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Low temperature microwave response of YBaCuO superconductors: narrow resonance-like peak in ESR
Synthetic Metals, 29 (1989) F 5 4 1 - F 5 4 6
F541
LOW T E M P E R A T U R E M I C R O W A V E R E S P O N S E OF Y B a C u O SUPERCONDUCTORS: N A R R O W R E S O N A N C E - L I K E P E A K IN ESR
ILIAS I. KHAIRULLIN,
A N V A R A. ZAKHIDOV and PULAT K. K H A B I B U L L A E V
Institute for N u c l e a r Physics, U z b e k SSR A c a d e m y of S c i e n c e s Ulugbek, Tashkent, 702132 (U.S.S.R.) LEVON S. G R I G O R I A N Institute of Physics, (U.S.S.R.)
A r m e n i a n A c a d e m y of Sciences,
Erevan,378410
ABSTRACT New f e a t u r e s of l o w - f i e l d signal (LFS) in Y B a C u O s u p e r c o n d u c t i n g c e r a m i c s (SC) and their c o m p o s i t e s w i t h p o l y a c e t y l e n e are studied in ESR. The n a r r o w r e s o n a n c e - l i k e c o m p o n e n t (RC) of LFS is r e v e a l e d at n e a r - z e r o field H ~ 0 . 5 G. RC is found to be p r o n o u n c e d at low T 10K, s e n s i t i v e to m a g n e t i c field (destroyed at H > Hb(T) ), to modulation amplitude ( H m ~ ) and m i c r o w a v e power level P. D e c r e a s e of RC and b a c k g r o u n d (BG) LFS intensity is o b s e r v e d at e v a c u a t i o n of the sample, r e v e r s i b l y i n c r e a s i n g in the atlmosphere of gaseous oxidants: ambient O~ or J2 -vapour. RC d e m o n s t r a t e s n e g l i g i b l e h y s t e r e s i s upon m a g n e t i c c y c l i n g c o n t r a r y to significant h y s t e r e s i s within BG i m p l y i n g thus the d i s t i n c t origin of two LFS components.
INTRODUCTION High Tc s u p e r c o n d u c t i n g oxides [I] have been shown r e c e n t l y to exhibit a v a r i e t y of unusual p r o p e r t i e s c o n n e c t e d with m i c r o w a v e (MW) absorption [2-10]. In the E S R - s t u d y b r o a d a s y m m e t r i c low field signal (LFS) is o b s e r v e d in all Y B a C u O family ceramics at T < Tc [2-6, 9-10] and r e c e n t l y studied in m o n o c r y s t a l s [7-8].In most papers LFS is i n t e r p r e t e d in terms of w e a k l y coupled s u p e r c o n d u c t i n g (SC) J o s e p h s o n junctiones [2-6]. Some other explanation's have been p r o p o s e d b a s e d on other ideas, i n c l u d i n g p a r a e l e c t r i c r e s o n a n c e [5], etc. [9]. Up to now h o w e v e r n e i t h e r the m e c h a n i s m of LFS is clarified, nor even m a i n LFS features are well studied. In this p a p e r we present some new e x p e r i m e n t a l results connected w i t h LFS at low t e m p e r a t u r e s and at influence of external gaseous oxidants and focus on two q u a l i t a t i v e l y new observationes: I) r e s o n a n c e like c o m p o n e n t (RC) is found at H 0.5 G. RC is very sensitive to e x p e r i m e n t a l condition,s and the sample quality and is easily q u e n c h e d by s c a n n i n g H> H (T) and by increase of m o d u l a t i o n amplitude, Hmod. 2) the gaseous oxidants are found to influence s i g n i f i c a n t l y LFS intensity and p a r t i c u l a r l y RC. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
F542 EXPERIMENT YBaCuO ceramics (with ~ 9 0 K and ~ T ¢ ~ I - 2 K) were p r e p a r e d using the standard method of solid-phase synthesis at 950 C. Composites with p o l y a c e t y l e n e (CH)~ were p r e p a r e d from Y B a C u O powders and, (CH)~- L u t t i n g e r gels m i x e d u l t r a s o n i c a l l y and formed into films by solvent evaporation. The samples placed in quartz tubes were sealed in the ambient atmosphere or in v a c u u m p=10 -= Tort. M e a s u r e m e n t s of MW a b s o r p t i o n were p e r f o r m e d on a Bruker ESP-300 spectrometer, equiped with ESP-1600 Data System, w o r k i n g in the X band (heO=9.5 GHz) and up to i00 kHz field m o d u l a t i o n frequency. T e m p e r a t u r e was controlled by He g a s - f l o w "Oxford instruments ESR-900 cryostat" that allowed T s t a b i l i z a t i o n w i t h ! 0.2 K p r e c i s i o n till 5 K. To obtain strict zero of H field and n e g a t i v e fields up to H= -50 G t h e magnet of s p e c t r o m e t e r was a d d i t i o n a l l y e q u i p p e d by external constantcurrent Helmholz coils.
RESULTS AND DISCUSSION
I. N a r r o w r e s o n a n c e - l i k e c o m p o n e n t
(RC)
As is well e s t a b l i s h e d all high Tc family SCs d e m o n s t r a t e the broad (aH~400 G) and a s y m m e t r i c LFS of MW a b s o r p t i o n b e l o w Tc [2-10] However in some good quality Y B a C u O samples we have found the appearance at H ~ 0 . 5 G of rather unusual n a r r o w structure [9,10] in LFS called b e l o w RC which appears in rather special conditions: c o o l i n g to SC state (T < ~ ) is p e r f o m e d in zero external magnetic field (H=0 with ~ 0 . 2 G accurancy, achieved by additional coils). So RC could h a r d l y be o b s e r v e d in usual ESR spectrometer, due to nonzero residual m a g n e t i z a t i o n of e l e c t r o m a g n e t H ~ 2 5 G -
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Fig. l a) R e s o n a n c e - l i k e c o m p o n e n t (RC) of LFS a p p e a r a n c e at scanning from -H to +H (f~r each + and - RC A and B components are shown at insert), P=2~10- W, H n ~ = ~ . 0 2 G b) -Overmodulation of RC at increase of H m o d at T=5 K, P=2xl0- W.
F543
H m o 6 should be low enough: Hmod<0.5 G, with optimal Hmod~0.02 G On the other hand RC is better observed at higher MW power levels. RC is characterised by A and B components ( F i g . l a ) , and could be most correctly studied at scanning from H=0 to certain H < H b (as in Fig.2a). RC peak symmetrically images into negative H and for convenience we sometimes below represent the full spectrum scanned from -H to +H showing decreased RC (due to H / 0) together with its negative image. The usual broad assymetric LFS that is observed always in SC states [2-10] of YBaCuO assumed below as background (BG) component. It should be noted that in the optimal conditions the intensity of RC is rather high and several times exceeds the BG intensity, BG being normally masked at narrow H-scanning, used for RC study. To confirm that Re is not experimental artifact, but is connected with some real physical process we have examined the behaviour of RC on H R o d , magnetization H, environment conditiones at various T, and compare the behaviour of RC (A and B parts) with BG: I) Re is very sensitive to H med. Fig.lb presents LFS at different values of Hmad. RC decrease with increase of Hmo3 and disappears above Hmod=0.5 G. This behaviour is quite similar to overmodulation of narrow lines in traditional ESR. 2) RC significantly depends on magnetic history. Most sharp and intensive lines at T=5-7 K are obtained after cooling in strict H=0 RC decreases with H-scanning and completely disappears above
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Fig.2. a) Decrease of RC (both A and B components) after magnetization in constant H field: 1-first scanning after cooling in H=0; 2-- Hma~=50 G' 3 - H m a _ ~ l S 0 G; 4- HmQ~%=300 G; b) Dependence of RC A-intenslty on Hmq~, at varlous temperatures; RC destroyed above boundary Hb: Hb(70 K)=I00 G, Hb(5 K)=320 G. •
.
'
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.
F544
boundary Hb=Hb(T): Hb(70 K)=100 G and H5(5 E)=320 G (Fig.2b). It should be e m p h a s i s e d that both A and B components of RC decrease and d i s a p p e a r with H (Fig.2a). This indicates that RC line could be viewed as assymetric Dyson shape a b s o r p t i o n signal w i t h the phase opposite to the phase of c o n v e n t i o n a l ESR (e.g. of C u Z ÷ p a r a m a g n e t i c ions in YBaCuO (insert in Fig.2a)). 3) The important o b s e r v a t i o n is that h y s t e r e s i s known for LFS ~6~] is strongly suppressed in RC region: at Hmo4=O.02 G (Fig. la) h y s t e r e s i s is p r a c t i c a l l y absent within RC region, while for Hmod=0.0OiG (when h y s t e r e s i s is r e v e r s e d in BG (Fig.4a)), it is also n a r r o w e r in RC region. It seems that h y s t e r e s i s is c o n n e c t e d with BG and this indicates that RC and BG have d i f f e r e n t origines, and the distinct sensitivity of both to e n v i r o n m e n t atmosphere, discussed below, also supports this idea. 2. Influence of gaseous oxidants
(02 and ~ !
If the tube w i t h Y B a C u O sample is evacuated, the intensity of LFS is s i g n i f i c a n t l y d e c r e a s e d [5,10]. At ambient a t m o s p h e r e intensity of LFS signal r e v e r s i b l y increases while in the atmosphere of inert gases e.g.in Ar [I0] (or Ne [5]) LFS is the s a m e as in v a c u u m . But if 02 is s u b s t i t u t e d by J a v a p o u r LFS increases again though to smaller values [I0]-. This means that only strong gaseous oxidants may influence the LFS intensity, after evacuation. The key result here is the different t e m p e r a t u r e b e h a v i o u r of A and B components of RC in v a c u u m and in O a a t m o s p h e r e clear from Fig.3. Intensity of A d e m o n s t r a t e s m a x i m u m uppon cooling in atmosphere (at T~50 K) while in vacuum A shows m o n o t o n o u s T-behaviour, so that ratio A w e / A Q ~
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TEMPERATURE Fig.3.
(K)
T e m p e r a t u r e d e p e n d e n c e of RC components in a~bient a t m o s p h e r e and in vacuum, Hm~=0.02 G, P = 2 ~ I O ° W.
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Fig.4. H y s t e r e s i s of LFS d e c r e a s e d in RC region at small Hmod=0.O01G (compare to Fig. la with H m 0 d = 0 . 0 2 G) Fig.5. H y s t e r e s i s of BG c o m p o n e n t (measured at H=50 G) versus H in: (a) ~ - i r r a d i a t e d samples, (b) [YBaCuO]~s[(CH)× ]01 composite. White symbols- dH/dt>0 (forward scan), b l a c k - dH/dt<0. T= I0 K.
3.
BG h y s t e r e s i s and shifting.
The h y s t e r e s i s of BG c o m p o n e n t is known [2,3,8] to be sensitive to Hm0d. We have o b s e r v e d that at low T in YBaCuO c o m p o s i t e s with (CH) and in ~ - i r r a d i a t e d samples the whole structure rf h y s t e r e s i s is changed (compared to standard Y B a C u O samples) as shown at Fig.5: h y s t e r e s i s is shifted in ~-irradiated samples [12], while in c o m p o s i t e - it is compressed.
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F546
Upon m a g n e t i c cycling the m a x i m u m of BG shifts to h i g h e r H as observed by many authors [2-4,7-10]. We have studied the b e h a v i o u r of Hm~fBG s h i f t i n g on the m a g n e t i z a t i o n field H,=9~at various temperatures (Fig.6a). S h i f t i n g is found to increase at low T and to saturate above certain H (Fig.6b). This b e h a v i o u r is a p p a r e n t l y c o n n e c t e d with flux t r a p p i n g and the c o r r e s p o n d i n g s h i f t i n g of the reference point for H. At low t e m p e r a t u r e s the flux is trapped more e f f e c t i v e l y [7] leading to more effective BG-shifting.
SUMMARY The origin of RC is not clear, but RC d i s a p p e a r a n c e w i t h small H implies the surface nature of RC since and p e n e r a t i o n of H into the granules destroys Re. The opposite phase of RC r e l a t i v e to conventional ESR means either the e m i s s i o n of MW [2], (that seems unreal), or the c o n t r i b u t i o n of some type of d i a m a g n e t i c resonance response at v e r y low H-fields. The last could be c o n n e c t e d with c i r c u l l a r SC currents on granules surfaces. On other hand unusual m a g n e t i c b e h a v i o u r of h i g h l y c o r r e l a t e d states of "spin liquid" or "quantum p a r a m a g n e t i c " p r o p o s e d recently for new SC family [ii] also could not be excluded. In any case s e n s i t i v i t y of RC and BG to ambient 02 means that upon e v a c u a t i o n in the surface layer of YBaCuO the 02 content is changed from the one optimal for SC-tM; and this "undopping" effect m o d u l a t e s b o t h the p r o p e r t i e s of t u n n e l i n g J o s e p h s o n junctions between granules (influence on BG), and the very surface p r o p e r t i e s that are proposed here to be c o n n e c t e d with RC [i0].
ACKNOWLEDGEMENTS We are indebted to A.I. G o l o v a s h k i n and K.V. Mitzen who prepared and kindly p r o v i d e d h i g h quality Y B a C u O SC samples, to M.H. A s h u r o v for Y B a C u O powders, and also gratefull to M.V. Yakovina for p r e p a r a t i o n of composites w i t h (CH)x. Useful d i s c u s s f o n s w i t h A.R. A r u t u n i a n are also appreciated.
REFERENCES 1 2
J.G. Bednortz and K.A. Muller. Z...Phys. B.64 (1986) 189. V.I. Alexandrov, A.G. B a d a l y a n et.al., Fisika. Tverd. Tela. 29 (1987) 3710. 3 A.R. Arutunian, L.S. Grigorian, Abstr. Intern. Conf. on "Low T e m p e r a t u r e Physics", B u d a p e s t (1987) 31. 4 C. Rettorl, D. Davidov, I. Belaish, I. Felner, Phys. Rev. B. t v.38 (1987) 4028. 5 B.V. Bhat, P. Ganuly, T.V. Ramakrishnan, C.N.R. Rao, J. Phys. C, 20 (1987) L559. 8 K. K h a c h a t u r y a n et. al., Phys. Rev. B.. 36 (1987) 8308. 7 V.I. Alexandrov, A.G. B a d a l y a n et. al., Pis'ma Zh. Eks. Teor. Fis. 4 7 (1988) 169. 8 S.V. Bogachev, et.al., Pis'ma JETF 47 (1988) 166. 9 A.R. Arutunian, L.S. Grigorian, A.V. Gevorkian, A.C. Kuzanian, P r e p r i n t Inst. Physics Arm. SSR A c a d e m y of Sciences 88 - 130 (1988) 30pp. I0 P.K. Khabibullaev, A.A. Zakhidov, I.I. Khairullin, S.V. Martinchenko, P r e p r i n t Inst. N u c l e a r Phys. R-9 - 381 (1988) 35 pp. ii P. Wiegman, P r e p r i n t of Landau Inst. Theor. Phys., M o s c o w (1988) Phys. Rev. Left. 59 (1988) 918.
F541
LOW T E M P E R A T U R E M I C R O W A V E R E S P O N S E OF Y B a C u O SUPERCONDUCTORS: N A R R O W R E S O N A N C E - L I K E P E A K IN ESR
ILIAS I. KHAIRULLIN,
A N V A R A. ZAKHIDOV and PULAT K. K H A B I B U L L A E V
Institute for N u c l e a r Physics, U z b e k SSR A c a d e m y of S c i e n c e s Ulugbek, Tashkent, 702132 (U.S.S.R.) LEVON S. G R I G O R I A N Institute of Physics, (U.S.S.R.)
A r m e n i a n A c a d e m y of Sciences,
Erevan,378410
ABSTRACT New f e a t u r e s of l o w - f i e l d signal (LFS) in Y B a C u O s u p e r c o n d u c t i n g c e r a m i c s (SC) and their c o m p o s i t e s w i t h p o l y a c e t y l e n e are studied in ESR. The n a r r o w r e s o n a n c e - l i k e c o m p o n e n t (RC) of LFS is r e v e a l e d at n e a r - z e r o field H ~ 0 . 5 G. RC is found to be p r o n o u n c e d at low T 10K, s e n s i t i v e to m a g n e t i c field (destroyed at H > Hb(T) ), to modulation amplitude ( H m ~ ) and m i c r o w a v e power level P. D e c r e a s e of RC and b a c k g r o u n d (BG) LFS intensity is o b s e r v e d at e v a c u a t i o n of the sample, r e v e r s i b l y i n c r e a s i n g in the atlmosphere of gaseous oxidants: ambient O~ or J2 -vapour. RC d e m o n s t r a t e s n e g l i g i b l e h y s t e r e s i s upon m a g n e t i c c y c l i n g c o n t r a r y to significant h y s t e r e s i s within BG i m p l y i n g thus the d i s t i n c t origin of two LFS components.
INTRODUCTION High Tc s u p e r c o n d u c t i n g oxides [I] have been shown r e c e n t l y to exhibit a v a r i e t y of unusual p r o p e r t i e s c o n n e c t e d with m i c r o w a v e (MW) absorption [2-10]. In the E S R - s t u d y b r o a d a s y m m e t r i c low field signal (LFS) is o b s e r v e d in all Y B a C u O family ceramics at T < Tc [2-6, 9-10] and r e c e n t l y studied in m o n o c r y s t a l s [7-8].In most papers LFS is i n t e r p r e t e d in terms of w e a k l y coupled s u p e r c o n d u c t i n g (SC) J o s e p h s o n junctiones [2-6]. Some other explanation's have been p r o p o s e d b a s e d on other ideas, i n c l u d i n g p a r a e l e c t r i c r e s o n a n c e [5], etc. [9]. Up to now h o w e v e r n e i t h e r the m e c h a n i s m of LFS is clarified, nor even m a i n LFS features are well studied. In this p a p e r we present some new e x p e r i m e n t a l results connected w i t h LFS at low t e m p e r a t u r e s and at influence of external gaseous oxidants and focus on two q u a l i t a t i v e l y new observationes: I) r e s o n a n c e like c o m p o n e n t (RC) is found at H 0.5 G. RC is very sensitive to e x p e r i m e n t a l condition,s and the sample quality and is easily q u e n c h e d by s c a n n i n g H> H (T) and by increase of m o d u l a t i o n amplitude, Hmod. 2) the gaseous oxidants are found to influence s i g n i f i c a n t l y LFS intensity and p a r t i c u l a r l y RC. 0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
F542 EXPERIMENT YBaCuO ceramics (with ~ 9 0 K and ~ T ¢ ~ I - 2 K) were p r e p a r e d using the standard method of solid-phase synthesis at 950 C. Composites with p o l y a c e t y l e n e (CH)~ were p r e p a r e d from Y B a C u O powders and, (CH)~- L u t t i n g e r gels m i x e d u l t r a s o n i c a l l y and formed into films by solvent evaporation. The samples placed in quartz tubes were sealed in the ambient atmosphere or in v a c u u m p=10 -= Tort. M e a s u r e m e n t s of MW a b s o r p t i o n were p e r f o r m e d on a Bruker ESP-300 spectrometer, equiped with ESP-1600 Data System, w o r k i n g in the X band (heO=9.5 GHz) and up to i00 kHz field m o d u l a t i o n frequency. T e m p e r a t u r e was controlled by He g a s - f l o w "Oxford instruments ESR-900 cryostat" that allowed T s t a b i l i z a t i o n w i t h ! 0.2 K p r e c i s i o n till 5 K. To obtain strict zero of H field and n e g a t i v e fields up to H= -50 G t h e magnet of s p e c t r o m e t e r was a d d i t i o n a l l y e q u i p p e d by external constantcurrent Helmholz coils.
RESULTS AND DISCUSSION
I. N a r r o w r e s o n a n c e - l i k e c o m p o n e n t
(RC)
As is well e s t a b l i s h e d all high Tc family SCs d e m o n s t r a t e the broad (aH~400 G) and a s y m m e t r i c LFS of MW a b s o r p t i o n b e l o w Tc [2-10] However in some good quality Y B a C u O samples we have found the appearance at H ~ 0 . 5 G of rather unusual n a r r o w structure [9,10] in LFS called b e l o w RC which appears in rather special conditions: c o o l i n g to SC state (T < ~ ) is p e r f o m e d in zero external magnetic field (H=0 with ~ 0 . 2 G accurancy, achieved by additional coils). So RC could h a r d l y be o b s e r v e d in usual ESR spectrometer, due to nonzero residual m a g n e t i z a t i o n of e l e c t r o m a g n e t H ~ 2 5 G -
b H
T-5K
120
t
~
-1o
I
I
o
i
'
4o
F,ELO (&)
I
'
20
|
,
-2O
,
.
.
I
.
-IO
.
.
.
I
.
.
O FIELD
.
.
I
10 (~)
.
.
.
.
!
20
Fig. l a) R e s o n a n c e - l i k e c o m p o n e n t (RC) of LFS a p p e a r a n c e at scanning from -H to +H (f~r each + and - RC A and B components are shown at insert), P=2~10- W, H n ~ = ~ . 0 2 G b) -Overmodulation of RC at increase of H m o d at T=5 K, P=2xl0- W.
F543
H m o 6 should be low enough: Hmod<0.5 G, with optimal Hmod~0.02 G On the other hand RC is better observed at higher MW power levels. RC is characterised by A and B components ( F i g . l a ) , and could be most correctly studied at scanning from H=0 to certain H < H b (as in Fig.2a). RC peak symmetrically images into negative H and for convenience we sometimes below represent the full spectrum scanned from -H to +H showing decreased RC (due to H / 0) together with its negative image. The usual broad assymetric LFS that is observed always in SC states [2-10] of YBaCuO assumed below as background (BG) component. It should be noted that in the optimal conditions the intensity of RC is rather high and several times exceeds the BG intensity, BG being normally masked at narrow H-scanning, used for RC study. To confirm that Re is not experimental artifact, but is connected with some real physical process we have examined the behaviour of RC on H R o d , magnetization H, environment conditiones at various T, and compare the behaviour of RC (A and B parts) with BG: I) Re is very sensitive to H med. Fig.lb presents LFS at different values of Hmad. RC decrease with increase of Hmo3 and disappears above Hmod=0.5 G. This behaviour is quite similar to overmodulation of narrow lines in traditional ESR. 2) RC significantly depends on magnetic history. Most sharp and intensive lines at T=5-7 K are obtained after cooling in strict H=0 RC decreases with H-scanning and completely disappears above
"dP [ FI
2 3 ,4
2+
LFS
Cu
o
i ~k,\4
PIELD (GAUSS)
II
\. 4ol,-.
_ k
Fig.2. a) Decrease of RC (both A and B components) after magnetization in constant H field: 1-first scanning after cooling in H=0; 2-- Hma~=50 G' 3 - H m a _ ~ l S 0 G; 4- HmQ~%=300 G; b) Dependence of RC A-intenslty on Hmq~, at varlous temperatures; RC destroyed above boundary Hb: Hb(70 K)=I00 G, Hb(5 K)=320 G. •
.
'
g
.
F544
boundary Hb=Hb(T): Hb(70 K)=100 G and H5(5 E)=320 G (Fig.2b). It should be e m p h a s i s e d that both A and B components of RC decrease and d i s a p p e a r with H (Fig.2a). This indicates that RC line could be viewed as assymetric Dyson shape a b s o r p t i o n signal w i t h the phase opposite to the phase of c o n v e n t i o n a l ESR (e.g. of C u Z ÷ p a r a m a g n e t i c ions in YBaCuO (insert in Fig.2a)). 3) The important o b s e r v a t i o n is that h y s t e r e s i s known for LFS ~6~] is strongly suppressed in RC region: at Hmo4=O.02 G (Fig. la) h y s t e r e s i s is p r a c t i c a l l y absent within RC region, while for Hmod=0.0OiG (when h y s t e r e s i s is r e v e r s e d in BG (Fig.4a)), it is also n a r r o w e r in RC region. It seems that h y s t e r e s i s is c o n n e c t e d with BG and this indicates that RC and BG have d i f f e r e n t origines, and the distinct sensitivity of both to e n v i r o n m e n t atmosphere, discussed below, also supports this idea. 2. Influence of gaseous oxidants
(02 and ~ !
If the tube w i t h Y B a C u O sample is evacuated, the intensity of LFS is s i g n i f i c a n t l y d e c r e a s e d [5,10]. At ambient a t m o s p h e r e intensity of LFS signal r e v e r s i b l y increases while in the atmosphere of inert gases e.g.in Ar [I0] (or Ne [5]) LFS is the s a m e as in v a c u u m . But if 02 is s u b s t i t u t e d by J a v a p o u r LFS increases again though to smaller values [I0]-. This means that only strong gaseous oxidants may influence the LFS intensity, after evacuation. The key result here is the different t e m p e r a t u r e b e h a v i o u r of A and B components of RC in v a c u u m and in O a a t m o s p h e r e clear from Fig.3. Intensity of A d e m o n s t r a t e s m a x i m u m uppon cooling in atmosphere (at T~50 K) while in vacuum A shows m o n o t o n o u s T-behaviour, so that ratio A w e / A Q ~
A__ A a.u.)
b
A ,__~
B
C
B__~
A o~
'~,-
6£ 5G /I£ 30 ~0 w
o 20 ~o 60 80
0 20 40 60 80
0 20 ~0 60 80
TEMPERATURE Fig.3.
(K)
T e m p e r a t u r e d e p e n d e n c e of RC components in a~bient a t m o s p h e r e and in vacuum, Hm~=0.02 G, P = 2 ~ I O ° W.
F545 ,2,
~
b
,,.J
rl
S
~'¸
~;
~ Z o
~0 13. e~ 0 ffl )
t
I
50
20
10
I
0
"10
I
20
I
30
,6' Id' 15' 10' 16' 1~ Id' Id' I0°
F I E L D (G)
Hmod(.G)
Fig.4. H y s t e r e s i s of LFS d e c r e a s e d in RC region at small Hmod=0.O01G (compare to Fig. la with H m 0 d = 0 . 0 2 G) Fig.5. H y s t e r e s i s of BG c o m p o n e n t (measured at H=50 G) versus H in: (a) ~ - i r r a d i a t e d samples, (b) [YBaCuO]~s[(CH)× ]01 composite. White symbols- dH/dt>0 (forward scan), b l a c k - dH/dt<0. T= I0 K.
3.
BG h y s t e r e s i s and shifting.
The h y s t e r e s i s of BG c o m p o n e n t is known [2,3,8] to be sensitive to Hm0d. We have o b s e r v e d that at low T in YBaCuO c o m p o s i t e s with (CH) and in ~ - i r r a d i a t e d samples the whole structure rf h y s t e r e s i s is changed (compared to standard Y B a C u O samples) as shown at Fig.5: h y s t e r e s i s is shifted in ~-irradiated samples [12], while in c o m p o s i t e - it is compressed.
,H,.,~-BG
T=TK
F'-
Ir~
300
m
600
'~ 60
,looo.
-r 40
~.~ 6ooq
20
/.X~ ,j
O u')
0
. . .100 ..
tk0 120
(.9
~
200
FIELD (G)
3
'o0
b
T=7 K
t6~
Hd25 G~
>_ Z O
1 1
100
8O
#: I
: i
i
BOO
: i
46o0
i
i
2O0O
H mo.gn.(G)
Fig.6 (a) S h i f t i n g of BG m a x i m u m - H m a x u p o n m a g n e t i z a t i o n in various Hmagn (sample cooled in }{=25 G, when RC absent); (b) Hmax- BG versus Hma~,at various T, H m a ~ s a t u r a t e s at certain H (T).
F546
Upon m a g n e t i c cycling the m a x i m u m of BG shifts to h i g h e r H as observed by many authors [2-4,7-10]. We have studied the b e h a v i o u r of Hm~fBG s h i f t i n g on the m a g n e t i z a t i o n field H,=9~at various temperatures (Fig.6a). S h i f t i n g is found to increase at low T and to saturate above certain H (Fig.6b). This b e h a v i o u r is a p p a r e n t l y c o n n e c t e d with flux t r a p p i n g and the c o r r e s p o n d i n g s h i f t i n g of the reference point for H. At low t e m p e r a t u r e s the flux is trapped more e f f e c t i v e l y [7] leading to more effective BG-shifting.
SUMMARY The origin of RC is not clear, but RC d i s a p p e a r a n c e w i t h small H implies the surface nature of RC since and p e n e r a t i o n of H into the granules destroys Re. The opposite phase of RC r e l a t i v e to conventional ESR means either the e m i s s i o n of MW [2], (that seems unreal), or the c o n t r i b u t i o n of some type of d i a m a g n e t i c resonance response at v e r y low H-fields. The last could be c o n n e c t e d with c i r c u l l a r SC currents on granules surfaces. On other hand unusual m a g n e t i c b e h a v i o u r of h i g h l y c o r r e l a t e d states of "spin liquid" or "quantum p a r a m a g n e t i c " p r o p o s e d recently for new SC family [ii] also could not be excluded. In any case s e n s i t i v i t y of RC and BG to ambient 02 means that upon e v a c u a t i o n in the surface layer of YBaCuO the 02 content is changed from the one optimal for SC-tM; and this "undopping" effect m o d u l a t e s b o t h the p r o p e r t i e s of t u n n e l i n g J o s e p h s o n junctions between granules (influence on BG), and the very surface p r o p e r t i e s that are proposed here to be c o n n e c t e d with RC [i0].
ACKNOWLEDGEMENTS We are indebted to A.I. G o l o v a s h k i n and K.V. Mitzen who prepared and kindly p r o v i d e d h i g h quality Y B a C u O SC samples, to M.H. A s h u r o v for Y B a C u O powders, and also gratefull to M.V. Yakovina for p r e p a r a t i o n of composites w i t h (CH)x. Useful d i s c u s s f o n s w i t h A.R. A r u t u n i a n are also appreciated.
REFERENCES 1 2
J.G. Bednortz and K.A. Muller. Z...Phys. B.64 (1986) 189. V.I. Alexandrov, A.G. B a d a l y a n et.al., Fisika. Tverd. Tela. 29 (1987) 3710. 3 A.R. Arutunian, L.S. Grigorian, Abstr. Intern. Conf. on "Low T e m p e r a t u r e Physics", B u d a p e s t (1987) 31. 4 C. Rettorl, D. Davidov, I. Belaish, I. Felner, Phys. Rev. B. t v.38 (1987) 4028. 5 B.V. Bhat, P. Ganuly, T.V. Ramakrishnan, C.N.R. Rao, J. Phys. C, 20 (1987) L559. 8 K. K h a c h a t u r y a n et. al., Phys. Rev. B.. 36 (1987) 8308. 7 V.I. Alexandrov, A.G. B a d a l y a n et. al., Pis'ma Zh. Eks. Teor. Fis. 4 7 (1988) 169. 8 S.V. Bogachev, et.al., Pis'ma JETF 47 (1988) 166. 9 A.R. Arutunian, L.S. Grigorian, A.V. Gevorkian, A.C. Kuzanian, P r e p r i n t Inst. Physics Arm. SSR A c a d e m y of Sciences 88 - 130 (1988) 30pp. I0 P.K. Khabibullaev, A.A. Zakhidov, I.I. Khairullin, S.V. Martinchenko, P r e p r i n t Inst. N u c l e a r Phys. R-9 - 381 (1988) 35 pp. ii P. Wiegman, P r e p r i n t of Landau Inst. Theor. Phys., M o s c o w (1988) Phys. Rev. Left. 59 (1988) 918.