NON-EQUILIBRIUM PHENOMENA IN SUPERCONDUCTING MICROBRIDGES

(1)

HAL Id: jpa-00217644

https://hal.archives-ouvertes.fr/jpa-00217644

Submitted on 1 Jan 1978

HAL is a multi-disciplinary open access

archive for the deposit and dissemination of

sci-entific research documents, whether they are

pub-lished or not. The documents may come from

teaching and research institutions in France or

abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est

destinée au dépôt et à la diffusion de documents

scientifiques de niveau recherche, publiés ou non,

émanant des établissements d’enseignement et de

recherche français ou étrangers, des laboratoires

publics ou privés.

NON-EQUILIBRIUM PHENOMENA IN

SUPERCONDUCTING MICROBRIDGES

J. Bindslev Hansen, P. Jespersen, P. Lindelof

To cite this version:

(2)

JOURNAL DE PHYSIQUE Colloque

C6, supplPment au no 8, Tome 39, aolit 1978, page

_{~ 6 - 5 2 0}

NON-EQUILIBRIUM PHENOMENA IN SUPERCONDUCTING MICROBRIDGES

J. Bindslev Hansen, P. Jespersen and P.E. Lindelof

Physics Laboratory

I,

H.C. 0rsted Institute, University of Copenhagen, DK-2100 Copenhagen

0,

Denmark

Rgsum6.- Nos rdsultats expgrimentaux sur des microponts supraconducteurs de Sn, In and A1 indiquent

la n6cessit6 de tenir compte de la th6orie des effets de non-gquilibre, pour leur description.

Abstract.- Measurements on superconducting microbridges (Dayem bridges) in Sn, In and A1 show syste-

matic departures from existing theories which may point to ways of incorporating non-equilibrium ef-

fects into the description of these high current density Josephson devices.

A superconductor with a constriction smaller

than the coherence length exhibits the Josephson

effects. Such small structures are obtained using

clean point contacts or using thin-film technology.

We have used variable thickness thin-film micro-

bridges in our investigations.

The resistively shunted Josephson junction (RSJ)

model, incorporating a sinusoidal current-phase re-

lation I

=

I sin

d

is central to the understan-

S

0 ding of superconducting microbridges. Recently, so-

me attempts have been made to incorporate non-equi-

librium phenomena into such

a

two-fluid description.

In the behaviour of superconducting microbridges

three distinct features show up which with all pro-

bability are of such origin, namely the large excess

current (shoulder) at low voltages/l/, a saturation

of the microwave power of the Josephson radiation/2/,

and the subharmonic energy gap structure/3/.

We have previously reported measurements on

indium microbridges/4/ showing that, as the tempe-

Voltage (5pV I

Div)

rature is lowereh from Tc, both the excess current

and the microwave power first increases (the latter

as

(1

-

T/T~)')

and then saturates in the same tem-

perature region around a reduced temperature of

0.95. Our variable thickness aluminium microbridges

exhibit the shoulder in the I-V characteristics.

The shoulder is positioned.at a lower voltage than

for bridges of tin and indium. The determination

of the shoulder in aluminium therefore presents a

special problem as it can easily be mixed up with

the effect of noise. Figure

1

shows a series of

I-V characteristics for one of our aluminium micro-

bridges.

As a measure of the position of the shoulder

along the voltage (frequency) axis we have taken the

Fig. I

:

Series of I-V characteristics for a varia-

ble thickness aluminium microbridge showing shoulder

Bridge parameters: length

X

width

=

0.2

X

0.2 ym2,

%

=

0.57

52,

dIc/dT

=

1.20

mA/K.

voltage, Vinfl., where d2v/d1'

= 0.

Figure 2 shows

a double logarithmic plot of this voltage versus

1

-

T/Tc. As seen,

varies as

(1

-

TITc)

112 'infl.

for bridges made of lead/5/, indium, ti.n and alumi-

nium. There is a clear difference in the position

of Vinfl. at a given temperature for the different

materials. This reflects probably the variation in

the inelastic relaxation time for the electrons,

which determines the characteristic times

tine^

.'

in the superconductor. However, the difference in

-cinel,

of four orders of magnitude between lead and

aluminium is not observed.

(3)

F i g . 2 : I n f l e c t i o n p o i n t v o l t a g e , V i n f l . v s . (l-T/ Tc) f o r a number of Pb, I n , Sn and A 1 m i c r o b r i d g e s .

A number of a t t e m p t s have been made t o e x p l a i n t h e s h o u l d e r a l l based on d i r e c t r e l a x a t i o n of t h e non-equilibrium i n t h e m i c r o b r i d g e and on d i f f u s i o n . E s p e c i a l l y t h e model s u g g e s t e d by Aslamazov and Larkin161 i s i n t e r e s t i n g a s i t i m p l i e s V a

i n f l. ( T ~ ~ ~ ~ ) - ~ . However, t h e model does n o t e x p l a i n t h e observed s a t u r a t i o n of t h e e m i t t e d microwave power.

The t h i r d f e a t u r e which we r e l a t e t o non-equi- l i b r i u m phenomena i s t h e subharmonic energy gap s t r u c t u r e , SHG, which a p p e a r s a s a s e r i e s of peaks i n t h e dynamic r e s i s t a n c e v e r s u s v o l t a g e c u r v e s . T h i s s t r u c t u r e i s presumably caused by p a i r b r e a k i n g due t o phonons c r e a t e d by r e l a x a t i o n of q u a s i p a r t i - c l e s t o t h e s i n g u l a r i t y i n t h e d e n s i t y of s t a t e . The e x c i t a t i o n of t h e q u a s i p a r t i c l e s can i n t u r n b e caused by Josephson r a d i a t i o n o r by q u a s i p a r t i c l e s c r o s s i n g t h e b r i d g e g i v i n g an image p i c t u r e of t h e d e n s i t y of q u a s i p a r t i c l e s on one s i d e of t h e b r i d g e a n energy eV h i g h e r on t h e o t h e r s i d e . The d e n s i t y of s t a t e on t h e two s i d e s of t h e b r i d g e i s modula- t e d by t h e o s c i l l a t i n g e l e c t r i c a l p o t e n t i a l on t h e two s i d e s .

We have found t h a t t h e SHG depends on t h e b r i d g e p r o p e r t i e s . T h i s i s i l l u s t r a t e d f o r aluminium i n f i g u r e s 3 and 4 which show c u r r e n t and dynamic r e s i s t a n c e v e r s u s v o l t a g e , and i n a d d i t i o n t h e vol-

VOLTAGE V IpVI

F i g . 3 : Aluminium m i c r o b r i d g e , same a s i n f i g u r e 1 . C u r r e n t and dynamic r e s i s t a n c e v s . v o l t a g e p l u s e n e r - gy gap determined from t h e v o l t a g e s of t h e peaks p l o t t e d v e r s u s v o l t a g e . The arrow i n d i c a t e s t h e BCS gap a t t h e same t e m p e r a t u r e .

11 J

1

I

-

4

voltage V [pvi Voltage V IpVI F i g . 4 : Aluminium m i c r o b r i d g e . Sane p l o t s a s i n f i - g u r e 3 . B r i d g e p a r a m e t e r s : l e n g t h x width = 0.3 x 0.5 pm2, RN=0.025

R,

dIc/dT = 28.6 mA/K.

t a g e dependence of t h e energy gap a s found from t h e p o s i t i o n of t h e peaks i n t h e dynamic r e s i s t a n c e . Bridges w i t h s m a l l c r i t i c a l c u r r e n t s ( a s i n f i g u r e

(4)

at finite voltage values'

.

In contrast, for bridges with large critical currents (figure 4) we see a general depression of the energy gap as we increase the current. The gap reduction stems from depairing caused by the large quasiparticle current and from heating effects.

ACKNOWLEDGEMENT.- We are indebted to Ole Eg for preparation of our microbridges.

References

/l/ Hbjgaard Jensen,H. and Lindelof,P.E., J. Low Temp. Phys.

3

(1976) 469

/2/ Sbrensen,O.H., Mygind,J., Pedersen,N.F., Guban- kov,V.N., Levinsen,M.T. and Lindelof,P.E., 3. Appl. Phys.

48

(1977) 5372

/3/ Gregers-Hansen,P.E., Hendricks,E., Levinsen,M.T. and Pickett,G.R., Phys. Rev. Lett.

2

(1973) 524 /4/ Lindelof,P.E., Hansen,J.B., Jespersen,P., Proc.

of Conf. on Future Trends in Superconductive Electronics, University of Virginia, Charlottes- ville 23-25 March 1978

/5/ Yeh,J.T.C. and Buhrman,R.A., J. Appl. Phys.

48

(1977) 5360

/6/ As1amazov.L.G. and Larkin.A.1.. Zh. EXD. Teor. Fiz. 70 (i976) 1340 (sov.'phys:

JETP

'J

(1976) 698)

-

/7/ Bindslev Hansen,J., Lindelof,P.E. and Pickett, G.R., In preparation.