Thermoelectric power in Bi2Sr2−xKxCaCu2Oy
Introduction
The bismuth based (Bi-2212) high temperature superconducting cuprates (HTSC) are currently useful for technological applications [1]. By realizing its potential importance the basic characterization is extensively studied by our group on alkali-doped samples in Bi-2212 system [2], [3], [4], [5], [6], [7], [8]. From our earlier reports, we have demonstrated that alkali dopants have significantly improved the amount of superconducting phases and it is also claimed that the Tc of the sample varies sensitively with the carrier concentration [6], [7], [8], [9].
Among the transport properties of high Tc superconducting oxides, intense research efforts have been devoted to understand the thermoelectric power of these materials [10], [11], [12], [13], [14], [15]. The thermoelectric power is a highly sensitive property to understand the nature of the carriers and the mechanism of transport. It also provides information about the scattering mechanism, band widths and band gaps governing the transport properties of the materials [10], [15]. Thermoelectric power (S) measurements are useful to test whether the charge carriers or holes or electrons in a given system. By studying the normal state transport mechanism, we could easily obtain the important information like altering the dopant of a particular compound and also we can estimate whether the particular compound is overdoped or underdoped. Their relationship with doping and temperatures show some common features. It is possible to use room temperature TEP as a probe to determine the doping state of any HTSC material [12].
Cooper et al. [12] emphasized that the TEP data of Bi-2212 and Tl-2201 cuprates show a parabolic variation of Tc with hole concentration p, there being a minimum and a maximum hole concentration for the occurrence of superconductivity. They reported TEP as a function of ‘p’ for some of the oxides and observed the following trends [12].
Close similarities have been observed in the TEP of several compounds. (a) The sign of the thermopower shows a change in its sign at room temperature and also near . (b) Continuity in the TEP when doping across the two superconducting–nonsuperconducting boundaries. (c) A universal correspondence of the room temperature TEP with ‘p’ (hole concentration) over the whole range of doping.
In the normal state, well above the superconducting transition, the Bi-based superconductors exhibit a thermopower that varies linearly with temperature with a negative slope. This feature is evident in the Tl- and Bi-based superconductors and does not seem to be dependent on the number (n) of CuO2 layers per unit cell [13]. The absolute magnitude of TEP tends to undergo a temperature-independent shift depending on the carrier concentration in the CuO2 layer. This was explained in Pb-substituted Bi-2212, Bi-2201 and Tl-1201 systems [10], [14]. In these systems, the hole concentration can be altered over a wide range by changing the dopant concentrations. The Tc has its maximum value at an optimal doping and decreases as the carrier concentration is reduced (the underdoped regime) or shows an increase (the overdoped regime) and obviously the normal state transport studies will provide clues for probing charge carrier concentration.
Earlier authors [15] have claimed that the effect of substituting alkali metal dopants at the Ca and Sr sites has a strong tendency to decompose the superconducting phase, due to this the volume fraction of the high Tc phase would decrease and the transition temperature of low Tc phase increases by 8–10 K. Kanai et al., claimed that all the doped samples were multiphase and some unknown peaks were present in their XRD spectrum [15]. Based on the above results we have systematically undertaken the TEP studies on potassium doped Bi-2212 system and especially the results on TEP are presented and such systematic studies hitherto are not reported in antiquity.
Section snippets
Experimental
Samples with the nominal composition Bi2Sr2−xKxCaCu2Oy x = 0, 0.1, 0.2, 0.3, 0.4 (labelled as P0, K1, K2, K3, K4) were prepared by solid-state reaction by taking constituent carbonates and oxides of high purity (99.999%). The pre-sintered powders were made into rectangular shape bars of approximate 1 cm in length and 1–2 mm thickness. Final sintering was carried out at 850 °C for 50 h and quenched directly to liquid nitrogen temperature. Room temperature X-ray diffraction data were collected on the
Results and discussion
All the samples were found to be single phase Bi-2212, The XRD, SEM, Tc(0) and magnetic measurements by SQUID were also undertaken and the results were reported elsewhere [17]. Neutron diffraction measurements were also carried out in order to see the structure, possible site occupancy and the results were reported elsewhere [17]. However, we are reporting the refined X-ray lattice parameters using Fullprof programme and these values are given in Table 1. The thermopower was measured as a
Acknowledgments
The authors thank Inter University Consortium for DAE facilities, Indore-India, for permitting to use TEP facilities and Dr. V. Ganesan for technical help. Chandrasekhar is also thankful to Prof. J.B. Goodenough, Superconductivity laboratories of Texas at Austin, USA for useful discussions.
References (37)
- et al.
Physica C
(1992) - et al.
Physica C
(2000) - et al.
Physica C
(1992) - et al.
Physica C
(1991) - et al.
Physica C
(1991) - et al.
Physica C
(1990) - et al.
Phys. Rev. B
(1987) - et al.
Solid State Commun.
(1990) - et al.
Phys. Rev. B
(1992) - et al.
Nature (London)
(1988)
Crystal Res. Tech.
Bull. Mater. Sci.
Mater. Sci. Eng. B
Supercond. Sci. Tech.
Supercond. Sci. Tech.
Phys. Rev. B
Phys. Rev. B
Phys. Rev. B
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