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CRITICAL CURRENT HYSTERESIS DUE TO
ELECTRON OVERHEAT IN NARROW
SUPERCONDUCTING FILMS
V. Volotskaya, L. Musienko, I. Dmitrenko
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C6, supplkment au no
8, Tome 39, ao6t 1978, page
~ 6 - 5 0 5CRITICAL CURRENT HYSTERESIS DUE TO ELECTRON OVERHEAT IN NARROW SUPERCONDUCTING FILMS
V.G. Volotskaya, L.E. Musienko and I.M. DmitrenkoPhysico-TechnicaZ I n s t i t u t e of Low Temperatures, UkrSSR Academy o f Sciences, 47, Lenin Prospect,
Kharkov, 310164, USSR.
Rdsumd.- On a dtudil l'hystlrdsis descaractdristiques courant-tension des couches minces supraconduc- trices d'8tain et d'indium dans un large intervalle de tempdrature. On a montrd, que si ce transfert de chaleur est parfait, la valeur de l1hyst8rdsis et sa variation avec latemperature s'accordentavec les rlsultats de la th6orie de Shklovski, expliquant l'apparition de l'hyst6rdsis par la surchauffe des llectrons par rapport au rdseau. Sur ces mgmes couches, on a dscouvert l'influence de La sur- chauffe des llectrons sur la valeur de la resistance thermique de l'interface mdtal-isolant.
Abstract.- The hysteresis of IVC has been studied on thin superconducting films of tin and iridium in a wide temperature range. It has been shown that under perfect heat transfer, the value of hysteresis and its temperature dependence are in agreement with the results of Shklovsky's theory according to which the hysteresis is due to electron overheat with respect to the lattice. The effect of electron overheat on thermal resistivity of a metal-insulator interface has been found.
The paper deals with the problems on the cri- tical current hysteresis in tin and indium films with no magnetic field applied and on the hysteresis produced by electron overheating. Until recently, the Joule heat evolution in a resistive film was considered to be the only cause of the critical cur- rent hysteresis in I-V-characteristics in the tempe- rature range of t ? 0 . 8 (t = TITc) /l/. A value of
the hysteresis is dependent considerably on the heat transfer conditions. But as it follows from /2/ the critical current hysteresis should be observed also under perfect heat transfer from a film, ifthe electron overheat with respect to the lattice is in- volved. In this case the hysteresis value is inde- pendent of heat transfer conditions and determined only by the metal parameters and temperature.
The experiments were carried out on narrow films = 0 . 5
+
7 . 0 p) deposited onto sapphire or quartz substrates optically polished. The critical current was found and the IVC's were taken in the temperature range of t = 1-0.4 K. As the tempera- ture decreases, the reversible smoothed IVC's become "discontinuous" when under reaching the critical current of pair breakdown, fcl, a sharp increase in voltage and a transition to a resistive state with the resistance .close to the normal film resistance were observed. In this case with the current de- creased, the superconductivity was restored atI=, < Icl. The relation a = Ic,/Icl defines the hys-
teresis value and is temperature dependent.
A comparison is made between the experimental
dependences a (t) for various films and the theore- tical dependence /l/
where 8, a reduced temperature of an electron gas (in Tc units), is determined by the equation 8'
-
t5 =B(1
-
82)3(1+
e2),6
is a dimensionless parameter expressed only in terms of bulk specimen characteristics. In figure 1 a solid curve presents the theoretical dependence a(t), points are the ex- perimental values ofa
(arrowed is the helium pointX).
For narrow films (w<2~) the values ofa
in the whole temperature range (t = 0.4-
0 . 9 ) fit well thetheoretical curve, while for wider specimens the agreement is observed only below the X-point and at t a 0 . 9 . A good agreement between the experimen- tal and the theoretical curves indicates that we succeededin obtaining the critical current hystere- sis due to the electron overheat, on narrow films. For wide films above the X-point the electron over- heat seems to be masked by heat effects. This is supported by a jump in the curve IC2(t) observed on wide films under transition through the X-point. There is no jump on films of 2 U width. It should be mentioned that the relation between the hysteresis value and the electron mean free path ( k decreases
as the parameter
6
reduces) was observed; it can be due to the fact that additional mechanisms of elec- tron scattering reducing their temperature are in-Fig. 1 : H y s t e r e s i s a s a f u n c t i o n of temperature. S o l i d curves p r e s e n t t h e o r e t i c a l d a t a / 2 / f o r
B
=6 and B = 35 and p o i n t s a r e experi-mental r e s u l t s : 0
-
W = 3 1.1-,
.
-
W'
0.5 p.Besides, t h e c r i t i c a l h e a t flow i n He-I1 on narrow f i l m s h a s been found, and t h e dependence of f i l m temperature T on power q, r e l e a s e d i n a c u r r e n t - loaded f i l m , has been measured a t a c o n s t a n t b a t h temperature To. The dependence q(T) were o b t a i n e d by t h e temperature curve of r e s i s t a n c e i n t h e 4.2
-
30 K range and t h e dependence R(q) a t To = 2 K and To = 2 . 2 K. The d a t a p e r m i t t e d t h e thermal re- s i s t i v i t y of a m e t a l - i n s u l a t l ~ r i n t e r f a c e t o be determined f o r our f i l m s and t h e r o l e of e l e c t r o n s i n t h e f o r m a t i o n of thermal r e s i s t i v i t y a t t h e in- t e r f a c e t o be found. The a c o u s t i c mismatch t h e o r y i n v o l v i n g only t h e a c o u s t i c c h a r a c t e r i s t i c s of a metal g i v e s t h e r e l a t i o n q = A (T'-
T ~ ) g i v i n g a s a t i s f a c t o r y t r e a t m e n t of our d a t a f o r q > 50 w/cm2 A t lower powers q T~ i s a p p r o p r i a t e t o t h e c a s e of o n l y e l e c t r o n o v e r h e a t / 3 / and c o n s i s t e n t w i t h t h e e l e c t r o n o v e r h e a t observed i n t h e v a l u e and i n t h e temperature dependence of c r i t i c a l c u r r e n t hys- t e r e s i s .References
/
1 / Skocpol, W. J . , Beasley, M.R. and Tinkham, M.,
J. Appl. Phys.65
(1974) 4054./ 2 / Shklovsky, V.A., F i z . Tverd, T e l a
