Physica C 235-240 (1994) 1471-1472 North-Holland PHYSICA $

Pressure Dependence of Tc of YI_ Pr Ba2Cu307_ (0.1 < z < 0.5, 0 < < 0.5)

M. Buchgeister a*, L. M. Paulius at, M. B. Maple a, D. Wagener ~ and S. M. Hosscini ~

a Insti tute for Pure and Applied Physical Sciences, Univ. of California at San Diego, 9500 Gilman Dr., La JoPa, CA 92093-0075, USA, b Insti tut fiir Strahlen und Kernphysik, Univ. Bonn, Nuf]allee 14-16. D-53115 Bonn, Germany

The system Yt-xPrxBa2CusOr-6 has been investigated for the pressure dependence of the superconducting critical temperature dTc/dP for a range of Pr concentrations x with varying oxygen deficiency 6 for each Pr doping level. The variation of both parameters indicates a dependence of the Tc suppression by Pr under pressure on the mobile hole concentration in the CuO2 planes. Furthermore, eff(,cts of Josephson weak links in the samples are observed for higher levels of Pr doping and oxygen deficiency.

Among the RBa2Cu307-6 (R = Y and a lanthanide, except Ce, Pm and Tb) ceramics, PrBa2Cu3OT-~ is the only one that does not be- come superconducting on cooling down. This phenomenon has been a stimulus for quite a num- ber of experiments in. order to determine the cause of the superconductivity suppressing effect of PI (ibr a review, see, e.g. [1]). In the sys- tem Yl_xPr , Ba2Cu3OT-~, a previous investiga- tion studied the effect of hydrostatic pressure on the critical temperature Te of these compounds [2]. The pressure dependence dTe/dP w ~ found to evolve sign from positive to increasingly nega- tive for increasing Pr content x with a change of sign at x ~ 0.2. In contrast, dTc/dP for oxygen deficient YBa2Cu3OT-6 samples is always posi- tive with increasing values for increasing oxygen deficiency [3]. This study now combines the vari- ation of both parameters, Pr doping level x and oxygen deficiency 6, for measurements of the shift of Tc for increasing hydrostatic pressure up to 20

S ~ a i i u & t u S O ~ L ~ kbar. Samptes were prepared by " ~ _.~ I:~, state reaction as described in ref. [4]. The resis- tivity measurements under hydrostatic pressure were performed as described in ref. [2].

Fig. 1 shows the resistance vs. tempera-

*Present address: IFW Dresden e.V., Postfach, D-01171 Dresden. t Present address: Department of Physics, Western Michi- gan University, Kalamazoo, MI 49008-3899, USA

ture arou~.~.d Tc for a sample of composition Y0.~Pr0.4Ba2Cu306.~0 for 15.0 kbar pressure in 0, 5 and 85 Gauss external magnetic field. The double step transition in zero field is typical for samples with higher Pr content and lov:er oxy- gen content and is attributed to Josephson we~k links. This is affirmed by the .,suppression of tile second step for magnotic fields of 5 and 85 Gauss in fig. 1. The step in resistivity at higher temper- ature is therefore identified as the intrinsic Tc of the correspouding compound. For later analysis, we define the initial slope of To(P) at P = 0 as the derivative dTc/dP.

A summary of ~i1-11 results on dTc/dP(x,6) in- cluding data of ref. [3,5] for x = 0 is shown in fig. 2. Since we were mostly interested in sam- ples that change sign of dTc/dP from negative to positive, we measured for x = 0.1 only samples with ~ = 0, 0.1 and 0.2, which all exhibited pos- itive pressure dependence of T~. >amples with x = 0.5 allowed only for 6 = 0 and 0.1 to determine A r l ~ l A D u x c / u L •

The model function of T~(x,P) of ref. [2] can be extended to include the results of ref. [3] for dT, /dP(6) by adding An(6) and dAn/dP(6)-P in the squared bracket that describes the q'~ de- pendence on the mobile hole concentration in the CuO2 planes. For comparison with our experi- mental results, dTc/dP(x,6) for x = 0.3 and 0.4 are shown together with ti~e corresponding model

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1472 M. Budigeister et al./Physica C 235-240 (1994) 1471--1472

calculations in fig. 3 (a) and (b), respectively. For x = 0.3 the change of sign from negative to positive is shifted clearly to a higher value in 6. Additionally, the calculated dT¢/de(6) values decrease for 6 > 0.2, whereas the experimental points show a continued increase to even higher positive values, dTe/dP(~) for x = 0.4 in fig. 3 (b) seems to show the predicted peak and return to negative values, but for higher oxygen deficiencies dTe/dP is clearly positive.

In summary, these measurements of dTe/d P(x, 6) are at variance with results of a simple charge transfer model based on previous results of dTe/dP(x) and dT~/dP(6) for higher oxygen deficiencies. A possible explanation for this could be a decrease of the Pr 4f hybridization with the Cu3d-O2p conduction band for l o ~ r nmbile ]|ole concentrations. This would weaken the localiza- tion of holes in the CuO~ planes as well as the additional pair breaking according to the luodel in ref. [2].

A c k n o w l e d g e m e n t

This research was supporled by the iTS l)e- partment of Energy mt(ter Grant No. DE-FG03- 86ER45230. M. B. gratefully acknowledges a Feodor-Lynen-Fellowship of the Alexander-yon II u lnboldt-Foundat ion.

R E F E R E N C E S

1. Ii. B. l/adousky, ,l. 3later. I/,'s. 7, !!) i7, (199~).

2. M. B. Maple, L. M. Paulius and ,1. J. Neun'ieier, Physica C 195 64 (1992).

3. C. C. Ahnasan, S. 11. llan, B. \\'. Lee, L. M. Paulius, M. B. Maple, B. \V. Veal, J. W. Downey, A. P. Paulikas, Z. Fisk and J. E. Schirber, Phys. Rev. Lctt. 69 680 (1992).

4. D. Wagener, M. Buchgeister, W. Ililler, S. M. ltosseini and K. Kopitzki, Supercond. Sci. Technol. 4 $211 (1991).

5. R. Benischke, T. Weber, W. tl. l:i~tz, ,l. Met- zger, K. Grube, T. Wolf and II. Wiihl, Pl/vs- ica C 203 2!);~; (1992).

Ya.eProt4ElaiCU:lCl(uso • _ - , t . . . . , . , . i , . . . . . . . . . .

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I ' ~ - • SO . ~ r i . • 0 (1 _ ~ j

1 .0 -

0.8

0.II

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Q , 2

TemperWture IKI Figure 1. E lec t r i ca l res is tance vs. t e m p e r a t u r e for a

Y0.6Pr0 .~Ba2Cu306.50 s amp le at a p r e s s u r e of 15.0 k b a r

in ex terna l m a g n e t i c fields of O, 5 a n d 85 Gauss .

Pressure dependence of Tc of Y~.xPrxBaiCuaOr4

~,o

'

, , % 0., - ,., 0.,

Figure 2. [ ( d T c / d P ) ( x , 6 ) ] v = 0 for P r c o n c e n t r a t i o n s x =

0.1, 0.2, 0.3, 0.,1, a n d 0.5. D a t a for x = 0 are taken f rom

ref. [3,5]. T h e c o n n e c t i n g lines are gu ides to the eye.

!

1.2 .-,m.- ~¢mQrlrc~Qn'l T 4. ~Aodlel !

1.0 :\,\ o . 8 I ' " 0 4 , ~ ' ' ~ '

0 A O ~ 4

o.o ...... ,'r. ........... s ................... ~--*- .,~- 4,,~ ........

, , ! , J ! . It,. i ~ - .

0,8 (hi Y Ui'V,~.~Q tln, a,~i 0 7.,S O~ l~.q~iment

,e, Model 4b ,~, ,e, 0A o 0-~ 41

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e-~ e.4 e-~ es e.7 e.a e~ r.o ~.~

l"ig~lr,; 3. l ' ;xl)eri,i , .nt al and c;dcu-

latcd v:tl~l,.>: f,,r (,t'l ',.;qlP)(~) for Yo 7Pro-~Bw2Cu'~()7_,~

(a) and Yo c, Pre ~ B<'i-2Cu;407_~ (b). "l'ile cl lnnccting lines

are guides i,) ilia: t:y'.r.