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RESISTANCE ANOMALY IN THE SUPERCONDUCTING FLUCTUATION REGION IN Bi2SrzCaCuzOs SINGLE CRYSTALS

S.H. Han,“ Y. Zhao,” H. Zhang,b G.D. Gu,~ G.J. Russell” and N. Koshizukad

‘Superconductivity Research Group, School of Materials Science and Engineering University of New South Wales, Sydney, NSW 2052, Australia

department of Physics, Peking University, Beijing 100871, P.R. China ‘AEM Group, School of Physics, University of New South Wales, Sydney 2052 NSW, Australia

dSuperconductivity Research Laboratory, ISTEC, lo-13 Shinonome l-chrome, Koto-ku, Tokyo 135, Japan

(Received and accepted 20 November 1996 by Z.Z. Gan)

A resistivity anomaly near the superconducting transition temperature is observed in the ab-plane of as-grown Bi&#aCu20s (Bi-2 2 12) single crystals. The resistivity at the peak is much larger than the normal state resistivity around 100 K. The peak vanished gradually with increasing the applied magnetic field. I-V curves of the Bi-2 2 1 2 samples also exhibit a peak close to the transition steps, vanishing gradually with increasing intrinsic granularity of high-rc superconductors. 0 1997 Elsevier Science Ltd. All rights reserved

1. INTRODUCTION

Superconductivity in granular and disordered systems has attracted a lot of attention during the last years in low-temperature superconductors [l]. In the case of high-temperature superconductors (HTS) the granularity behaviour has been strongly enhanced, mainly due to a very short superconducting coherence-length which is of the order of the interatomic distances. Granularity has been observed in the single crystals of HTS systems, such as LZ_XCe,Cu04-~ (L = Pr, Nd, Sm) [2], YBa2Cu107-,, (YBCO) [3], etc. and it has been argued that intrinsic granularity can be at the origin of the controversy between Lorentz-force vs non-Lorentz-force-,based dissipation mechanism in some HTS [4].

A typical behaviour of the granularity is the resi;stivity anomalies close to the superconducting transition. This is due to the single particle hmnelling across the resistive barriers between superconducting islands and the charging effects below T,, which increases the resistivity above its extrapolated normal-state value. A sharp peak near T, has been observed in YBCO and other HTS cuprates. For Bi-2 2 1 2 system because of its very strong anisotropic and extremely short superconducting coherence-length, it is expected that the intrinsic granularity would be more pronounced.

In this paper we report the observation, in Bi-2 2 1 2 single crystal, of the granularity behaving as a giant peak close to the superconducting transition in resistivity and I-V curve measurements. Our results further demon- strate that the granularity is an intrinsic property of the HTS.

2. EXPERIMENTAL

The Bi-2 2 1 2 single crystal were prepared by a travelling solvent floating-zone method. An infrared radiation furnace equipped with one ellipsoidal mirror and one 750 W halogen lamp placed in the focus of the mirror were used to melt the rods and to create a floating molten zone between a feed rod and a seed rod. The feed rod and the seed rod are contra-rotated to achieve homogeneous heating of the floating zone and to promote mixing of the elements in the zone, which is useful to maintain the steady growth.

Sintered rods of the oxide powder Bi-2 2 1 2 were prepared as feed rods for floating zone growth. The powders of Bi203, SrCOj, CaC03 and CuO (99.99%) in their metal ratios were mixed and ground in agate mortar, then placed in an Al203 crucible and calcined first for 48 h at 810°C. The calcined powders were then reground and calcined for 72 h at 840°C for feed rods and

899

900 RESISTANCE ANOMALY IN Bi~Sr&aCuzOs SINGLE CRYSTALS Vol. 101, No. 12

820°C for solvent materials. The powders were again reground and placed into a rubber tube and then hydro- statically pressed under 2000 Kg cm-‘. The pressed rods of 97.5 mm and 150 mm in length were sintered for 72 h at 860°C for seeds and at 840°C for solvent materials, in air. The sir&red rod were zone-melted at a velocity of 25 mm h-’ to increase the density of the feed rods and to keep the molten zone stable. Premelted rods were used as feed rods.

The crystal rod was grown in air at air flow rate of 11 min-‘. The bars lower and upper the molten zone were counter rotated with a 30 cylces min-‘. The growth velocity of the crystals was at a rate of 0.5 mm h-‘. The as-grown rod consisted of platelets which extended up to centimetres along the pulling direction (u-axis of the crystal) and 2-4mm width along the b-axis of the crystals. The single crystal platelets with typical dimen- sions of 4 X 2 X 0.1 mm3 were cleaved from the cut sections with 4 mm length of as-grown rod.

The superconducting transition were measured by using a SQUID magnetometer at 10 Oe. The resistivity and I-V measurements were undertaken using a.c. and d-c. four-probe technique. The current and voltage con- tacts are evaporated silver strips on the surface of the crystals, which have contact resistances less than 1 B. The magnetic field was oriented the c axis of the crystal and perpendicular to the transport current.

3. RESULTS AND DISCUSSION

Figure 1 shows the temperature dependence of resistivity along ab plane for the Bi-2 2 12 single crystal from below T, to about 200 K The sample shows a metallic behaviour of resistivity in all temperatures except a giant peak close to superconducting transition. This is similar to what is observed in other HTS single crystals [2,3]. The &-plane resistivity in the normal state can be fitted as pN(T) = AT + B (see the dashed lines in Fig. 2). Furthermore, by fitting the Aslamosov-Larkin [5] formula p-r -Pi? = (e2/16hd)(Td - T)-‘, it is

p.0 v

r.z “80 100 120 140 160 180

Temperature (K)

Fig. 1. Temperature dependence of resistivity at zero field. A resistive peak can be seen near T,.

1 4.0 $3.0

0

Fig. 2. Resistive peaks under different magnetic fields. The dashed line is the normal state resistivity.

estimated that the mean-field superconducting transition temperature is Td = 94.82 K.

Figure 2 shows the resistive transition peak under different magnetic fields. It is evident that all the resistive peaks appear at the temperature, Tp which is below the mean-field superconducting transition, Td. Tp and the amplitude of the peak are magnetic field dependent, in particular, the amplitude of the peak decreases with magnetic field and finally vanishes as the magnetic field is higher than 90 Qe. In order to characterize the variation of the resistive peak with the applied field, it is defined that Ap = (PO) - p(H)) at each peak temperature, Tp. Figure 3 shows the variation of Ap with magnetic field. A nearly linear relation between Ap and H has been obtained.

As shown in Fig. 2, the resistance anomaly appears between Td and T,, where the superconducting fluctua- tions are the strongest. Because the fluctuations are weakened by a small magnetic field [6] and thus a reduction of the resistance anomaly has been observed. Our results shown above suggest the correlation of the resistive anomaly with the superconducting fluctuations. Further to demonstrate the correlation, we investigated

B-221 2 single crystal

----guide of eyes

J 01

Fig. 3. Amplitude of the peak, &, vs magnetic field, H. The dashed line is the guide of eyes.

Vol. 101, No. 12 RESISTANCE ANOMALY IN Bi&aCaCuaOs SINGLE CRYSTALS 901

o’5 (a) T = 81.67 K

OB (W T=84.85 85.84 88.84 88.10.89.19, 9038,9f.‘27 91:84, 92.>3 K From left to hght

0.0 0 IO 20 30 40 50

Current (mA)

Fig. 4. (a) I-V curves under different magnetic field at T = 81.67 K. A peak can be seen as the field is lower than 70 Oe. (b) I-V curves at different temperatures and zero field.

the I-V characteristics under different fields and at different temperatures for Bi-2 2 1 2 single cryst.sl. The I-V curves at 81.67 K and under different magnetic fields are shown in Fig. 4(a); and the I-V curves under zero field and at different temperatures are shown in Fig. 4(b). It is very interesting to note that a peak appears in the I-V curve as the field is low. The amplitude of the peak also decreases with increasing field and vanishes above 70 Oe. Except the peak, each I-V curve ‘can be regarded as a two-step transition, which is similar to the I-V curve of a granular system in which the transition at lower transport current represents the superconducting- to-normal-state transition in the weak-link region and that at higher current represents the transition of the superconducting islands or grains. This further indicates that the origin of the resistive peak in the present study is related to the fluctuation of a granular system. However, the sample examined here is single crystal other than polycrystalline samples. This paradox can be understood if taking into account the possible chemical and struc- tural inhomogeneities as well as the extremely short superconducting coherence-length in Bi-2 2 1 2 single crystal. Because the complication of structure and

chemical composition in Bi-2 2 12, the oxygen distribu- tion is very easy to cause the inhomogeneities. Also because the superconductivity is sensitive to the oxygen content, the inhomogeneously distributed oxygen con- tent will bring about the non-superconducting regions in the Bi-2 2 12 single crystal. Together with the very short coherence-length which enhances the weak-link effect of the structural inhomogeneities, the crystal thus can be viewed as a granular superconducting system. This is so-called intrinsic granularity of the I-ITS oxides [2].

Resistive peak was observed in the superconducting fluctuation region of disordered Cu-Zr alloys with dilute magnetic impurities by Lindqvist et al. [7]. Also, it was observed in high-Tc granular materials [8] and single crystals [2, 31. For Cu-Zr system, the peak effect is explained to be due to the interaction between superconducting fluctuation and spin. For the high-T, granular system, the grain-boundary is believed to be the cause of the resistive peak. For high-T, single crystals, Mosqueira et al. [3] has established a two-dimensional electrical-circuit model by assuming the presence of T, inhomogeneities associated with small stoichiometric (oxygen-content) inhomogeneities, at long length scales, non-uniformly distributed in the crystals. This model has given a good explanation for the resistive anomaly in YBCO system. However, the phenomena in the present case are much more complicated than what can be described by that model, because the anomaly appears not only in the resistive transition, but also in the I-V characteristics. This new phenomenon can not be simply explained by the two-dimensional electrical- circuit model. In addition, two-step transition in the I-V curve strongly suggests the existence of weak-link effect in the samples. In our opinion, the resistive peak can be viewed as the quasi-reentrant behaviour resulting from an intrinsic granularity of high-Tc superconductors. In a layered system with Josephson coupling, it is possible that the reentrant behaviour can be observed, as been reported recently by Zhao et al. [9].

As described in [9], in Bi-Sr-Ca-Cu-0, the Josephson coupling energy between two superconducting islands can be described as:

EJ = EJo( 1 - T/T&2exp ( - 2ErIksT) (1)

where EJo is a constant, E, is the energy gap for the insulator between superconducting islands. In this case, the changes of the coupling energy with the temperature is not monotonous and a reentrant phenomenon may occur. If using this model to our present system, the resistive peak can be interpreted as a quasi-reentrant behaviour.

In summary, a remarkable resistivity anomaly near the superconducting transition temperature is observed in the &-plane of as-grown Bi-2 2 1 2 single crystals. The I-V curves of the samples exhibit a peak close to the

902 RESISTANCE ANOMALY IN BizSrzCaCuzOS SINGLE CRYSTALS Vol. 101, No. 12

transition steps, vanishing gradually with increasing 4. magnetic field. The observed phenomena can be 5. interpreted as a quasi-reentrant behaviour resulting from an intrinsic granularity of high-T= superconductors. ‘.

1. 2.

3.

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