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PHYSICAL REVIEW B
VOLUME 46, NUMBER 21
Superconductivity
at 86 K in LuBa2Cu3Q7
R. Pinto, S. P. Pai, A. S. Tamhane, P. R. Apte, L. C. Gupta,
Tata Institute of Fundamental
Department
1
DECEMBER 1992-I
s thin films
and
R. Vijayaraghavan
Research, Homi Bhabha Road, Bombay 400005, India
K. I. Gnanasekar and H. V. Keer
of Chemistry, Indian Institute of Technology, Bombay 400076, India
(Received 20 May 1992)
Superconductivity has been observed in thin films of LuBa2Cu307 z, which otherwise does not form
the 1:2:3 superconducting phase in bulk, presumably due to the small ionic radius of Lu. From this
work, it is clear that the substrate inAuences the nucleation of the 1:2:3 superconducting phase in thin
films. Results indicate that LuBa2Cu307 z thin films with as good a quality as that achievable with
YBa2Cu307 q thin films can be realized; zero resistance transition temperature ( T, ) of 86 K and
critical-current density ( J, ) of 5 X 10' A cm at 77 K have been obtained.
Early in the history of high-temperature superconductors, it was found that the superconducting properties
of orthorhombically
distorted
and oxygen-deficient
perovskites 8 BazCu307 & were nearly independent of the
rare-earth element R.
Exceptions to this behavior are
Ce, Tb, Pr, and possibly Lu. R =Ce and Tb do not form
the superconducting
1:2:3 phase. In the case of R =Pr,
the phase does form but is semiconducting and orders
magnetically. ' Experimental evidence of superconductivity in LuBa2Cu307 & is rather controversial. Early reports have indicated that LuBaCuO forms the superconducting 1:2:3 phase with transition temperature T, above
Later reports, however, have shown absence of
77 K.
in this system down to very low temsuperconductivity
peratures. ' It has also been shown by Somasundaram
et al. that due to the small ionic radius of Lu (0.985 A),
LuBaCuO does not form a pure 1:2:3 phase and is multiphasic.
It is in this context that we undertook the growth and
study of the LuBaCuO system in the thin-film form,
which, to the best of our knowledge, has not been reported to date. Thin films, because of their proximity to the
substrate, are expected to behave considerably different
when compared to the bulk materials. In the case of LuBaCuO, which cannot be formed in the pure 1:2:3 phase
because of the small ionic radius of Lu, the substrate may
be expected to significantly stabilize the LuBaCuO 1:2:3
phase in the thin-film form. Our results have shown that
the pure LuBaCuO 1:2:3 phase can indeed be formed in
situ by pulsed-laser ablation on many substrates commonly used to grow YBa2Cu307 & films. In this paper, we
and study of superconducting
discuss the growth
LuBa2Cu307 & thin films.
Bulk LuBaCuO pellets and targets for pulsed-laser ablation were prepared by mixing stoichiometric amounts
of 99.99% purity Luz03, Ba2CO3, and CuO and by using
solid-state reaction similar to that
a high-temperature
used to prepare R 1:2:3 pellets. Since partial melting
occurs for Lu 1:2:3 if the temperature exceeds 910 C, all
the reactions were carried out at 910'C followed by annealing stages. Thin films of Lu 1:2:3 material were
'
'
46
prepared by pulsed-laser ablation as has been described
elsewhere.
Polished MgO, SrTi03, and LaA103 substrates, all with (100) orientation, were used for the
growth of the films. The KrF excimer laser deposition
parameters were optimized to realize in situ grown
Lu 1:2:3 thin films of the best possible quality. The optimized conditions are: 3 mm X1 mm laser spot, 3 J cm
Auence, 4. 5 cm target-substrate distance, and 200 mTorr
oxygen pressure. The substrate temperature used varied
from 680 to 720'C. The growth of Lu 1:2:3 films under
optimized conditions has been found to be 50 —70 A/min
at the beginning of the growth of the films. This is considerably slower than the deposition rate with the Y 1:2:3
target which gives a film growth rate of —100 A/min under similar growth conditions.
The thickness of Lu 1:2:3
films studied was in the range of 300 —2000 A. The composition of the films analyzed using energy-dispersive xray (EDX) analysis in the thin-film mode indicated 1:2:3
stoichiometry in the films similar to the bulk targets.
The films were initially characterized using x-ray
diffraction (XRD) to determine the crystallinity
and
phase purity. Shown in Fig. 1(b) is the XRD spectrum of
a 700 A-thick Lu 1:2:3 film grown on (100) MgO subc-axis-oriented orthorhombic
strate. A predominantly
phase is clearly seen with the presence of (001 ) lines.
This is compared to the XRD spectrum of the bulk
Lu 1:2:3 target in Fig. 1(a) which shows the absence of
the Lu 1:2:3 orthorhombic phase. Figure 1(c) shows the
XRD spectrum of a thicker (1500 A) Lu 1:2:3 film, indicating once again a predominantly c-axis orientation.
In realizing films of LuBa2Cu307 & crystallizing in the
1:2:3 phase, the substrate seems to be playing a crucial
role. It acts like a template and thus aids the formation
of the 1:2:3 phase. The quality of the films improves as
their thickness increases (see following). This shows that
a few initial layers of the material must be under a strain
which gets progressively relaxed as the film thickness in-
creases.
The resistivity of films was measured using the standard four-probe technique and evaporated silver film contacts. A closed cycle He cryocooler with a temperature
14 242
1992
The American Physical Society
BRIEF REPORTS
46
JJ„J1,. ~&i.,
(b)
CA
O
CD
O
O
cO
O
O
O
O
CV
O
t
,
i
2
h
53
57
21
8 (deg)
FIG. 1. X-ray diffraction spectra of LuBaCuO system: (a)
bulk pellet; (b) 700 A-thick film on (100) MgO; and (c) 1500A-thick film on (100) MgO. Films show a predominantly caxis orientation with (001 ) lines.
14 243
crease. The highest value of T, of 86 K has been found
for films with thicknesses in the range 800 —2000 A.
These films also show a narrow transition width hT of
about 1.5 K indicating the high quality of the films. As
the thickness increases further, critical current density J,
has been found to drop considerably. This is true in general with Y 1:2:3 films as well, since it is known that as
the film thickness increases above 2000 A, the a-axis fraction in the film also increases. This reduces J, .
The results obtained on the structural and superconducting transition of the films grown on (100) SrTi03
and (100) LaA103 substrates have been found to be
comparable to those obtained with the films grown on
(100) MgO substrates. The films were highly oriented
in both cases with T, -86 K and AT-1. 5 K with optimized conditions.
Shown in Fig. 3 is the variation of J, with temperature
obtained with 1000-A-thick films grown on (100) MgO.
J, was measured using patterned 100-pm-wide microbridges and by using the 1 pV/mm voltage criterion. The
best value Of J, obtained with Lu1:2:3 films is 5X10
Acm
at 77 K. Both LsA103 and SrTi03 substrates
have shown similar results. It can be seen in Fig. 3 that
the J, vs T plot of Lu 1:2:3films is linear and is similar to
that of Y 1:2:3 films. ' This indicates that the thermally
activated self-field-induced flux-creep mechanism operating in Lu 1:2:3 thin films is similar to that existing in the
Y1 2:3 films. '
The fact that we can prepare superconducting
thin
of LuBazCu307 & implies an important relationship
between superconductivity
and structure:
among the
compounds RBa2Cu307 & in which the R ion is in the
trivalent state, LuBa2Cu307 & is the only one which in
bulk does not crystallize in the 1:2:3 phase and does not
superconduct.
The present work shows that once the
structure is restored, superconductivity
is also restored.
This should
have an important
on the
bearing
phenomenon of superconductivity in the 1:2:3materials.
To summarize, we have reported the successful growth
of thin films of an LuBaCuO system which normally does
not form the superconducting 1:2:3 phase in bulk due to
films
controller
was used to electrically characterize
the
in the temperature range 10—300 K. All
the Lu 1:2:3 films showed metallicity with a resistivity
-3X10 cm at 300 K. Shown in Fig. 2 are resistancetemperature
plots of Lu 1:2:3 films with various
thicknesses in situ grown on ( 100) MgO substrates. The
transition temperature for zero-resistance T, is about 72
K for the thinnest films (-300 A) studied. As the film
thickness increases, the value of T, has been found to in-
Lu1:2:3 films
100
100 X10
80—
LuBa&Cu307 gThin Film
80o 60-
60-
CD
E
CD
4Q
20—
20-
Q
0
0
20
I
)00
200
t
I
30
t
I
40
l
I
50
300
Temperature
Temperatue
(K )
FIG. 2. Resistance-temperature plots of LuBa2Cu307
in situ grown on ( 100) MgO with various thicknesses.
z
films
FIG.
3. Variation of J,
0
I
1
1
60
I
70
80
90
(K)
with temperature
s film grown on
1000-A-thick LuBa2Cu307
patterned into 100-pm-wide microbridge.
obtained with a
MgO and
(100)
14 244
BRIEF REPORTS
the small ionic radius of Lu. The in situ growth of highquality LuBa2Cu30~ & thin films by pulsed laser deposition on MgO, SrTi03, and LaA103 substrates indicates
that the substrate strongly influences the nucleation and
stabilization of the superconducting Lu 1:2:3phase.
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46
The authors would like to thank Dhananjay Kumar, C.
P. D'Souza, B. S. Amin, and O. B. Fernandes for experimental assistance. One of the authors would like to
thank the Council of Scientific and Industrial Research
for financial support.
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Tarascon, W. R. McKinnon, L. H. Greene, G. W. Hull,
and E. M. Vogel, Phys. Rev. B 36, 226 (1987).
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