Probing quasiparticle excitations in a hybrid single electron transistor

Abstract

We investigate the behavior of quasiparticles in a hybrid electron turnstile with the aim of improving its performance as a metrological current source. The device is used to directly probe the density of quasiparticles and monitor their relaxation into normal metal traps. We compare different trap geometries and reach quasiparticle densities below 3 µm −3for pumping frequencies of 20 MHz. Our data show that quasiparticles are excited both by the device operation itself and by the electromagnetic environment of the sample. Our observations can be modelled on a quantitative level with a sequential tunneling model and a simple diffusion equation.Applications of superconductors generally rely on the fact that electronic excitations can be generated only if energy higher or equal to the gap energy ∆ is available. Hence the number of the excitations is ideally exponentially small at low temperatures and the properties specific for superconductors appear. If the number of excitations, typically characterized by the density of quasiparticles, increases, the superconducting features degrade. Such an effect has been studied in several devices such as superconducting qubits 1-4 , superconductor-insulator-normal metal-insulatorsuperconductor (SINIS) microcoolers 5-8 and kinetic inductance detectors 9,10 . In this letter we focus on the effects of quasiparticles on a hybrid single-electron transistor. We use the Coulomb blockaded transistor for direct and simple probing of the quasiparticle excitation density. The device operates as a charge pump and is a promising candidate for the realization of a metrological current source 11 . We show experimentally that the quasiparticle excitations limit the current quantization. By optimizing the quasiparticle relaxation, however, we estimate that it is possible to reach metrological accuracy.In order to observe how quasiparticles influence the performance of the turnstile we designed samples (type A) where the quasiparticle relaxation in normal metal traps was purposefully delayed by extending the bare superconducting lines that connect the junctions to the traps. The beginning of this isolated superconducting line can be seen in the scanning electron microscope image of the turnstile in Fig. 1. (a) where it connects to the normal metal island via oxide junctions that appear as lighter areas. Its extension is visible in (c) all the way through to the wide traps of overlapping superconductor and normal metal separated by an oxide layer. The sample shown in (b) (type B) has wide leads with normal metal traps close to the junctions to enable efficient quasiparticle evacuation. The samples were fabricated with the standard electronbeam lithography and shadow mask technique 12 . We compare the behaviour of quasiparticles in SINIS turnstile samples with different geometries. The length of the isolated superconducting line (given by the separation of the transistor junction from the trap) was varied between l = 200 nm and l = 20 µm. Measurements were performed in a dilution refrigerator a...

Excitation of Single Quasiparticles in a Small Superconducting Al Island Connected to Normal-Metal Leads by Tunnel Junctions

“…Later on, as the fabrication techniques progressed, it became possible to bring N S close to unity to reveal the parity effect of electrons on a superconducting island [10][11][12][13][14]. In recent years, the tunneling and relaxation dynamics of quasiparticles, which we address in this Letter, have become a topical subject because of their influence on practically all superconducting circuits in the low temperature limit [15][16][17][18][19][20][21][22].…”

Experimental realization of a Coulomb blockade refrigerator

“…These refrigerators work most of the time in the temperature range of 300-100 mK. They perform suboptimally at lower temperatures, due to several reasons, including excess quasiparticle population in a superconductor [17,18], leakage of the junctions [19], and a low electron-electron scattering rate [13].…”

Proximity effect in normal-metal quasiparticle traps

“…(38)- (41), starting from the definitions for n and p in Eqs. (12) and (13), respectively. According to Eq.…”

“…These approaches include engineering the spatial profile of the superconducting gap to steer quasiparticles away from Josephson junctions [7,8], and cooling down a device in a magnetic field in order to generate vortices that trap quasiparticles in their cores [9][10][11]. Here, we consider a different way to capture quasiparticles using normal-metal traps, a proposal which has been implemented in single-electron turnstiles [12], hybrid multilayer coolers [13,14], superconducting resonators [15], and qubits [16].…”