TRANSMISSION OF BALLISTIC LONGITUDINAL AND TRANSVERSAL PHONON PULSES THROUGH A SAPPHIRE-HeII INTERFACE AT T = 0.25 K

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TRANSMISSION OF BALLISTIC LONGITUDINAL

AND TRANSVERSAL PHONON PULSES THROUGH

A SAPPHIRE-HeII INTERFACE AT T = 0.25 K

H. Salemink, H. van Kempen, P. Wider

To cite this version:

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JOURNAL DE PHYSIQUE _{Colloque C6, supplPment au}

no

8, Tome 39, aozit 1978, page C6-326

TRANSMISSION OF B A L L I S T I C LONGITUDINAL AND TRANSVERSAL PHONON PULSES THROUGH A SAPPHIRE-HkI I INTERFACE AT T = 0.25 K

H.W.M. Salemink, H. van Kempen and P. Wider

Research I n s t i t u t e of Materials, University of Nijmegen, ToernooiueZd, N i jmegen, The Netherlands

Rdsum6.- En employant des techniques de "heat-pulse1' rapides, nous avons observe la transmission de phonons balistiques longitudinaux (L) et transversaux (T) du saphir aux phonons dans 11h61ium su- perfluide P une tempdrature de T = 0,23 K. Le rapport d1intensit6 des modes L et T converties dtait de 0,22 2 0,02, tandis qu'une valeur de 0,31 f 0,01 pour cette mGme grandeur a 6t6 observde

dans le saphir.

Abstract.- Using fast heat-pulse techniques, the transmission of ballistic longitudinal (L) and transversal (T) phonons from sapphire into phonon in superfluid He-I1 at T = 0.23 K has been obser- ved. The intensity ratio of the converted L and T mode in He-I1 was found to be 0.22 f 0.02 whereas a value of 0.31 f 0.01 was obtained for the sapphire crystal.

MIXING

C

HAMaER

J$

FIu-UNE

KAPTON

SPACER HeII CEL HEATER He-CHAMBER

AI-FILM

DETECTOR

The transmission probability of phonons from heater. solids to liquid He still raises many questions

both theoretically as experimentally. To gain more insight one has tried to study the transmission of longitudinal (L) and transversal

(T)

phonons sepa- rately by means of time-of-flight techniques. Up to now in most experiments the reflection from the surface under study has been measured /I/. However, among others because of the presence of phonon £0- cusing, the interpretation of the reflection experiments is not straight forward, so direct transmission experiments are preferable/2/.

In this paper experiments are described on transmission of ballistic phonon pulses from single

crystal sapphire to superfluid He-I1 at environmen- Fig. 1 : Experimental arrangement. tal temperatures below 0.25 K and under hydrosta-

tic pressures up to 25 bar. Under these conditions The heater is backed by a vacuum to ensure that all phonon transport in He-I1 is purely ballistic 131. energy is directed into the crystal. After

Time-of-flight techniques are used to resolve the through the sapphire-He interface, the converted phonon pulses in He-11, originating from longitudi- phonon pulses are detected by a superconductive nal (L) and transverse (T) mode phonons in the sap- Al-film bolometer in the superfluid, placed at a phire crystal. Since the converted phonon pulses distance of 0.5 mm from the sapphire surface under in He-I1 propagate under identical conditions, the study. The whole assembly is mounted in a pressure conversion of L and T mode phonons at the solid- cell and cooled to T9.25 i( by means of a dilution liquid-interface can be studied, thus providing a refrigerator. The space between the detector and

new technique for investigation of the Kapitza the sapphire surface is filled with high-purity li-

resistance. quid helium under pressures of up to 25 bar.

The experimental arrangement is shown in fi- A typical detected time-of-flight signal at a gure 1 . A high time resolution has been obtained temperature of T = 0.22 K and a He-I1 pressure of by using thin film transducers. Heat pulses are ge- 24 bar is shown in figure 2. Distinct phonon pul- nerated into the source material (single crystal ses are seen, whose arrival times coincide well sapphire, along c-axis) by an evaporated thin film with the expected transit times for converted L,T

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and sidewall reflected (SW) pulses as well as echo's from these pulses in the He-I1 cell (L3,T3).

DETECTOR SIGNAL 1

Fig. 2 : Time-of-flight signal. T = 0.22 K, p = 24 bar. Vertical scale in arbitrary units.

The peak signal intensity of the converted L and T phonon modes, detected in He-11, as a function of the heater power density is shown in figure 3.

HEATER TEMP. ( K )

Fig. 3 : Signal peak intensity vs. heater power density. Vertical scale in arbitrary units ; circles : L mode

,

triangles : T mode. T = 0.23 K, p = 24.8 bar.

60

LO

20

The indicated heater temperatures are calculated from the input power density 141.. The measured ratio of the converted L and T mode signals is 0.22 t 0.02 over the input power density range of 1 to 3w/mm2, whilst a value of 0.31 f 0.01 is found far the LIT ratio in the sapphire itself. This is in contrast with the results of transmission experi-

A

-

A

-

AA A AA A

-

0 0 0 O O -0 0 000 O

ments

,

using second sound excitations at 1.5 K, where no change in the converted LIT intensity ra-

tio relative to the ratio in the solid was obser- ved 151.

Upon reduction of the He-I1 pressure the converted L and T mode signals decrease in intensity, with a large step around p = 14 bar. The converted

LIT

ratio decreases too, leading to a value of 0.15 f 0.02 at p = 1 bar. On the basis of the acous- tic mismatch theory and its modifications, the ob- served values and the shifts in the detected LIT ratio is unexpected. However, an explanation can be attempted taking into account the broad energy spec- trum of the phonons involved. Experiments with mono- chromatic phonons should certainly be very useful to unravel this problem.

Part of this work has been supported by the "Stichting voor Fundamenteel Onderzoek der Materiel' (FOM) with financial support from the "Nederlandse Organisatie voor Zuiver Wetenschappelijk Onderzoek"

(ZWO)

.

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References

/I/ Guo,C.-J., Maris,H.J., Phys. Rev. E(1974)960 /2/ Taylor,B., Maris,H.J., Elbaum,C., Phys Rev.

(1971) 1462

/3/ Narayanamurti,V., Dynes,R.C., Andres,K., Phys. Rev. (1 975) 2500

/4/ Weiss,O., Z. Angew. Phys.

2

(1969) 325 151 Swanenburg,T.J.B., Wolter,J., Phys. Rev. Lett.

31 (1 973) 693