Energy resolution of strip superconducting detectors with two tunnel junctions

A mathematical model of a strip X-ray detector has been developed. The detector consists of a long superconducting strip with two tunnel junctions at both ends. The model developed takes into account the diffusion of nonequilibrium quasiparticles, their capture to the region of the tunnel junctions, and quasiparticle losses in the bulk and at the boundaries of the superconducting strip. Analytical solutions have been obtained for the boundary conditions of the third kind. It is shown that the quasiparticle losses at the strip boundaries lead to a dependence of the detector signal amplitude on the transverse coordinate of the photon absorption point and deteriorate the energy and spatial resolution. The spectral line has been calculated with allowance for the contribution of electronic noise.     

Electronic cooling in superconducting tunnel junctions

The cooling power provided by the current through a superconductor/insulator/superconductor tunnel junction is studied theoretically. The influence of non-equilibrium distributions of the quasi-particles on the heat flow is analysed within a simple relaxation model. A superconducting gap enhancement can be explained within the equilibrium as well as the non-equilibrium model.     

Superconducting tunnel junctions as very fast detectors

Superconducting tunnel junction detectors biassed on the gap edge have a response time of the order of 1 ns and a sensitivity comparable with that achieved in the conventional mode of operation.     

Subharmonic gap structures in asymmetric superconducting tunnel junctions

Tunneling in the asymmetric superconducting junctions of PbIn and PbTl shows sharp current steps at voltages eV = Δ₁ + Δ₂,$\text{2Δ}{}_2\text{2n}\text{and}\text{2Δ}{}_1\text{2n}$ (structures for n > 1 not readily observed); 2Δ₁ and 2Δ₂ are the energy gaps and n is an integer. We obtain new results for the relative magnitude of the structures, where the larger of the two structures is observed at the larger of the two voltages $\text{Δ}{}_1\text{ρ}\text{and}\text{Δ}{}_2\text{ρ}$. These results, in contrast to other available experimental results on different superconductors, seem to exclude self-coupling theory as the mechanism.     

High-performance electronic cooling with superconducting tunnel junctions

When biased at a voltage just below a superconductor's energy gap, a tunnel junction between this superconductor and a normal metal cools the latter. While the study of such devices has long been focused to structures of submicron size and consequently cooling power in the picowatt range, we have led a thorough study of devices with a large cooling power up to the nanowatt range. Here we describe how their performance can be optimized by using a quasi-particle drain and tuning the cooling junctions' tunnel barrier.     

Phonon emission spectra of superconducting tunnel junctions

Resonant scattering of phonons by the stress split acceptor ground state in Si:B has been used to investigate the phonon emission spectra of superconducting tunnel junctions. Symmetric Sn, Pb and PbBi alloy junctions have been studied in the single particle tunneling regime. The spectra show many features as expected from relaxation and recombination of quasiparticles via one-phonon emission. The results are compared with the steady state solutions of the kinetic equations for quasiparticles and phonons. Spatially homogeneous distribution functions are assumed. This approach describes the spectra of tin junctions fairly well. However, there are marked deviations for Pb and PbBi junctions which are attributed to a decay of transverse phonons within the junction.Dedicated to K. Dransfeld on the occasion of his 60th birthday     

Two-dimensional imaging of the current-density distribution in superconducting tunnel junctions

Using a scanning electron microscope equipped with a low-temperature stage, we show that under the electron irradiation a superconducting tunnel junction can operate regularly. The electron beam generates a voltage signal across the current-biased junction. By recording the small voltage change synchronously with the coordinate irradiated by the beam, a two-dimensional “voltage image” of the density distribution of the junction tunneling current can be obtained. The distributions are in agreement with the magnetic interference patterns of the dc-Josephson currents in the junctions.     

Quasiparticle relaxation and phonon emission in superconducting tunnel junctions

Using SnISn tunnel junctions as generators and detectors of high frequency phonons, the temperature dependence (down to 0.3°K) of the derivative of the detected signal as a function of generator bias is reported. For T?1°K we have observed the expected BCS singularity in the signal at a generator bias of 4Δ. An additional singularity has been observed at a bias of 2Δ when the number of injected particles greatly exceeds the number thermally excited. Unlike the 4Δ peak, the temperature of occurrence of the 2Δ peak depends markedly on generator impedance, (injection rate) as expected. The data provide strong evidence that Tewordt's model (strictly valid for T = 0°K), in which an excited quasiparticle relaxes predominantly in a single step to the gap edge before recombination, is correct in the temperature range $\text{T}\text{T}{}_{\mathrm{c}}\text{< 0.3}$ and for low injection rates.     

Dynamics of radiation induced quasiparticles in superconducting tunnel junction detectors

In order to achieve a high resolution in the spectroscopy of low energy X-rays, detectors based on superconducting tunnel junctions as sensors are presently investigated. The knowledge of the processes affecting the signal generation in such sensors is essential for the interpretation of the detector response. Starting from a diffusion model including decay and tunneling of excess quasiparticles in the metal layers of a superconducting tunnel junction detector, the detector response is determined as a function of absorption position and of rate constants. Model predictions agree very well with experimental data. The advantages of a detector employing quasiparticle trapping are pointed out and the parameters determining the signal gain are deduced. The linearity of the detector signal is much more affected by pair recombination of the quasiparticles during their tunneling rather than during their diffusive propagation into the tunneling region.     

Electron and phonon effects in superconducting tunnel detectors of x-radiation

Nb/Al/AlO_x/Nb superconducting tunnel junctions were investigated in the role of x-ray detectors. Amplitude spectra of pulses arising upon irradiation of tunnel junctions of different sizes by ⁵⁵Mn x-radiation were recorded at a temperature T=1.4 K. We also analyzed the temporal shape of the pulses. We considered the influence of diffuse motion of nonequilibrium quasiparticles, the inverse tunneling effect, and exchange of 2Δ phonons between electrodes, on the characteristics of the tunnel detectors. It is shown that phonon processes can bring about changes in the amplitude, duration, and polarity of the signal. Fiz. Tverd. Tela (St. Petersburg) 41, 1168–1175 (July 1999)     

Quantum coherent effects and Zener tunneling in superconducting tunnel junctions

We calculate the current-voltage characteristics of a small capacitance underdamped superconducting tunnel junction and find the value of the critical current which corresponds to switching between coherent voltage oscillations and uncorrelated single electron tunneling. Both Zener tunneling and dissipative relaxation are important in this context.     

Direct detection at submillimetre wavelengths using superconducting tunnel junctions

Superconducting tunnel-junction direct detectors are considered in some detail. For frequencies below twice that of the gap there is some bias voltage for which the input impedance is real, the responsivity quantum limited, and the dynamic range high. A susperconducting detector saturates for two reasons: intrinsic saturation due to the relative increase in two-photon tunnelling processes, and extrinsic saturation due to the input match changing with bias voltage. The responsivity of a detector with a resistive RF source is least sensitive to bias-voltage changes and has the greatest dynamic range when operating with a sloping load line. In the case of an inductive source, the dynamic range can be higher than the intrinsic saturation rate would suggest. Ideally, superconducting tunnel-junction detectors should be biased in a constant-voltage mode. If the responsivity is to be depressed by no more than a few percent, the photon step should have a height which is no more than one quarter of the total current turn-on at the gap. Superconducting direct detectors can be used to make precise and well-calibrated optical measurements at submillimetre wavelengths.     

On the properties of photovoltaic X-ray detectors based on GaAs epitaxial structures

Operation of a new photovoltaic detector of X-ray bremsstrahlung based on GaAs epitaxial structures at room temperature without bias is studied. The efficiency of the absorbed energy conversion into short-circuit current is calculated from the measured photoresponses for the photon energies in the range from 12 to 120 keV. In this energy range, the absorption in GaAs is governed by photoelectric effect. The efficiency of the X-ray bremsstrahlung energy conversion in GaAs peaks at 80 keV. It is suggested that the X-ray absorption of a thin 50-μm detector can be enhanced by applying an inclined irradiation scheme. The effect is most pronounced in the region of hard X-rays.     

Imaging of spatial structures in superconducting tunnel junctions by electron-beam scanning

The voltage change caused in a current-biased superconducting tunnel junction by scanning the junction surface with an electron beam can serve to generate a two-dimensional image of spatial structures within the specimen. Depending upon the bias point during the scanning process, an image of local variations in the tunneling current density due to variations of the tunnel barrier resistance or in the energy gaps of the electrodes is obtained. PbAuIn/PbBi cross-line and SiO-window junctions have been used to demonstrate this imaging technique.     

Nb/Al-AlO_x/Nb超导隧道结的制备

在高阻硅衬底上采用光刻、直流磁控溅射、反应离子刻蚀(RIE)、等离子体增强化学气相沉积法(PECVD)等方法研究制备了高质量的Nb/Al-Al Ox/Nb超导隧道结。在4.2K下,测量了直径8μm的圆形结样品,得到临界电流密度约为1.6k A/cm2,漏电流约为50μA。制结工艺流程的重复性较好。     

Determination of the quasiparticle energy distribution in superconducting tunnel junctions under microwave irradiation

The influence of microwave irradiation on the quasiparticle energy distribution in a superconductor is determined by unfolding the change of the current-voltage characteristics of small tunnel junctions. The results are in good agreement with the theoretical predictions by Chang and Scalapino.     

Fast nonequilibrium induction detectors based on thin superconducting films

A new type of fast detector is proposed. The operation of the detector is based on the change induced in the kinetic inductance of NbN and YBa₂Cu₃O_7−δ superconducting films by nonequilibrium quasiparticles produced by electromagnetic radiation. The speed of a NbN detector is essentially temperature-independent and is less than 1 ps. A model based on the Omen-Scalapino scheme describes well the experimental dependence of the voltage-power sensitivity of a NbN detector on the modulation frequency of the radiation. A low equilibrium quasiparticle density and a high quantum yield give detecting power D*=10¹²W⁻¹ · cm · Hz^1/2 at temperature T=4.2 K and D*=10¹⁶ W⁻¹ · cm · Hz^1/2 at temperature T=1.6 K. The time constant of the low-temperature YBaCuO induction detector is determined only by the electron-phonon interaction time τ _e-ph ^d in the nodal regions. Zh. Tekh. Fiz. 68, 63–69 (October 1998)