAFS data measured for Cu K (8979 eV) absorption edge with the IFEFFIT program. The k-weighted Fourier transform functions of the Cu K-edge EXAFS for all samples are shown in Fig. 4 and the radial distribution of neighbors of the Cu atom can be observed by inspecting the peaks of the Fourier transforms. Fig. 4 represents the 1st and the 2nd shells located at a
351
distance of ~1.5 Å and ~2.6 Å, respectively, which correspond to the contribution of the CueO and the CueCa/Sr scatterings (coordinated with 4 O and 4 Ca/Sr atoms). Even though all the EXAFS oscillations exhibited similar shapes, a systematic increase/decrease in the amplitude of the CueO bond in the 1st shell with increasing the Na/K contents is clearly visible. This signifies that the ordering of the CueO bond is improved with the Na substitution while it is degraded with the K substitution. In order to quantatively study local atomic displacement, twoshell-model fitting including the contribution of the CueO and the CueCa/Sr bonds were carried out for the Fourier transforms of the Cu K-edge EXAFS oscillations. During the fitting process, only two parameters, the bond distance (Ri), and the Debye-Waller factor (DWF) indicating the mean square relative displacement (for each shell), were allowed to vary and the other parameters were fixed [33]. The fitting k and R ranges were 2.8e11.5 Å1 and 1.0e3.2 Å (for the Cu K-edge) and the fitting result for the pure sample is shown in the inset of Fig. 4 as a representative. The R factor estimated as in Refs. [33,34] for the EXAFS fits was found to be 15% for all the samples, confirming high goodness of fits. Fig. 5 (a)-(d) show the variation of local bond distances and the values of DWF, which provide information on the atomic disorder, as a function of the doping contents. Owing to that the radii of Na1þ and K1þ are slightly different from those of Ca2þ and Sr2þ, the distances of the CueO, the CueCa and the CueSr bonds exhibit small changes as the doping contents vary. The local bond distances and the values of DWF showed a similar behavior depending upon the doping content, infering that the local atomic disorder is correlated with the bond distance. By increasing the Na contents, the CueO bond gradually contracted from 1.906 Å (pure) to 1.897 Å (Na006) with smaller values of DWF, which indicates stabilization of the CuO2 layer. On the other hand, by increasing the K contents, the CueO bond distance monotonically elongated from 1.906 Å (pure) to 1.916 Å (K006) with larger DWF values, which may be due to the reduction of carrier density in the CuO2 layer. These results showed that both alkaline substitutions induced distortions in the CuO2 layer. Unlike the CueO bond, the CueCa bond distance was constant to be ~3.097 Å for whole range of dopants level, indicating that the Ca layer was the least affected by the Na/K substitution. Therefore, this result suggests that a relatively stable Ca layer is a suitable candidate for alkaline substitution with no loss of interlayer interaction. On the other hand, the CueSr bond distance changed differently with the Na/K substitutions; the CueSr bond distances were constant to be ~3.144 Å for the case of Na substitution, while those of the K substitution were increased from 3.144 Å to 3.216 Å. The elongated CueSr bond distance with larger value of DWF by the K substitution may cause a reduced interaction between the SrO and the CuO2 layers, which weakens local superconductivity. As a result, the enhancement/suppression of Tc values by different alkaline substitutions may be explained by the improved/degraded carrier supply from the space layer (SrO and Ca) to the conduction CuO2 layer due to the enhanced/weakend CuO2 interlayer coupling caused by local structural alterations. 5. Conclusion
Fig. 4. Fourier transforms of Cu K-edge EXAFS oscillations weighted by k3 which represent the radial distribution of neighboring atoms around Cu atom. Inset: The solid line represents the fit to the two-shell-model of the EXAFS.
The effects of partial substitution of Na and K at the Ca and Sr sites respectively, on the superconductivity and the interlayer coupling of the Bi1$6Pb0$4Sr2Ca2Cu3O10þd system were investigated. The Na substitution systematically increased the Tc values, while the K substitution lead to opposite results. The excess conductivity analysis by the AL and the LD theories showed that dimesionality crossover from 2D to 3D in the mean field region depends upon what the doping contents are. The systematic increase of the 2D-3D
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Fig. 5. The bond distances and the corresponding Debye-Waller factors for CueO, CueSr, and SreO bonds as a function of dopant content, which were determined by the two-shellmodel fitting to the Cu K-edge EXAFS oscillations.
transition temperature TLD of the Na-substituted samples indicated a gradual expansion of the 3D fluctuation region due to the enhanced CuO2 interlayer coupling. It should be noted, however, that exactly the opposite results were also observed in the Ksubstituted cases, which might be attributed to the weakened CuO2 interlayer coupling. The EXAFS data measured at the Cu K-absorption edge were analyzed to probe alteration on the local structue of the CuO2 layer and the space layers (SrO and Ca). The EXAFS data showed that the CueO bond distance systematically decreased for the Na substitution, while it increased for the K substitution, which implies that distortions in the CuO2 layer occcurs in two different directions depending on the dopants. The CuO2 distortion may lead to deformation only of the SrO layer, not to that of the Ca layer, thereby resulting in either an improved or reduced interlayer interaction of the CuO2 with the Ca or the SrO layers respectively, which accounts for the enhancement or suppression of Tc values by different alkaline substitutions. Acknowledgments This work was supported by the National Research Foundation of Korea grant funded by the Korean government (MSIT) (NRF2018R1A2B6004784). References [1] H. Maeda, Y. Tanaka, M. Fukutumi, T. Asano, Jpn. J. Appl. Phys. 27 (2) (1988) L209eL210. [2] S.E. Mousavi Ghahfarokhi, M. Zargar Shoushtari, Phys. B 405 (2010) 4643. [3] I.H. Gul, M.A. Rehman, M. Ali, A. Maqsood, Phys. C 432 (2005) 71e80. € [4] C. Terzioglu, H. Aydin, O. Ozkurt, E. Bekiroglu, I. Belenli, Phys. B 403 (2008)
3354e3359. [5] K. Maarit, H. Yamauchi, Philos. Mag. A B 79 (1999) 343e366. [6] M. Karppinen, H. Yamauchi, Y. Morita, M. Kitabatake, T. Motohashi, R.S. Liu, J.M. Lee, H.M. Chen, J. Solid State Chem. 177 (2004) 1037e1043. [7] N. Kijima, N. Endo, J. Tsuchiya, A. Sunmiyama, M. Mizino, Y. Oguri, Jpn. J. Appl. Phys. 27 (1998) L821. [8] G. Ilonca, A.V. Pop, T.R. Yang, I. Gr Deac, C. Lung, R. Stiufiuc, G. Stiufiuc, Int. J. Inorg. Mater. 3 (2001) 769e772. [9] O. Bilgili, Y. Selamet, K. Kocabas, J. Supercond. Nov. Magnetism 21 (2008) 439. € [10] B. Ozçelik, M. Gürsul, A. Sotelo, M.A. Madre, J. Mater. Sci. Mater. Electron. 26 (2015) 441e447. [11] T. Kawai, T. Horiuchi, K. Mitsui, K. Ogura, S. Takagi, S. Kawai, Phys. C 161 (1989) 561. [12] L.D. Sykorova, O. Smrckova, V. Jakes, Phys. Status Solidi 1 (7) (2004) 1952. [13] E. Silva, S. Sarti, R. Fastampa, M. Giura, Phys. Rev. B 65 (2001) 144508. [14] L.G. Aslamazov, A.I. Larkin, Sov. Phys 10 (1968) 875. [15] W.E. Lawrence, S. Doniach, in: Proc. 12th International Conf. Low Temperature Physics (Keigaku, Tokyo) Eizo Kanada, 1970, p. 361. [16] S.M. Khalil, J. Low Temp. Phys. 143 (2006) 31e44. [17] D.R. Mishra, Pramana - J. Phys. 70 (3) (2008) 535e541. [18] R.K. Nkum, W.R. Datars, Phys. C 192 (1992) 215e222. [19] N. Mori, J.A. Wilson, H. Ozaki, Phys. Rev. B 45 (1992) 10633e10638. [20] S.A. Saleh, S.A. Ahmed, E.M.M. Elshikh, J. Supercond. Nov. Magnetism 21 (2008) 187. € Rapp, Phys. C 408 (2004) 679. €s, B.R. Zhao, O. [21] S.H. Han, I. Bryntse, J. Axna [22] C.J. Lobb, Phys. Rev. B 36 (1987) 3930. [23] P.W. Anderson, Z. Zou, Phys. Rev. Lett. 60 (1988) 132. [24] B. Ravel, J. Synchrotron Radiat. 8 (2001) 314e316. [25] B. Ravel, M. Newville, J. Synchrotron Radiat. 12 (2005) 537e541. € Rapp, Phys. C 282 (1997) 1571. [26] S.H. Han, P. Lundqvist, O. [27] A.L. Solovjov, H.U. Habermeier, T. Haage, Low Temp. Phys. 28 (2002) 17. [28] M. Mun, S. Lee, S.S. Salk, H.J. Shin, M.K. Joo, Phys. Rev. B 48 (1993) 6703. [29] L. Reffiani, R. Vaglio, A.A. Varlamo, Phys. Rev. B 44 (1991) 9543. [30] W. Lang, G. Heine, Phys. Rev. B 51 (1995) 9180. [31] P.C. Hohenberg, B.I. Halperin, Rev. Mod. Phys. 49 (1977) 435. [32] A. Mohanta, D. Behera, Phys. B 406 (2011) 877e884. [33] R. Prins, D.C. Koningsberger, X-ray Absorption: Principles, Applications, Techniques of EXAFS, SEXAFS, XANES, Wiley, New York, 1988. [34] M. Newville, J. Synchrotron Radiat. 8 (2) (2001) 96e100.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Role of interlayer coupling in alkaline-substituted (Bi, Pb)-2223 superconductors J.Y. Oh a, Tien M. Le b, d, A.T. Pham b, D.H. Tran b, D.S. Yang c, B. Kang a, * a
Department of Physics, Chungbuk National University, Cheongju, South Korea Faculty of Physics, VNU University of Science, Hanoi, Viet Nam c Department of Physics Education, Chungbuk National University, Cheongju, South Korea d Department of Physics, Sungkyunkwan University, Suwon, South Korea b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2019 Received in revised form 1 July 2019 Accepted 3 July 2019 Available online 4 July 2019
In this work, we investigated a correlation between superconductivity and interlayer coupling of two different alkaline (Na and K)-substituted Bi1$6Pb0$4Sr2Ca2Cu3O10þd (BSCCO) polycrystalline samples. The excess conductivity analysis by the Aslamazov-Larkin (AL) and Lawrence-Doniach (LD) theories showed that Na substitution at the Ca site induced a gradual broadening of 3D fluctuation region with increasing interlayer coupling strength, which explains a systematic increase of Tc and a decrease of normal state resistivity. On the other hand, exactly the opposite results were observed in the K-substituted samples in place of Sr. Extended x-ray absorption fine structure (EXAFS) studies revealed that substitution of Na and K generated completely different effects on the local structure around Cu atoms. It is noticeable that the CueO bond distance was found to decrease monotonically with the varying amounts of Na, which indicates that the CuO2 layer is stabilized. On the while, the opposite was observed to occur with the varying amounts of K. Unlike the CueCa bond which was the least affected by the substitution, the CueSr bond distance increased drastically with K substitution. All these findings indicate that Na substitution at the Ca site enhances superconductivity with no loss of interlayer interaction, while K substitution at the Sr site weakens superconductivity due to the diminished interlayer interaction. © 2019 Published by Elsevier B.V.
Keywords: BSCCO Critical temperature Excess conductivity Interlayer coupling EXAFS
1. Introduction Ever since the discovery of high temperature superconductor (HTS) in layered cuprates, intensive studies have been carried out to identify the mechanism of high-Tc superconductivity in these systems [1]. In recent years, there have been a lot of studies on the investigation of BSCCO with a general formula of Bi2Sr2Can1CunO2nþ4þy, which is identified as three different phases depending on the numbers of CuO2 layers (n ¼ 1, 2, or 3). Each phase yields a different value of the superconducting transition temperature (Tc), i.e., Bi2Sr2CuO6 (Bi-2201, Tc ¼ 20 K), Bi2Sr2CaCu2O8 (Bi-2212, Tc ¼ 80 K), and Bi2Sr2Ca2Cu3O10 (Bi-2223, Tc ¼ 110 K) [2,3]. Among them, the Bi-2223 phase has attracted a large amount of attention as a potential candidate for wire/cable applications not only due to remarkably high Tc but also due to larger values of the critical current density Jc and the critical magnetic field Hc2, as
* Corresponding author. E-mail address: [email protected] (B. Kang). https://doi.org/10.1016/j.jallcom.2019.07.029 0925-8388/© 2019 Published by Elsevier B.V.
compared to other phases [3,4]. It is well known that the Bi-2223 system consists of a conducting CuO2 layer and a space layer (SrO, BiO, and Ca). The conducting CuO2 layer is in charge of the electrical conduction, while the space layer is to transfer charge to the conducting CuO2 layer in order to generate superconductivity [4,5]. Therefore, carrier density is the most important parameter of this system, which determine both the normal state and the superconducting state properties. Due to a strong dependence of superconductivity on carrier density, many scientists have tried to modify the space layer in order to improve superconductivity. Among the various ways of studies, cationic substitution is known to be an effective method to change the number of carriers in the CuO2 layer and to understand the underlying mechanism of superconductivity. The effect of cationic substitution on the Bi-2223 system has been studied by several groups [6e12]. Substitution of rare-earth (RE) materials into the Bi2þ, the Sr2þ, and the Ca2þ sites has been studied, and some groups displayed that there is a close relationship between rare-earth substitution and hole filling effect, thereby resulting in a decrement in the Tc values [4,8]. As an alternative,
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substitution of alkaline materials, which have a lower valence (þ1) state and similar ionic radii with Bi, Sr, Ca, and Cu [9e12], was carried out to change carrier concentration. Some of these substitutions (Ca replacement by Na and Cu replacement by Li) promoted an optimum carrier density, hence the Tc values were enhanced [9e11], while the case of Sr replacement by K showed an opposite trend [11,12]. For the BSCCO system, intrinsic characteristics such as a short coherence length and a high anisotropy induce superconducting fluctuation which arises from the formation of Cooper pairs above Tc [13]. In order to understand the underlying mechanism between optimization of charge carriers and Tc, excess conductivity studies based on the Aslamazov and Larkin (AL), and Lawrence and Doniach (LD) theories [14,15] were performed by several groups. Previous studies reported that excess conductivity was greatly affected by cationic substitution [16e19], and that the Tc suppression was probably due to weakened interlayer coupling between the CuO2 layers. Despite intense experimental studies carried out, the origin of weakened interlayer coupling and the mechanism of Tc enhancement or suppression by alkaline substitution have not been clearly resolved yet [9e12]. In this work, we report a comparative study of the excess conductivities of Na- and K- substituted Bi-2223 system, i.e., Bi1$6Pb0$4Sr2Ca2-xNaxCu3O10þd (Na-substituted) and Bi1$6Pb0$4Sr2yKyCa2Cu3O10þd (K-substituted). Na and K were chosen as dopants due to the following reasons: (i) their ionic radii are similar to those of Ca and Sr, and (ii) they have lower oxidation state (þ1) than Ca and Sr (þ2). Therefore, carrier concentration is expected to change [9e12], which may produce alteration in local structure. To identify the distribution of atomic displacements by two differnet alkaline substitutions, the local atomic structure of the CuO2 plane was characterized by using extended X-ray absorption fine structure (EXAFS) spectroscopy. We found a close correlation between Tc and the local structure and proposed an effective doping mechanism which may improve the superconductivity of the Bi-2223 system.
Ds ¼
e2 4J 0:5 1 þ ε 16Zdε
349
(2)
Where J ¼ ½2xc ð0Þ=d2 . For the superconducting system with weak coupling, this expression can be reduced to Ds ¼ Aε1:0 ð2DÞ. This equation well suits for the BSCCO system and predicts a crossover of conduction dimensionality from 2D to 3D at TLD, where TLD ¼ Tc ð1 þ JÞ [15]. 3. Experimental details Polycrystalline Bi1$6Pb0$4Sr2Ca2-xNaxCu3O10þd (x ¼ 0.00, 0.02, 0.04 and 0.06) and Bi1$6Pb0$4Sr2-yKyCa2Cu3O10þd (y ¼ 0.02, 0.04 and 0.06) samples were prepared by a solid-state reaction technique. The Pb content was chosen to be 0.4 based on the optimized value for the highest Tc [7]. Appropriate weights of high-purity (99.99%) Bi2O3, PbO, SrCO3, CuO and Na2CO3 (for Na-doped), and Bi2O3, PbO, SrCO3, CuO and K2CO3 (for K-doped) were thoroughly mixed and ground. The mixed powders were calcined by four steps in air at 670 C/48 h þ750 C/48 h þ800 C/48 h þ820 C/48 h with several intermittent grindings in order to ensure homogeneity. The powders were then pressed into pellets and sintered in air at 850 C for 160 h. According to the alkaline material contents x and y, the samples were labeled as Pure, Na002, Na004, Na006, K002, K004, and K006. The temperature dependence of electrical resistivity was measured in the temperature range from 50 K to 300 K by using a four probe method in a closed-loop Helium system. The local atomic displacements were investigated by analyzing extended Xray absorption fine structure (EXAFS) data with 8C Beam line of the Pohang Light Source (PLS) at room temperature. The collected data were analyzed by the ATHENA and ARTEMIS codes of the IFEFFIT (Newville, 2001) software program [24,25]. 4. Results and discussion
2. Theoretical approaches Aslamazov and Larkin provided a theoretical expression of excess conductivity Ds by using microscopic approach in the mean field region (MFR), where the fluctuations are small. Ds is expressed to diverge as a power-law given by Ds ¼ Aεl , where ε is the reduced temperature given by the relation ε ¼ (T - Tc)/Tc with Tc corresponding to the maximum peak in the dr/dT vs. T plot, and l is the critical exponent related to the conduction dimensionality expressed as; l ¼ 0.3 for critical region (CR), l ¼ 0.5 for 3D, l ¼ 1.0 for 2D, and l ¼ 3.0 for short wave fluctuations (SWF). A is a temperature independent constant and is expressed as A ¼ e2 = 32Zxc ð0Þ and A ¼ e2 =16Zd for 3D and 2D, respectively, where xc ð0Þ is the c-axis coherence length and d is the effective inter-layering spacing between the two CuO2 layers [14,15,20e22]. From an experimental point of view, excess conductivity can be calculated:
Ds ¼
1
rðTÞ
1
rn ðTÞ
The temperature dependences of the ab-plane resistivity rab ðTÞ of all samples are shown in Fig. 1. The resistivity vs. temperature plot is separated into two regimes by different behaviors; the first is attributed to the characterization of the normal state, and the other is characterized by the superconducting fluctuation of Cooper pairs above Tc. In the first regime, resistivity exhibited a metallic behavior
(1)
where rn ðTÞ is extrapolated normal state resistivity obtained from the Anderson and Zou relation [23]. Lawrence and Doniach (LD) modified the AL theory to be suitable for strong anisotropic superconductor by introducing the interlayer coupling strength J, and excess conductivity is expressed as:
Fig. 1. The temperature dependence of the resistivity of pure, Na-substituted and Ksubstituted samples. The inset shows a plot of dr =dT as a function of temperature and the solid line is the best fit to the Anderson-Zou relation to obtain Tc and rn .
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with a nearly constant slope, while in the second regime where rðTÞ is deviating from a linearity, the fluctuation of Cooper pairs contributed to the conductivity near Tc [12,26e28]. The values of Tc were determined from the temperatures corresponding to the maximum dr =dT as shown in the inset of Fig. 1. The resistivity curves for Na substitution and the K substitution displayed opposite behaviors as dopant contents increased. For the samples with Na substitution, the values of Tc were found to increase systematically from 105.4 K (pure) to 108.5 K (Na006), whereas the normal state resistivity at 250 K decreased from 0.037 Ucm (pure) to 0.025 Ucm (Na006). On the other hand, the resistivity curves for K substitution showed an exactly opposite trend. Unlike the expectation that the substitution of alkaline ions with þ1 valence state would increase the carrier density in the CuO2 layer, the carrier density seemed to be systematically reduced with K substitution. These behaviors may be explained as a result of different modifications in the space layers by introducing Na and K ions. In the HTS cuprates, strong interlayer coupling facilitates transferring charge carriers from the space layer to the CuO2 layer, and may increase carrier density in the CuO2 layer, manifested by a reduction of the normal state resistivity [15,20]. Therefore, the rðTÞ behaviors may indicate an increase or a decrease of the CuO2 interlayer coupling induced by introducing Na and K ions into the space layer. To investigate a change in the CuO2 interlayer coupling by alkaline substitution, the excess conductivity data of all samples were analyzed according to the LD theory [15]. The inset of Fig. 1 presented a fit of the normal state resistivity from 150 K to 250 K to the Anderson and Zou's linear relation to obtain an extrapolated resistivity, which was used to calculate the excess conductivity. The double logarithmic plots of calculated excess conductivity ðDsÞ as a function of reduced temperature ðεÞ of five representative samples (Pure, Na002, Na006, K002 and K006) were displayed in Fig. 2. By fitting the data with the critical exponent values of the AL model, the reduced temperature can be separated into three regions i.e., short wave fluctuation (SWF), mean field region (MFR) and critical region (CR). In the SWF, the excess conductivity rapidly decreased above Tc with an exponent value of ly3, indicating that short wave fluctuations play a dominant role [29]. In the MFR, the critical exponent decreased to ly1:0, which is characterized by 2D fluctuations, indicating that conductivity in this region mainly occurs from Cooper pairs limited in the CuO2 layer [15,30]. As temperature is further decreased close to Tc, the critical exponent decreases to ly 0:5 (3D fluctuation), where Cooper pairs cross a barrier layer to reach the conducting CuO2 layer [15]. The final region with ly 0:3 is the dynamic CR, where the GL theory breaks down and interaction between Cooper pairs is considered [22,31]. As shown in Fig. 2, the critical exponents of the Ds data in the CR for all samples were found to be ~0.3 in good agreement with the theoretical prediction of dynamic scaling effect [22], which justify the adaption of the LD theory. In addition, the Ds data in the MFR for all samples represented two distinct linear parts with different exponents, indicating that a crossover from 2D to 3D was occurred at TLD. It is noted that the values of 2D-3D crossover temperature TLD and the interlayer coupling J calculated from TLD turned out to depend upon the doping contents and increased systematically for the Na-substituted case, while they decreased for the K-substituted case, as summarized in Table 1. The increase of TLD prolonged the 3D fluctuation region and resulted in an increase of interlayer coupling strength. The Fermi velocity of charge carriers could be deduced from the excess conductivity data in MFR. According to the LD theory, the excess conductivity in 2D is reduced to Ds ¼ Aε1:0 ð2DÞ and the slope b of Ds1 as a function of temperature is given as b ¼ 16Zd= e2 Text , where Text is extrapolated temperature obtained from the
Fig. 2. Double logarithmic plot of excess conductivity Ds as a function of reduced temperature ε for a) Pure, b) Na002, c) Na006, d) K002, and e) K006 samples. The black dot-dashed lines with a slope of ~0.3 correspond to the CR in the LD model. The red and the blue lines are the fits to the 3D and the 2D models in the MFR, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Table 1 Parameters calculated from the excess conductivity analysis of pure, Na-substituted and K-substituted samples. Sample
T mf c (K)
TLD (K)
J
b(K1)
d (Å)
xc ð0Þ(Å)
Vf (106 m/s)
Pure Na002 Na004 Na006 K002 K004 K006
105.4 107.1 107.6 108.5 104.6 103.6 102.9
106.6 108.6 109.3 111.1 105.6 104.5 103.7
0.0028 0.0036 0.0040 0.0061 0.0024 0.0021 0.0019
0.041 0.036 0.032 0.028 0.037 0.040 0.046
65.4 62.8 57.2 49.1 68.7 72.9 88.9
1.59 1.67 1.74 1.76 1.58 1.51 1.46
1.44 1.53 1.61 1.63 1.43 1.33 1.29
slope b. The temperature dependence of Ds1 and an enlarged plot near Text for all samples are shown in Fig. 3 and the inset of Fig. 3, respectively. The values of d and xc ð0Þ determined from the extrapolation of the Ds1 plots revealed an inverse relationship with each other. By varying amounts of Na, the former decreased and the latter increased, while exactly the opposite trend was observed in the K-substituted case, as listed in Table 1. The increase of xc ð0Þ by Na substitution may reduce the anisotropy of the Bi2223 system and leads to an improved superconducting property, consistent with the result of increasing Tc. The Fermi velocity VF of carriers was calculated by using the following expression:
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Fig. 3. The temperature dependence of Ds1 plots for all samples. Inset: The linear fit line determines the value of b.
VF ¼
5pkB Tc xc ð0Þ 2KZ
(3)
where kB is the Boltzman constant, K is a coefficient of proportionality (K y0:12Þ, and Z is the reduced Planck constant. The values of VF were found to increase with increasing Na content, while they were found to decrease with varying amounts of K, as listed in Table 1. The increase or decrease of VF indicates that the efficiency of charge carrier transfer to the CuO2 layer is enhanced or reduced by different substitutions, which may lead to an increase or a decrease of carrier numbers in the CuO2 layer. This result provides a plausible explanation on enhanced/weakened CuO2 interlayer coupling by different dopants, which is known to be sensitive to the local structure in the normal state [32]. To have a direct access to the local structural change, we carried out interactive analysis of EXAFS data measured for Cu K (8979 eV) absorption edge with the IFEFFIT program. The k-weighted Fourier transform functions of the Cu K-edge EXAFS for all samples are shown in Fig. 4 and the radial distribution of neighbors of the Cu atom can be observed by inspecting the peaks of the Fourier transforms. Fig. 4 represents the 1st and the 2nd shells located at a
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distance of ~1.5 Å and ~2.6 Å, respectively, which correspond to the contribution of the CueO and the CueCa/Sr scatterings (coordinated with 4 O and 4 Ca/Sr atoms). Even though all the EXAFS oscillations exhibited similar shapes, a systematic increase/decrease in the amplitude of the CueO bond in the 1st shell with increasing the Na/K contents is clearly visible. This signifies that the ordering of the CueO bond is improved with the Na substitution while it is degraded with the K substitution. In order to quantatively study local atomic displacement, twoshell-model fitting including the contribution of the CueO and the CueCa/Sr bonds were carried out for the Fourier transforms of the Cu K-edge EXAFS oscillations. During the fitting process, only two parameters, the bond distance (Ri), and the Debye-Waller factor (DWF) indicating the mean square relative displacement (for each shell), were allowed to vary and the other parameters were fixed [33]. The fitting k and R ranges were 2.8e11.5 Å1 and 1.0e3.2 Å (for the Cu K-edge) and the fitting result for the pure sample is shown in the inset of Fig. 4 as a representative. The R factor estimated as in Refs. [33,34] for the EXAFS fits was found to be 15% for all the samples, confirming high goodness of fits. Fig. 5 (a)-(d) show the variation of local bond distances and the values of DWF, which provide information on the atomic disorder, as a function of the doping contents. Owing to that the radii of Na1þ and K1þ are slightly different from those of Ca2þ and Sr2þ, the distances of the CueO, the CueCa and the CueSr bonds exhibit small changes as the doping contents vary. The local bond distances and the values of DWF showed a similar behavior depending upon the doping content, infering that the local atomic disorder is correlated with the bond distance. By increasing the Na contents, the CueO bond gradually contracted from 1.906 Å (pure) to 1.897 Å (Na006) with smaller values of DWF, which indicates stabilization of the CuO2 layer. On the other hand, by increasing the K contents, the CueO bond distance monotonically elongated from 1.906 Å (pure) to 1.916 Å (K006) with larger DWF values, which may be due to the reduction of carrier density in the CuO2 layer. These results showed that both alkaline substitutions induced distortions in the CuO2 layer. Unlike the CueO bond, the CueCa bond distance was constant to be ~3.097 Å for whole range of dopants level, indicating that the Ca layer was the least affected by the Na/K substitution. Therefore, this result suggests that a relatively stable Ca layer is a suitable candidate for alkaline substitution with no loss of interlayer interaction. On the other hand, the CueSr bond distance changed differently with the Na/K substitutions; the CueSr bond distances were constant to be ~3.144 Å for the case of Na substitution, while those of the K substitution were increased from 3.144 Å to 3.216 Å. The elongated CueSr bond distance with larger value of DWF by the K substitution may cause a reduced interaction between the SrO and the CuO2 layers, which weakens local superconductivity. As a result, the enhancement/suppression of Tc values by different alkaline substitutions may be explained by the improved/degraded carrier supply from the space layer (SrO and Ca) to the conduction CuO2 layer due to the enhanced/weakend CuO2 interlayer coupling caused by local structural alterations. 5. Conclusion
Fig. 4. Fourier transforms of Cu K-edge EXAFS oscillations weighted by k3 which represent the radial distribution of neighboring atoms around Cu atom. Inset: The solid line represents the fit to the two-shell-model of the EXAFS.
The effects of partial substitution of Na and K at the Ca and Sr sites respectively, on the superconductivity and the interlayer coupling of the Bi1$6Pb0$4Sr2Ca2Cu3O10þd system were investigated. The Na substitution systematically increased the Tc values, while the K substitution lead to opposite results. The excess conductivity analysis by the AL and the LD theories showed that dimesionality crossover from 2D to 3D in the mean field region depends upon what the doping contents are. The systematic increase of the 2D-3D
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Fig. 5. The bond distances and the corresponding Debye-Waller factors for CueO, CueSr, and SreO bonds as a function of dopant content, which were determined by the two-shellmodel fitting to the Cu K-edge EXAFS oscillations.
transition temperature TLD of the Na-substituted samples indicated a gradual expansion of the 3D fluctuation region due to the enhanced CuO2 interlayer coupling. It should be noted, however, that exactly the opposite results were also observed in the Ksubstituted cases, which might be attributed to the weakened CuO2 interlayer coupling. The EXAFS data measured at the Cu K-absorption edge were analyzed to probe alteration on the local structue of the CuO2 layer and the space layers (SrO and Ca). The EXAFS data showed that the CueO bond distance systematically decreased for the Na substitution, while it increased for the K substitution, which implies that distortions in the CuO2 layer occcurs in two different directions depending on the dopants. The CuO2 distortion may lead to deformation only of the SrO layer, not to that of the Ca layer, thereby resulting in either an improved or reduced interlayer interaction of the CuO2 with the Ca or the SrO layers respectively, which accounts for the enhancement or suppression of Tc values by different alkaline substitutions. Acknowledgments This work was supported by the National Research Foundation of Korea grant funded by the Korean government (MSIT) (NRF2018R1A2B6004784). References [1] H. Maeda, Y. Tanaka, M. Fukutumi, T. Asano, Jpn. J. Appl. Phys. 27 (2) (1988) L209eL210. [2] S.E. Mousavi Ghahfarokhi, M. Zargar Shoushtari, Phys. B 405 (2010) 4643. [3] I.H. Gul, M.A. Rehman, M. Ali, A. Maqsood, Phys. C 432 (2005) 71e80. € [4] C. Terzioglu, H. Aydin, O. Ozkurt, E. Bekiroglu, I. Belenli, Phys. B 403 (2008)
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