suppression of superconductivity and normal state electrical transport in y1−xprxbasrcu3o7−δ...

phys. stat. sol. (b) 241, No. 4, 895–901 (2004) / DOI 10.1002/pssb.200301975

© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Suppression of superconductivity and normal state electrical transport in Y1–xPrxBaSrCu3O7–δ and Y1–xPrxBa2Cu3O7–δ systems

Anurag Gupta*, 1, A. Sedky2, and A. V. Narlikar3

1 National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi-110012, India 2 Physics Department, Faculty of Science, Assiut University, Assiut, Egypt 3 IUC for DAE Facilities, University Campus, Khandwa Road, Indore-452 001, India

Received by Peter Deák 1 June 2003, revised 3 November 2003, accepted 6 November 2003 Published online 19 February 2004

PACS 74.25.Fy, 74.62.Dh, 74.72.Bk

The structure, normal state electrical transport and superconducting critical temperatures (Tc) of Y1–xPrxBaSrCu3O7–δ (i.e., Y(Pr)-1113) and Y1–xPrxBa2Cu3O7–δ (i.e., Y(Pr)-123) systems, with 0.00 ≤ x ≤ 1.00 and 6.93 ≤ 7 – δ ≤ 6.95, have been investigated. Residual resistivity (ρ0) and resistivity slope ((dρ/dT)cc) corresponding to the linear ρ-T region are determined from the normal state resistivity measurements. It is found that an increase in ρ0 and (dρ/dT)cc correlates with a decrease and enhancement of Tc(x), respec-tively. Interestingly, in both the systems, the destruction of superconductivity seems to occur at the same value of x where (dρ/dT)cc tends to zero. The observed correlations suggest a possible mechanism of su-perconductivity in these systems, which is discussed.


1 Introduction

Among the RBa2Cu3O7–δ (i.e. R-123, R = rare earth) high temperature superconductors (HTSC), Pr-123 has been an exception and intriguing both by its usual non-superconducting behaviour [1, 2] and some reports demonstrating superconductivity (Meissner fraction ≈7%) [3, 4] in it. The latter was shown to be highly dependent on preparation conditions, where, even for the same batch of crystals the superconduct-ing critical temperature (Tc) and metallicity in the normal state varied a lot [4]. In the case of sintered polycrystalline samples there is just one report [5] of possible indication of superconductivity in Pr-123 when the paramagnetic background is subtracted from the magnetization data. However, otherwise all the polycrystalline samples and flux grown crystals of R1–xPrxBa2Cu3O7–δ (i.e. R(Pr)-123) show a mono-tonic decrease in Tc with increasing Pr, when at a critical concentration (xc) of Pr the Tc goes to zero. For Y(Pr)-123, the reported value of xc ≈ 0.55 [1, 2]. The value of xc has been found to increase with a de-crease in ionic radii of cation R [6, 7]. It was also shown that partial substitution of Ba2+ by a smaller cation Sr2+/Ca2+ in R1–xPrxBa(Sr, Ca)Cu3O7–δ (i.e. R(Pr)-1113) increases xc [8, 9]. However, depending on the sample preparation, the understanding of destruction and observation of superconductivity in R(Pr)-123 both stay elusive. Although, several possible mechanisms have been invoked to understand the former: (1) depletion [10]/localization [11] of holes due to the presence of Pr4+ at R3+ sites [12]; (2) the presence of Pr3+ at Ba2+ [13, 3] anti-sites; and (3) magnetic pair breaking by the presence of Pr3+ moments [14, 15].

* Corresponding author: e-mail: [email protected]

896 A. Gupta et al.: Suppression of superconductivity and normal state electrical transport


In the present work we address the question of Pr substitution by studying the differences in structural details, and normal and superconducting state properties of Y(Pr)-123 and Y(Pr)-1113 systems. It was previously shown by some of us [16] that Pr substitution in R-123 may lead to creation of oxygen vacan-cies in CuO2 planes, which may cause the destruction of superconductivity. Besides, in other substituted R-123 systems, we had reported [17] a correlation between Tc and normal state resistivity. Thus it would be interesting to investigate Y(Pr)-1113 and Y(Pr)-123 systems along similar lines.

2 Experimental details

Polycrystalline samples of the series Y(Pr)-123 and Y(Pr)-1113, with x ranging from 0 to 1, were syn-thesised through a solid state reaction method. The ingredients Y2O3, Pr6O11, BaCO3 or (BaCO3 + SrCO3) and CuO of 4 N purity were thoroughly mixed in required proportions and calcined at 910 °C in air for a period of 24 hours. This exercise was repeated three times with intermediate grinding at each stage. The resulting powders were ground, mixed and pelletized. The pellets were annealed in flowing oxygen at 960 °C for a period of 24 hours, followed by furnace cooling to room temperature with an intervening annealing for 24 hours at 600 °C. The samples were then characterized for their phase purity and lattice parameters by X-ray diffraction. The resistivity measurements were obtained using standard four-probe technique. The oxygen content in all the samples was determined by iodometric titrations.

3 Experimental results

Figure 1 shows X-ray patterns of both Y(Pr)-1113 and Y(Pr)-123 samples with varying x. All the sam-ples showed a single phase nature, except for two samples of Y(Pr)-1113 series with x = 0.75 and 1.0, which show some low intensity peaks probably belonging to an impurity phase. Table 1 shows the varia-tion of lattice parameters a, b and c, and orthorhombic distortion for both the systems. The c-parameter increases with Pr in both the systems, indicating the substitution of larger Pr3+ ion in the place of smaller Y3+ ion. Also, the c parameter of Y(Pr)-1113 samples is smaller than Sr-free Y(Pr)-123 samples, in agreement with the reported data [18–21]. This substantiates that smaller Sr (1.32 Å) ion substitutes the bigger Ba (1.52 Å) ion with the same 10-fold coordination in Y(Pr)-1113. However, as a function of increase in x (see Table 1), the orthorhombic distortion decreases and goes to zero at x ≈ 0.6 in the case of Y(Pr)-1113, whereas, the same is found to be affected much less in the case of Y(Pr)-123. The evolution of the crystallographic splitting (see Figs. 1a and 1b) of [020], [200] and [123], [213] supports the change in the orthorhombic distortion for both the systems. As seen from the splitting, the samples of Y(Pr)-1113 with x = 0.0 and 0.25 are clearly orthorhombic. For higher val-ues of x the splitting of both the doublets is reduced, before vanishing completely for the samples with x ≥ 0.6 revealing a tertragonal structure. Whereas, in the case of Y(Pr)-123 the splitting of both [020], [200] and [123], [213] is maintained, revealing an orthorhombic structure for all the samples with x = 0 to 1. The measured oxygen content by iodometeric titrations show no significant change with increasing Pr for both Y(Pr)-1113 and Y(Pr)-123 series (see Table 1). This observation is in agreement with previ-ous reports [1, 8, 9]. Figures 2a–2d show the measured resistivity as a function of temperature for all the samples of both Y(Pr)-1113 and Y(Pr)-123. As is evident from the figure, the normal state resistively increases with Pr in both systems. The ρ(T) behaviour turns from metallic to semiconductor- like for values of x ≥ 0.70 for Y(Pr)-1113 and x ≥ 0.6 for Y(Pr)-123. All samples of Y(Pr)-1113 (Y(Pr)-123) with x < 0.6 (x < 0.7) show a transition to superconducting state. Figure 3 shows the variation of reduced critical temperature (Tc(x)/Tc(0)) with Pr concentration for both the systems. For comparison, similar data from literature of polycrystalline Y(Pr)-1113 [9], Y(Pr)-123 [7] and single crystal Y(Pr)-123 [22] are also plotted in Fig. 3. Slightly lower values of Tc(ρ = 0) for single crystals are mainly due to tail-like broadening of ρ(T) when ρ goes to zero in them [22]. The values of xc, to quench superconductivity is ∼0.7 for Y(Pr)-1113 system and ∼0.55 for Y(Pr)-123 system.

phys. stat. sol. (b) 241, No. 4 (2004) / www.pss-b.com 897


Table 1 Lattice parameters (in Å) a, b and c, orthorhombic distortion (OD = [(b – a)/b] × 100) and total oxygen content (7 – δ ). The lattice parameters are rounded off at last decimal place and maximum error in δ is ±0.02.

Y1–xPrxBaSrCu3O7–δ Y1–xPrxBa2Cu3O7–δ x a b c OD 7 – δ a b c OD 7 – δ 0 3.784 3.842 11.523 1.59 6.94 3.825 3.886 11.667 1.57 6.94 0.1 3.790 3.841 11.524 1.33 6.94 3.832 3.893 11.672 1.57 6.94 0.2 3.805 3.840 11.530 0.91 6.93 3.836 3.898 11.682 1.60 6.94 0.25 3.812 3.839 11.535 0.70 6.93 – – – – – 0.3 3.818 3.838 11.540 0.52 6.93 3.838 3.900 11.687 1.59 6.94 0.4 3.821 3.837 11.549 0.42 6.93 – – – – 6.93 0.5 3.826 3.835 11.552 0.24 6.93 3.842 3.902 11.698 1.54 6.93 0.6 3.831 3.834 11.563 0.08 6.92 – – – – 6.93 0.75 3.834 3.834 11.573 0 6.92 3.844 3.900 11.705 1.44 6.93 1.0 3.841 3.841 11.584 0 6.92 3.843 3.904 11.711 1.56 6.92

Fig. 1 X-ray diffraction patterns with various x values for a) Y1–xPrxBaSrCu3O7– δ

; b) Y1–xPrxBa2Cu3O7– δ

.



Figs. 2 a–d) Resistivity as a function of temperature with various x values for Y1–xPrxBaSrCu3O7–δ and Y1–xPrxBa2Cu3O7–δ .

4 Discussion

In general the ρ(T) curves involve a high temperature (T > Tc, see Figs. 2a–2d) linear region for all the samples of both Y(Pr)-1113 and Y(Pr)-123 series, except for values of x in the vicinity of xc. The linear part of the ρ(T) curves has a positive slope (dρ /dT)cc and its extrapolation to T = 0 K provides the resid-ual resistivity, say ρ0. While the ρ0 is connected with impurity scattering, the slope (dρ /dT)cc determined from the linear region of the ρ-T curve is connected with carrier-carrier scattering. In the following we shall consider only such a resistivity slope. To appreciate the changes in ρ0 and (dρ/dT)cc as a function of x, in Figs. 4a and 4b we plot these quan-tities for both Y(Pr)-1113 and Y(Pr)-123. For comparison we plot similar data determined for single crystals of Y(Pr)-123 from the literature [22]. The data of ρ0 and (dρ/dT)cc for single crystals was normal-ized by factors 0.1 and 0.45. Normalization factors less than 1 are actually expected as a result of effec-tively smaller cross section area and longer length for electrical transport in polycrystalline samples as compared to the crystals. As seen from the Fig. 4a, ρ0(x) increases monotonically for both the systems. Since, simultaneously, the Tc(x) shows a decrease for both the series (see Fig. 3), it may be concluded that Tc does inversely correlate with the ρ0 in these systems. However, we note that although the values of ρ0(x) are higher and increase faster for the Y(Pr)-1113 system as compared to Y(Pr)-123 system, the decrease of Tc(x) is faster in the latter case. This indicates that there are other parameter(s) influencing Tc(x) in these systems.



We now discuss the behaviour of resistivity slope (dρ/dT)cc for both Y(Pr)-1113 and Y(Pr)-123. From Fig. 4b we see that an increase of Pr content in both the series results in a non-monotonic behaviour. In case of Y(Pr)-1113, the slope (dρ/dT)cc increases until x ∼ 0.4 and tends to zero at x ∼ 0.7 (note that (dρ/dT)cc becomes negative for x = 0.75, see Fig. 2c). Whereas, in case of Y(Pr)-123, it increases only until x ∼ 0.2 and goes to zero at x ∼ 0.5. Now we would like to point out a couple of observations: a) for all the values of x shown in Fig. 4b, the values of slope (dρ/dT)cc are remarkably higher in case of Y(Pr)-1113 as compared to Y(Pr)-123. For instance, at x = 0.4, (dρ/dT)cc is around factor five higher in Y(Pr)-1113 than that in Y(Pr)-123. b) The values of x ∼ 0.7 and 0.5, where (dρ/dT)cc goes to zero, match quite well with the observed xc ∼ 0.7 and 0.55 where superconductivity is destroyed for Y(Pr)-1113 and Y(Pr)-123, respectively. These observations, along with the fact that the suppression of Tc(x) is slower in Y(Pr)-1113 when compared with Y(Pr)-123, reveal that an increase of slope (dρ/dT)cc correlates with an enhancement of Tc(x) in these systems.

Fig. 3 Reduced critical temperature [Tc(x)/Tc(0)] as a function of Pr concentration x for Y1–xPrxBaSrCu3O7–δ and Y1–xPrxBa2Cu3O7–δ. Similar data on both the systems taken from the literature is also included in the figure.

Fig. 4 a) Residual resistivity ρ0 and b) slope (dρ/dT)cc as a function of Pr concentration x for Y1–xPrxBaSrCu3O7–δ and Y1–xPrxBa2Cu3O7–δ. Similar data on single crystals of the latter system taken from the literature is also included in the figure.



The above correlations indicate that underlying processes responsible for a change of ρ0 and (dρ/dT)cc are affecting the superconductivity in opposite ways in both Y(Pr)-1113 and Y(Pr)-123 systems. Now we discuss these processes and how they throw light on a possible mechanism of superconductivity. The increase of ρ0(x) clearly indicates an increase of temperature independent impurity scattering introduced by substitution of Pr in both Y(Pr)-1113 and Y(Pr)-123. We had earlier reported that Y(Pr)-123 is vul-nerable to creation of oxygen vacancies, in CuO2 planes [16], that increase with Pr concentration and constitute an additional scattering source. It has been recently [23–26] suggested that pinning of the dynamically fluctuating striped phase may lead to suppression of superconductivity. The impuri-ties/vacancies in the lattice tend to pin the dynamically fluctuating striped phase and can explain the observed correlation between Tc and ρ0, where ρ0 would signify the extent of pinning. Now turning to-wards (dρ/dT)cc, we have to remember that it has been defined only for the observed linear ρ-T region. A linear ρ-T dependence especially in HTSC due to classical electron–phonon scattering can be ruled out, and the physical reason for it may lie only in carrier-carrier scattering. Thus the increase of (dρ/dT)cc implies mainly an increase in hole carrier concentration and/or an increase in carrier-carrier scattering rate. There are no reports till date showing an increase of hole carriers with Pr substitution in these sys-tems. The increase of (dρ/dT)cc thus primarily implies an increase in carrier-carrier scattering rate. Now, unlike impurity/vacancy that is fixed in the sample, the carrier is mobile and will tend to depin the striped phase. An increase in carrier-carrier scattering would mean enhanced depinning of the striped phase. In this way, suppression of Tc due to pinning of the striped phase appears consistent with the ob-served correlation of Tc with (dρ/dT)cc also. We would also mention here that, for samples with x > xc of both Y(Pr)-1113 and Y(Pr)-123, the observed semiconductor-like upturn of the ρ-T curves at low tem-peratures is also in accord with pinned striped phase [27–29].

5 Conclusion

In summary, the gradual destruction of superconductivity with increasing Pr, in both Y(Pr)-1113 and Y(Pr)-123 systems, seems to show up in the change of normal state parameters ρ0 and (dρ/dT)cc, which can be extracted from the ρ(T) curves. In general, it is found that increase in ρ 0 correlates with a suppres-sion in Tc , while an increase in (dρ /dT)cc correlates with an enhancement of Tc. The difference in the values of critical Pr concentration for destruction of superconductivity in Y(Pr)-1113 and Y(Pr)-123 coincides with the values of x where (dρ/dT)cc tends to zero. The origin of superconductivity in these systems looks consistent with a presence of dynamically fluctuating striped phase, the pinning and de-pinning of which can explain the observed correlation of Tc with ρ0 and (dρ/dT)cc, respectively.

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suppression of superconductivity and normal state electrical transport in y1−xprxbasrcu3o7−δ...

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