Normal-metal Tunnel Junctions Research Articles

Superconducting terahertz receivers for space-borne and balloon-borne radio telescopes

A superconducting heterodyne receiver based on superconductor–insulator–superconductor (SIS) junctions and a bolometric receiver based on superconductor–insulator–normal metal (SIN) tunnel junctions developed at the Kotelnikov Institute is described. The noise temperature and noise equivalent power of such a receiver fit the requirements for the TELIS, BOOMERANG, OLIMPO, and LSPE aerostatborne radio telescopes, and the space-based SPICA and MILLIMETRON projects, among others. Applications of such receivers for medical diagnostics and remote control are described.

Influence of the in-plane magnetic field on the Andreev conductance of a superconductor–insulator–normal metal structure

Pronounced conductance due to electrons experiencing Andreev reflection from a superconducting condensate has been observed in superconductor (aluminum)–insulator (aluminum oxide)–normal metal (copper) tunnel junctions at low voltages, along with single-electron tunneling. It has been discovered experimentally that the collective current is suppressed in the magnetic field parallel to the tunnel junction plane and the Andreev conductance decreases nearly twofold in a field of ∼20–30 mT.

Effect of a magnetic field on the conductivity of superconductor–insulator–normal metal tunnel junctions

The influence of a magnetic field on the conductivity of superconductivity–insulator–normal metal tunnel junctions with aluminum superconducting electrodes has been investigated. The area of the junctions was 2 µm2 and the thickness of the superconducting aluminum electrodes was 80 nm. The nonlinearity of the current–voltage characteristics has been observed at temperatures of 100–400 mK in a transverse field of up to 28 mT, which is approximately three times greater than the critical field for pure bulk aluminum. The conductivity at zero bias voltage increased with the field by 2–4 orders of magnitude. The effect is hysteretic. It can be caused by the emergence of a vortex structure in the magnetic field above the first critical field in the aluminum film, which becomes a type-II superconductor owing to its small thickness, impurities, and structural defects.

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.

Spin injection from a normal metal into a mesoscopic superconductor

We report on nonlocal transport in superconductor hybrid structures, with ferromagnetic as well as normal-metal tunnel junctions attached to the superconductor. In the presence of a strong Zeeman splitting of the density of states, both charge and spin imbalance is injected into the superconductor. While previous experiments demonstrated spin injection from ferromagnetic electrodes, we show that spin imbalance is also created for normal-metal injector contacts. Using the combination of ferromagnetic and normal-metal detectors allows us to directly discriminate between charge and spin injection, and demonstrate a complete separation of charge and spin imbalance. The relaxation length of the spin imbalance is of the order of several $\mu$m and is found to increase with a magnetic field, but is independent of temperature. We further discuss possible relaxation mechanisms for the explanation of the spin relaxation length.

Long hold times in a two-junction electron trap

The hold time τ of a single-electron trap is shown to increase significantly due to suppression of photon assisted tunneling events. Using two rf-tight radiation shields instead of a single one, we demonstrate increase of τ by a factor exceeding 103, up to about 10 h, for a trap with only two superconductor (S)—normal-metal (N) tunnel junctions and an on-chip resistor R ∼ 100 kΩ (R-SNS structure). In the normal state, the improved shielding made it possible to observe τ ∼ 100 s, which is in reasonable agreement with the quantum-leakage-limited level expected for the two-electron cotunneling process.

Environmentally activated tunneling events in a hybrid single-electron box

We have measured individual tunneling events and Coulomb step shapes in single-electron boxes with opaque superconductor--normal metal tunnel junctions. We observe anomalous broadening of the Coulomb step with decreasing temperature in a manner that is consistent with activation of first-order tunneling events by an external dissipative electromagnetic environment. We demonstrate that the rates for energetically unfavorable tunneling events saturate to finite values at low temperatures, and that the saturation level can be suppressed by more than an order of magnitude by a capacitive shunt near the device. The findings are important in assessing the performance limits of any single-electronic device. In particular, master-equation-based simulations show that the electromagnetic environment realized in the capacitively shunted devices allows for a metrologically accurate charge pump based on hybrid tunnel junctions.

An array of SIN tunnel junctions as a sensitive thermometer

We have fabricated and measured an array of superconductor–insulator–normalmetal (SIN) tunnel junctions for the purpose of using it as asensitive thermometer. An increase in the temperature responsivity ofdV/dT∼5 µV mK−1 for ten junctionsis observed from dV/dT∼1 µV mK−1 for a single junction. We then used such an array thermometer to measure the temperature stabilityof the Heliox AC-V cryogen-free refrigerator. We have been able to measure a temperature stabilityof ± 250 µK over a period of 8 h with a temperature resolution of ± 100 µK.

An array of 100 Al–Al2O3–Cu SIN tunnel junctions in direct-write trilayer technology

We present superconductor–insulator–normal metal (SIN) tunnel junction thermometers made of arrays of4–100 Al–Al2O3–Cu SIN tunnel junctions fabricated in direct-write technology. The technology is based onin situ evaporation of the superconductive electrode followed by the oxidation and thenormal counter-electrode as a first step and deposition of normal metal absorber as asecond one.This approach allows one to realize any geometry of the tunnel junctions and of theabsorber with no limitation related to the size of the junctions or the absorber, which is notpossible using the shadow evaporation technique.Measurements performed at 300 mK showed the high quality ofthe fabricated tunnel junctions, low leakage currents, and that anRd/Rn ratio of 500 has been achieved at that temperature.The junctions were characterized as temperature sensors, andvoltage versus temperature dependence measurements showed adV/dT of0.5 mV K−1 for each single junction, which is typical for this kind of tunnel junction. A temperature resolutionof ± 5 µK has been achieved which is much better than the previously reported value of ± 30 µK for this type of thermometer.

Temporal measurement of hot-electron relaxation in a phonon-cooled metal island

We report temporal measurements of the dynamic electronic temperature and the electron-phonon thermal relaxation rate in a micron-scale metal island, with an electronic heat capacity of order 1 fJ/K $(C\ensuremath{\sim}{10}^{7}{k}_{B}).$ We employed a superconductor--insulator--normal-metal tunnel junction, embedded in a radio-frequency resonator, as a fast $(\ensuremath{\sim}20\mathrm{MHz})$ thermometer. A resistive heater coupled to the island allowed us to pulse the electronic temperature well above the phonon temperature. Using this device, we have determined the thermal relaxation rate of a hot-electron population in a thin normal-metal film with a measurement bandwidth that exceeds the low-temperature thermal bandwidth.

Effective electron microrefrigeration by superconductor–insulator–normal metal tunneljunctions with advanced geometry of electrodes and normal metal traps

We demonstrate effective electron cooling of the normal metal strip bysuperconductor–insulator–normal metal (SIN) tunnel junctions. The improvement wasachieved by two methods: first, by using an advanced geometry of the superconductingelectrodes for more effective removal of the quasiparticles; and second, by adding a normalmetal trap just near the cooling junctions. With simple cross geometry and without normalmetal traps, the decrease in electron temperature is 56 mK. With the advanced geometry ofthe superconducting electrodes, the decrease in electron temperature is 129 mK. With theaddition of the normal metal traps, the decrease in electron temperature is 197 mK.

Electrical and thermal modeling of the non-Ohmic differential conductance in a tunnel junction containing a pinhole

To test the quality of a tunnel junction, one sometimes fits the bias-dependent differential conductance to a theoretical model, such as Simmons’s formula. Recent experimental work by Åkerman and collaborators, however, has demonstrated that a good fit does not necessarily imply a good junction. Modeling the electrical and thermal properties of a tunnel junction containing a pinhole, we extract an effective barrier height and effective barrier width even when as much as 88% of the current flows through the pinhole short rather than tunneling. A good fit of differential conductance to a tunneling form therefore cannot rule out pinhole defects in normal metal or magnetic tunnel junctions.

Magnon-assisted transport and thermopower in ferromagnet–normal-metal tunnel junctions

We develop a theoretical model of magnon-assisted transport in a mesoscopic tunnel junction between a ferromagnetic metal and a normal (non-magnetic) metal. The current response to a bias voltage is dominated by the contribution of elastic processes rather than magnon-assisted processes and the degree of spin polarization of the current, parameterized by a function $P (\Pi_{\uparrow (\downarrow)},\Pi_{N})$, $0 \leq P \leq 1$, depends on the relative sizes of the majority $\Pi_{\uparrow}$ and minority $\Pi_{\downarrow}$ band Fermi surface in the ferromagnet and of the Fermi surface of the normal metal $\Pi_{N}$. On the other hand, magnon-assisted tunneling gives the dominant contribution to the current response to a temperature difference across the junction. The resulting thermopower is large, $S \sim - (k_B/e) (k_BT/\omega_{D})^{3/2} P (\Pi_{\uparrow (\downarrow)},\Pi_{N})$, where the temperature dependent factor $(k_{B}T/\omega_{D})^{3/2}$ reflects the fractional change in the net magnetization of the ferromagnet due to thermal magnons at temperature $T$ (Bloch's $T^{3/2}$ law) and $\omega_{D}$ is the magnon Debye energy.

Nanoscale radio-frequency thermometry

We experimentally demonstrate the high bandwidth readout of a thermometer based on a superconductor–insulator–normal metal (SIN) tunnel junction, embedded in a rf resonant circuit. Our implementation enables basic studies of the thermodynamics of mesoscopic nanostructures. It can also be applied to the development of fast calorimeters, as well as ultrasensitive bolometers for the detection of far-infrared radiation. We discuss the operational details of this device, and estimate the ultimate temperature sensitivity and measurement bandwidth.

Fabrication and optical characterization of the normal metal hot-electron microbolometer with Andreev mirrors

The normal metal hot-electron microbolometer (NHEB) is a direct detector for mm and sub-mm wavelengths. Theoretical estimations indicate that such a device should be able to achieve a sensitivity of around 10 −17 W/(Hz) 1/2 at 0.3 K and a time constant less than 1 μs. The temperature of the electron gas is monitored using superconductor–insulator–normal metal (SIN/NIS) tunnel junctions. We have fabricated such an NHEB where the normal metal strip is coupled to a double slot antenna for optical qualification (normal metal strip size (5.6×0.25×0.03) μm 3). The antenna is designed for 300 GHz and has an impedance of 30 Ω at the center frequency. The actual optical responsitivity and the response time of our NHEB has been measured at 0.5 K using a hot/cold load (black body radiation at room temperature and at liquid nitrogen temperature). DC-current measurements performed on other samples earlier indicate that our NHEB can achieve an electrical noise equivalent power (NEP) of at least 3×10 −16 W/(Hz) 1/2. We conclude that the optical responsitivity value obtained in the measurement is consistent with the electrical NEP value at the same temperature and in agreement with the estimated efficiency of the quasioptical coupling.

Analytic Results for the Cotunneling Current in the Double Junction System in Series With RC Line

We study the effect of the finite RC line on the electron cotunneling through two serially connected small normal-metal tunnel junctions. We obtain the analytic expressions for the cotunneling current in the cases of small and large line resistance compared to the quantum resistance.

Coulomb-blockade transport in single-crystal organic thin-film transistors

Coulomb-blockade transport--whereby the Coulomb interaction between electrons can prohibit their transport around a circuit--occurs in systems in which both the tunnel resistance, Rb between neighbouring sites is large (>>h/e2) and the charging energy, E(C) (E(C) = e2/2C, where C is the capacitance of the site), of an excess electron on a site is large compared to kT. (Here e is the charge of an electron, k is Boltzmann's constant, and h is Planck's constant.) The nature of the individual sites--metallic, superconducting, semiconducting or quantum dot--is to first order irrelevant for this phenomenon to be observed. Coulomb blockade has also been observed in two-dimensional arrays of normal-metal tunnel junctions, but the relatively large capacitances of these micrometre-sized metal islands results in a small charging energy, and so the effect can be seen only at extremely low temperatures. Here we demonstrate that organic thin-film transistors based on highly ordered molecular materials can, to first order, also be considered as an array of sites separated by tunnel resistances. And as a result of the sub-nanometre sizes of the sites (the individual molecules), and hence their small capacitances, the charging energy dominates at room temperature. Conductivity measurements as a function of both gate bias and temperature reveal the presence of thermally activated transport, consistent with the conventional model of Coulomb blockade.

Quasiparticle and Josephson tunneling of overdopedBi2Sr2CaCu2O8+δsingle crystals

The point contact tunneling technique is used to examine quasiparticle and Josephson currents in overdoped ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi-2212) single crystals with bulk ${T}_{c}$ values ranging from 82 K down to 62 K. Superconductor--insulator--normal-metal (SIN) tunnel junctions are formed between Bi-2212 crystals and a Au tip, which display well-resolved quasiparticle gap features including sharp conductance peaks. Reproducible superconductor-insulator-superconductor (SIS) tunnel junctions are also obtained between two pieces of the Bi-2212 crystals, resulting in simultaneous quasiparticle and Josephson currents. The dynamic conductances of both SIN and SIS junctions are qualitatively similar to those found on optimally doped Bi-2212, but with reduced gap values, e.g., $\ensuremath{\Delta}=15--20\mathrm{meV}$ for ${T}_{c}=62\mathrm{K}.$ Fits to the conductance data in the gap region are obtained using a model with ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ symmetry, and it is shown that this provides a better fit than s-wave symmetry. Both SIN and SIS tunneling conductances also display dip and hump features at high bias voltages similar to those found on optimal and underdoped crystals, indicating that these are intrinsic properties of the quasiparticles. The SIS data indicate that these features appear to be part of a larger spectrum that extends out to 300--400 mV. The Josephson current has been measured for 13 SIS junctions on the 62 K crystals with resistances varying over two decades. It is found that the maximum value depends on junction resistance in a manner consistent with Ambegaokar-Baratoff theory, but with a reduced ${I}_{c}{R}_{n}$ product.

Tunneling spectroscopy of Tl 2Ba 2CuO 6

New results from tunneling spectroscopies on near optimally doped single crystals of Tl 2Ba 2CuO 6 (Tl-2201) junctions are presented. The superconductor–insulator–normal metal (SIN) tunnel junctions are obtained using the point-contact technique with a Au tip. The tunneling conductances reproducibly show a sharp cusp-like subgap, prominent quasiparticle peaks with a consistent asymmetry, and weakly decreasing backgrounds. A rigorous analysis of the SIN tunneling data is performed using two different models for the d x 2– y 2 (d-wave) density of states (DOS). Based on these and earlier results, the tunneling DOS of Tl-2201 has exhibited the most reproducible data that are consistent with a d-wave gap symmetry. We show that the dip feature at 2 Δ that is clearly seen in SIN tunneling data of Bi 2Sr 2CaCu 2O 8+ δ is also present in Tl-2201, but at a weaker level. The gap values for crystals with a bulk T c=86 K are in the range of 19–25 meV.

Intrinsic limits of thermometers based on Coulomb charging effect in arrays of tunnel junctions

Arrays of normal metal tunnel junctions have been found to be suitable for a thermometer in the regime where thermal energy is comparable to charging energy. Using a functional integral approach, we have calculated the quantum conductance of an array of asymmetric double junctions including the effect of background charges. Our theoretical results allow us to investigate the influence of asymmetry and background charge, as well as the nonlinear correction to the linear calibration formula for the Coulomb blockade thermometer. We have also analyzed the current noise spectra and discovered that the noise to conductance ratio is proportional to temperature. Hence, this ratio provides a new kind of Coulomb blockade thermometer.