EVIDENCE FOR ELECTRONIC LOCALIZATION IN YBA2-YLAYCU3OX

Abstract Electrical resistivity, thermoelectric power, magnetic susceptibility and neutron scattering experiments on YBa 2− y La y Cu 3 O x , with y = 1, show that this compound is neither superconductive nor magnetically ordered but are consistent with it being an electronically localized system.

In efforts to understand the physics of the RBa2Cu3Ox high temperature superconductors, substitutlonal studies have concentrated on interchanging rare earth R atoms [ 1 ] and replacing some of the Cu atoms with various transition elements [2]. Although these studies have proven useful in revealing aspects important for an understanding of high temperature superconductivity, surprisingly httle efford [ 3 ] has been given to investigating the role that the Ba sites play m these materials. Because trlvalent La is nearly the same s~ze as divalent Ba, it is possible to prepare RBa2_ vLavCu30~ with y at least equal to 1. With thts substitution the nominal valence of Cu is reduced from 2.3 (for y=0) to 2.0 (for y= 1 ), assuming that the oxygen content x~ 7 in both cases. Here we report characteristics of YBaLaCu3Ox that are distinctively different than those of YBa2Cu30~ and suggest a substituuonally-lnduced transition to a localized state.
Polycrystalhne samples of YBaLaCu3Ox were prepared using standard ceramic techniques with starting materials ofY203, BaCO3, La203, and CuO. The starting materials were thoroughly ground together, calcined m air, reground, fired at 1080 °C in oxygen and slowly cooled to 400°C before being removed from the furnace. X-ray analysis of the resulting pellets showed a dominant (<90%) orthorhombic phase, characteristic of RBa2Cu3Ox materials, with lattice parameters a=3.840 /k, b=3.856 /k and c= 11.55 A. These values, compared to a= 3.8203/~, b= 3.8855 A and c= 11.6835/k for YBa2Cu3069 [4], indicate that La addition reduces the orthorhombl-tlClty. The presence of a minority ( > 5%) phase that could be indexed to Y2Cu205 and at least one additional unidentified phase (<5%) also was detected. The oxygen content of these samples was not determined, although a similar preparation procedure is known to give x~ 7 in YBa2Cu3Ox. In this regard, however, a crude analysis of the c-lattice parameter change with La substitution in terms of observations by Segre et al. [3 ] for LaBa2_xLa~Cu3OT+o suggests that our sample may be slightly oxygen deficient. Sections from the pellets were used for electrical resistivity, thermoelectric power and magnetic susceptibility measurements. Several large pellets, combined weight ~ 50 g, were powdered for mag-neUc neutron scattering experiments at the National Bureau of Standards reactor. The susceptlblhty of random pieces from the neutron samples all showed the same temperature dependence.
The inverse magnetic susceptibility 1/Z is shown in fig. 1 as a function of temperature. The Curie-Weiss behavior and maximum in Z at 12 K (see inset) suggest the possibility of antlferromagnetlc order. Neutron diffraction at low temperatures failed to reveal any evidence for an antiferromagnetic transmon in the majority phase YBaLaCu3Ox. However, clear evidence for antiferromagnetic order at 12 K was found in the second phase Y2Cu2Os. Recent suscepubdity measurements on this compound by Troc et al. [ 5 ] show a maximum m X at 13 K and at higher temperatures Z(T) oc/t2rf/( T-38) where /zefr= 2.81 #B/formula unit. An analysis of the data in fig. 1 in light of their observations leads to the conclusion that 8.5% of the mass of our sample is Y2Cu205 and that the remaining fraction has a temperature Independent, or at most a weakly temperature dependent, susceptibility of magnitude Zo ~ 1.6 X 10-7 emu/g. Results of this one-adj ustableparameter analysis are shown by the solid line in fig.  1 and by the square in the inset, which are in good agreement with the data and with the estimate obtained by X-ray analysis of the fractional amount of Y2Cu205 present. Assuming that Zo is due to a Pauhlike contribution from YBaLaCu3Ox implies that the effective density-of-states at the Fermi energy in YBaLaCu3Ox is about 1/4 that of the high-To superconductor YBa2Cu30~ [ 6 ].
The electrical resistance increases monotonically with decreasing temperature and shows no evidence for a superconducting transition above 4 K or for magnetic ordering in Y2Cu2Os. In fig. 2 we plot the logarithm of the resistance as a function of T-~/4 The linear variation from 9< T< 125 K covers over three orders-of-magnitude increase in resistance and is highly suggestive of transport by three-dimensional (3D) variable-range hopping [7]. Taking In Roc (To~T) ~/4 yields a characteristic temperature To~ 5 × l05 K, which is comparable in magnitude to values of To found [8] in oxygen-deficient poly- F~g. 2. Reststance of YBaLaCu3Ox on a logarithmic scale as a function of temperature to the -1/4 power crystalline samples of La~ 8Sro 2CUO4_ ~. Such behavior is contradistinctive to the linearly temperature dependent resistance observed in polycrystalline YBa2Cu307 and is indicative of the significant role that Ba atoms play in the conduction process. Although we emphasize that the precise temperature dependence may be influenced by unknown anisotropy effects [9], YBaLaCu3Ox is not unique in displaying localized-like transport: the resistivity in the Cu-O planes of single crystalline La2CuO4 is that expected of correlated 2D or 3D variable-range hopping [ 9]. For non-superconducting RBa2Cu3Ox samples in which x~<6.2 or in which 5 at% Zn has been substituted for Cu, one finds respectively variable-range hopping [10] and weak-3D localizationtype conductivitles [ 11 ].
The thermoelectric power S ( fig. 3) of this ma- The reset shows that S is a hnear function of T TM for the temperature interval 25 < T< 220 K terial is large and positive, which m a single-band model implies that entropy transport is dominated by a small number of hole-hke carriers. The room temperature value of S is nearly 50 Umes larger than In RBa2Cu307 [6] and comparable to that found in oxygen-deficient samples of this material [ 10]. Unlike the nearly temperature independent thermopower of polycrystalline high-Tc compounds [ 6 ], we find Soc T 1/4 for 25 < T<220 K, roughly the same temperature interval over which in Roc T-i/4. For 3D variable-range hopping, S should be proportional to T 1/2 [ 7,12 ], clearly not the temperature dependence observed here. However, as m the case of the electrical resistance, anlsotropy effects could distort the intrinsic temperature dependence.
Although conductivity in the high-To superconductors is generally regarded as being dominated by the Cu-O planes/chains, it is apparant from these results that changes in valence and spatial order of the Ba-slte atoms can alter significantly the transport characteristics. In all cases of which we are aware, the approach to a localized-like state (at the expense of superconductiwty) is accompanied by a decrease in the state-density at the Fermi level, whether it be produced by oxygen depletion [ 10 ], Cu substitution [ 1 l] or, in this case, a replacement for Ba, as well as the introduction of disorder on one of these s~tes. Clearly, these observations should be considered in attempts to raise Tc above 100 K.