ELSEVIER Physica C 282-287 (1997) 1177-1178

PHYSICA @

Observation of anisotropy of in-plane resistivities in PrBa=CuaO7 single crystals

Xuanjia Zhang, Guanghan Cao, Jiangping He, Waihua Yu, Hongqing Song, Zhengkuan Jiao and Qirui Zhang

Department of Physics, Zhejiang University, Hangzhou 310027, P. R. CHINA

We report the anisotropic resistivities of PrBa2Cu307 single crystals. It was observed that both pa and Pb exhibit semiconducting behavior, but Pa is 10 times higher than pb at room temperature. The anisotropic behavior of the in-plane resistivity suggests the conductivity contribution from the CuO chains in PrBa2Cu3OT. We also analyze the results in terms of the variable-range-hopping model.

1. INTRODUCTION

PrBa2Cu307 (PBCO) is the unique member that exhibits anomalous properties among RBa2Cu307 (R is a rare earth element) family [11. Earlier work [2,3] indicated that PBCO is a semiconductor associated with variable-range-hopping (VI-IR) conduction. So, it was regarded as a localized and/or underdoped semiconductor. However, the fact does not seem to be so simple. Optical reflectivity [4] and positron annihilation [5] experiments suggested that the hole-doping state of CuO2 planes and CuO chains were quite different: CuO2 planes were insulating with no hole carriers while the hole concentration on CuO chains was comparable to that of 123 superconductors. Fehrenbacher and Rice [6] proposed a Pr4f-O2p hybridization model, which succeeded in explaining many experiments. Their model predicted that, in PBCO, 1D CuO chains are intrinsically metallic, 2D CuO2 planes are insulating, and the conductivity of the Pr4f-O2p hybridization state is not obvious. So, direct measurements of the anisotropic in-plane resistivities are of great significance. Lee et al. [7] has reported the resistivity of PBCO film, showing that the resistivity was different for the thin films with different alignments. In this paper, we report the observation of anisotropic in-plane resistivities on the same specimen of PBCO single crystals.

2. EXPERIMENTAL

PBCO crystals were grown by a conventional self-flux method using alumina crucibles. The crystals were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), and

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energy dispersive x-ray spectrum (EDX) techniques. XRD and SEM showed that the crystals were of high quality in structural and morphological aspects. The chemical formula of the crystals was PrBa2Cu2.92Alo.osOz from the EDX determination.

The as-grown and oxygen-annealed crystals generally contain (110) twinning. In order to obtain twin-free crystals we developed a simple detwinning method, inspired by the work of Rice and Ginsberg [81. The as-grown crystals were first annealed at 1023K in flowing pure nitrogen for 24h to ensure the formation of the tetragonal phase. Oxygen was then replaced for nitrogen when the furnace was cooled to 573K-673K. To get fully oxygenated crystals, long time annealing in oxygen atmosphere is absolutely necessary. Further characterization of the detwinned crystals is undertaken.

Resistivity was measured by a two-probe method, applying a fixed voltage. The I-V characteristic of the measuring system is essentially linear. Rectangular crystals with natural cleaved planes were chosen. Low contact resistance is achieved with silver paste. The resistivities along the two perpendicular directions were measured one by one.

3. RESULT AND DISCUSSIONS

We observed that the in-plane resistivities of O2- annealed crystals and the detwinned crystals were quite different. The former showed almost identical resistivities and temperature dependence of resistivities along the two directions, but the latter exhibited anisotropy, as indicated in Fig. 1. These results were reproducible. Considering the feature of the crystal structure and electronic structure for 123 compounds, it is reasonable that smaller resistivity

1178 X.. Zhang et al./Physica C 282-287 (1997) 1177-1178

5

~ 3

8'2 ,.J

1

100 150 200 250 300 Temperature (K)

Fig. 1 Semilog plot of resistivity versus Temperature for detwinned PBCO crystals.

corresponds to b (CuP chains) direction. It can be seen that p, is 10 times higher than Pb at room temperature. With decreasing temperature, Pb increases faster than pa until they are almost at 100K. This observation seems to be different from the result of Lee et al. [7]. They found that the anisotropy occurred only at low temperature. This controversy may be due to the M-incorporation, since the resistivities are distinctly higher than those of PBCO ceramics 121 and film [7].

Fitting the data with the equation Iogp=A+BT p, we found that when p=l/4 the best linearity of logp versus T -p could be achieved. It is noted that the data of pa gives better linearity, as shown in Fig.2. This analysis suggests that the conductivity of the detwinned crystals belong to the 3D VRH. Similar analysis on the p-T data of the O2-annealed crystals gives the p value of 1/2, suggesting that the crystal is a Coulomb-gap semiconductor. In fact, there were discrepancies on the p value of oxygenated PBCO. Ref.2 got p=l/3 or 1/4 while Ref.3 suggested p=l/2. We think that the discrepancy possibly comes from the different oxygen content and the atomic ordering.

Assuming that Pa represents the 2D planar resisitivity and Pb contains the part of CuP chains, we have 1/pb -- 1/pa = l/poh~, i.e., pCh~ =p,Pb /(Pa-- Pb). It is easy to estimate that Pch~m increases fast with decreasing temperature from the equation, which indicates that the CuP chains has insulating nature. This conclusion does not conflict with the

I

3

8'2 1

, , B . . . . t . . . . , . . . . , . . . . ' • • , , I , , , ,

0.24 0.25 0.26 0.27 0.28 0.29 0.3 0.31 T-1/4 (K-I/,)

Fig. 2 logp vs T p for detwinned PBCO crystals.

previous work [4-7]. Reminding that the crystals contain AI impurity (8% of Cu(1) was occupied by AI atoms provided AI all entered the chain site), it is comprehensible that Poh~ exhibits insulating behavior. Detailed work are in progress.

4. CONCLUDING REMARKS

We have observed anisotropic in-plane resistivity on PBCO single crystals. The results indicated that CuP chains contribute the conductivity. The insulating behavior of pch~a~ is probably due to the AI contamination. It is expected that study on the AI- free detwinned PBCO crystals should give further insight on the intrinsic transport properties of PBCO.

REFERENCES

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M. Lee et al., Phys. Rev. B, 54 (1996) R3776. 8. J.P. Rice and D. M. Ginsberg, J. Cryst. Growth,

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