Physiea C 208 (1993) 259-262 North-Holland PHYSICA

Study of the electric hysteresis loop from properties of the Bi-O2 layer on the cleaved surface of a BiESr2CaCuEOy single crystal

Xiaolin Wang, Zhuo Wang, Hong Wang and Minhua Jiang Institute of Crystal Materials, Shandong University, Jinan 250100, China

Received 16 October 1992

The electric hysteresis loop of a Bi~Sr2CaCu2Oy (BSCCO) single crystal was measured at high frequency at room temperature in air and at 77 K in liquid nitrogen to determine whether the BSCCO superconductor has ferroelectricity. It has been found that the cleaved plate-like crystals have a hysteresis loop under low external voltage. However, it can be destroyed together with the formation of a insulating layer on the sample local surface around the electrode at a certain high external voltage. And the single crystal also shows a hysteresis loop at 77 K in liquid nitrogen in spite of the superconducting state of the sample. Based on some detailed results, it is conducted that the BSCCO superconductor has no ferroelectricity. The formation of the hysteresis loop arises from semiconducting properties of the surface state of the Bi-O2 layer which is revealed on the single crystal surface, rather than the bulk behavior of the sample. Furthermore, the superconducting electron is in the Cu-O2 network and the difference between properties ofth Bi-O2 layer in the superconducting state and in the normal state are emphasized in this paper.

1. Introduction

Since Yu et al. [ 1 ] suggested that high-T¢ super- conductors may have ferroelectric propert ies, many other researchers have drawn more or less s imi lar conclusions [ 2,3 ]. Unfor tunate ly , this result was ob- ta ined in an indirect way. Roy et al. [4] have con- c luded that the YBa2Cu3Oy (YBCO) c o m p o u n d is a ferroelectric with Curie t empera ture Tc o f 483 K. Undoubted ly , it is impor t an t to s tudy the relat ion- ship between ferroelectric and superconduct ing proper t ies in high-T¢ superconductors . To our knowledge, one o f the impor t an t factors for the probing o f possible ferroelectr ici ty o f certain mate- rials is the measurement o f the electric hysteresis loop o f the sample. I f the sample shows a hysteresis loop, it may have ferroelectr ici ty ( F E ) . In this paper , our original goal was to determine whether BSCCO shows FE if it has a hysteresis loop. In addi t ion , we note that the t ip o f the electrode employed in the STS/ STM measurement is very small, o f a tomic d imen- sions [ 5 ]. It is the key to the de te rmina t ion o f the surface state o f the sample. So, a needle-like copper rod with a sharp t ip was used as the electrode. It has been found that the BSCCO single crystal c leaved

along (001) di rect ion showed a hysteresis loop. However , this is not due to its FE propert ies. It is suggested that the hysteresis loop is due to the prop- erties o f the Bi-O2 layer revealed on the cleaved BSCCO single crystal surface. Fur thermore , we ob- served that the crystal surface, i.e. Bi-O2 layer, showed semiconduct ing behav iour below Tc. The su- perconducting electron is in the two dimensional C u - O2 network and the differences between proper t ies o f the Bi-O2 layer in the superconduct ing state and in the normal state are emphasized.

2. Experimental

Single crystals o f 2212 phase were grown by the h igh- temperature gradient technique. The as-grown crystal has a m a x i m u m size o f 16 × 10 × 5 m m 3 and could be easily cleaved into plate samples with a large size o f at least 8 X 5 X 1 m m 3. The sample showed su- perconduct ivi ty with Tc zero of 91 K and Tc < 3 K from the R - T and the x - T data, and the X-ray diffract ion pat terns has shown that the cleaved sample contains single 2212 phase, which indicates that the as-grown crystal is a high-quali ty single crystal o f 2212 phase.

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260 X. Wang et al. / Electric hysteresis loop of the Bi-02 layer

The equipment for the experiment on the electric hysteresis loop for a BSCCO crystal is the same as that employed in the measurements of conventional ferroelectrics. A diagram of the Sawyer-Tower cir- cuit is shown in fig. 1. The measurement frequency is 500 Hz. Three samples were prepared with or without a silver layer on the surface for each side of the cleaved plate-like samples (see fig. 2). The mea- surements o f the hysteresis loop of the sample were carried out at room temperature in air and at 77 K in liquid nitrogen. The electrode was a needle-like copper rod with a tip at the end of the electrode.

3. Results and discussion

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- 4 0 - 2 0 0 20 40

¥×(mV) Fig. 3. Hysteresis loop for sample A pattern with different exter- nal voltage from curves 1 to 3, measured at room temperature in air. 4: insulating state measurement. Frequency is 500 Hz.

The hysteresis loop measurement for sample A was first carried out. Figure 3 illustrates a series o f step- by-step and uncompensated hysteresis loops ob- tained from sample A. It seems like FE behaviour since it is very similar to that o f ferroelectrics. How-

Sawyer-Tower Circuit

Sine- ( ~ Wave Generator

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C

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Fig. 1. Diagram of the Sawyer-Tower circuit.

[ _

2

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1 11 A B C

Fig. 2. Three sample patterns employed for the measurement of the hysteresis loop. 1: electrode, 2: cleaved BSCCO single crystal, 3: thin silver layer.

ever, if Vx was increased to a certain value, the hys- teresis loop abruptly disappeared. Meanwhile, Vy be- came zero, which shows that it is an insulating behavior. This result is very similar to the conven- tional breakdown effect of the ferroelectrics. In this case, when we measure the electrical resistance on sample A from two electrodes at each side o f the sample, the experimental data show the sample to be insulating. This result seems to be a possible bulk be- havior o f the BSCCO single crystal. However, for sample C with silver layers on the two sides of the crystal surface, there appeared no hysteresis loop at all by the same measurement. No matter how high the external voltage was increased, the sample could not be changed to an insulator. Figure 4 shows the hysteresis loop for sample C. The hysteresis loop ex- periments of the ceaved single crystal were carried out on different thicknesses and areas both for sam- ple A and for sample B. It was found that they had the same loop, independently of their thicknesses or areas. This enabled us to suppose that the hysteresis loop for sample A without a silver layer may arise from the surface properties. Based on this view, the insulating crystal sample, by the effect o f a certain higher external voltage, was cleaved again on the crystal surface, and its hysteresis loop was measured again. We could observe a new hysteresis loop and found that the re-cleaved sample A can also become insulating with a certain high external voltage. The result of sample B with only one silver layer on one side is the same as that obtained in sample A and the

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- 2 0 J

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V×(mV)

Fig. 4. Hysteresis Imp ~r sample C,f= 500 Hz.

1

re-cleaved sample A. I f we changed the position of the electrode tip on the sample surface, the hysteresis loop still appeared for sample A, thus indicating that the insulating area is in a position below the tip.

Now let us discuss why the hysteresis loop ap- peared in samples A and B rather than C. It is well known that the Bi-O2 layer is revealed at the surface of a cleaved single crystal of BSCCO [6,7 ], and the STS measurements established that the properties of the Bi-O2 layer are semiconducting, while those of Cu-O2 are metallic [ 5 ]. It should be noted that in the STS measurement the Bi-O2 plane is not covered with any metallic electrode, and the experimental data were obtained with a very small tip above the Bi-O2 layer surface. I f we deposite a metallic layer on the cleaved surface of the BSCCO single crystal, its behavior at the electrode on the metal-deposited surface must be metallic. Let us go back to the pres- ent phenomenon. The pattern of the electrode is very similar to that employed in STS area scanning mea- surements. However, the tip of the needle-like elec- trode is larger than that used in STS measurement, the appearance of the hysteresis loop indicating that the surface of the sample must be of capacity. In fact, from sample patterns (see fig. 2), it is shown that there is a triangular air gap between the electrode and the sample surface. The sample capacity shown by the hysteresis loop is formed by this gap. This re- sulted in the appearance of a hysteresis loop on the crystal surface. We can conclude that the behavior of the Bi-O2 layer revealed on the sample surface must be semiconductive. Otherwise, the charges induced

by the external voltage could not accumulate on the surface at the high frequency, 500 Hz, of the external voltage, and the charges may diffuse rapidly on the sample surface if the properties of the sample surface are metallic. As to why a certain value of external voltage can result in the insulating behavior of the crystal surface, there are two possible reasons. One is the breakdown effect of the air in the gap, in which the high electric field breaks down the oxygen in the air into active oxygen atoms, which then react with the copper atoms of the tip to form copper oxides, or react with the Bi-O2 complex to form new insu- lating Bi-containing compounds on the crystal sur- face. The other is the high external electric field de- stroys the structure of the Bi-O2 layer on the sample surface, thus enabling a rearrangement of Bi and O atoms to take place which may form an insulating layer on the sample surface.

The hysteresis loop measurements were also per- formed in liquid nitrogen for sample A (see fig. 5 ). The hysteresis loop shows clearly if compared with that observed at room temperature, even at the tem- perature of 77 K. From fig. 5, it can be seen that the hysteresis loop of sample A at room temperature in air (curve 1 ) changed to curve 2 at 77 K in liquid nitrogen, which still shows a behavior of the capacity of the sample surface rather than the superconduct- ing state. It should be noted that the sample must be in the superconducting state at 77 K because the sample's T¢ .... is 91 K. I f the B-O2 layer is the su-

4 0

2O

o

- 2 0

X. Wang et al. / Electric hysteresis loop of the Bi-02 layer 261

-40 -20 0 20 40

Vx ( mV ) Fig. 5. Hysteresis loop for sample A pattern at the same fre- quency of 500 Hz. 1: at room temperature in air. 2: at 77 K in liquid nitrogen. The external voltage is the same for both curve 1 and curve 2. 3: insulating state.

262 X. Wang et al. / Electric hysteresis loop of the Bi-02 layer

perconducting layer, there should be no accumulated charges on its surface. Thus, the hysteresis loop of sample A should have disappeared at 77 K, similarly to what was observed for sample C (see fig. 4). We can suggest that, even though in the superconducting state, the surface properties, i.e. the Bi-O2 layer properties, are semiconducting in superconducting state. This implies that the superconducting electron is in the Cu-O2 network rather than the Bi-O2 layer. This results is in agreement with that concluded from the STS measurement [ 7 ].

Comparing curve 2 and curve 1 in fig. 5, we can also see that Vymax or Vxm~x of curve 2 (at 77 K) are smaller or larger than those o f curve 1 (at room tem- perature). This indicates that the amount of accu- mulated charge on the sample surface in the super- conducting state is lower than that in the normal state, while the stay periods of the accumulated charge are larger. This illustrates that the properties o f the Bi-O2 layer on the sample surface in the su- perconducting state are different f rom those in the normal state. It might be a key factor for the origin o f BSCCO superconductivity. It is interesting that, if the external voltage were increased to a high value in the liquid nitrogen, the hysteresis loop disap- peared abruptly, and the sample surface became in- sulating, which is similar to the results of sample A determined at room temperature. Since there is no oxygen or air in the triangular gap, because it is filled with liquid nitrogen, so there is no air breakdown effect in the gap and it is impossible for insulating copper oxides to be formed on the copper electrode. There is a very small possibility of active nitrogen atoms being produced by a high external voltage and reacting with the BiO complex to form nitrides which may be insulating. So, it is most likely that, by the effect o f high electric field, the Bi-O2 layer varied at the sample surface and formed a new insulating layer. Similar results as shown in fig. 5 can he reproduced for different cleaved single crystals which have Tc .... above 85 K. In addition, if we take the sample out o f the liquid nitrogen after the sample surface has become insulating and move the electrode to an- other position on the sample surface or recleave a new surface, then put this sample into liquid nitro- gen, the results of fig. 5 can again be produced. This

indicates that the insulating area of the sample, even in liquid nitrogen, is also at a position below the tip o f the electrode, as discussed for sample A. In this study, the BSCCO superconductor shows no ferroelectricity.

4. Conclusions

The BSCCO superconductor has no ferroelectric- ity. The formation of an electric hysteresis loop of a BSCCO single crystal is related to the properties of the Bi-O2 layer on the cleaved surface rather than its bulk behaviour. The superconducting electron is in the Cu-O2 network for the BSCCO superconductor. The properties o f the Bi-O2 layer in the supercon- ducting state are different from those in the normal state. The formation of an insulating layer on a local surface area below the electrode tip may be due to the variation o f the Bi-O2 layer on the crystal surface due to the effect o f a certain high external voltage.

Acknowledgements

The first author is grateful to Min Wang for his very helpful discussions. This work is supported by Youth Science Fund of Shandong University.

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