Electronic structure of (TaSe4)2I studied by positron annihilation and Ultraviolet Photoemission Spectroscopy (UPS)

Physica 143B (1986) 192-194 North-Holland, Amsterdam

192

ELECTRONIC STRUCTURE OF (TaSe~)21 STUDIED BY POSITRON A N N I H I L A T I O N AND U L T R A V I O L E T P H O T O E M I S S I O N S P E C T R O S C O P Y (UPS)

K. OHTAKE, E. SATO, S. KOSHIBA, R. YAMAMOTO, M. D O Y A M A K. SODA,* T. M O R I , * S. SUGA* and K. ENDO**

Department of M e t a l l u r g y and M a t e r i a l s Science, Faculty of Engineering, The U n i v e r s i t y of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan * S y n c h r o t r o n R a d i a t i o n Laboratory, Institute for Solid State Physics, The U n i v e r s i t y of Tokyo, Tanashi-shi, Tokyo 188, Japan * * E l e c t r o t e c h n i c a l Laboratory, Umezono, Sakura-mura, Niihari-gun, Ibaraki

305, Japan

The e l e c t r o n i c structure of (TaSe~)21 has b e e n studied by positron a n n i h i l a t i o n e x p e r i m e n t and u l t r a v i o l e t p h o t o e m i s s i o n s p e c t r o s c o p y (UPS). By positron study, the Fermi surface is o b s e r v e d only for the high c o n d u c t i n g c-axis direction, e l u c i d a t i n g the o n e - d i m e n s i o n a l i t y of the e l e c t r o n i c structure. The Fermi wave vector k F is also determined. By UPS study, it is found that the h y b r i d i z a t i o n of Ta 5d orbitals with Se 4p orbitals is remarkable at shallow bindin 9 energies in the valence bands.

i. I N T R O D U C T I O N

FWHM.

(TaSe4)21 is w e l l k n o w n to be a quasi oned i m e n s i o n a l c o n d u c t o r w h i c h undergoes density w a v e study,

the charge

(CDW) transition at 263 K. I In this

the m e a s u r e m e n t s of the p o s i t r o n annihi-

lation and the u l t r a v i o l e t p h o t o e m i s s i o n spectroscopy

(UPS) were p e r f o r m e d to investigate

e l e c t r o n i c structure of

(TaSe~)21.

the

It is expect-

ed that positron study gives the i n f o r m a t i o n of the e l e c t r o n m o m e n t u m distribution,

In UPS study,

resonant p h o t o e m i s s i o n measure--

ment was carried out at the second beam line ot SOR-RING,

the 400 MeV e l e c t r o n storage ring oi

S y n c h r o t r o n Radiation Laboratory, Solid State Physics,

Institute

fo~"

The University of Tokyo.

The resolution of the m o n o c h r o m a t o r was set to 0.2 eV at the photon energy

(h~') of 50 eV, and

that of the analyzer to 0.2 eV.

especially,

of the Fermi surface, w h i l e the e l e c t r o n density of states is e x a m i n e d from UPS study.

3. RESULTS AND D I S C U S S I O N 3.1. P O S I T R O N S'£UDY The m e a s u r e d angular c o r r e l a t i o n curves are

2. E X P E R I M E N T A L

presented in Figure i.

Single crystals used in this experiment was prepared by direct vapor transport.

near the peak are 4-5 x 10 ~ for c// and 0.5-1 x 104

In p o s i t r o n study, an angular c o r r e l a t i o n

The coincidence counts

for c~.

The anisotropy in the elec-

tron m o m e n t u m d i s t r i b u t i o n was

found near 0 mrad

m e a s u r e m e n t was p e r f o r m e d both for cff (parallel

as shown in the inset.

to the c-axis) and c ~ (perpendicular to the c-

be called the s h o u l d e r structure, was o b s e r v e d

Sharp cut off, which may

axis) c o n f i g u r a t i o n at room temperature.

near the peak of the curve for c// , whereas no

(TaSe4)21 shows the high c o n d u c t i v i t y along the

s h o u l d e r was o b s e r v e d for c i .

c-axis.

late both c o n d u c t i o n electrons and core elec-

Usual long slit technique was used and

p o s i t r o n source was

22NaCI of 10mCi.

The angu-

lar r e s o l u t i o n of the s y s t e m was 0.63 m r a d in

0 3 7 8 - 4 3 6 3 / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division) and Yamada Science Foundation

trons.

The wave

Positrons annihi-

functions of c o n d u c t i o n elec-

trons are spread in real space,

then the momen-

K. Ohtake et al. 'Electronic structure of ( TaSe4) 21

(Ta Se4 )2 I

193

A

1.005 C .ID L.

U.l •

• •

uJ (J

,u 1.000

• •

m,

o @ o u

z uJ c3

o

o

G

o

o

z

(mrad) o

o

C/

o

o

9

o

5

(A,R)

r(M.X)

O000090QO C #

|

0.995

0 0.5 1.0 1.5

~| ~

10 15 ANGLE ( m r a d )

FIGURE 1 Angular correlation curves of positron annihilation for c// and c± direction in (TaSe4)21

FIGURE 2 Projected density of states in k-space for c// of (TaSe4)21 obtained after the application of the LCW theorem

momentum distribution for c// , the Gc// is the projection of the reciprocal lattice vector to the direction c// .

rum distribution is rather sharp compared with

D(kc//) consists of the two contributions,

those for core electrons.

from the full-filled bands and core electrons,

are localized, broad.

Since core electrons

the momentum distribution is

Roughly speaking,

electron momentum

and the other from the conduction bands. former is fairly smooth.

one

The

The latter, Dc(keH),

distribution of the filled bands is smooth,

is given only by n i (k), the occupation number

while sharp cut off at the Fermi momentum in the

of the conduction bands, as follows ;

angular correlation curve can be expected for conduction electrons.

Dc(kc//) = f dkc± ~ - n i (k) BZ i

For this reason, it is

concluded that the Fermi surface could be observed only for c// .

sionality of the electronic structure of

tively,

To obtain the Fermi surface quantita-

the Lock-Crisp-West

(LCW) theorem 2 was

applied to the measured electron momentum distribution curve for c// in the following way. We reduced the electron momentum distribution curves into the k-space.

(2)

Observed angular corre-

lation curves clearly show the quasi one-dimen-

(TaSe4)21.

,

The superposition

where the integration is over the Brillouin zone (BZ).

The result of the projected electron

momentum density inside the BZ , D(kc//),is shown in Figure 2.

A very sharp cut off indi-

cated by the arrow can be attributed to the existence of the electron Fermi surface.

The

Fermi wave vector k F was determined to be 0.35 c*. 3.2. UPS STUDY

D(kc//) is defined as ;

Figure 3 shows the energy distribution curve D(kc//) = ~- N (Pc//+ Gc//) Gc//

,

(i)

(EDC) of angle integrated UPS of (TaSe4)21 for photon energy of i00 eV measured at room temperature.

where N(Pcff) is the experimental electron

The structures with the binding energy E~

smaller than 18 eV are related to the valence

K. Ohtake et al. / Electronic structure of ( TaSe4) 21

194

i

i

i

!

i

i

i

(TaSe4)2 ! ~.

c-

•i

eV

h~= 100eV Z O

.d r~

I,u

UJ

~ )2 I

(TaSe

tD

r ~,

LLI

O

60

40

20

BINDING

eV

-

0

j

Es:0.5eV

rr

ENERGY(eV)

13_

30

FIGURE 3 The angle integrated UPS spectrum of (TaSe~+)2] for photon energy of 100 eV measured at room temperature

40

50

PHOTON E N E R G Y FIl; LrRiL :.

Constant-initial-state and conduction bands, and four sets of doublet

60 (or)

(CIS)

curves correspond-

i n g t o EB = 0 . 5 , 3 . 2 a n d 5 . 5 eV i n t h e integrated UPS s p e c t r a of (TaSe4)2I

angle

peak with E B larger than 20 eV can be assigned to individual core states as shown in Table I.

sponds to the local electronic densit7 of Ta 5d states at the corresponding binding energies.

TABLE 1 Core level binding energies of (TaSe~)2I determined from Figure 3

From the negligible of E B = 5.5 eV,

resonance

we have

for the structuru

found that the Ta 5d

orbitals are not appreciabl\ hybridized with the Se 4p orbitals,

the deep binding energy region

(TaSe4)21 in the valence bands.

(eV) Ta 4f7/2 4f5/2 Ta 5P3/2 5Pl/2

23.8 25.7 35.2 45.2

I

4d5/2 4d3/2 Se 3d5/2 3d3/2

48.2 49.9 55.0 55.5

4. CONCLUSION In conclusion, we have obtained the information of both the electron momentum distribution and electron energy distribution

from combined and ultraviolet

The measured constant-initial-state

(CIS) curves

studies of positron annihilation

are shown in Figure 4 for E B = 0.5,

3.2, and 5.5

photoemission.

eV, respectively.

The resonant enhancements

due

The one-dimensionality

of the

electronic structure of (TaSe~)21 was confirmed

to the Ta 5d state were observed around hv = 39

from positron study.

and 48 eV for E B = 0.5 and 3.2 eV, but not for

the Ta 5d orbitals are hybridized with the Se 4p

E B = 5.5 eV.

orbitals in the valence bands from UPS study.

The resonant effects can be quali-

We have also observed how

tatively explained by an interference via the super Coster-Kronig decay of the 7a 5p core

REFERENCES

excited states, in which the additional photoemission intensity arises as the photon energy becomes greater than the Ta 5p core excitation threshold.

i. Z.Z° Wang, M.C. Saint-Lager, P. Monceau, M. Renard, P. Gressier, A. Meerschaut, h. Guemas and J. Rouxel, Solid State Commun. 46 (1983) 325.

The magnitude of the resonance in

the CIS spectra around h~ = 39 and 48 eV corre-

2. D.G. Lock, V.H.C. Crisp and R.N. West, J. Phys. F 3 (1973) 561.