TESTS OF CRITERIA FOR THE EXISTENCE OF COLLECTIVE EXCITATIONS IN LIQUID METALS

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TESTS OF CRITERIA FOR THE EXISTENCE OF

COLLECTIVE EXCITATIONS IN LIQUID METALS

O. Söderström, J. Copley, J.-B. Suck, B. Dorner

To cite this version:

JOURNAL DE PHYSIQUE Colloque C8, suppZdment au n o 8 , Tone 41, aoiit 1980, page

Cg-230

TESTS OF C R I T E R I A FOR THE EXISTENCE OF COLLECTIVE EXCITATIONS I N L I Q U I D METALS

Y X Y X Y

0 . SBderstrSm. J.R.D. Copley

.

J . - 6 . S u c k and 8. Dorner

Department of Reactor Physics, The Royal I n s t i t u t e of Technology, Fack, 5-10044 S t o c k h o b , Sweden

x Department o f Physics, McMaster U n i v e r s i t y , Hamilton, OntcuYio L8S 4M1, Canada

x x I n s t i t u t Laue-Langevin, 156 X Centre de R.i, 38042 Crenoble Cedes, France

A b s t r a c t . - C o l l e c t i v e d e n s i t y f l u c t u a t i o n s have been observed i n l i q u i d lead by thermal neutron s c a t t e r i n g experiments. Comparisons a r e made w i t h observations i n o t h e r l i q u i d s and some c r i t e r i a a r e t e s t e d . The i n f l u e n c e of t h e e f f e c t i v e p a i r p o t e n t i a l i s a l s o discussed. 1 . I n t r o d u c t i o n E x p e r i m e n t a l e v i d e n c e c u r r e n t l y a v a i l a b l e s u g g e s t s t h a t , f o r l i q u i d a l k a l i m e t a l s , r a t h e r l o n g - l i v e d c o l l e c t i v e e x c i t a t i o n s e x i s t w i t h w a v e l e n g t h s o f t h e same o r d e r o f m a g n i t u d e 2s t h e d i s t a n c e b e t w e e n t h e a t o m s . S u c h e x c i t a t i o n s a r e o b s e r v e d i n c o n d e n s e d r a r e g a s e s o n l y f o r much l o n g e r w a v e l e n g t h s . The e v i d e n c e d e r i v e s f r o m n e u t r o n i n e l a s t i c s c a t t e r i n g ( N I S I m e a s u r e m e n t s on f l u i d neon ( 1 1 a n d l i q u i d r u b i d i u m ( 2 ) a s w e l l a s mo- l e c u l a r d y n a m i c s ( M D ) s t u d i e s o f a L e n n a r d - J c n o s (L.2) s y s t c ~ n ( 3 1 a n d a s y s t z n i r . p r e - s e n t i n g l i q u i d r u b i d i ~ m ( 4 ) . A p o s s i b l e e x p l a n a t i o n i s t h a t t h e e f f e c t i v e p a i r p o - t e n t i a l b e t w e e n a t o m s ( t o g e t h e r w i t h t h e p o s i t i o n o f t h e s y s t e m i n i t s own p h a s e d i a g r a m ] i s r e s p o n s i b l e f o r t h i s d i f f e r e n - c e , b u t t h e r e l a t i v e i m p o r t a n c e o f d i f f e - r e n t f e a t u r e s o f t h e p o t e n t i a l i s n o t com- p l e t e l y c l e a r . Though t h e h a r d n e s s o f t h e r e p u l s i v e c o m p o n e n t i s a m a j o r f a c t o r , t h e i n f l u e n c e o f t h e s h a p e o f t h e p o t e n t i a l m i - nimum i s much h a r d e r t o a s s e s s . I n o r d e r t o g a i n f u r t h e r i n s i g h t i n t o t h i s p r o b l e m

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