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Raman scattering on single crystals of YBa2Cu3O7−δ
PhysicaC 153-155 (1988) 262-263 North-Holland,Amsterdam
RAMAN SCATTERING ON SINGLE CRYSTALS OF YBa2Cu307_6
C. THOMSEN, M. CARDONA, B. GEGENHEIMER, and R. LIU M a x - P l a n c k - l n s t i t u t fur Festkbrperforschung, Heisenbergstrasse I , D-7000 S t u t t g a r t 80, Federal R e p u b l i c of Germany
We p r e s e n t the Raman t e n s o r elements o f the f i v e Raman a c t i v e phonons of Y B a 2 C u 3 0 7 _ 6 d e t e r m i n e d in p o l a r i z a t i o n dependent measurements on s i n g l e c r y s t a l s . Two d e f e c t - i n d u c e d i n f r a r e d a c t i v e modes i n v o l v i n g the OI atom are a l s o i d e n t i f i e d .
I.
INTRODUCTION The identification of the role p l a y e d by phonons in the high Tc s u p e r conductors r e q u ir e s a d e f i n i t e assignment of the v i b r a t i o n a l modes and a det a i l e d i n v e s t i g a t i o n of t h e i r temperature dependence. For this reason, a number of Raman measurements have been performed on single crystals of YBa2Cu307_6(1-4 ). Among the modes which are commonly observed at ~80, 500, 435~ 335, 220, 150 and 120 cm- " , t h e r e i s d i s a g r e e m e n t over the assignment of the 220 and 580 cm - I phonon mode~ and the s e l e c t i o n r u l e s of the 335 cm mode. In t h i s paper we p r e s e n t an unambiguous e x p e r i m e n t a l d e t e r m i n a t i o n of the Raman selection rules for single crystals, the d e t a i l s of the Raman t e n s o r s , and two ir a c t i v e m o d e s based on Raman scattering results. 2.
EXPERIMENTAL Unannealed single crystals of YBa2Cu307_ 6 w e r e g r o w n f r o m a nons t o i c h i o m e t r i c melt by slow c o o l i n g . Backscattering Raman measurements, e x c i t e d w i t h the 541.5 nm Ar + l a s e r l i n e , were taken w i t h the i n c i d e n t and s c a t t e r e d l i g h t p a r a l l e l and p e r p e n d i c u l a r t o the c - a x i s . Since the c r y s t a l s s t u d i e d here were n e a r l y t e t r a g o n a l , no d i f f e r e n c e was observed w h e n both the polarization o f t h e i n c i d e n t ( ~ i ) and analyzed (~s) light were r o t a t e d by 90°; we d i s p l a y here o n l y one of e v e r y two i d e n t i c a l s p e c t r a . 3.
DISCUSSION In s p e c t r u m ( i ) ~ i and ~s a r e p a r a l l e l t o the x a x i s . Spectrum ( i i ) for crossed polarizations shows no detectable depolarized scattering sign a l . T h u s a l l Raman t e n s o r elements a i are equal t o zero f o r i ~ j and r e f l e c ~
0921-4534/88/$03.50 ©Elsevier Science Publishers B.V, (North-Holland Physics Publishing Division)
t h e A a s y m m e t r y o f t h e modes in t h e orthor~ombic s t r u c t u r e . When we r o t a t e d the c r y s t a l by 45 ° w i t h r e s p e c t t o the l i g h t p o l a r i z a t i o n we f o u n d t h a t t h e peak a t 335 cm-~ i s present for c r o s s - p o l a r i z e d l i g h t while i t i s absent f o r p a r a l l e l p o l a r i z a t i o n . T h i s i s p o s s i b l e o n l y w i t h ~.. = T~yyi t h e d i f f e r e n c e between . a x x . ^X and ~ g ~ t may depend on the o r t h o r h o m b i c di tion as w e l l as the e x t e n t of the t w i n n i n g . From the absence of t h i s peak in s p e c t r u m ( v i i i ) , taken f o r ~ i l l ~ s l l ~ , we i n f e r t h a t a z = O. A l l Raman t e n s o r components of t ~ i s mode are thus s p e c i f i e d ( t o a f a c t o r ) , and t h e y are consistent with the measurements using c i r c u l a r l y p o l a r i z e d l i g h t ( s p e c t r a (v) and ( v i ) ) . In agreement w i t h the Raman r e s u l t s and the symmetry of the c r y s t a l we assign t h i s mode t o the O I I - O I I I v i b r a t i o n in z d i r e c t i o n where O i l and O I I I are out of phase ( 5 ) . The t e n s o r elements f o r the o t h e r Raman a c t i v e modes and t h e i r assignments are deduced s i m i l a r l y from the s p e c t r a in F i g . 1 and can be found in Table I . We w i l l now focus on two Raman f o r b i d d e n peaks. There has been c o n t r o v e r s y a b o y t Raman f e a t u r e s a t ~220 and ~580 cm - . We conclude from arguments l i k e the ones used above t h a t the Raman t e n s o r s o f these peaks are not c o m p a t i b l e w i t h the c r y s t a l symmetry. While d i a g o n a l , a c c o r d i n g t o the data f o r p o l a r i z a t i o n p a r a l l e l t o the c r y s t a l axes, the data for circular polarizations and those f o r p o l a r i z a t i o n s p a r a l l e l t o the base d i a g o n a l ( x + y , x - y ) r e q u i r e t h a t ~x and a y y be i n c o h e r e n t . Spectrum ( v i i 1 ~ shows t h a t t h e a z z = O. I n c o h e r e n t components suggest d e f e c t - i n d u c e d s c a t t e r i n g . Thus one p o s s i b l e assignment of t h e 220 and 580 cm -1 modes i s t o t h e
I
I
C. Thomsen et a L / Single crystals of YBa2Cu307 a
(VII),(VIII): killkslc
263
TABLE I : Raman f re q u e n cie s ( t h e i r symm e t r i e s and a c t i v i t i e s ) , the t e n s o r elements ~ i j ( ~ i j = 0 f o r i ~ j ) and the assignment of observed modes.
Y
Frequency (cm-1 )
~ij
120 (A ,R) (from ~ef 4)
(II), ~.-
~
=0.6~ xx
•
~%~~(IV),/.\ i--
Assignment
~ZZ
145 (Ag,R)
~x =~ ~zz
335 (Ag,R)
~xx =-~ yy ~zz=O
YY
Ba (z) symm. stretching Cu2 (z) symm. stretching OII-OIII
(z)
out-of-phase
435 (Ag,R)
~xx'~yy<<~zz
O I I - O I I I (z) in-phase
480 (Ag,R)
~xx=~yy<<~zz
OIV symm. stretching
z w
220 (B2u,IR)
~ , ~ xx yy incoherent
OI-Cul (y) in-phase
zz 580 (B2u,IR)
100
200
300
400
500
600
ditto
OI-Cul (y) out-of-phase
700
RAMAN SHIFT (era -1) FIGURE 1: P o l a r i z a t i o n dependent Raman spectra of single crystalline YBa2Cu307-6. OI-Cul in-phase and o u t - o f - p h a s e s t r e t ching modes- which are i r - a c t i v e and have B2u s y m m e t r y . These modes have been c ~ l c u l a t e d t o be a t 156 and 576 c m - ' , r e s p e c t i v e l y ( 5 ) . The d e f e c t induced nature of the t w i n n i n g would cause the i n c o h e r e n c e of ~xx and ~yy. This assignment has been r e c e n t l y confirmed by Raman measurements on an untwinned s i n g l e c r y s t a l of YBa2Cu307_6 (~ = O) (4) where these two peaks were only observed f o r p o l a r i z a t i o n s p a r a l l e l to the OI-Cul chain• 4. CONCLUSIONS We h a v e determined the Raman t e n sors of the Raman a c t i v e phonons and assigned two d e f e c t - i n d u c e d i r modes in
s u p e r c o n d u c t i n g YBa2Cu307_~ by p o l a r i z a t i o n dependent Raman s c a t t e r i n g on single c r y s t a l s . REFERENCES 1) D . M . K r o l , M. S t a v o l a , W. Weber, L.F. Schneemeyer, J.V. Waszczak, S.M. Zahurak, and S.G. K o s i n i s k i , Phys. Rev. B 36, 8325 (1987). 2) A. Yamanaka, F. Minami , K. Watanabe, K. Inoue, S. Takekawa, and N. l y i , Jap. J. of Appl. Phys. 26, L1404 (1987). 3) R. L i u , C. Thomsen, W. Kress, M. Cardona, B. Gegenheimer, F.W. de Wette, J. Prade, and U. Schr~der, Phys. Rev. B, in p r i n t . 4) C. Thomsen, M. Cardona, B. Gegenheimer, R. L i u , and A. Simon, to be p u b l i s h e d . 5) W. Kress, U. Schr~der, J. Prade, A.D. K u l k a r n i , and F.W. de Wette, t h i s volume.
RAMAN SCATTERING ON SINGLE CRYSTALS OF YBa2Cu307_6
C. THOMSEN, M. CARDONA, B. GEGENHEIMER, and R. LIU M a x - P l a n c k - l n s t i t u t fur Festkbrperforschung, Heisenbergstrasse I , D-7000 S t u t t g a r t 80, Federal R e p u b l i c of Germany
We p r e s e n t the Raman t e n s o r elements o f the f i v e Raman a c t i v e phonons of Y B a 2 C u 3 0 7 _ 6 d e t e r m i n e d in p o l a r i z a t i o n dependent measurements on s i n g l e c r y s t a l s . Two d e f e c t - i n d u c e d i n f r a r e d a c t i v e modes i n v o l v i n g the OI atom are a l s o i d e n t i f i e d .
I.
INTRODUCTION The identification of the role p l a y e d by phonons in the high Tc s u p e r conductors r e q u ir e s a d e f i n i t e assignment of the v i b r a t i o n a l modes and a det a i l e d i n v e s t i g a t i o n of t h e i r temperature dependence. For this reason, a number of Raman measurements have been performed on single crystals of YBa2Cu307_6(1-4 ). Among the modes which are commonly observed at ~80, 500, 435~ 335, 220, 150 and 120 cm- " , t h e r e i s d i s a g r e e m e n t over the assignment of the 220 and 580 cm - I phonon mode~ and the s e l e c t i o n r u l e s of the 335 cm mode. In t h i s paper we p r e s e n t an unambiguous e x p e r i m e n t a l d e t e r m i n a t i o n of the Raman selection rules for single crystals, the d e t a i l s of the Raman t e n s o r s , and two ir a c t i v e m o d e s based on Raman scattering results. 2.
EXPERIMENTAL Unannealed single crystals of YBa2Cu307_ 6 w e r e g r o w n f r o m a nons t o i c h i o m e t r i c melt by slow c o o l i n g . Backscattering Raman measurements, e x c i t e d w i t h the 541.5 nm Ar + l a s e r l i n e , were taken w i t h the i n c i d e n t and s c a t t e r e d l i g h t p a r a l l e l and p e r p e n d i c u l a r t o the c - a x i s . Since the c r y s t a l s s t u d i e d here were n e a r l y t e t r a g o n a l , no d i f f e r e n c e was observed w h e n both the polarization o f t h e i n c i d e n t ( ~ i ) and analyzed (~s) light were r o t a t e d by 90°; we d i s p l a y here o n l y one of e v e r y two i d e n t i c a l s p e c t r a . 3.
DISCUSSION In s p e c t r u m ( i ) ~ i and ~s a r e p a r a l l e l t o the x a x i s . Spectrum ( i i ) for crossed polarizations shows no detectable depolarized scattering sign a l . T h u s a l l Raman t e n s o r elements a i are equal t o zero f o r i ~ j and r e f l e c ~
0921-4534/88/$03.50 ©Elsevier Science Publishers B.V, (North-Holland Physics Publishing Division)
t h e A a s y m m e t r y o f t h e modes in t h e orthor~ombic s t r u c t u r e . When we r o t a t e d the c r y s t a l by 45 ° w i t h r e s p e c t t o the l i g h t p o l a r i z a t i o n we f o u n d t h a t t h e peak a t 335 cm-~ i s present for c r o s s - p o l a r i z e d l i g h t while i t i s absent f o r p a r a l l e l p o l a r i z a t i o n . T h i s i s p o s s i b l e o n l y w i t h ~.. = T~yyi t h e d i f f e r e n c e between . a x x . ^X and ~ g ~ t may depend on the o r t h o r h o m b i c di tion as w e l l as the e x t e n t of the t w i n n i n g . From the absence of t h i s peak in s p e c t r u m ( v i i i ) , taken f o r ~ i l l ~ s l l ~ , we i n f e r t h a t a z = O. A l l Raman t e n s o r components of t ~ i s mode are thus s p e c i f i e d ( t o a f a c t o r ) , and t h e y are consistent with the measurements using c i r c u l a r l y p o l a r i z e d l i g h t ( s p e c t r a (v) and ( v i ) ) . In agreement w i t h the Raman r e s u l t s and the symmetry of the c r y s t a l we assign t h i s mode t o the O I I - O I I I v i b r a t i o n in z d i r e c t i o n where O i l and O I I I are out of phase ( 5 ) . The t e n s o r elements f o r the o t h e r Raman a c t i v e modes and t h e i r assignments are deduced s i m i l a r l y from the s p e c t r a in F i g . 1 and can be found in Table I . We w i l l now focus on two Raman f o r b i d d e n peaks. There has been c o n t r o v e r s y a b o y t Raman f e a t u r e s a t ~220 and ~580 cm - . We conclude from arguments l i k e the ones used above t h a t the Raman t e n s o r s o f these peaks are not c o m p a t i b l e w i t h the c r y s t a l symmetry. While d i a g o n a l , a c c o r d i n g t o the data f o r p o l a r i z a t i o n p a r a l l e l t o the c r y s t a l axes, the data for circular polarizations and those f o r p o l a r i z a t i o n s p a r a l l e l t o the base d i a g o n a l ( x + y , x - y ) r e q u i r e t h a t ~x and a y y be i n c o h e r e n t . Spectrum ( v i i 1 ~ shows t h a t t h e a z z = O. I n c o h e r e n t components suggest d e f e c t - i n d u c e d s c a t t e r i n g . Thus one p o s s i b l e assignment of t h e 220 and 580 cm -1 modes i s t o t h e
I
I
C. Thomsen et a L / Single crystals of YBa2Cu307 a
(VII),(VIII): killkslc
263
TABLE I : Raman f re q u e n cie s ( t h e i r symm e t r i e s and a c t i v i t i e s ) , the t e n s o r elements ~ i j ( ~ i j = 0 f o r i ~ j ) and the assignment of observed modes.
Y
Frequency (cm-1 )
~ij
120 (A ,R) (from ~ef 4)
(II), ~.-
~
=0.6~ xx
•
~%~~(IV),/.\ i--
Assignment
~ZZ
145 (Ag,R)
~x =~ ~zz
335 (Ag,R)
~xx =-~ yy ~zz=O
YY
Ba (z) symm. stretching Cu2 (z) symm. stretching OII-OIII
(z)
out-of-phase
435 (Ag,R)
~xx'~yy<<~zz
O I I - O I I I (z) in-phase
480 (Ag,R)
~xx=~yy<<~zz
OIV symm. stretching
z w
220 (B2u,IR)
~ , ~ xx yy incoherent
OI-Cul (y) in-phase
zz 580 (B2u,IR)
100
200
300
400
500
600
ditto
OI-Cul (y) out-of-phase
700
RAMAN SHIFT (era -1) FIGURE 1: P o l a r i z a t i o n dependent Raman spectra of single crystalline YBa2Cu307-6. OI-Cul in-phase and o u t - o f - p h a s e s t r e t ching modes- which are i r - a c t i v e and have B2u s y m m e t r y . These modes have been c ~ l c u l a t e d t o be a t 156 and 576 c m - ' , r e s p e c t i v e l y ( 5 ) . The d e f e c t induced nature of the t w i n n i n g would cause the i n c o h e r e n c e of ~xx and ~yy. This assignment has been r e c e n t l y confirmed by Raman measurements on an untwinned s i n g l e c r y s t a l of YBa2Cu307_6 (~ = O) (4) where these two peaks were only observed f o r p o l a r i z a t i o n s p a r a l l e l to the OI-Cul chain• 4. CONCLUSIONS We h a v e determined the Raman t e n sors of the Raman a c t i v e phonons and assigned two d e f e c t - i n d u c e d i r modes in
s u p e r c o n d u c t i n g YBa2Cu307_~ by p o l a r i z a t i o n dependent Raman s c a t t e r i n g on single c r y s t a l s . REFERENCES 1) D . M . K r o l , M. S t a v o l a , W. Weber, L.F. Schneemeyer, J.V. Waszczak, S.M. Zahurak, and S.G. K o s i n i s k i , Phys. Rev. B 36, 8325 (1987). 2) A. Yamanaka, F. Minami , K. Watanabe, K. Inoue, S. Takekawa, and N. l y i , Jap. J. of Appl. Phys. 26, L1404 (1987). 3) R. L i u , C. Thomsen, W. Kress, M. Cardona, B. Gegenheimer, F.W. de Wette, J. Prade, and U. Schr~der, Phys. Rev. B, in p r i n t . 4) C. Thomsen, M. Cardona, B. Gegenheimer, R. L i u , and A. Simon, to be p u b l i s h e d . 5) W. Kress, U. Schr~der, J. Prade, A.D. K u l k a r n i , and F.W. de Wette, t h i s volume.