PHONON EMISSION CAUSED BY SUBHARMONIC GAP STRUCTURE OF SUPERCONDUCTING SN-I-SN TUNNEL JUNCTIONS

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PHONON EMISSION CAUSED BY SUBHARMONIC

GAP STRUCTURE OF SUPERCONDUCTING

SN-I-SN TUNNEL JUNCTIONS

W. Forkel, H. Schenk

To cite this version:

JOURNAL DE PHYSIQUE Colloque C6, suppldment au no 8, Tome 39, aotit 1978, page C6-593

PHONON E M I S S I O N CAUSED B Y SUBHARMONIC GAP STRUCTURE OF SUPERCONDUCTING SN-I-SN TUNNEL JUNCTIONSn

W. Forkel and H.Schenk

PhysikaZisches I n s t i t u t der Universitiit S t u t t g m t , Germany 7000 S t u t t g a r t 80,PfaffenmZdring 57

RQsum6.- On a dtudid le spectre des phonons dmis par des jonctions Sn-I-Sn supraconductrices ayant une structure sous-harmonique de la frdquence du gap. On obtient par ces spectres des informations expdrimentales qui n'dtaient pas accessibles auparavant, par exemple une indication sur le nombre des excitations dues au transfert d'une charge dlectronique 1 travers la barrigre.

Abstract.- The phonon spectrum emitted by superconducting Sn-I-Sn tunnel junctions with different kinds of subharmonic gap structure (SGS) contains previously unavailable experimental information, e.g. on the number of excitations created per electronic charge transferred across the barrier. For a certain type of junctions our results agree with the multiparticle tunnelling model.

Recently, it was shown /I/ that the most impor- tant properties of SGS reproduced both by the Joseph- son self-coupling (JSC) model /2/ and by the multi- particle tunnelling (MPT) model/3/. Since work based on measurements of the I-V characteristics ledto contrary statements concerning the existence of MPT / 4 , 5 / we looked for new arguments by measuring the phonon emission spectrum.

BASIC IDEA.- As shown in figure 1 on the left, MPT produces 2/m excitations per electron transferred across the barrier (m= 1,2,3

...).

Before recombina- tion, the quasiparticles loose part of their excess kinetic energy (maximum m.eV -2A ) by electron- phonon interaction, giving rise to a voltage-tune- able continuum of relaxation phonons ("phonon brems- spectrum")which discontinuously breaks off at mmax=m.eV-2A

.

The m = 1 case is well known from

phonon spectroscopy/6/ In addition, a continuum of recombination phonons is generated with a peak at iiC$ = 2A

.

For MPT the phonon generation rate is obviously proportional to l/m.

On the righthand side of figure 1 we have se- lected. JSC processes in which a single pair is broken with the assistance of n Josephson photons, two quasiparticles being distributed among both films in different ways depending on the tunnelling probability. In this case no simple rule can be given which relates the number of excitations to the d.c. current for each m, since I(V) depends on various parameters /2/ Further-more, we would expect a different shape of the relaxation spectrum since e.g. the coherence factors entering the pro- cesses on both sides of figure 1 are different.

Fig.1: Schematical representation of the quasipar- ticle distribution generated by multiparticle tun- nelling (left) and pair-breaking by Josephson pho- tons combined with tunnelling (right); m,n and are integers.

m -PARTICLE TRANSFER (MULTIPARTICLE TUNNELING) relaxation phonon energy : hi2 a m.eV -2A

&

-L,

?IR -2eV-2A

EXPERIMENTS.- The Sn-I-Sn junctions used as phonon generators were placed on one side of a 3...6 mm

thick single crystal silicon substrate. A supercon- ducting A1-I-A1 tunnel junction on the other side served as the detector, counting only phonons abo- ve the pairbreaking threshold A 0 = 2AA1. We measu- red the derivative dS/dI of the detector signal S(V) with respect to the generator current I(V)

8-PARTICLE TRANSFER

ASSISTED BY n-PHOTONS

(m=2n+P) relaxation phonon energy : hR=nhwo+leV -2A hwo= 2eV - - h R sm.eV

-

2A

""

J&JJ -nnwo (PAT 1

-15-

nbw0

using a constant a.c. current modulation technique

161.

vior, however, we could stabilize an a.c. Josephson

...

' .

I , I , I I I . . I

0 0 5 1 15 2

VOLTAGE ( m V )

Fig.2 : I-V characteristics (dotted lines) and dif- ferential phonon signal dS/dI of a junction with a "patchy" oxide layer.

In figure 2 typical results for Sn-junctions with impure or "patchy" oxide layers-generated by direct exposure to a d.c. glow discharge 171-are shown. The I-V characteristics (dotted lines) show magnetic-field insensitive step-like SGS up to m = 3. The phonon signal dS/dI shows corresponding

step structure /8/ at eV = (2ASn+ 2A )/m, i.e. A1

when the maximum relaxation phonon energy meV-2A Sn just reaches the detector threshold 2AA1. We seein figure 1 that in the m =2 process quite exactly half the number of excitations is generated as in the m = 1 case. The m = 3 step is somewhat smaller than one third of the m = 1 step, probably because the three particle current is comparable in magni tude to the thermal single-particle current which "shunts away" part of the modulation current. There is little doubt that we deal with mltiparticle tun- nelling in this case, Further details of the spec- trum, such as the line shape, are also in accordan- ce with calculations based on the MPT model /9/

Sometimes we accidentally obtained "leaky" Sn-I-Sn junctions showing roughly a resistively shunted junction behavior (similar to a weak link) at low voltages and magnetic-field insensitive cur- rent bumps at eV = 2A/m, slowly decreasing in magni- tude up to m = 5. The maxima in dV/dI were reflec- ted as steps in dS/dV showing abrupt enhancement of quasiparticle production rate and recombination phonon emission at eV = 2A/m. A weak m = 2 relaxa- tion phonon structure at eV = A Sn +

AAl

was also visible/9/. In this cas possibly both mechanisms

(MPT and JSC) are present.

For both types of junctions the Josephson current was much less than the theoretical maximum value. With junctions showing a more ideal beha-

mode by a weak magnetic field, causing the junction to draw a large d.c. current /lo/. In this case we observed not only recombination phonons /11/ but again a relaxation structure which now was diffe- rent in line shape and magnitude from the predic- tions based on the MPT model/9/.

We are thus led to the conclusion that both mechanisms,Josephson self-coupling and multiparti- cle tunnelling, are met with real junctions and can be distinguished by their phonon spectra. It would be interesting to knowwhethera unified des-

cription is possible.

References

/I/ Hasselberg,L.E., Levinsen,M.T. and Samuelsen, M.R. Phys.Rev. (1974) 3757

131 Schrieffer,J.R. and Wilkins,J.W.,Phys.Rev.Lett.

I0(1963) 17

-

/4/ Giaever,I. and Zel-ler,H.R., Phys. Rev; (1970) 4278

151 Mukhopadhyay,P., Solid State Commun.3 (1977) 35 1

/6/ Forkel,W.,Welte,M.,and Eisenmenger,W. Phys. Rev.Lett.

2

(1973) 215. Kinder,H.,Phys.Rev.Lett

.

28 (1972) 1564

-

/7/ Shen,L.Y.L., Phys.Rev.Lett.2 (1968) 361 Our junctions also showed the "voltage shift" phe- nomenon.

/8/ Kinder,H., Low Temperature Physics-LT 13, Vo1.3,p.341

/9/ Forkel,W., to be published

/ lo/ Eck,R.E

.

,s;alapino ,D. J. and Taylor ,B.N.

,

Phys,Rev.Lett.g (1964) 15