Normal Metal-insulator-superconductor Junction Research Articles

Single-junction quantum-circuit refrigerator

We propose a quantum-circuit refrigerator (QCR) based on photon-assisted quasiparticle tunneling through a single normal-metal–insulator–superconductor (NIS) junction. In contrast to previous studies with multiple junctions and an additional charge island for the QCR, we directly connect the NIS junction to an inductively shunted electrode of a superconducting microwave resonator making the device immune to low-frequency charge noise. At low characteristic impedance of the resonator and parameters relevant to a recent experiment, we observe that a semiclassical impedance model of the NIS junction reproduces the bias voltage dependence of the QCR-induced damping rate and frequency shift. For high characteristic impedances, we derive a Born–Markov master equation and use it to observe significant non-linearities in the QCR-induced dissipation and frequency shift. We further demonstrate that, in this regime, the QCR can be used to initialize the linear resonator into a non-thermal state even in the absence of any microwave drive.

Stability, sub-gap current, 1/f-noise, and elemental depth profiling of annealed Al:Mn-AlOX-Al normal metal-insulator-superconducting tunnel junctions

In this paper we report a study of the effect of vacuum annealing at 400°C on the properties of normal metal-insulator-superconductor (NIS) tunnel junctions, with manganese doped aluminium (Al:Mn) as the normal metal, aluminum as the superconductor and amorphous aluminum oxide as the tunneling barrier (Al:Mn-AlOx-Al). The annealing treatment improves the stability of the junctions, increases their tunneling resistance and does not have a negative impact on the low-temperature current-voltage characteristics. The measured 1/f resistance noise of the junctions also changes after annealing, in the best case decreasing by over an order of magnitude. All these observations show that annealing is a viable route to improve NIS junction devices after the sample has been fabricated.

A Robust Cooling Platform for NIS Junction Refrigeration and sub-Kelvin Cryogenic Systems

Recent advances in Normal metal–insulator-superconductor (NIS) tunnel junctions (Clark et al. Appl Phys Lett 86: 173508, 2005, Appl Phys Lett 84: 4, 2004) have proven these devices to be a viable technology for sub-Kelvin refrigeration. NIS junction coolers, coupled to a separate cold stage, provide a flexible platform for cooling a wide range of user-supplied payloads. Recently, a stage was cooled from 290 to 256 mK (Lowell et al. Appl Phys Lett 102: 082601 2013), but further mechanical and electrical improvements are necessary for the stage to reach its full potential. We have designed and built a new Kevlar suspended cooling platform for NIS junction refrigeration that is both lightweight and well thermally isolated; the calculated parasitic loading is $$<\!\!300$$ pW from 300 to 100 mK. The platform is structurally rigid with a measured deflection of 25 $$\upmu $$ m under a 2.5 kg load and has an integrated mechanical heat switch driven by a superconducting stepper motor with thermal conductivity G $$ = 4.5 \times 10^{-7}$$ W/K at 300 mK. An integrated radiation shield limits thermal loading and a modular platform accommodates enough junctions to provide nanowatts of continuous cooling power. The compact stage size of 7.6 cm $$\times $$ 8.6 cm $$\times $$ 4.8 cm and overall radiation shield size of 8.9 cm $$\times $$ 10.0 cm $$\times $$ 7.0 cm along with minimal electrical power requirements allow easy integration into a range of cryostats. We present the design, construction, and performance of this cooling platform as well as projections for coupling to arrays of NIS junctions and other future applications.

Leakage current of a superconductor–normal metal tunnel junction connected to a high-temperature environment

We consider a voltage-biased Normal metal-Insulator-Superconductor (NIS) tunnel junction, connected to a high-temperature external electromagnetic environment. This model system features the commonly observed subgap leakage current in NIS junctions through photon-assisted tunneling which is detrimental for applications. We first consider a NIS junction directly coupled to the environment and analyze the subgap leakage current both analytically and numerically; we discuss the link with the phenomenological Dynes parameter. Then we focus on a circuit where a low-temperature lossy transmission line is inserted between the NIS junction and the environment. We show that the subgap leakage current is exponentially suppressed as the length, $\ell$, and the resistance per unit length, $R_0$, of the line are increased. We finally discuss our results in view of the performance of NIS junctions in applications.

Theory of tunneling conductance of graphene normal metal-insulator-superconductor junctions

We calculate the tunneling conductance of a graphene normal metal-insulator-superconductor (NIS) junction with a barrier of thickness $d$ and with an arbitrary voltage $V_0$ applied across the barrier region. We demonstrate that the tunneling conductance of such a NIS junction is an oscillatory function of both $d$ and $V_0$. We also show that the periodicity and amplitude of such oscillations deviate from their universal values in the thin barrier limit as obtained in earlier work [Phys. Rev. Lett. {\bf 97}, 217001 (2006)] and become a function of the applied voltage $V_0$. Our results reproduces the earlier results on tunneling conductance of such junctions in the thin [Phys. Rev. Lett. {\bf 97}, 217001 (2006)] and zero [Phys. Rev. Lett. {\bf 97}, 067007 (2006)] barrier limits as special limiting cases. We discuss experimental relevance of our results.

Tunneling Conductance of Graphene NIS Junctions

We show that, in contrast with conventional normal metal-insulator-superconductor (NIS) junctions, the tunneling conductance of a NIS junction in graphene is an oscillatory function of the effective barrier strength of the insulating region, in the limit of a thin barrier. The amplitude of these oscillations is maximum for aligned Fermi surfaces of the normal and superconducting regions and vanishes for a large Fermi surface mismatch. The zero-bias tunneling conductance, in sharp contrast to its counterpart in conventional NIS junctions, becomes maximum for a finite barrier strength. We also suggest experiments to test these predictions.

Normal metal–insulator–superconductor junction technology for bolometers

Normal metal–insulator–superconductor tunnel junctions are candidates for large-format arrays of ultra-low noise equivalent power bolometers. Using a wafer scale process we have fabricated devices with the required sub μ m 3 volumes and ohmic superconductor–normal metal contacts for delivering power loads. Additionally, we demonstrate simultaneous tunnel junction thermometry and SQUID-based Johnson current noise thermometry.

Electron-phonon multiplier involving a cascade of NIS tunnel junctions and hot-electron microcalorimeters

A device for amplifying weak electric signals arising in a normal-or a superconducting-metal absorber upon the absorption of photons or phonons is proposed. The device consists of a cascade of normal-metal-insulator-superconductor (NIS) tunnel junctions, where the normal metal is used as an electrode and as a hot-electron microcalorimeter. Tunneling through the barrier, a particle transports energy of about Δ (energy gap in the superconductor) from one microcalorimeter to another and amplifies the current through the next NIS junction. The device operates as a solid-state electron multiplier at low temperatures.

Nonequilibrium theory of a hot-electron bolometer with normal metal-insulator-superconductor tunnel junction

The operation of the hot-electron bolometer with normal metal-insulator-superconductor (NIS) tunnel junction as a temperature sensor is analyzed theoretically. The responsivity and the noise equivalent power (NEP) of the bolometer are obtained numerically for typical experimental parameters. Relatively simple approximate analytical expressions for these values are derived. The time constant of the device is also found. We demonstrate that the effect of the electron cooling by the NIS junction, which serves as a thermometer, can improve the sensitivity. This effect is also useful in the presence of the finite background power load. We discuss the effect of the correlation of the shot noise and the heat flow noise in the NIS junction.

Electronic refrigerators: optimization studies

Electric current flowing through a normal metal-insulator-superconductor (NIS) junction accompanied by the heat transfer from normal metal to superconductor is able to cool electrons in the normal metal electrode below the lattice temperature under certain conditions. The tunneling electric current dependence on biasing voltage at different normal metal electrode electron temperatures is calculated. Refrigerators with one and with two NIS junctions are investigated. Further, the cooling power of SNIS and SINIS refrigerators, its dependence on biasing voltage and on the temperature of normal metal electrode electrons are investigated. With reduced insulator layer thickness the resistivity of a NIS junction decreases. Thus, the cooling power of a refrigerator increases. But simultaneously the probability for Andreev reflections which diminish the efficiency of the refrigerator increases. Therefore, the optimal thickness of the insulator layer needs to be found. A method for calculating an optimal insulator layer thickness based on the introduction of a new function that needs to be minimized is described. Geometrical limits on the influence of Andreev reflection are determined.

Coherence effects in a normal-metal-insulator-superconductor junction.

Coherence effects arising due to Andreev reflections in a superconductor--insulator--normal-metal junction are considered. It is shown that in the absence of electron-electron interaction in the metal at low temperatures, the electrical potential drop on the insulator ${\mathit{I}}_{0}$${\mathit{R}}_{\mathit{I}}$, caused by the current density ${\mathit{I}}_{0}$ through the junction, vanishes, although the resistance of the device ${\mathit{R}}_{\mathrm{\ensuremath{\infty}}}$ measured by the two-probe method can be large. The electron-electron interaction determines the zero-temperature value of ${\mathit{R}}_{\mathit{I}}$. The implications of these effects for the theory of the superconductor--normal-metal--superconductor junction are discussed.