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Superconducting properties as a function of thickness in NbN films
SUPERCONDUCTING PROPERTIES THICKNESS IN NbN J. R. GAVALER, Westinghouse
M. A. JANOCKO,
Research
Laboratories,
AS A FUNCTION FILMSt,S
J. K. HULM Pittsburgh,
OF
and C. K. JONES
Pennsylvania
15235, USA
Synopsis Niobium nitride, NbN, films with thicknesses ranging from 5900 A down to approximately 10 .& have been prepared by a high purity reactive sputtering prdcess and thdir superconducting properties measured. A depression in transition temperature, T,, with decreasing thickness has been observed, starting from a maximum T, of = 15.2 K in the thicker films down to ~6.5 K in a 2.5 A film. The critical current vs. applied magnetic field curves were found to be independent of thickness or even to increase slightly down to a thickness of 100 A. Below this point the critical current at high fields,decreased sharply, the films apparently becoming discontinuous. The average resistivity of the NbN films was = 1000 p ohm cm.
1. Introduction. In the study of superconductivity, the effect on superconducting properties produced by changes in specimen dimensions or geometry, has long been a subject of considerable interest. Recently, for example, a significant part of the effort to produce higher transition temperature materials has been centered on the study of superconductors prepared in thin film formlW4). In general, the work in this area has been concentrated on the study of superconducting elements - all of which have transition temperatures in bulk form of 9 K or lower. In a few cases, substantial increases in transition temperature have been reported. However, all of these enhancements were observed with elements having low transition temperatures, and in no case have materials been prepared with transition temperatures in excess of 10 K. Very limited data is presently available on thin films of the high T, compounds such as occur in the rocksalt or P-tungsten structures, even though enhancement of any of the superconducting properties of these materials would be of immediate practical importance. This lack of data is undoubtedly associated with the considerable difficulties involved in the preparation of these compounds in thin film form. The effect on T, of preparing very thin films of some of these compounds has been considered by a number of theorists. The conclusions which have been drawn, however, have been dissimilar, or even ,contradictory5,6). t This work supported in part by the National Aeronautics and Space Administration. $ Part of this work was performed at the Francis Bitter National Magnet Laboratory which is supported at M.I.T. by the Air Force Office of Scientific Research. 585
586
_I. R. GAVALER,
M. A. JANOCKO,
J. K. HULM
and
C. K. JONES
Recently, we have successfully prepared films of the rocksalt compound NbN, with Tc’s of approximately 15 K and with enhanced high field properties relative to those of the bulk material’). The thicknesses of these films were approximately 2000 A. Films of NbN have now been prepared with thicknesses ranging from 5000 A down to about 10 A. In this paper, we report on thesuperconducting properties of these films. 2. Experimental procedure. The NbN films used in this study were deposited by a high-purity reactive sputtering process which has previously been described in detaiP). Briefly, this process involves sputtering niobium in an argon-nitrogen atmosphere with a total background impurity pressure of 5 X lo-lo Torr or less. The NbN which is thus formed is deposited on substrates which are maintained at approximately 500°C. The thicknesses of the films were measured with an interferometer which has an accuracy off 10 A. The thicknesses of the very thin films (- 50 A and less) were estimated from the duration of the growth experiments. Transition temperatures were measured with a silicon resistance thermometer in a system using helium exchange gas to cool the samples. Films with Tc’s above 13.7 K were also measured in pumped liquid hydrogen by observing the vapor pressure of the hydrogen at the transition. The agreement between the two measurements was 0.1 K or better. High field data were obtained in the 220 kG Bitter-solenoid located at the National Magnet Laboratory. In all of these measurements the samples were mounted so that the magnetic field was applied perpendicular to the plane of the film and to current flow. 3. Results and discussion. Figure 1 shows transition temperature data on NbN films deposited on both metallic and insulating substrates. This figure illustrates the depression in T, which occurs with decreasing film thickness. The last point shown on the curve is for a = 25 A thick film on sapphire with a T, of = 6.5 K. NbN films approximately 10 A thick have been prepared; these films, however, remained normal down to a temperature of 1.2 K. Following the usual convention, the value for T, used to plot this data is that temperature at which the specimen has lost one-half of its normal-state resistivity. To illustrate the entire transitions, some representative curves are reproduced in fig. 2. As typified by curves 1, 2 and 3 in this figure, all of the NbN layers down to a thickness of 100 A became resistanceless. Starting at approximately 50 A, however, the films began to retain some of their normalstate resistance. These results indicate that between 50 and 100 A, the deposited films start to become discontinuous and as the thickness is further decreased the discontinuity becomes increasingly greater. An interesting feature of the T, curves in this figure is that with the exception of the 25 A film, the onset of superconductivity in every case is at approximately 15 K. Therefore, if the highest temperature at which superconductivity is observed
EFFECT
OF FILM THICKNESS
16-, , , , , ,
,
,
ON SUPERCONDUCTIVITY ’
I”“”
’
’
I”“’
’
587
’
16 - 15
14 -
- 14
l3-
- 13
=12 5 52 11 _
- 11
- 12
HlO -
e E
- 10
0 5.5. Substrate o Sapphire Substrate
-9
a-
-8
l-
-1
cl
6-
-6
5-
-5
’
4-J”“’ lo4
1111ll1
’
103
I
I
11lllI
Thickness, A
I I
I
lo2
-4 10
Tc as a function of NbN film thickness
Fig. 1. Transition
temperature
as a function of thickness down to 25 A thick.
in NbN films approximately
5000 A
were plotted against film thickness, no depression in T, would occur until some thickness below 50 A. Regardless of what criterion for T, is used, however, our results demonstrate that no enhancement in T, occurs with decreasing thickness in NbN for films on the substrates employed so far. The reason
I
2
,,,I
4
I
6
I
I
a 10 Temperature,
I
I,
1
12
14
I,
16
K
Typical Tc curves for NbN films of various thickness
Fig. 2. Transition temperature curves for NbN films of five different thicknesses.
588
J. R. GAVALER,
M. A. JANOCKO,
J. K. HULM
and
C. K. JONES
for the observed depression in transition temperature is not known with certainty at present. The deleterious effect on T, of residual gas impurities in the growth system has been shown for NbN thin films prepared in the earlier work. Our present growth techniques greatly minimize the level of residual gases; however, these gases have not been completely eliminated. The portion of these remaining impurities, which originate from outgassing in the substrate region could be expected to exert a maximum effect on the initially deposited NbN. Thus the degradation in T, in the thinnest films could be interpreted as resulting from this type of contamination. Alternatively, the changes in T, could be attributable to a greater lattice disorder, which may be present to a greater degree in the thinner films. The character of this disorder would be sensitive to the conditions of nucleation. A mechanism by which lattice disorder influences transition temperature has been proposed by Garland et ~1.~). Since no evidence exists to confirm either of these possibilities, the explanation for the depression in T, is still an open question. We are currently preparing a deposition system in which we hope to lower the background impurity level by at least another two orders of magnitude. The influence of various substrate materials will also be investigated. Figure 3 shows J, VS. If data for three NbN films. Both the 5000 A film on stainless steel and the 100 A film on sapphire have higher current density values than the 5000 A film on sapphire. These results suggest that the current carrying capacities of these samples were limited by heating effects at the current contacts. Using this assumption the higher .I, values for the 100 A
1041 ’ 100 Fig. 3. J,-H
curves
’ ’ 120
of: (1) 100 .& NbN
’
140
’
’
160
’
Field, kOe
film on sapphire,
’
I@
’
’
zm
’
(2) 5000 A NbN
and (3) 5000 A NbN film on sapphire.
’
220
film on stainless
steel
EFFECT
OF FILM
THICKNESS
589
ON SUPERCONDUCTIVITY
T =4.2 K H =15OkOe
IO4 ““I lo4
Fig. 4. Current
density
’ ’
’
11111II ld
as a function
I
I
Thickness, A
of thickness
in seven
llllllI
I
I
13
NbN
10
films deposited
on sapphire
substrates.
sample on sapphire probably result from the lower currents required to drive this film normal, compared to the 5OOOA film. In the case of the NbN film on stainless steel, the contact problem is apparently minimized by the stabilizing effect of the conducting substrate. This ~-would account for the higher values in this sample. From these results, it is probable that our J, values are somewhat conservative, especially in the thicker films measured at lower fields. Our most accurate current densities should be those measured in the thinnest continuous films and at the highest field. Using this criterion, our most reliable value is the = 3 x lo5 A/cm2 observed in the 100 8, film in a 210 kG field. It is worth noting that this is the highest current density, at this field, yet reported for any material. In fig. 4 current density versus thickness data on seven NbN films deposited on sapphire substrates are shown. Films on insulating substrates were used for these measurements since the detection of changes in resistance in very thin NbN films deposited on metallic substrates is greatly complicated by electrical noise problems. The figure shows an upward trend in J, values in films between 5000 A and 100 A thick. This apparent enhancement may be due to the heating effects at the current contacts. At a minimum, however, these results demonstrate that there is no decrease in current densities down to a thickness of about 100 A. This, in turn, suggests that the NbN films down to 100 A are continuous with uniform thickness and composition. At thicknesses below 100 A, there is a sharp drop in the .I,-H curves. In addition, the character of the transition from the superconducting to the normal state is found to
590
J. R. GAVALER,
M. A. JANOCKO,
J. K. HULM
and C. K. JONES
change radically from that observed in the thicker films. This difference is shown in fig. 5.. Curve A typifies the transitions for films 100 A thick and greater, while curve B is a transition for a 50 A film. As previously observed in the T, measurements, only a part of the 50 A thick NbN film enters the superconducting state, suggesting physical discontinuity in this film. It would seem probable that the sharp drop in current density, as well as the broad transition, are related to this apparent discontinuity. An alternative possibility is that at 50 A, the film thickness is less than the optimum size for the pinning centers required to maintain superconductivity at the high fieldsa). The drop in Jc-H properties in the 50 A film could also simply be related to the low ‘T, of this film (= 10 K). However, a drop in T, from 15 K to 12.5 K in the 100 A film did not produce a similar depression in its J,-H properties. I
loo
I
,
I
I
I
I
I
I
I
120
140
160
180
MO
22u
Field, kOe A) Typical transition curve for continuous NbN films f-la, to > Ma) A thick) 5) Typicaltransition curve for discontinuous NbN films MO Athick and less)
Fig. 5. Typical
transitions
with field for: (A) continuous
NbN
films between
100 A and 5000 A
thick and (B) for NbN films = 50 A thick or less.
In addition to the measurement of the superconducting properties of NbN, we have also determined the normal-state resistivity of these films as a function of thickness. These measurements were made at room temperature, 77 K and 20 K. It was found that in every film measured the resistivity either stayed constant or rose slightly with decreasing temperature. The resistivity of these films showed no apparent dependence on film thickness down to and including
EFFECT
OF FILM THICKNESS
ON SUPERCONDUCTIVITY
591
films approximately 20 A thick. The average resistivity, (p,), of all the samples measured was approximately 1000 p ohm cm. The upper critical field, H&O), of these films is estimated to be about 250 kG. This is considerably below the value suggested by the GLAG theory for a material with this very high resistivity. Using the GLAG formulation, H&?(O)= 3.1 x 104p,y T,,
(1)
where y is the electronic specific heat coefficient; a value of 1400 kG is calculated for the upper critical field. (The value for y used in this calculation is 3.2 X lo3 erg/cc K2 the bulk value.) Taking into account paramagnetic effects, according to the equation of MakilO), H,,(O) = 3.1 X 104Pn y T,( 1 + c?)-1’2, where (Y,the term used to characterize the paramagnetic effects, equals 7.7; the calculated upper critical field at zero temperature equals approximately 180 kG. This result indicates that H,,(O) is probably Limited by the electron spin paramagnetisms). This effect would be consistent with the proposed band structure for this compound”).
REFERENCES I) Biickel, W. and Hilsch, R., Z. Phys. 138(1954) 109. 2) Strongin, M., Kammerer, 0. F., Douglas, D. H., Jr. and Cohen, M. H., Phys. Rev. Letters 19(1967) 121. 3) Bond, W. L., Cooper, A. S., Andres, K., Hall, G. W., Geballe, T. H. and Matthias, 9. T., Phys. Rev. Letters 15 (1965) 260. 4) McConnell, H. M., Hoffman, 9. M., Thomas, D. D. and Gamble, R. F., Proc. Natl. Acad. Sci. (U.S.)54(1965) 371. 5) Garland, J. W., Bennemann, K. H. and Mueller, F. M., Phys. Rev. Letters 21(1968) 1315. 6) Strongin, M., Kammerer, 0. F., Crow, J. E., Parks, R. D., Douglass, D. H., Jr. and Jensen, M. A., Phys. Rev. Letters 21(1968) 1320. 7) Gavaler, J. R., Hulm, J. K., Janocko, M. A. and Jones, C. K., Journal Vat. Science and Technology 6 (1969) 177. 8) Friedel, J., De Gennes, P. G. and Matricon, J., Appl. Phys. Letters 2 (1963) 119. 9) Deis, D. W., Gavaler, J. R., Hulm, J. K. and Jones, C. K., J. appl. Phys. 40 (1969)2 153. 10) Maki, K., Physics 1(1964) 21 and 127. 11) Geballe, T. H., Matthias, 9. T., Remeika, A. M., Clogston, A. M., Compton, V. B., Maita, J. P. and Williams, H. J., Physics 2 (1966) 293.
M. A. JANOCKO,
Research
Laboratories,
AS A FUNCTION FILMSt,S
J. K. HULM Pittsburgh,
OF
and C. K. JONES
Pennsylvania
15235, USA
Synopsis Niobium nitride, NbN, films with thicknesses ranging from 5900 A down to approximately 10 .& have been prepared by a high purity reactive sputtering prdcess and thdir superconducting properties measured. A depression in transition temperature, T,, with decreasing thickness has been observed, starting from a maximum T, of = 15.2 K in the thicker films down to ~6.5 K in a 2.5 A film. The critical current vs. applied magnetic field curves were found to be independent of thickness or even to increase slightly down to a thickness of 100 A. Below this point the critical current at high fields,decreased sharply, the films apparently becoming discontinuous. The average resistivity of the NbN films was = 1000 p ohm cm.
1. Introduction. In the study of superconductivity, the effect on superconducting properties produced by changes in specimen dimensions or geometry, has long been a subject of considerable interest. Recently, for example, a significant part of the effort to produce higher transition temperature materials has been centered on the study of superconductors prepared in thin film formlW4). In general, the work in this area has been concentrated on the study of superconducting elements - all of which have transition temperatures in bulk form of 9 K or lower. In a few cases, substantial increases in transition temperature have been reported. However, all of these enhancements were observed with elements having low transition temperatures, and in no case have materials been prepared with transition temperatures in excess of 10 K. Very limited data is presently available on thin films of the high T, compounds such as occur in the rocksalt or P-tungsten structures, even though enhancement of any of the superconducting properties of these materials would be of immediate practical importance. This lack of data is undoubtedly associated with the considerable difficulties involved in the preparation of these compounds in thin film form. The effect on T, of preparing very thin films of some of these compounds has been considered by a number of theorists. The conclusions which have been drawn, however, have been dissimilar, or even ,contradictory5,6). t This work supported in part by the National Aeronautics and Space Administration. $ Part of this work was performed at the Francis Bitter National Magnet Laboratory which is supported at M.I.T. by the Air Force Office of Scientific Research. 585
586
_I. R. GAVALER,
M. A. JANOCKO,
J. K. HULM
and
C. K. JONES
Recently, we have successfully prepared films of the rocksalt compound NbN, with Tc’s of approximately 15 K and with enhanced high field properties relative to those of the bulk material’). The thicknesses of these films were approximately 2000 A. Films of NbN have now been prepared with thicknesses ranging from 5000 A down to about 10 A. In this paper, we report on thesuperconducting properties of these films. 2. Experimental procedure. The NbN films used in this study were deposited by a high-purity reactive sputtering process which has previously been described in detaiP). Briefly, this process involves sputtering niobium in an argon-nitrogen atmosphere with a total background impurity pressure of 5 X lo-lo Torr or less. The NbN which is thus formed is deposited on substrates which are maintained at approximately 500°C. The thicknesses of the films were measured with an interferometer which has an accuracy off 10 A. The thicknesses of the very thin films (- 50 A and less) were estimated from the duration of the growth experiments. Transition temperatures were measured with a silicon resistance thermometer in a system using helium exchange gas to cool the samples. Films with Tc’s above 13.7 K were also measured in pumped liquid hydrogen by observing the vapor pressure of the hydrogen at the transition. The agreement between the two measurements was 0.1 K or better. High field data were obtained in the 220 kG Bitter-solenoid located at the National Magnet Laboratory. In all of these measurements the samples were mounted so that the magnetic field was applied perpendicular to the plane of the film and to current flow. 3. Results and discussion. Figure 1 shows transition temperature data on NbN films deposited on both metallic and insulating substrates. This figure illustrates the depression in T, which occurs with decreasing film thickness. The last point shown on the curve is for a = 25 A thick film on sapphire with a T, of = 6.5 K. NbN films approximately 10 A thick have been prepared; these films, however, remained normal down to a temperature of 1.2 K. Following the usual convention, the value for T, used to plot this data is that temperature at which the specimen has lost one-half of its normal-state resistivity. To illustrate the entire transitions, some representative curves are reproduced in fig. 2. As typified by curves 1, 2 and 3 in this figure, all of the NbN layers down to a thickness of 100 A became resistanceless. Starting at approximately 50 A, however, the films began to retain some of their normalstate resistance. These results indicate that between 50 and 100 A, the deposited films start to become discontinuous and as the thickness is further decreased the discontinuity becomes increasingly greater. An interesting feature of the T, curves in this figure is that with the exception of the 25 A film, the onset of superconductivity in every case is at approximately 15 K. Therefore, if the highest temperature at which superconductivity is observed
EFFECT
OF FILM THICKNESS
16-, , , , , ,
,
,
ON SUPERCONDUCTIVITY ’
I”“”
’
’
I”“’
’
587
’
16 - 15
14 -
- 14
l3-
- 13
=12 5 52 11 _
- 11
- 12
HlO -
e E
- 10
0 5.5. Substrate o Sapphire Substrate
-9
a-
-8
l-
-1
cl
6-
-6
5-
-5
’
4-J”“’ lo4
1111ll1
’
103
I
I
11lllI
Thickness, A
I I
I
lo2
-4 10
Tc as a function of NbN film thickness
Fig. 1. Transition
temperature
as a function of thickness down to 25 A thick.
in NbN films approximately
5000 A
were plotted against film thickness, no depression in T, would occur until some thickness below 50 A. Regardless of what criterion for T, is used, however, our results demonstrate that no enhancement in T, occurs with decreasing thickness in NbN for films on the substrates employed so far. The reason
I
2
,,,I
4
I
6
I
I
a 10 Temperature,
I
I,
1
12
14
I,
16
K
Typical Tc curves for NbN films of various thickness
Fig. 2. Transition temperature curves for NbN films of five different thicknesses.
588
J. R. GAVALER,
M. A. JANOCKO,
J. K. HULM
and
C. K. JONES
for the observed depression in transition temperature is not known with certainty at present. The deleterious effect on T, of residual gas impurities in the growth system has been shown for NbN thin films prepared in the earlier work. Our present growth techniques greatly minimize the level of residual gases; however, these gases have not been completely eliminated. The portion of these remaining impurities, which originate from outgassing in the substrate region could be expected to exert a maximum effect on the initially deposited NbN. Thus the degradation in T, in the thinnest films could be interpreted as resulting from this type of contamination. Alternatively, the changes in T, could be attributable to a greater lattice disorder, which may be present to a greater degree in the thinner films. The character of this disorder would be sensitive to the conditions of nucleation. A mechanism by which lattice disorder influences transition temperature has been proposed by Garland et ~1.~). Since no evidence exists to confirm either of these possibilities, the explanation for the depression in T, is still an open question. We are currently preparing a deposition system in which we hope to lower the background impurity level by at least another two orders of magnitude. The influence of various substrate materials will also be investigated. Figure 3 shows J, VS. If data for three NbN films. Both the 5000 A film on stainless steel and the 100 A film on sapphire have higher current density values than the 5000 A film on sapphire. These results suggest that the current carrying capacities of these samples were limited by heating effects at the current contacts. Using this assumption the higher .I, values for the 100 A
1041 ’ 100 Fig. 3. J,-H
curves
’ ’ 120
of: (1) 100 .& NbN
’
140
’
’
160
’
Field, kOe
film on sapphire,
’
I@
’
’
zm
’
(2) 5000 A NbN
and (3) 5000 A NbN film on sapphire.
’
220
film on stainless
steel
EFFECT
OF FILM
THICKNESS
589
ON SUPERCONDUCTIVITY
T =4.2 K H =15OkOe
IO4 ““I lo4
Fig. 4. Current
density
’ ’
’
11111II ld
as a function
I
I
Thickness, A
of thickness
in seven
llllllI
I
I
13
NbN
10
films deposited
on sapphire
substrates.
sample on sapphire probably result from the lower currents required to drive this film normal, compared to the 5OOOA film. In the case of the NbN film on stainless steel, the contact problem is apparently minimized by the stabilizing effect of the conducting substrate. This ~-would account for the higher values in this sample. From these results, it is probable that our J, values are somewhat conservative, especially in the thicker films measured at lower fields. Our most accurate current densities should be those measured in the thinnest continuous films and at the highest field. Using this criterion, our most reliable value is the = 3 x lo5 A/cm2 observed in the 100 8, film in a 210 kG field. It is worth noting that this is the highest current density, at this field, yet reported for any material. In fig. 4 current density versus thickness data on seven NbN films deposited on sapphire substrates are shown. Films on insulating substrates were used for these measurements since the detection of changes in resistance in very thin NbN films deposited on metallic substrates is greatly complicated by electrical noise problems. The figure shows an upward trend in J, values in films between 5000 A and 100 A thick. This apparent enhancement may be due to the heating effects at the current contacts. At a minimum, however, these results demonstrate that there is no decrease in current densities down to a thickness of about 100 A. This, in turn, suggests that the NbN films down to 100 A are continuous with uniform thickness and composition. At thicknesses below 100 A, there is a sharp drop in the .I,-H curves. In addition, the character of the transition from the superconducting to the normal state is found to
590
J. R. GAVALER,
M. A. JANOCKO,
J. K. HULM
and C. K. JONES
change radically from that observed in the thicker films. This difference is shown in fig. 5.. Curve A typifies the transitions for films 100 A thick and greater, while curve B is a transition for a 50 A film. As previously observed in the T, measurements, only a part of the 50 A thick NbN film enters the superconducting state, suggesting physical discontinuity in this film. It would seem probable that the sharp drop in current density, as well as the broad transition, are related to this apparent discontinuity. An alternative possibility is that at 50 A, the film thickness is less than the optimum size for the pinning centers required to maintain superconductivity at the high fieldsa). The drop in Jc-H properties in the 50 A film could also simply be related to the low ‘T, of this film (= 10 K). However, a drop in T, from 15 K to 12.5 K in the 100 A film did not produce a similar depression in its J,-H properties. I
loo
I
,
I
I
I
I
I
I
I
120
140
160
180
MO
22u
Field, kOe A) Typical transition curve for continuous NbN films f-la, to > Ma) A thick) 5) Typicaltransition curve for discontinuous NbN films MO Athick and less)
Fig. 5. Typical
transitions
with field for: (A) continuous
NbN
films between
100 A and 5000 A
thick and (B) for NbN films = 50 A thick or less.
In addition to the measurement of the superconducting properties of NbN, we have also determined the normal-state resistivity of these films as a function of thickness. These measurements were made at room temperature, 77 K and 20 K. It was found that in every film measured the resistivity either stayed constant or rose slightly with decreasing temperature. The resistivity of these films showed no apparent dependence on film thickness down to and including
EFFECT
OF FILM THICKNESS
ON SUPERCONDUCTIVITY
591
films approximately 20 A thick. The average resistivity, (p,), of all the samples measured was approximately 1000 p ohm cm. The upper critical field, H&O), of these films is estimated to be about 250 kG. This is considerably below the value suggested by the GLAG theory for a material with this very high resistivity. Using the GLAG formulation, H&?(O)= 3.1 x 104p,y T,,
(1)
where y is the electronic specific heat coefficient; a value of 1400 kG is calculated for the upper critical field. (The value for y used in this calculation is 3.2 X lo3 erg/cc K2 the bulk value.) Taking into account paramagnetic effects, according to the equation of MakilO), H,,(O) = 3.1 X 104Pn y T,( 1 + c?)-1’2, where (Y,the term used to characterize the paramagnetic effects, equals 7.7; the calculated upper critical field at zero temperature equals approximately 180 kG. This result indicates that H,,(O) is probably Limited by the electron spin paramagnetisms). This effect would be consistent with the proposed band structure for this compound”).
REFERENCES I) Biickel, W. and Hilsch, R., Z. Phys. 138(1954) 109. 2) Strongin, M., Kammerer, 0. F., Douglas, D. H., Jr. and Cohen, M. H., Phys. Rev. Letters 19(1967) 121. 3) Bond, W. L., Cooper, A. S., Andres, K., Hall, G. W., Geballe, T. H. and Matthias, 9. T., Phys. Rev. Letters 15 (1965) 260. 4) McConnell, H. M., Hoffman, 9. M., Thomas, D. D. and Gamble, R. F., Proc. Natl. Acad. Sci. (U.S.)54(1965) 371. 5) Garland, J. W., Bennemann, K. H. and Mueller, F. M., Phys. Rev. Letters 21(1968) 1315. 6) Strongin, M., Kammerer, 0. F., Crow, J. E., Parks, R. D., Douglass, D. H., Jr. and Jensen, M. A., Phys. Rev. Letters 21(1968) 1320. 7) Gavaler, J. R., Hulm, J. K., Janocko, M. A. and Jones, C. K., Journal Vat. Science and Technology 6 (1969) 177. 8) Friedel, J., De Gennes, P. G. and Matricon, J., Appl. Phys. Letters 2 (1963) 119. 9) Deis, D. W., Gavaler, J. R., Hulm, J. K. and Jones, C. K., J. appl. Phys. 40 (1969)2 153. 10) Maki, K., Physics 1(1964) 21 and 127. 11) Geballe, T. H., Matthias, 9. T., Remeika, A. M., Clogston, A. M., Compton, V. B., Maita, J. P. and Williams, H. J., Physics 2 (1966) 293.