Study on excess magnetoconductivity in (1−x)YBa2Cu3O7 + xBaTiO3 composites

Physica C 483 (2012) 45–50

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Study on excess magnetoconductivity in (1x)YBa2Cu3O7 + xBaTiO3 composites A. Kujur ⇑, D. Behera Department of Physics, National Institute of Technology, Rourkela 769 008, India

a r t i c l e

i n f o

Article history: Received 4 June 2012 Received in revised form 27 June 2012 Accepted 28 June 2012 Available online 17 July 2012 Keywords: Layered compounds Superconductors Composites Electrical properties microstructure

a b s t r a c t (1x)YBa2Cu3O7 + xBaTiO3 composites were synthesized following the standard solid state reaction route. Analysis of the resulting morphological structure confirmed that BaTiO3 resides at the grain boundary of the granular matrix of YBCO. Fluctuation conductivity obtained for zero magnetic field confirms that superconducting region is affected by the BaTiO3 incorporation. The values of the crossover temperature from 2D to 3D behavior are found to shift towards higher temperatures with increasing BaTiO3 concentration. The excess magnetoconductivity measured at 8 T revealed a dimensionality crossover in accordance with the Aslamazov–Larkin theory. The overall analysis of the obtained experimental results suggests manifestation of Gaussian, critical and short wavelength type fluctuations in the behavior of the observed paraconductivity. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction YBa2Cu3O7d (YBCO) is one of the most widely studied superconductors (SC) because of its high transition temperature (Tc) and possibility of large critical current density (Jc). However grain boundaries and the poor flux pinning causes deterioration in current conduction and thus reduces the Jc in layered compounds i.e. high temperature superconductor (HTSC) [1]. There have been several reports on improving the pinning efficiency in YBCO system by chemical doping and introduction of secondary phases [2,3]. Nonsuperconducting inclusions of nanoparticles like BaZrO3, BaTiO3, etc. in YBCO are means of generating artificial pinning centers [4]. The composite YBCO + xBTO result in improvement in the values of Jc and flux pinning (Fp) not only at lower fields but even at higher fields and temperatures [4]. Hence YBCO/xBTO composites are of immense value to be studied. The improvement of the Jc under magnetic fields is strongly desired for producing efficient, low-cost HTSC coated conductors. Vortex pinning technology holds the key for this purpose [3]. The vortex state in HTSC is an interesting research area. Thermal fluctuation effects in HTSC are enhanced by high temperatures, small coherence length and strong anisotropy structure [5]. These fluctuations are not observed for conventional superconductors because their order parameter does not fluctuate until the temperature becomes of order of 106 of transition temperature (Tc). The CuO2 planes are common structural element of HTSC’s responsible for their dimensionality and their anisotropic properties [6,7]. Thermodynamic fluctuations are observed in HTSCs over

a temperature interval above and below the normal-superconductor transition. Excess conductivity or paraconductivity is related to study of fluctuation phenomena in HTSC which have been intensively studied since their discovery in different composites [8– 11]. Different models are proposed to explain the high magnitude of fluctuation effects in these HTSCs. The most discussed models are Aslamazov and Larkin (AL) [12], Maki and Thompson (MT), Lawrence and Doniach model (LD) [13]. Although above Tc electron pairing is not favored because of the high associated energy, there are always pairing fluctuations. Far above Tc, experiments can be interpreted in terms of Gaussian fluctuations of the order parameter [14]. Closer to Tc appears the critical fluctuations indicating 3D XY fluctuations by universality class [15]. The 3D-XY model describes the critical behavior of a neutral superfluid with a two-component scalar order parameter, such as liquid 4He [16]. In this report, attempts have been made to establish the thermal effects on the pairing mechanism. Fluctuation analysis is performed using the logarithm of the excess conductivity identifying the Gaussian and critical fluctuation regimes. Critical exponents of all fluctuation temperature regions are correlated with the respective dimensionality of system by means of the AL model and critical fluctuations are interpreted within the 3D-XY model. Close to the zero resistance state precursor evidences of the coherence transition are detected. Our paper reports precision measurements of the zero-field and 8 T fluctuation conductivity of YBCO/ xBTO composite samples.

2. Experimental details ⇑ Corresponding author. Tel.: +91 6612462724; fax: +91 6612462999. E-mail addresses: [email protected] (A. Kujur), [email protected] (D. Behera). 0921-4534/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.physc.2012.06.011

The bulk YBCO powder was prepared by solid state reaction route. Stoichiometric amount of high-purity powders of Y2O3,

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BaCO3 and CuO precursors were grinded and calcined at 870 °C. A series of polycrystalline composite samples of (1x)YBCO + xBTO (x = 0.0, 1.0, 2.5, 5.0 and 10.0 wt.%) were then pressed into pellets for final sintering at 920 °C for 12 h. The process was followed by oxygen annealing at 500 °C for 12 h to obtain well oxygenated samples. DC electrical resistivity measurement was done by four probe set up with a Nanovoltmeter (Keithley-2182) supplied with constant current source (Keithley-2400). Low temperature was achieved by liquid helium. Data acquisition was carried out with computer controlled program. To generate high magnetic field a superconducting magnet system is used. The surface morphology of the pressed pellets was investigated by scanning electron microscope (Model No. JSM-6480 LV, Make JEOL). 3. Result and discussion 3.1. Morphological outlook Scanning Electron Microscopy (SEM) reveals microstructural changes occurring within the samples. The morphologic images of YBCO pristine and (1x)YBCO + xBTO samples are shown in Fig. 1. The sample exhibits well defined elongated rod like structure randomly oriented in all directions with varying length. With increase in the concentration of BTO microstructural changes are observed. BTO occupies the spaces between the elongated grains which is evident from images (c) and (d) leading to increase in grain connectivity and filling up of cracks and voids. In the compositions of 2.5 and 5 wt.% extra deposition of BTO layers is observed giving supporting evidence that BTO stays sticking to the grain boundaries. 3.2. Electrical properties 3.2.1. Temperature dependent resistivity (at zero magnetic field) Temperature dependent resistivity is shown in Fig. 2 from which we may conclude that (i) Metallicity increases with the increase of concentration of BTO as revealed from the plot. (ii) The room temperature resistance increases to high value i.e. 10 times

Fig. 2. Temperature dependence of the resistivity for YBCO + xBTO composites (x = 0, 1, 2.5, 5 and 10 wt.%). The linear fitting of the resistivity in the temperature range 150–250 K, extrapolated to 0 K gives resistivity slope (dq/dT) and residual resistivity (q0).

the pristine for addition of 10 wt.% of BTO. (iii) The ratio of

q300K/q100K is greater than 2 for pristine and 3 for 1 wt.% BTO addition. For higher concentration of BTO the ratio falls indicating that resistance decreases. (iv) The graph exhibits two different regimes: one corresponding to the normal state behavior (above 2Tc) and follows Anderson and Zou relation qn(T) = A + BT [17]. Linear fitting of resistivity within temperature range 150–250 K gives slope of resistivity (dq/dT = B) and extrapolation to 0 K determines residual resistivity (q0 = A). The other is non ohmic region characterized by non linearity and is due to the contribution of Cooper pairs. (v) The normal state resistivity goes on increasing with addition of BTO to YBCO matrix (see Table 1). For detailed analysis of microscopic parameter the temperature derivative of resistivity is plotted in Fig 3.

Fig. 1. SEM micrographs for YBCO + xBTO composites (x = 0.0, 1.0, 2.5, 5.0 wt.% marked as a, b c, d respectively).

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A. Kujur, D. Behera / Physica C 483 (2012) 45–50 Table 1 Superconducting parameters associated with resistivity for different concentration of BTO wt.% added. BTO (wt.%)

q300 (lohm cm)

dq/dT(lX cm K1)

q0 (lX cm)

q100 (lX cm)

nc (A°)

J

0.0 1.0 2.5 5 10

1209.72 3199.54 4219.89 3733.74 9039.00

3.31 7.78 9.95 8.48 16.04

257.35 924.72 1346.63 1180.35 4209.31

537.77 1062.66 1922.29 2227.24 5644.43

1.70587 1.78774 2.46033 1.7317 1.73228

0.08532 0.09371 0.17748 0.08793 0.08799

Fig. 3. Temperature derivative of resistivity for YBCO + xBTO composites labeled as a, b, c, d, e for different concentration x = 0, 1, 2.5, 5 and 10 wt.%.

Firstly we observe a sharp peak at Tc, which is related to intragranular fluctuation (fluctuations in the amplitude of the order parameter). The temperature value of this maximum is close to the bulk critical temperature value Tc called Tc mean field (Tcmf) or pairing transition [18]. The sharp peak is followed by a hump or secondary peak which broadens for composite samples. Tc is not severely damaged by the addition of non superconducting inclusion BTO as observed from graph. At the same time, ferroelectric BTO is affecting the intergranular percolation of cooper pairs in YBCO matrix. It means that the samples consist of two phases i.e. superconducting phase + ferroelectric phase. The transition width (DT) increases with doping concentration. This can be related to the morphological fact that BTO resides between the grains and increases grain boundary resistance. BTO adds to both intra as well as intergranular modifications. However Tc0 is significantly reduced as the grains are packed by non superconducting BTO. Tc0 is defined as onset of global superconductivity in the sample. 3.2.2. Method of analysis Excess-conductivity (Dr) above Tc is generated by the thermodynamic fluctuations is given by AL model as [11]

Dr ¼ Aek

Fig. 4. Log–log plot of excess conductivity as a function of reduced temperature e for YBCO + xBTO composites in zero magnetic field.

exponent linked to the dimension (D) of the system in which the fluctuations in high-Tc materials occur. The exponent value is related to D through the expression

k ¼ 2  D=2

ð3Þ

The k value was found to be limited in the interval from 0.5 to 1.0 [14]. For 3D and 2D k values are 0.5, 1 respectively. Dimensional crossover takes place between 2D and 3D above Tc [19]. The linear fits of 2D and 3D intersect at a point called TLD which is defined as crossover temperature (TLD). These relations are based on GL theory and are valid only for mean field temperature region (1.01–1.1TC). Lawrence and Doniach (LD) [12] extended the AL model for layered superconductors, where conduction occurs mainly in 2D CuO2 planes and these planes are coupled by Josephson tunneling. According to LD model TLD is given by TLD = TC(1 + J2) where interlayer coupling strength (J) is expressed as J = 2nc(0)/d. The excess conductivity parallel to layers in the LD model is given by Eq. (4)

DrðTÞLD ¼ e2 =16hdef1 þ ðJÞ2 e1 g1=2

ð4Þ

3.3. Excess conductivity studies

ð1Þ

ð2Þ

Fig. 4 shows excess conductivity plot of Dr as function of e. Three distinct regimes are observed in the inset: critical fluctuations, mean-field region or the Gaussian fluctuations and short wave fluctuation region.

Here qm and rm is the measured resistivity and conductivity. qn is the extrapolated normal state resistivity and rn(T) is the conductivity. e is defined as the reduced temperature given by e = (TTc)/ Tc. A is a temperature dependent parameter related to dimensionality of the fluctuating charge carriers. A = e2/32 hn(0) for 3D and e2/ 16 hd for 2D, where n(0) is the zero-temperature coherence length, d is the separation of CuO2 layers. k is the Gaussian critical

3.3.1. Gaussian fluctuations In the mean-field region we represent two linear fits, one with slope value 1.0 and the other with value 0.5. The whole plot is dominated by 3D and 2D regime. The first exponent value is close to 1 and lies in the normal region at log e (0.7 P log e P 1.1) in the pristine sample. This indicates that the order parameter

where Dr is defined by

Dr ¼ ð1=qm  1=qn Þ ¼ rm  rn

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Table 2 Various temperatures (zero resistance, mean field, Ginzburg, Lawrence–Doniach and 2D SWF crossover temperature) in zero magnetic field. BTO (wt.%)

Tc0 (K)

Tc (K)

TG (K)

TLD (K)

T2D-SW (K)

0.0 1.0 2.5 5.0 10.0

90.17 85.21 85.72 87.47 84.66

92.12 91.56 91.05 91.44 89.56

94.32 93.54 93.96 94.76 92.02

99.98 100.14 107.21 99.48 97.44

109.169 107.459 118.563 110.851 103.143

3.4. Magnetoconductivity According to the AL model, the fluctuation magnetoconductivity diverges as a power law of the type:

DrðT; BÞ ¼ Aek

dimensionalities (OPD) are 2D. The second exponent lies within a range of log e (1.1 P log e P 2) and its values are close to 0.5, which signifies that the OPD are 3D. TLD (dimensionality crossover occurs from 3D to 2D) values are higher than the Tc values as observed from the Table 2. It reveals that the activated Cooper pairs produced within the grains are formed at comparatively higher temperatures but disturbances caused in the intragranular region lowers Tc value. It is possible to infer that this 3D Gaussian regime, determines the spatial limit for the obtainment of long range order of the superconductivity in the bulk material. When temperature is reduced first superconductivity is established in the CuO2 planes, as a 2D regime, and crosses up to a well-defined 3D regime. However with the increase of BTO concentration up to 2.5 wt.% TLD values increases to 107.21 K and then TLD decreases for higher concentration. The 2D regime increases with BTO concentration up to 5 wt.% i.e. confinement of cooper pairs in planes increases. The 3D regime also increases for BTO concentration up to 2.5 wt.%. The cross over temperature TLD yields microscopic parameter called c axis coherence length (nc) and interlayer coupling strength (J) shown in Table 1. nc increases with addition of BTO concentration up to 2.5 wt.% and then decreases. 3.3.2. Critical fluctuations Closer to Tc, a critical region is observed in Fig. 4. This regime, which is labeled as kcr is described by the 3D-XY model, and corresponds to genuine critical fluctuations. The critical exponent for this type of conductivity fluctuation is given by [15,20]

kcr ¼ mð2 þ Z  D  lÞ

fluctuation effects may add to the vortex motion contribution at high fields, we have analyzed the data for 8 T.

ð5Þ

where ‘m’ is the critical exponent for the coherence length, ‘z’ is the dynamical exponent, ‘D’ is the dimensionality, and ‘l’ is exponent for the order-parameter correlation function. According to renormalization group calculations, m = 0.67 and l = 0.03 are expected and z = 0.32 being predicted by the theory of dynamical critical scaling [21]. Using these values with D = 3 yields kcr = 0.33 which is called the 3D XY-E because of the model-E dynamics [22]. The critical fluctuation and 3D fluctuation regions intersect at the so-called Ginzburg temperature TG. Still closer to the so-called Ginzburg temperature TG, a critical scaling regime beyond 3D-XY is observed, labeled by the exponent kcr = 0.16. This result is similar to that obtained by many authors [23–25]. The regime beyond 3D XY with kcr = 0.17 was first observed in YBCO single crystal [26]. This exponent is known to characterize the critical resistive transition in classical granular arrays formed by metallic superconducting particles embedded in a poorly conducting matrix. 3.3.3. Short wavelength fluctuations The GL theory breaks down for higher values of e, because e no longer remains a small parameter and short wavelength fluctuations start to play a dominant role [27]. The excess conductivity varies sharply as e3 indicating the presence of short wave fluctuations. T2D-SW is the crossover temperature from 2D to SWF as indicated in Table 2. Short-wavelength fluctuations (SWF) effects appear when the characteristic wavelength of the order parameter becomes of the order of coherence length. Assuming that the

ð6Þ

where A is a constant, e = (TTc(B))/Tc(B) represent the reduced temperature and is field dependent and k is the critical exponent. Determination of the measured conductivity and the normal state conductivity was done in similar manner by measuring resistivity as a function of temperature under the application of 8 Tesla (8 T). The resistivity shows remarkable broadening in the presence of magnetic field when compared with zero field resistivity that shows a sudden drop to zero shown in Fig. 5. Fig. 6 shows temperature derivative of resistivity vs temperature plot measured at 8 T. This shows an increase of transition width. Magnetic field induced broadening was first shown quantitatively by Tinkham [28]. This feature has been explained successfully by several authors [29,30] based on the Ambegaokar–Halperin (AH) phase-slip theory. The broadening of the resistive transition is well analyzed through thermally assisted flux motion theory by Liu [31]. With increasing magnetic field, the mean field transition temperature Tc is not affected much; rather Tc0(R = 0) varies significantly moving towards lower temperature side. Broadening of peak is evident indicating transition width is increasing. A secondary peak is marked for 10 wt.% of BTO composite for which Tc0 is 48 K, much below the transition temperature. Tc and Tc0 are given in Table 3. Fig. 7 shows excess conductivity plot Dr as a function of reduced temperature (e) at 8 T field. Close to Tc the conductivity fluctuation analysis reveals the occurrence of two fluctuation regimes characterized by the critical exponents k1 (3D)  0.50 and k2 (2D)  1. This is attributed to two and three-dimensional Gaussian fluctuations respectively. The plot is dominated by k2 exponents giving a clue that superconductivity is confined to 2D planes due to field effect for wide range of temperature. Then the superconducting nature spreads to 3D on further lowering the temperature. Mohanta group highlights the presence of 2D–3D regime at 8 T magnetic field in YBCO/BaZrO3 composites [32]. The TLD shifts to lower temperature side and lies below the transition temperature. It has strikingly different behavior from zero field. It has been

Fig. 5. Resistivity versus temperature plots of YBCO + xBTO (x = 0, 1, 2.5, 5 and 10 wt.%) composite samples at 8 T field.

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field application and below Tc data is interpreted in terms of Josephson effects [37–39]. 3.5. The approach to zero-resistance state

Fig. 6. Temperature derivative of resistivity for YBCO + xBTO composites (x = 0, 1, 2.5, 5 and 10 wt.%) in 8 T magnetic field.

Table 3 Various temperatures (zero resistance, mean field, Ginzburg, Lawrence–Doniach and 2D SWF crossover temperature) at 8 T field application. BTO (wt.%)

Tc0 (K)

Tc (K)

TLD (K)

nc (A°)

J

0 1.0 2.5 5.0 10

71.26 48.33 49.33 56.31 48.08

89.06 85.52 82.71 86.19 86.10

92.19 89.42 86.50 90.28 88.56

1.09482 1.24713 1.25013 1.27217 0.98714

0.03514 0.0456 0.04582 0.04745 0.02857

At Tc0 the electrical resistivity vanishes and the phase of the order parameter acquires long range order between the grains of the system. This critical temperature characterizes the phase-locking coherent transition. The onset of global resistivity decreases with addition of BTO indicating that BTO adheres to grain boundary forming weak links. The finite tailing is observed in the superconducting transition for all the YBCO + xBTO composites before the resistance attains zero value. Finite tailing suggests that superconducting grains are getting progressively coupled to each other via weak links by Josephson tunneling across the grain boundary. The zero-resistance Tc0, characterizes the onset of global superconductivity in the samples where the long range superconducting order is achieved. It is thus suggested that the BTO, probably acts as a weak link, which causes the global resistivity transition temperature to decrease with addition of extra BTO. 4. Conclusion In this work, the effects of non superconducting ferroelectric material BTO on conductivity fluctuation have been studied. Zero field analysis revealed the occurrence of two fluctuation regimes close to and above Tc characterized by the critical exponents k1 = 0.5 and k2 = 1, respectively. These regions were interpreted as corresponding to 3D and 2D Gaussian regimes, respectively. Critical fluctuation region with kcr = 0.16 was identified closer to Tc. Near the zero resistance state, the two-stage nature of the superconducting transition was evident due to the occurrence of a shoulder below the maximum in the temperature derivative of resistivity. We have interpreted this behavior as an evidence of precursor effects on the coherence transition, which occurs for the zero-resistance critical temperature. Varying the magnetic field up to 8 T marked the broadening of transition in the resistivity plot. Excess conductivity plot at 8 T identified the presence of 2D–3D Gaussian regimes followed by critical regime and short wavelength fluctuation regime occurring on the higher temperature side. The decrease of onset of zero global resistivity in presence of field indicated the dissipation caused by flux motion hence Tc0 was drastically affected. Acknowledgements We acknowledge Dr. Rajiv Rawat of UGC-DAE CSR, Indore for carrying out Magnetoresistivity measurements. We are grateful to Mr. S.N. Dash for support in data collection.

Fig. 7. Log–log plot of excess conductivity as a function of reduced temperature e = (TTc)/Tc for YBCO + xBTO composites (x = 0, 1, 2.5, 5 and 10 wt.%) in 8 T magnetic field.

described by Jurelo et al. [24] that 3D-XY scaling behavior is unaffected by the fields up to 50 mT. However in the present study we have applied a field beyond 50 mT. We observe critical region beyond 3D with kcr = 0.16 beside this we also observe the short wavelength fluctuation region. The various crossover temperatures are presented in Table 3. Within the framework of superconducting glass model, Sergeenkov predicts a crossover between 2D and 3D in presence of field [33,34]. A higher order of critical exponent kcr  3 has been observed below Tc in Bi2212 samples which is accounted to granularity of the HTSC system [35,36] and is absent in our represented data. 2D Gaussian fluctuation above Tc in Bi2212 samples is shown in thermo electrical power studies with

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