Ž .Physica C 337 2000 285–287www.elsevier.nlrlocaterphysc

Anelastic relaxation near T in Zn-substituted YBCOc

Li Ang), Ying Xuenong, Xu Xiaoshan, Zhang Qingming, Li Baiqin, Chen Wuming,Wang Yening

National Laboratory of Solid State Microstructures, Nanjing UniÕersity, Nanjing 210093, People’s Republic of China

Abstract

The anelastic properties of Zn-doped ceramic YBa Cu O superconductors were measured and two thermally2 3 6qd

activated relaxation peaks were observed in acoustic-frequency internal friction spectrum at 100 and 110 K. The intensity ofthe lower-temperature peak was found to decrease rapidly upon zinc substitution while the other one remains almostunaffected. The relaxation mechanisms for the two relaxation peaks are studied and the deteriorative effect of zincsubstitution on superconductivity is also discussed. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Anelastic property; Zn doping; Superconductor

1. Introduction

By now a great number of experiments have beendevoted to investigation of the structure of supercon-ductors and their relationship to superconductivity.Mechanical energy dissipation measurement is a goodprobe for change in microstructures and relativedynamic processes. The internal friction spectra ofyttrium 1-2-3 superconductors have been long stud-

w xied 1–3 but to our knowledge there is no work yetreported on Zn-doped YBCO. The substitution of Cuby Zn in YBCO causes a drastic depression on

w xsuperconductivity 4 and has been attracting exten-sive attention for many years. Investigating the ori-gin of such suppression might throw light on themystery of high-T superconductivity and for thisc

sake we systematically studied the anelastic behaviorŽ .of YBa Cu Zn O .2 1yx x 3 6qd

) Corresponding author. Tel.: q86-25-359-32-02.Ž .E-mail address: wyn@netra.nju.edu.cn L. Ang .

2. Experimental

Ž .Samples of YBa Cu Zn O with x vary-2 1yx x 3 6qd

ing from 0 to 0.1 were prepared under the samecondition with high purity Y O , BaCO , CuO and2 3 3

ZnO powders by a standard solid reaction method.X-ray diffraction analysis shows all samples are ofsingle phase and the superconducting transition tem-perature was determined by a four-probe electricalresistivity measurement.

The mechanical energy dissipation, Qy1, wasŽ 3.measured on a rectangular bar 44=6=0.8 mm ,

which was electronically driven on a flexural vibra-tion mode, from liquid nitrogen temperature to roomtemperature in the frequency range 1.5–1.9 kHz. Therate of temperature increase was about 1 Krmin.

3. Results and discussion

The superconducting transition temperatures ofeach sample are shown in Fig. 1 from which we can

0921-4534r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0921-4534 00 00118-0

( )L. Ang et al.rPhysica C 337 2000 285–287286

Fig. 1. The superconducting transition temperature T as a func-cŽ .tion of zinc content x in YBa Cu Zn O . T decreases2 1yx x 3 6qd c

rapidly upon zinc doping that the superconducting transitions inxs0.05 and 0.1 samples were not detected above 20 K.

see that T drops rapidly with the increase of zincc

content and the superconducting transitions in thesamples with xs0.05 and 0.1 were not detected

Ž .within our temperature range TG20 K at all. Thereason why Zn doping depresses T so strongly willc

be discussed later.ŽFig. 2 shows the internal friction spectra 90–160

. Ž .K of YBa Cu Zn O with xs0, 0.002,2 1yx x 3 6qd

Ž0.005, 0.02 and 0.05. Two peaks labeled as P1 100. Ž .K and P2 110 K were observed which were

reported to be thermally activated relaxation peakswith activation energies being 0.16 and 0.19 eV

w xrespectively 1,2 . We studied the evolution of P1and P2 upon zinc doping and found the peak temper-atures of P1 and P2 do not shift with doping but theintensity of P1 shows unambiguously the tendencyof decreasing as Zn2q ions enter while that of P2remains almost unaffected.

w xAccording to Cannelli et al. 1 P2 originates fromŽ .the jump of chain oxygen atoms O 1 between off-

centered positions in a zigzag-chain configuration.Our results seem to coincide with this proposal that

2q Ž .Zn ions mainly take the Cu 2 sites in the CuO2w xplane 4,5 and have little effect on chain oxygen

atoms; thus P2 remains unchanged on zinc substitu-tion.

In the case of P1, possible relaxation speciesinvolve movement of lattice defects like dislocationsw x w x Ž .3 and twin boundaries 6 , electron or hole polaron

w x w xrelaxation 3,7 and jump of off-centered atoms 1,3 .Due to the perfect reproducibility and systematic

evolution on Zn doping, the movement of latticedefects should have nothing to do with P1. Further-more, the polaron relaxation mechanism can be alsoruled out by the fact that P1 shows no systematicchange on Pr doping which modifies the hole con-

w x Ž .centration by a factor of 2 7 . Because the Cu 2 –Ž .O 3 bond is the strongest chemical bond in this

w xmaterial 8 and the activation energy of P1 is justabout 0.16 eV, the atoms’ jump in the CuO plane2

seems impossible for P1 either. From this point wesuppose the jump of off-centered apical oxygen atomsŽ .O 2 should be taken into account. EXAFS and

inelastic neutron diffraction experiments have foundŽ . w xtracks of site splitting in O 2 9,10 and many

theoretical attempts have been proposed involvingw x w xPeierls dimerization 11 , Jahn–Teller effect 12 and

Ž .Fig. 2. Internal friction spectra of YBa Cu Zn O with2 1yx x 3 6qd

xs0, 0.002, 0.005, 0.02 and 0.05 in the temperature range90–160 K.

( )L. Ang et al.rPhysica C 337 2000 285–287 287

w x Ž .pseudo Jahn–Teller effect 13 . The jump of O 2between off-centered positions can be visualized as athermally activated relaxation peak in internal fric-

Ž . Ž .tion spectrum and the site splittings of O 2 and O 1˚w x w xwere reported to be 0.13 9 and 0.16 A 14 , which

are in good agreement with the activation energies ofŽ .P1 and P2 0.16 and 0.19 eV .

When Cu2q is substituted by Zn2q, the electronicstate of nearby ions is changed, and the perturbationitself exhibits a strong and extended nature in case of

w xzinc substitution 5 . As a result the charge distribu-Ž .tion on several apical O 2 surrounding zinc impuri-

ties takes a change, thus the site splitting mechanismŽ .like Peierls dimerization or pseudo Jahn–Teller

effect is strongly suppressed. In the case of Nisubstitution in YBCO, which also could redistributethe charges on neighboring atoms, the intensity of P1

w xdecreases much slowly 15 due to the weak andw xlocalized character of the perturbation 5 .

Furthermore, we think the change in electronicstate produced by Zn2q is the main reason whysuperconductivity is depressed by zinc doping so

Ž .strongly see Fig. 1 . Zinc doping does not changeŽ . w xthe overall carrier hole density 16 but it gives rise

w xto a redistribution of charges 5 that produces strongelectron scattering not only from Zn2q but also fromnearby sites which breaks the Cooper pairs andtherefore decreases T rapidly. In addition, interlayerc

charge transfer across rock-salt layer in layered high-T superconductors is crucial for a stable quasi-2Dc

superconductivity and the mediating role is attributedŽ . w xto apical oxygen O 2 in YBCO 13 . The appear-

Ž . Ž .ance of an extra positive charge hole on O 2 issupposed to be indispensable for superconductivityw x Ž .17 . Zinc substitution changes the charge on O 2and further modulates its position then places adetrimental effect on interlayer charge transfer whichalso leads to the degradation of superconductivity.

4. Conclusion

In this work we studied the anelastic behavior ofZn-doped ceramic YBa Cu O . Two thermally2 3 6qd

activated peaks were observed and the relaxationŽ .mechanism of the lower-temperature one 100 K is

interpreted as the jump of apical oxygen atoms be-tween off-centered positions caused by Peierls

Ž .dimerization or pseudo Jahn–Teller effect. The de-pression of P1 on zinc doping stems from the strongand extended nature of the perturbation created byZn2q on the electronic state, which is also responsi-ble for the degradation of superconductivity.

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