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Polarization effects in the magneto-reflection spectrum
Volume33A, number 5
POLARIZATION
PHYSICS
EFFECTS
IN
THE OF
LETTERS
16 November 1970
MAGNETO-REFLECTION CuC1
SPECTRUM
M. C E R T I E R , C. WECKER, S. NIKITINE and J. C. M E R L E Laboratoire de Spectroscopie et d'Optique du Corps Solide Groupe de R e c h e r c h e no. 15, lnstitut de Physique, Universit$ de Strasbourg, France Received 15 October 1970
The reflection s p e c t r a of single c r y s t a l s (110) and (100) of CuCI, have been studied under the action of high magnetic fields (120 kG). The polarization of the excitonic r e s i d u a l ray of the Vlf ( F ~ ) line as obs e r v e d on the m i s s i n g ray RM (25 810 c m - 1) for the t h r e e orientations (1t//( 110}, /'/77(10~) } and !1//(111 }) is in good a g r e e m e n t with the hypothesis of a paraexciton t r a n s i t i o n F 1 ~ F 2.
W e h a v e p r e v i o u s l y s t u d i e d t h e e f f e c t of h i g h m a g n e t i c f i e l d on t h e r e f l e c t i o n s p e c t r u m of t h i c k p o l y c r i s t a l l i n e s a m p l e s of CuC1 [1]. We h a v e o b served that a sharp anomaly, called RM, located a t 2 5 8 1 0 c m -1 a p p e a r s i n a m a g n e t i c f i e l d of 119 kG. T h i s a n o m a l y w a s a s c r i b e d to t h e i g n i t i o n of t h e f o r b i d d e n p a r a e x c i t o n l i n e Vlf(F2) a s s o c i a t e d to t h e a l l o w e d o r t h o e x c i t o n l i n e Vlf(F4) (25 860 c m - 1 ) . In t h i s l e t t e r , t h e p o l a r i z a t i o n e f f e c t s a s o b s e r v e d o n t h e R M a n o i n a l y a r e d e s c r i b e d . We u s e s i n g l e c r y s t a l s (100) a n d (110), p u r i f i e d by z o n e m e l t i n g t e c h n i c (10 p p m ) a n d o r i e n t e d b y X r a y s diffractions. The magnetic field H is directed a l o n g t h e m e a n a x i s (110}, (100} ( r o t a t i o n r e f l e c t i o n a x i s $4) a n d (111) ( r o t a t i o n a x i s C3). T h e angle between the incident light beam and the n o r m a l to t h e s a m p l e i s a b o u t 45 ° . T h e e l e c t r i c v e c t o r of l i g h t ~ s e l e c t e d b y a g l a z e b r o o k p o l a r i z e r i s e i t h e r p a r a l l e l o r p e r p e n d i c u l a r to t h e m a g n e t i c f i e l d H (n a n d (r c o m p o n e n t s ) . Table 1 shows the observed results. The a n o m a l y R M a p p e a r s w i t h t h e m a g n e t i c f i e l d only w h e n ~ i s p a r a l l e l to H ( u c o m p o n e n t ) , w h a t e v e r t h e o r i e n t a t i o n , t h e r e f o r e t h e p o l a r i z a t i o n of t h e r e s i d u a l r a y a s s o c i a t e d w i t h t h e l i n e Vlf(F2) i s also parallel to/'/. T h e s y m m e t r y g r o u p of t h e CuC1 c r y s t a l i s t h e t e t r a h e d r o n T d one. T h e t r a n s i t i o n b e t w e e n the ground state F 1 and the paraexciton state F 2 is forbidden in an electric dipole approximation, whereas the orthoexcition transition F 1 ~ F4 is a l l o w e d in t h e s a m e a p p r o x i m a t i o n . The magnetic field H reduces the symmetry of the c r y s t a l T d to C 3 w h e n H i s p a r a l l e l to C 3 a n d to S4 w h e n H i s p a r a l l e l to S4. A c c o r d i n g to 270
Table 1 Ignition of the paraexciton line Ylf(ro) (25 810 cm -1 at 4.2°K) in a strong magnetic fie~d (120 kG). Single c r y s t a l
(001) (110)
without H
component 7r
(r
~(110~
0
1
0
Y (111} (C3)
0
1
0
(100} ($4)
0
1
0
t h e p r o p e r t i e s of t h e e l e c t r i c d i p o l e t r a n s i t i o n , i t i s e a s y to find t h a t t h e d e g e n e r a c y of t h e F 4 e x citonic level will be lifted, giving a normal L o r e n t z t r i p l e t (one u c o m p o n e n t , two cr c o m p o nents) for all orientations. Furthermore the transition F 1 ~ F2 will become allowed in a magn e t i c f i e l d a n d t h e p a r a e x c i t o n l i n e a p p e a r s in t h e field with the same polarization as the u compon e n t of t h e n o r m a l L o r e n t z t r i p l e t of t h e F 1 -~ F 4 transition. Our results agree well with previous interp r e t a t i o n of t h e R M a n o m a l y . T h e e x i s t e n c e of t h e p a r a e x c i t o n t r a n s i t i o n F 1 -~ F 2 (25 810 c m -1) i s now w e l l e s t a b l i s h e d [ 1 - 3 ] .
References [1] M. C e r t i e r , C . W e c k e r and S. Nikitine, Phys. L e t t e r s 28A (1968) 307. [2] S. Nikitine, M. C e r t i e r , J. R i n g e i s s e n a n d J . C . M e r l e J. Physique 27 (1966) C2-]04. [3] W. Staude, Phys. L e t t e r s 29 (1969) 228.
POLARIZATION
PHYSICS
EFFECTS
IN
THE OF
LETTERS
16 November 1970
MAGNETO-REFLECTION CuC1
SPECTRUM
M. C E R T I E R , C. WECKER, S. NIKITINE and J. C. M E R L E Laboratoire de Spectroscopie et d'Optique du Corps Solide Groupe de R e c h e r c h e no. 15, lnstitut de Physique, Universit$ de Strasbourg, France Received 15 October 1970
The reflection s p e c t r a of single c r y s t a l s (110) and (100) of CuCI, have been studied under the action of high magnetic fields (120 kG). The polarization of the excitonic r e s i d u a l ray of the Vlf ( F ~ ) line as obs e r v e d on the m i s s i n g ray RM (25 810 c m - 1) for the t h r e e orientations (1t//( 110}, /'/77(10~) } and !1//(111 }) is in good a g r e e m e n t with the hypothesis of a paraexciton t r a n s i t i o n F 1 ~ F 2.
W e h a v e p r e v i o u s l y s t u d i e d t h e e f f e c t of h i g h m a g n e t i c f i e l d on t h e r e f l e c t i o n s p e c t r u m of t h i c k p o l y c r i s t a l l i n e s a m p l e s of CuC1 [1]. We h a v e o b served that a sharp anomaly, called RM, located a t 2 5 8 1 0 c m -1 a p p e a r s i n a m a g n e t i c f i e l d of 119 kG. T h i s a n o m a l y w a s a s c r i b e d to t h e i g n i t i o n of t h e f o r b i d d e n p a r a e x c i t o n l i n e Vlf(F2) a s s o c i a t e d to t h e a l l o w e d o r t h o e x c i t o n l i n e Vlf(F4) (25 860 c m - 1 ) . In t h i s l e t t e r , t h e p o l a r i z a t i o n e f f e c t s a s o b s e r v e d o n t h e R M a n o i n a l y a r e d e s c r i b e d . We u s e s i n g l e c r y s t a l s (100) a n d (110), p u r i f i e d by z o n e m e l t i n g t e c h n i c (10 p p m ) a n d o r i e n t e d b y X r a y s diffractions. The magnetic field H is directed a l o n g t h e m e a n a x i s (110}, (100} ( r o t a t i o n r e f l e c t i o n a x i s $4) a n d (111) ( r o t a t i o n a x i s C3). T h e angle between the incident light beam and the n o r m a l to t h e s a m p l e i s a b o u t 45 ° . T h e e l e c t r i c v e c t o r of l i g h t ~ s e l e c t e d b y a g l a z e b r o o k p o l a r i z e r i s e i t h e r p a r a l l e l o r p e r p e n d i c u l a r to t h e m a g n e t i c f i e l d H (n a n d (r c o m p o n e n t s ) . Table 1 shows the observed results. The a n o m a l y R M a p p e a r s w i t h t h e m a g n e t i c f i e l d only w h e n ~ i s p a r a l l e l to H ( u c o m p o n e n t ) , w h a t e v e r t h e o r i e n t a t i o n , t h e r e f o r e t h e p o l a r i z a t i o n of t h e r e s i d u a l r a y a s s o c i a t e d w i t h t h e l i n e Vlf(F2) i s also parallel to/'/. T h e s y m m e t r y g r o u p of t h e CuC1 c r y s t a l i s t h e t e t r a h e d r o n T d one. T h e t r a n s i t i o n b e t w e e n the ground state F 1 and the paraexciton state F 2 is forbidden in an electric dipole approximation, whereas the orthoexcition transition F 1 ~ F4 is a l l o w e d in t h e s a m e a p p r o x i m a t i o n . The magnetic field H reduces the symmetry of the c r y s t a l T d to C 3 w h e n H i s p a r a l l e l to C 3 a n d to S4 w h e n H i s p a r a l l e l to S4. A c c o r d i n g to 270
Table 1 Ignition of the paraexciton line Ylf(ro) (25 810 cm -1 at 4.2°K) in a strong magnetic fie~d (120 kG). Single c r y s t a l
(001) (110)
without H
component 7r
(r
~(110~
0
1
0
Y (111} (C3)
0
1
0
(100} ($4)
0
1
0
t h e p r o p e r t i e s of t h e e l e c t r i c d i p o l e t r a n s i t i o n , i t i s e a s y to find t h a t t h e d e g e n e r a c y of t h e F 4 e x citonic level will be lifted, giving a normal L o r e n t z t r i p l e t (one u c o m p o n e n t , two cr c o m p o nents) for all orientations. Furthermore the transition F 1 ~ F2 will become allowed in a magn e t i c f i e l d a n d t h e p a r a e x c i t o n l i n e a p p e a r s in t h e field with the same polarization as the u compon e n t of t h e n o r m a l L o r e n t z t r i p l e t of t h e F 1 -~ F 4 transition. Our results agree well with previous interp r e t a t i o n of t h e R M a n o m a l y . T h e e x i s t e n c e of t h e p a r a e x c i t o n t r a n s i t i o n F 1 -~ F 2 (25 810 c m -1) i s now w e l l e s t a b l i s h e d [ 1 - 3 ] .
References [1] M. C e r t i e r , C . W e c k e r and S. Nikitine, Phys. L e t t e r s 28A (1968) 307. [2] S. Nikitine, M. C e r t i e r , J. R i n g e i s s e n a n d J . C . M e r l e J. Physique 27 (1966) C2-]04. [3] W. Staude, Phys. L e t t e r s 29 (1969) 228.