Quasiparticle Diffusion Research Articles

Experimental Test of Theories of the Detection Mechanism in a Nanowire Superconducting Single Photon Detector

We report an experimental test of the photodetection mechanism in a nanowire superconducting single photon detector. Detector tomography allows us to explore the 0.8-8 eV energy range via multiphoton excitations. High accuracy results enable a detailed comparison of the experimental data with theories for the mechanism of photon detection. We show that the temperature dependence of the efficiency of the superconducting single photon detector is determined not by the critical current but by the current associated with vortex unbinding. We find that both quasiparticle diffusion and vortices play a role in the detection event.

Numerical analysis of detection-mechanism models of superconducting nanowire single-photon detector

The microscopic mechanism of photon detection in superconducting nanowire single-photon detectors is still under debate. We present a simple but powerful theoretical model that allows us to identify essential differences between competing detection mechanisms. The model is based on quasi-particle multiplication and diffusion after the absorption of a photon. We then use the calculated spatial and temporal evolution of this quasi-particle cloud to determine detection criteria of three distinct detection mechanisms, based on the formation of a normal conducting spot, the reduction of the effective depairing critical current below the bias current, and a vortex-crossing scenario, respectively. All our calculations as well as a comparison to experimental data strongly support the vortex-crossing detection mechanism by which vortices and antivortices enter the superconducting strip from the edges and subsequently traverse it thereby triggering the detectable normal conducting domain. These results may therefore help to reveal the microscopic mechanism responsible for the detection of photons in superconducting nanowires.

Hotspot Temperatures Reached in Current-Driven Superconducting Niobium Filaments

In a superconducting microbridge too narrow to support vortex motion, the current-induced resistance occurs non-uniformly at definite spots designated as Phase-Slip-Centres (PSC). Further, if the core of a PSC happens to heat above the critical temperature Tc, a PSC may evolve into a normal propagating zone, or hotspot. The PSC’s time of nucleation and the HS minimum current Ih are determined, which allows deriving without ambiguity the rate of heat transfer to the substrate, the latter is compatible with a phonon blackbody radiation model at the Nb/R–sapphire interface. The computation of the HS temperature is then straightforward, while the PSC case is more involved. However, the PSC core temperature can be obtained through an independent determination of the inelastic quasi-particle diffusion length Λqp∼2.8 μm. The results of these computations are consistent with all the specific cases, PSCs and HSs, measured experimentally.

Field-suppressed interfacial spin scattering in YBa2Cu3O7−δ/La0.67Sr0.33MnO3 heterostructures

The superconductivity and magnetoresistance (MR) of epitaxial bilayers consisting of YBa2Cu3O7−δ (YBCO) and La0.67Sr0.33MnO3 (LSMO) are investigated with respect to the proximity and spin polarization effects. In the region of the mixed vortex state near the onset temperature of the superconducting transition, the diffusion of the spin-polarized quasiparticles from the LSMO to the YBCO is evidenced by their distinct magnetotransport behaviors, which are characterized by two superconducting transitions. It is revealed that the second transition is strongly dependent on the applied magnetic field, demonstrating an abnormal pair-breaking effect. This effect, together with the reduced MR, is believed to arise from an increase in the spin injection efficiency induced by the suppressed interfacial spin scattering, disclosing a new perspective for understanding the magnetotransport phenomena in proximity-coupled superconductor/ferromagnet (S/F) hybrids.

Efficiency of quasiparticle evacuation in superconducting devices

We have studied the diffusion of excess quasiparticles in a current-biased superconductor strip in proximity to a metallic trap junction. In particular, we have measured accurately the superconductor temperature at a near-gap injection voltage. By analyzing our data quantitatively, we provide a full description of the spatial distribution of excess quasiparticles in the superconductor. We show that a metallic trap junction contributes significantly to the evacuation of excess quasiparticles.

Reentrant resistance via spin-polarized quasiparticle self injection in CMR/HTSC heterostructures

Epitaxial superlattices of half-metal, colossal magnetoresistive La 2/3Ca 1/3MnO 3 (HM-CMR) and high- T c superconducting YBa 2Cu 3O 7- δ (HTSC) are grown with thick and thin modulation lengths ( Λ) of YBCO/LCMO, with Λ = 280 nm and 12.5 nm; respectively, on SrTiO 3 (0 0 1) single-crystalline substrates by pulsed laser deposition. Transport measurements R( T) show a resistive state below T = 35 K although the superconducting transition temperature is found to be T c = 60 K and 63 K for both different superlattices, respectively. The onset of the resistive state coincides with a magnetic transition of the samples. This can be explained by a diffusion of spin-polarized quasiparticles into the superconducting film. Which can be considered as evidence for inverse-proximity effects over a wide temperature range in HM-CMR/HTSC heterostructures.

Magnetic field and contact resistance dependence of non-local charge imbalance

Crossed Andreev reflection (CAR) in metallic nanostructures, a possible basis forsolid-state electron entangler devices, is usually investigated by detecting non-local voltagesin multi-terminal superconductor/normal metal devices. This task is difficult because othersubgap processes may mask the effects of CAR. One of these processes is the generation ofcharge imbalance (CI) and the diffusion of non-equilibrium quasi-particles in thesuperconductor. Here we demonstrate a characteristic dependence of non-local CIon a magnetic field applied parallel to the superconducting wire, which can beunderstood by a generalization of the standard description of CI to non-localexperiments. These results can be used to distinguish CAR and CI and to extract CIrelaxation times in superconducting nanostructures. In addition, we investigatethe dependence of non-local CI on the resistance of the injector and detectorcontacts and demonstrate a quantitative agreement with a recent theory using onlymaterial and junction characteristics extracted from separate direct measurements.

Strong influence of nonlocal nonequilibrium effects on the dynamics of the order parameter in a phase-slip center: Ring studies

We study the influence of the inelastic relaxation time ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{E}$ of the quasiparticle distribution function $f(E)$ on the phase slip process in quasi-one-dimensional superconducting rings at a temperature close to the critical temperature ${T}_{c}$. We find that the initial time of growth of the order parameter $|\ensuremath{\Delta}|$ in the phase slip core after the phase slip is a nonmonotonic function of ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{E}$ which has a maximum at ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{E}\ensuremath{\simeq}{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{GL}=\ensuremath{\pi}\ensuremath{\hbar}/8{k}_{B}({T}_{c}\ensuremath{-}T)$ and has a tendency to saturate for large ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{E}⪢{\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{GL}$. The effective ``heating'' of the electron subsystem due to the increase in $|\ensuremath{\Delta}|$ in the phase slip center together with the above effect result in a nonmonotonic dependence of the number of subsequent phase slips on ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{\ensuremath{\tau}}}_{E}$ in rings of relatively large radius (in which each phase slip reduces the current density to a small fraction of its initial value). During the phase slip process the order parameter distribution has two peaks near the phase slip core due to the diffusion of the nonequilibrium quasiparticles from that region.

Experimental and theoretical response of distributed read-out imaging devices with imperfect charge confinement

We present a model to describe the responsivity of distributed read-out imaging devices following photon absorption in the absorber or in the base or top film of the superconducting tunnel junctions at either end of the absorber. The model describes the processes most relevant for photon detection, taking into account diffusion of quasiparticles across the absorber and imperfect confinement in the superconducting tunnel junctions via exchange of quasiparticles between absorber and the junction. It incorporates diffusion mismatch between superconducting tunnel junction and absorber, possible asymmetry between the two junctions and asymmetry between base and top electrodes within each junction. We have conducted dedicated experiments in which different experimental conditions were varied in order to test the model. A good agreement was found between the experimental results and model predictions.

Quasiparticle-diffusion-based heating in superconductor tunneling microcoolers

In a hybrid Superconductor-Insulator-Normal metal tunnel junction biased just below the gap, the extraction of hot electrons out of the normal metal results in electronic cooling effect. The quasiparticles injected in the superconductor accumulate near the tunnel interface, thus increasing the effective superconductor temperature. We propose a simple model for the diffusion of excess quasiparticles in a superconducting strip with an additional trap junction. This diffusion model has a complete analytic solution, which depends on experimentally accessible parameters. We find that the accumulated quasiparticles near the junction reduce the efficiency of the device. This study is also relevant to more general situations making use of superconducting tunnel junctions, as low temperature detectors.

Energy Decay in Superconducting Josephson-Junction Qubits from Nonequilibrium Quasiparticle Excitations

We calculate the energy decay rate of Josephson qubits and superconducting resonators from nonequilibrium quasiparticles. The decay rates from experiments are shown to be consistent with predictions based on a prior measurement of the quasiparticle density n(qp) = 10/microm(3), which suggests that nonequilibrium quasiparticles are an important decoherence source for Josephson qubits. Calculations of the energy-decay and diffusion of quasiparticles also indicate that prior engineered gap and trap structures, which reduce the density of quasiparticles, should be redesigned to improve their efficacy. This model also explains a striking feature in Josephson qubits and resonators-a small reduction in decay rate with increasing temperature.

Localized phase-slip centers in proximity-induced long superconducting nanowires

Current-voltage (I-V) curves have been measured on 60-μm-long Zn nanowires in contact with bulk film electrodes of Sn or Pb. The Zn nanowires become superconducting at the transition temperatures of the respectively electrodes, which are well above the transition temperature of bulk Zn. At temperatures well above the transition temperature of bulk Zn but below the transition temperatures of the electrodes, the I-V curves display a reproducible steplike pattern characteristic of the establishment of multiple localized phase-slip centers. The typical quasiparticle diffusion length associated with a phase-slip center is estimated to be 5–10μm. These results demonstrate that the observed proximity effect occurs along the entire length of a nanowire, rather than only at the nanowire-electrode interfaces.

Feynman formula for a diffusion of particles with a variable mass in a domain

In the present note we consider a class of second order parabolic equations with position dependent coefficients; such equations describe a diffusion of (quasi) particles with a variable mass. We represent a solution of Cauchy-Dirichlet problem for such class of equations in a bounded domain in the form of a limit of finite dimensional integrals of elementary functions. Such kind of a representation is usually called Feynman formula and can be used for calculations. Finite dimensional integrals in our Feynman formula give approximations for a functional integral over a probability measure on a set of trajectories in the domain where the solution of the considered problem is investigated; this measure is generated by a diffusion process with variable diffusion coefficient and absorption on the boundary, hence, to get Feynman formula also means to get a representation of the solution of the considered problem with the help of a functional integral (such kind of a representation is usually called Feynman-Kac formula).

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.

Toward the optimization of electronic refrigerators

Normal metal–insulator–superconductor (NIS) junction can act as refrigerator of electron gas in a normal metal electrode under certain conditions. If majority of high energy electrons in a normal metal is moved with a bias voltage to energy levels above the superconductor's energy gap, their extraction from a normal metal is enabled by tunnelling. Their substitution with lower energy electrons results in lowering of the normal metal electron gas energy. Due to excess quasiparticle density in the superconductive electrode in vicinity of the junction, which is a result of tunnelling of quasiparticles from the normal metal and low capability of removal of these particles from the junction area the cooling capability of junctions deteriorates at lower environment temperatures. This is mainly the results of back tunnelling of excess quasiparticles to a normal metal and of dissipated energy, released by Coooper pair formation in proximity of the junction. Quantitative description of quasiparticle behavior in the superconductive electrode of the NIS junction is given. Energy dependent diffusion of quasiparticles in superconductor, their consumption due to Cooper pair creation and inflow of additional quasiparticles from a normal metal are considered. The model results in position dependence of quasiparticles' density in the junction proximity.

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.

Advances in ${\hbox{YBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7-\delta}$ Grain Boundary Biepitaxial Josephson Junctions: Transport Properties and Mesoscopic Effects

We report on the latest developments in the study of transport properties of YBa2Cu3O7-delta (YBCO)-based grain boundary (GB) biepitaxial Josephson junctions. We have successfully extended the fabrication process, originally developed for (110) oriented SrTiO3 substrates, to (110) oriented LaAlO3 substrates in view of microwave applications. We will report, also, on the observation of conductance fluctuations, we believe related to the coherent diffusion of quasi-particles across the GB. These results, together with the recent observation of macroscopic quantum tunneling and energy level quantization in these junctions, could help cast some light on the mechanism involved in transport across grain boundaries and in revising the role and the effect of quasi particles in d-wave transport.

Fabrication of array detectors with 100 superconducting tunnel junctions and Ta x‐ray absorbers

AbstractWe have developed superconducting tunnel junction (STJ) x‐ray array detectors with a large detection area, high detection efficiency, and high counting rate. The STJ array detectors consist of 100 Nb/Al‐AlOx/Al/Nb STJs with a size of 200 × 200 µm. Each STJ has a 2 µm thick Ta absorber connected to the top Nb layer through a thin insulating layer. The absorbers act as an energy converter from high x‐ray energies to phonon energies, which are compatible with superconducting energy gaps. The array detectors realize a total detection area of 4 mm2 and a detection efficiency of ∼70% for x‐rays of energy up to 6 keV. In addition, it is expected that the spatial uniformity of the x‐ray signal outputs of the Nb‐based STJ by using the Ta absorber may be improved by a fast diffusion of quasiparticles in the Ta absorbers. In order to minimize cross talk, we have adapted a coplanar transmission waveguide (CPW) structure having an 8 µm wide signal Nb line, 7 µm wide ground gaps, 12 µm wide ground Nb lines, and a 50 Ω impedance. The fabrication processes and the detector response to x‐rays are discussed. Copyright © 2007 John Wiley & Sons, Ltd.

Spin diffusion versus proximity effect at ferromagnet/superconductorLa0.7Ca0.3MnO3∕YBa2Cu3O7−δinterfaces

We report on the interplay between magnetism and superconductivity in La_(0.7)Ca_(0.3)MnO_(3)/YBa_(2)Cu_(3)O_(7) structures. We have grown heterostructures (bilayers and trilayers) with a constant thickness of the ferromagnetic layer of 40 unit cells (15 nm) and changing the thickness of the superconductor between 1 (1.2 nm) and 40 unit cells (48 nm). The critical temperature of the bilayers decreases when the thickness of the superconductor is reduced below 10 unit cells, thus providing an estimate of the length scale of superconductivity suppression by spin-polarized quasiparticles in YBa_(2)Cu_(3)O_(7-δ) (YBCO) of 10 nm, much larger than the coherence length. For thickness of the YBCO layer smaller than 4 unit cells; a second mechanism of superconductivity depression comes into play, probably related to the ferromagnetic/superconducting proximity effect. The relative importance in depressing the critical temperature of intrinsic mechanisms (quasiparticle diffusion and proximity effect) and extrinsic ones (intralayer disorder, interface roughness, or reduced dimensionality of ultrathin layers) is discussed.

The role of spin diffusion quasiparticle in CMR/HTSC heterostructures

Half metal-colossal magnetoresistive La 2 / 3 Ca 1 / 3 MnO 3 (HM-CMR) and YBa 2 Cu 3 O 7 - δ high-T c superconductors (HTSC) bilayer structures are grown on SrTiO 3 (100) single crystalline substrates using pulsed laser deposition. Magnetization and magneto-optical measurements are carried out that show the coexistence of ferromagnetism and superconductivity at low temperatures. Using the HM-CMR layer as an electrode for spin polarized electrons, we investigate spin polarized self injection into the HTSC layer. The experimental results are in good agreement with a theoretical model, that is developed with respect to spin polarized quasiparticle diffusion.