Thermoelectric power of Hg-based superconductor

ELSEVIER

Synthetic Metals 71 (1995) 1571-1572

Thermoelectric power of Hg-based superconductor Y.S. Ha,H.M. Chung, Y.W. Park Seoul National University, Seoul Korea, 15 l-742 Mi-Ckk Munay Sergey Leea>c, Myoung-Kwang Baea, and Sung-n< Leeayb aPohang University of Science and Technology, Pohang, 790-784, Korea bResearch Institute of Science and Technology, Pohang, 790-784, Korea cMoscow State University, Moscow, 119899, Russia Abstract Thermoelectric power(TEP) of a single phase Hg-I 223 superconductor was measured as a function of temperature. We found several interesting features of the TEP. First, the sign of the TEP is positive. Secondly, the TEP increases sharply just above Tc but demostrates the semiconductor-like behavior at the normal state. Thirdly, the zero-crossing of the TEP coincides with the resistive transition temperature, T,. The obvious explaination can be attributed to the phenomenological mixed state of charge carriers. It is clarified by the localized bound pairs and the free carriers in the normal state. This model explains the semiconductor-like temperature dependency, as well as the sharp increase preceding and just above the transition temperature.

The thermoelectric power measurement is a demonstratively powerful probe into study of the electronic properties of conductors. If it is interpreted properly from this measurement, the information of scattering nature can be ascertained and evaluated. However, it is known that the behavior of the TEP in high-T, superconductors is quite different from conventional superconducting materials[ 11. The magnitude of the TEP is not much different from that of normal metals, but the temperature dependence is rather

/+-

E

5

s

-x

- 01

004 I

-Oo6 I _oo81.., 50

.I.

100

R

e

1

...lo.oo

0

P

I....

,....I

150

200

250

300

VW

Fig. 1. The R(T) and x,(T) of Hg-1223. temperature T, is 130 K.

0379-6779/95/$09.50

0

1995 Elsevier

SSDI 0379-6779(94)02954-W

Science

In this paper, we report the measurement of the TEP of almost single phase Hg-1223. We found that several features of the TEP observed in high-T, materials were also observed for this superconductor. We analyzed the relative behaviors of the TEP with the mixed state model of single-particle-like carriers and bound pairs above T,.

.02

.G-

Y

semiconductor-like. Moreover, the sign of the TEP depends on the superconducting materials. As far as the mercury based superconductor is concerned, the single phase of Hg-1223 is not yet successfully produced, and the TEP of this phase is not as yet investigated.

The transition

S.A.

All rights reserved

For the superconductivity properties, the electrical resistance and the AC susceptibility were measured. The pure Hg-1223 phase exhibited a sharp transition at 130 K in the electrical resistance measurement. The AC susceptibility revealed diamagnetic behavior with T, = 131 K (Fig. 1). The sign of the TEP indicates that the major charge carriers have hole-like character. We found that the zero-crossing of the TEP coincides with the resistive transition temperature. The typical behavior of the sharp increase just above T,, and the subsequent decrease are also observed in this material. However, the magnitude of the TEP is order of uV/K, which is much less than the typical value (mV/K) of semiconductors. This is comparable to the value of broad band metals. From this measurement, no hysteretic behavior is observed in the cooling

1572

YS.

Ha et al. I Synthetic Metals 71 (1995) 1571-1572

J---

Fig. 2. The experimental data(hollow circles) and the theoretical tit with the mixed state model (solid line) of the TEP for Hg-1223. and heating procedure. To interpret the observed maximum in the TEP data, Devaux et a/.[41 have proposed the existence of “inactive’ and ‘active’ superconducting layers of charge transfer. Although the idea seems to indicate a deep insight to explain the superconducting mechanism of high T, materials, it is somewhat ambiguous in explaining the most ‘metallic’ samples. Instead we applied the mixed state model which also has been applied to explain the TEP of other high-T, matcrials[2,4]. The basic idea of this model is following; This node1 assumes the coexistence of singleparticle-like carriers and bound pairs and combines the effect of these two charge carriers. The number of bound pairs and that of single carriers are not independent. This model explains the general behaviors of the TEP of the

cuprate superconductors such as the sharp increase of TEP near T, and the subsequently observed negative slope of the TEP, dS/dT < 0 as shown in Fig. 2. We obtained a best parameter tit with y = 0.10 and p = 0.47. y is defined as the fraction of the total number of conducting charge carriers contributing to the contiguration entropy, and the parameter p is defined as the ratio of the number of total charge carriers contributing to the configuration entropy and the number of available states[2]. In conclusion, we successfully produced a stable and almost pure phase Hg-1223 superconductor with superconducting transition temperature of T, = 130 K, and measured the thermoelectric power. The TEP has a positive value at room temperature. We also found the generally observed TEP behaviors of the cuprams in this Hg-1223 superconductor. And we explained this behavior utilizing the phenomenological model, where the paired or single carriers contribute the TEP separately. We express sincere appreciation for the financial support of the Korea Science and Engineering Foundation(KOSEF), the Ministry of Science and Technology(MOST), and the Ministry of Education(MOE) of Korea.

References 1. Z.S. Lim, K.H. Han, Sung-Ik Lee, Y.H. Jeong, S.H. Salk, Y.S. Song and Y.W. Park, Phys. Rev. B 40 (1989) 7310 2. Y.S. Song, Y.S. Choi, Y.W. Park, M.!3. Jang and SK. Han, Physica C 185-189 (1991) 1341 3. F. Devaux, A. Manthiram and J.B. Goodenough, Phys. Rev. B 4 1 (1990) 8723 4. Y.S. Song, Y.S. Choi, Y.W. Park, MS. Jang and S.K. Han, J. Moscow Phys. Sot. 1 (1991) 293