content_image/368682.jpg
current in the sub-gap region. 

<img src='content_image/368682.jpg'>

The method to determine the voltage position V$_{1}$ of the first Fiske step is shown in Fig.2. We note that the voltage position can also be estimated as the voltage at current I F c (see Fig.2) but the difference between this method and the one represented in Fig.2 is negligible. 

The experimental technique to measure the Josephson critical current I$_{c}$ was as follows. The current I was increased linearly in time from zero value, and the junction voltage V was simultaneously recorded. When the current reached the Josephson critical current value, a non zero junction voltage appeared. Under a condition that the junction voltage equals to the threshold voltage V I$_{c}$ (criterion of the Josephson critical current, see Fig.2), the current value was recorded and the current returned back to zero and then the next measuring cycle started again. The current sweep frequency was 10 Hz and the value of V I$_{c}$ was of 10 µV . 

A cycle to measure the amplitude of the first Fiske step was similar to the Josephson critical current measurements but the starting value of the current was chosen to satisfy the condition that the junction voltage was greater then zero and less then voltage position of the first Fiske step. From this starting point the current was increased, the first Fiske step 

