Specific heat and correlation between resistivity and thermoelectric power of GdBa2(Cu1−xMnx)3O7−δ HTSC system for x ⩽ 0.02
Introduction
The substitutional effects in high Tc superconductors have been extensively investigated by several researchers. Primary aim of such studies has been essentially to explore the mechanism of superconductivity and to improve various properties so as to enhance the applicability of these materials. As far as transition metal ions are concerned, relatively less work seems to have been done on substitution of Mn, primarily due to its small solubility [1], [2], [3], [4], [5], [6], [7]. However, Mn doping is of great physical importance from the theoretical point of view due to some common features between various transition metal ions. Mn doping exhibits an unusual behavior in the sense that the superconducting transition temperature of the YBa2Cu3O7−δ system is seen to only slightly affected by the Mn content. There are several reports on the measurements of the thermal behavior of YBa2Cu3O7−δ with other dopants (Co, Zn, etc.) at Cu-sites. Loram et al. [8] have studied the specific heat of Co-doped YBa2Cu3O7−δ, while Sisson et al. [9] have studied the specific heat of Zn-doped YBa2Cu3O7−δ system. As far as the thermoelectric power of doped Y- or Gd-based 123 systems is concerned, there exist studies with Zn [10], Pr [11], and Ca [12] doping.
In this paper we present theoretical studies on the GdBa2(Cu1−xMnx)3O7−δ superconducting system and highlight some important features of the specific heat (CP) and thermoelectric power (S) [13], [14] with a view to understand the role of Mn in GdBa2(Cu1−xMnx)3O7−δ on the basis of the connections of its effects on the specific heat and thermoelectric power. The Mn+k ion (k = 2, 3, 4) is magnetic. For k > 2, Mn will donate electrons to the overall system, which results in the reduction of the carrier (hole) density. Since the superconducting transition temperature TC is known to decrease with the reduction of carrier (hole) density with respect to the optimal doping case [15], a decrease in Tc with Mn may be considered as an indication for the existence of Mn in valence states higher than +2.
Section snippets
Experimental techniques
Samples of the GdBa2(Cu1−xMnx)3O7−δ system in the nominal doping range 0 ⩽ x ⩽ 0.02 were synthesized by a solid state reaction route. Synthesis details and measurement techniques for thermopower and specific heat have already been described elsewhere [13], [14].
Results and discussion
Superconducting transition temperature TC deduced through resistivity measurements for our GdBa2(Cu1−xMnx)3O7−δ samples have already been reported [13]. Since the cuprate systems are d-wave superconductors [16], paramagnetic impurity is not expected to drastically reduce TC. However we observe drastic reduction of TC for the sample with x = 0.02 which is either due to potential scattering [17] or due to reduction in the carrier (hole) density (caused by Mn+k, k > 2 ions) [15]. In our earlier
Conclusions
We have investigated specific heat and thermoelectric power of the Mn-doped GdBa2Cu3O7−δ superconductors both in the superconducting state and in the normal state. The substitution of Mn in the system has been found to effectively suppress the specific heat jump observed in the pristine compound. Thermoelectric power data analysis suggests that for low Mn content (x ⩽ 0.0075) Mn+k ions cause mainly electronic effects in the system. A crossover takes place at x = 0.0075 so that for higher x (x >
Acknowledgement
The authors are thankful to Prof. S.K. Joshi, NPL, New Delhi for valuable discussions. One of us (BG) is grateful to CSIR for financial help under scheme No.CSIR-80(0056)/05/EMR-II. The experimental work is supported by the National Science Council of Taiwan under Contracts Nos. NSC-94-2112-M-259-012 (YKK).
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Magneto-resistance, thermal conductivity, thermo-electric power and specific heat of superconductor Gd<inf>0.95</inf>Pr<inf>0.05</inf>Ba <inf>2</inf>Cu<inf>2.94</inf>M<inf>0.06</inf>O<inf>7-δ</inf> (M=Fe, Ni, Zn and Mn)
2011, Solid State CommunicationsCitation Excerpt :It is further seen that Pr and Gd undergo magnetic ordering independently although they are present in the same magnetic sub-lattice [27]. We had carried out some work on thermal and electrical properties of Mn substitutions at the Cu site [29–31]. It is observed that HC2(0) for pristine sample of Gd-123 was estimated at 84 T and with 5% Pr-doping, HC2(0) increases to about 95 T.
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