PHYSlCA E L S E V I E R P h y s i c a C 3 4 1 - 3 4 8 ( 2 0 0 0 ) 9 3 5 - 9 3 6

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The rma l exci ta t ion m e a s u r e d b y resis t ivi ty m e a s u r e m e n t on the M g - d o p e d h igh t empera tu re

superconduc tors

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 =

0 9 2 1 - 4 5 3 4 / 0 0 / $ - s ee f ron t m a t t e r © 2 0 0 0 E l s e v i e r S c i e n c e B.V. A l l r i gh t s r e s e r v e d .

PII S 0 9 2 1 - 4 5 3 4 ( 0 0 ) 0 0 7 4 3 - 7

936 ,'1. Vyas et al./P/o,sica C" 341-348 (2000) 935 936

330 K, and Tc (max) = 91.3 K. It can be seen that this fitting curve fits the experimental data very well. The inset in Fig. 2 shows a relationship between Tc and Mg doping. The critical temperature T~ is suppressed by the Mg doping. This relationship

3 4 0 , • t - , • , . , • , • , • ,

320 ~

300 ~ ~ o

280 ~ = 0

260 6 ~ 0 0

240 & ~a 0 0

220 ~ o ~ T~oz--o.~- ~ ' o o 5

200

180

160

140

120

78 80 02 84 86 88 90 92

Tc (K)

Fig. 2. A plot between pseudo-gap temperature T* and critical temperature T c. The inset shows a plot between the critical temperature and Mg doping.

implies that the charge carrier density in CuO2 planes is greater than that in optimum doping case. Ref. [7] suggested that T* increases linearly with increasing n in the over-doped regime. Our experimental result is in agreement with the existing result.

** (B)ldg = 0.0000 (C)Mg " 0.0120

30 ~ 6 (D)Mg = 0'0102 (E)Mg = 0.0205

E" ~" , A (F) Idg = 0.0318 o .~ (G)ldg = 0.0410

2 0

"19002 ' o.~, ' o . ~ ' o.~ ' o.d,o ' o.d,2 'o.d, 1/'1" (K-')

Fig. 3. A relationship between In(1/pN(T ) - Up(T)) and 1/T for Mg doped YBCO samples.

In addition, a plot of logarithmic deviation of ln[1/�rq(T) - Up(T)] against 1/T was constructed, see the inset of Fig. 3, where ON(T) is the extrapolation of the normal state resistivity that exhibits linear T-dependence and 9(T) is the actual

resistivity of the sample. This curve shows some special characteristics. At first, the curve goes up from l/T" and then at 1/Ts it starts showing a linear behavior within the range ofTs-~ < T -1 < TF -~, where TF is close to Tc (°nset). We suggest that the slope in this linear part represents the pseudo-gap since the deviation in the resistivity is associated with pseudo-gap. Hence, the pseudo-gap can be determined from the In[1/pN(T)-I/p(T)] vs 1/T curve. For various Mg doped samples, the curves of ln[1/ON(T)-I/p(T)] vs 1/T are displayed in Fig.3. Thus, we obtain the relationship between Apc and doping level as shown in Fig. 4. Our result shows that the estimated value for Apo is Mg doping level dependent in YBCO system.

lOOO

800

600

400

2oo o

o o o

0 . 0 0 o . 0 1 0 . 0 2 0 . 0 3 0 . 0 4

Mg Doping (x)

Fig. 4. A relationship between the magnitude (slope) of the pseudo-gap and Mg doping (x).

To summarize, we can conclude that T* increases with increasing Mg doping x. This is in agreement with the previous reports. We found that the magnitude of pseudo-gap is increasing with Mg doping level.

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