Normal-state out-of-plane and in-plane resistivities of Y1xPrxBa2Cu3O7 (0 < x 1) single crystals
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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
~Department of Physics, Kent State University, Kent OH 44242, USA
bDepartment of Physics and Institute 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
1130 C.C Almasan et aL/Physica C 282-287 (1997) 1129-1130
I ~ ' I I I ' I Y~ Pr Ea2Cu307. ~
60(3 /~ . - * -100 I ~ x=0.42 o 500 / \~150 t~
o ~40C / ~-200 :3oo /~y-3oo~
o o 20C
. I00 ~l x=0.53 ,.x=0.22 0 0 , , . , , . I~ , ,
aaA~. ,0 0.2 0 .40 .6 0.8 I
"4~: o o ooot t~ . . . . . J x=0.55 A A a - - ~ ~
x VVvv ~a x x x x x x x : .~v ~. ,~x ; i '~'.=~(,, .x,x ~ " J
X=I _~oo o "ovo~ ~ ~, vOvO o "o"m ~' B#a~ t ,l~a~jl I , j ,x-r~3, , , " ; " ' : " r ' , ; , , , I
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).
that 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 . 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 . 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 that 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 transport 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  or the properties of the density of states of the blocking layers .
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).