Different shape normal metal interlayers between niobium based SIS junctions and niobium titanium nitride leads and their influence on the electron temperature

Heat transfer coefficient saturation in superconducting Nb tunnel junctions contacted to a NbTiN circuit and an Au energy relaxation layer

“…In our previous measurements, we found no clear evidence of a geometrical influence of the Au layer on the relaxation of nonequilibrium quasiparticles [9]. This is due to the fact that we performed our measurements in the linear regime, shown in Fig.…”

“…This is not necessarily the best geometry for a practical heterodyne mixer because the large overlap of the Au layer and the NbTiN layer which is one of the electrodes of the radio frequency (RF) tuning circuit of the SIS junction, causes RF signal losses, that depend on the length of the overlap. However, we have found in a different experiment, that the relaxation of nonequilibrium quasiparticles does not depend on the particular geometry of the Au layer as long as the volume of the Au layer is sufficient for the relaxation [9]. Possible remaining influences of the geometry on the relaxation process could not be identified so far.…”

“…In this article we present our ongoing work and focus in particular on a thorough systematic study of the relaxation of nonequilibrium quasiparticles in various circular and halfcircular shaped Au layers, added between the Nb junction and the NbTiN embedding circuit. Earlier work [9] showed a linear dependence between the volume of the Au layer and the heattransfer coefficient alpha. On further increasing the volume of the Au-layer while keeping its thickness fixed to d Au = 60 nm, we find in dc-measurements of the I-V characteristic a saturation in the nonequilibrium quasiparticle relaxation.…”

“…In case that this increases T e , Eq. ( 2) implies that the relevant energy gap for the tunneling decreases leading to the possibility of back-bending [6,8,9] (the slope of I qp becomes negative around V g ). Therefore, the slope of I qp directly probes T e over the reduction in ∆(T e ).…”

“…( 2) we obtain T e and similarly α. More details are described in [8,9]. In order to describe our device geometries, i.e.…”