Fietz, W. H.

Grube, K.

Hornung, F. W.

Ludwig, H. A.

Quenzel, R.

Schlachter, S. I.

Wolf, T.

English

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H.A. Ludwig et al.change hereby the hole concentration nh. By partly substituting Ca for Y, nh can be increased further which reduces T, in the overdoped region. lt is generally accepted that T,(nh) has a parabolic shape for most HT,SC's6, but the maximum T, is limited to different T,.max for different materials. For a given structure Tccannot only be changed by oxygen and Ca doping, but also by hydrostatic pressure which increases nh, too. In contrast to chemical doping, pressure allows to vary Tc without changing the chemical composition. Therefore, pressure experiments are a good choice to investigate the variation of T, with nh. If the nh-increase under pressure dn/dp is nearly independent of nh, dT/dp gives a measure for the derivative ofthe TJnh) curve: dTc(nh) dTc dnh dTc - = · =--const. (1) dp dnh dp dnh If T,(nh) has a parabolic shape dT/dp is proportional to nh and, therefore, a qualitative measure for the hole concentration l .7. Hydrostatic pressure experiments showed two problems with this explanation: First, dT/dp is not exactly zero for optimal doping, i.e., maximal T,, and second, dT/dp drops at x=6.7 from 7.4 K/GPa to 3 K/GPa for decreasing oxygen content x. The hydrostatic pressure effect can be obtained by the sum of the uniaxial pressure effects along the three crystal axes. Therefore, uniaxial pressure experiments allow a more detailed investigation of the changes under pressure, possibly a separation of charge transfer effects from other effects. 2. EXPERIMENTALFig. 1 shows the setup to investigate ac-susceptibility under uniaxial pressure. A bellow, loaded with Helium gas, supplies the uniaxial force, allowing a smooth pressure application to the brittle samples and pressure changes at low temperatures. The latter is important to have the possibility to avoid pressure induced oxygen ordering leading to a drastically enhanced pressure effect for oxygen deficient samples8. The pistons are kept parallel by a parallelogram spring suspension avoiding abrupt movements due to friction of the piston during pressure application. The pistons are sapphire cylinders that give no background signal. W e selected this hard material, because smooth pistons would be deformed during the experiment resulting in an uncontrolled support of the sample perpendicular to the pressure axis. In this case the sample would be exposed to a mixture of the uniaxial pressures along all three crystal axes. The ac-susceptibility is measured by a compensated pickup coil system. The change of T, under uniaxial pressure has been measured on four YBa2Cu3Ox single crystals. All samples were prepared as described in 9 using ZrO2 crucibles. Sample 1 and 2 had dimensions of 0.90*1.24*0.12 mm3 and 1.09* 1.63*0.12 mm3 with T, of 77.5 K and 87.8 K, respectively. Sample 2 74 76 78 80 T[K] 82 H.A. Ludwig et al.78.0 .------- �-- �---� � 77,8 r 77,6 77,4 � -�---�---� 84 0 00 0,05 0 10 0,15 P, [GPa] Fig. 3. ac-susceptibility of sample 1 measured at various uniaxial Fig. 4. T,.so¾ of sample 1 versus uniaxial pressure p,. pressures p,. sample. This measurement can be done with an accuracy of about ± 5% depending on the sample geometry and the quality of the surfaces. The accuracies for determining x', x", T and PHelium are better than ± 1 %. For all investigated samples we estimate the error in dT/dp to be below ± 20% indicated by the error bars in fig. 5. Fig. 5 shows dT/dp, values in dependence on the initial T,(p=0) of the samples. The black solid squares show the results of our measurements. For comparison, values calculated from • high-resolution thermal expansion measurements3.4 and values from uniaxial pressure experiments5 are shown. Fig. 5 also includes dT/dp values from hydrostatic pressure experiments 13 • 14 represented by the crosses. The dashed line is a guide for the eye for the hydrostatic data. The origin ofthe solid curve will be discussed later. 4. DISCUSSIONThe charge transfer picture assumes that holes from the CuO-chains are transferred to the CuO2-planes2 under pressure. Usually, the position of the apical oxygen atom and the position of the Ba ion are thought to be responsible for the amount of transferred holes. Unfortunately, there are no experimental data available monitoring the atomic positions under uniaxial pressure. Therefore, an ionic model has been calculated to get a qualitative estimation of the change of the atomic positions under uniaxial pressure11 . This model predicts a strong shift ofthe apical oxygen atom towards the CuO2-planes and ofthe Ba ion towards the CuO-chains for pressure along the c-axis, resulting in a very effective charge transfer. Assuming T,(nh) being ofparabolic shape and dn/dp, being nearly constant for all oxygen contents we get for dT/dp, dTc = J Tc,max - Tc(p = 0) + a Tc(p = 0) .dp, Y Tc,max (2)H.A. Ludwig et al.5. CONCLUSIONSAn experimental setup has been built to detect the Tc vanat10n under uniaxial pressure by ac-susceptibility measurements. T, of several YBa2Cupxsingle crystals with different oxygen contents has been investigated under uniaxial pressure along the c-axis. The effect ofuniaxial pressure along the c-axis can be described within the charge transfer model which is consistent with hydrostatic pressure data12 for overdoped samples. Hydrostatic pressure data 13• 14 ofunderdoped samples do not fit to this model. From this we conclude that in the underdoped region only a part of the hydrostatic pressure effect, namely uniaxial pressure in c-axis direction, can be explained by charge transfer. The remaining part has to be ascribed to uniaxial pressure effects along the a- and b-axis with an increasing contribution to the hydrostatic pressure effect for decreasing oxygen content. Further investigations are necessary to find out, whether there is a change in the c-axis pressure effect at the Ortho I / Ortho II transition at x=6. 7. REFERENCES 1. T. Weber, diploma thesis Universität Karlsruhe ( 1993 ).2. J.D. Jorgensen, S. Pei, P. Lightfoot, D.G. Hinks, B.W. Veal, B. Dabrowski, A.P. Paulikas,R. Kleb, Physica C 171, 93 (1990).3. 0. Kraut, C. Meingast, G. Bräuchle, H. Claus, A. Erb, G. Müller Vogt, H. Wühl, PhysicaC 205(1/2), 139 (1993).4. C. Meingast, T. Wolf, M. Kläser, G. Müller Vogt, this conference. 5. U. Welp, M. Grimsditch, S. Fleshler, W. Nessler, B.W. Veal and G.W. Crabtree, Journal ofSuperconductivity, 7(1 ), 159 (1994 ).6. J.L. Talion, C. Bernhard, H. Shaked, R.L. Hitterman and J.D. Jorgensen, Phys. Rev.B51,12911 (1995).7. J.J. Neumeier and H.A. Zimmermann, Phys. Rev. B 47, 8385 (1993 ).8. W.H. Fietz, R. Quenzel, K. Grube, J. Metzger, H.A. Ludwig, Physica C 235 240 (1994) 1785.9. Th. Wolf, W. Goldacker, B. Obst, G. Roth, R. Flükinger, J. Crystal Growth 96, 1010 ( 1989).10. T.B. Lindemer, J.F. Hunley, J.E. Gates, A.L. Sutton Jr., J. Brynestad, C.R. Hubbard, J. AmCeram. Soc. 72, 1775 (1989). 11. H.A. Ludwig, W.H. Fietz, H. Wühl, Physica C 197, 113 (1992).12. S.l. Schlachter, H.A. Ludwig, K. Grube, F.W. Hornung, W.H. Fietz, B. Obst, P. Schweiss, T. Wolf, M. Kläser, G. Müller Vogt, this conference.13. R. Benischke, T. Weber, W.H. Fietz, J. Metzger, K. Grube, T. Wolf, H. Wühl, Physica C 203, 293 (1992). 14. R. Quenzel, diploma thesis Universität Karlsruhe (1995).

Hydrostatic and uniaxial pressure effect on Tsub(c) of YBa₂Cu₃Oₓ

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Hydrostatic and uniaxial pressure effect on Tsub(c) of YBa₂Cu₃Oₓ

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