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Transport measurements on granular YBaCuO films
PhysieaC 162-164 (1989) 365-366 No~h-Holland
TRANSPORT MEASUREMENTS ON GRANULAR Y - B A - C U - O FILMS M. A. Stan*, S. A. Alterovitz, D. Ignjatovic, and K. B. Bhasln NASA-Lewls Research Center, 21000 Brookpark Road Cleveland, OH 44135 USA G. C. Valco and N. J. Rohrer, Department of Electrical Engineering, The Ohio State University Columbus, OH 43210 USA The c r i t i c a l current in granular YBaCuOfilms has been measured at temperatures very near the c r i t i c a l temperature, Tc. The c r i t i c a l current is proportional to (I-T/Tc)~ for T<.9 Tc. The current-voltage (I-V) characteristics of the films have also been measured as a function of temperature and indicate that the observed deviation from the (I-T/Tc)2 dependencefor T > .9Tc is a natural consequenceof the constant f i e l d criterion ( e . g . . 2 5 ~V/mm) used to define the c r i t i c a l current. I.
Introduction
been published elsewhere.3
Reports 1,2 have shown that the measured
Scanning-electron-micrographs of the f i l m
c r i t i c a l current, I c, near Tc is proportional
surface showed irregularly shaped grains .5 pm
to (I-T/Tc)2 for T < .9Tc and exhibits a
in size having no epitaxy with the substrate.
weaker temperature dependencefor temperatures nearer to Tc.
Our I c data exhibits
A d.c. four probe method was used for a l l transport measurements. Electrlcal contact to
q u a l i t a t i v e l y the same behavior and we w i l l show, with the aid of the (I-V)
the films was made by In-soldering to previously deposlted IMm Ag electrodes
characteristics, that the variation in
the
extending across the short dimenslon of the
power law near TC is a result of the
f11ms. The c r i t i c a l temperature for the film
measurement criterion.
described below was 62.3K, determined wlth a
Next we discuss
several different c r i t e r i a for defining the
measuring current density of .2A/cm2.
onset of dissipation as i t applies to I c measurements. Finally we show that i) the
3. R e s u l t s
temperature dependence of the I c can be
In Flg.
and D i s c u s s i o n 1 the upper f o u r sets o f I c data
obtained by f i t t i n g the I-V data taken at
were o b t a i n e d using d i f f e r e n t
different temperatures to the simple
criteria.
expression V=A(I-Ic )m, i i ) the temperature
eye.
voltage
The s o l i d l i n e s are guides t o the
The lowest l i n e in Fig. 1 was obtained
dependence of I c obtained using this procedure
from a least-squares f i t of the I-V data and
is in good agreement with that of the measured
w i l l be discussed later in thls section.
data.
Notice that the upper four curves have a slope of 2 at low temperatures, and show a departure
2.
Experimental
from the straight line behavior at different
The superconducting films were prepared by
temperatures depending upon the voltage
sequential evaporation of Cu,Y, and BaF2 onto
criterion.
(lO0) SrTiO3.
÷ Tc can be understood with the aid of the I-V
The films were deposited onto
The departure from (I-T/Tc)2 as T
.5 cmx].O cm substrates and had a nominal
data in Fig. 2.
thickness of I ~m prior to annealing.
I-V data obtained at six different
Details
Here we show a sequence of
of the f i l m preparation and annealing have *Also Department of Physics, Kent State University, Kent, Ohio, 44242 USA
0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
366
M.A. Stan et al. / Granular Y - B a - C u - O films
101
...........................
,
10:
A
.< v
Et
10,.
o
io. /
v
o.O f/
El
,,,,,,,i
,
,,
F
10 =
10= 10-=
.wa
10-= _. 10-s
I0 -~
10-'
10-t 1 -
Fig.
1
10-=
T/Te
10 o
101
10 =
Current (mA)
I C against (1-TIT c) obtained using different criteria. From top to bottom 2.5 pV/mm, ]pV/mm, .5pV/mm, .25pV/mm, and data obtained from a least-squares f i t of the I-V data to V=A(I-Ic )m.
temperatures.
10 -1
The upper and lower dashed
lines represent the 2.5 pV/mm and .25pV/mm
Fig.
2
Voltage against current obtained at a sequence of temperatures. Fromright to l e f t : 20, 40, 51, 58, 62, and 64K.
represent the lowest quotable values of I c for the 2.5 pV/mm and .25pV/mm c r i t e r i a respectively. We believe there is yet another way to
c r i t e r i a respectively, and a vertical llne drawn from the intersection of these lines
obtain the temperature dependenceof I c.
with the data yields the I c.
has previously been shown that the I-V data
At 20K the two
It
c r i t e r i a result in the same I c while at 58K
can be described by V=A(I-Ic)m where I c and m
they yield numbers differing by a factor of
are temperature dependent4.
two. The important parameter for evaluatlng
least-squares f i t of our data to this relation
the accuracy of a constant voltage criterion
we obtain I c and m as a function of
By performing a
is apparently the 1ogarithmlc derivative
temperature.
dlog Vl dLog I.
are the points on the lowest line in Fig. I.
The smaller the 1ogarlthmlc
The I c data obtained in this way
derivative at the chosen voltage criterion the
The exponent is 2.2, in good agreement with
more one overestimates I c.
the measured data satifying the resistive
Since dLogVldLogI
is rapidly diminishing as T~Tc
i t is
criterion, and I c vanishes at 63.8K.
In
]nevltable that one overestimates I c which
conclusion, we found that I c is proportional
results in the apparent weakeningof the
to (I-T/Tc)2 for T~Tc.
temperature dependence near Tc.
In principle
one would have to define I c at the same value
REFERENCES
of the logarithmic derivative
l)
S. B. Ogale, D. Dijkkamp, and T. Venkatesan, Phys. Rev. B36, (1987) 7210
2)
3. N. C. de Vrles, M. A. M. Gijs, G. M. Stollman, T. S. Baller, and G. N. A. Vern, O. Appl. Phys. 64 (1988) 426.
3)
G. J. Valco, N. 3. Rohrer, J. D. Warner, and K. B. Bhasin, A.I.P. Proceedings No. 182 (]989) 147.
4)
P. England, T. Venkatesan, X. D. Hu, and A. Inam, Phys. Rev. B38 (1988) 7125.
( i . e . lower voltages) in order to maintain the same level of accuracy. Since this method is not practical one could use the less stringent resistive criterion whereby a supercurrent is present i f Vc/Ic
TRANSPORT MEASUREMENTS ON GRANULAR Y - B A - C U - O FILMS M. A. Stan*, S. A. Alterovitz, D. Ignjatovic, and K. B. Bhasln NASA-Lewls Research Center, 21000 Brookpark Road Cleveland, OH 44135 USA G. C. Valco and N. J. Rohrer, Department of Electrical Engineering, The Ohio State University Columbus, OH 43210 USA The c r i t i c a l current in granular YBaCuOfilms has been measured at temperatures very near the c r i t i c a l temperature, Tc. The c r i t i c a l current is proportional to (I-T/Tc)~ for T<.9 Tc. The current-voltage (I-V) characteristics of the films have also been measured as a function of temperature and indicate that the observed deviation from the (I-T/Tc)2 dependencefor T > .9Tc is a natural consequenceof the constant f i e l d criterion ( e . g . . 2 5 ~V/mm) used to define the c r i t i c a l current. I.
Introduction
been published elsewhere.3
Reports 1,2 have shown that the measured
Scanning-electron-micrographs of the f i l m
c r i t i c a l current, I c, near Tc is proportional
surface showed irregularly shaped grains .5 pm
to (I-T/Tc)2 for T < .9Tc and exhibits a
in size having no epitaxy with the substrate.
weaker temperature dependencefor temperatures nearer to Tc.
Our I c data exhibits
A d.c. four probe method was used for a l l transport measurements. Electrlcal contact to
q u a l i t a t i v e l y the same behavior and we w i l l show, with the aid of the (I-V)
the films was made by In-soldering to previously deposlted IMm Ag electrodes
characteristics, that the variation in
the
extending across the short dimenslon of the
power law near TC is a result of the
f11ms. The c r i t i c a l temperature for the film
measurement criterion.
described below was 62.3K, determined wlth a
Next we discuss
several different c r i t e r i a for defining the
measuring current density of .2A/cm2.
onset of dissipation as i t applies to I c measurements. Finally we show that i) the
3. R e s u l t s
temperature dependence of the I c can be
In Flg.
and D i s c u s s i o n 1 the upper f o u r sets o f I c data
obtained by f i t t i n g the I-V data taken at
were o b t a i n e d using d i f f e r e n t
different temperatures to the simple
criteria.
expression V=A(I-Ic )m, i i ) the temperature
eye.
voltage
The s o l i d l i n e s are guides t o the
The lowest l i n e in Fig. 1 was obtained
dependence of I c obtained using this procedure
from a least-squares f i t of the I-V data and
is in good agreement with that of the measured
w i l l be discussed later in thls section.
data.
Notice that the upper four curves have a slope of 2 at low temperatures, and show a departure
2.
Experimental
from the straight line behavior at different
The superconducting films were prepared by
temperatures depending upon the voltage
sequential evaporation of Cu,Y, and BaF2 onto
criterion.
(lO0) SrTiO3.
÷ Tc can be understood with the aid of the I-V
The films were deposited onto
The departure from (I-T/Tc)2 as T
.5 cmx].O cm substrates and had a nominal
data in Fig. 2.
thickness of I ~m prior to annealing.
I-V data obtained at six different
Details
Here we show a sequence of
of the f i l m preparation and annealing have *Also Department of Physics, Kent State University, Kent, Ohio, 44242 USA
0921-4534/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
366
M.A. Stan et al. / Granular Y - B a - C u - O films
101
...........................
,
10:
A
.< v
Et
10,.
o
io. /
v
o.O f/
El
,,,,,,,i
,
,,
F
10 =
10= 10-=
.wa
10-= _. 10-s
I0 -~
10-'
10-t 1 -
Fig.
1
10-=
T/Te
10 o
101
10 =
Current (mA)
I C against (1-TIT c) obtained using different criteria. From top to bottom 2.5 pV/mm, ]pV/mm, .5pV/mm, .25pV/mm, and data obtained from a least-squares f i t of the I-V data to V=A(I-Ic )m.
temperatures.
10 -1
The upper and lower dashed
lines represent the 2.5 pV/mm and .25pV/mm
Fig.
2
Voltage against current obtained at a sequence of temperatures. Fromright to l e f t : 20, 40, 51, 58, 62, and 64K.
represent the lowest quotable values of I c for the 2.5 pV/mm and .25pV/mm c r i t e r i a respectively. We believe there is yet another way to
c r i t e r i a respectively, and a vertical llne drawn from the intersection of these lines
obtain the temperature dependenceof I c.
with the data yields the I c.
has previously been shown that the I-V data
At 20K the two
It
c r i t e r i a result in the same I c while at 58K
can be described by V=A(I-Ic)m where I c and m
they yield numbers differing by a factor of
are temperature dependent4.
two. The important parameter for evaluatlng
least-squares f i t of our data to this relation
the accuracy of a constant voltage criterion
we obtain I c and m as a function of
By performing a
is apparently the 1ogarithmlc derivative
temperature.
dlog Vl dLog I.
are the points on the lowest line in Fig. I.
The smaller the 1ogarlthmlc
The I c data obtained in this way
derivative at the chosen voltage criterion the
The exponent is 2.2, in good agreement with
more one overestimates I c.
the measured data satifying the resistive
Since dLogVldLogI
is rapidly diminishing as T~Tc
i t is
criterion, and I c vanishes at 63.8K.
In
]nevltable that one overestimates I c which
conclusion, we found that I c is proportional
results in the apparent weakeningof the
to (I-T/Tc)2 for T~Tc.
temperature dependence near Tc.
In principle
one would have to define I c at the same value
REFERENCES
of the logarithmic derivative
l)
S. B. Ogale, D. Dijkkamp, and T. Venkatesan, Phys. Rev. B36, (1987) 7210
2)
3. N. C. de Vrles, M. A. M. Gijs, G. M. Stollman, T. S. Baller, and G. N. A. Vern, O. Appl. Phys. 64 (1988) 426.
3)
G. J. Valco, N. 3. Rohrer, J. D. Warner, and K. B. Bhasin, A.I.P. Proceedings No. 182 (]989) 147.
4)
P. England, T. Venkatesan, X. D. Hu, and A. Inam, Phys. Rev. B38 (1988) 7125.
( i . e . lower voltages) in order to maintain the same level of accuracy. Since this method is not practical one could use the less stringent resistive criterion whereby a supercurrent is present i f Vc/Ic