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Nuclear relaxation and Knight shift studies of63Cu in 90 K-and 60 K-class YBa2Cu3O7−y
Physica C 153-155 (1988) 83 86 North-Holland, Amsterdam
NUCLEAR RELAXATION AND KNIGHT SHIFT STUDIES OF 63Cu IN
90 E-AND 60 E-CLASS YBa2Cu3OT_y
Y. KITAOKA, S. HIRAMATSU, Y. KOHORI, K. ISHIDA, T. KONDO, H. SHIBAI, K. ASAYAMA, H. TAKAG[ ~, S. UCHIDA*, H. IWABUCH[ ~ and S. TANAKA *~
D e p a r t m e n t of M a t e r i a l P h y s i c s , 560. J a p a n *Engineering Research Institute, U n i v e r s i t y of Tokyo, B u n k y o - k u ,
Faculty
of E n g i n e e r i n g
Science,
Osaka U n i v e r s i t y ,
and * * D e p a r t m e n t of A p p l i e d P h y s i c s , Tokyo 113, J a p a n .
Faculty
Toyonaka,
Osaka
of E n g i n e e r i n g ,
The n u c l e a r r e t a x a t l o n r a t e ( I / T 1) and K n i g h t s h i f t f o r Cu of YBa2Cu307-y were m e a s u r e d by u s i n g t h e n u c l e a r q u a d r u p o l e (NQR) and m a g n e t i c r e s o n a n c e (NHR) t e c h n i q u e s . For 90 K - s u p e r c o n d u c t o r w i t h yn0, 1/T 1 a t t h e CuO 2 p l a n e s i t e d e c r e a s e s m a r k e d l y w i t h o u t t h e e n h a n c e m e n t j u s t b e l o w T c and a p p r o x i m a t e s to a T3 b e h a v i o r b e t w e e n 40K and 10 K. K n i g h t s h i f t shows a d i s t i n c t decrease below T c, g i v i n g an e v i d e n c e f o r d o m i n a n t s i n g l e t p a i r i n g . For an e x p l a n a t i o n of u n c o n v e n t i o n a l n u c l e a r [elaxation behavior, it seems plausible to a p p l y a d-wave model w i t h t h e gap z e r o s of l i n e s on t h e Fermi surface. For 80 E - s u p e r c o n d u c t o r , we f o u n d a s u r p r i s i n g r e s u l t t h a t 1/T 1 a t t h e CuO 2 p l a n e site i s by t h r e e o r d e r o f m a g n i t u d e s u p p r e s s e d a s c o m p a r e d to t h a t of 90 K s u p e r c o n d u c t o r and shows no a n o m a l i e s a t Tc = 60 K. T h i s marked d i f f e r e n c e of r e l a x a t i o n b e h a v i o r s b e t w e e n 90K and 60K s u p e r c o n d u c t o r s implies a strong correlation w i t h t h e m e c h a n i s m of h i g h T c s u p e r c o n d u c t i v i t y .
1.
INTRODUCTION Since the discovery of high-T c superconducting oxides, much a t t e n t i o n and e f f o r t a r e focused on t h e m e c h a n i s m a n d t h e o r i g i n of high-T e superconductivity. A m o n g them, YBa2Cu307-,, is of particular interest because of its highest T c value wlth 90 K. I) This T c value is invariant with the replacement of Y by other m a g n e t i c rare earth elements. The c r y s t a l s t r u c t u r e is of a oxygen-deficient perovsklte type. There are two copper sites, i.e. Cu(l) sites forming the Cu-O chain and Cu(2) sites in the CuO 2 plane. The nuclear spin-lattice relaxation time, T 1 probes the excited state in the superconducting state. The Knight shift proportional to the local, spin s u s c e p t i b i l i t y gives an important i n s i g h t into the p a r i t y of the p a i r i n g function. Both behaviors of T I and K n i g h t shift b e l o w T c for the conventional superconductors gave d e c i s i v e evidences to v e r i f y the BCS p r e d i c t i o n s . As f o r T I m e a s u r e m e n t s , f o u r g r o u p s r e p o r t ed that I/T I for 89y of NMR and 63Cu of NOR at 31.5 MHz d e c r e a s e much q u i c k l y w i t h o u t t h e enhancement just below Tc.2-5) Then from fitting a simple relation o f I / T I = A exp ( - h / k B T ) to t h e d a t a , a ratio of 2A/kBT c was estimated to be l a r g e r t h a n t h e BCS v a l u e o f 3 . 5 on an i m p l i c i t assumption of a uniform superconducting e n e r g y g a p . 2 , 4 ) H o w e v e r , We should take into consideration that the
0921-4534/88/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
presence o f u n i f o r m e n e r g y gap on t h e F e r m i surface always accompanies both characteristic features of the enhancement just below Tc and subsequent e x p o n e n t i a l decrease of l / T 1 in lower t e m p e r a t u r e . T h e r e f o r e , no o b s e r v a t i o n of the e n h a n c e m e n t j u s t b e l o w Tc m e a n s that a simple exponential fit to t h e T I d a t a is not a p p r o p r i a t e . 2 , 4 ) Thus, t h e r e r e m a i n s some q u e s t i o n in the analysis o f T 1. As a further serious problem, there is still no c l e a r consensus on the s i t e - a s s i g n m e n t of the observed NQR s i g n a l s to t h e two d i f f e r e n t C u ( l ) and (2) s i t e s . Maiiet a l . 3) and we 6) assigned the signals a t 3 1 . 5 and 2 2 . 0 5 MHz to Cu(2) and (I) sites, respectively, while Nalstedt et at. concluded the reverse assignment. 2) Quite recently, Kohori et at. have established the unambiguous a s s i g n m e n t f r o m t h e c o m p a r i s o n o f T l b e t w e e n Cu (1) a n d (2) s i t e s of ReBa2Cu307-y w i t h Re=Nd, Sm and Gd. 7) In t h e p a r a m a g n e t i c s t a t e a t low t e m p e r a t u r e , the nuclear relaxation process is governed o n l y by an i s o t r o p l c fluctuation of rare earth magnetic moment through the dipole c o u p l i n g b e t w e e n e a c h Cu n u c l e a r s p i n a t Cu(1) and (2) s i t e s and r a r e e a r t h moments. Hence 1/T 1 i s p r o p o r t i o n a l to t h e l a t t i c e sum of Z I ( 1 / R I )S where R i i s t h e d i s t a n c e b e t w e e n 63Cu and each rare earth i o n s and t h e n a r a t i o , R=(I/TI)Cu(2)/(I/TI)Cu(I ) b e t w e e n Cu (2) and (1) sites is independent of rare earth elements. The r a t i o R is e x a c t l y calculated
84
}1. Kitaoka et aL / Nuclear relaxation and Knight shift studies
4O
.,7 v
x :Gd e:Sm ,t:Nd
--X--~
--
"T I'.'--~ v
%
I
s
I
I
20
FIGURE 1 The c a l c u l a t e d r a t i o of 1/T 1 f o r Cu(2) v e r s u s C u ( 1 ) o f 63Cu f o r R e B a 2 C u 3 0 7 _ v. ( d a s h e d l i n e ) The e x p e r i m e n t a l r a t i o of I / T I at 31.5 HHz v e r s u s a t 22.05 HHz i s p l o t t e d f o r Re=Gd, Sm and Nd. to be 2 1 . 6 by t h e l a t t i c e sum. 7) The e x p e r i monte[ results of a ratio o f ( 1 / T 1) a t 3 1 . 5 MHz v e r s u s ( t / T 1) a t 2 2 . 0 5 MHz on Sm, Nd and Cd a r e s h o w n by s o l i d c i r c l e s , triangle and c r o s s m a r k s in F i g . 1 . , r e s p e c t i v e l y together with the calculated value( dashed line). At 4 . 2 K, t h e e x p e r i m e n t and c a l c u l a t i o n a r e in quantitative agreement without a serious d e p e n d e n c e of r a r e e a r t h e l e m e n t s . T h i s r e s u l t supports unambiguously the correctness of o u r assignment from a quantitative p o i n t of view. As f o r t h e Sm c o m p o u n d ( s o l i d c i r c l e s ) , the ratio is gradually deviating with increasing temperature, which suggests the necessity to m e a s u r e T 1 a t low t e m p e r a t u r e for the siteassignment. We a l r e a d y r e p o r t e d t h e m e a s u r e m e n t s of TI f o r b o t h Cu s i t e s and t h e f i r s t Knight shift d a t a b e l o w T c . 5 ) We f o u n d t h a t t h e d i s t i n c t difference of r e l a x a t i o n behaviors between Cu(2) and (1) a r e a t t r i b u t e d to t h e d i f f e r e n c e of r e l a x a t i o n process, namely, the magnetic and q u a d r u p o l e p r o c e s s , respectively. Furtherm o r e , f r o m the s i g n i f i c a n t decrease of the Knight shift b e l o w To, we g a v e an i m p o r t a n t evidence for dominant singlet pairing, if the p a i r was formed. 5) Here We p r e s e n t t h e e x t e n s i v e and p r e c i s e measurements of T 1 at Cu(2) sites f o r 90 K superconductors of three s a m p l e s p r e p a r e d by the different methods. The Knight shift is r e - e x a m i n e d . We also p r e s e n t t h e systematic TI m e a s u r e m e n t s of 60 K s u p e r c o n d u c t i n g compound with y=0.35 and of semiconducting specimen with y=0.7 which has been recently f o u n d to e x h i b i t an a n t i f e r r o m a g n e t l c o r d e r a t 20 K. 8) 2.
MATERIALS Two s a m p l e s having the superconducting transition temperature Tc=92 K w e r e p r e p a r e d by t h e s o l i d s t a t e r e a c t i o n in a i r and flowing
oxygen atmosphere ( h e r e a f t e r denoted as sample (a) a n d ( b ) , r e s p e c t i v e l y ) and were slowly c o o l e d in f u r n a c e . YBa2Cu307-y with y=O.07 (denoted as sample (c)), y=0.35 and 0.7 were prepared by q u e n c h i n g the sintered pellets in p u r e 02 a t m o s p h e r e from the temperatures of 300 K, 700 K a n d 950 K, r e s p e c t i v e l y . 9) The absolute v a l u e o f y was d e t e r m i n e d from the quenching temperature, using the relationship between temperature a n d y. lO) The c o m p o u n d s w i t h y=O. 07 a n d 0 . 3 5 h a v e To=92 K a n d 60 K, w h i l e one w i t h y = 0 . 7 p o s s e s s e s no T c w i t h t h e structural phase transition from o r t h o r h o m b i c to t e t r a g o n a l . 3. Cu NQR SPECTRUM The Cu NQR s p e c t r a at 1.3 K and zero external f i e l d a r e shown in F i g . 2 f o r y=O. 07, 0 . 3 5 and 0 . 7 . 8) Two p a i r s of Cu NQR s i g n a l s were o b s e r v e d f o r a l l s p e c i m e n s a t a r o u n d 31 HHz and 22MHz, w h i c h r e s u l t from Cu(2) and (1) sites, respectively. Each p a i r c o r r e s p o n d s to two i s o t o p e s of 63Cu and GSCu w i t h t h e ratio of electric quadrupole moment, (63Q/G5Q) = 1 . 0 8 2 . As s e e n in t h e f i g u r e , the resonance frequencies at both sites are unchanged appreciably f o r y = O . 0 7 and 0 . 3 5 , w h i l e t h o s e f o r y=O. 7 h a v i n g the tetragonal structure shift to l o w e r a n d h i g h e r s i d e f o r C u ( 2 ) a n d (i), respectively. A most s t r i k i n g feature in these spectra i s t h a t t h e l i n e w i d t h a t Cu(2) sites for y=0.7 is considerably broadened. As shown a l r e a d y , t h i s l a r g e i n c r e a s e of t h e l i n e w i d t h i s d u e to an a n t i f e r r o m a g n e t i e order b e l o w a b o u t 20 K. 8) Thus t h e v a r i a t i o n o f NQR spectra with different oxygen contents is associated with the change in t h e c r y s t a l structure and m o r e o v e r in t h e e l e c t r o n i c state from s u p e r e o n d u c t l v i t y to a n t l f e r r o m a g n e t i s m .
(~ y=oo7 ~=92K
C~ozP~e
~'~,..:2,.,...
,\ ic) Y=Q7
~ .~. :
p
/ - ~ " : . $.
~=0 ' I.
~.:-..-,'._,., 20
24 ~ Fr~uencyIMHz)
NQR s p e c t r a for external field.
?~, 32.
FIGURE 2 YBa2Cu3OT_y a t
1 . 3 K and z e r o
4. NUCLEAR SPIN-LATTICE RELAXATION TIME TI In T 1 m e a s u r e m e n t by u s i n g NQR t e c h n i q u e , the relaxation originates from t h e electronspin fluctuation through the magnetic
Y. Kitaoka et al. / Nuclear relaxation and Knight shift studies
hyperfine interaction and the c'harge fluctuation through the electrle coupling between the n u c l e a r e l e c t r i c quadrupote moment and e l e c t r i c field gradient. The f o r m e r and l a t t e r c o r r e s p o n d to the magnetic and q u a d r u pole relaxation process, which are proportional to the square of the gyromagnetic ratio y n and of electric quadrupole moment (I of nuclei, respectively. Cu has two isotopes of 83Cu and 65Cu with different Y and (I whose r a t i o is ( S S y / 8 3 y ) = l . 071 and (8501830)=0.924 ,respectlvety. Accordingly, the r e l a x a t i o n p r o c e s s is p r e c i s e l y i d e n t i f i e d from the r a t i o of the relaxation rate lIT 1 for both
isotopes, i.e.
(85(I/TI)I83(IITI)=(85 y/83 7
)2=i. 15 and (65Q/B3Q)2=0.85 for the magnetic and quadrupoIe r e t a x a t i o n p r o c e s s , r e s p e c t i v e l y . In general, the o b s e r v e d (llTl)obs. consists of both contributions of (L/TI) M and (I/T[) o. In order to obtain a valid information about the excited state s p e c t r u m of electron system, we should separate only the magnetic contribution 8 3 ( I / T I ) M from the observed one 83(I/Tl)obs., foIlowlng the relation of
63(l/TI)M=
63 (I/TI) obs.
((65 7163 Y)2-Z)I(Z-(G5QIG3Q)2)÷l
(1)
with Z=85(t/Tt)obs 163(llTOobs In the prevlous paper, Z) we showed that T l at Cu(1) sites is governed by the quadrupoIe mechanism between 4.2 K and 130 K, while T I at Cu(2) sites by the magnetic relaxation down to 50K and the temperature independent relaxation with TL=4 sec below 4.2 K is dominated by the quadrupoIe relaxation. The three different samples (a), (b) and (c) with Tc=92 K are used here. Figure 3 shows the T l for the compounds with y ~ O by solid circles. It should be stressed that T l measured for all samples is longer than the reported one, 2,4) showing to be less affected by some extrinsic contribution to T l such as the fluctuation of the dilute magnetic moments and/or quadrupoIe relaxation. In normal state, LIT 1 shows a s m a l l sample dependence, 5) which is not w e | I resolved in this figure. The important feature is no t to o b e y the K o r r i n g a relation (TlT=const.). Below T c, lIT I exhibits no distinct sample-dependence and decreases markedly with n_& enhacement just below T c. The retaxation process Is magnetic down to 25 K, w h i l e the quadrupole r e l a x a t i o n p r o c e s s s t a r t s gradualty to c o n t r ' i b u t e to the o b s e r v e d T 1 with d e c r e a s i n g t e m p e r a t u r e below 20 K. In the f i g u r e , we show by open c i r c l e s the m a g n e t i c relaxation rate 63(IITI)H extracted from the observed T I below 20 K by measuring precisely the ratio of Z=85(I/TL)/S3(I/TI ) and applying thls to eq. (I). Below lO K, it Is, however, so hard to separate (I/TL) M because of the comparable order of both contributions.
.
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,
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•
S/ I
I IIIllll
10 T(K)
I
100
I
I
III
1000
FIGURE 3 Temperature dependence of I/T l at Cu(2) sites for YBa2Cu307-y with y ~ O , y=0.35 and 0.7. Open circles indicate the magnetic contributlon to the relaxation evaluated from the observed one (solid circles) by using eq. (i). A solid curve indicates a calculation based on a d wave model (see text).
As seen in the f i g u r e , I / T 1 a t Cu(2) s i t e s a p p r o x i m a t e s to a T b e h a v i o r below 40 K. This result gives a direct proof that the superconducting s t a t e p o s s e s s e s n.~o u n i f o r m e n e r g y gap, b u t a l o w e n e r g y e x c i t a t i o n at l o w e r t e m p e r a t u r e , though the l a r g e decrease of I / T 1 j u s t b e l o w T c means the opening of a l a r g e gap in the e x c i t e d energy spectrum. We a r e now p r o c e e d i n g the further accurate measurements below 20 K. As f o r 80 E s u p e r c o n d u c t o r with y=0.35, I/T I is shown by t r i a l g l e marks in Fig. 3. We obtalneJ the surprising result that I/T l at Cu(2) sites is by three orders of magnitude suppressed as compared to that for 90 K s u p e r c o n d u c t o r . Furthermore, no anomaly is observed near T c. This dramatic change of I/T I may give a crucial insight into the origin of high T c s u p e r c o n d u c t i v i t y . I n c l u d i n g an identification of the relaxation process, the T 1 measurement is now in p r o g r e s s in a considerable wide temperature range. I/T l for the antlferromagnetic compound with y=0.7 is also indicated in the figure by cross marks. Although the recovery behavior is of not simple- but multi-exponential type, the largest component of I/T l diverges around 20 K, reproducing the previous result from the
Y. Kitaoka et al.
86
/ Nuclear relaxation and Knight shift studies
temperature dependence of the nuclear t r a n s v e r s e r e l a x a t i o n r a t e I/T2 . 8 ) Thus, the nuclear relaxation behavior verifies the marked variety of electronic states on the oxygen contents in YBa2Cu307_y and presents a crucial information in clarifying the mechanism of superconductivity.
5. KNIGHT SHIFT K n i g h t s h i f t of 63Cu was m e a s u r e d a t t h e p l a n e s i t e in the t e m p e r a t u r e range of 4 . 2 K120 K by 63Cu NMR a t 39 1 M H z . The r e s u l t of sample (c) (open c i r c l e s ) is i n d i c a t e d tog e t h e r w i t h t h a t of sample (b) ( s o l i d c i r c l e s ) r e p o r t e d p r e v i o u s l y . 5) The p e r p e n d i c u l a r s h i f t K ~ to the e - a x i s was e x t r a c t e d from the peak o f the powder p a t t e r n of 63Cu NMR due to (+1/2 (---)I/2) t r a n s i t i o n with a large quadrupole s h i f t . S i n c e K_i_ i s e v a l u a t e d from t h e e x a c t s u b t r a c t i o n of q u a d r u p o l a r s h i f t by u s i n g NQR frequency, the s m a l l t e m p e r a t u r e dependence o f NQR f r e q u e n c y y i e l d s some e r r o r in t h e K n i g h t s h i f t v a l u e , w h i c h may be r e s p o n s i b l e f o r t h e o b s e r v e d s a m p l e d e p e n d e n c e of t h e shift. Nevertheless, i t is e v i d e n t t h a t K..L of 0 . 4 2 ~ in normal s t a t e d e c r e a s e s below Tc, i m p l y i n g t h a t the p a i r is of dominant s i n g l e t t y p e . We s h o u l d e m p h a s i z e d t h a t t h e p o s i t i v e K n i g h t s h i f t p r o p o r t i o n a l to the s u s c e p t i b i l i t y i s due to t h e d i p o l e h y p e r f i n e c o u p l i n g originating from the d(x2-y 2) like wave function on the Fermi surface. 5) So the Knight shift is expected to be anisotroplc.
~0~
--
Cu(2) s i t e s • :s~ple(b) o :
,.
(c)
T~=92K . ~ - , ' - - 042"/.
2,&
=0.3 T: tn
°+ 01
I
50 T(K)
I
10o
150
FIGURE 4 T e m p e r a t u r e d e p e n d e n c e of K n i g h t s h i f t p e r p e n d i c u l a r to the c - a x i s m e a s u r e d a t 3 9 . 1 M H z for Cu(2) s i t e s .
6. ANALYSIS In o r d e r to i n t e r p r e t the u n c o n v e n t i o n a l behavior of nuclear relaxation o u t o f BCS scheme w i t h s-wave t o g e t h e r w i t h the d e c r e a s e of the K n i g h t s h i f t , we t r i e d t e n t a t i v e l y a dwave m o d e l h a v i n g t h e p a i r function of dsymmetry and hence, gap zeros of l i n e s on the Fermi surface In t h i s simple model, the a n o m a l o u s d e n s i t y of s t a t e s in the e x p r e s s i o n
o f I / T 1, w h i c h results from so-called coherence effect characteristic f o r BCS superconductor, disappears and h e n c e the e n h a c e m e n t of 1/T l j u s t below Tc is s u p p r e s s e d t o g e t h e r w i t h a f u r t h e r e f f e c t of the . a n i s e t r o p i c e n e r g y gap w i t h the gap z e r o s of l i n e s . The c a l c ' u l a t e d r e s u l t i s shown by a s o l i d c u r v e in Fig. 3. Here for s i m p l i c i t y , the gap z e r o s of l i n e s a r e a p p r o x i m a t e d by a r e l a t i o n of h = h o cos 0 w i t h 2 h o = 1 2 k B T c and the temperature dependence of t h e e n e r g y gap i s assumed to be the same as t h a t of BCS t h e o r y . As s e e n in the f i g u r e , the o v e r a l l f e a t u r e of the experiment i s in q u a l i t a t i v e agreement with a simple calculation, though the d e v i a tion is appearing b e l o w 40 K. I t s h o u l d be n o t i c e d t h a t 1/T 1 b e l o w 40 K a p p r o x i m a t e s to t h e T3 b e h a v i o r w h i c h i s p r e d i c t e d from t h e anisotropic e n e r g y gap w i t h t h e gap z e r o s of l i n e s . Thus, i t seems p l a u s i b l e t h a t the o r d e r p a r a m e t e r i s of d - s y m m e t r y t y p e w i t h a l a r g e r a t i o of 2 A / k B T c f o r YBa2Cu3OT-y with Tc=92 K. REFERENCES (1) H.K. Wu, J.R. A s h b u r n , C.J. Torng, P.H. Her, R.L. Meng, L. Gap, Z . J . Gap, Z . J . Huang, Y.Q. Wang and C.W. Chu: Phys. Rev. L e t t . 58 (1987) 908. (2) R.E. W a l s t e d t , W.W. Warren, J r . , R.F. B e l l , G.F. B r e n n e r t , G.P. E s p i n o s a , J . P . Remeika, R. J. Cava, and E. A. R i e t m a n : Phys. Roy. B36 (1987) 5727. (3) I. Furo, A. J a n o s s y , L. Mihaly, P. B a n k l , I. P r o c s i c , I. Bakonyi, [ . H e i n m a a , E. Joon and E. L i p m a a : P h y s . Rev. B36 (1987) 5690. (41 M. Mall, D. Brinkman, L. P a u l i , J. Roos, H. Zimmermann and J. H u l l i g e r : Phys. L e t t . A124 (1987) 112. (5) Y. K i t a o k a , S. H i r a m a t s u , T. Kondo and K. Asayama: J. Phys. Soc. Jpn. 5-7 (1988) 30. (6) Y. K i t a o k a , S. H i r a m a t s u , K. I s h i d a , T. Kohara, Y. Oda, K. Amaya and K. Asayama: P h y s i c a 148B (1987) 298; T. Kohara, Y. K o h o r i , Y. K i t a o k a , S. H i r a m a t s u , K. Ueda, M. K a b u r a g i , Y. Oda and K. Asayama : i b i d , 292. (7] Y. K o h o r i , H. S h i b a i , Y. Oda, T. Kohara, Y. K i t a o k a and K. Asayama: J. Phys. See. Jpn. 57 (1988) No 3. (8) Y. K i t a o k a , S. H i r a m a t s u , K. I s b l d a , K. Asayama, H. T a k a g i , H. I w a b u c h i , S. Uchida and S. Tanaka: J. Phys. Soc. Jpn. 57 (1988) No. 3. {9] H. T a k a g l , S. Uchida, H. I w a b u c h i , H. E i s a k i , K. K i s h i o , K. K i t a z a w a , K. Fueki and S. Tanaka: P h y s l c a 148B (1987) 349. fIO] K. K i s h i o , J. Shimoyama, T. Hasegawa, K. E i t a z a w a and K. F u e k i : Jpn. J. Appl. Phys. 26 (1987) L~228.
NUCLEAR RELAXATION AND KNIGHT SHIFT STUDIES OF 63Cu IN
90 E-AND 60 E-CLASS YBa2Cu3OT_y
Y. KITAOKA, S. HIRAMATSU, Y. KOHORI, K. ISHIDA, T. KONDO, H. SHIBAI, K. ASAYAMA, H. TAKAG[ ~, S. UCHIDA*, H. IWABUCH[ ~ and S. TANAKA *~
D e p a r t m e n t of M a t e r i a l P h y s i c s , 560. J a p a n *Engineering Research Institute, U n i v e r s i t y of Tokyo, B u n k y o - k u ,
Faculty
of E n g i n e e r i n g
Science,
Osaka U n i v e r s i t y ,
and * * D e p a r t m e n t of A p p l i e d P h y s i c s , Tokyo 113, J a p a n .
Faculty
Toyonaka,
Osaka
of E n g i n e e r i n g ,
The n u c l e a r r e t a x a t l o n r a t e ( I / T 1) and K n i g h t s h i f t f o r Cu of YBa2Cu307-y were m e a s u r e d by u s i n g t h e n u c l e a r q u a d r u p o l e (NQR) and m a g n e t i c r e s o n a n c e (NHR) t e c h n i q u e s . For 90 K - s u p e r c o n d u c t o r w i t h yn0, 1/T 1 a t t h e CuO 2 p l a n e s i t e d e c r e a s e s m a r k e d l y w i t h o u t t h e e n h a n c e m e n t j u s t b e l o w T c and a p p r o x i m a t e s to a T3 b e h a v i o r b e t w e e n 40K and 10 K. K n i g h t s h i f t shows a d i s t i n c t decrease below T c, g i v i n g an e v i d e n c e f o r d o m i n a n t s i n g l e t p a i r i n g . For an e x p l a n a t i o n of u n c o n v e n t i o n a l n u c l e a r [elaxation behavior, it seems plausible to a p p l y a d-wave model w i t h t h e gap z e r o s of l i n e s on t h e Fermi surface. For 80 E - s u p e r c o n d u c t o r , we f o u n d a s u r p r i s i n g r e s u l t t h a t 1/T 1 a t t h e CuO 2 p l a n e site i s by t h r e e o r d e r o f m a g n i t u d e s u p p r e s s e d a s c o m p a r e d to t h a t of 90 K s u p e r c o n d u c t o r and shows no a n o m a l i e s a t Tc = 60 K. T h i s marked d i f f e r e n c e of r e l a x a t i o n b e h a v i o r s b e t w e e n 90K and 60K s u p e r c o n d u c t o r s implies a strong correlation w i t h t h e m e c h a n i s m of h i g h T c s u p e r c o n d u c t i v i t y .
1.
INTRODUCTION Since the discovery of high-T c superconducting oxides, much a t t e n t i o n and e f f o r t a r e focused on t h e m e c h a n i s m a n d t h e o r i g i n of high-T e superconductivity. A m o n g them, YBa2Cu307-,, is of particular interest because of its highest T c value wlth 90 K. I) This T c value is invariant with the replacement of Y by other m a g n e t i c rare earth elements. The c r y s t a l s t r u c t u r e is of a oxygen-deficient perovsklte type. There are two copper sites, i.e. Cu(l) sites forming the Cu-O chain and Cu(2) sites in the CuO 2 plane. The nuclear spin-lattice relaxation time, T 1 probes the excited state in the superconducting state. The Knight shift proportional to the local, spin s u s c e p t i b i l i t y gives an important i n s i g h t into the p a r i t y of the p a i r i n g function. Both behaviors of T I and K n i g h t shift b e l o w T c for the conventional superconductors gave d e c i s i v e evidences to v e r i f y the BCS p r e d i c t i o n s . As f o r T I m e a s u r e m e n t s , f o u r g r o u p s r e p o r t ed that I/T I for 89y of NMR and 63Cu of NOR at 31.5 MHz d e c r e a s e much q u i c k l y w i t h o u t t h e enhancement just below Tc.2-5) Then from fitting a simple relation o f I / T I = A exp ( - h / k B T ) to t h e d a t a , a ratio of 2A/kBT c was estimated to be l a r g e r t h a n t h e BCS v a l u e o f 3 . 5 on an i m p l i c i t assumption of a uniform superconducting e n e r g y g a p . 2 , 4 ) H o w e v e r , We should take into consideration that the
0921-4534/88/$03.50 ©Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
presence o f u n i f o r m e n e r g y gap on t h e F e r m i surface always accompanies both characteristic features of the enhancement just below Tc and subsequent e x p o n e n t i a l decrease of l / T 1 in lower t e m p e r a t u r e . T h e r e f o r e , no o b s e r v a t i o n of the e n h a n c e m e n t j u s t b e l o w Tc m e a n s that a simple exponential fit to t h e T I d a t a is not a p p r o p r i a t e . 2 , 4 ) Thus, t h e r e r e m a i n s some q u e s t i o n in the analysis o f T 1. As a further serious problem, there is still no c l e a r consensus on the s i t e - a s s i g n m e n t of the observed NQR s i g n a l s to t h e two d i f f e r e n t C u ( l ) and (2) s i t e s . Maiiet a l . 3) and we 6) assigned the signals a t 3 1 . 5 and 2 2 . 0 5 MHz to Cu(2) and (I) sites, respectively, while Nalstedt et at. concluded the reverse assignment. 2) Quite recently, Kohori et at. have established the unambiguous a s s i g n m e n t f r o m t h e c o m p a r i s o n o f T l b e t w e e n Cu (1) a n d (2) s i t e s of ReBa2Cu307-y w i t h Re=Nd, Sm and Gd. 7) In t h e p a r a m a g n e t i c s t a t e a t low t e m p e r a t u r e , the nuclear relaxation process is governed o n l y by an i s o t r o p l c fluctuation of rare earth magnetic moment through the dipole c o u p l i n g b e t w e e n e a c h Cu n u c l e a r s p i n a t Cu(1) and (2) s i t e s and r a r e e a r t h moments. Hence 1/T 1 i s p r o p o r t i o n a l to t h e l a t t i c e sum of Z I ( 1 / R I )S where R i i s t h e d i s t a n c e b e t w e e n 63Cu and each rare earth i o n s and t h e n a r a t i o , R=(I/TI)Cu(2)/(I/TI)Cu(I ) b e t w e e n Cu (2) and (1) sites is independent of rare earth elements. The r a t i o R is e x a c t l y calculated
84
}1. Kitaoka et aL / Nuclear relaxation and Knight shift studies
4O
.,7 v
x :Gd e:Sm ,t:Nd
--X--~
--
"T I'.'--~ v
%
I
s
I
I
20
FIGURE 1 The c a l c u l a t e d r a t i o of 1/T 1 f o r Cu(2) v e r s u s C u ( 1 ) o f 63Cu f o r R e B a 2 C u 3 0 7 _ v. ( d a s h e d l i n e ) The e x p e r i m e n t a l r a t i o of I / T I at 31.5 HHz v e r s u s a t 22.05 HHz i s p l o t t e d f o r Re=Gd, Sm and Nd. to be 2 1 . 6 by t h e l a t t i c e sum. 7) The e x p e r i monte[ results of a ratio o f ( 1 / T 1) a t 3 1 . 5 MHz v e r s u s ( t / T 1) a t 2 2 . 0 5 MHz on Sm, Nd and Cd a r e s h o w n by s o l i d c i r c l e s , triangle and c r o s s m a r k s in F i g . 1 . , r e s p e c t i v e l y together with the calculated value( dashed line). At 4 . 2 K, t h e e x p e r i m e n t and c a l c u l a t i o n a r e in quantitative agreement without a serious d e p e n d e n c e of r a r e e a r t h e l e m e n t s . T h i s r e s u l t supports unambiguously the correctness of o u r assignment from a quantitative p o i n t of view. As f o r t h e Sm c o m p o u n d ( s o l i d c i r c l e s ) , the ratio is gradually deviating with increasing temperature, which suggests the necessity to m e a s u r e T 1 a t low t e m p e r a t u r e for the siteassignment. We a l r e a d y r e p o r t e d t h e m e a s u r e m e n t s of TI f o r b o t h Cu s i t e s and t h e f i r s t Knight shift d a t a b e l o w T c . 5 ) We f o u n d t h a t t h e d i s t i n c t difference of r e l a x a t i o n behaviors between Cu(2) and (1) a r e a t t r i b u t e d to t h e d i f f e r e n c e of r e l a x a t i o n process, namely, the magnetic and q u a d r u p o l e p r o c e s s , respectively. Furtherm o r e , f r o m the s i g n i f i c a n t decrease of the Knight shift b e l o w To, we g a v e an i m p o r t a n t evidence for dominant singlet pairing, if the p a i r was formed. 5) Here We p r e s e n t t h e e x t e n s i v e and p r e c i s e measurements of T 1 at Cu(2) sites f o r 90 K superconductors of three s a m p l e s p r e p a r e d by the different methods. The Knight shift is r e - e x a m i n e d . We also p r e s e n t t h e systematic TI m e a s u r e m e n t s of 60 K s u p e r c o n d u c t i n g compound with y=0.35 and of semiconducting specimen with y=0.7 which has been recently f o u n d to e x h i b i t an a n t i f e r r o m a g n e t l c o r d e r a t 20 K. 8) 2.
MATERIALS Two s a m p l e s having the superconducting transition temperature Tc=92 K w e r e p r e p a r e d by t h e s o l i d s t a t e r e a c t i o n in a i r and flowing
oxygen atmosphere ( h e r e a f t e r denoted as sample (a) a n d ( b ) , r e s p e c t i v e l y ) and were slowly c o o l e d in f u r n a c e . YBa2Cu307-y with y=O.07 (denoted as sample (c)), y=0.35 and 0.7 were prepared by q u e n c h i n g the sintered pellets in p u r e 02 a t m o s p h e r e from the temperatures of 300 K, 700 K a n d 950 K, r e s p e c t i v e l y . 9) The absolute v a l u e o f y was d e t e r m i n e d from the quenching temperature, using the relationship between temperature a n d y. lO) The c o m p o u n d s w i t h y=O. 07 a n d 0 . 3 5 h a v e To=92 K a n d 60 K, w h i l e one w i t h y = 0 . 7 p o s s e s s e s no T c w i t h t h e structural phase transition from o r t h o r h o m b i c to t e t r a g o n a l . 3. Cu NQR SPECTRUM The Cu NQR s p e c t r a at 1.3 K and zero external f i e l d a r e shown in F i g . 2 f o r y=O. 07, 0 . 3 5 and 0 . 7 . 8) Two p a i r s of Cu NQR s i g n a l s were o b s e r v e d f o r a l l s p e c i m e n s a t a r o u n d 31 HHz and 22MHz, w h i c h r e s u l t from Cu(2) and (1) sites, respectively. Each p a i r c o r r e s p o n d s to two i s o t o p e s of 63Cu and GSCu w i t h t h e ratio of electric quadrupole moment, (63Q/G5Q) = 1 . 0 8 2 . As s e e n in t h e f i g u r e , the resonance frequencies at both sites are unchanged appreciably f o r y = O . 0 7 and 0 . 3 5 , w h i l e t h o s e f o r y=O. 7 h a v i n g the tetragonal structure shift to l o w e r a n d h i g h e r s i d e f o r C u ( 2 ) a n d (i), respectively. A most s t r i k i n g feature in these spectra i s t h a t t h e l i n e w i d t h a t Cu(2) sites for y=0.7 is considerably broadened. As shown a l r e a d y , t h i s l a r g e i n c r e a s e of t h e l i n e w i d t h i s d u e to an a n t i f e r r o m a g n e t i e order b e l o w a b o u t 20 K. 8) Thus t h e v a r i a t i o n o f NQR spectra with different oxygen contents is associated with the change in t h e c r y s t a l structure and m o r e o v e r in t h e e l e c t r o n i c state from s u p e r e o n d u c t l v i t y to a n t l f e r r o m a g n e t i s m .
(~ y=oo7 ~=92K
C~ozP~e
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,\ ic) Y=Q7
~ .~. :
p
/ - ~ " : . $.
~=0 ' I.
~.:-..-,'._,., 20
24 ~ Fr~uencyIMHz)
NQR s p e c t r a for external field.
?~, 32.
FIGURE 2 YBa2Cu3OT_y a t
1 . 3 K and z e r o
4. NUCLEAR SPIN-LATTICE RELAXATION TIME TI In T 1 m e a s u r e m e n t by u s i n g NQR t e c h n i q u e , the relaxation originates from t h e electronspin fluctuation through the magnetic
Y. Kitaoka et al. / Nuclear relaxation and Knight shift studies
hyperfine interaction and the c'harge fluctuation through the electrle coupling between the n u c l e a r e l e c t r i c quadrupote moment and e l e c t r i c field gradient. The f o r m e r and l a t t e r c o r r e s p o n d to the magnetic and q u a d r u pole relaxation process, which are proportional to the square of the gyromagnetic ratio y n and of electric quadrupole moment (I of nuclei, respectively. Cu has two isotopes of 83Cu and 65Cu with different Y and (I whose r a t i o is ( S S y / 8 3 y ) = l . 071 and (8501830)=0.924 ,respectlvety. Accordingly, the r e l a x a t i o n p r o c e s s is p r e c i s e l y i d e n t i f i e d from the r a t i o of the relaxation rate lIT 1 for both
isotopes, i.e.
(85(I/TI)I83(IITI)=(85 y/83 7
)2=i. 15 and (65Q/B3Q)2=0.85 for the magnetic and quadrupoIe r e t a x a t i o n p r o c e s s , r e s p e c t i v e l y . In general, the o b s e r v e d (llTl)obs. consists of both contributions of (L/TI) M and (I/T[) o. In order to obtain a valid information about the excited state s p e c t r u m of electron system, we should separate only the magnetic contribution 8 3 ( I / T I ) M from the observed one 83(I/Tl)obs., foIlowlng the relation of
63(l/TI)M=
63 (I/TI) obs.
((65 7163 Y)2-Z)I(Z-(G5QIG3Q)2)÷l
(1)
with Z=85(t/Tt)obs 163(llTOobs In the prevlous paper, Z) we showed that T l at Cu(1) sites is governed by the quadrupoIe mechanism between 4.2 K and 130 K, while T I at Cu(2) sites by the magnetic relaxation down to 50K and the temperature independent relaxation with TL=4 sec below 4.2 K is dominated by the quadrupoIe relaxation. The three different samples (a), (b) and (c) with Tc=92 K are used here. Figure 3 shows the T l for the compounds with y ~ O by solid circles. It should be stressed that T l measured for all samples is longer than the reported one, 2,4) showing to be less affected by some extrinsic contribution to T l such as the fluctuation of the dilute magnetic moments and/or quadrupoIe relaxation. In normal state, LIT 1 shows a s m a l l sample dependence, 5) which is not w e | I resolved in this figure. The important feature is no t to o b e y the K o r r i n g a relation (TlT=const.). Below T c, lIT I exhibits no distinct sample-dependence and decreases markedly with n_& enhacement just below T c. The retaxation process Is magnetic down to 25 K, w h i l e the quadrupole r e l a x a t i o n p r o c e s s s t a r t s gradualty to c o n t r ' i b u t e to the o b s e r v e d T 1 with d e c r e a s i n g t e m p e r a t u r e below 20 K. In the f i g u r e , we show by open c i r c l e s the m a g n e t i c relaxation rate 63(IITI)H extracted from the observed T I below 20 K by measuring precisely the ratio of Z=85(I/TL)/S3(I/TI ) and applying thls to eq. (I). Below lO K, it Is, however, so hard to separate (I/TL) M because of the comparable order of both contributions.
.
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.
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.
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,
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0"11
,
t I Illlll
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S/ I
I IIIllll
10 T(K)
I
100
I
I
III
1000
FIGURE 3 Temperature dependence of I/T l at Cu(2) sites for YBa2Cu307-y with y ~ O , y=0.35 and 0.7. Open circles indicate the magnetic contributlon to the relaxation evaluated from the observed one (solid circles) by using eq. (i). A solid curve indicates a calculation based on a d wave model (see text).
As seen in the f i g u r e , I / T 1 a t Cu(2) s i t e s a p p r o x i m a t e s to a T b e h a v i o r below 40 K. This result gives a direct proof that the superconducting s t a t e p o s s e s s e s n.~o u n i f o r m e n e r g y gap, b u t a l o w e n e r g y e x c i t a t i o n at l o w e r t e m p e r a t u r e , though the l a r g e decrease of I / T 1 j u s t b e l o w T c means the opening of a l a r g e gap in the e x c i t e d energy spectrum. We a r e now p r o c e e d i n g the further accurate measurements below 20 K. As f o r 80 E s u p e r c o n d u c t o r with y=0.35, I/T I is shown by t r i a l g l e marks in Fig. 3. We obtalneJ the surprising result that I/T l at Cu(2) sites is by three orders of magnitude suppressed as compared to that for 90 K s u p e r c o n d u c t o r . Furthermore, no anomaly is observed near T c. This dramatic change of I/T I may give a crucial insight into the origin of high T c s u p e r c o n d u c t i v i t y . I n c l u d i n g an identification of the relaxation process, the T 1 measurement is now in p r o g r e s s in a considerable wide temperature range. I/T l for the antlferromagnetic compound with y=0.7 is also indicated in the figure by cross marks. Although the recovery behavior is of not simple- but multi-exponential type, the largest component of I/T l diverges around 20 K, reproducing the previous result from the
Y. Kitaoka et al.
86
/ Nuclear relaxation and Knight shift studies
temperature dependence of the nuclear t r a n s v e r s e r e l a x a t i o n r a t e I/T2 . 8 ) Thus, the nuclear relaxation behavior verifies the marked variety of electronic states on the oxygen contents in YBa2Cu307_y and presents a crucial information in clarifying the mechanism of superconductivity.
5. KNIGHT SHIFT K n i g h t s h i f t of 63Cu was m e a s u r e d a t t h e p l a n e s i t e in the t e m p e r a t u r e range of 4 . 2 K120 K by 63Cu NMR a t 39 1 M H z . The r e s u l t of sample (c) (open c i r c l e s ) is i n d i c a t e d tog e t h e r w i t h t h a t of sample (b) ( s o l i d c i r c l e s ) r e p o r t e d p r e v i o u s l y . 5) The p e r p e n d i c u l a r s h i f t K ~ to the e - a x i s was e x t r a c t e d from the peak o f the powder p a t t e r n of 63Cu NMR due to (+1/2 (---)I/2) t r a n s i t i o n with a large quadrupole s h i f t . S i n c e K_i_ i s e v a l u a t e d from t h e e x a c t s u b t r a c t i o n of q u a d r u p o l a r s h i f t by u s i n g NQR frequency, the s m a l l t e m p e r a t u r e dependence o f NQR f r e q u e n c y y i e l d s some e r r o r in t h e K n i g h t s h i f t v a l u e , w h i c h may be r e s p o n s i b l e f o r t h e o b s e r v e d s a m p l e d e p e n d e n c e of t h e shift. Nevertheless, i t is e v i d e n t t h a t K..L of 0 . 4 2 ~ in normal s t a t e d e c r e a s e s below Tc, i m p l y i n g t h a t the p a i r is of dominant s i n g l e t t y p e . We s h o u l d e m p h a s i z e d t h a t t h e p o s i t i v e K n i g h t s h i f t p r o p o r t i o n a l to the s u s c e p t i b i l i t y i s due to t h e d i p o l e h y p e r f i n e c o u p l i n g originating from the d(x2-y 2) like wave function on the Fermi surface. 5) So the Knight shift is expected to be anisotroplc.
~0~
--
Cu(2) s i t e s • :s~ple(b) o :
,.
(c)
T~=92K . ~ - , ' - - 042"/.
2,&
=0.3 T: tn
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I
50 T(K)
I
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150
FIGURE 4 T e m p e r a t u r e d e p e n d e n c e of K n i g h t s h i f t p e r p e n d i c u l a r to the c - a x i s m e a s u r e d a t 3 9 . 1 M H z for Cu(2) s i t e s .
6. ANALYSIS In o r d e r to i n t e r p r e t the u n c o n v e n t i o n a l behavior of nuclear relaxation o u t o f BCS scheme w i t h s-wave t o g e t h e r w i t h the d e c r e a s e of the K n i g h t s h i f t , we t r i e d t e n t a t i v e l y a dwave m o d e l h a v i n g t h e p a i r function of dsymmetry and hence, gap zeros of l i n e s on the Fermi surface In t h i s simple model, the a n o m a l o u s d e n s i t y of s t a t e s in the e x p r e s s i o n
o f I / T 1, w h i c h results from so-called coherence effect characteristic f o r BCS superconductor, disappears and h e n c e the e n h a c e m e n t of 1/T l j u s t below Tc is s u p p r e s s e d t o g e t h e r w i t h a f u r t h e r e f f e c t of the . a n i s e t r o p i c e n e r g y gap w i t h the gap z e r o s of l i n e s . The c a l c ' u l a t e d r e s u l t i s shown by a s o l i d c u r v e in Fig. 3. Here for s i m p l i c i t y , the gap z e r o s of l i n e s a r e a p p r o x i m a t e d by a r e l a t i o n of h = h o cos 0 w i t h 2 h o = 1 2 k B T c and the temperature dependence of t h e e n e r g y gap i s assumed to be the same as t h a t of BCS t h e o r y . As s e e n in the f i g u r e , the o v e r a l l f e a t u r e of the experiment i s in q u a l i t a t i v e agreement with a simple calculation, though the d e v i a tion is appearing b e l o w 40 K. I t s h o u l d be n o t i c e d t h a t 1/T 1 b e l o w 40 K a p p r o x i m a t e s to t h e T3 b e h a v i o r w h i c h i s p r e d i c t e d from t h e anisotropic e n e r g y gap w i t h t h e gap z e r o s of l i n e s . Thus, i t seems p l a u s i b l e t h a t the o r d e r p a r a m e t e r i s of d - s y m m e t r y t y p e w i t h a l a r g e r a t i o of 2 A / k B T c f o r YBa2Cu3OT-y with Tc=92 K. REFERENCES (1) H.K. Wu, J.R. A s h b u r n , C.J. Torng, P.H. Her, R.L. Meng, L. Gap, Z . J . Gap, Z . J . Huang, Y.Q. Wang and C.W. Chu: Phys. Rev. L e t t . 58 (1987) 908. (2) R.E. W a l s t e d t , W.W. Warren, J r . , R.F. B e l l , G.F. B r e n n e r t , G.P. E s p i n o s a , J . P . Remeika, R. J. Cava, and E. A. R i e t m a n : Phys. Roy. B36 (1987) 5727. (3) I. Furo, A. J a n o s s y , L. Mihaly, P. B a n k l , I. P r o c s i c , I. Bakonyi, [ . H e i n m a a , E. Joon and E. L i p m a a : P h y s . Rev. B36 (1987) 5690. (41 M. Mall, D. Brinkman, L. P a u l i , J. Roos, H. Zimmermann and J. H u l l i g e r : Phys. L e t t . A124 (1987) 112. (5) Y. K i t a o k a , S. H i r a m a t s u , T. Kondo and K. Asayama: J. Phys. Soc. Jpn. 5-7 (1988) 30. (6) Y. K i t a o k a , S. H i r a m a t s u , K. I s h i d a , T. Kohara, Y. Oda, K. Amaya and K. Asayama: P h y s i c a 148B (1987) 298; T. Kohara, Y. K o h o r i , Y. K i t a o k a , S. H i r a m a t s u , K. Ueda, M. K a b u r a g i , Y. Oda and K. Asayama : i b i d , 292. (7] Y. K o h o r i , H. S h i b a i , Y. Oda, T. Kohara, Y. K i t a o k a and K. Asayama: J. Phys. See. Jpn. 57 (1988) No 3. (8) Y. K i t a o k a , S. H i r a m a t s u , K. I s b l d a , K. Asayama, H. T a k a g i , H. I w a b u c h i , S. Uchida and S. Tanaka: J. Phys. Soc. Jpn. 57 (1988) No. 3. {9] H. T a k a g l , S. Uchida, H. I w a b u c h i , H. E i s a k i , K. K i s h i o , K. K i t a z a w a , K. Fueki and S. Tanaka: P h y s l c a 148B (1987) 349. fIO] K. K i s h i o , J. Shimoyama, T. Hasegawa, K. E i t a z a w a and K. F u e k i : Jpn. J. Appl. Phys. 26 (1987) L~228.