HAL Id: jpa-00217664
https://hal.archives-ouvertes.fr/jpa-00217664
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.
CROSS-LIKE FILM STRUCTURES, MICROWAVE
IRRADIATION STIMULATED
SUPERCONDUCTIVITY AND JOSEPHSON EFFECT
Yu. Bevza, V. Karamushko, I. Dmitrenko
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplhent au no 8, Tome 39, au&t 1978, page C6-537
CROSS-LIKE F I L M STRUCTURES, MICROWAVE I R R A D I A T I O N S T I M U L A T E D SUPERCONDUCTIVITY AND JOSEPHSON EFFECT
Yu.G.Bevza, V.I.Karamushko and 1.M.Dmitrenko
Physico-Technical Institute of Low Temperatures, UkrSSR Academy of Sciences, 47,Lenin Prospect, Kharkov, 310164, USSR.
R6sumd.- On a dtudid les struchures en couches cruciformes de l'ltain. Les observations de la supra- conductivitd stimulde par irradiation d'hyperfrdquence sur les couches homogsnes dans un large inter- valle de libre parcours Q sont en accord quantitatif avec la thdorie dlEliashberg. L'effet relatif augmente quand une zone faible est crdde par le passage du courant transversal i. Aux forts i et au voisinage de Tc, on observe des marches Josephson induites sur les caractdristiques I-V par I'irra- diation.
Abstract.- Cross-like tin film structuresare studied. Superconductivity stimulated by microwave ir- radiation was observed on homogeneous films in a wide range of mean free paths Q and agree quanti-
tatively with Eliashberg's theory. In a weak portion created by passing transverse current i throu- gh it, the relative effect increases. At high i near T irradiation-induced Josephson steps appear in V-I curves.
Fig. 1 a) :
Cross-like thin film structures were studied experimentally (figure la, insert) on tin films
0
800-1000 A thick with mean free path Q = 350-2000 A
The preset sample geometry was obtained photolitho- graphically. Small Q was provided using Ar ion irra- diation. Studies were made on both homogeneous and
riment and Galaiko's theory / l / served as a crite- rion. For most samples the second critical current, step widths corresponding to the appaerance of pha- se slip centre (PSC) and the voltage intervals bet- ween the steps were consistent with the theory to within 5-10%. The agreement between experimental
and theoretical currents corres~onding to the appea- rance of separate PSC was somer~hat worse (+ 20%).
locally weakened films, the latter being obtained
Fig. 1 b) :
by passing the transverse control current i
.
In both .cases the critical current Ic increa-Another evidence for homogeneity was a quantitati- ses due to microwave irradiation (y = 9-23 GHz).
ve agreement of I-V curves for two 30 um films of The film homogeneity was examined by several me-
a single sample at different T with and without thods in addition to the visual one. To provide a
irradiation. Experimental I-V curves agree quali- higher probability of homogeneity over the film
tatively and quantitatively (to within less than length, a separate examination of 30 um arms of the
10%). The Tc rise under irradiation was observed cross was made. Quantitative agreement between expe-
for no tin samples. Qualitatively, the stimulation
effect to a high extent was determined by the mean 0
free path. With R =2000 A an Ic increase under irra- diation was revealed starting with T
.
WithR
=350 0A this increase is observed at T = 3.74 K (Tc= 3.804 K).
We solved numerically Eliashberg's equation for tin samples. The calculated T-dependence (
+i2
at different K is shown in figure 2. The notationY
is similar to that in /2/. The maximum experimental
a % 0.001 ( R=2000 A,Aw = 500 m,RN = l ohm, ~=10-"
henry). The calculation shows the absence of Tcrise and proves that with increasing R the stimulation effect grows too. The experimental maximum I in- crease was achieved at certain Pm, then as P grows, Ic falls, very sharply at the end (almost a jump). In the region of Pm,Ic hysteresis starts to appear even near T i.e. much earlier than the heat hys-
C'
teresis occurs in the I-V curves without irradia- tion. A sharp drop in Ic(P) occurs as the reverse critical current reaches zero. The temperature of the stimulation effect enset T 1 and Ic(P) maximum may be accounted for assuming that the experimental-
ly observed critical current making allowance for stimulation Ice=Ic-I,
,
where I, is the microwave current amplitude, I is found on the bases of Eliashberg's equation solution. We assume that A*islinearly dependent on AT and cc As seen in figure 2, it is quite
a
good approximation. Withdice
= 0dI% 1
: magnitude is found, which corresponds to the effect maximum. T at which the Ic(P) maximum first
1
m
appears, is found from the condition I > 0 with ce
1
: substituted. The agreement of calculated and ex- 0
perimsntal T for a sample with
R
= 350 A is sur- 1prisingly good (to within 0.001 K). For a pure sam- 0
ple (
R
= 2000 A) the calculation gives Tc-T =0.001K (experimentally the effect onset is difficult to find because of fluctuations). The calculated beha- viour of the IT):( dependence agree qualitatively with experiment (the match with the microwave trackis unknown).
If i 0, the relative stimulation effect increases 1 4 1 . It is likely that in addition to the Eliashberg mechanism, some mechanism of "smearing" theorder parameter minimum of type 131 must be in- volved. Weak link in our samples was produced at the expense of additional break of pairs in the region of crossing /4,5/ (as the transverse film transforms into a resistive state). In figure la) the sample I-V curves are shown for different control currents i, i.e. the dynamics of a transition from a homo-
eneous film to a microbridge. At T/Tc 0.99 I-V curves of such weak links displayed microwave irra- diation-induced steps. As i grows, the coupling energy of the controlled Josephson elements decrea- ses and, as seen in figure Ib), the dependence of the 1st step height on the microwave power P approa- ches the standard Bessel one.
Fig. 2 :
References
/3/ Lindeloff,P.E.,Solid.State Cormnun.= (1976) 253 /4/ Bevza,Yu.G., Karamushko,V.I.,Tsakh,G.G.
Dmitrenko,I.M., Pisma Zh Techn.Fiz.2 (1976); Fiz.Nizk.Temp.(to be published)
