Physica C 153-155 (1988) 984-985 North-Holland, Amsterdam

EVIDENCE FOR TWIN PLANE SUPERCONDUCTIVITY IN SMALL PARTICLES OF YBa2Cu307_

Sebastian VIEIRA(#), S.A. SOLIN, NicolAs GARClA(*), Mariano HORTAL(*) and Ana AGUILO(*).

Department of Physics and Astronomy and Center for Fundamental Materials Research, Michigan State University, E. Lansing, MI, USA and (*) Dpto. de Materia Condensada, C- I l l , Universidad Aut6noma de Madrid, Cantoblanco, 2804g-Madrid, Spain.

We have performed magnetic orientation experiments with small grains (d < 2ON) of YBa2Cu307-~, below T c. Our results of magnetic susceptiblity and levi tat ion, can not be explained with the assumption of bulk anisotropic superconductivity; however, they can on the hypothesis that the twin boundary planes carry supercurrents.

The physical understanding of recent ly discovered High Temperature superconduct iv i ty (HTS) (1,2) is a matter of controversy between two explanat ions consider ing the character of the superconducting currents e i t he r bulk (3) or i n t e r f a c i a l (4,5) (non-bulk) super- conduc t i v i t y .

For these experiments, quasihomogeneous powders of YIBa2Cu307-~, 8~0, were synthesized by the usual method (2) and sieved, without any further sintering.

In a previous paper (6) we have studied the f ie ld orientation of small grains (D < 5N) and obtained evidence by means of X-ray studies that both the twin planes (TP) and the basal (ab-CuO) planes carry supercurrents. We found that some 60% of the (110) axes of small grains orient at T < T c under small applied f ields H=O.ST while for T > T c this does not occur. On the other hand (7), levitat ion experiments show that the force of a magnetic gradient on small grains (D < 2ON) is smaller than the force on large grains (D > 4ON) or on a pel let made from smal l grains s l ight ly compacted without sintering nor chemical alteration.

In order to check the pos ib i l i ty of bulk anisotropic superconduction (BAS) in the sp i r i t of the Ginzburg-Landau theory (GLT) we have performed quantitative levi tat ion and magnetic susceptibi l i ty measurements. In the f i r s t we measure the levi tat ion height as a function of temperature under a characterized horizontal magnetic f ie ld Hy(z! with a vertical gradient ?zHy(z) where z IS the vertical coordinate, i .e. the direction antiparallel to the gravitational f ie ld g.

As can easily be shown (7) the magnetic force acting on the particles is

Fz(z)= [V/4~ Hy(z)] @Hy(z)/~z [1]

and the l e v i t a t i o n cond i t ion for a gra in of volume V is that

(#) Permanent address: same as (*)

Fz(Z)=(P-PLN)gV [2]

where p = 6.38 g/cc is the mass dens i ty of YBa2Cu307_ 6 and p = 0.6 g/cc the dens i ty of l i q u i d n i t rogen.

One can def ine a magnetic dens i ty

Pmag = Fz(z)/(Vg ) [3]

and so the p a r t i c l e w i l l l e v i t a t e at a heigh where (Pmag(Z))max = P -PLN.

When the app l ied magn@tic f i e l d exceeds the lower c r i t i c a l f i e l d Hcl J along on or more of the p r i nc ipa l axes, j , of the p a r t i c l e , i t enters the vor tex s tate and we must replace ,(7) the bracketed term in equat ion [1] by V/8~ Hc l .

The lower c r i t i c a l f i e l d fo r YBaCuO is H~I known to be p a r a l l e l to the basal ab plane. Using H~I(77K ) = 90±8G, computed from the value Hcl(5K) = 300 ± 25G at which H (T=5K) of a random or or ien ted powder departs from constancy ( f i g . I ) , and the value of the magnetic g rad ient we use, namely ~zHy)max = 6.5 ± 0.8 KG/cm one obtains (Pmag)max about 4 times the mass densi ty of the p a r t i c l e s and there fore they should l e v i t a t e , however they do not.

By reducing the temperature s l i g h t l y we f i nd tha t , at 74 ± 2K, the grains s t a r t l e v i t a t i n g . We can understand these resu l t s i f on ly a small f r a c t i o n (10 to 20%) of the bulk of the mater ia l is superconducting.

To fu r the r confirm th i s view, we have performed s u s c e p t i b i l i t y measurements as a func t ion of temperature and, fo r a given T, as a func t ion of appl ied f i e l d H. These measurements have been made on both random and oriented powders. The orientation was achieved by applying a magnetic f ie ld of 5KG at T=5K during some 3 minutes while the sample was mechanically oscilated to ease orientation.

Fig. 2 shows values of× for f ie ld cooling and zero-f ield cooling, for oriented and random small grains. The decrease in x for f ie ld cooling is very drastic when the grains are

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S. Vieira et aL / Evidence for twin plane superconductivity 985

oriented, i t s value being of some 10% of the maximum value i f one asumes that only 60% of the grains are oriented as suggested by the X-rays measurements. Notice that for the random powder in ze ro - f ie ld cooling, the low temperature values of x are near the maximum due to the fact that the non-oriented grains almost f u l l y screen the f i e l d , t h i s not implying that the whole volume is superconducting.

Fig. 1 shows the value of x(H) for T=5K for random and oriented powders. The response is almost f u l l screening up to approximately 300 G, where the curve departs from constancy. This value is the c r i t i c a l f i e l d para l le l to (a,b) planes Hc I because starts weakening at the smaller c r i t i c a l f i e l d response. This is confirmed by X(H) for the oriented powder in the same f igure where th is function is seen to remain constant also up to 300G.

The c r i t i c a l f i e l d , nevertheless, is very much dependent on the d is t r ibu t ion of the oxigen contents. We have measured x(H) at T=5K in two samples (random and oriented) of powder (D < 2ON) obtained by grounding a piece of material which was quenched in a i r a f ter the sinthesis at 950°C. These samples showed negative suscep t i b i l i t y up to 9OK. Their lower c r i t i c a l f i e l d , derived from a p lot s imi lar to Fig 2, is some IOOG.

In f ig . 1 we can again observe the drast ic e f fect of the or ienta t ion in reducing the absolute value of the suscep t ib i l i t y when a f ract ion of the grains or ient with an external f i e l d .

The dramatic drop in Meissner e f fect for the oriented materials and the changes of suscep t ib i l i t y shown in f igs 1 and 2, together with the l ev i t a t i on results consti tutes a mass of data that cannot be j o i n t l y explained using the BAS model. We think that , i f the super- currents are p a r t i a l l y local ized on the TP's, or on other slabs of ma te r i a l~ i cha l i gn with the or ient ing f i e l d , in a s imi lar way as observed for the TP's, the magnetic f lux w i l l penetrate between the superconducting zones in the oriented s i tua t ion . However, these planes w i l l screen the external magnetic f i e l d when in the random state.

ACKNOWLEDGEMENTS The thank M. Pazos and J. Eglin for

technical assistence. One of us (S.A,S.) acknowledge support by the US NSF and NASA.

(6) S.A. Solin (1988) 744

(7) S. V ie i ra (submitted).

et a l . , Phys. Rev. Lett 60

et a l . , Phys. Rev. Lett

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REFERENCES (1) G. Berdnoz and K.A. MUller, Z. Phys. B64 FIGURE 2

(1986) 189. The temperature dependence of the mass (2) M.K. Wu et a l . , Phys. Rev. Lett. 58 suscep t i b i l i t y of YBa2Cu307~ powder (d <

(1987) 908 20Nm), in a random (circ leS) and a f i e l d (3) K.A. MUller et a l . , Physica 1488(1987) oriented conf igurat ion (squares). Open

149. (so l id) simbols represent f i e l d cooling at (4) N. Garcfa et a l . , Z. Phys. B70 (1988) 9. 18G (zero f i e l d cooling fol lowed by f i e l d (5) M.V. Fang et a l . , Phys. Rev. ( in press), heating at 25G).