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FOUR-PHONON PROCESSES IN SOLIDS
T. Singh, G. Verma
To cite this version:
T. Singh, G. Verma. FOUR-PHONON PROCESSES IN SOLIDS. Journal de Physique Colloques, 1981, 42 (C6), pp.C6-155-C6-157. �10.1051/jphyscol:1981647�. �jpa-00221583�
JOURNAL DE PHYSIQUE
CoZZoque C6, suppZ6ment au n012, Tome 42, de'cembre 1981 page C6-155
FOUR-PHONON PROCESSES I N S O L I D S
T.J. Singh and G.S. Verma
Physics Department, Bmzaras Hindu Ilniversity, Varanasi, 221 005, India
A b s t r a c t . - The r e l a x a t i o n r a t e o f four-Phonon P r o c e s s e s i s d e r i v e d u s i n g a diagraminatic f i e l d - t h e o r e t i c approach. The r e s u l t s a r e i n agreement w i t h t h e e a r l i e r p r e d i c t i o n s of Pomeranchuk and r e c e n t p e r t u r b a t i o n r e s u l t s o f Ecsedy and Klemens f o r a r e l a t i v e l y s i m p l e model o f a s o l i d f o r which
i t i s p o s s i b l e t o o b t a i n a n a l y t i c a l e x p r e s s i o n .
Four-Phonon p r o c e s s e s c a n a r i s e i n two ways v i a t h e
p e r t u r b a t i o n t e r m s (i H41 f ) and <i H31
1)(11~~~
~ ) / ( E ~ - E ~ ) . H and H a r e r e s p e c t i v e l y t h e c u b i c and q u a r t i c anharmonic p a r t s3 4
o f t h e c r y s t a l H a m i l t o n i a n . The e q u i v a l e n t diagrams a r e shown i n F i g . 1. F i g . l ( b ) h a s a v e r y i n t e r e s t i n g f e a t u r e . I t i s p o s s i b l e t o c o n s e r v e energy a t t h e i n t e r a c t i o n v e r t e x g i v i n g r i s e t o a
r e s o n a n c e o f t h e Briet-Wigner t y p e i n t h e Phonon-Phonon i n t e r a c t i o n . P r o c e s s e s d e p i c t e d i n F i g . l ( c ) a r e well-behaved and hence t h e r e w i l l be no r e s o n a n c e . We d i s c u s s r e s o n a n c e e f f e c t s i n a s e p a r a t e p a p e r 1
.
So f o r t h e p r e s e n t we c o n c e r n t r a t e on F i g . l ( a ) .We s t a r t by d e f i n i n g a one-Phonon G r e e n ' s f u n c t i o n and c a l c u l a t i n g t h e phonon s e l f energy a s s o c i a t e d w i t h t h e d i a g r a m s ( F i g . 2 . ) . The s e l f - e n e r g y i s found t o b e a complex q u a n t i t y ; t h e r e a l p a r t merely s e r v i n g t o r e n o r m a l i z e t h e phonon f r e q u e n c i e s and t h e i m a g i n a r y p a r t i s o u r r e l a x a t i o n r a t e f o r four-phonon p r o c e s s e s . T h i s r a t e when s i m p l i f i e d f o r h i g h t e m p e r a t u r e c a s e , becomes
-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981647
JOURNAL DE PHYSIQUE
where A($) i s t h e l a t t i c e Kronecker d a l t a , t h e summations over 1 2 3 i n d i c a t e summations over a l l wave v e c t o r s q l s and o v e r a l l
p o l a r i s a t i o n branches S 1 s , K i s t h e Boltzmann c o n s t a n t , N i s t h e number of u n i t c e l l s i n t h e c r y s t a l and y ( q ) I s a r e t h e atomic f o r c e c o n s t a n t s and c),
--
d ( q l s 1 ) and e t c .For a f . c . c . c r y s t a l w i t h c e n t r a l n e a r e s t neighbour i n t e r a c t i o n s t h e atomic f o r c e c o n s t a n t s y ( q ) I s become, i n t h e l o n g wavelength l i m i t
2n4aaO4 A
4
-
IT(-+ ,:lsl. 92s2. a3s3)1 = M sq1q2q3 ( 2 )
where a. i s t h e l a t t i c e c o n s t a n t , and M mass o f each atom, a i s t h e e q u i l i b r i u m f o r c e c o n s t a n t and t h e f a c t o r A c o n t a i n s a l l t h e
F i g . 1 : Diagrammatic r e p r e s e n t a t i o n s of 4-phonon p r o c e s s e s .
F i g . 2 : Diagrams c o n t r i b u t i n g t o t h e s e l f - e n e r g y .
i n f o r m a t i o n r e g a r d i n g a n g u l a r dependences of t h e v a r i o u s modes p r o p a g a t i n g t h r o u g h t h e l a t t i c e . Using ( 2 ) i n ( l ) , we g e t
r ( Z s , ~ )
= ( ; s ) T ~where A' = 2n16 a 2 ~ 2 ~ 2 a 0 9 / 3 v 9 ~ 4 and v i s a s u i t a b l y a v e r a g e d phonon v e l o c i t y .
E q u a t i o n ( 3 ) a g r e e s w i t h t h a t s u g g e s t e d by ?omeranchuk2 and t h e r e c e n t p e r t u r b a t i o n r e s u l t o f Ecsedy and Klemens3. However, t h e i r t r e a t m e n t s u f f e r s from t h e b a s i c assumption t h a t a l l phonons e x c e p t t h e one u n d e r c o n s i d e r a t i o n a r e i n t h e r m a l e q u i l i b r i u m . Such a n a s s u m p t i o n i s u n n e c e s s a r y i n t h e p r e s e n t d e r i v a t i o n which a l s o t a k e s c a r e o f a l l p o s s i b l e modes of c o m b i n a t i o n s of phonons i n a
n a t u r a l waye Moreover, f o r any model o f a s o l i d t h e r e l a x g t i o n r a t e o f 4-phonon p r o c e s s e s c a n be c a l c u l a t e d by u s i n g eqn. (1) B e s i d e s t h e r e a l p a r t of t h e s e l f - e n e r g y f u n c t i o n , t h e r e i s a n o t h e r s o u r c e of r e n o r m a l i z a t i o n of phonon f r e q u e n c i e s . Becaxse o f t h e r m a l e x p a n s i o n t h e f r e q u e n c i e s o f phonons may be changed4 and s u c h a change can be e a s i l y c a l c u l a t e d . We f i n d t h a t f o r a p u r e l y anharmonic c r y s t a l t h e change i s o f n e g l i g i b l e o r d e r and t h a t so f a r a s t e m p e r a t u r e dependence o f t h e r e l a x a t i o n r a t e i s concerned t h e e f f e c t of t h e r m a l e x p a n s i o n i s n e g l i g i b l e . Thus t h e r m a l e x p a n s i o n a l e n e c a n n o t e x p l a i n t h e T 2 -dependence o f t h e observed phonon c o n d u c t i v i t y i n s e m i c o n d u c t o r , especially i n S i 5
.
R e f e r e n c e s .
1. T.J.Singh and G=S.Verma, Phys. Rev. B (Accepted)
2. I.Pomeranchuk, J.Phys. VSSR,
4 ,
259(1941) ; 7 , 1 9 7 ( 1 9 4 3 ) . 3. D.J.Ecsedy and P.G.Klemens, Phys. Rev.M ,
5957(1977).4 . A.A.Marad.udin and A.E.Fein, Phys. Rev.
128.
2589(1962).5. Y.P.Joshi, M.D.Tewari and G.S.Verma, Phys. Rev.