Electrical transport properties of YPd5B3C0.3

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Synthetic

Metals 71 (1995) 1567-1568

Electrical Transport Properties of YPdsB$o,3 Y. S. Choi, D. J. Lee and Y. W. Park Dept. of Physics, Seoul National University,

Seoul 15l-742, Korea

Abstract We have measured the electrical resistrvity and the thermoelectric power (TEP) of a superconducting intermetallic compound, YPdsB3CO.3; as a function of temperature between 4K and 300K. The bulk sample was prepared by arc-melting. The electrical resistivrty shows quasi linear temperature dependence and drops sharply to zero showing the superconductivity at 23K. TEP at room temperature is positive (2.1 uV/K) and shows zero-crossing behavior at SOK. At T>lOOK, TEP decreases linearly as the temperature is lowered. Unlike the cuprate superconductors, TEP does not drop sharply to zero at the superconducting transition temperature, 23K, 1. INTRODUCTION Cava et al reported superconductivity at 23K in an quatemary intermetallic system based on yttrium, palladium, boron and carbon[l]. ‘This transition temperature is equal to that of the sputtered films ofNb3Ge[2] and is the highest ever reported for the bulk mtermetallic compound. The composition of the superconducting phase was: however, not identified. They obtained the multiphase superconducting samples from the melt composition with an average stoichiometry of YPdSB3CO 3. They proposed that this compound represent the first of a new family of superconducting intermetallic compounds wrth relatively hrgh T,s Superconductivity has been also reported for the quaternary inter-metallic compounds, YNi2B2C[3], LnNi2B2C(Ln=Y, Tm, Er, Ho and Lu (Tc=16.6K))[4] and more recently RPt2B2C(R=La, Y)]S]_ The crystal structure for these materials has been determined as a layered structure which can be considered a derivative of the ThCr2Si2 type[6]. Electronic structure calculations have been also carried out for the superconducting Ln-Ni-B-C inter-metallic compounds to clarify the issue as to whether or not these materials constitute a class ofnew high critical temperature superconductors[7-91. Electronic structure calculations for the superconducting Ln-NiB-C intermetallic compounds suggests that these materials might be placed in the category of conventional rather than high‘I‘, superconductors However, the experimental studies are not sutficient to determine whether these materials are the conventional superconductors or the new high T, superconductors. Further experimental studies are necessary to investigate the superconducting mechanism for the materials. The thermoelectric power(TEP) measurement as a function of temperature IS a good experimental tool for investigating the electronic structure because the temperature dependence of TEP is sensitive to the electronic structure of a material. We measured the electrical resistivity and the thermoelectric power results firstly for the Y-Pd-B-C intermetallic compounds to understand the superconducting mechanism of the material. In this paper, we present the results. 2. EXPERtMENTALS Our sample was prepared bv arc-melting. The starting materials were Y chips (99.9% purity), Pd shot (99.99% purity), B crystalline pieces (99.6% purity) and C powder (99.99% purity). They were weighed appropriately for the stoichiometry of Wd5R3C(j 3. They were then arc-melted under Ar on a

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standard water-cooled copper hearth. We remelted the firstly melted button type sample four or five times turning it over between each melt. The electrical resistivity as a function of- temperature between 4K and 300K was measured by the four probe method with silver paint contact. The thermoelectric power was measured by using the method described earlier[lO] in the temperature range between 4K and 300K. 3. RESULTS AND DISCUSSIONS Figure 1 shows the temperature dependence of the electrical resistivity for the sample of composition YPdsB3CO 3 in the temperature range between 4K and 300K. The resi$tivity at room temperature is of the order of lOO@cm which is the same as the result reported by Cava et al[l]. The normal state resistivity decreases monotonically with decreasing temperature and exhibits a broad upward curvature. The slope of the resistivity plot versus temperature is changed at lOOK. 150

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YS. Choi et al. I Synthetic Metals 71 (1995) 1567-1568

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The quasi linear temperature dependence of normal state TEP is similar to that of broad band metals but different from the anomalous TEP of the cuprate superconductors[ 111. This system seems to have free charge carriers. The temperature dependence change in TEP at 1OOK is an noticeable feature. This reminds us of the curvature change in the temperature dependence of resistivity at the same temperature. This is not found in the In the resistivity data of the rare-earth nickel boride carbide. present alloy, Y-Pd-B-C, the scattering -mechanism for the charge carriers or the electronic structure seems to change significantly around that temperature. This needs to be more studied. The drop in the absolute value of TEP below 20K seems to reflect the superconducting transition. It is strange for us that TEP in the present alloy does not drop sharply to zero and remains positive, unlike in the cuprate superconductors, although the resistivity drops sharply to zero at 23K. To clarify the origin of the peculiar temperature dependence of TEP in our sample at low temperature, the thermoelectric property for Niand Pt- based boro carbides as well as the Pdbased boro carbides are currently under investigation.

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Fig. 2 Temperature dependence of thermoelectric power (TEP) for YPd5B3CO.3. Inset shows the TEP at low temperature. The resistivity starts to drop sharply showing the superconductivity at 23K. The inset to figure 1 shows the resistivity drop in more detail. The lo-90% transition width is less than 1K. Figure 2 shows the temperature dependence of the thermoelectric power (TEP) for the same sample. TEP at room temperature is of the order of 2.1 pV/K and positive. Its positive sign indicates that the charge carriers are hole-like. The features shown in the temperature dependence of TEP can be summarized as follows. (I) TEP decreases quasi linearly between 300K and 1OOK as temperature is lowered. (2) The linear temperature dependence of TEP is changed into the complicated one at 1OOK. (3) The zero-crossing behavior at 80K and the negative deep at 40K can be seen. (4) The absolute value of TEP drops, although not as sharply as in the resistivity, to 0.2uV/K between l5K and 20K (5) Below 15K, TEP is temperature independent. (see inset to figure 2)

This work was supported by the Korean Engineering Foundation (KOSEF), Ministry (MOE) and the Hyundai Motor Company, Korea.

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REFERENCES 1. R. J. Cava et al, Nature 367 (1994) 146 2. J. R. Gavaier, M. A. Janocko and C. J. Jones, J. Appl. Phys. 45 (1974) 3009 3. R. Nagarajan et al, Phys. Rev. Lett. 72 (1994) 274 4. R. J. Cava et al, Nature 367 (1994) 252 5. R. J. Cava et al, Phys. Rev. B 49 (1994) 12384 6. T. Siegrist, H. W. Zandbergen, R. J. Cava, J. J. Krajewski and W. F. Peck Jr., Nature 367 (1994) 254 7. L. F. Mattheiss, Phys. Rev. B, 49 (1994) 13279 8. W. E. Pickett and D. J. Singh, Phys. Rev. Lett, 72 (1994) 3702 9. L. F. Mattheiss, T. Siegrist and R. J. Cava, Solid State commun. 91 (1994) 587 10. Y. W. Park, Synth. Met. 45 (1991) 173 11. Y. S. Song, Y. S. Choi, Y. W. Park, M. S. Jang and S. K. Han, Physica C 185-189 (1991) 1341