Improved Nb SIS devices for heterodyne mixers between 700 GHz and 1.3 THz with NbTiN transmission lines using a normal metal energy relaxation layer

Abstract

In this paper we demonstrate experimentally the implementation of a niobium-trilayer junction with an aluminum-oxide tunnel barrier, embedded in a high-gap superconducting niobium-titanium-nitride circuit. Previously reported heating by quasiparticle trapping is removed by inserting a normal metal layer of gold between the niobium junction and the niobium-titanium-nitride layer. We analyze in dc-characterization measurements the cooling of the nonequilibrium quasiparticles in various device geometries having different gold layer thickness and shape. Our work is concluded with remarks for future heterodyne mixer experiments using our device technology.

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

“…We experimentally found the relaxation of these nonequilibrium quasiparticles in an added small normal-metal layer of Au between the Nb junction and the NbTiN embedding circuit [8]. In this proof-of-principle study, the Au layer had a circular shape with a diameter of several µm, centered on the SIS junction.…”

“…( 3), for a distributed layer of Au where quasiparticle relaxation via diffusion and electron-phonon scattering occurs simultaneously, α contains both relaxation contributions. This is in contrast to the definition of α we used in our earlier work [8] where α was used to describe relaxation only via heat transfer to the phonon system. By calculating T e from Eq.…”

“…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 ).…”

“…In order to deal with non-idealities in the sharpness of the I-V curve compared to the ideal situation described by Eq. ( 1), a phenomenological broadening parameter Γ is introduced into the expression for N S allowing to estimate the smearing of I qp around V g due to quasiparticle energy broadening [8,11], e.g. caused by an external magnetic field which we use to suppress the Josephson effect during our measurements.…”

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