High-energy phonon dispersion in La1.85Sr0.15CuO4

PHYSICA@ ELSEVIER

Physica B 213&214 (1995) 72 74

High-energy phonon dispersion in Lal.s5Sro.1sCuO4 A.H. Moudden a'*, P.M. Gehring b' l, G. Shirane b, M. Matsuda b, L. Vasiliu-Doloc a, B. Hennion a, Y. Endoh c, I. Tanaka d, H. Kojima d a LLB, CEA-CNRS, CE/Saclay F-91191 Gif/Yvette, France b Brookhaven National Laboratory, Upton, N Y 11973, USA c Tohoku University, Aoba-ku, Sendal 980, Japan d Yarnanashi University, Miyamae 7, Kofu 400, Japan

Abstract We have measured two phonon-dispersion curves above 50meV in a superconducting (T~ = 33 K) single crystal of Lal.85Sro.tsCuO4. In the normal phase (T = 4 5 K), we show that the A, mode compatible with the Raman A, g(425 cm - 1) mode is weakly dispersive, falling from 53.7 meV at the zone center to about 51 meV at the zone boundary along the tetragonal (0, 0, ~) direction. For the A1 mode compatible with the infra-red A2, (480 cm- 1) mode, we observe a significant dispersion of the phonon energy which drops monotonically from 60 to 56 meV. The global features of these dispersion curves are similar to those previously reported in studies of both pure and 10% Sr doped La2CuO4. However significant differences in the shape of the dispersion of the highest A1 branch is shown, as well as a significant softening with increasing carrier concentration.

1. Introduction The complete phonon dispersion curves of pure and 10% Sr-doped LazCuO4 compounds have been mapped out by Pintschovius et al. [1] at room temperature using inelastic neutron scattering. The most puzzling feature of these studies was the observation of 'extra branches' which were at first interpreted as the manifestation of electronic degrees of freedom interacting with the lattice vibrations. Although some of these extra branches were later explained by the orthorhombic distortions, others are still not considered to be normal phonons and thus remain a puzzle [2]. In this paper we will be mainly interested in the lattice vibrations in the energy range

* Corresponding author, i Permanent address: National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.

from 50 to 70meV which largely covers the Raman A~g (425cm-1) and the infra-red A2, (480cm -1) modes for which extensive optical studies [3] have been performed, and for which the polarization at finite propagating wave vector involves the vibrations of all the oxygen atoms.

2. Experimental The inelastic neutron scattering studies were performed on the H4M and H8 spectrometers at the HFBR at BNL and at the 1 T triple-axis at the LLB. Constant final neutron energies of 14.7 and 30.5 meV were selected using PG(0 0 2) crystals as monochromator and analyzer. A pyrolytic-graphite filter was used to suppress higherorder contamination. Appropriate collimations were used for the phonon dispersion and for the line width

0921-4526/95/$09.50 .~c" 1995 Elsevier Science B.V. All rights reserved SSDI 0 9 2 1 - 4 5 2 6 ( 9 5 } 0 0 0 6 5 - 8

73

AH. Moudden et al./'Physica B 213&214 (1995) 72 74

studies. The single crystals we used were grown by the travelling-solvent floating-zone m e t h o d described elsewhere [4]. Two crystals with a total volume estimated to a b o u t 1 cm 3 were precisely oriented a n d m o u n t e d in a displex refrigerator such that reflections of the form (h, h, l) (tetragonal indexing) lay in the scattering plane. At r o o m t e m p e r a t u r e the lattice p a r a m e t e r s were found to be a = b = 3.776 A a n d c = 13.22 ,~. The t e m p e r a t u r e dependence of the superlattice peak associated with the t e t r a g o n a l - t o - o r t h o r h o m b i c distortion shows a s h a r p a n d unique structural phase transition at a b o u t 220 K.

Laz_xSrxCuO4 I

I

I

}

I

I

65.0

;i

.10 (l~fereaee 1) ? = ~ 5 K

62.5

60.0 &

57.5 e~

55.0

3. R e s u l t s

Fig. 1 shows a set of constant-Q scans from 50 to 7 0 m e V in the n o r m a l state (T = 45 K) for various mom e n t u m transfers t h r o u g h o u t the Brillouin zone in the (0, 0, ~) direction. O n e can clearly see two peaks in the p h o n o n g r o u p for Q = (0, 0, 13.5), one at to1 = 52.8 meV a n d a n o t h e r at ~o2 = 59.3 meV. W h e n interested in the dispersion relation only, we used a simple, s t a n d a r d gaussian fitting of the profiles to determine the frequencies. F o r detailed analysis of the p h o n o n line width, we fit the d a t a with Lorentzian line shapes with a p p r o p r i a t e p h o n o n t e m p e r a t u r e occupation factors as represented by the solid lines in the figures. Note that, as the scattering vector is set closer to the zone center (0,0, 14), the intensity of the COl m o d e decreases whereas t h a t of toz

52.5 50.0 [

I

o

o,2

I

I

0.4 0.6 [o o t]

L

I

o.6

1.o

Fig. 2. Selected phonon-dispersion curves for La,.ssSro.lsCuO4 measured between 50 and 65 meV along the (0, 0, ~) direction. A comparison is made with the corresponding dispersion in a lesser doped compound.

Q = (o' o (3)-

11o 90

/

0 45 K *8K

70 50

T=45K

Lal.asSr.15CuO4 125

(o,o,ta.75)

100 t

ao

lO

5O 2

m

125 l- . . ' . . '

~1oo

I-

'

('o,oj3.5)'

50~ i

100

o 45 K * BK

o

70 ,

,

i

(0,0,13.25

.~tO0 75 50

125

Q = (0 0 14)

-~ 90

50 30 10

p

*

p

q

,

p

q=(o,o63)

,

75 5O ~5 0 45 50 55 60 65 70 75 Energy [meY]

Fig. 1. Phonon groups of the second and third highest branches along the tetragonal (0,0, 4) direction measured at 45 K in Lal.ssSro.~sCuO4. The solid lines are fits to Lorentzian line shapes.

45 50 55 60 65 70 75 Energy (meV) Fig. 3. Zone-center infrared mode and the corresponding zone boundary mode, measured above (45 K) and below (8 K) the superconducting transition temperature (33).

increases. Right at the (0,0,14) zone center, only the (02 peak is readily visible a n d is centered at 60 +_ 0.5 meV. The frequencies of these two zone-center m o d e s are in excellent agreement with those of the R a m a n (425 c m - 1)

74

A.H. Moudden et al. Phvsica B 213&214 (1995) 72 74

and infra-red (480cm 1) modes. No additional unidentified peaks were detected in this energy range. In Fig. 2 we report the resulting dispersion curves along the (0, 0, ~) direction for these two modes, which correspond to the A~ irreducible representation of the space group I4/mmm. The solid dots represent our data for La2 :,Sr:,CuO4 with x = 0.15. In the same figure, for comparison, we show previously published data [1] on La2 xSrxCuO4 with x = 0.1. Within experimental accuracy, there is no significant difference between the two sets of data for the Raman branch. This demonstrates that the Alg mode, which at least near the zone center involves mainly vibration of the apical oxygens, is insensitive to the carrier concentration. As we will see below, it also remains unmodified across the superconducting transition. In contrast, a significant modification in the dispersion of the polar A2u mode occurs when changing the carrier concentration. The dispersion curve for the lesser doped system exhibits a pronounced minimum around the wave vect~r t0, 0, 0.7) that is washed out at the higher concentr~lon. In Fig. 3 we show, for temperatures above (45 K) and below (8K) the superconducting transition temperature (33 K), the phonon groups measured at the zone center (0,0, 14) and at the zone boundary (0,0, 13). Note that at the zone center the phonon profile, apart from a small reduction of the intensity consistent with the Bose occupation factor, shows no significant change in line width or frequency for either the Raman or infra-red modes. But near the zone boundary there is however some evidence of a slight softening of the highest A~ branch of about 2 meV.

4. Discussion

We have measured the phonon-dispersion curves above 50 meV in the normal phase of a superconducting

single crystal of L a l 8~Sro. ~sCuO4. We have shown that the global features of the phonons in this energy range are quite similar to those reported previously [1,2] for LazCuO4 and La~.gSro.~CuO4. We have, however, observed several basic differences which may or may not be related to the novel superconductivity. A significant softening of the 'infrared' branch, by about 2meV, is observed near the zone boundary when increasing the carrier concentration and lowering the temperature. No such change is observed for the 'Raman branch'. Moreover we have shown that the minimum present in the dispersion curve of the 'Raman' branch along (0, 0, ~) at low carrier concentration is no longer present in our sample with high carrier concentration. We suggest that the minimum in this dispersion curve at low carrier concentration indicates that a single nearest-neighbour interlayer is not sufficient to describe the dynamics. Longer-range interlayer couplings are required. This is not unreasonable if we believe that long-range Coulomb forces, which dominate to some extent the dynamics of the pure compound, are screened by the incorporated carrier and act as short-range forces. We would like to thank Drs. J.D. Axe, R. Birgeneau and K. Yamada for useful discussions. References

[l] L. Pintschovius et al., in: Proc. Int. Sem. on High Temp. Supcrcond., Dubna, June (1989) (World Scientific, Singapore~ 1990) p. 36. [2] L. Pintschovius et al., Physica C 185-189 (1991) 156. [3] S. Sugai, S. Shamoto, M. Sato, T. Ido, H. Takagi and S. Uchida, Solid State Comm. 76 (1990) 371. [4] 1. Tanaka, K. Yamane and H. Kojima, Nature 337 (1989) 21.