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The thermal gradient can be turned on or off accordingly to the presence or not of the electrical current flux through the supporting tube. The function p ( t ) determines when the current would be on or off. We choose a square wave-like function to represent p ( t ) with the form 

where 

<img src='content_image/358591.jpg'>

<img src='content_image/358592.jpg'>

with P being the period of p ( t ). The modulo function mod was used to provide the p ( t ) periodicity. The parameters β (= 10 ps − $^{1}$), t$_{1}$ = P/ 6, and t$_{2}$ = 4 t$_{1}$ characterize p ( t ). An example of p ( t ) is depicted in Fig. 2 (B) for P = 400 ps. 

<img src='content_image/358594.jpg'>

