INVESTIGATION OF THE CHARGE DENSITY WAVES IN 2H-TaSe2 BY ONE AND TWO PHONON RAMAN SCATTERING

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INVESTIGATION OF THE CHARGE DENSITY WAVES IN 2H-TaSe2 BY ONE AND TWO PHONON

RAMAN SCATTERING

S. Sugai, K. Murase, S. Uchida, S. Tanaka

To cite this version:

S. Sugai, K. Murase, S. Uchida, S. Tanaka. INVESTIGATION OF THE CHARGE DENSITY WAVES

IN 2H-TaSe2 BY ONE AND TWO PHONON RAMAN SCATTERING. Journal de Physique Collo-

ques, 1981, 42 (C6), pp.C6-728-C6-730. �10.1051/jphyscol:19816213�. �jpa-00221293�

JOURNAL DE PHYSIQUE

CoZZope C6, suppZ6ment au n o 12, Tome 42, de'cembre 1981

INVESTIGATION OF THE CHARGE DENSITY WAVES IN 2H-TaSe2 BY ONE AND TWO PHONON RAMAN SCATTERING

S. suga<*; K. Murase, S. uchidat:nd

S. ~anaka*"

* Department of Physics, Osaka University, Toyonaka, 560 Japan

** Department of Applied Physics, University o f Tokyo, Tokyo, 123 Japan

Abstract. - One and two phonon Raman scattering was investigated in 2H-TaSeZ at the normal and the charge density wave (CDW) phases. The temperature dependence of the generalized susceptibility was obtained from the integrated scattering intensity of the two phonon peaks relating to the Kohn anomaly mode. The temperature dependence of the phonon energies calculated from this susceptibility shows excellent agreement with the experimental data obtained from one phonon Raman scattering in the soft A and E - modes in the CDW phase and the Kohn anomaly mode in the normal pft8se. Zg

0 100 0 100

ENERGY SHIFT (cm-1)

Transition metal dichalcogenide ZH-TaSe2 is a typical layered compound with a CDW phase transition. The crystal structure changes from the original hex- agonal phase D6h to the incommensurate 4 charge density wave (CDW) phase at 122 K, followed by the triply commensurate CDW (TCCDW) phase of 3ao x 3ao x co at 90 K. C1) On warming only the stripe domain struc- ture (SCCDW), in which one CDW is commen- surate and the other two are incommensur- ate, was observed by x-ray diffraction between 90 K to 112 K.(2)

The experiment was made in the back scattering configuration on the layer

0

plane with 5145 A Ar-ion laser and micro- computer associated system. Using this experimental configuration, one A and

1 g two E modes are observed in the normal

2g

phase. In the TCCDW phase six points on the C-lines and the two K-points in the Fig.1 : Lower energy part of Raman

- spectra in the normal (127 K), ICDW original phase are folded into the r-

(117 K) , SCCDW (107, 96 K) and TCCDW point.

(85, 66 K) phases. The dotted curves are duplications of the 127 K spectra.

X

Present address: Div.Engineering, Brown University, Providence, RI-02912

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19816213

Figure 1 shows t h e Raman s p e c t r a below 100 cm-l. The A - s p e c t r a were o b t a i n e d

ZH-TaSe2 l g

by s u b t r a c t i n g t h e p e r p e n d i c u l a r l y p o l a r -

Fig.2 : Temperature dependence of t h e

- phonon e n e r g i e s i n t h e CDW phase. The d o t t e d c u r v e s a r e c a l c u l a t e d from t h e o b t a i n e d s u s c e p t i b i l i t y .

\ . I f ,

0 100 2W MO

TEMPERATURE (K)

m -

.-.

7

k

F i g . 3 : Temperature dependence o f t h e energy s h i f t o f t h e maximum p o s i t i o n and t h e edge o f t h e lower energy f o o t of t h e two phonon Raman peak. The d o t t e d curve i s c a l c u l a t e d using t h e s u s c e p t i b i l i t y .

2H-TaSq (2-PHONON PROCESS)

-

Fig.4 : Temperature dependence of t h e g e n e r a l i z e d s u s c e p t i b i l i t y o b t a i n e d from 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 of t h e two phonon Raman peak.

i z e d s p e c t r a Z ( X Y ) ~ from t h e p a r a l l e l p o l a r i z e d s p e c t r a z ( x x ) ~ . The normalized i n t e n s i t y was o b t a i n e d by d i v i d i n g t h e e x p e r i m e n t a l l y o b t a i n e d i n t e n s i t y by n + 1, where n i s t h e Bose-Einstein f a c t o r . The small peak a t 24 cm-I i n t h e E

2g-spectra i s due t o t h e n a t i v e l y a c t i v e r i g i d l a y e r mode. The broad peak a t 110 cm-I i s due t o t h e two phonon p r o c e s s . On d e c r e a s i n g t h e temperature, two A -mode3 and f o u r

1 g

E -modes become s t r o n g . Figure 2 shows 2g

t h e temperature dependent phonon e n e r g i e s of t h e s e modes. The A -mode a t 82 cm-I

l g

and t h e E -mode a t 50 cm-l, a t low temp- 2g

e r a t u r e s , show s o f t e n i n g toward t h e CDW t o t h e normal phase t r a n s i t i o n temperature a t 122 K . The 50 cm-I mode i s overdamped above 110 K . The 82 cm-I A -mode ap-

1 g

proaches t h e 42 cm-I A -mode and merges l g

a t 100 K.

The s t r o n g two phonon s c a t t e r i n g i n t e n s i t y r e l a t i n g t o t h e Kohn anomaly i s t h e d i s t i n c t i v e f e a t u r e of 2H-compounds with CDW phase t r a n s i t i o n s . The maximum p o s i t i o n and t h e edge of t h e f o o t of t h e two phonon s c a t t e r i n g peaks a r e shown i n Fig. 3 . The energy of t h e edge shows s t r o n g temperature dependence. The h a l f of t h i s energy corresponds t o t h e energy of t h e Kohn anomaly C1LA-mode. The 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 i n c r e a s e s with d e c r e a s i n g temperature, i f t h e i n t e n s i t y i s normalized by t h e s t a t i s t i c a l f a c t o r of t h e two phonon e m i t t i n g p r o c e s s . The two phonon s c a t t e r i n g i n t e n s i t y i s r e l a t e d t o t h e g e n e r a l i z e d e l e c t r o n i c s u s c e p t i b i l i t y X (4) by

CIA

2 2

I aXqh(n + 1 ) .

-

C

'A.

-

I

"'1m- b

Z

W -

___....-

__..-.

.... *...-

,*.

.i'

C6-730 JOURNAL DE PHYSIQUE

Figure 4 shows the obtained susceptibility. Phonon energies renormalized by the electron - phonon interaction are expressed(5) by

where zL ^and w are the renormalized and bare phonon frequency, respectively.

9 qh

The calculated phonon energies are shown in Fig. 2 and Fig. 3 by dotted curves.

The agreement with the experimental results is excellent.

Our results are summarized in the following.

1. The generalized electronic susceptibility was obtained from the second order Raman scattering intensity. The energy of the Kohn anomaly mode was calcu- lated kbove and below the CDW phase transition temperature using the susceptibility.

The agreement with the experimental data was excellent.

2. In the CDW phase, the A -mode at 82 cm-I at 12 K and the E -mode at

lg 2g

50 cm-' shows strong softening and their intensity curves have shoulders at about 90 K. They might be assigned to the amplitude modes. The 42 cm-' A -mode and the

lg 64 cm-' E -mode might be assigned to the phase modes.

2g

3 . No hysteresis was observed in the phonon energy, scattering intensity

and line width in the temperature range of the SCCDW phase. This suggests the existence of the domain structure in which the CDW is almost commensurate.

4. The Kohn anomaly phonon energy above the CDW transition temperature was obtained from the lower energy foot of the second order Raman peak.

References

1. D . E. Moncton, J. D. Axe and F. J. DiSalvo, Phys. Rev. B16 (1977) 801.

2. R. M. Fleming, D. E. Moncton, D. B. McWhan and F. J. DiSalvo, Phys. Rev.

Letters 9 (1980) 576.

3. J. A. Holy, M. V. Klein, W. L. McMillan and S.. F. Meyer, Phys. Rev. Letters 17 (1976) 1145.

-

4. P. F. Maldague and J. C. Tsang, Proc. Int. Conf. on Lattice Dynamics, edited by M. Balkanski, Flammarion Science (19781, p. 602.

5. S, K. Chan and V. Heine, J. 'Phys. F: Metal Phys., 3 (1973) 795.