We have studied the voltage response of superconducting NbTiN filaments to a step-pulse of over-critical current I > Ic. The current induces the destruction of the Cooper pairs and initiates different mechanisms of dissipation depending on the bath temperature T. For the sample investigated, and for T above a certain T*, not far from Tc, the resistance manifests itself in the form of a phase-slip center, which turns into a normal hot spot (HS) as the step-pulse is given larger amplitudes. However, at all temperatures below T*, the destruction of superconductivity still occurs at Ic(T), but leads directly to an ever-growing HS. By lowering the current amplitude during the pulse, one can produce a steady HS and thus define a threshold HS current Ih(T). That is achieved by combining two levels of current, the first and larger one to initiate an HS, the second one to search for constant voltage response. The double diagram of the functions Ic(T) and Ih(T) was plotted in the T-range Tc/2 < T < Tc, and their crossing found at T* = (8.07 ± 0.07) K.

We have measured in superconducting niobium filaments the delay time t d that separates the initiation of a pulse of overcritical current (I > I c ) from its first resistive response. The experiments were performed at various temperatures, typically 4, 5, and 6 K, well below the critical temperature T c , for delays in the range 0.3 ns <; t d <; 80 ns, divided into two parts for technical reasons. The data t d (T/T c , I/I c ) were analyzed through a time-dependent Ginzburg-Landau theory leaving the gap relaxation time τ d as an unknown parameter. In a restricted range of current amplitudes (I/I c <; 1.15), a fit is obtained by choosing τ d between 1.65 and 1.75 ns, which we interpret as a film cooling time of 23 ps per nm thickness, almost independently of the temperature.

We have investigated the voltage response of superconducting NbTiN strips to a step-pulse of overcritical current in the range of temperatures 0.4 < T/T c < 0.9, where the critical temperature, T c , is 8.7 K. The current-induced destruction of the Cooper pairs leads to the nucleation of a phase-slip center. The response appears after a certain delay time t d , which we analyze through a Time-Dependent Ginzburg-Landau (TDGL) theory according to Tinkham’s approach. The experimental findings can be fitted by inferring a film cooling time of about 1.8 ns for a 20-nm-thick film, very little dependent upon sample width and temperature. Assuming a definite ratio between the electron and phonon specific heats, one deduces an average phonon escape time of 90 ps per nm thickness of NbTiN film sputtered on sapphire.

We have observed the voltage response of superconducting niobium strips to overcritical (I > I c ) step pulses of electrical current. The resistive response appears after a certain delay time td function of the temperature and of the ratio I/I c (T), which can be analyzed through a time-dependent Ginzburg- Landau equation, along the method introduced by Tinkham. The experimental data can be fitted by using an effective gap relaxation time of a few nanoseconds, independently of the sample widths and of the temperature. Assuming proportionality to sample thickness, this indicates a phonon escape time of about 8 ps per nanometer of thickness for a Nb film dc sputtered on polished crystalline Al 2 O 3 .

Using the anisotropic time-dependent Ginzburg–Landau theory we study the effect of ordered and disordered pinning on the time response of superconducting strips to an external current that switched on abruptly. The pinning centers result in a considerable delay of the response time of the system to such abrupt switching on of the current, whereas the output voltage is always larger when pinning is present. The resistive state in both cases are characterized either by dynamically stable phase-slip centers/lines or expanding in-time hot-spots, which are the main mechanisms for dissipation in current-carrying superconductors. We find that hot-spots are always initiated by the phase-slip state. However, the range of the applied current for the phase-slip state increases significantly when pinning is introduced. Qualitative changes are observed in the dynamics of the superconducting condensate in the presence of pinning.

Phase-slip centers/lines and hot spots are the main mechanisms for dissipation in current-carrying superconducting thin films. The pulsed-current method has recently been shown to be an effective tool in studying the dynamics of phase-slip centers and their evolution to hot spots. We use the time-dependent Ginzburg-Landau theory in the study of the dynamics of the superconducting condensate in superconducting strips under external current and zero external magnetic field. We show that both the flux-flow state (i.e., slow-moving vortices) and the phase-slip line state (i.e., fast-moving vortices) are dynamically stable dissipative units with temperature smaller than the critical one, whereas hot spots, which are localized normal regions where the local temperature exceeds the critical value, expand in time, resulting ultimately in a complete destruction of the condensate. The response time of the system to abrupt switching on of the overcritical current decreases with increasing both the value of the current (at all temperatures) and temperature (for a given value of the applied current). Our results are in good qualitative agreement with experiments we have conducted on Nb thin strips.

For over a decade, ultrathin superconducting films have been developed for the detection of single photons at optical or near infrared frequencies, with competitive performances in terms of quantum efficiency, speed, and low dark count rate. In order to avoid the requirement of helium refrigeration, we consider here the use of high temperature materials, known to achieve very fast responsiveness to laser irradiation. We excite thin filaments of the cuprate \(\hbox {YBa}_{2} \hbox {Cu}_{3} \hbox {O}_{7}\) by rectangular pulses of supercritical current so as to produce either a phase-slip centre (PSC) or a normal hot spot (HS), according to the temperature and the current amplitude selected. That procedure provides information about the maximum bias current to be used in a particle detector, about the return current back to the quiescent state after excitation, and about the rate of growth and decay of a HS. We also measure the time of PSC nucleation. A unique feature of that approach is to provide the rate of heat transfer between the film and its substrate at whatever temperature, in the superconducting state, in the practical conditions of operation.

The one-dimensional heat flow equation controlling the temperature of a current-driven hotspot (HS) in a long superconducting microbridge is reexamined in all its components. The resulting nonlinear differential system, which admits temperature-dependent thermal conductivities, and a blackbody-like phonon radiation into the substrate, is solved numerically. In this work, the phonon escape rate is not the outcome of a best-fitting procedure, but rather is derived from the dependence, in a pulse experiment, of the HS nucleation time upon the current intensity. As a result, the temperature profile of a self-heating HS in a niobium strip can be computed without any adjustable parameter for each choice of the bath temperature. One notes a severe limitation of the HS temperature as compared to previous models. The minimum current sustaining a stable HS thus determined is in close agreement with direct measurements even far from the critical temperature. The method is applied to a NbN filament typical of the superconducting single photon detectors.

WE REPORT THE DETECTION OF SINGLE ELECTRONS USING A 6 NM-THICK, 100 NM-WIDE, NB0.7TI0.3N SUPERCONDUCTING STRIP DEPOSITED ON A SIOX/SI SUBSTRATE. WHEN BIASED SLIGHTLY BELOW THE CRITICAL CURRENT, A MEANDER-SHAPED DEVICE, NOT ONLY DETECTS SINGLE PHOTONS, BUT ALSO COUNTS THE SINGLE KEV ELECTRONS ISSUED FROM A SCANNING ELECTRON MICROSCOPE (SEM) WITH AN EFFICIENCY APPROACHING UNITY. THE RESPONSE TIME IS SHORT ENOUGH TO DISCRIMINATE THE INCIDENT ELECTRONS FROM THOSE BACKSCATTERED FROM THE UNDERLYING MATERIAL. IT IS THEREFORE POSSIBLE TO MAP THE ELECTRON DETECTIVITY AS WELL AS THE PHOTON DETECTIVITY ON THE SAME DEVICE. A CLEAR CORRELATION BETWEEN THE TWO MEASUREMENTS IS OBSERVED, WITH A SUPERIOR SPATIAL RESOLUTION THOUGH (AROUND 100 NM) FOR THE SEM MAPPING. IT ILLUSTRATES THE POTENTIAL USE OF THIS SINGLE ELECTRON MAPPING BY THE SEM METHOD TO CHARACTERIZE THE DETECTION HOMOGENEITY OF SSPDS

We report the detection of single electrons using a Nb0.7Ti0.3N superconducting wire deposited on an oxidized silicon substrate. While it is known that this device is sensitive to single photons, we show that it also detects single electrons with kilo-electron-volt energy emitted from the cathode of a scanning electron microscope with an efficiency approaching unity. The electron and photon detection efficiency map of the same device are in good agreement. We also observe detection events outside the active area of the device, which we attribute to sensitivity to backscattered electrons.

Superconducting NbN wires have recently received attention as detectors for visible and infrared photons. We present experiments in which we use a NbN wire for high-efficiency (40 use the beam of a scanning electron microscope as a focussed, stable, and calibrated electron source. Scanning the beam over the surface of the wire provides a map of the detection efficiency. This map shows features as small as 150 nm, revealing wire inhomogeneities. The intrinsic resolution of this mapping method, superior to optical methods, provides the basis of a characterization tool relevant for photon detectors.

Using a sequence of stepped bias currents in the nanosecond range, we have studied the destruction of superconductivity in c-axis textured YBa2Cu3O7 strips of various thickness and structure (single or multi-layers). Sufficiently far from T c, vortex flow fades out and gradually gives way to localized dissipative structures, which can be interpreted either as Phase-Slip Centers (PSC) or normal Hot Spots (HS). A plot of the corresponding threshold currents in the current–temperature (I–T) plane indicates when each of them will occur, and how to switch over from one to the other, in a manner similar to that demonstrated for metallic materials (cf. Ladan et al., J. Low Temp. Phys. 153:103, 2008). The capability of some YBCO strips to support PSC’s at arbitrarily low temperatures escapes the common picture.

The resistance induced by nanosecond pulses of electric currrent or of light has been studied in superconducting Nb and NbN thin films. Narrow bridges exhibit the features expected from quasi-ID transport, namely, localized phase-slip centers (PSC) and hot spots (HS), depending upon the prescribed conditions of temperature and applied current. PSC and HS are clearly identified from their characteristic delay and their evolution in time. We then analyze the response to combined pulsed illumination and current. Using very thin (4 nm) niobium-nitride films, it was possible to detect the visible light delivered photon by photon.

The resistive response of c-axis textured YBa2Cu3O7 bridges (100 nm thick and 10 mu m wide) has been studied dynamically on the nanosecond scale over a wide span of temperatures and currents. From T, down to 75 K, dissipation is dominated by the viscosity of vortices spontaneously generated in the earth's magnetic field. For 70 K < T < 75 K, vortex flow coexists with phase-slip centers (PSCs) characteristic of one-dimensional (1-D) transport. Eventually, below about 70 K, vortex flow is inhibited while, independently, PSCs transform into normal hot spots. All these resistive modes are identified by using a different time-varying voltage consecutive to the application of a step function of the current to the bridge.

By applying nanosecond current pulses to narrow superconducting Nb strips, we have observed the induced resistive states expected for quasi 1-D transport, namely localized phase-slip centres (PSC) and hot spots (HS). The current-controlled drive discriminates stable-in-time PSC structures near T c from expanding HS at lower temperatures. HS-PSC exchange and return towards equilibrium are studied by using two-step current pulses. Remarkably, it appears that a hot spot never forms unless a PSC has first been nucleated. Then from a plot of the threshold currents I c ( T ) and I h ( T ), corresponding to PSC and HS, respectively, one can predict the response to current, temperature, or luminous excitation, as well as the effect of an applied magnetic field.

By electrodeposition of tin inside the pores of a polycarbonate template, nanowires were grown, out of which a single one was contacted electrically so as to form a quasi-one-dimensional superconductor (diameter: 50 nm). A resistive phase-slip center appears as a response to a pulse of supercritical current. Measuring its resistance, and its time of nucleation, gives access to the electron inelastic lifetime, and to the gap relaxation time, respectively, both in the nanosecond range. Finally, the threshold current for passage into the fully normal state indicates a good thermal contact with the polycarbonate matrix. (C) 2004 American Institute of Physics.

Topological defects such as vortices subsist for some time in a rapidly frozen superconducting film (W. H. Zurek, Phys. Rep. 276, 177, 1996). We propose to use as a probe of the vortex density the finite delay T d , which, in narrow strips, exists between a current step and the voltage response. Technically, this amounts to driving a bridge into the localized hot-spot regime by means of a pump pulse (laser or electrical). Cooling of such films as epitaxial YBCO-on-MgO, or niobium-on-sapphire, requires only a few nanoseconds at low T . A time Δ t later, a probe pulse is applied to measure T d . The dependence Δ t → T d is interpreted as a witness of the fossil vorticity, long after quenching into the zero-resistance regime.

By means of nanosecond pulse techniques, we have studied the current-induced dissipation in one-dimensional superconducting bridges, namely, metallic and high-Tc films. It is well known that narrow strips dissipate through phase-slip centers (PSC) close to Tc, or hot spots (HS) at low T, rather than by flux flow. When driven by step pulses of current, PSC give stable voltages, while HS produce a voltage linearly changing with time. By using two-step pulses of current, we have studied the decay of a HS into another HS, or a PSC, or into a zero-resistance state. It was thus found possible to reach the PSC state at arbitrary low temperatures.

In addition to the critical current Ic (T), which generates Phase-Slip Centers (PSCs), thin superconducting films possess a well-defined second limiting current Ih, or current intensity able to maintain a preestablished hot spot. By pulsing step functions of the current and monitoring the voltage response on the nanosecond scale, we have determined (T ↔ Ic) and (T ↔ Ih). From a dynamic study of the two main modes of dissipation in YBCO and Nb films, it is concluded that PSCs are stable structures in current-biased bridges. In contrast, hot spots grow at a constant rate of a few tens of meters per second, determined by the thermal diffusivity of the material and by its bolometric response time. On reducing the current from Ih, the so-called healing length, or minimum normal length, was found, of the order of 0.2 μm in YBCO and 2 μm in Nb. In summary, the experiment provides three independent measurements (PSC nucleation time, velocity of growth, and minimum length) for only two parameters (D and τ).