PHYSICA ELSEVIER Physica C 282-287 (1997) 1129-1130

Normal-State Out-of-Plane and In-Plane Resistivities of Yl_xPrxBa2Cu307- (0 < x G 1) Single Crystals. *

C. C. Almasan, a G. A. Levin, a C. N. Jiang, a T. Stein, ~ D. A. Gajewski, b S. H. Han, b and M. B. Maple b

~Depar tment of Physics, Kent State University, Kent OH 44242, USA

bDepar tment of Physics and Inst i tute for Pure and Applied Physical Sciences, University of California at San Diego, La Jolla, CA 92093, USA

Out-of-plane resistivity and anisotropy of fully-oxygenated YI-xPr=Ba2Cu307-e (0 < x <_ 1) single crystals demonstrate a unique non-monotonic variation with Pr concentration x. The strongly temperature-dependent anisotropy reaches a maximum at x ~ 0.4 (T~ ~ 45 K), and decreases for both higher and lower T(: samples.

The number of charge carriers in bilayer cuprates can be varied either by chang- ing the oxygen content of the blocking lay- ers (e.g., YBa2Cu3Ou, Bi2Sr2CaCu2Os+~), or by elemental substi tutions inside the bi- layers (as in Bi2Sr2Cal-~YxCu208+6 or in Yl_=PrzBa2Cu307-~). I t is well established tha t the in-plane resistivity Pab decreases mono- tonically (at a given tempera ture T) from an un- doped insulator to overdoped metal, regardless of the type of doping. This is because the in-plane t ranspor t is determined only by the number of carriers in the Cu02 planes. The way in which hole doping occurs has little influence on the mag- nitude of flab and its t empera tu re dependence as long as the Cu02 planes remain structurally un- damaged [1].

On the other hand, the t ranspor t in the c- direction is determined by both the condition of the Cu02 planes and blocking layers. There- fore, the evolution of Pc in YBa2Cu30y or Bi2Sr2CaCu2Os+~ reflects a mixture of two phe- nomena: the t ransformat ion of the Cu02 planes (a fundamental ly impor tan t process, relevant to the understanding of superconductivity) and the introduction of oxygen vacancies in the blocking layers. Given the very impor tant role of the c- axis t ranspor t in the overall picture of the normal

*The work at KSU was supported by DOE Grant No. DE-FG02-90ER45427 through Midwest Superconductiv- ity Consortium and by NSF Grant No. DMR-9601839, and at UCSD by DOE Grant No. DE-FG03-86ER-45230.

0921-4534/97/$17.00 © Elsevier Science B.V All rights reserved. PII S0921-4534(97)00689-8

state of the cuprates, it is necessary to investigate how Pc evolves in systems in which the composi- tion of the blocking layers is maintained constant while the Cu02 planes evolve under tile influence of the "internal charge reservoir".

The investigation of the c-axis resistivity of Yl-xPrxBa2Cu307-~ and similar cuprates has been hampered by the unavailability of large single crystals suitable for measurements by a conventional four-point method. We have over- come this problem by using the six-terminal tech- nique and developing an algorithm for calcu- lating Pc and Pab from measured voltages [2,3]. We had presented in Ref. 2 the data on Pc and Pab of fully-oxygenated single crystals of Yl-xPr~Ba2Cu307-~. Here we discuss a few more aspects of the same data.

With increasing Pr concentration, Tc decreases [4] while P~b monotonically increases; this com- pound becomes nonsuperconducting at x ~ 0.55 and PrBa2Cu307-~ is an insulator [3]. This evolution is similar to tha t of oxygen deficient YBa2Cu30~, but the important difference is tha t the CuO chains of Yl-zPrzBa2Cu307-~ re- main fully formed. The T dependence of Pc/Pab is a sensitive probe of the type of the ground state since Pc/Pab ~ cons$ in anisotropic three- dimensional (3-D) Fermi liquids. Figure 1 and the inset are plots of Pc/Pab VS T and Pr concen- tration, respectively, of six Yl-zPrzBa2Cu307-~ crystals. There are two aspects of these da ta

1130 C.C Almasan et aL/Physica C 282-287 (1997) 1129-1130

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50 1 O0 150 200 250 300 T (K)

Figure 1. The anisotropy Pc/Pab of single crystals of YI-=Pr=Ba2Cu307-6 vs temperature T and Pr concentration x (inset).

tha t distinguish them from those of conven- tional anisotropic metals or oxygen deficient YBa2Cu30 u. First, Pc/Pab is strongly temper- ature dependent and shows no sign of saturation at low T's, especially in moderately-doped sam- ples. Second, Pc/Pab and Pc change nonmonoton- ically with Pr content (or with T¢) while P~b in- creases monotonically with x [2]. In contrast, the anisotropy of oxygen deficient YBa2Cu30y sam- ples with similar Tc's is much greater than that of Y1- z Pr= Ba2 Cu307- ~, and increases monoton- ically with decreasing Tc [5]. The inset shows that p¢/p~b(T, x) changes strongly only within a nar- row range of Pr concentrations.

The T-dependence of Pc/Pab is the strongest in the x = 0.42 sample (To ~ 45K), which is also the most anisotropic. The same sample demon- strates a coexistence of metallic Pab and semicon- ducting Pc- The anisotropy is appreciably lower for both higher and lower T~ samples with insu- lating PrBa2Cu307-~ being about as anisotropic as the high-T¢ x = 0.13 sample. The characteris- tic energy E = dln(pc/Pab)/d(1/T), which deter- mines the T dependence of Pc/Pab, has roughly the same behavior as a function of Pr concentra-

tion as Pc/Pab itself (shown in the inset). A comparison of these results with the data on

YBa2Cu30y clearly demonstrates that the out- of-plane transport in cuprates can not be under- stood within the framework of a model which is only concerned with the properties of the Cu02 planes and ignores material aspects, such as the composition of the blocking layers and the type of doping. Our results also cast doubt on the be- lief that the evolution of Pc is intimately related to that of superconductivity; it appears tha t the values of Tc are much more strongly correlated with the in-plane resistivity than the out-of-plane resistivity. Also, these data do not support the idea that increasing disorder is responsible for increased anisotropy in .underdoped systems; if the concentration of Pr is a measure of disorder, the x = 0.53 and 0.55 samples should be more anisotropic than the x = 0.42 sample, which, ob- viously, is not the case. The different tempera- ture dependences of Pc and pab which lead to a T-dependent anisotropy exclude 3-D anisotropic transport as well.

All these features can be understood, at least qualitatively, if the c-direction t ransport is de- termined by tunneling with the main contribu- tion coming from electrons with energies different from the Fermi energy by more than kBT (in the tails of the Fermi distribution). This may be due to either the presence of resonant tunneling cen- ters [6] or the properties of the density of states of the blocking layers [2].

R E F E R E N C E S

1. The "damaging" types of doping include irra- diation which causes atom displacements and elemental substitutions for Cu on the planes.

2. C. N. Jiang et al., Phys. Rev. B 55, R3390 (1997).

3. G . A . Levin et al., Physica C: these proceed- ings and references therein.

4. M. B. Maple et al., J. Superconductivity 7, 97 (1994).

5. K. Takenaka et al., Phys. Rev. B 50, 6534 (1994).

6. A. A. Abrikosov, Phys. Rev. B 52, R7026 (1995).