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GAP SUPPRESSION AND INSTABILITY IN
NONEQUILIBRIUM SUPERCONDUCTING LEAD
FILMS
I. Iguchi, F. Shiota, K. Hara
To cite this version:
JOURNAL D E PHYSIQUE
Colloque C6, supplkment au no
8,
Tome
39,
aofit
1978,
page
C6-521
GAP SUPPRESSION AND INSTABILITY IN NONEQUILIBRIUM SUPERCONDUCTING LEAD FILMS
I . I g u c h i , F. S h i o t a and K. Hara
Department o f MathematicaZ Engineering and Instrumentation Physics,
University o f Tokyo, Tokyo 113, Japan
R6sumd.- Cet a r t i c l e c o n t i e n t une d t u d e s u r l a v a r i a t i o n d ' d n e r g i e i n t e r d i t e du plomb i n f l u e n c g e p r l ' i n j e c t i o n i n t e n s e d ' u n c o u r a n t t u n n e l P t e m p d r a t u r e d i f f d r e n t e e n u t i - l i s a n t l a j o n c t i o n d o u b l e - t u n n e l . Une i n s t a b i l i t d e s t o b s e r v d e P t o u t e t e m p d r a t u r e e n t r e 4, 2 K e t TA. Les r d s u l t a t s o b t e n u s s o n t c o m p a t i b l e s a v e c l a diagramme de p h a s e d e Chang e t S c a l a p i n o .
A b s t r a c t . - We p r e s e n t t h e measurements of l e a d e n e r g y gap r e d u c t i o n under i n t e n s e t u n n e l c u r r e n t i n j e c t i o n n e a r gap edge a t v a r i o u s t e m p e r a t u r e s u s i n g a d o u b l e - t u n n e l - j u n c t i o n . An i n s t a b i l i t y i s o b s e r v e d a t any t e m p e r a t u r e between 4 . 2 K and TA. The r e s u l t s a r e con- s i s t e n t w i t h t h e phase d i a g r a m o f Chang and S c a l a p i n o .
I t h a s been s u g g e s t e d by Chang and S c a l a p i - no / I / and Smith / 2 / t h a t a n o n e q u i l i b r i u m supercon- d u c t o r d r i v e n f a r from t h e t h e r m a l e q u i l i b r i u m e x h i b i t s a n i n s t a b i l i t y toward t h e inhomogeneous s t a t e b e f o r e t h e c o n d i t i o n f o r t h e homogeneous t r a n s i t i o n i s f u l f i l l e d . The Chang-Scalapino t h e o r y was r e c e n t l y developed by S c a l a p i n o and Huberman
/ 3 / and s u b s e q u e n t l y by Hida / 4 / . One o f t h e r e s u l t s i s t h a t t h e i n s t a b i l i t y t h r e s h o l d i s i n c r e a s e d by a c e r t a i n amount of q u a n t i t y r e l a t e d t o t h e phonon t r a p p i n g f a c t o r . I n t h e r e c e n t e x p e r i m e n t s / 5 , 6 / , we have r e p o r t e d t h e o c c u r r e n c e of a f i r s t o r d e r t r a n s i t i o n below TA and a n i n s t a b i l i t y above TA i n n o n e q u i l i b r i u m l e a d f i l m s due t o t u n n e l c u r r e n t i n j e c t i o n , t h e f a c t t h a t i n d i c a t e s dependence o f t h e n o n e q u i l i b r i u m t r a n s i t i o n o n t h e sample s u r r o u n d i n g s . We h e r e p r e s e n t t h e d e t a i l e d measurements of gap r e d u c t i o n o f s u p e r c o n d u c t i n g l e a d f i l m s i n terms o f t u n n e l r e s i s t a n c e under i n t e n s e t u n n e l c u r r e n t i n j e c t i o n n e a r gap edge u s i n g a N-I-S-I-N t y p e d o u b l e - t u n n e l - j u n c t i o n . The i n j e c t o r j u n c t i o n was a Pb-I-NiCr-Cu j u n c t i o n w i t h low t u n n e l - r e s i s t a n c e (a10-S 52cm2) / 5 , 6 / . The f i l m s a r e 0.1-0.5 m wide and a 2000
1
t h i c k . Sample geometry i s s i m i l a r t o t h o s e used i n r e f e r e n c e s / S / and / 6 / , i n which a p a r t of t h e d e t e c t i n g a r e a (20-50 %) i s exposed un- d e r q u a s i p a r t i c l e i n j e c t i o n . We used b o t h g l a s s and s a p p h i r e s u b s t r a t e s . The sample s u r f a c e had d i - r e c t t h e r m a l c o n t a c t w i t h l i q u i d h e l i u m . (AakBT l n ( R /R ) ) , a s a f u n c t i o n of i n j e c t i o n s n nn c u r r e n t I t a t v a r i o u s t e m p e r a t u r e s u s i n g a l o c k - i n a m p l i f i e r . F i g u r e 1 shows s u c h a n example f o r t h e sample w i t h a g l a s s s u b s t r a t e , t o g e t h e r w i t h t h e t u n n e l c h a r a c t e r i s t i c o f t h e i n j e c t o r j u n c t i o n . The t u n n e l r e s i s t a n c e P. s n f i r s t d e c r e a s e s r a p i d l y w i t h i n j e c t i o n c u r r e n t I t , t h e n becoming a l m o s t c o n s t a n t F o r f u r t h e r i n c r e a s e o f I t , Rsn a g a i n s t a r t s t o d e c r e a s e r a p i d l y a t a c e r t a i n i n j e c t i o n c u r r e n t I u n t i l i t f i n a l l y r e a c h e s t h e normal s t a t e v a l u e . According t o Hida 141, t h e t u n n e l c o n d u c t a n c e ( o r r e s i s t a n c e ) i s c o n t i n u o u s and i t s d e r i v a t i v e w i t h r e s p e c t t o I t i s d i s c o n t i n u o u s a t t h e i n s t a b i l i t y t h r e s h o l d i n c a s e o f second o r d e r t r a n s i t i o n . 0 50 100 150 2d0INJECTION CURRENT (mA)
rJe measured the differential tunnel resis- F i g . 1 : V a r i a t i o n o f t h e d e t e c t o r t u n n e l r e s i s -
t a n c e a t z e r o b i a s v o l t a g e w i t h i n j e c t i o n t a n c e o f t h e d e t e c t o r j u n c t i o n Rsn a t z e r o b i a s v o l - c u r r e n t , t o g e t h e r w i t h t h e t u n n e l c h a r a c - t a g e , which i s d i r e c t l y r e l a t e d t o t h e e n e r g y gap t e r i s t i c of t h e i n j e c t o r j u n c t i o n .
Hence we i d e n t i f y Ic a s an i n s t a b i l i t y t h r e s h o l d i n t h a t Rsn h a s d i s c o n t i n u o u s s l o p e a t I = Ic and
t
r a p i d d e c r e a s e of t h e gap i s observed j u s t a f t e r Ic. The measured gap v a l u e a t I t = I was A =0.6- -0.7A0. Note t h a t no v o l t a g e i s recorded i n t h e superconducting f i l m a t t h i s t h r e s h o l d . The s t r u c - t u r e s e e n i n lower c u r r e n t r e g i o n developed w i t h d e c r e a s i n g temperature toward T A , whose b e h a v i o r i s unexplained a t t h e p r e s e n t time.
The i n s t a b i l i t y t h r e s h o l d Ic e x h i b i t e d an anomalous temperature dependence. I n f i g u r e 2 , we show one of t h e r e s u l t s o b t a i n e d f o r t h e samples w i t h s a p p h i r e s u b s t r a t e s . S i m i l a r behavior was a l s o observed f o r those w i t h g l a s s s u b s t r a t e s . J u s t below
TA,
Ic i n c r e a s e s very r a p i d l y due t o l a r g e phonon e x t r a c t i o n from the sample. The system under- goes a f i r s t o r d e r t r a n s i t i o n i n t h i s c a s e .DETECTOR AREA
(25% EXPOSED)
0 100 200 300
INJECTION CURRENT (mA)
F i g . 2 : The t u n n e l r e s i s t a n c e v s . i n j e c t i o n current a t v a r i o u s temperatures. The i n s e t shows temperature dependence of t h e i n s t a b i l i t y t h r e s h o l d Ic.
The gap r e d u c t i o n below TA approximately obeys t h e simple r e l a t i o n A(n)/ A(0) = 1-211 (n : e x c e s s q u a s i p a r t i c l e number d e n s i t y i n u n i t s of 4N(D)Ao,N(0) : s i n g l e s p i n d e n s i t y of s t a t e s a t Fermi l e v e l ) 151.
We compare t h e experimental d a t a w i t h t h e phase diagram of Chang and S c a l a p i n o / l / . Using the observed v a l u e s of Ic and Ac
,
t h e d a t a were ana- lyzed by t h e modified Rothwarf-Taylor e q u a t i o n 1 6 1 .The d a t a were f i t t e d t o t h e theory a t t h e f i r s t o r d e r t r a n s i t i o n t h r e s h o l d of Gwen and Scalapino / 7 / . In f i g u r e 3 , the r e s u l t s t o g e t h e r w i t h t h e i n s t a b i l i t y t h r e s h o l d nc and t h e f i r s t o r d e r t h r e - shold n a r e shown. f F i g . 3 : Phase diagram f o r t h e i n s t a b i l i t y t h r e - shold nc and t h e f i r s t o r d e r t r a n s i t i o n t h r e s h o l d n
.
The d a t a p o i n t s were f i t - t e d t o t h e t h e o r y a t t h e f i r s t o r d e r t h r e s h o l d .The experimental p o i n t s l i e w i t h i n the i n s t a b i l i t y r e g i o n , i n d i c a t i n g t h a t t h e d a t a a r e q u a l i t a t i v e l y c o n s i s t e n t w i t h t h e t h e o r y . The t h e o r e t i c a l v a l u e of n c i s i n c r e a s e d by a c e r t a i n amount o f v a l u e a c c o r d i n g a s t h e s t r e n g t h of phonon t r a p p i n g / 3 , 4 / . Hence i t i s p o s s i b l e t o have a f i r s t o r d e r t l a n s i - t i o n whenever l a r g e phonon e x t r a c t i o n i s r e a l i z e d s o t h a t n exceeds n f . A p u z z l i n g n a t u r e i s t h a t t h e r e l a t i o n ~ ( n ) / ~ ( 0 ) = 1 - 2 n does n o t h o l d even qua- l i t a t i v e l y f o r t h e d a t a above T A . We c o n s i d e r t h a t t h e nonequilibrium s t a t e above TA i s probably non- thermal and t h e
v*
model may n o t be a p p l i c a b l e .Reference
/ I / Chang, J . J . and S c a l a p i n o , D . J . , Phys.Rev. BM) (1974) 4047.
/ 2 / Smith, L.N., J.Low Temp. P h y s . 2 (1977) 519. 131 Scalapino, I).J.and Huberman, B.A., Phys.Rev.
L e t t .
2
(1977) 1365. / 4 / Hida, K., p r e p i n t .151
I g u c h i , I., Phys. L e t t . (1978) 415; J.Low Temp. Phys.2
(1978) 605./6/ I g u c h i , I . , Phys. Rev. B 1 6 (1977) 1954.
/ 7 / h e n , C.S. and Scalapino, D . J . , Phys. Rev.Lett. 28 (1972) 1559.
