Prueba de Hipótesis General

V arious n e u t ro n - s c a tte r in g te c h n iq u e s have b e e n used to inve stiga te the s u p e rc o n d u c tin g and m a g n e ti c p r o p e r tie s o f b o t h c o n v e n tio n a l and high-Tc s u p e r c o n d u c t o r s .

In our s tu d ie s o f t h e m a g n e ti c o r d e r in a sin g l e c r y s ta l o f non-

s u p e r c o n d u c t i n g P r B a 2C u 30 7. 5> in three different oxygenation states 8 ,, 8 2 and

8 3 (where 8 , < 8 2 < 8 3), the Cu spins in the C u 0 2 planes were found to be a n tiferro m a g n etica lly o r d e r e d at ro o m te m p e r a tu re with the individual spins lying in the a - b plane. For the 8 2 = 0.27 state, the only state which 8 was

d e t e r m i n e d p r e c i s e l y , th e o r d e r i n g t e m p e r a t u r e , T N 1 , o f this high-

t e m p e r a t u r e Cu o r d e r e d p h a s e w as e x t r a p o l a t e d fro m the te m p e r a tu re

dependence o f a magnetic Bragg peak to be (400 ± 25) K and the saturated Cu moment was found to be (0.55 ± 0.04) p.B. A lthough accurate determination o f

T N , was n o t p o s s i b l e d u e to the u p p e r t e m p e r a t u r e li m i t o f the

c r y o r e f r i g e r a t o r , T N1 a p p e a re d to be rath er in s e n s iti v e to changes in the

o x y g en co n c en tra tio n o f the c ry sta l. A s im ila r i n s e n s iti v ity o f T N, to the

o x y g e n d e f ic ie n c y 8 is ob se rv e d in the s u p e r c o n d u c ti n g m em bers o f the

R e B a 2C u 30 7.6 family o f compounds.

At a lo w e r t e m p e r a t u r e , T N2, the Cu m a g n e t i c m o m e n ts also order

a n t if e r ro m a g n e tic a lly a l o n g the Cu ch a in s in tr o d u c i n g fu rth e r com plication

into the spin c o n f ig u r a tio n . The spin c o n f ig u r a t io n o f this low-tem peratu re Cu ordered phase co n s ists o f a non-collinear ar r a n g e m e n t o f spins along the c direction. The planar C u moments either side o f a chain Cu ion are each rotated at an angle, the s a m e in magnitude but op p o site in sense, to the chain Cu moment. For the 8 2 = 0.73 state, at 2K, the p la n ar and chain Cu moments were found to be (0.54 ± 0.06) p B and (0.25 ± 0.03) | i B respectively and orientated at a relative angle of 23° ± 2°. In contrast to T NI, T N2 was found to be strongly depe ndent on the o x y gena tion state o f the crystal, falling from (40 ± 3) K to

(11 ± 1) K with an increase in oxygen concentration o f the crystal from 8, to 52 = 0.73.

A d d itio n a lly , at low t e m p e r a t u r e s , below 20 K , d if fu s e s c a tt e r in g was

o b s e r v e d w hich can be a ttr ib u te d to tw o - d im e n s io n a l and three -d im en sio n al short-range correlations o f the Pr spins. For the 8 2 = 0.73 crystal at 2K, the Pr

s p i n s w e re fo u n d to be a n t i f e r r o m a g n e t i c a l l y o r d e r e d , w ith th e spin

d irec tio n s along the c direction, o v e r a correlation length o f - 2 0 0 reciprocal la ttic e spacings in the a - b p la n e and a correlation length o f < 2 in the c direction. At no temperature, d o w n to the expe rim ental limit o f 2K, was any lo n g -ra n g e magnetic ordering o f th e Pr spins obse rve d. The o nse t o f diffuse scattering at such a high te m p eratu re coupled with th e absence o f long range m a g n e ti c o rd e r is u n c h a r a c t e r is ti c o f typic al q u a s i-tw o - d im e n s io n a l m agnets a n d the failure o f the Pr spins to order may be explained due to disorder in t h e crystal. Disorder also pro v id es a m echanism for the failure of the low- te m p e r a tu re Cu ordered phase to form a sim ple c o llin e a r spin config uratio n a t te m p eratu re s near OK, u n lik e the sa m e p hase in other R e B a 2C u 30 7. 8 com pounds. Possible sources o f disorder may be im purities, e.g. A1 present due to the growth of the crystal in an A120 3 cruc ible, a n d su b stitu tio n a l defects, e .g . the presence o f Pr on the B a site. I f the crystal was o f a higher quality a n d behaved in a sim ilar m a nner to othe r qu as i-tw o -d im en sio n a l magnets we w o u ld expect the Pr spins to o rd e r in the vicinity o f 10K. This would be an o r d e r o f m agnitude higher than w ould be expected i f one scaled T N for the o t h e r rare earths and would in d ic a te a stronger in te ractio n b etw e en the Pr sp in s . A mechanism for this in teraction could be th e hybrid isation o f the Pr 4 f electrons with the conduction electrons o f the C u 0 2 plane.

Analysis o f the induced m agnetisation density distribution in PrB a2C u 30 6 73 r e v e a le d a strongly aspherical m a g n e tis a tio n d e n s ity d istrib u tio n around the P r site. The magnetisation density was found to be ex tended towards the Cu

sites in the C u 0 2 planes. O ne cause o f this d ev iatio n from a spherical free ion d is trib u tio n m ay be again th e h y b rid isa tio n o f th e Pr 4 f elec tro n s with th e c o n d u c tio n elec tro n s o f the C u 0 2 plane. T he re su lts o f this experim ent sh o u ld n o t be o v e rly in te rp re ted as th e m a g n etisatio n d e n s ity d istrib u tio n may be sim ila r to th a t for a su p e rc o n d u c tin g com pound o f the sam e fam ily. T h is

th e re fo r e w o u ld m ean th a t o u r a ssu m p tio n th a t th e m a g n e tisa tio n d e n s ity

su rro u n d in g th e Re site, in R e B a2C u 30 7> w ould n o rm ally be ro ughly sp h erical

in n a tu re w as in c o rrec t. T h e c o n se q u e n c e s o f th e o b se rv e d m a g n e tisa tio n

d e n s ity d is trib u tio n aro u n d th e Pr s ite co u ld b e c la rifie d if a s im ila r

e x p e rim e n t w as perform ed on a superconducting com pound, e.g. N dB a2C u 30 7, w hich has sim ilar structural param eters to P rB a2C u 30 7.

A n a ly s is o f the induced m a g n e tisa tio n den sity d istrib u tio n also revealed a siz e a b le m om ent on the Ba site . This m ay be ex p lain ed by the presence o f a sm a ll p e rc e n ta g e o f P r3* io n s upon this site. It h a s been p rev io u sly show n th at the R e3* ions may be d elib erately substituted on this site and hence due to th e sim ila r radii o f P r3* and B a2* ions it would seem highly feasible that P r 3 * io n s m ay be u n intentionally p rese n t on th e Ba site. If we assum e the Pr and Ba site s are sim ilar in respect to the m agnetic m om ent which may be induced

u p o n th e se site s then from o u r in itia l a n a ly sis, assum ing sp h e ric al fo rm

factors, (7.8 ± 0.03) % o f Pr w as found to substituted on to the Ba site. (Note: the p re se n c e o f th is am ount o f P r on the Ba site and the subsequent additional p o s itiv e ch a rg e is not th o u g h t to play a sig n ific a n t role in the absence o f s u p e rc o n d u c tiv ity in P rB a2C u 30 7.) A dditio n ally sm a ll m agnetic m om ents w ere fo u n d to be p rese n t on the tw o Cu site s and in our sph erical form fa c to r

a p p ro a c h to m o d e llin g th e m a g n e tisa tio n d e n s ity d is trib u tio n th e in d u c ed

m om ents on the Pr, C u ( l) and C u(2) sites were found to be (0.34 ± 0.01) | i B, (0.024 ± 0.003) p B and (0.006 ± 0.004) p B respectively.

P o la rise d -n e u tro n refle c tro m e try has proved to be an accurate tool in the in v e stig a tio n o f the m ag n etic induction pro files o f Pb film s in both the bulk

and su rfa c e su p e rc o n d u c tin g sta te s. In the b u lk su p e rc o n d u c tin g sta te th e

o b se rv e d sp in -d e p e n d e n t re fle c tiv ity p ro file s can be a c c u ra te ly d escrib ed by an e x p o n e n tia l m agnetic induction d ecay w ith a p e n e tra tio n depth o f (390 ±

10) A, as p re d ic te d b y L o n d o n s' th e o ry . N o d e v ia tio n s from a p u re ly

e x p o n e n tia l d ec ay , as p re d ic te d to o cc u r at a depth o f several p e n e tra tio n d e p th s in type I su p e rc o n d u cto rs due to the n o n -lo cal re la tio n sh ip betw een

th e in d u c e d c u r r e n t an d field, w ere o b se rv e d d u e to th e s e n s itiv ity o f

p o la r is e d - n e u tr o n r e f le c tr o m e tr y to o n ly s u r fa c e e f f e c ts . T h e o b s e rv e d

p e n e tra tio n d ep th and th e related L o n d o n s' p en e tratio n d ep th , \ L(0), (390 ±

10) A and (280 ± 10) A re sp e c tiv e ly , are re la tiv e ly sm all co m p ared to

p re v io u s ly re p o rte d v alu e s but are in very good a g re e m e n t w ith th e o re tic al

c a lc u la tio n s b ased upon a stro n g -co u p lin g in te ra c tio n , (S w ilh a rt and S haw .

1 9 7 2 ).

In th e su rface su p e rc o n d u ctin g reg io n it w as im p o ssib le to m odel all th e

s p in - d e p e n d e n t r e f le c tiv ity p ro file s u sin g G in s b u rg -L a n d a u th e o ry w ith a

s in g le se t o f c o n s ta n t p a ra m e te rs th e G in s b u r g -L a n d a u p a ra m e te r, th e

p e n e tra tio n d ep th and th e low er and u p p er c ritic a l a p p lie d fields for th e

s u r f a c e s u p e rc o n d u c tin g sta te , K ,A ,p, Hc and Hcj re s p e c tiv e ly . By sim p ly

a llo w in g K to increase w ith the applied field, from 0.497 at H c to 0.56 at Hc3, we

w ere a b le to ac cu rately m odel all the m agnetic induction profiles betw een Hc and H c3. Only near to Hc3 was K found to increase rapidly and for the m ajority

o f th e s u rfa c e s u p e rc o n d u c tin g re g io n , aw ay from H c3, th e m a g n e tic

in d u c tio n p r o file s co u ld be a c c u ra te ly d e s c rib e d u s in g G in sb u rg -L a n d a u

theory w ith K = 0.501 ± 0.0 0 4 . The observed critical fields Hc and Hc3, (6.13 ± 0.15) x 104 A m '1 and (8.12 ± 0.25) x 104 A m '1 resp e ctiv ely , agreed w ell w ith p re v io u sly p u b lish ed re su lts for Pb.