Switching Current Distribution Research Articles

Josephson radiation threshold detector

A Josephson radiation threshold detector (JRTD) that is based on the threshold behaviour of a current bias Josephson junction (CBJJ) is designed and fabricated for infrared radiation (IR@1550nm) detection at low temperatures. To achieve the optimal performance, we develop a binary hypothesis detection method to calibrate Josephson threshold behaviours (i.e. the switching current distributions of the CBJJ with the Al/AlOx/Al junction) in the absence and presence of radiation. In the absence of IR radiation, the junction transitioned with a measurable voltage drop across the junction, and this signal was treated as the events of hypothesis H0. The events of junction transition observed in the presence of the IR radiation served as hypothesis H1. Considering the usual Gaussian noise and based on statistical decision theory, the accumulated data of the measured switching current distributions are processed, and the threshold sensitivity of the demonstrated JRTD device is estimated. The minimum detectable IR radiation power of the proposed detector is approximately 0.74 pW, which corresponds to the photon rate of 5.692 × 106 photons/second. Further optimisation of JRTDs to implement the desired binary detection of a single photon is still a subject of argument, at least theoretically.

Switching current distributions in ferromagnetic anomalous Josephson junctions

We investigate the switching current distributions of ferromagnetic anomalous Josephson junctions subjected to a linearly increasing bias current. Our study uncovers a significant correlation between the position of the switching current distributions and crucial system parameters, such as the strength of the spin–orbit coupling and the Gilbert damping parameter. This indicates that these parameters can be directly determined through experimental measurements. By conducting a comprehensive analysis of the interplay among noise, magnetization, phase dynamics, and the statistical properties of the switching current distribution, we deepen our understanding of these intriguing cryogenic spintronics devices. These findings hold potential for applications in the field of quantum computing architectures and information processing technologies.

Signatures of Gate-Driven Out-of-Equilibrium Superconductivity in Ta/InAs Nanowires.

Understanding the microscopic origin of the gate-controlled supercurrent (GCS) in superconducting nanobridges is crucial for engineering superconducting switches suitable for a variety of electronic applications. The origin of GCS is controversial, and various mechanisms have been proposed to explain it. In this work, we have investigated the GCS in a Ta layer deposited on the surface of InAs nanowires. Comparison between switching current distributions at opposite gate polarities and between the gate dependence of two opposite side gates with different nanowire-gate spacings shows that the GCS is determined by the power dissipated by the gate leakage. We also found a substantial difference between the influence of the gate and elevated bath temperature on the magnetic field dependence of the supercurrent. Detailed analysis of the switching dynamics at high gate voltages shows that the device is driven into the multiple phase slips regime by high-energy fluctuations arising from the leakage current.

Activation Energies in MoSi/Al Superconducting Nanowire Single-Photon Detectors

Superconducting nanowire single-photon detectors (SNSPDs) are receiving the interest of the scientific and industrial communities due to their unparallel high performances in the infrared. However, even though the fabrication process permits the achievement of about 98% efficiency and less than 1 cps dark-count rate, the physical mechanism inducing the detection remains unclear. It is clear however that normal core vortices play a crucial role. In this work we investigate the role of vortices in two-dimensional (2D) molybdenum silicide SNSPDs of different widths covered with an $\mathrm{Al}$ layer through the analysis of the switching current distributions from the superconducting to resistive regime, in a wide interval of temperatures from 4.5 K down to 10 mK. This analysis provides the energy scales of different mechanisms that are responsible for fluctuations and dark counts in SNSPDs. We consider two models based on vortices, the unbinding of vortex-antivortex pairs (VAPs) and vortices hopping over the edge barrier (VH) and we underline the differences among different devices made by different materials. We also estimate the energy scales of similar $\mathrm{Nb}\text{\ensuremath{-}}\mathrm{Ti}\text{\ensuremath{-}}\mathrm{N}$ and $\mathrm{Nb}\mathrm{N}$ devices and compare the results. The lower activation energies obtained for $\mathrm{Mo}\mathrm{Si}/\mathrm{Al}$ devices, explain the peculiarity of this material to work at longer wavelengths with a higher quantum detection efficiency.

Photon-assisted Phase Slips in Superconducting Nanowires

Phase slip is the intrinsic fluctuation of the order parameter and leads to the dissipation of superconductors. Here we propose a photon-assisted phase-slip model in superconducting nanowires to explain the phase-slip rate under photon irradiation. In this phenomenological model, incident photons destroy large quantities of Cooper pairs and reduce the free-energy barrier of phase slips, resulting in proliferation in the phase-slip events and leading to superconducting transition. The switching rates from the superconducting state of a niobium nitride nanowire under various photon irradiation (4.4 mW/m${}^{2}$--4.4 W/m${}^{2}$) and temperatures (2.3--3.7 K) are investigated through the distribution of switching currents in the experiment. The experimental data can be well fitted by our deduced expression of phase-slip rate after eliminating the influence of external noise. Our model develops the phase-slip theory under incident photons and is promising to reveal the intrinsic detection mechanism of the superconducting nanowire single-photon detector.

Lévy noise effects on Josephson junctions

We review three different approaches to investigate the non-equilibrium stochastic dynamics of a Josephson junction affected by Lévy-distributed current fluctuations. First, we study the lifetime in the metastable superconducting state of current-biased short and long junctions, in the presence of Gaussian and Lévy noise sources. We highlight the noise-induced nonmonotonic behavior of the mean switching time as a function of noise intensity and driving frequency, that is the noise enhanced stability and the stochastic resonant activation, respectively. Then, we characterize the Lévy noise source through the average voltage drop across a current-biased junction. The voltage measurement versus the noise intensity allows to infer the value of the stability index that characterizes Lévy-distributed fluctuations. The numerical calculation of the average voltage drop across the junction well agrees with the analytical estimate of the average velocity for Lévy-driven escape processes from a metastable state. Finally, we look at the distribution of switching currents out of the zero-voltage state, when a Lévy noise signal is added to a linearly ramped bias current. The analysis of the cumulative distribution function of the switching currents gives information on both the Lévy stability index and the intensity of fluctuations. We present also a theoretical model to catch the features of the Lévy signal from a measured distribution of switching currents. The phenomena discussed in this work can pave the way for an effective and reliable Josephson-based scheme to characterize Lévy components eventually embedded in an unknown noisy signal.

Phase slips dynamics in gated Ti and V all-metallic supercurrent nano-transistors

The effect of electrostatic gating on metallic elemental superconductors was recently demonstrated in terms of modulation of the switching current and control of the current phase relation in superconducting quantum interferometers. The latter suggests the existence of a direct connection between the macroscopic quantum phase (φ) in a superconductor and the applied gate voltage. The measurement of the switching current cumulative probability distributions is a convenient and powerful tool to analyze such relation. In particular, the comparison between the conventional Kurkijärvi–Fulton–Dunkleberger model and the gate-driven distributions give useful insights into the microscopic origin of the gating effect. In this paper, we summarize the main results obtained in the analysis of the phase slip events in elemental gated superconducting weak-links in a wide range of temperatures between 20 mK and 3.5 K. Such a large temperature range demonstrates both that the gating effect is robust as the temperature increases, and that fluctuations induced by the electric field are not negligible in a wide temperature range.

Detection of bias inhomogeneity in Josephson junctions by switching current distributions

We demonstrate the double-peaked switching current distributions of a long Josephson junction from the zero to the finite voltage state in the case of asymmetrical bias feed profile. The considered asymmetry stands to be a natural feature of YBCO junctions fabricated on bicrystal substrates. We argue that such a picture is associated with the formation of solitons at the opposite junction edges for slightly different bias currents, arisen due to asymmetry. System parameters such as junction length, noise intensity and character of bias feed inhomogeneity leading to atypical distribution of switching currents are discussed.

Gate Control of Superconductivity in Mesoscopic All-Metallic Devices.

The possibility to tune, through the application of a control gate voltage, the superconducting properties of mesoscopic devices based on Bardeen–Cooper–Schrieffer metals was recently demonstrated. Despite the extensive experimental evidence obtained on different materials and geometries, a description of the microscopic mechanism at the basis of such an unconventional effect has not been provided yet. This work discusses the technological potential of gate control of superconductivity in metallic superconductors and revises the experimental results, which provide information regarding a possible thermal origin of the effect: first, we review experiments performed on high-critical-temperature elemental superconductors (niobium and vanadium) and show how devices based on these materials can be exploited to realize basic electronic tools, such as a half-wave rectifier. Second, we discuss the origin of the gating effect by showing gate-driven suppression of the supercurrent in a suspended titanium wire and by providing a comparison between thermal and electric switching current probability distributions. Furthermore, we discuss the cold field-emission of electrons from the gate employing finite element simulations and compare the results with experimental data. In our view, the presented data provide a strong indication regarding the unlikelihood of the thermal origin of the gating effect.

Gate-Controlled Suspended Titanium Nanobridge Supercurrent Transistor.

Under standard conditions, the electrostatic field-effect is negligible in conventional metals and was expected to be completely ineffective also in superconducting metals. This common belief was recently put under question by a family of experiments that displayed full gate-voltage-induced suppression of critical current in superconducting all-metallic gated nanotransistors. To date, the microscopic origin of this phenomenon is under debate, and trivial explanations based on heating effects given by the negligible electron leakage from the gates should be excluded. Here, we demonstrate the control of the supercurrent in fully suspended superconducting nanobridges. Our advanced nanofabrication methods allow us to build suspended superconducting Ti-based supercurrent transistors which show ambipolar and monotonic full suppression of the critical current for gate voltages of VGC ≃ 18 V and for temperatures up to ∼80% of the critical temperature. The suspended device architecture minimizes the electron-phonon interaction between the superconducting nanobridge and the substrate, and therefore, it rules out any possible contribution stemming from charge injection into the insulating substrate. Besides, our finite element method simulations of vacuum electron tunneling from the gate to the bridge and thermal considerations rule out the cold-electron field emission as a possible driving mechanism for the observed phenomenology. Our findings promise a better understanding of the field effect in superconducting metals.

Electrostatic Control of Phase Slips in Ti Josephson Nanotransistors

The investigation of the switching current probability distribution of a Josephson junction is a conventional tool to gain information on the phase slips dynamics as a function of the temperature. Here we adopt this well-established technique to probe the impact of an external static electric field on the occurrence of phase slips in gated all-metallic titanium (Ti) Josephson weak links. We show, in a temperature range between 20 mK and 420 mK, that the evolution of the phase slips dynamics as a function of the electrostatic field starkly differs from that observed as a function of the temperature. This fact demonstrates, on the one hand, that the electric field suppression of the critical current is not simply related to a conventional thermal-like quasiparticle overheating in the weak-link region. On the other hand, our results may open the way to operate an electrostatic-driven manipulation of phase slips in metallic Josephson nanojunctions, which can be pivotal for the control of decoherence in superconducting nanostructures.

The Role of Multiple Fluctuation Events in NbN and NbTiN Superconducting Nanostrip Single-Photon Detectors

We report on measurements of the switching current distributions on two-dimensional NbN superconducting nanostrip single-photon detectors (SNSPD), 5 nm thick and 80 nm wide, in an interval of temperatures from 6 K down to 0.3 K and compare the data with those obtained for similar NbTiN nanostrips. The standard deviations of the switching distributions show an extended region at high temperatures where multiple phase slip switching events occur. This is probably related to a decreasing critical current and an increasing electron and phonon heat capacities. In this temperature region, the width of the switching distribution, and therefore the dark count rate, is considerably reduced down to values below those observed at the lowest temperature. Finally, we also quantify the energy scale of the fluctuation phenomena. The proposed experimental approach may result in a powerful tool for the diagnostic of SNSPD operation mode.

Superconductor to resistive state switching by multiple fluctuation events in NbTiN nanostrips

We report on measurements of the switching current distributions on two-dimensional superconducting NbTiN strips that are 5 nm thick and 80 nm wide. We observe that the width of the switching current distributions has a non-monotonous temperature dependence, where it is constant at the lowest temperatures up to about 1.5 K, after which it increases with temperature until 2.2 K. Above 2.5 K any increase in temperature decreases the distribution width which at 4.0 K is smaller than half the width observed at 0.3 K. By using a careful analysis of the higher order moments of the switching distribution, we show that this temperature dependence is caused by switching due to multiple fluctuations. We also find that the onset of switching by multiple events causes the current dependence of the switching rate to develop a characteristic deviation from a pure exponential increase, that becomes more pronounced at higher temperatures, due to the inclusion of higher order terms.

Josephson-based Threshold Detector for Lévy-Distributed Current Fluctuations

We propose a threshold detector for L\'evy distributed fluctuations based on a Josephson junction. The L\'evy noise current added to a linearly ramped bias current results in clear changes in the distribution of switching currents out of the zero-voltage state of the junction. We observe that the analysis of the cumulative distribution function of the switching currents supplies information on both the characteristics shape parameter $\alpha$ of the L\'evy statistics and the intensity of the fluctuations. Moreover, we discuss a theoretical model which allows to extract characteristic features of the L\'evy fluctuations from a measured distribution of switching currents. In view of this results, this system can effectively find an application as a detector for a L\'evy signal embedded in a noisy background.

Topological supercurrents interaction and fluctuations in the multiterminal Josephson effect

We study the Josephson effect in the multiterminal junction of topological superconductors. We use the symmetry-constrained scattering matrix approach to derive band dispersions of emergent sub-gap Andreev bound states in a multidimensional parameter space of superconducting phase differences. We find distinct topologically protected band crossings that serve as monopoles of finite Berry curvature. Particularly, in a four-terminal junction the admixture of $2\pi$ and $4\pi$ periodic levels leads to the appearance of finite energy Majorana-Weyl nodes. This topological regime in the junction can be characterized by a quantized nonlocal conductance that measures the Chern number of the corresponding bands. In addition, we calculate current-phase relations, variance, and cross-correlations of topological supercurrents in multiterminal contacts and discuss the universality of these transport characteristics. At the technical level these results are obtained by integrating over the group of a circular ensemble that describes the scattering matrix of the junction. We briefly discuss our results in the context of observed fluctuations of the gate dependence of the critical current in topological planar Josephson junctions and comment on the possibility of parity measurements from the switching current distributions in multiterminal Majorana junctions.

Josephson-like dynamics of the superconducting LaAlO3/SrTiO3 interface

In this article, we show that the two-dimensional electron gas formed at the ${\mathrm{LaAlO}}_{3}/{\mathrm{SrTiO}}_{3}$ interface behaves as a Josephson junction array. In particular, it exhibits a stochastic switching of the superconducting critical current, which qualitatively follows the dynamics of the resistively and capacitively shunted Josephson junction model. The switching current distribution (SCD) has been measured as a function of temperature and back-gate voltage. At low temperatures a clear saturation of the standard deviation of the SCD is observed, possibly indicating the presence of a macroscopic quantum tunneling regime with phase diffusion. Through the gate voltage we modify the damping of the array and compare it to artificial arrays of junctions.

Electrodynamics of Josephson junctions containing strong ferromagnets

Triplet supercurrents in multilayer ferromagnetic Josephson junctions with misaligned magnetization can penetrate thicker ferromagnetic barriers compared to the singlet component. Although the static properties of these junctions have been extensively studied, the dynamic characteristics remain largely unexplored. Here we report a comprehensive electrodynamic characterization of multilayer ferromagnetic Josephson junctions composed of Co and Ho. By measuring the temperature-dependent current-voltage characteristics and the switching current distributions down to 0.3 K, we show that phase dynamics of junctions with triplet supercurrents exhibits long (in terms of proximity) junction behavior and moderately damped dynamics with renormalized capacitance and resistance. This unconventional behavior possibly provides a different way to dynamically detect triplets. Our results show new theoretical models are required to fully understand the phase dynamics of triplet Josephson junctions for applications in superconducting spintronics.

Anomalous Switching Current Distributions in Superconducting Weak Links

Anomalous Switching Current Distributions in Superconducting Weak Links

Parallel Simulation of Josephson Junctions With Multiplicative Noise

Parallel graphic processing units have been employed for fast simulations of the switching dynamics of Josephson junctions subject to critical current fluctuations. Such a system is modeled by a nonequilibrium washboard model with multiplicative noise, for which analytical results are lacking. The proposed approach allows us to execute extensive numerical simulation in short time and with relatively inexpensive resources. This allows us to fully characterize the effect of the noise on the junction switching current distributions at realistic bias current sweeprates.

Dynamics of Phase Switch in the Intrinsic Josephson Junctions Made of Bi2212 with Perfectly-stoichiometric Cation Compositions

We study the phase switching dynamics in the intrinsic Josephson junctions (IJJs) of Bi2Sr2CaCu2Oy (Bi2212) cuprate superconductors, focusing on the effect of stoichiometry of cation composition. The current-voltage characteristics for the IJJs with the cation composition approaching to be perfectly stoichiometric (Bi:Sr:Ca:Cu=2:2:1:2) show the highly-underdamped nature with a very small retrapping current. We found that the switching current distribution P(I) for the IJJs with perfectly stoichiometric composition and less damage had double peaks or broadened. This suggests that the phase switched junction is not always specified in the case of IJJs with stoichiometric composition, since more than one junction with almost the same switching current is associated with the observed switching events. We obtained the magnitude of a critical current density Jc and an effective temperature Tesc for the phase escape composed of a single component, by analysing the bias current dependence of the switching rate Γ(I). The results indicate that the perfectly-stoichiometric cation composition can greatly enhance the magnitude of Jc, although a crossover temperature to the macroscopic quantum tunnelling (MQT) for the higher-ordered phase switches is not influenced by the enhancement of Jc, in contrast to the conventional MQT theory.