Local trap spectroscopy in superconducting tunnel junctions

Kozorezov, A. G.; Wigmore, J. K.; Peacock, A.; Poelaert, A.; Verhoeve, P.; Hartog, R. den; Brammertz, Guy

doi:10.1063/1.1377624

Cited by 21 publications

(33 citation statements)

References 12 publications

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“…22,23 We showed earlier 24,25 that the kinetics of qps and phonons in nonequilibrium superconductors cannot be adequately described without taking explicit account of the interaction between the mobile qps and phonons and the trapped qps. The microscopic nature of the defects responsible for the trapping states in a particular superconductor is often uncertain.…”

Section: Kinetic Description Of Nonequilibrium Quasiparticles Andmentioning

confidence: 99%

Dynamics of nonequilibrium quasiparticles in narrow-gap superconducting tunnel junctions

et al. 2008

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The latest generation of high quality, narrow gap, superconducting tunnel junctions ͑STJs͒ exhibits a steadystate and time-dependent behavior which cannot be described satisfactorily by previous treatments of nonequilibrium quasiparticle ͑qp͒ dynamics. These effects are particularly evident in experiments using STJs as detectors of photons, over the range from near infrared to x ray. In this paper, we present a detailed theoretical analysis of the spectral and temporal evolution of the nonequilibrium qp and phonon distributions in such STJs excited by single photons, over a wide range of excitation energy, bias voltage, and temperature. By solving the coupled set of kinetic equations describing the interacting excitations, we show that the nonequilibrium qp distribution created by the initial photoabsorption does not decay directly back to the initial undisturbed state in thermal equilibrium. Instead, it undergoes a rapid adiabatic relaxation to a long-lived, excited state, the spectral distribution of which is nonthermal, maintained by a balance between qp creation, recombination, and trapping. The model is able to describe successfully photoabsorption data taken on several different aluminum STJs, using a single set of parameters. Of particular note is the conclusion that the local traps responsible for qp loss are situated specifically in the region of Nb contacts.

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Section: Kinetic Description Of Nonequilibrium Quasiparticles Andmentioning

confidence: 99%

Dynamics of nonequilibrium quasiparticles in narrow-gap superconducting tunnel junctions

et al. 2008

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“…A second experiment highlighting the role of QP trapping is the measurement of the temperature dependence of responsivity, and therefore, of QP lifetime, 11 as illustrated in Fig. 11 for x-ray and optical photons for a tantalum sample, with the data for both normalized to the value at 600 mK showing the effects both of detrapping and thermal recombinations.…”

Section: Analysis Of Existing Experimental Datamentioning

confidence: 99%

“…15͒ so that trap ϳ 1.8/ c ns. The observed lifetimes in Ta, at such low temperatures that thermal recombination is absent, are typically several tens of microseconds 11,[18][19][20][21][22][23] leading to an estimate for c of between 1 ϫ 10 −5 and 1 ϫ 10 −4 or a range of impurity concentration of 10-100 ppb. Thus even if the state were not strongly localized, for example, say ͑a 2 / a 0 ͒ 4 Ӎ 100, in order to produce the observed QP lifetimes in Ta at low temperature, it would require a concentration of only 1-10 ppm.…”

Section: B Continuum-bound Trappingmentioning

confidence: 99%

“…Finally, activation of localized QPs into the continuum may result in an anomalous temperature dependence of observable parameters characterizing the nonequilibrium state, such as QP lifetime. 11 It might be expected that any such effects of isolated magnetic impurities in superconductors, while being of significant academic interest, could hardly influence the macroscopic properties of a nominally pure device or sample. However, an important conclusion from our calculations is that the QP processes described above show up at much lower impurity concentrations than has been previously realized.…”

Section: Introductionmentioning

confidence: 99%

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Inelastic scattering of quasiparticles in a superconductor with magnetic impurities

et al. 2008

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We show that inelastic scattering of quasiparticles by trace concentrations of magnetic impurities may result in significant changes in the nonequilibrium properties of superconductors. We used the approach of Müller-Hartmann and Zittartz to model Kondo scattering of conduction electrons by the magnetic impurities, and hence, to calculate the rates of ͑i͒ quasiparticle trapping into the localized impurity states, ͑ii͒ trap-enhanced recombination, ͑iii͒ pair breaking, and ͑iv͒ detrapping of localized quasiparticles by phonons, including both deformation-potential and spin-lattice couplings. Our results indicate that these processes will give rise to anomalies in the temperature dependence of kinetic parameters, which should be easily observable.

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“…The increase in charge output at higher temperatures is explained by the enhanced de-trapping mechanism due to phonon absorption, which is of course more effective at higher temperatures. Strong temperature dependencies of the charge output have already been observed in Ta based STJs [11], but these results were obtained at optical wavelengths. At higher photon energies the temperature dependence for such Ta based devices vanishes because the number of traps is no longer of importance when compared to the number of QPs in the junction.…”

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confidence: 84%

Strong quasiparticle trapping In A 6×6 array of vanadium-aluminum superconducting tunnel junctions

Brammertz

Peacock

Verhoeve

et al. 2002

AIP Conference Proceedings

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Abstract.A 6x6 array of symmetrical V/Al/AlOx/Al/V Superconducting Tunnel Junctions (STJs) was fabricated. The base electrode is a high quality epitaxial film with a residual resistance ratio (RRR) of ~30. The top film is polycrystalline with an RRR of ~10. The leakage currents of the 25x25 µm 2 junctions are of the order of 0.5 pA/µm 2 at a bias voltage of 100 µV, which corresponds to a dynamical resistance of ~ 3 10 5 Ω. When the array was illuminated by 6 keV X-ray photons from a 55 Fe radioactive source the single photon charge output was found to be low and strongly dependent on the temperature of the devices. This temperature dependence at X-ray energies can be explained by the existence of a very large number of quasiparticle (QP) traps in the Vanadium. QPs are confined in these traps, having a lower energy gap than the surrounding material, and are therefore not available for tunneling. The number of traps can be derived from the energy dependence of the responsivity of the devices (charge output per electron volt of photon input energy).As a process-route precursor to the fabrication of low T c STJs based on Mo and Al for use as optical to X-ray detectors in Astronomy, V-Al-AlOx-Al-V STJs were produced. Their characteristics from a film quality, leakage and spectral performance point of view are presented in this paper.The multilayer is deposited on a sapphire substrate without breaking vacuum. First a 100 nm thick epitaxial V film is sputtered at 550 o C in an UHV chamber. After cooling the sample a 30 nm thick Al film is deposited on top of the base V. 10 Å of Al are then oxidized to form the AlOx barrier of the junction. On top of this barrier another 30 nm of Al and 100 nm of V is deposited. The trilayer wafer then is removed from the UHV chamber and a ~7 nm thick VOx film naturally forms on top of the multilayer. Figure 1a shows a TEM image of the multilayer.Despite considerable lattice mismatch between the substrate and the V film, TEM has shown the base electrode to comprise an epitaxial film with a residual resistance ratio ρ 293K /ρ 10K (RRR) of ~30, as opposed to the top film which is polycrystalline with an RRR of ~10. This corresponds to a mean free path λ 10K in the base V of ~120 nm and a normal state diffusion constant D 10K of ~60 cm 2 /sec. The polycrystalline top film has a 3 times lower mean free path and diffusion constant. The junction's geometry is defined by using a photolithographical etching mask. Two different mask layouts are

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Local trap spectroscopy in superconducting tunnel junctions

Cited by 21 publications

References 12 publications

Dynamics of nonequilibrium quasiparticles in narrow-gap superconducting tunnel junctions

Dynamics of nonequilibrium quasiparticles in narrow-gap superconducting tunnel junctions

Inelastic scattering of quasiparticles in a superconductor with magnetic impurities

Strong quasiparticle trapping In A 6×6 array of vanadium-aluminum superconducting tunnel junctions

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