Proximity effects in CeCu6/Nb bilayers
Otop, A.; Süllow, S.; Hesselberth, M.B.S.; Aarts, J.
Citation
Otop, A., Süllow, S., Hesselberth, M. B. S., & Aarts, J. (2003). Proximity
effects in CeCu6/Nb bilayers. Acta Physica Polonica B, 34(2),
1169-1172. Retrieved from https://hdl.handle.net/1887/45422
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PROXIMITY EFFECTS IN CeCu 6 /Nb BILAYERS A. Otop a;b , S. Süllow a , M.B.S. Hesselberth b and J. Aarts b a
InstitutfürMetallphysikundNukleareFestkörperphysik Te hnis heUniversitätBrauns hweig
Mendelssohnstr. 3,38106Brauns hweig,Germany b
KamerlinghOnnesLaboratory,Leiden University Niels Bohrweg2,2300RALeiden,TheNetherlands
(Re eivedJuly10,2002)
We studied theproximityee t in order to probethe response of the heavy-fermion metal CeCu
6
to indu ed super ondu tivity. High-quality CeCu
6
lmswere grownbysputter deposition. Resistivity measurements showthat the orrelatedele tronstateis formed. Bilayerswereprepared ofCeCu
6
(75nm)/Nb(d Nb
),withtheNbthi knessd Nb
between10nmand 50nm. Measuredwerethesuper ondu tingtransitiontemperatureT
and theparallel riti al elds H
jj 2
. We nd that theinterfa etransparen y is high, indi ating that Fermi-velo ity mismat h ee ts are not signi ant, butthe oheren e lengthoftheindu edsuper ondu tivityissmall, whi h isprobablyduetotheverylowFermi-velo ityintheheavyfermionmetal.
PACSnumbers: 71.27.+a,74.50.+r
1. Introdu tion
Insimplenormalmetals su hasAuor Pd, questionsabout their ability to sustainsuper ondu ting orrelations orunbalan edspins areofmu h in-terest. Similar questions an be raised about a metalli ground state with strong intrinsi orrelationssu h asinheavy fermion(HF)systems. Spe if-i ally, ifa metal isin onta t witha super ondu tor, it an be asked what the me hanism is for pair breaking of Cooper pairs diusing out of the su-per ondu tor. For normal metals (N) the me hanism is dephasing due to a nite temperature; for ferromagnets (F) it is the ee t of the ex hange eld; fora HFgroundstatetheanswerisnotknown,and ould involve e.g. spin u tuations. Anintrinsi problemfor su hstudies isthatthematerial has to be available in thin lm form. Herewe present the rst results of a
study of the proximity ee t in asystem onsistingof thin lms of theHF material CeCu
6
ombined with thin lms of a super ondu tor (S), in this ase Nb. Among the dierent HF systems, CeCu
6
is of parti ular interest sin e itdoesnot order magneti allydown to mKtemperatures[1,2℄.
2. Sample preparation
SetsofCeCu 6
lmsandbilayersofCeCu 6
/Nbweremadeby DC-magnet-ron sputtering. We used an ultra high va uum system with a ba kground pressure of the order of 510
10
mbar, and an Ar sputtering pressure of 2:510
3
mbar. Crystalline CeCu 6
was grown as reported before [3℄, at a temperature of 350
Æ
C using Si-substrates with amorphous Si 3
N 4
buer layers to prevent Cudiusion at thosetemperatures.
The Nb was deposited on top of the CeCu 6
after ooling the substrate holder with old nitrogen gas to lose to room temperature. Composi-tion, thi kness and rystallinity of the lms were determined by Ruther-ford ba ks attering (RBS) measurements together with X-ray dira tion measurements at low and highangles. The RBS measurements show good agreement with the expe ted stoi hiometry for CeCu
6
and no diusion is found eitherofCeor Cuinto thesubstrate or ofNb intotheCeCu
6
. Fig.1 showspartoftheRBSspe trumforCeCu
6
(75nm)/Nb(15nm)onaSi/Si 3
N 4 substrate and a t of thedata withouttakinganydiusion into a ount.
340
360
380
400
420
440
460
Channel
0
10
20
30
40
50
60
N
o
rm
al
iz
ed
Y
ie
ld
1.4
1.5
1.6
1.7
1.8
Energy (MeV)
Cu
Nb
Ce
Fig.1. RBSmeasurementusing 4
He-ionsof2MeV onasample Si/Si 3 N 4 /CeCu 6
(75 nm)/Nb(15 nm). The dierent elements are indi ated. The thin smoothlineisa tto themeasured urve.
Fig.2(a)showstheele tri alresistivityasafun tionoftemperatureT for two single lms ofCeCu
6
and inprevious investigations on sputteredCeCu 6
lms [3℄. We de idedto use 75 nm thi k CeCu
6
layer, whi h with T max
= 4 K suggest only little deviation from thebulkproperties.
0
10
20
30
40
50
0
1
2
3
4
5
6
7
8
9
10
0
5
10
15
20
25
30
35
40
76
78
80
82
84
86
88
90
92
T
c
(K
)
d
Nb
(nm)
CeCu
6
/Nb
Nb
(b)
(a)
r
(
m
W
cm
)
T(K)
CeCu
6
d=188 nm
CeCu
6
d=75 nm
Fig.2. (a)Resistivityasafun tion oftemperatureforCeCu 6
lmsof75 nmand 188 nm. (b) Criti al temperature asa fun tion of Nb thi kness d
Nb
for bilayers CeCu
6
/Nb andsingleNb lms. Thesolid linerepresentsthetypi albehaviorfor Nbsingle lmsfromdierentdepositionsystems,trianglesshowvaluesforNb lms made inthesamesystemastheCeCu
6
/Nb bilayers,thedotted lineis aguide to theeye.
3. CeCu 6
/Nb bilayers; results and dis ussion
The super ondu ting transition temperature T
was determined from resistivity measurements on a set of CeCu
6 (75 nm)/Nb(d Nb ) withvariable Nb thi kness d Nb
in the range 10 nm to 50 nm. As seen in Fig. 2(b), T
shows a strong suppression, with a riti al thi kness d r
for onset of super ondu tivityrea hedaround12nm. Thiswouldbeequivalentto24nm inatrilayer on guration,andofasimilarmagnitudeasfoundinFe/Nb/Fe trilayers [5℄. It has to be kept in mind however that single thin Nb lms also show a de rease of T
at small thi kness (see Fig. 2(b)), whi h makes the orre t determination of d
r
more di ult. The result shows, however, that the interfa e does allow parti le ex hange, oppositeto what might be expe tedfromthehugeFermivelo itymismat hinthesystemoforder10
3 .
Also theparallel riti al eldH jj 2
2
3
4
5
6
7
8
9
0
1
2
3
4
2
3
4
5
0
1
2
3
4
(a)
(b)
m
0
H
c
2
(T
)
T (K)
d
Nb
=50 nm
d
Nb
=31 nm
d
Nb
=25 nm
d
Nb
=17 nm
d
Nb
=15 nm
d
Nb
=13.5 nm
T
DCO
m
0
H
c
2
(T
)
T (K)
Cu/Nb(15 nm)
CeCu
6
/Nb(15 nm)
Fig.3. Upperparallel riti al eld as afun tion of temperature for: (a) bilayers CeCu
6
(75 nm)/Nb, d Nb
as indi ated; (b) bilayersof CeCu 6
(75 nm)/Nb(15 nm) andCu(75nm)/Nb(15nm) withT
DCO
indi ated.
Absen eoftheDCOmeansthatnoappre iableamountof super ondu -tivity leaks into the HF metal; together with the nite transparen y this means either appre iable pair breaking by the HF metal, or a very small diusivity due to the low Fermi velo ity v
F
. Whether a small v F
alone is able to explainthe experimentsispresently under investigation.
WorkatLeidenUniversityispartoftheresear hprogramofthe `Sti ht-ing F.O.M.'. We a knowledgete hni al supportbyT.J. Gortenmulder and R.W.A. Hendrikx. A.O. a knowledges support of the European S ien e Foundation(programmeFERLIN)andofF.O.M.forshorttermfellowships.
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