INELASTIC SCATTERING FROM 3He UNDER PRESSURE

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INELASTIC SCATTERING FROM 3He UNDER PRESSURE

P. Hilton, R. Cowley, W. Stirling, R. Scherm

To cite this version:

P. Hilton, R. Cowley, W. Stirling, R. Scherm. INELASTIC SCATTERING FROM 3He UNDER PRESSURE. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-208-C6-210.

�10.1051/jphyscol:1978692�. �jpa-00218372�

JOURNAL DE PHYSIQUE Colloque C6, supplement au n° 8, Tome 39, aout 1978, page C6-208

INELASTIC SCATTERING FROM 3He UNDER PRESSURE

P.A. H i l t o n , R.A. Cowley, W.G. S t i r l i n g t and R. Scherm t

t Institut LaUe-Langevin, Grenoble, France.

Résumé.- Nous présentons des résultats obtenus en diffusion inélastique des neutrons sur 3He liqui- de à la P.V.S. et à des pressions de iO et 20 bars. Les mesures ont été effectuées à une tempéra- ture de 0,7 K dans une gamme de transfert de moment de 1,1 à 2,4 S ' et pour des transferts d'éner- gie allant jusqu'à 27 K. Aux petites valeurs de transfert de moment utilisées il n'y avait pas d'évidence pour un mode zéro son bien défini, même à une pression de 20 bars. A 1,9 p~ (la posi- tion du "roton minimum" dans le ''He superfluide), la diffusion observée a une distribution large, dont la fréquence décroît d'un facteur supérieur à 2, quand la pression augmente jusqu'à 20 bars.

Abstract.- Results are presented for the inelastic neutron scattering from liquid 3He at S.V.P., 10 bars and 20 bars pressure. Measurements were obtained for wavevector transfers in the region 1.1 - 2.4 A_ 1> a n d f °r energy transfers up to 27 K. The experiments were performed at a tempera- ture of 0.7 K. At low wavevector transfers, Jhere was no evidence for a well defined zero sound mode, even at a pressure of 20 bars. At 1.9 A- 1 (the position of the roton minimum in superfluid

^He), the observed scattering has a broad distribution, whose peak frequency decreases by more than a factor of 2, on increasing the pressure form S.V.P. to 20 bars.

Neutron scattering experiments on liquid 3H=

at the saturated vapour pressure have shown that none of the currently available microscopic theories of 3He is wholly satisfactory. Measurements were initially performed using the INS time of flight spectrometer at the Institut Laiie-Langevin, at 1.3 K /I/, and 0.6 K HI. These measurements showed that the scattering has a broad spectrum for wavevector transfers, Q, between I.I and 2.5 A . No evidence was obtained in these measurements for scattering which could be at- tributed to a well defined zero sound mode. Subse- quent experiments /3/, performed at 0.015 K, suggest that there is a fairly well defined zero sound mode for wavevectors between 0.8 and 1.4 A- 1. Despite se- veral theoretical efforts /4/, there has not yet been in our view, a convincing explanation of this tempe- rature dependence. Since the velocity of sound and the effective mass, m , of the % e atoms are known to increase with increasing pressure, it is to be expected that at higher pressures, it will be more difficult for the zero sound modes to decay into par- ticle hole pairs than at the saturated vapour pres- sure. This report is on measurements performed at higher pressures in the hope of elucidating the na- ture of the zero sound mode in pure 3He.

The experiments were performed at 0.7 K; the only significant change from our earlier experiments was that a single crystal of sapphire was chosen as

the window of the sample container /5/. The choppers determining the incident neutron energy were phased

to produce neutrons of wavelength 4.5 A.. The 400 counters available, were arranged in eight blocks, chosen so as to give a range of wavevector trans- fers between 1.1 and 2.4 A . Under these conditions the full width oi the incoherent vanadium calibra- tion peak was 3.65 K (FWHM;. After substraction of an appropriate background the time of flight spec- tra were corrected for the variation of counter efficiency with scattered neutron energy, the varia- tion of absorption in the 3H e with neutron wavelen- gth, and the variation of scattered intensity with the density of 3H e . Conversion w a s then made to energy spectra. Typical results are shown in figure

1 for three different scattering angles.

WAVEVECTOR TRANSFER (X"')

•,. 124 121 1.19 1.16 I B 1.67 1.62 156 I S U i 2.15 2.10 203 196 1.89

I "i J4-.-..L. i «•; f"""-<-.... i 41 ..••-""""•j

| °-t-5VP ^r o-rsip — °-£4VP 1

§ » r f i 1 1 1 <-. 2 0 — J - H 1 1—-i <-. va'—t-, , 1 1 , J

| J JY . \ 4 &S-^ X {,\ \ .*-~\i... J

fc ; . j 10BARS ^ f • 1QBARS ^ ^ Q I 10BARS f l S J O r - j - i 1 1 1 1 - 2B 1 1 1 -)- r- 2 . 0 — L . 1——i 1 A

g i IV '• h '• ! J " . - 4

0 5 10 15 20 25 0 5 1 0 1 5 2 0 2 5 0 5 10 15 20 2§*

54* 75* 104*

ENERGY TRANSFER (K)

Fig. 1 : Typical energy distributions for liquid at 0.7 K, for S.V.P., 10 bars and 2 0 bars pressure The three angles shown correspond to wavevector trangfers of approximately 1.2 A- 1, !

.55A

and

2.0 A- 1 respectively.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1978692

All of our data is summarized in figures 2 and

with our earlier measurements and we do not observe Figure 2 shows the peak and half-height frequencies a well defined zero sound mode at 0.7 K. Calcula- for each of the energy spectra, while figure

shows tions of the excitation spectrum of 3 ~ e at high pres- the distribution of scattered intensity at the three sures suggest that the zero sound mode at 20 bars different pressures. occurs at Q =1.2l-l, for energy transfers in the re-

gion of 20 K, and at Q =

If1, the zero sound mo- de appears to merge with the particle hole continu- um. Our results show no evidence of a well defined excitation, neither at

bars nor at 20 bars, for energy transfers up to 25 K.

A qualitative understanding of our results may be obtained when it is remembered that the scattered neutron intensity is proportional to the sum of the mass density correlation function and

0

1

1.0 1.5 20 ID 15 20 ID l5 2.0

~ ( x ' ) the spin density correlation function. Calculations then suggest that for Q

the scattering at S.V.P. consists almost equally of the spin density Pia. 2

Peak positions and half height positions of

the energy disiributions at S.V.P.,

birs and 20 and mass density components. At elevated pressures bars pressure. The dotted lines refer to fi2~2/2m*, the effective mass increases, so that the spin den- with mX

3.08 m. at S.V.P.,

m, at

bars and

5.2 m, at 20 bar;. sity component may be expected to decrease in ener- gy, with a corresponding increase in peak intensity,

20 BARS 2 5 2 0 1 5

1.0 1.5

a(&')

while at high energy transfers, the scattering is dominated by the mass density fluctuations. If the zero sound modes are broadened by decay into multi- particle hole states, it may well be that the scat- tered intensity from the mass density fluctuations is spread over such a wide frequency range that the zero sound excitation cannot be detected above the background in our experiment. These arguments pro- vide a qualitative understanding of the behaviour at smaller wavevector transfers. At larger wavevec- tor transfers (Q

the decrease in peak energy with increasing pressure isgore rapid than can be accounted for solely by the change in effec- tive mass. This suggests that the appropriate effec- tive mass for these energy and wavevector transfers is somewhat larger than is obtained in macroscopic measurements. We hope that these results will sti- mulate further calculations of the pressure depen- dence of the excitations in liquid 3 ~ e .

Fig. 3

Plots of equal intensity contours for

S(Q,w) for liquid 3 ~ e at 0.7 K, shown for

S.V.P.,

bars and 20 bars pressure.

References

/ l / S c h e m , R . , Stirling, W.G., Woods, A.D.B., Cowley, R.A. and Coombs, G.J., J. Phys. C : Solid State Phys.

7

/2/ Stirling, W.G., Scherm, R., Hilton, P.A., Cowley, R.A., J.Phys. C : Solid State Phys.

9

/3/ Skold, K., Pelizzari, C.A., Kleb,

R.

37

/4/ Khanna, F.C. and Glyde, H.R., Can. J. Phvs.

56

Cryogenics

17