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Submitted on 1 Jan 1978
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DETERMINATION OF THE ENERGY GAP IN
Nb0.75Zr0.25 USING A PROXIMITY TUNNELING
METHOD
E. Wolf, R. Noer
To cite this version:
JOURNAL DE PHYSIQUE Colloque C6, supplément au n" 8, Tome 39, août 1978, page C6-454
DETERMINATION OF THE ENERGY GAP I N
N b0
.
7 5Z r0 - 2
5
USING A PROXIMITYTUNNELING METHOD
E.L. Wolf and R.J. Noer
Ames Laboratory-USWE, Iowa State University, Ames, Iowa 5001J, U.S.A.
Cavleton College, Northfield, Minnesota 55057, U.S.A.
Résumé.- Nous avons observé une structure bien définie dans la densité tunnel d'états N T ( E ) des jonc-tions en forme de In/AljOs/AlNbo 7sZro 25- Nos observajonc-tions nous ont permis de déterminer 2A/kTc=4,l à Tç=10,7 K dans Nbo.7sZro 25. Cette méthode peut s'appliquer à d'autres supraconducteurs qui ne for-ment pas de barrière bunnel par oxydation.
Abstract.- We have observed a well defined structure in the tunneling density of states Nx(E) for junctions of the form In/Al203/AlNbo.7sZro#25. Our observation allows us to determine 2A/kTc=4.! at Tc=t0.7 K. in Nbo#75Zro 25- The method may be applicable to other superconductors which do not form a suitable thermal oxide tunnel barrier.
The excitation spectrum of the clean normal metal-superconductor (NS) intimate contact, of nor-mal metal thickness d , was first discussed by de Gennes ans Saint-James/l/, who showed the existen-ce of one or more quasiparticle bound states in the N-metal at energies E less than the superconductor /pair potential A . In the limit of thin normal
la-yers the highest bound state energy E approaches the value A as an upper bound/1/.
The detailed structure of Nm(E) near A has
T s been quantitatively predicted by a recent exact Green's function calculation/2/ for the NS bilayer in the clean, thin-N limit. This structure may be described as a large bound state peak at E abrup-tly terminated by a narrow gap of width 2e = A -E , followed by a lower peak rising beyond E = A . The width 2e is governed by. d^ and can readily be esti-mated/2/ to be £10 viV for Al layers of d =200 A and A = 1 . 9 meV. A more observable feature of the
s
structure is the negative discontinuity in N (E) at E„, owing to the fact that the bound state peak at E is higher than the N (E) structure rising beyond A . Numerical stimulations of normal
metal-insula-s
tor-superconductor experiments (of kT resolution) o using Arnold's N (E)/3/ show that for d = 200 A the discontinuity but not the gap would be resolva-ble at 0.1 K, while neither would be seen at 1 K.
The tunnel junctions were prepared by deposi-ting 75 to 300 A of Al in ultra-high vacuum on foils of Nb0#9Zro#i which were cleaned by
electro-polishing and outgassing to about 2100 K at 2x10 9
torr. The foils were masked, oxidized, and crossed
t . .
Ames Laboratory Summer Visitor 1977
with In counterelectrode strips/4/. A magnetic field parallel to the foil was available to drive the In into the normal state, in conventional measurements of dV/dl in an immersion cryostat.
An Auger profiling analysis was carried out after the vacuum annealing on each foil sample. The quoted concentration x = Zr/Zr+Nb =0.25 is deter-mined as its average over the first coherence length into the alloy form the Al/NbZr interface, based upon the Auger profile and taking £ = 50 A. The T of foil sample NbZr-5 was measured subsequent to the tunneling measurements as 10.67 K. This value is consistent with the data of Hulm and Blaugher/5/ who find a broad maximum at about 11 K in I for
c Nb Zr in the range O.Kx<0.3.
In dV/dl measurements at 1.3 K, H = 0 on sets of such junctions fabricated on four Nbo 9 Zro 1 foils outgassed under slightly differing con-ditions we always observe a sharp dip in -dV/dl at 2.44 ± 0.04 mV. As shown in figure 1, the dip wea-kens and disappears as the In is driven normal by the parallel magnetic field. Evidently the N (E) structure giving rise to the dip is so narrow as to be observed only when probed by the BCS singulari-ty of the superconducting In : the resolution of kT - 100 )lV when the In is normal is insufficient. The data indicate a sharp drop in N (E) for the Ns bilayers at an energy E' = 2.44 ± 0.04 meV-A-. As
shown in figure 2 and in other data, the position of this feature is quite independent of the thick-ness d in the range studied. We identify this fea-ture with the negative discontinuity in N (E) at E for the NS bilayers at an energy E'=l.91±0.04meV
of
if some reflection of quasiparticles occurs
at the NS interface
;this will increase
A
-E and
s N
will make the discontinuity observable at smaller
values of dN, as shown by our numerical work. The
method here demonstrated
maybe applicable to other
materials which do not oxidize properly to form con-
ventional tunnel junctions/4/.
ACKNOWLEDGEMENT.- This work was supported by the
U.S
Department of Energy, Division of Basic Energy
Sciences. We thank F.A. Schmidt for assistance in
preparing the alloy foils and Dr. A. Bevolo for the
Auger measurements.
Fig.
1 :The diffegential resistance dV/dI of a
junction of N=135 A showing a peak too narrow to be
observed in the normal
metal-insulation-superconduc-tor configuration at 1.26 K. For H
5
300 Oe the In
is also superconducting.
References
Fig. 2
:The dV/dI peak shown for three junction of
varying d~ on foil NbZrAl-5. The zeros dV/dI have
been arbitrari.1~
off-set
.
taking
A
=0.53
20.02 meV/6/. Evidently the
gap
In
A -E is too narrow to detect in these samples
:s N
thus we conclude E'
=A to experimental accuracy.
The same conclusion may be drawn from the observed
independence of
E'on dN (figure 2), implying that
all samples are in the small dN limit where EN=
As
/1,2/. We thus take E'
-
ANbZr
=1.91
f0.04 mV,
which corresponds to 2A/kTC
=4.1 for Nb0,75Zr0,25.
This value agrees with the value 2A/kTC
=4.05 ob-
tained from infrared measurements on Nbo,
7s7Zro,253
171.
The'
observability of the feature may be ex-
pected to depend upon the cleanliness of the NS
/ I /
De Gennes,P.G. and Saint-James,D., Phys. Lett.
4 (1963) 151
-
/2/ Arnold,G.B., Phys. Rev. B (to be published)
/3/ We thank G.B. Arnold for providing a program
for the calculation of NT(E)
/4/ Wolf,E.L. and Zasadzinski,J., Phys. Lett.
9
(1977) 165
;Proc. Conf. Physics of Transition
Metals, Toronto1977 (The Institute of Physics
Conf. Series No.
29)/5/ Hulm,J.K. and Blaugher,R.D., Phys. Rev.
123
(1961) 1569
/6/ Bostock,J., Agyeman,K. Frommer,M.H. and Mac
Vicar,M.L.A., J. Appl. Phys.
46
(1973) 5567
/7/ Cappelletti,R.L., Ginsberg,D.M. and Hulm,J.K.,
Phys. Rev. =(I
967) 340
/8/
