PHYSlCA ELSEVIER Physica C 341-348 (2000) 935-936 www.elsevier.nl/Iocate/physc Thermal excitation measured by resistivity measurement on the Mg-doped high temperature superconductors Anand Vyas, C.C. Lam, LJ. Shen Department of Physics and Materials Science, City University of Hong Kong, Hong Kong, P.R.C. The thermal excitation that starts to occur from the opening temperature T* in the normal state of Mg-doped YBCO superconductors was studied by the resistivity measurement. There are some characteristic temperatures T* and TF related to the thermal excitation of quasi-particles during the cooling process. The data of TF are close to the onset temperature of the superconductors. The magnitude of the pseudo-gap (Ape) can be measured from the ln[1/gN(T) - 1/9(T)] vs 1/T curve, it depends on the concentration of the magnesium. 1. INTRODUCTION With reference to previous studies, we link a drop in p below the extrapolation of its high temperature linear T dependence with reduction in scattering due to the opening of the pseudo-gap (PG). Many investigations in the past showed the existence of the opening of PG, see Refs. [1-4]. As well known, T" decreases linearly with increasing charge carrier density (n). On the other hand, the Mg doping effect on T¢ shows that n changes due to doping, [5]. In this work we investigate the relationship between PG behavior and Mg doping. 2. RESULTS, DISCUSSION & CONCLUSION Several cuprate samples of nominal composition YBa~Cu~.xMg~O7.~, with 0.002 < x < 0.048, were prepared. We show the temperature dependence of resistivity in Fig. 1. The characteristic temperatures T~ and T* can be determined from this figure. Obviously, T ° increases with Mg doping, while Tc decreases. It is clearly seen that the pseudo-gap phenomenon is closely related to Mg doping level. In addition, the p-T curves for Mg doped samples show a similar positive curvature as seen in the over-doped regime [6]. Hence, we believe that the Mg doping in YBCO is favorable to exhibit the over-doped behavior. To describe the Mg doping effect on T', T~ is used as an effective indicator of charge carrier density n. This is based on a fact that 1.0 " ' " ' . . . . G"(x= 0',041 ) "~ 0.9 ~.~,~= 0,032} i 08 ~ '~'~ _-- o o=s~ "N o.4 m ~ oo) -~ 0.3 (~ 0.2 0,1 0.0 41.1 i L h = i 50 100 150 200 250 Temperature (K) Fig. 1. A plot of resistance vs. T for Mg-doped YBCO. The T* is indicated in the figure by arrows. T~ decreases monotonously as n increases in over doped regime. For studying the relationship between T* and n, we plot T* against T c in Fig. 2. In over- doped regime, the relationship between T* and n shows a linear behavior as suggested by [7] while that between Tc and n is parabolic. Hence, we can describe T* in terms of To by: T* = T ,toe) +a(1 - T / Tc~"=x y 2 (1) where T *~°PI is the pseudo-gap temperature at optimum doping, Tc (max) is the maximum critical temperature of pure system, ct is a fitting parameter. In Fig. 2, the bold line represents the fitting result using the above Eq. (1) where T *(°vl = 125 K, et = 0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)00743-7