ELSEVIER Physica C 282-287 (1997} 1293--1294

PHYSICA

The effect of Pr ion concentration on superconducting parameters in Yl-xPr=Ba2Cu307-s single crystals Yi Zhuo ~,b, Jae-Hyuk Choi ~, Mun-Seog Kim a, Jin-Nam Park ~, Myoung-Kwang Bae ~, Ze Xianyu Sung-Ik Lee a

Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea

b Department of Physics, Northeastern University, Shenyang 110006, China

b

Yl-zPr=Ba2Cu3Ot-~ single crystals with x = 0.1, 0.2 and 0.3 were grown by the flux method. We analyzed the reversible magnetization with the external field parallel to the c-axis and obtained the various thermody- namic parameters using the Hao-Clem model. Comparing thermodynamic parameters of pure YBa2CuzOr_~, we discussed the effect of Pr ion on the superconducting properties.

1. I n t r o d u c t i o n

The Yl-zPr=Ba2Cu3OT-6 system is isostruc- tural with YBa2Cu3OT-~, but Tc decreases monotonically with increasing z[1]. In order to understand the suppression of Te, systematic studies of the physical parameters is quite neces- sary.

In this paper, we have prepared YI-zPr=Ba2- Cu307-6 single crystals with z = 0.1, 0.2 and 0.3 (called as YPr-0.1, YPr-0.2 and YPr-0.3, respec- tively). We have analyzed the reversible magne- tization and have obtained a series of supercon- ducting parameters with different x [2].

2. E x p e r i m e n t s

Single crystals were grown by the flux method described in the literature[3]. The magnetization were measured by a SQUID magnetometer. The normal-state background was appropriately sub- tracted from the observed the magnetization val- ues in the high-temperature region.

3. Resu l t s a n d Discuss ion

Figure 1 shows temperature dependence of re- versible magnetization for YPr-0.1 single crys- tal. The crossover of magnetization is observed at 85 K below Te. This phenomenon, due to vor- tex fluctuation, was also observed in YPr-0.2 and YPr-0.3 single crystals.

Equation (20) of Ref [2] is used to analyze the

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i i , , | , i , , [ , 1 , i

O

CO -4 ~X~o

vX 2~o~o o T I - 8 a o o 2 T

v& o o ~ 3

- 1 2 O ° o ° v 5 o

o - 1 6 . . . . I ~ , , ~ I w a , =

75 80 85 90 T (K)

Figure 1. Temperature dependence of the re- versible magnetization 47rM(T) with applied field parallel to the c axis for Yo.gPr0 aBa2Cu3OT-6.

experimental data. The detailed description of this procedure has been given in Refs. [2,4]. For YPr-0.1, the ~(T) value is nearly constant at the temperature region of 75K < T < 80K. The average value of x(T), the xa., is 79 in this tem- perature range. In the same way, n~. is 88 for YPr-0.2 and is 101 for YPr-0.3.

Figure 2 shows -4~rM' = -4~rM/v/2He(T) versus H' = H/x/2He(T) of the experimental data and theoretical curve for YPr-0.1. Rather good agreement is obtained between the experi- mental data and theoretical predictions. Similar results was also observed for YPr-0.2 and YPr- 0.3. Insets in Figure 2 shows He(T) and the the- oretical curve[5]. From this analysis, the He(0) = 0.78 T with Te = 86.6 K is obtained.

The (dHe2(T)/dT)To and He2(0) are estimated

1 2 9 4 Y Zhuo et aL/Physica C 282-287 (1997) 1293-1294

Table 1 Thermodynamic parameters of YBCO, YPr-0.1, YPr-0.2 and YPr-0.3 obtained from Hao-Clem model analysis ( Superscripts a, b represent clean and dirty limit, respectively. )

YBCO YPr-0.1 YPr-0.2 YPr-0.3 57 (Ref.[2]) 79 88 101

Tc (K) 94. l(Ref.[2]) 86.6 77.8 64.1 He(0) (T) 1.10(Ref.[2]) 0.78 0.57 0.31

dHc2(T)/dT -1.65(Ref.[2]) -1.64 -1.48 -1.09 He2(0) (T) 112 (Ref.[2]) 103 a (98 ~') 84" (80 b) 51 '~ (48 b) ~,b(0) (-~) 17.2 ~ (17.6b,Ref.[2]) 17.9" (18.3b) 19.8" (20.3 b) 25.5 ~ (26.1 b) Aab(0) ()t) 1650 ~ (1870 b) 2040" (2300 b) 2980 a (3340 b)

0 . 0 2 0 ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' '

0.015 ~" ISOO e ~

0.010: \ K : = 7 9 ~ 1

0.005' -

O . 0 0 0 , , , , I , i i , I . . . . I , , ~ ' ~ - - ~ -

0 20 40 60 80

H' Figure 2. Magnetization vs applied field scaled by v~H¢(T) for Y0.9Pr0jBa2Cu307-~. Solid line represents theoretical curve. Inset: The He(T) obtained from the Hao-Clem model, solid lines represent the theoretical curve.

according to the relation H¢2(T ) = x/2gH¢(T) and the WHI-I equation[6]. The coherence length ~,b(0) is obtained from the expression H¢~(0) = ¢0/27r~b(0 ) and the magnetic penetra- tion depth )~ab(T) is calculated through the re- lation x/2H¢(T) = ~¢0[27r,~2b(T) [7]. From the theoretical prediction of dirty and clean limit, we obtain the magnetic penetration depth ~ab(0). These values are listed in Table I.

Compared with the case of pure YBCO, the pa- rameters, such as He(0), Hc~(0), and T¢, decrease monotonically with increasing Pr concentration. The changes of these parameters reflect the sup- pression of superconductivity. Prom Table I, we also notice that the parameters ~, ,~,b(0), and ~ab(0) increase with the Pr content.

It is known that the ,k,b(0) is proportional to (m*ab/n)W2 for the clean superconductor[7]. A

decrease of n could directly result in the increase of the penetration depth. Assuming the valence state of Pr ion is greater than +3, the substitution of Pr for trivalent Y may bring extra electrons to fill holes in the CuO~ planes, which would cause a decrease of n and the increase of the penetration depth.

4. Conclus ion

By applying the Hao-Clem model, we studied the effects of the Pr ion on the thermodynamic parameters. The obvious change of these parame- ters was observed. The results show that the crit- ical fields decrease with Pr concentration, while the parameters, such as ~, A,b(0) and ~ab(O), in- crease with Pr doping. Based on the change of ,~ab(0), the mixed-valence state of Pr ion is most likely responsible for the suppression of supercon- ductivity in YI-xPr~Ba2Cu3OT-6.

R E F E R E N C E S

1. Y. Dalichaouch et al., Solid State Commun. 65, 1001(1988).

2. Zhidong tIao et al., Phys. Rev. B 43, 2844 (1991).

3. J. P. Rice et al., J. Low. Temp. Phys. 72, 345(1988).

4. Mun-Seog Kim et al., Phys. Rev. B 51, 3261 (1995).

5. J. R. Clem, Ann. Phys. (N. Y.) 40, 268 (1966).

6. N.R. Werthamer et al., Phys. Rev. 147, 295 (1966).

7. M. Tinkham, Introduction to Superconduc- tivity (McGraw-iiill, New York, 1980).