Complete Analysis of STJ Detector Performance via Absorption of Phonon Pulses

For the first time a complete analysis of the tunnel and loss parameters of superconducting tunnel junction photon detectors has been made solely by the use of nanosecond phonon pulse excitation. Previously only a partial characterization, requiring supplementary information from photo-excitation measurements, was possible. The present results have been achieved by a more realistic model for the energy spectrum of the phonon pulses and greatly improved (nanosecond) time resolution of the detected signal. The value determined for the tunnel rate is in good agreement with calculations based on the device layer structure. It is believed that the relatively high values of loss time obtained are the result of trap-enhanced recombination due to the high quasiparticle densities attained in the experiments.      

Bias Voltage Dependence of Quasiparticle Recombination in STJ Detectors with Killed Electrode

Superconducting tunnel junction X-rays detectors Ti/Nb/Al/AlO _x /Al/Nb/NbN with the Ti/Nb/Al/ killed electrode has been studied under irradiation by X-rays photons of different energies produced by the fluorescence method. The nonlinearity of the STJ-detector response versus photon energy has been studied as a function of the bias voltage. The minimum of the nonlinearity is observed when the tunneling probability P ₁ has maximum and the detector signals have shortest rise times. The experimental data were analyzed on the basis of the diffusion model taking into account the quasiparticle self-recombination. The simple approximate expression for the self-recombination contributions to STJ-detector signal was obtained. The nonlinearity of the response depends on the ratios of the recombination constant to the diffusion coefficient R/D and the diffusion length to the initial radius of the quasiparticle distribution Λ/a ₀ and is inverse proportional to the thickness of the electrode.      

Quasiparticle Edge Losses in Double STJs Strip X-Rays Detectors

The 2D diffusion model of the strip X-ray detector was developed. The detector consists of a long superconducting strip, which is ended by the trapping layers and superconducting tunnel junctions at each end. The model takes into account the diffusion of the excess quasiparticles, quasiparticle trapping at the tunnel junctions and quasiparticle losses in the volume of the strip and at the strip boundaries. The analytical solution was obtained. It has been shown that quasiparticle losses at the strip boundaries caused the dependence of the signals on the photon absorption site in transverse direction. The latter worsens the energy resolution and transforms the spectral line of the detector to nongaussian shape.      

Time Dependence of Partial Integrated Charge Fluctuations in STJs

Multiple tunneling of quasiparticles in a superconducting tunnel junction (STJ) detector increases the fluctuations in the measured charge. To optimize the energy resolution of an STJ detector, it is necessary to know the time dependence of the integrated charge and its noise. In this work, the theory of branching cascade processes was applied to the process of quasiparticle multitunneling. The duality nature of quasiparticles and the coupling of the quasiparticle and the phonon subsystems were taken into account. The formulae for time dependences of the mean value and the relative variance of the integrated charge were derived. Influence of a shaping amplifier on the relative variance of an STJ detector was also considered.     

Influence of the Quasiparticle and Phonon Subsystem Coupling on the Multitunneling in STJ Detectors

The scope of this work is to account for the phonon exchange in the process of multitunneling in superconducting tunnel junction (STJ) detector. The theory of branching cascade processes was applied to the process of multitunneling in STJ. The duality nature of quasiparticles and the coupling of the quasiparticle and phonon subsystems were taken into account. The extreme cases of the general formula were analyzed. It was shown that the general formula encloses all specific cases published in literature.     

The role of epitaxy in superconducting tunnel junction detectors

We have experimentally analysed the use of epitaxial and polycrystalline tantalum trapping layers in tunnel junction detectors, using epitaxial niobium based double tunnel junction devices. We have shown that the trapping rate is enhanced by having a low mean-free-path in the Ta trap film and an epitaxial Nb absorber film. This effectively increases the proportion of time spent by a quasiparticle in the trap film. Phonon-quasiparticle scattering, which reduces the effective trapping rate, has been observed.     

Effect of electron-beam irradiation on superconducting films

Scanning electron microscopy performed at cryogenic conditions can be used for the two-dimensional display of various properties of superconducting thin-film samples. The Rothwarf-Taylor phenomenological equations are used to treat the spatial and temporal decay of the perturbation generated by the electron-beam irradiation. In the limit of high phonon trapping in the superconducting film, the spatial extension of the perturbed area determining the resolution of this imaging method is given by the thermal healing length. In the case of low phonon trapping, however, the healing length is given by the quasiparticle diffusion length. Also calculated is the characteristic time scale of the temporal decay after switching off the electron-beam irradiation. Modulating the electron beam at frequencies high compared to the inverse of this characteristic time can reduce the healing length considerably below the low-frequency value.     

Superconducting Nb-Ta-Al-AlOx-Al tunnel junctions for x-ray detection

We report progress on the microlithographic fabrication of Nb-Ta-Al-AlOx-Al structures designed for x-ray detection. These structures use bandgap engineering both for quasiparticle trapping to increase the collection efficiency and to prevent quasiparticle diffusion out through the leads. Non-standard tunnel junction geometries are used to reduce the magnetic field needed to suppress the Josephson current for stable biasing. The performance of these devices as alpha particle detectors is presented.     

Superconducting tunnel junctions and quasiparticle trapping

The interaction of a nuclear particle or X-ray with a superconductor leads to the breaking of Cooper pairs and the creation of excess phonons and quasiparticles. The basic physics and the use of superconducting tunnel junctions as detectors of excess quasiparticles are reviewed. For superconducting absorbers of appreciable mass intrinsic limitations require the use of the phenomenon of quasiparticle trapping. The relaxation phonons released in the trapping process can also be detected and they can lead to amplification via further pair breaking. Superconducting tunnel junctions can also detect phonons produced by particle interactions in a substrate absorber. The use of series-connected arrays of junctions to achieve high sensitivity and good energy resolution is discussed.     

The constriction model for SNS Josephson junctions

The approach developed by Blonder, Tinkham and Klapwijk (BTK) for calculation of the quasiparticle (qp) current in NcS contacts is generalized to the case of ScNS and SNcNS contacts with disordered NS electrodes. Amplitudes of Andreev and normal reflections at the ballistic constriction are calculated. The relation between the qp current and energy spectrum of NS proximity sandwich is found for arbitrary transparencies of the constriction and NS interfaces. The appearance of new series of subharmonic peaks associated with the two-gap structure in the density of states of NS electrodes is demonstrated. The model is also applied to calculation of the critical and the excess currents in ScNS and SNcNS for different interface parameters. The relevance of the model to transport mechanisms in step-edge HTS SNS contacts is discussed.     

Energy Resolution of STJ X-Ray Detectors with Killed Electrode. Simple Model

Superconducting tunnel junctions X-ray detectors Ti/Nb/Al,AlO _x /Al/Nb(2)/NbN with killed Ti/Nb electrode was studied as a function of bias voltage, energy of the absorbed quantum, and thickness of the Nb(2) layer. The data was compared with a simple diffusion model including the losses of excess quasiparticles due to self-recombination. It was shown that increasing of the electrode thickness reduces the self-recombination contribution and improves the linearity of the detector response.     

Quasiparticle Dynamics in FeSe Superconductors Studied by Femtosecond Spectroscopy

The ultrafast quasiparticle dynamics in FeSe single crystals were measured by using dual-color transient reflectivity measurements (ΔR/R) from 4.4 to 290 K. In general, the typical ΔR/R of FeSe includes two significant components. One is the relaxation of photoinduced quasiparticles, which has been used to estimate the electron–phonon coupling strength (λ=0.16). The other is the oscillation component due to the acoustic phonon. Moreover, the acoustic phonon’s energy estimated from the period of oscillation in ΔR/R markedly shrinks around 90 K, which is the so-called phonon softening.     

Development of a TES-Based Anti-coincidence Detector for Future x-Ray Observatories

Microcalorimeters onboard future x-ray observatories require an anti-coincidence detector to remove environmental backgrounds. In order to most effectively integrate this anti-coincidence detector with the main microcalorimeter array, both instruments should use similar read-out technology. The detectors used in the Cryogenic Dark Matter Search (CDMS) use a phonon measurement technique that is well suited for an anti-coincidence detector with a microcalorimeter array using SQUID readout. This technique works by using a transition-edge sensor (TES) connected to superconducting collection fins to measure the athermal phonon signal produced when an event occurs in the substrate crystal. Energy from the event propagates through the crystal to the superconducting collection fins, creating quasiparticles, which are then trapped as they enter the TES where they produce a signal. We are currently developing a prototype anti-coincidence detector for future x-ray missions and have recently fabricated test devices with Mo/Au TESs and Al collection fins. We present results from the first tests of these devices which indicate a proof of concept that quasiparticle trapping is occurring in these materials.     

Quasiparticle loss rates in Ta-based superconducting tunnel junctions

The quasiparticle (QP) lifetime in superconducting tunnel junctions (STJs) at sufficiently low temperature is usually found to be governed by loss processes associated with edges, interfaces or contacts. Such losses are closely related to fabrication issues and difficult to control. In our Ta-based STJs we observe variations in pulse decay time up to a factor of 2–3 for nominally the same devices. Nevertheless, experiments with STJs in which the thickness of the Al layers adjacent to the barrier is progressively increased, show a clear trend of increasing QP lifetime. Alternatively, introducing Nb capping layers below and on top of the Ta STJs gives a significant reduction of QP lifetime.     

Electron–Phonon Coupling in Ti/TiN MKIDs Multilayer Microresonator

Over the last few years, there has been a growing interest toward the use of superconducting microwave microresonators operated in quasi-thermal equilibrium mode, especially applied to single particle detection. Indeed, previous devices designed and tested by our group with X-ray sources in the keV range evidenced that several issues arise from the attempt of detection through athermal quasiparticles produced within direct strikes of X-rays in the superconductor material of the resonator. In order to prevent issues related to quasiparticles self-recombination and to avoid exchange of athermal phonons with the substrate, our group focused on the development of thermal superconducting microresonators. In this configuration, resonators composed of multilayer films of Ti/TiN sense the temperature of an absorbing material. To maximize the thermal response, low-critical-temperature films are preferable. By lowering the critical temperature, though, the maximum probing power bearable by the resonators decreases abruptly because of the weakening of the electron–phonon coupling. A proper compromise between the value of critical temperature (and hence sensitivity to energy deposition) and readout power bearable by the device has to be found in order to avoid signal-to-noise ratio degradation. In this contribution, we report the latest measurement of the electron–phonon coupling.     

The effect of hotspot formation on quasiparticle yields in superconducting aluminium films

Existing models show that any hotspot created in an Al film following a particle interaction is rapidly diluted. Quasiparticles and phonons decouple at high energies so that on short time-scales quasiparticle diffusion determines the energy transport. Few quasiparticles have energies near the energy gap and there is little gap suppression. We have measured the initial quasiparticle yields in very thin Al films and find somewhat surprisingly that this yield is reduced. It appears that a short time-scale quasiparticle loss mechanism, probably associated with localised over-injection, exists as quasiparticles scatter to low energies within the phonon hotspot.     

Thermal resistance between phonon and quasiparticle excitations in dilute mixtures of3He in4He

As part of a Kapitza resistance study, the thermal resistance between the phonon and³He quasiparticle excitations in three dilute³He-⁴He mixtures (0.03%, 0.1%, and 0.3%³He) has been measured over the temperature range 20–50 mK. The results are compared with a theory based upon the Baym-Ebner interaction between the excitations. The temperature dependences agree, but the measured resistance is smaller than the theoretical resistance by a factor of 2 that varies inversely with concentration. This concentration-dependent phonon-quasiparticle coupling discrepancy is consistent with the results of recent heat pulse absorption experiments by Husson et al. An indirect measurement was made of the same thermal resistance in³He-⁴He mixtures confined within the pores of a submicrometer sintered copper powder, with the results confirming a size-effect model for the resistance.     

Heating of Superconducting Tunnel Junction Detector by X-Ray Flux

Superconducting tunnel junction (STJ) detectors are in common use for high-resolution soft X-ray spectroscopy at high count rates. Each quasiparticle in superconductor should be treated as quantum superposition of electron-like and hole-like excitations. This duality nature of quasiparticle leads to multitunneling in STJ. Because of the multitunneling process, one quasiparticle can transfers into heat the energy that equals to the difference in quasiparticle energy between two electrodes more than once. As a result, the energy transferred into heat in STJ detector is several times grater the energy of X-ray quantum. In this work, the theory of branching cascade processes is applied to the process of energy transfer caused by quasiparticle multitunneling. The mean and the variance of STJ temperature shift from the substrate temperature caused by heating of STJ by X-rays flux were derived.     

The effect of quasiparticle self-generation on the performance of small gap multiple tunnelling STJs

Due to the slowness of the electron–phonon interaction in small gap STJs currently being developed as the latest generation of photon detectors, during the device operation a broad statistical distribution of interacting quasiparticles and phonons is established. This gives rise to new physical phenomena and also renders the traditional Rothwarf–Taylor model completely inadequate for the analysis of quasiparticle and phonon dynamics for all traditional superconductors except unproximised Nb. We have developed a new kinetic approach replacing the Rothwarf–Taylor scheme. We demonstrate the results of experimental and theoretical studies of the new physical phenomenon of quasiparticle self-generation in a heavily proximised multiple tunneling Ta/Al STJ and discuss the implications of the observed quasiparticle self-generation for photon detection.     

Aluminum thickness dependence of spatial profile in niobium-based superconducting tunnel junctions

Spatial profiles of low-temperature detectors can be measured with Low-Temperature Scanning Synchrotron Microscopy directly. The dependence of the spatial profiles on the bias current, the magnetic field strength, and the size of junctions have been already studied in previous reports. In this study, we fabricated Nb-based junctions having Al layers of different thicknesses, which are located at the both sides of the tunneling barrier, by using a lift-off technique. It has been found that the spatial uniformity is improved by increasing the Al thickness. Therefore, it is demonstrated that the Al layers play a important role more than quasiparticle trapping.