Study of phonon anomalies in Tl-system superconductor near Tc

PHYSICA © ELSEVIER

Physica C 282-287 (1997) 1087-1088

Study of phonon anomalies in TI-system superconductor near TIc Yun-Bo Wang 8 , Guo-Hui Cao b, Yang Lib, Xin Ju a, Long Wei 8 and Wei-Fang Wu8 8

b

Institute of High Energy Physics, Chinese Academy of Science, Beijing 100080 Department of Materials Physics, University of Science and Technology Beijing, Beijing 100083

The phonon softening near Tcon iron-doped TI-1223 phase samples were observed by 57Fe Mtissbauer measurements at temperatures ranging from 77K to 250K. Simultaneously, the temperature dependence of positron annihilation parameters on an iron-free sample show the anomaly of the electronic behavior in the temperature range from Tc to Tc+30K.. 1. INTRODUCTION Mtissbauer studies on the high Tc superconductors have been carried out widely, and valuable information has been obtained[IJ. Since the difficulty in the synthesizing of the samples, the Mtissbauer study of TI-system is much less extensive than those of Y -system and Bi-system. In this paper, the Mtissbauer measurements at temperatures ranging from 77K to room temperature on iron-doped Tl-system 1223 phase samples are reported. Simultaneously, the electronic properties of an iron-free sample were investigated by positron annihilation are technique (pAn, and some new ideas proposed.

2. EXPERIMENTAL The superconductors with nominal of composition (TIO.5Pbo.5)(Sro.sBao.2)2Ca2(CUI.xFexhOy (0 ~ x ~ 0.05) were prepared by solid state reaction method under the same technological condition as reported in reference[2J. The prepared samples were characterized by X-ray

diffraction and shown that the dominant phase is of 1-2-2-3 type. The 57Fe Mtissbauer spectra on the samples were obtained in an Oxford MS-500 Mtissbauer spectrometer The Doppler-broadened line-shape measurements were carried out using an intrinsic germanium detector. The positron lifetime spectra were collected by using a fast-fast coincidence spectrometer. 3. RESULTS AND DISCUSSION The recoilless fraction f(T), which is proportional to the relative resonance absorption areas A(T) ,can be obtained from the Lorentzian areas. In the Debye model, the ffactor represents the strength of the lattice binding. An anomalous dip near T cO can be observed in these three samples, which indicates that the phonon softening occurs in the temperature range from TcO+ 15K to TcO. Another drop of f-factor can also be found around 200K (The temperature dependence of the f-factor for the sample with x=O.03 is shown in figure 1 as an example. The solid line is a plot of f assuming a

1 Project supported by the National Center for Research and Development on Superconductivity of China and Laboratory of Nuclear Analysis Techniques Academia Sinica.

0921-4534197/517.00 ~ Elsevier Science B.Y. PH S0921-4534(97)00648.5

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Y.-B. Wang et a/.f Physica C 282-287 (1997) 1087-1088

1088

Debye -like density of states with the indicated Debye temperature.).

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,

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... . ... .

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100

120

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1110

180

200

210

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280

280

T (K)

Fig. 1. The temperature dependence of f-factor for (Tlo.5Pbo.5)(SrO.8Bao.2hCa2(Cul-xFe,J30y Since iron which doped in these superconductors is suggestive occupy the copper sites[3], thus the anomalous softening reflect the nature of lattice vibration on Cu-O plain. The similar phenomenon have been observed in some Mossbauer, XRD and Internal studies on La-system, Y -system and Bi-system[4-6]. It need to be noticed: the Tc of TI-1223 phase is higher than Y-123 phase and Bi-2223 phase, the softening on Tl-system 1223 phase occurs near TcO rather than in the temperature range above Tc 15-30K. The positron lifetime spectra were fitted on two-state trapping model. The mean lifetime

r m (= r / 1 + r 2 12 ) at various temperatures was evaluated (shown in fig.2.). It can be found that with the temperature decreasing there is an anomalous increment on r m in temperature range from 150K(Tco+30K) to 120K(TcO). Meanwhile, the temperature dependence of SIW shows an obvious dip in the same temperature range[7]. Since the positrons mainly probe the Cu-O layers in Tl-system superconductors[8], our results implies the tendency that before the superconducting transItion there is a transformation from free electrons to localized electrons on around local structure of Cu-O plain.

120

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160

T (K)

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Fig.2 The temperature dependence lifetime of the (Tlo.5Pbo.5)(Sro.8Bao.2hCa2Cu30y

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of mean sample

As a conclusion, the lattice softening accompanied by the anomalous electronic behavior in Tl-system 1223 phase indicates a strong electronic-phonon interaction before superconducting transition. The temperature range of the softening in Tl-system is more closer to Tc than in Y -system and Bi-system .

REFERENCE 1. P.Boolchand,

D.McDanied, Hyperfine Interactions, 72, 125(1992). 2. Wang Yunbo, Li Yang, Cao Guohui et al., Chinese Science Bulletin, 40, 1955(1995). 3. Physical Properties of High Temperature Superconductors II, ed. Donald M. Ginsberg, (World Scientific Publishing Co. Pte. Ltd., 1990)532. 4. P.Boolchand, R.N.Enzweiler, Ivan Zitkovsky et al., Phys. Rev., B37, 3766(1988). 5. V. W. Cherepanov, M. A. Chuev et aI., JEPT Lett., 47, 424(1988). 6. Tan Yuen, C.L.Lin, J.E.Crow, G.N.Myer et al., Phys. Rev. B, 37, 3770(1988). 7. Yun-bo Wang, Guo-hui Cao, Yang Li, Solid State Comm. ,96,717(1995). 8. C.S.Sundar, A.Bharatbi, W.Y.Ching et al., Physical Review B, 43, 13019(1991).