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Submitted on 1 Jan 1981
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INVESTIGATION OF THE CHARGE DENSITY
WAVES IN IT-VSe2 BY RAMAN SCATTERING
S. Sugai, K. Murase, S. Uchida, S. Tanaka
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C6, suppldment au no 12, Tome 42, dicembre 1982 page c6-740
INVESTIGATION O F T H E CHARGE D E N S I T Y WAVES
I N IT-VSe2 BY RAMAN
SCATTERING
*W
*
*XS. Sugai ,K. Murase
,
S. Uchida and S. ~anaka**X
Department of physics, Osaka University, Toyonaka, 560 Japan
*%
Department of AppZied Physics, University o f Tokyo, Tokyo, 113 Japan
Abstract.- The l a t t i c e dynamics i n IT-VSe2 was i n v e s t i g a t e d by Raman s c a t t e r - i n g i n t h e normal phase (> 112 K ) , t h e incommensurate charge d e n s i t y wave
(ICDW) phase (80 - 112 K) and t h e commensurate charge d e n s i t y wave (CCDW) phase of 4 a x 4a ( < 80 K). Recently t h e s t a c k i n g o f t h e l a y e r s was observed to0be ingommensurate even i n t h e lowest temperature phase. I n t h e normal phase two modes were observed. If t h e CDW phase new peaks appear i n t h e A - symmetry 50, 62 and 174 cm- due t o t h e formation of t h e super- l a t t i 6 8 . The 50 cm mode showed s o f t e n i n g toward t h e CDW t o normal phase t r a n s i t i o n temperature. The observed CDW peaks a r e broad r e f l e c t i n g t h e incommensurability along t h e d i r e c t i o n perpendicular t o t h e l a y e r . A c e n t r a l peak was observed i n t h e CDW phase.
The two dimensional t r a n s i t i o n metal d i s e l e n i d e 1T-VSe i s known t o show t h e 2
charge d e n s i t y wave (CDW) phase t r a n s i t i o n . On cooling, t h e ICDW i s formed a t 112 K and s u c c e s s i v e l y t h e CCDW of 4ao x 4ao i s constructed i n a l a y e r , but t h e s t a c k i n g o f l a y e r s along t h e C
-
a x i s i s s t i l l incommensurate.(') The c a l c u l a t i o n of t h e band s t r u c t u r e shows a pancake-like Fermi s u r f a c e near t h e I'-point(2) a s i n t h e c a s e of IT-TaSe2. This might be t h e o r i g i n of t h e incommensurate s t a c k i n g . The e l e c t r o n i c r e s i s t i v i t y i s m e t a l l i c even a t low temperature, and t h e temperature dependence resembles 2H-TaSe2.The experiment was made i n t h e back s c a t t e r i n g c o n f i g u r a t i o n on a cleaved
0
l a y e r s u r f a c e , u s i n g a 5145 A A r - i o n l a s e r o f 100 mii and a microcomputer a s s o c i - a t e d Raman spectroscopy system. The s c a t t e r i n g i n t e n s i t y from 1T-VSe2 was weak by about one-tenth o f t h a t o f 2H-TaSe2.
3
The c r y s t a l s t r u c t u r e i n t h e normal phase i s t r i g o n a l DSd and t h e u n i t c e l l c o n t a i n s one formula u n i t . One A and one E - mode a r e Raman a c t i v e . The A -
l g g l g
mode i s observed i n t h e p a r a l l e l p o l a r i z a t i o n c o n f i g u r a t i o n o f t h e i n c i d e n t and t h e s c a t t e r e d l i g h t s . The E
-
mode i s a c t i v e both i n t h e p a r a l l e l and perpendicularg
p o l a r i z a t i o n c o n f i g u r a t i o n s . The A - symmetry s p e c t r a were obtained by s u b t r a c t -
1 g
i n g t h e perpendicular p o l a r i z a t i o n s p e c t r a from t h e p a r a l l e l p o l a r i z a t i o n s p e c t r a a f t e r c o r r e c t i n g f o r t h e apparatus e f f i c i e n c y about t h e p o l a r i z a t i o n d i r e c t i o n f o r each wavelength p o i n t .
* ? r e s e n t a d d r e s s : D i v . Engineering, Brown U n i v e r s i t y , Providence,
RI
029127 . ........................
IT-VSe2
A E
Fig. 1 : The A and E symmetry s p e c t r a of 1 ~ - + 8 e ~ i n ghe normal
(> 112 K) incommensurate (80 K
-
11 2 K) and commensurate-in-a- l a y e r ( < 80 K) phases.
Fig. 2 : Temperature dependence of t h e e n e r g i e s of t h e A - modes which appear i n t h e phase.
Figure I shows t h e A and'E -
l g g
symmetry s p e c t r a i n IT-VSe2. The s c a t t e r - ing i n t e n s i t y was c o r r e c t e d f o r t h e phonon d i s t r i b u t i o n f u n c t i o n n, a s
I(w) = Io(w)/(n(w,T) + 11,
where Io(w) is t h e observed s c a t t e r i n g i n t e n s i t y . A t room temperature t h e A
-
l g mode i s observed a t 211 cm-' and t h e E
-
g mode a t 143 cm-l. The peak of t h e E
-
g mode i s small and broad a t room tempera- t u r e , but becomes s t r o n g with c o o l i n g t h e c r y s t a l . The braad peak near 130 cm-' i s assigned t o t h e second o r d e r Raman peak r e l a t i n g t o t h e Kohn anomaly. Below t h e
CDW phase t r a n s i t i o n temperature new peaks appear because of t h e i n c r e a s e o f t h e u n i t c e l l . The new peaks were observed o n l y i n t h e A
-
symmetry with t h e energy o fl g
SO cm-l, 6 2 cm-' and 174 cm-' a t 20 K. These peaks a r e broad even a t 20 K , re- f l e c t i n g t h e incommensurability along t h e C- a x i s . The 50 cm-I mode a t low .temp- e r a t u r e s shows s t r o n g s o f t e n i n g toward t h e ICDW t o t h e normal phase t r a n s i t i o n temp- e r a t u r e of 112 K a s shown i n Fig. 2.
Figure 3 shows t h e normalized i n t e - g r a t e d s c a t t e r i n g i n t e n s i t y and t h e f u l l l i n e width a t h a l f maximum. Figure 3Ca) i s concerned with t h e CDW s t a t e a c t i v a t e d modes and Fig. 3(b) t h e n a t i v e modes. A t
low temperatures t h e CDW s t a t e a c t i v a t e d modes of 50 cm-' and 62 cm-' decrease t h e i r s c a t t e r i n g i n t e n s i t y and i n c r e a s e t h e i r l i n e width with temperature. The i n t e n s i t y of t h e modes a c t i v e even i n t h e o r i g i n a l phase d e c r e a s e s with temperature. The p o i n t of i n f l e c t i o n i s about 130 K. The 147 cm-I E
-
mode almost merges i n t og
t h e background a t room temperature. This temperature behavior i s s i m i l a r t o t h e n a t i v e mode of 240 cm-I A - mode i n
JOURNAL DE PHYSIQUE
Fig. 3(a)
Fig. 3(b)
Fig. 3 : Temperature dependence o f t h e i n t e g r a t e d s c a t t e r i n g i n t e n s i t y and t h e f u l l l i n e width a t h a l f maxi- mum of t h e CDW
-
a c t i v a t e d modes'(a)and t h e o r i g i n a l l y a c t i v e modes (b)
.
2H-TaSeq .
Below t h e CDW phase t r a n s i t i o n temp- e r a t u r e a c e n t r a l peak appears below 10 em-', and i t s i n t e n s i t y i n c r e a s e s with decreasing temperature. The c e n t r a l peaks accompanying t h e CDW phase t r a n - s i t i o n was observed by Sooryakumer, Bruns and Klein i n t h e ICDW phase of
M-TaSe2. I n t h e naive theory of t h e one dimensional CDW, t h e phase mode has vanishing energy a t zero wave v e c t o r . I n two dimensional m a t e r i a l s , t h e energy of t h e phase mode a r e n o t well known due t o t h e i n t e r a c t i o n o f t h e t r i p l e CDWSs. The observed c e n t r a l peak might be r e l a t e d t o t h e phonon o r t o some d e f e c t s i n t h e c r y s t a l .
The authors thank A. Toriumi f o r t h e p r e p a r a t i o n o f good c r y s t a l s .
References
(1) K . Tsutsumi, T. Sambongi, A . Toriumi and S . Tanaka, Physica, (1981)419 (2) A. M. Wolley and G. Wexler, J. Phys.C:
S o l i d S t a t e Physics (1977) 2601.
( 3 ) R. Sooryakumer, D. G. Burns and M. V.
