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Josephson steps produced by the modulated heat flow
~
Solid State Communications, Vol.57,No.2, pp.99-I02, 1986. Printed in Great Britain.
0038-1098/86 $3.00 + .00 Pergamon Press Ltd.
JOSEPHSON STEPS PRODUCED BY THE MODULATED HEAT FLOW G.Yu.Logvenov, N.V.Osherov,
V.V.Ryazanov
Institute of Solid State Physics, the Academy of Sciences of the USSR, Chernogolovka, Noscow district, 142432, USSR (Received
11 October
1985 by V.M.Agranovich)
Current steps have been found in the I-V characteristics of SNS junctions when the heat flow through the junction was periodically modulated. The features observed are shown to be due to the alternating thermoelectric supercurrents which appear in the junction.
istic of a Josephson SNS junction due to a periodically modulated heat flow P through the junction with the modulation frequency f ~ f ~ . The heat flow through the junction was modulated by periodically interrupting the infrared laser ray focused on one of the SNS sandwich superconducting plates; the second plate was in thermal contact with the helium bath. If the laser irradiation then caused considerable junction overheat our technique would not be different from the technique suggested in 6 to observe Josephson current steps due to the periodic modulation of the junction critical current l~(t) by heating. "The amplitude modulation" of the alternating Josephson current due to the periodic suppression of I c has been shown in our experimental conditions to be negligible in comparison with the Josephson oscillations"phase modulation" caused by the alternating thermoelectric currents in the junction.
I. Introduction Thermoelectric analogues of Josephson effects in SNS sandwiches I . were discovered and investigated in "-4 When the heat flow P through the SNS sandwich exceeds some critical value P there appears a nonstationary state w~ich is characterized by the timeaverage thermoelectric voltage V at the junction and by Josephson generation with the frequency f=V/@ o (@o is the magnetic flux quantum). This phenomenon is due to the appearance of thermoelectric quasiparticle currents and compensating supercurrent counterflows in the junction. The heat flow achieves its critical value P when the quasiparticle current (an~ an equal supercurrent) in the junction achieves the critical value I . At temperatures close to T c the valu~ Pc=(K/LT)Ic , where K and L T are specific thermal conductivity and thermoelectric coefficient of the weak link material in the normal state 5 at T=Tc; T c is the
2. Experimental
critical temperature of SNS sandwich superconducting plates, I c is the Josephson junction critical current. In I-4 low-resistance SNS junctions
Fig. 1 shows a block diagram of our experimental setup. Heat flow P through the junction was created using the continuous-action C02-1aser (1) with the radiation wave length 10,6 ~m, heat flow modulation was achieved using a mechanical laser interrupter (2) with the rotation frequency determined by the voltage supplied to the direct current generator. Laser radiation output power was attenuated by several silicium parallel-plane plates (3) and two diffraction polarizers (4) and was ~I0-3W. The laser ray was then directed into the cryostat to fall on the sandwich upper plate. A considerable radiation power loss seemed to occur in the stainless light pipe which served for continuous gas evacuation from the working volume. The laser radiation
~r~Uu~d~hga~llo~dI~~)the critical heat flow P ~ IO-5W when the overheat in SNS sandwich superconducting plates was negligible ( ~ I O -2 K). In constant voltage steps in the voltage-heat flow characteristic (V-P characteristic) of the SNS junction were observed when it was irradiated by a low-frequency electromagnetic field with the frequency f ~ f c = V / ~ o ~ 50 Hz. The purpose of our work was to observe current steps (constant voltage steps) in the ordinary I-V character99
JOSEPHSON STEPS PRODUCED BY THE MODULATED HEAT FLOW
100
t
!
2
3 4
5
Vol. 57, No. 2
,,.,:~
T
~5
v_.J Fig. 1. Schematic drawing of the instrumentation for the experiment: (I) Laser, (2) Nechanical ray interrupter, (3) Silicium plates, (4) Diffraction polarizers, (5) Copper mirror, (6) Frequency meter, (7) Sample, (8) Vacuum can, (9) Copper heat conductor. power that reached the sample was not measured directly but could be evaluated from the experimental data obtained as is described in Section 3. A simple scheme with an electric lamp, photodiode and frequency meter (6) allowed an independent measurement of the laser ray interruption frequency. A n SNS sandwich (7) was placed into a vacuum can (8) and attached by the lower superconducting plate to a copper heat conductor (9) which was in contact with the helium bath. The gas pressure in the can throughout3the experiment was kept about 5.10 - torr by the baking pump. As SNS Josephson junctions, we used Ta-Cu-Ta sandwiches prepared by 7 mutual hot rolling of metals in vacuum'. The thickness of the upper superconducting plate was ~ I 0 ~m, the junction copper normal layer had approximately the same thickness. The massive lower superconducting plate of ~ 5 0 0 ~ m thickness had an additional outer copper layer which was necessary for solding the sample to the heat conductor. The junction area w a s ~ O . 2 x O . 2 c m 2. At T=3.6K (T/T =0.8) the characteristic parameters 5f the junctions were as follows: critical current I =2-I0-5A, c normal resistance R=5-10 - 9 ~ , characteristic voltage Vc=10-13V, characteristic Josephson frequency f~-5OHz. At that temperature the Josephs~n penetration depth was estimated to be about 0.2cm, namely, it was close to the junction width, Our junctions were thus large enough to provide a uniform supercurrent flow through the junction. Fig.2 shows an exponential temperature dependence of the critical current I for one of the junctions.
.4o
As
•
TEHPEI~ATUI~,T (10 Fig.2. Temperature as a function of the junction critical current (experimental points and interpolating curve). (A logarithmic scale has been used for the current axis.) The RF SQUID voltmeter inserted in an ordinary feedback arrangement was used to obtain I-V characteristics. The voltmeter sensitivity was ~ 1 0 - 1 4 V with the time constant of 2 s. The experiments were performed in a screened cryostat so that the residual magnetic field was less than I0-3G. The temperature was varied by helium vapour pumping and stabilized by a diaphragm pressure regulator. Fig.3 gives I-V characteristics at the helium bath temperature of 3.58K for the SNS junction with the Ic(T) dependence shown in Fig.2. In
,~> 2O 'to
~-2.0 -~0
CVRRgNT Pig.3. I-V characteristics of the Ta-Cu-Ta junction in the presence of the ac heat flow modulated at the frequen£y: (I) OHz, (2) 30 Hz, (3) 70 Hz, (4) 92 Hz. The heat flow amplitude is fixed. T/Tc=0.81. Arrows indicate the voltage V n = n f ~ . Curves are shifted along the current axis.
JOSEPHSON STEPS PRODUCED BY THE MODULATED HEAT FLOW
Vol. 57, No. 2
Fig.3 curves for different ray interruption frequencies are shifted along the current axis. The junction was irradiated by the laser ray of fixed radiation power, the interruption frequenc~ f was 30, 70 and 92Hz (curves 2,3,4). Curve 1 gives the I-V characteristic for a junction which was not irradiated by the infrared light. The position of current steps is seen to be defined strictly enough by the relationship Vn=nf~ o. (The calculated values V n are marked by arrows). The steps on I-V characteristics were observed within the interruption frequency range from 30 to 500Hz but to observe steps for frequencies larger than 3OOHz the radiation power had to be increased considerably. Fig.4 shows I-V characteristics for different radiation powers and the fixed interruption frequency of 33Hz. The results represented in Figs, 3 and 4 have been obtained in the same experiment for the same sample and at the same temperature. The curves in Fig.4 are shifted along the voltage axis so that the upper curve corresponds to the largest radiation power.
101
as a whole shift to the left along the current axis. When the radiation power was further increased any regularity in the step position and their size disappeared and the results became unreproducible. To study the effect of the junction overheat on its critical current during the above experiments (without the laser readjustment and the helium bath temperature change) the dependence of I on the unmodulated laser radiationCpower was measured. The critical current suppression was not found to exceed 10-15% even at the maximum power used to obtain the results (see Fig.4). Using the dependence I~(T) in Fig.2 it is easy to see that th~ junction overheat did not exceed 3.10-2K. Certainly, I-V characteristics in this case did not have current steps but they were also shifted to the left along the current axis when the unmodulated laser radiation was increased. This shift is attributed, as is further discussed, to the appearance of stationary thermoelectric currents in the junction. It was also observed when the heat flow was introduced from the heater placed directly inside the vacuum can so as to have a reliable thermal contact with the upper SNS sandwich plate. The experimental conditions in the la~t~case were analogous to those described and the shift was proportional to the heat flow P and reached I c at P ~ 10-5W, which is in satisfactory agreement with the relationship Pc(Ic) (see Section I). 3. Discussion Let us first discuss the possibility of the steps appearance on the I-V characteristics as a result of the Josephson alternating current amplitude modulation caused by the periodic suppression of I when the junction is irradiated by th~ modulated infrared light. The critical current suppression 6 in this case is comparable with the first step size. The critical currents and the first step in the high freque-
N
O >
-6
-4
~
CU~RENT,
0
2
4
I(40~A~
Fig. 4. I-V characteristics of the junction in the presence of the ac heat flow modulated at 33 Hz. The curves are shifted along the voltage axis so that the upper curve position corresponds to the largest radiation power. One can see that when the power increases the critical current (i.e. the zeroth step) reduces and the higher number steps grow larger. Besides, the curves
ncy limit ( f ~ f c ) are I°'(1-a/2)c and ic.o a/2, respectively, where I °c is the undisturbed critical current, a is the depth of the critical current modulation due to the sample periodical heating. As it was pointed out in the previous Section the critical current suppression in the case presented in Pigs.3 and 4 must be less than 10-15%, i.e. 3 - 4 ~ A . The z ~ s t e p change and the f i r s ~ s t e p size are seen to reach IO-20 ~ A in our experiment. Thus the appearance of the constant voltage steps seems to be associated with the phase modulation of the Josephson oscillations. Let us suppose now that there
102
JOSEPHSON STEPS PRODUCED BY THE MODVLATED HEAT FLOW
exist alternating thermoelectrical currents with the ray interruption frequency in the Josephson SNS junction and then discuss the results obtained. The thermal power supplied to the upper plate of the SNS junction in our experiment was the time function close to a rectangular pulse. According to our estimates, time which characterizes heat relaxation to a uniform temperature distributio~ along the sample did not exceed 10- 9s and, consequently, was comparable with the rectangular signal period. ~or the time being, we take the heat flow through the junction to be approximately described by the following time function: (I) i.e, the mean heat flow through the sample is half of the peak value P and depth of the heat flow modulation is 100%. The supercurrent in the junction (see Section I) must display a similar time dependence. Although the thermal power supplied to the sample has not been measured directly we could estimate P~ by the mean supercurrent in the junction whmch was equal to the curves shift Ish along the current axis
P=(Po/2)/1-sin(2~ft)/
•
.
O
Vol. 57, No. 2
processes in the junction can be thus approximately described by the equation: Icsin ~ +V/R=I+Ish+IshSin(2~ft) (2) where ~ is the difference of the superconducting phases at the junction. Calculation of zer~h and first step sizes on the basis of (2) is in satisfactory agreement with the experiment. (To solve (2) numerical results ° were used). More exact results could be obtained with an alternating signal amplitude s o m e w h a t smaller than I ,. Such a value is quite possible. As th@nhea~ relaxation time for our sample (10- 9s) is rather large the upper sandwich plate has been somewhat overheated all the time in comparison with the lower one. The mean values of P and I . must thus rise as the laser ray integ~uption frequency increases, and the depth of the heat flow modulation and alternating signal amplitude must reduce. This also explains the experimental fact that in order to obtain steps at fre ~ quences close to 500Hz we had to increase P considerably to reach sufficient alternating signal amplitudes.
.
in Fig.4. According to (I) the amplitude of the alternating signal interacting with the Josephson oscillations must have a similar value Ish. The
Acknowledgements - Authors are grateful t o ~ . V . S c h m i d t l f o r his attention to our work. It is a pleasure to express our gratitude to N.S.Stepakov for his assistance in preparation of the equipment.
References I. ~.V.Kartzovnik, V.V.Ryazanov, V.V.Shmidt, JETP Lett, 33, 356
(1981). 2. V.V.Ryazanov, V.V.Shmidt, Solid State Communs, 40, 1055 (1981). 3. O.I.Panaitov, V.V.Ryazanov, A.V.Ustinov, V.V.Schmidt, Phys.Lett., IOOA, 301 (1984). 4..~Arutunian, G.F.Zharkov, G.Yu. Logvenov, V.V.Ryazanov, V.V.Schmidt, Sov.Phys.JEPT, 8 7 596 (1984). 5. It should be noted that both the copper Josephson layer and the nearest nonequilibrium layers of the superconducting tantalum can
6. 7. 8. 9.
participate in the thermoelectric processes that occur in the region of the Josephson weak link at temperatures close to T~ 1,9. C.Vanneste, A.Gilabe~t, P.Sibillot, D.B.0strowsky, J.Low Temp.Phys., 45, 517 (1981). V.V.Ryazanov, V.V.S~hmidt, L.A.Ermolaeva, J.Low Temp.Phys., 45, 507 (1981). P.Russer, J.Appl.Phys., 43, 2008 (1972). S.N.Artemenko, A.F.Volkov Sov. Phys. JETP, 4__3, 548 (1976)~
Solid State Communications, Vol.57,No.2, pp.99-I02, 1986. Printed in Great Britain.
0038-1098/86 $3.00 + .00 Pergamon Press Ltd.
JOSEPHSON STEPS PRODUCED BY THE MODULATED HEAT FLOW G.Yu.Logvenov, N.V.Osherov,
V.V.Ryazanov
Institute of Solid State Physics, the Academy of Sciences of the USSR, Chernogolovka, Noscow district, 142432, USSR (Received
11 October
1985 by V.M.Agranovich)
Current steps have been found in the I-V characteristics of SNS junctions when the heat flow through the junction was periodically modulated. The features observed are shown to be due to the alternating thermoelectric supercurrents which appear in the junction.
istic of a Josephson SNS junction due to a periodically modulated heat flow P through the junction with the modulation frequency f ~ f ~ . The heat flow through the junction was modulated by periodically interrupting the infrared laser ray focused on one of the SNS sandwich superconducting plates; the second plate was in thermal contact with the helium bath. If the laser irradiation then caused considerable junction overheat our technique would not be different from the technique suggested in 6 to observe Josephson current steps due to the periodic modulation of the junction critical current l~(t) by heating. "The amplitude modulation" of the alternating Josephson current due to the periodic suppression of I c has been shown in our experimental conditions to be negligible in comparison with the Josephson oscillations"phase modulation" caused by the alternating thermoelectric currents in the junction.
I. Introduction Thermoelectric analogues of Josephson effects in SNS sandwiches I . were discovered and investigated in "-4 When the heat flow P through the SNS sandwich exceeds some critical value P there appears a nonstationary state w~ich is characterized by the timeaverage thermoelectric voltage V at the junction and by Josephson generation with the frequency f=V/@ o (@o is the magnetic flux quantum). This phenomenon is due to the appearance of thermoelectric quasiparticle currents and compensating supercurrent counterflows in the junction. The heat flow achieves its critical value P when the quasiparticle current (an~ an equal supercurrent) in the junction achieves the critical value I . At temperatures close to T c the valu~ Pc=(K/LT)Ic , where K and L T are specific thermal conductivity and thermoelectric coefficient of the weak link material in the normal state 5 at T=Tc; T c is the
2. Experimental
critical temperature of SNS sandwich superconducting plates, I c is the Josephson junction critical current. In I-4 low-resistance SNS junctions
Fig. 1 shows a block diagram of our experimental setup. Heat flow P through the junction was created using the continuous-action C02-1aser (1) with the radiation wave length 10,6 ~m, heat flow modulation was achieved using a mechanical laser interrupter (2) with the rotation frequency determined by the voltage supplied to the direct current generator. Laser radiation output power was attenuated by several silicium parallel-plane plates (3) and two diffraction polarizers (4) and was ~I0-3W. The laser ray was then directed into the cryostat to fall on the sandwich upper plate. A considerable radiation power loss seemed to occur in the stainless light pipe which served for continuous gas evacuation from the working volume. The laser radiation
~r~Uu~d~hga~llo~dI~~)the critical heat flow P ~ IO-5W when the overheat in SNS sandwich superconducting plates was negligible ( ~ I O -2 K). In constant voltage steps in the voltage-heat flow characteristic (V-P characteristic) of the SNS junction were observed when it was irradiated by a low-frequency electromagnetic field with the frequency f ~ f c = V / ~ o ~ 50 Hz. The purpose of our work was to observe current steps (constant voltage steps) in the ordinary I-V character99
JOSEPHSON STEPS PRODUCED BY THE MODULATED HEAT FLOW
100
t
!
2
3 4
5
Vol. 57, No. 2
,,.,:~
T
~5
v_.J Fig. 1. Schematic drawing of the instrumentation for the experiment: (I) Laser, (2) Nechanical ray interrupter, (3) Silicium plates, (4) Diffraction polarizers, (5) Copper mirror, (6) Frequency meter, (7) Sample, (8) Vacuum can, (9) Copper heat conductor. power that reached the sample was not measured directly but could be evaluated from the experimental data obtained as is described in Section 3. A simple scheme with an electric lamp, photodiode and frequency meter (6) allowed an independent measurement of the laser ray interruption frequency. A n SNS sandwich (7) was placed into a vacuum can (8) and attached by the lower superconducting plate to a copper heat conductor (9) which was in contact with the helium bath. The gas pressure in the can throughout3the experiment was kept about 5.10 - torr by the baking pump. As SNS Josephson junctions, we used Ta-Cu-Ta sandwiches prepared by 7 mutual hot rolling of metals in vacuum'. The thickness of the upper superconducting plate was ~ I 0 ~m, the junction copper normal layer had approximately the same thickness. The massive lower superconducting plate of ~ 5 0 0 ~ m thickness had an additional outer copper layer which was necessary for solding the sample to the heat conductor. The junction area w a s ~ O . 2 x O . 2 c m 2. At T=3.6K (T/T =0.8) the characteristic parameters 5f the junctions were as follows: critical current I =2-I0-5A, c normal resistance R=5-10 - 9 ~ , characteristic voltage Vc=10-13V, characteristic Josephson frequency f~-5OHz. At that temperature the Josephs~n penetration depth was estimated to be about 0.2cm, namely, it was close to the junction width, Our junctions were thus large enough to provide a uniform supercurrent flow through the junction. Fig.2 shows an exponential temperature dependence of the critical current I for one of the junctions.
.4o
As
•
TEHPEI~ATUI~,T (10 Fig.2. Temperature as a function of the junction critical current (experimental points and interpolating curve). (A logarithmic scale has been used for the current axis.) The RF SQUID voltmeter inserted in an ordinary feedback arrangement was used to obtain I-V characteristics. The voltmeter sensitivity was ~ 1 0 - 1 4 V with the time constant of 2 s. The experiments were performed in a screened cryostat so that the residual magnetic field was less than I0-3G. The temperature was varied by helium vapour pumping and stabilized by a diaphragm pressure regulator. Fig.3 gives I-V characteristics at the helium bath temperature of 3.58K for the SNS junction with the Ic(T) dependence shown in Fig.2. In
,~> 2O 'to
~-2.0 -~0
CVRRgNT Pig.3. I-V characteristics of the Ta-Cu-Ta junction in the presence of the ac heat flow modulated at the frequen£y: (I) OHz, (2) 30 Hz, (3) 70 Hz, (4) 92 Hz. The heat flow amplitude is fixed. T/Tc=0.81. Arrows indicate the voltage V n = n f ~ . Curves are shifted along the current axis.
JOSEPHSON STEPS PRODUCED BY THE MODULATED HEAT FLOW
Vol. 57, No. 2
Fig.3 curves for different ray interruption frequencies are shifted along the current axis. The junction was irradiated by the laser ray of fixed radiation power, the interruption frequenc~ f was 30, 70 and 92Hz (curves 2,3,4). Curve 1 gives the I-V characteristic for a junction which was not irradiated by the infrared light. The position of current steps is seen to be defined strictly enough by the relationship Vn=nf~ o. (The calculated values V n are marked by arrows). The steps on I-V characteristics were observed within the interruption frequency range from 30 to 500Hz but to observe steps for frequencies larger than 3OOHz the radiation power had to be increased considerably. Fig.4 shows I-V characteristics for different radiation powers and the fixed interruption frequency of 33Hz. The results represented in Figs, 3 and 4 have been obtained in the same experiment for the same sample and at the same temperature. The curves in Fig.4 are shifted along the voltage axis so that the upper curve corresponds to the largest radiation power.
101
as a whole shift to the left along the current axis. When the radiation power was further increased any regularity in the step position and their size disappeared and the results became unreproducible. To study the effect of the junction overheat on its critical current during the above experiments (without the laser readjustment and the helium bath temperature change) the dependence of I on the unmodulated laser radiationCpower was measured. The critical current suppression was not found to exceed 10-15% even at the maximum power used to obtain the results (see Fig.4). Using the dependence I~(T) in Fig.2 it is easy to see that th~ junction overheat did not exceed 3.10-2K. Certainly, I-V characteristics in this case did not have current steps but they were also shifted to the left along the current axis when the unmodulated laser radiation was increased. This shift is attributed, as is further discussed, to the appearance of stationary thermoelectric currents in the junction. It was also observed when the heat flow was introduced from the heater placed directly inside the vacuum can so as to have a reliable thermal contact with the upper SNS sandwich plate. The experimental conditions in the la~t~case were analogous to those described and the shift was proportional to the heat flow P and reached I c at P ~ 10-5W, which is in satisfactory agreement with the relationship Pc(Ic) (see Section I). 3. Discussion Let us first discuss the possibility of the steps appearance on the I-V characteristics as a result of the Josephson alternating current amplitude modulation caused by the periodic suppression of I when the junction is irradiated by th~ modulated infrared light. The critical current suppression 6 in this case is comparable with the first step size. The critical currents and the first step in the high freque-
N
O >
-6
-4
~
CU~RENT,
0
2
4
I(40~A~
Fig. 4. I-V characteristics of the junction in the presence of the ac heat flow modulated at 33 Hz. The curves are shifted along the voltage axis so that the upper curve position corresponds to the largest radiation power. One can see that when the power increases the critical current (i.e. the zeroth step) reduces and the higher number steps grow larger. Besides, the curves
ncy limit ( f ~ f c ) are I°'(1-a/2)c and ic.o a/2, respectively, where I °c is the undisturbed critical current, a is the depth of the critical current modulation due to the sample periodical heating. As it was pointed out in the previous Section the critical current suppression in the case presented in Pigs.3 and 4 must be less than 10-15%, i.e. 3 - 4 ~ A . The z ~ s t e p change and the f i r s ~ s t e p size are seen to reach IO-20 ~ A in our experiment. Thus the appearance of the constant voltage steps seems to be associated with the phase modulation of the Josephson oscillations. Let us suppose now that there
102
JOSEPHSON STEPS PRODUCED BY THE MODVLATED HEAT FLOW
exist alternating thermoelectrical currents with the ray interruption frequency in the Josephson SNS junction and then discuss the results obtained. The thermal power supplied to the upper plate of the SNS junction in our experiment was the time function close to a rectangular pulse. According to our estimates, time which characterizes heat relaxation to a uniform temperature distributio~ along the sample did not exceed 10- 9s and, consequently, was comparable with the rectangular signal period. ~or the time being, we take the heat flow through the junction to be approximately described by the following time function: (I) i.e, the mean heat flow through the sample is half of the peak value P and depth of the heat flow modulation is 100%. The supercurrent in the junction (see Section I) must display a similar time dependence. Although the thermal power supplied to the sample has not been measured directly we could estimate P~ by the mean supercurrent in the junction whmch was equal to the curves shift Ish along the current axis
P=(Po/2)/1-sin(2~ft)/
•
.
O
Vol. 57, No. 2
processes in the junction can be thus approximately described by the equation: Icsin ~ +V/R=I+Ish+IshSin(2~ft) (2) where ~ is the difference of the superconducting phases at the junction. Calculation of zer~h and first step sizes on the basis of (2) is in satisfactory agreement with the experiment. (To solve (2) numerical results ° were used). More exact results could be obtained with an alternating signal amplitude s o m e w h a t smaller than I ,. Such a value is quite possible. As th@nhea~ relaxation time for our sample (10- 9s) is rather large the upper sandwich plate has been somewhat overheated all the time in comparison with the lower one. The mean values of P and I . must thus rise as the laser ray integ~uption frequency increases, and the depth of the heat flow modulation and alternating signal amplitude must reduce. This also explains the experimental fact that in order to obtain steps at fre ~ quences close to 500Hz we had to increase P considerably to reach sufficient alternating signal amplitudes.
.
in Fig.4. According to (I) the amplitude of the alternating signal interacting with the Josephson oscillations must have a similar value Ish. The
Acknowledgements - Authors are grateful t o ~ . V . S c h m i d t l f o r his attention to our work. It is a pleasure to express our gratitude to N.S.Stepakov for his assistance in preparation of the equipment.
References I. ~.V.Kartzovnik, V.V.Ryazanov, V.V.Shmidt, JETP Lett, 33, 356
(1981). 2. V.V.Ryazanov, V.V.Shmidt, Solid State Communs, 40, 1055 (1981). 3. O.I.Panaitov, V.V.Ryazanov, A.V.Ustinov, V.V.Schmidt, Phys.Lett., IOOA, 301 (1984). 4..~Arutunian, G.F.Zharkov, G.Yu. Logvenov, V.V.Ryazanov, V.V.Schmidt, Sov.Phys.JEPT, 8 7 596 (1984). 5. It should be noted that both the copper Josephson layer and the nearest nonequilibrium layers of the superconducting tantalum can
6. 7. 8. 9.
participate in the thermoelectric processes that occur in the region of the Josephson weak link at temperatures close to T~ 1,9. C.Vanneste, A.Gilabe~t, P.Sibillot, D.B.0strowsky, J.Low Temp.Phys., 45, 517 (1981). V.V.Ryazanov, V.V.S~hmidt, L.A.Ermolaeva, J.Low Temp.Phys., 45, 507 (1981). P.Russer, J.Appl.Phys., 43, 2008 (1972). S.N.Artemenko, A.F.Volkov Sov. Phys. JETP, 4__3, 548 (1976)~