Phonons in high-Tc superconductivity mechanism

Physica C 235-240 (1994) 2395-2396 North-Holland

Phonons In HI M.F.Limonov,

A.F.loffe

PHYSICA

-T c Superconductivity Mechanism and A.G.Panfilov

Phys.-Tech.

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T h e c o r r e l a t i o n b e t w e e n critical t e m p e r a t u r e s Tc and freq u e n c i e s of intense lines in Raman s p e c t r a of per'ovskite-like s u p e r c o n d u c t o r s has been discussed. It has been revealed that there is a good agreement between experimental TO and t h o s e c a l c u l a t e d in the f r a m e w o r k of the E l i a s h b e r g theory at the same k ~ 1.5-2 and ~" ~ 0.1-0.2 for all HTSC families. There is increasing evid e n c e in a f a v o u r of the p h o non nature of superconductivity in HTSC. Among such indications, we distinguish those in v i b r a t i o n a l spectra. Electron-phonon interaction to m a n i f e s t itself just in the Haman s p e c t r a is connected with the nature of the H a m a n s c a t t e r i n g . The p o s i t i ons, i n t e n s i t i e s and shapes of the H a m a n lines immediately d e p e n d on the strength of this interaction. That is w h y the most intense Raman lines s h o u l d be the focus of attention. The correlation [i] b e t w e e n T o and f r e q u e n c i e s of the characteristic, intense H a m a n lines seems to be a pronounced manifestation of the i m p o r t a n c e of p h o n o n s in high-T o superconductivity mechanism. F i g u r e 1 explai n s considering the c o r r e l a t i o n , that the h i g h e s t T e in each superconducof four h i g h - T c tot families a r e the followin L a -based ing: T o , m a x ~ 36K (123) one, family, 92K in Th%a

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IIOK in Bi-and finally 125K in Tl-family. Two lines are are marked with arrows: the m o s t intense line with the h i g h e s t f r e q u e n c y in the s p e c t r u m with z z - p o l a r i z a t i o n and the intense one with xx(or y y - or xy-) polarization. E m p h a s i z e that the var~able units of superconductor lattices give rise to the intense Haman lines of the h i g h e s t frequency. The latter are the A I s / A g v i b r a t i o n s of o x y g e n atoms that belong to the layers s e p a r a t i n g c o p p e r o x i d e planes (see F~gure i). One can see that the higher characteristic frequen--cies of intense lines the h i g h e r Tc,ma x of s u p e r c o n d u c tor family. We believe that the c o r r e l a t i o n appears to be not accidental and to reflect a coiim~on proportionality between TO and an average frequency describing the w h o l e v i b r a t i o n a l spectrum. Considering the Haman s p e c t r a as a s o u r c e of inform a t i o n about the Eliashberg function a?F(~), it turned

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F i g u r e i. Raman spectra of the La-, Y-, Bi-, TIHTSC, and Nb3Sn [1-4]. Vertical dashed lines correspond to the f r e q u e n c i e s ~c = 2 ~ T c . m a x. T h e f r a g m e n t s of lattices of the H T S C c o m p o u n d s are given. out to be possible to describe this correlation. For that, the T c v a l u e s were c a l c u l a t e d ( G . V . K l i m o v i c h and A . V . R y l y a k o v ) u s i n g the Eliashberg equations [5], the frequencies of Raman-active m o d e s being t a k e n as a basis. The c a l c u l a t i o n s w e r e p e r f o r m e d using several models of the spectral density of electron-phonon interaction S(u) (see F i g u r e 2) w i t h d i f ferent c o u p l i n g c o n s t a n t s A C o u l o m b p s e u d o p o t e n t i a l H~. One can see f r o m F i g u r e 2 that the shape of S(~) is not so essential as the S(u) s p e c t r u m basic f r e q u e n c y v a l u e s . The m a i n r e s u l t consists in a good agreement between exp e r i m e n t a l and c a l c u l a t e d Tc for all s u p e r c o n d u c t o r families at the s a m e k ~ 1.5 ÷ 2 and H ~ = 0.I + 0.2.

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F i g u r e 2. C a l c u l a t e d c r i t i c a l t e m p e r a t u r e T c vs parameters k and ~* dependences. H o r i z o n t a l d a s h e d lines corr e s p o n d to T c , m a x of the La-, Y-, Bi-, and T l - b a s e d superc o n d u c t o r families. The region of agreement between calculated and experimental T o , m a x v a l u e s is e n c l o s e d by dotted lines. Insert shows t h r e e m o d e l s of ~2F(p) used in the c a l c u l a t i o n s .

REFERENCES

i. M . F . L i m o n o v et al. , Sol.St.Comm., 75 (1990) 511. --. H oc- h i anz e t a ] P h y s . R e v . B, 22 ( 1 9 8 0 ) 2 3 8 6 . 3. L . V . G a s p a r o v et al., Sov. JETP L e t t . , 49 ( 1 9 8 9 ) 6 8 . 4. J . S . P o n o s o v , G.A.Bolotin, Sov.JETP Lett., 49 ( 1 9 8 9 ) 16. 5. @ . M . E l i a s h b e r g , Sov.-JETP, 99 ( 1 9 6 0 ) 1 4 3 7 . .

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